Landslides constitute one of the most destructive geological hazards in the world today. One of the main reasons for this is because of the high speeds that slides can reach, up to 160 km/hour (100 mph). Another is the fact that these slides can carry quite a bit of debris with them that serve to amplify their destructive force.
Taken together, this is what is known as a Debris Flow, a natural hazard that can take place in many parts of the world. A single flow is capable of burying entire towns and communities, covering roads, causing death and injury, destroying property and bringing all transportation to a halt. So how do we deal with them?
Definition:
A Debris Flow is basically a fast-moving landslide made up of liquefied, unconsolidated, and saturated mass that resembles flowing concrete. In this respect, they are not dissimilar from avalanches, where unconsolidated ice and snow cascades down the surface of a mountain, carrying trees and rocks with it.
Images of a Debris Flow Chute and Deposit, taken by the Arizona Geological Survey (AZGS). Credit: azgs.com
A common misconception is to confuse debris flows with landslides or mudflows. In truth, they differ in that landslides are made up of a coherent block of material that slides over surfaces. Debris flows, by contrast, are made up of “loose” particles that move independently within the flow.
Similarly, mud flows are composed of mud and water, whereas debris flows are made up larger particles. All told, it has been estimated that at least 50% of the particles contained within a debris flow are made-up of sand-sized or larger particles (i.e. rocks, trees, etc).
Types of Flows:
There are two types of debris flows, known as Lahar and Jökulhlaup. The word Lahar is Indonesian in origin and has to do with flows that are related to volcanic activity. A variety of factors may trigger a lahar, including melting of glacial ice due to volcanic activity, intense rainfall on loose pyroclastic material, or the outbursting of a lake that was previously dammed by pyroclastic or glacial material.
Jökulhlaup is an Icelandic word which describes flows that originated from a glacial outburst flood. In Iceland, many such floods are triggered by sub-glacial volcanic eruptions, since Iceland sits atop the Mid-Atlantic Ridge. Elsewhere, a more common cause of jökulhlaups is the breaching of ice-dammed or moraine-dammed lakes.
Debris flow channel in Ladakh, near the northwestern Indian Himalaya, produced in the storms of August 2010. Credit: Wikipedia Commons/DanHobley
Such breaching events are often caused by the sudden calving of glacier ice into a lake, which then causes a displacement wave to breach a moraine or ice dam. Downvalley of the breach point, a jökulhlaup may increase greatly in size by picking up sediment and water from the valley through which it travels.
Causes of Flows:
Debris flows can be triggered in a number of ways. Typically, they result from sudden rainfall, where water begins to wash material from a slope, or when water removed material from a freshly burned stretch of land. A rapid snowmelt can also be a cause, where newly-melted snow water is channeled over a steep valley filled with debris that is loose enough to be mobilized.
In either case, the rapidly moving water cascades down the slopes and into the canyons and valleys below, picking up speed and debris as it descends the valley walls. In the valley itself, months’ worth of built-up soil and rocks can be picked up and then begin to move with the water.
As the system gradually picks up speed, a feedback loop ensues, where the faster the water flows, the more it can pick up. In time, this wall begins to resemble concrete in appearance but can move so rapidly that it can pluck boulders from the floors of the canyons and hurl them along the path of the flow. It’s the speed and enormity of these carried particulates that makes a debris flow so dangerous.
Deforestation (like this clearcut in Sumatra, Indonesia) can result in debris flows. Credit: worldwildlife.org
Another major cause of debris flows is the erosion of steams and riverbanks. As flowing water gradually causes the banks to collapse, the erosion can cut into thick deposits of saturated materials stacked up against the valley walls. This erosion removes support from the base of the slope and can trigger a sudden flow of debris.
In some cases, debris flows originate from older landslides. These can take the form of unstable masses perched atop a steep slope. After being lubricated by a flow of water over the top of the old landslide, the slide material or erosion at the base can remove support and trigger a flow.
Some debris flows occur as a result of wildfires or deforestation, where vegetation is burned or stripped from a steep slope. Prior to this, the vegetation’s roots anchored the soil and removed absorbed water. The loss of this support leads to the accumulation of moisture which can result in structural failure, followed by a flow.
Sarychev volcano, (located in Russia’s Kuril Islands, northeast of Japan) in an early stage of eruption on June 12, 2009. Credit: NASA
A volcanic eruption can flash melt large amounts of snow and ice on the flanks of a volcano. This sudden rush of water can pick up ash and pyroclastic debris as it flows down the steep volcano and carry them rapidly downstream for great distances.
In the 1877 eruption of Cotopaxi Volcano in Ecuador, debris flows traveled over 300 kilometers down a valley at an average speed of about 27 kilometers per hour. Debris flows are one of the deadly “surprise attacks” of volcanoes.
Prevention Methods:
Many methods have been employed for stopping or diverting debris flows in the past. A popular method is to construct debris basins, which are designed to “catch” a flow in a depressed and walled area. These are specifically intended to protect soil and water sources from contamination and prevent downstream damage.
Some basins are constructed with special overflow ducts and screens, which allow the water to trickle out from the flow while keeping the debris in place, while also allowing for more room for larger objects. However, such basins are expensive, and require considerable labor to build and maintain; hence why they are considered an option of last resort.
Aerial view of the destruction caused by a debris-flow in the Venezuelan town of Caraballeda. Credit: US Geological Survey
Currently, there is no way to monitor for the possibility of debris flow, since they can occur very rapidly and are often dependent on cycles in the weather that can be unpredictable. However, early warning systems are being developed for use in areas where debris flow risk is especially high.
One method involves early detection, where sensitive seismographs detect debris flows that have already started moving and alert local communities. Another way is to study weather patterns using radar imaging to make precipitation estimates – using rainfall intensity and duration values to establish a threshold of when and where a flows might occur.
In addition, replanting forests on hillsides to anchor the soil, as well as monitoring hilly areas that have recently suffered from wildfires is a good preventative measure. Identifying areas where debris flows have happened in the past, or where the proper conditions are present, is also a viable means of developing a debris flow mitigation plan.
NASA's new app, the Globe Observer, will allow users to collect observations of clouds, and engage in a little citizen science. Image: NASA GLOBE Observer
It’s long been humanity’s dream to do something useful with our smartphones. Sure, we can take selfies, and post pictures of our meals, but true smartphone greatness has eluded us. Until now, that is.
Thanks to NASA, we can now do some citizen science with our ubiquitous devices.
For over 20 years, and in schools in over 110 countries, NASA’s Global Learning and Observations to Benefit the Environment (GLOBE) program has helped students understand their local environment in a global context. Now NASA has released the GLOBE Observer app, which allows users to capture images of clouds in their local environment, and share them with scientists studying the Earth’s climate.
“With the launch of GLOBE Observer, the GLOBE program is expanding beyond the classroom to invite everyone to become a citizen Earth scientist,” said Holli Riebeek Kohl, NASA lead of GLOBE Observer. The app will initially be used to capture cloud observations and images because they’re such an important part of the global climate system. But eventually, GLOBE Observer will also be used to observe land cover, and to identify types of mosquito larvae.
GLOBE has two purposes. One is to collect solid scientific data, the other is to increase users’ awareness of their own environments. “Once you collect environmental observations with the app, they are sent to the GLOBE data and information system for use by scientists and students studying the Earth,” said Kohl. “You can also use these observations for your own investigations and interact with a vibrant community of individuals from around the world who care about Earth system science and our global environment.”
Clouds are a dynamic part of the Earth’s climate system. Depending on their type, their altitude, and even the size of their water droplets, they either trap heat in the atmosphere, or reflect sunlight back into space. We have satellites to observe and study clouds, but they have their limitations. An army of citizen scientists observing their local cloud population will add a lot to the efforts of the satellites.
“Clouds are one of the most important factors in understanding how climate is changing now and how it’s going to change in the future,” Kohl said. “NASA studies clouds from satellites that provide either a top view or a vertical slice of the clouds. The ground-up view from citizen scientists is valuable in validating and understanding the satellite observations. It also provides a more complete picture of clouds around the world.”
The observations collected by GLOBE users could end up as part of NASA’s Earth Observatory, which tracks the cloud fraction around the world. Image: NASA/NASA Earth Observation.
The GLOBE team has issued a challenge to any interested citizen scientists who want to use the app. Over the next two weeks, the team is hoping that users will make ground observations of clouds at the same time as a cloud-observing satellite passes overhead. “We really encourage all citizen scientists to look up in the sky and take observations while the satellites are passing over through Sept. 14,” said Kohl.
The app makes this easy to do. It informs users when a satellite will be passing overhead, so we can do a quick observation at that time. We can also use Facebook or Twitter to view daily maps of the satellite’s path.
“Ground measurements are critical to validate measurements taken from space through remote sensing,” said Erika Podest, an Earth scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California, who is working with GLOBE data. “There are some places in the world where we have no ground data, so citizen scientists can greatly contribute to advancing our knowledge this important part of the Earth system.”
The app itself seems pretty straightforward. I checked for upcoming satellite flyovers and was notified of 6 flyovers that day. It’s pretty quick and easy to step outside and take an observation at one of those times.
I did a quick observation from the street in front of my house and it took about 2 minutes. To identify cloud types, you just match what you see with in-app photos of the different types of clouds. Then you estimate the percentage of cloud cover, or specify if the sky is obscured by blowing snow, or fog, or something else. You can also add pictures, and the app guides you in aiming the camera properly.
The GLOBE Observer app is easy to use, and kind of fun. It’s simple enough to fit a quick cloud observation in between selfies and meal pictures.
Download it and try it out.
You can download the IOS version from the App Store, and the Android version from Google Play.
An illustration showing the 8 planets of the Solar System to scale Credit: NASA
At one time, humans believed that the Earth was the center of the Universe; that the Sun, Moon, planets and stars all revolved around us. It was only after centuries of ongoing observations and improved instrumentation that astronomers came to understand that we are in fact part a larger system of planets that revolve around the Sun. And it has only been within the last century that we’ve come to understand just how big our Solar System is.
And even now, we are still learning. In the past few decades, the total number of celestial bodies and moons that are known to orbit the Sun has expanded. We have also come to debate the definition of “planet” (a controversial topic indeed!) and introduced additional classifications – like dwarf planet, minor planet, plutoid, etc. – to account for new finds. So just how many planets are there and what is special about them? Let’s run through them one by one, shall we?
Mercury:
As you travel outward from the Sun, Mercury is the closest planet. It orbits the Sun at an average distance of 58 million km (36 million mi). Mercury is airless, and so without any significant atmosphere to hold in the heat, it has dramatic temperature differences. The side that faces the Sun experiences temperatures as high as 420 °C (788 °F), and then the side in shadow goes down to -173 °C (-279.4 °F).
MESSENGER image of Mercury from its third flyby. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Like Venus, Earth and Mars, Mercury is a terrestrial planet, which means it is composed largely of refractory minerals such as the silicates and metals such as iron and nickel. These elements are also differentiated between a metallic core and a silicate mantle and crust, with Mercury possessing a larger-than-average core. Multiple theories have been proposed to explain this, the most widely accepted being that the impact from a planetesimal in the past blew off much of its mantle material.
Mercury is the smallest planet in the Solar System, measuring just 4879 km across at its equator. However, it is second densest planet in the Solar System, with a density of 5.427 g/cm3 – which is the second only to Earth. Because of this, Mercury experiences a gravitational pull that is roughly 38% that of Earth’s (0.38 g).
Mercury also has the most eccentric orbit of any planet in the Solar System (0.205), which means its distance from the Sun ranges from 46 to 70 million km (29-43 million mi). The planet also takes 87.969 Earth days to complete an orbit. But with an average orbital speed of 47.362 km/s, Mercury also takes 58.646 days to complete a single rotation.
Combined with its eccentric orbit, this means that it takes 176 Earth days for the Sun to return to the same place in the sky (i.e. a solar day) on Mercury, which is twice as long as a single Hermian year. Mercury also has the lowest axial tilt of any planet in the Solar System – approximately 0.027 degrees – compared to Jupiter’s 3.1 degrees, which is the second smallest.
The MESSENGER spacecraft has been in orbit around Mercury since March 2011 – but its days are numbered. Credit: NASA/JHUAPL/Carnegie Institution of Washington
Mercury has only been visited two times by spacecraft, the first being the Mariner 10 probe, which conducted a flyby of the planet back in the mid-1970s. It wasn’t until 2008 that another spacecraft from Earth made a close flyby of Mercury (the MESSENGER probe) which took new images of its surface, shed light on its geological history, and confirmed the presence of water ice and organic molecules in its northern polar region.
In summary, Mercury is made special by the fact it is small, eccentric, and varies between extremes of hot and cold. It’s also very mineral rich, and quite dense!
Venus:
Venus is the second planet in the Solar System, and is Earth’s virtual twin in terms of size and mass. With a mass of 4.8676×1024 kg and a mean radius of about 6,052 km, it is approximately 81.5% as massive as Earth and 95% as large. Like Earth (and Mercury and Mars), it is a terrestrial planet, composed of rocks and minerals that are differentiated.
But apart from these similarities, Venus is very different from Earth. Its atmosphere is composed primarily of carbon dioxide (96%), along with nitrogen and a few other gases. This dense cloud cloaks the planet, making surface observation very difficult, and helps heat it up to 460 °C (860 °F). The atmospheric pressure is also 92 times that of Earth’s atmosphere, and poisonous clouds of carbon dioxide and sulfuric acid rain are commonplace.
Venus’ similarity in size and mass has led to it being called “Earth’s sister planet’. Credit: NASA/JPL/Magellan
Venus orbits the Sun at an average distance of about 0.72 AU (108 million km; 67 million mi) with almost no eccentricity. In fact, with its farthest orbit (aphelion) of 0.728 AU (108,939,000 km) and closest orbit (perihelion) of 0.718 AU (107,477,000 km), it has the most circular orbit of any planet in the Solar System. The planet completes an orbit around the Sun every 224.65 days, meaning that a year on Venus is 61.5% as long as a year on Earth.
When Venus lies between Earth and the Sun, a position known as inferior conjunction, it makes the closest approach to Earth of any planet, at an average distance of 41 million km. This takes place, on average, once every 584 days, and is the reason why Venus is the closest planet to Earth. The planet completes an orbit around the Sun every 224.65 days, meaning that a year on Venus is 61.5% as long as a year on Earth.
Unlike most other planets in the Solar System, which rotate on their axes in an counter-clockwise direction, Venus rotates clockwise (called “retrograde” rotation). It also rotates very slowly, taking 243 Earth days to complete a single rotation. This is not only the slowest rotation period of any planet, it also means that a single day on Venus lasts longer than a Venusian year.
Venus’ atmosphere is also known to experience lightning storms. Since Venus does not experience rainfall (except in the form of sulfuric acid), it has been theorized that the lightning is being caused by volcanic eruptions. Several spacecraft have visited Venus, and a few landers have even made it to the surface to send back images of its hellish landscape. Even though there were made of metal, these landers only survived a few hours at best.
Venus is made special by the fact that it is very much like Earth, but also radically different. It’s thick atmosphere could crush a living being, its heat could melt lead, and its acid rain could dissolve flesh, bone and metal alike! It also rotates very slowly, and backwards relative to the other plants.
Earth:
Earth is our home, and the third planet from the Sun. With a mean radius of 6371 km and a mass of 5.97×1024 kg, it is the fifth largest and fifth most-massive planet in the Solar System. And with a mean density of 5.514 g/cm³, it is the densest planet in the Solar System. Like Mercury, Venus and Mars, Earth is a terrestrial planet.
But unlike these other planets, Earth’s core is differentiated between a solid inner core and liquid outer core. The outer core also spins in the opposite direction as the planet, which is believed to create a dynamo effect that gives Earth its protective magnetosphere. Combined with a atmosphere that is neither too thin nor too thick, Earth is the only planet in the Solar System known to support life.
The Earth’s layers, showing the Inner and Outer Core, the Mantle, and Crust. Credit: discovermagazine.com
In terms of its orbit, Earth has a very minor eccentricity (approx. 0.0167) and ranges in its distance from the Sun between 147,095,000 km (0.983 AU) at perihelion to 151,930,000 km (1.015 AU) at aphelion. This works out to an average distance (aka. semi-major axis) of 149,598,261 km, which is the basis of a single Astronomical Unit (AU)
The Earth has an orbital period of 365.25 days, which is the equivalent of 1.000017 Julian years. This means that every four years (in what is known as a Leap Year), the Earth calendar must include an extra day. Though a single solar day on Earth is considered to be 24 hours long, our planet takes precisely 23h 56m and 4 s to complete a single sidereal rotation (0.997 Earth days).
Earth’s axis is also tilted 23.439281° away from the perpendicular of its orbital plane, which is responsible for producing seasonal variations on the planet’s surface with a period of one tropical year (365.24 solar days). In addition to producing variations in terms of temperature, this also results in variations in the amount of sunlight a hemisphere receives during the course of a year.
Earth has only a single moon: the Moon. Thanks to examinations of Moon rocks that were brought back to Earth by the Apollo missions, the predominant theory states that the Moon was created roughly 4.5 billion years ago from a collision between Earth and a Mars-sized object (known as Theia). This collision created a massive cloud of debris that began circling our planet, which eventually coalesced to form the Moon we see today.
A picture of Earth taken by Apollo 11 astronauts. Credit: NASA
What makes Earth special, you know, aside from the fact that it is our home and where we originated? It is the only planet in the Solar System where liquid, flowing water exists in abundance on its surface, has a viable atmosphere, and a protective magnetosphere. In other words, it is the only planet (or Solar body) that we know of where life can exist on the surface.
In addition, no planet in the Solar System has been studied as well as Earth, whether it be from the surface or from space. Thousands of spacecraft have been launched to study the planet, measuring its atmosphere, land masses, vegetation, water, and human impact. Our understanding of what makes our planet unique in our Solar System has helped in the search for Earth-like planets in other systems.
Mars:
The fourth planet from the Sun is Mars, which is also the second smallest planet in the Solar System. It has a radius of approximately 3,396 km at its equator, and 3,376 km at its polar regions – which is the equivalent of roughly 0.53 Earths. While it is roughly half the size of Earth, it’s mass – 6.4185 x 10²³ kg – is only 0.151 that of Earth’s. It’s density is also lower than Earths, which leads to it experiencing about 1/3rd Earth’s gravity (0.376 g).
It’s axial tilt is very similar to Earth’s, being inclined 25.19° to its orbital plane (Earth’s axial tilt is just over 23°), which means Mars also experiences seasons. Mars has almost no atmosphere to help trap heat from the Sun, and so temperatures can plunge to a low of -140 °C (-220 °F) in the Martian winter. However, at the height of summer, temperatures can get up to 20 °C (68 °F) during midday at the equator.
However, recent data obtained by the Curiosity rover and numerous orbiters have concluded that Mars once had a denser atmosphere. Its loss, according to data obtained by NASA’s Mars Atmosphere and Volatile Evolution (MAVEN), the atmosphere was stripped away by solar wind over the course of a 500 million year period, beginning 4.2 billion years ago.
At its greatest distance from the Sun (aphelion), Mars orbits at a distance of 1.666 AUs, or 249.2 million km. At perihelion, when it is closest to the Sun, it orbits at a distance of 1.3814 AUs, or 206.7 million km. At this distance, Mars takes 686.971 Earth days, the equivalent of 1.88 Earth years, to complete a rotation of the Sun. In Martian days (aka. Sols, which are equal to one day and 40 Earth minutes), a Martian year is 668.5991 Sols.
Like Mercury, Venus, and Earth, Mars is a terrestrial planet, composed mainly of silicate rock and metals that are differentiated between a core, mantle and crust. The red-orange appearance of the Martian surface is caused by iron oxide, more commonly known as hematite (or rust). The presence of other minerals in the surface dust allow for other common surface colors, including golden, brown, tan, green, and others.
Although liquid water cannot exist on Mars’ surface, owing to its thin atmosphere, large concentrations of ice water exist within the polar ice caps – Planum Boreum and Planum Australe. In addition, a permafrost mantle stretches from the pole to latitudes of about 60°, meaning that water exists beneath much of the Martian surface in the form of ice water. Radar data and soil samples have confirmed the presence of shallow subsurface water at the middle latitudes as well.
MSL Curiosity selfie on the surface of Mars. Image: NASA/JPL/Cal-Tech
Mars has two tiny asteroid-sized moons: Phobos and Deimos. Because of their size and shape, the predominant theory is that Mars acquired these two moons after they were kicked out of the Asteroid Belt by Jupiter’s gravity.
Mars has been heavily studied by spacecraft. There are multiple rovers and landers currently on the surface and a small fleet of orbiters flying overhead. Recent missions include the Curiosity Rover, which gathered ample evidence on Mars’ water past, and the groundbreaking discovery of finding organic molecules on the surface. Upcoming missions include NASA’s InSight lander and the Exomars rover.
Hence, Mars’ special nature lies in the fact that it also is terrestrial and lies within the outer edge of the Sun’s habitable zone. And whereas it is a cold, dry place today, it once had an thicker atmosphere and plentiful water on its surface.
Jupiter:
Mighty Jupiter is the fouth planet for our Sun and the biggest planet in our Solar System. Jupiter’s mass, volume, surface area and mean circumference are 1.8981 x 1027 kg, 1.43128 x 1015 km3, 6.1419 x 1010 km2, and 4.39264 x 105 km respectively. To put that in perspective, Jupiter diameter is roughly 11 times that of Earth, and 2.5 times the mass of all the other planets in the Solar System combined.
Jupiter has spectacular aurora, such as this view captured by the Hubble Space Telescope. Credit: NASA, ESA, and J. Nichols (University of Leicester)
But, being a gas giant, it has a relatively low density – 1.326 g/cm3 – which is less than one quarter of Earth’s. This means that while Jupiter’s volume is equivalent to about 1,321 Earths, it is only 318 times as massive. The low density is one way scientists are able to determine that it is made mostly of gases, though the debate still rages on what exists at its core (see below).
Jupiter orbits the Sun at an average distance (semi-major axis) of 778,299,000 km (5.2 AU), ranging from 740,550,000 km (4.95 AU) at perihelion and 816,040,000 km (5.455 AU) at aphelion. At this distance, Jupiter takes 11.8618 Earth years to complete a single orbit of the Sun. In other words, a single Jovian year lasts the equivalent of 4,332.59 Earth days.
However, Jupiter’s rotation is the fastest of all the Solar System’s planets, completing a rotation on its axis in slightly less than ten hours (9 hours, 55 minutes and 30 seconds to be exact). Therefore, a single Jovian year lasts 10,475.8 Jovian solar days. This orbital period is two-fifths that of Saturn, which means that the two largest planets in our Solar System form a 5:2 orbital resonance.
Much like Earth, Jupiter experiences auroras near its northern and southern poles. But on Jupiter, the auroral activity is much more intense and rarely ever stops. The intense radiation, Jupiter’s magnetic field, and the abundance of material from Io’s volcanoes that react with Jupiter’s ionosphere create a light show that is truly spectacular.
The Juno spacecraft isn’t the first one to visit Jupiter. Galileo went there in the mid 90’s, and Voyager 1 snapped a nice picture of the clouds on its mission. Credit: NASA
Jupiter also experiences violent weather patterns. Wind speeds of 100 m/s (360 km/h) are common in zonal jets, and can reach as high as 620 kph (385 mph). Storms form within hours and can become thousands of km in diameter overnight. One storm, the Great Red Spot, has been raging since at least the late 1600s. The storm has been shrinking and expanding throughout its history; but in 2012, it was suggested that the Giant Red Spot might eventually disappear.
Jupiter is composed primarily of gaseous and liquid matter. It is the largest of the gas giants, and like them, is divided between a gaseous outer atmosphere and an interior that is made up of denser materials. It’s upper atmosphere is composed of about 88–92% hydrogen and 8–12% helium by percent volume of gas molecules, and approx. 75% hydrogen and 24% helium by mass, with the remaining one percent consisting of other elements.
The interior contains denser materials, such that the distribution is roughly 71% hydrogen, 24% helium and 5% other elements by mass. It is believed that Jupiter’s core is a dense mix of elements – a surrounding layer of liquid metallic hydrogen with some helium, and an outer layer predominantly of molecular hydrogen. The core has also been described as rocky, but this remains unknown as well.
Jupiter has been visited by several spacecraft, including NASA’s Pioneer 10 and Voyager spacecraft in 1973 and 1980, respectively; and by the Cassini and New Horizons spacecraft more recently. Until the recent arrival of Juno, only the Galileo spacecraft has ever gone into orbit around Jupiter, and it was crashed into the planet in 2003 to prevent it from contaminating one of Jupiter’s icy moons.
Illustration of Jupiter and the Galilean satellites. Credit: NASA
In short, Jupiter is massive and has massive storms. But compared to the planets of the inner Solar System, is it significantly less dense. Jupiter also has the most moons in the Solar System, with 67 confirmed and named moons orbiting it. But it is estimated that as many as 200 natural satellites may exist around the planet. Little wonder why this planet is named after the king of the gods.
Saturn:
Saturn is the second largest planet in the Solar System. With a mean radius of 58232±6 km, it is approximately 9.13 times the size of Earth. And at 5.6846×1026 kg, it is roughly 95.15 as massive. However, since it is a gas giant, it has significantly greater volume – 8.2713×1014 km3, which is equivalent to 763.59 Earths.
The sixth most distant planet, Saturn orbits the Sun at an average distance of 9 AU (1.4 billion km; 869.9 million miles). Due to its slight eccentricity, the perihelion and aphelion distances are 9.022 (1,353.6 million km; 841.3 million mi) and 10.053 AU (1,513,325,783 km; 940.13 million mi), on average respectively.
With an average orbital speed of 9.69 km/s, it takes Saturn 10,759 Earth days to complete a single revolution of the Sun. In other words, a single Cronian year is the equivalent of about 29.5 Earth years. However, as with Jupiter, Saturn’s visible features rotate at different rates depending on latitude, and multiple rotation periods have been assigned to various regions.
This portrait looking down on Saturn and its rings was created from images obtained by NASA’s Cassini spacecraft on Oct. 10th, 2013. Credit: NASA/JPL-Caltech/Space Science Institute/G. Ugarkovic
As a gas giant, Saturn is predominantly composed of hydrogen and helium gas. With a mean density of 0.687 g/cm3, Saturn is the only planet in the Solar System that is less dense than water; which means that it lacks a definite surface, but is believed to have a solid core. This is due to the fact that Saturn’s temperature, pressure, and density all rise steadily toward the core.
Standard planetary models suggest that the interior of Saturn is similar to that of Jupiter, having a small rocky core surrounded by hydrogen and helium with trace amounts of various volatiles. This core is similar in composition to the Earth, but more dense due to the presence of metallic hydrogen, which as a result of the extreme pressure.
As a gas giant, the outer atmosphere of Saturn contains 96.3% molecular hydrogen and 3.25% helium by volume. Trace amounts of ammonia, acetylene, ethane, propane, phosphine and methane have been also detected in Saturn’s atmosphere. Like Jupiter, it also has a banded appearance, but Saturn’s bands are much fainter and wider near the equator.
On occasion, Saturn’s atmosphere exhibits long-lived ovals that are thousands of km wide, similar to what is commonly observed on Jupiter. Whereas Jupiter has the Great Red Spot, Saturn periodically has what’s known as the Great White Spot (aka. Great White Oval). This unique but short-lived phenomenon occurs once every Saturnian year, roughly every 30 Earth years, around the time of the northern hemisphere’s summer solstice.
The huge storm churning through the atmosphere in Saturn’s northern hemisphere overtakes itself as it encircles the planet in this true-color view from NASA’s Cassini spacecraft. Image credit: NASA/JPL-Caltech/SSI
The persisting hexagonal wave pattern around the north pole was first noted in the Voyager images. The sides of the hexagon are each about 13,800 km (8,600 mi) long (which is longer than the diameter of the Earth) and the structure rotates with a period of 10h 39m 24s, which is assumed to be equal to the period of rotation of Saturn’s interior.
The south pole vortex, meanwhile, was first observed using the Hubble Space Telescope. These images indicated the presence of a jet stream, but not a hexagonal standing wave. These storms are estimated to be generating winds of 550 km/h, are comparable in size to Earth, and believed to have been going on for billions of years. In 2006, the Cassini space probe observed a hurricane-like storm that had a clearly defined eye. Such storms had not been observed on any planet other than Earth – even on Jupiter.
Of course, the most amazing feature of Saturn is its rings. These are made of particles of ice ranging in size from a grains of sand to the size of a car. Some scientists think the rings are only a few hundred million years old, while others think they could be as old as the Solar System itself.
Saturn has been visited by spacecraft 4 times: Pioneer 11, Voyager 1 and 2 were just flybys, but Cassini has actually gone into orbit around Saturn and has captured thousands of images of the planet and its moons. And speaking of moons, Saturn has a total of 62 moons discovered (so far), though estimates indicate that it might have as many as 150.
A collage of Saturn (bottom left) and some of its moons: Titan, Enceladus, Dione, Rhea and Helene. Credit: NASA/JPL/Space Science Institute
So like Jupiter, Saturn is a massive gas giant that experiences some very interesting weather patterns. It also has lots of moons and has a very low density. But what really makes Saturn stand out is its impressive ring system. Whereas every gas and ice giant has one, Saturn’s is visible to the naked eye and very beautiful to behold!
Uranus:
Next comes Uranus, the seventh planet from the Sun. With a mean radius of approximately 25,360 km and a mass of 8.68 × 1025 kg, Uranus is approximately 4 times the sizes of Earth and 63 times its volume. However, as a gas giant, its density (1.27 g/cm3) is significantly lower; hence, it is only 14.5 as massive as Earth.
The variation of Uranus’ distance from the Sun is also greater than that any other planet (not including dwarf planets or plutoids). Essentially, the gas giant’s distance from the Sun varies from 18.28 AU (2,735,118,100 km) at perihelion to 20.09 AU (3,006,224,700 km) at aphelion. At an average distance of 3 billion km from the Sun, it takes Uranus roughly 84 years (or 30,687 days) to complete a single orbit of the Sun.
The standard model of Uranus’s structure is that it consists of three layers: a rocky (silicate/iron–nickel) core in the center, an icy mantle in the middle and an outer envelope of gaseous hydrogen and helium. Much like Jupiter and Saturn, hydrogen and helium account for the majority of the atmosphere – approximately 83% and 15% – but only a small portion of the planet’s overall mass (0.5 to 1.5 Earth masses).
Uranus as seen through the automated eyes of Voyager 2 in 1986. (Credit: NASA/JPL)
The third most abundant element is methane ice (CH4), which accounts for 2.3% of its composition and which accounts for the planet’s aquamarine or cyan coloring. Trace amounts of various hydrocarbons are also found in the stratosphere of Uranus, which are thought to be produced from methane and ultraviolent radiation-induced photolysis. They include ethane (C2H6), acetylene (C2H2), methylacetylene (CH3C2H), and diacetylene (C2HC2H).
In addition, spectroscopy has uncovered carbon monoxide and carbon dioxide in Uranus’ upper atmosphere, as well as the presence icy clouds of water vapor and other volatiles, such as ammonia and hydrogen sulfide. Because of this, Uranus and Neptune are considered a distinct class of giant planet – known as “Ice Giants” – since they are composed mainly of heavier volatile substances.
The rotational period of the interior of Uranus is 17 hours, 14 minutes. As with all giant planets, its upper atmosphere experiences strong winds in the direction of rotation. Hence its weather systems are also broken up into bands that rotate around the planet, which are driven by internal heat rising to the upper atmosphere.
As a result, winds on Uranus can reach up to 900 km/h (560 mph), creating massive storms like the one spotted by the Hubble Space Telescope in 2012. Similar to Jupiter’s Great Red Spot, this “Dark Spot” was a giant cloud vortex that measured 1,700 kilometers by 3,000 kilometers (1,100 miles by 1,900 miles).
Huge storms on Uranus were spotted by the Keck Observatory on Aug. 5 and Aug. 6, 2014. Credit: Imke de Pater (UC Berkeley), Pat Fry (University of Wisconsin), Keck Observatory
One unique feature of Uranus is that it rotates on its side. Whereas all of the Solar System’s planets are tilted on their axes to some degree, Uranus has the most extreme axial tilt of 98°. This leads to the radical seasons that the planet experiences, not to mention an unusual day-night cycle at the poles. At the equator, Uranus experiences normal days and nights; but at the poles, each experience 42 Earth years of day followed by 42 years of night.
Uranus was the first planet to be discovered with a telescope; it was first recognized as a planet in 1781 by William Herschel. Beyond Earth-based observations, only one spacecraft (Voyager 2) has ever studied Uranus up close. It passed by the planet in 1986, and captured the first close images. Uranus has 27 known moons.
Uranus’ special nature comes through in its natural beauty, its intense weather, its ring system and its impressive array of moons. And it’s compositions, being an “ice” giant, is what gives its aquamarine color. But perhaps mist interesting is its sideways rotation, which is unique among the Solar planets.
Neptune:
Neptune is the 8th and final planet in the Solar System, orbiting the Sun at a distance of 29.81 AU (4.459 x 109 km) at perihelion and 30.33 AU (4.537 x 109 km) at aphelion. With a mean radius of 24,622 ± 19 km, Neptune is the fourth largest planet in the Solar System and four times as large as Earth. But with a mass of 1.0243×1026 kg – which is roughly 17 times that of Earth – it is the third most massive, outranking Uranus.
Neptune’s system of moons and rings visualized. Credit: SETI
Neptune takes 16 h 6 min 36 s (0.6713 days) to complete a single sidereal rotation, and 164.8 Earth years to complete a single orbit around the Sun. This means that a single day lasts 67% as long on Neptune, whereas a year is the equivalent of approximately 60,190 Earth days (or 89,666 Neptunian days).
Due to its smaller size and higher concentrations of volatiles relative to Jupiter and Saturn, Neptune (much like Uranus) is often referred to as an “ice giant” – a subclass of a giant planet. Also like Uranus, Neptune’s internal structure is differentiated between a rocky core consisting of silicates and metals; a mantle consisting of water, ammonia and methane ices; and an atmosphere consisting of hydrogen, helium and methane gas.
The core of Neptune is composed of iron, nickel and silicates, with an interior model giving it a mass about 1.2 times that of Earth. The pressure at the center is estimated to be 7 Mbar (700 GPa), about twice as high as that at the center of Earth, and with temperatures as high as 5,400 K. At a depth of 7000 km, the conditions may be such that methane decomposes into diamond crystals that rain downwards like hailstones.
Because Neptune’s axial tilt (28.32°) is similar to that of Earth (~23°) and Mars (~25°), the planet experiences similar seasonal changes. Combined with its long orbital period, this means that the seasons last for forty Earth years. Also owing to its axial tilt being comparable to Earth’s is the fact that the variation in the length of its day over the course of the year is not any more extreme than it on Earth.
Reconstruction of Voyager 2 images showing the Great Black spot (top left), Scooter (middle), and the Small Black Spot (lower right). Credit: NASA/JPL
Just like Jupiter and Saturn, Neptune has bands of storms that circle the planet. Astronomers have clocked winds on Neptune traveling at 2,100 km/hour, which is believed to be due to Neptune’s cold temperatures – which may decrease the friction in the system, During its 1989 flyby, NASA’s Voyager 2 spacecraft discovered the Great Dark Spot on Neptune.
Similar to Jupiter’s Great Red Spot, this is an anti-cyclonic storm measuring 13,000 km x 6,600 km across. A few years later, however, the Hubble Space Telescope failed to see the Great Dark Spot, but it did see different storms. This might mean that storms on Neptune don’t last as long as they do on Jupiter or even Saturn.
The more active weather on Neptune might be due, in part, to its higher internal heat. Although Neptune is much more distant than Uranus from the Sun, receiving 40% less sunlight, temperatures on the surface of the two planets are roughly similar. In fact, Neptune radiates 2.61 times as much energy as it receives from the Sun. This is enough heat to help drive the fastest winds in the Solar System.
Neptune is the second planet discovered in modern times. It was discovered at the same time by both Urbain Le Verrier and John Couch Adams. Neptune has only ever been visited by one spacecraft, Voyager 2, which made a fly by in August, 1989. Neptune has 13 known moons. Th largest and most famous of these is Triton, which is believed to be a former KBO that was captured by Neptune’s gravity.
Global Color Mosaic of Triton, taken by Voyager 2 in 1989. Credit: NASA/JPL/USGS
So much like Uranus, Neptune has a ring system, some intense weather patterns, and an impressive array of moons. Also like Uranus, Neptune’s composition allows for its aquamarine color; except that in Neptune’s case, this color is more intense and vivid. In addition, Neptune experiences some temperature anomalies that are yet to be explained. And let’s not forgt the raining diamonds!
And those are the planets in the Solar System thank you for joining the tour! Unfortunately, Pluto isn’t a planet any more, hence why it was not listed. We know, we know, take it up with the IAU!
Earth's axial tilt (or obliquity) and its relation to the rotation axis and plane of orbit. Credit: Wikipedia Commons
In ancient times, the scholars, seers and magi of various cultures believed that the world took a number of forms – ranging from a ziggurat or a cube to the more popular flat disc surrounded by a sea. But thanks to the ongoing efforts of astronomers, we have come to understand that it is in fact a sphere, and one of many planets in a system that orbits the Sun.
Within the past few centuries, improvements in both scientific instruments and more comprehensive observations of the heavens have also helped astronomers to determine (with extreme accuracy) what the nature of Earth’s orbit is. In addition to knowing the precise distance from the Sun, we also know that our planet orbits the Sun with one pole constantly tilted towards it.
Earth’s Axis:
This is what is known axial tilt, where a planet’s vertical axis is tilted a certain degree towards the ecliptic of the object it orbits (in this case, the Sun). Such a tilt results in there being a difference in how much sunlight reaches a given point on the surface during the course of a year. In the case of Earth, the axis is tilted towards the ecliptic of the Sun at approximately 23.44° (or 23.439281° to be exact).
Earth’s axis points north to Polaris, the northern hemisphere’s North Star, and south to dim Sigma Octantis. Credit: Bob King
Seasonal Variations:
This tilt in Earth’s axis is what is responsible for seasonal changes during the course of the year. When the North Pole is pointed towards the Sun, the northern hemisphere experiences summer and the southern hemisphere experiences winter. When the South Pole is pointed towards the Sun, six months later, the situation is reversed.
In addition to variations in temperature, seasonal changes also result in changes to the diurnal cycle. Basically, in the summer, the day last longer and the Sun climbs higher in the sky. In winter, the days become shorter and the Sun is lower in the sky. In northern temperate latitudes, the Sun rises north of true east during the summer solstice, and sets north of true west, reversing in the winter. The Sun rises south of true east in the summer for the southern temperate zone, and sets south of true west.
The situation becomes extreme above the Arctic Circle, where there is no daylight at all for part of the year, and for up to six months at the North Pole itself (known as a “polar night”). In the southern hemisphere, the situation is reversed, with the South Pole oriented opposite the direction of the North Pole and experiencing what is known as a “midnight sun” (a day that lasts 24 hours).
The four seasons can be determined by the solstices (the point of maximum axial tilt toward or away from the Sun) and the equinoxes (when the direction of tilt and the Sun are perpendicular). In the northern hemisphere, winter solstice occurs around December 21st, summer solstice around June 21st, spring equinox around March 20th, and autumnal equinox on or about September 22nd or 23rd. In the southern hemisphere, the situation is reversed, with the summer and winter solstices exchanged and the spring and autumnal equinox dates swapped.
Changes Over Time:
The angle of the Earth’s tilt is relatively stable over long periods of time. However, Earth’s axis does undergo a slight irregular motion known as nutation – a rocking, swaying, or nodding motion (like a gyroscope) – that has a period of 18.6 years. Earth’s axis is also subject to a slight wobble (like a spinning top), which is causing its orientation to change over time.
Known as precession, this process is causing the date of the seasons to slowly change over a 25,800 year cycle. Precession is not only the reason for the difference between a sidereal year and a tropical year, it is also the reason why the seasons will eventually flip. When this happens, summer will occur in the northern hemisphere during December and winter during June.
Artist’s rendition of the Earth’s rotation and the precession of the Equinoxes. Credit: NASA
Precession, along with other orbital factors, is also the reason for what is known as “length-of-day variation”. Essentially, this is a phenomna where the dates of Earth’s perihelion and aphelion (which currently take place on Jan. 3rd and July 4th, respectively) change over time. Both of these motions are caused by the varying attraction of the Sun and the Moon on the Earth’s equatorial region.
Needless to say, Earth’s rotation and orbit around the Sun are not as simple we once though. During the Scientific Revolution, it was a huge revelation to learn that the Earth was not a fixed point in the Universe, and that the “celestial spheres” were planets like Earth. But even then, astronomers like Copernicus and Galileo still believed that the Earth’s orbit was a perfect circle, and could not imagine that its rotation was subject to imperfections.
It’s only been with time that the true nature of our planet’s inclination and movements have come to be understood. And what we know is that they lead to some serious variations over time – both in the short run (i.e. seasonal change), and in the long-run.
The eccentricity in Mars' orbit means that it is . Credit: NASA
With the Scientific Revolution, astronomers became aware of the fact that the Earth and the other planets orbit the Sun. And thanks to Copernicus, Galileo, Kepler, and Newton, the study of their orbits was refined to the point of mathematical precision. And with the subsequent discoveries of Uranus, Neptune, Pluto and the Kuiper Belt Objects, we have come to understand just how varied the orbits of the Solar Planets are.
Consider Mars, Earth’s second-closest neighbor, and a planet that is often referred to as “Earth’s Twin”. While it has many things in common with Earth, one area in which they differ greatly is in terms of their orbits. In addition to being farther from the Sun, Mars also has a much more elliptical orbit, which results in some rather interesting variations in temperature and weather patterns.
Perihelion and Aphelion:
Mars orbits the Sun at an average distance (semi-major axis) of 228 million km (141.67 million mi), or 1.524 astronomical units (over one and a half times the distance between Earth and the Sun). However, Mars also has the second most eccentric orbit of all the planets in the Solar System (0.0934), which makes it a distant second to crazy Mercury (at 0.20563).
This means that Mars’ distance from the Sun varies between perihelion (its closest point) and aphelion (its farthest point). In short, the distance between Mars and the Sun ranges during the course of a Martian year from 206,700,000 km (128.437 million mi) at perihelion and 249,200,000 km (154.8457 million mi) at aphelion – or 1.38 AU and 1.666 AU.
Speaking of a Martian year, with an average orbital speed of 24 km/s, Mars takes the equivalent of 687 Earth days to complete a single orbit around the Sun. This means that a year on Mars is equivalent to 1.88 Earth years. Adjusted for Martian days (aka. sols) – which last 24 hours, 39 minutes, and 35 seconds – that works out to a year being 668.5991 sols long (still almost twice as long).
Mars in also the midst of a long-term increase in eccentricity. Roughly 19,000 years ago, it reached a minimum of 0.079, and will peak again at an eccentricity of 0.105 (with a perihelion distance of 1.3621 AU) in about 24,000 years. In addition, the orbit was nearly circular about 1.35 million years ago, and will be again one million years from now.
Axial Tilt:
Much like Earth, Mars also has a significantly tilted axis. In fact, with an inclination of 25.19° to its orbital plane, it is very close to Earth’s own tilt of 23.439°. This means that like Earth, Mars also experiences seasonal variations in terms of temperature. On average, the surface temperature of Mars is much colder than what we experience here on Earth, but the variation is largely the same.
Mars eccentric orbit and axial tilt result in considerable seasonal variations. Credit and Copyright: Encyclopedia Britannica
All told, the average surface temperature on Mars is -46 °C (-51 °F). This ranges from a low of -143 °C (-225.4 °F), which takes place during winter at the poles; and a high of 35 °C (95 °F), which occurs during summer and midday at the equator. This means that at certain times of the year, Mars is actually warmer than certain parts of Earth.
Orbit and Seasonal Changes:
Mars’ variations in temperature and its seasonal changes are also related to changes in the planet’s orbit. Essentially, Mars’ eccentric orbit means that it travels more slowly around the Sun when it is further from it, and more quickly when it is closer (as stated in Kepler’s Three Laws of Planetary Motion).
Mars’ aphelion coincides with Spring in its northern hemisphere, which makes it the longest season on the planet – lasting roughly 7 Earth months. Summer is second longest, lasting six months, while Fall and Winter last 5.3 and just over 4 months, respectively. In the south, the length of the seasons is only slightly different.
Mars is near perihelion when it is summer in the southern hemisphere and winter in the north, and near aphelion when it is winter in the southern hemisphere and summer in the north. As a result, the seasons in the southern hemisphere are more extreme and the seasons in the northern are milder. The summer temperatures in the south can be up to 30 K (30 °C; 54 °F) warmer than the equivalent summer temperatures in the north.
Mars’ south polar ice cap, seen in April 2000 by the Mars Odyssey probe. Credit: NASA/JPL/MSSS
It also snows on Mars. In 2008, NASA’s Phoenix Landerfound water ice in the polar regions of the planet. This was an expected finding, but scientists were not prepared to observe snow falling from clouds. The snow, combined with soil chemistry experiments, led scientists to believe that the landing site had a wetter and warmer climate in the past.
And then in 2012, data obtained by the Mars Reconnaissance Orbiter revealed that carbon-dioxide snowfalls occur in the southern polar region of Mars. For decades, scientists have known that carbon-dioxide ice is a permanent part of Mars’ seasonal cycle and exists in the southern polar caps. But this was the first time that such a phenomena was detected, and it remains the only known example of carbon-dioxide snow falling anywhere in our solar system.
For starters, soil samples and orbital observation have demonstrated conclusively that roughly 3.7 billion years ago, the planet had more water on its surface than is currently in the Atlantic Ocean. Similarly, atmospheric studies conducted on the surface and from space have proven that Mars also had a viable atmosphere at that time, one which was slowly stripped away by solar wind.
Scientists were able to gauge the rate of water loss on Mars by measuring the ratio of water and HDO from today and 4.3 billion years ago. Credit: Kevin Gill
Weather Patterns:
These seasonal variations allow Mars to experience some extremes in weather. Most notably, Mars has the largest dust storms in the Solar System. These can vary from a storm over a small area to gigantic storms (thousands of km in diameter) that cover the entire planet and obscure the surface from view. They tend to occur when Mars is closest to the Sun, and have been shown to increase the global temperature.
The first mission to notice this was the Mariner 9 orbiter, which was the first spacecraft to orbit Mars in 1971, it sent pictures back to Earth of a world consumed in haze. The entire planet was covered by a dust storm so massive that only Olympus Mons, the giant Martian volcano that measures 24 km high, could be seen above the clouds. This storm lasted for a full month, and delayed Mariner 9‘s attempts to photograph the planet in detail.
And then on June 9th, 2001, the Hubble Space Telescope spotted a dust storm in the Hellas Basin on Mars. By July, the storm had died down, but then grew again to become the largest storm in 25 years. So big was the storm that amateur astronomers using small telescopes were able to see it from Earth. And the cloud raised the temperature of the frigid Martian atmosphere by a stunning 30° Celsius.
These storms tend to occur when Mars is closest to the Sun, and are the result of temperatures rising and triggering changes in the air and soil. As the soil dries, it becomes more easily picked up by air currents, which are caused by pressure changes due to increased heat. The dust storms cause temperatures to rise even further, leading to Mars’ experiencing its own greenhouse effect.
NASA's Deep Space Climate Observatory captured a series of images of the Moon passing in front of the Earth on July 5th. Image: NASA/NOAA
We’re accustomed to seeing stunning images of both the Moon and Earth floating in space. It’s the age we live in. But seeing them together is rare. Now, NASA’s Deep Space Climate Observatory (DSCOVR) has captured images of the Moon passing between itself and the Earth, in effect photo-bombing Earth.
The image was captured with the Earth Polychromatic Imaging Camera (EPIC) camera on DISCOVR, and is the second time this has been captured. EPIC is a 4 megapixel camera on board DSCOVR, and DSCOVR is in orbit about 1.6 million km (1 million miles) from Earth, between the Earth and the Sun.
“For the second time in the life of DSCOVR, the moon moved between the spacecraft and Earth,” said Adam Szabo, DSCOVR project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
Cool pictures of the Moon are a bonus, though, as DSCOVR’s primary mission is to monitor the solar wind in real time for the National Oceanic and Atmospheric Administration (NOAA). It does so while inhabiting the first LaGrange point between the Earth and the Sun, where the gravitational pull of the Sun and the Earth balance each other. To do so requires a complex orbit called a Lissajous orbit, a non-recurring orbit which takes DSCOVR from an ellipse to a circle and back.
DSCOVR occupies the LaGrange point 1 between the Earth and the Sun. Image: NOAA
DSCOVR has other important work to do. From its vantage point, DSCOVR keeps a constantly illuminated view of the surface of the Earth as it rotates. DSCOVR provides observations of cloud height, vegetation, ozone, and aerosols in the atmosphere. This is important scientific data in monitoring and understanding Earth’s climate.
DSCOVR is a partnership between NASA, NOAA and the U.S. Air Force. As mentioned above, its primary objective is maintaining the nation’s real-time solar wind monitoring capabilities, which are critical to the accuracy and lead time of space weather alerts and forecasts from NOAA. The DSCOVR website also has daily color pictures of the Earth, for all your eye-candy needs.
A sunrise from the edge of space. Credit: Project Soar
You may have heard the saying at some point in your life: “The Sun will still rise in the east and set in the west tomorrow.” You get the point, it means it’s not the end of the world. But have you ever wondered why the Sun behaves this way? Why does – and always has, for that matter – the Sun rise in the east and set in the west? What mechanics are behind this?
Naturally, ancient people took the passage of the Sun through the sky as a sign that it was revolving around us. With the birth of modern astronomy, we have come to learn that its actually the other way around. The Sun only appears to be revolving around us because our planet not only orbits it, but also rotates on its axis as it is doing so. From this, we get the familiar passage of the Sun through the sky, and the basis for our measurement of time.
Earth’s Rotation:
As already noted, the Earth rotates on its axis as it circles the Sun. If viewed from above the celestial north, the Earth would appear to be rotating counter-clockwise. Because of this, to those standing on the Earth’s surface, the Sun appears to be moving around us in a westerly direction at a rate of 15° an hour (or 15′ a minute). This is true of all celestial objects observed in the sky, with an “apparent motion” that takes them from east to west.
Earth’s axial tilt (or obliquity) and its relation to the rotation axis and plane of orbit. Credit: Wikipedia Commons
This is also true of the majority of the planets in the Solar System. Venus is one exception, which rotates backwards compared to its orbit around the Sun (a phenomena known as retrograde motion). Uranus is another, which not only rotates westward, but is inclined so much that it appears to be sitting on its side relative to the Sun.
Pluto also has a retrograde motion, so for those standing on its surface, the Sun would rise in the west and set in the east. In all cases, a large impact is believed to be the cause. In essence, Pluto and Venus were sent spinning in the other direction by a large impact, while another struck Uranus and knocked it over on its side!
With a rotational velocity of 1,674.4 km/h (1,040.4 mph), the Earth takes 23 hours, 56 minutes and 4.1 seconds to rotate once on its axis. This means, in essence, that a sidereal day is less than 24 hours. But combined with its orbital period (see below), a solar day – that is, the time it takes for the Sun to return to the same place in the sky – works out to 24 hours exactly.
Earth’s Orbit Around the Sun:
With an average orbital velocity of 107,200 km/h (66,600 mph), the Earth takes approximately 365.256 days – aka. a sidereal year – to complete a single orbit of the Sun. This means that every four years (in what is known as a Leap Year), the Earth calendar must include an extra day.
Viewed from the celestial north, the motion of the Earth appears to orbit the Sun in a counterclockwise direction. Combined with its axial tilt – i.e. the Earth’s axis is tilted 23.439° towards the ecliptic – this results in seasonal changes. In addition to producing variations in terms of temperature, this also results in variations in the amount of sunlight a hemisphere receives during the course of a year.
Basically, when the North Pole is pointing towards the Sun, the northern hemisphere experiences summer and the southern hemisphere experiences winter. During the summer, the climate warms up and the sun appears earlier in the morning sky and sets at a later hour in the evening. In the winter, the climate becomes generally cooler and the days are shorter, with sunrise coming later and sunset happening sooner.
Above the Arctic Circle, an extreme case is reached where there is no daylight at all for part of the year – up to six months at the North Pole itself, which is known as a “polar night”. In the southern hemisphere, the situation is exactly reversed, with the South Pole experiencing a “midnight sun” – i.e. a day of 24 hours.
And last, but not least, seasonal changes also result in changes in the Sun’s apparent motion across the sky. During summer in the northern hemisphere, the Sun appears to move from east to west directly overhead, while moving closer to the southern horizon during winter. During summer in the southern hemisphere, the Sun appears to move overhead; while in the winter, it appears to be closer to the northern horizon.
In short, the Sun rises in the east and sets in the west because of our planet’s rotation. During the course of the year, the amount of daylight we experience is mitigated by our planet’s tilted axis. If, like Venus, Uranus and Pluto, a large enough asteroid or celestial object were to strike us just right, the situation might be changed. We too could experience what it is like to watch the Sun rise in the west, and set in the east.
Planets and other objects in our Solar System. Credit: NASA.
It is a well known fact that the planets of the Solar System vary considerably in terms of size. For instance, the planets of the inner Solar System are smaller and denser than the gas/ice giants of the outer Solar System. And in some cases, planets can actually be smaller than the largest moons. But a planet’s size is not necessarily proportional to its mass. In the end, how massive a planet is has more to do with its composition and density.
So while a planet like Mercury may be smaller in size than Jupiter’s moon Ganymede or Saturn’s moon Titan, it is more than twice as massive than they are. And while Jupiter is 318 times as massive as Earth, its composition and density mean that it is only 11.21 times Earth’s size. Let’s go over the planet’s one by one and see just how massive they are, shall we?
Mercury:
Mercury is the Solar System’s smallest planet, with an average diameter of 4879 km (3031.67 mi). It is also one of its densest at 5.427 g/cm3, which is second only to Earth. As a terrestrial planet, it is composed of silicate rock and minerals and is differentiated between an iron core and a silicate mantle and crust. But unlike its peers (Venus, Earth and Mars), it has an abnormally large metallic core relative to its crust and mantle.
All told, Mercury’s mass is approximately 0.330 x 1024 kg, which works out to 330,000,000 trillion metric tons (or the equivalent of 0.055 Earths). Combined with its density and size, Mercury has a surface gravity of 3.7 m/s² (or 0.38 g).
Internal structure of Mercury: 1. Crust: 100–300 km thick 2. Mantle: 600 km thick 3. Core: 1,800 km radius. Credit: MASA/JPL
Venus:
Venus, otherwise known as “Earth’s Sister Planet”, is so-named because of its similarities in composition, size, and mass to our own. Like Earth, Mercury and Mars, it is a terrestrial planet, and hence quite dense. In fact, with a density of 5.243 g/cm³, it is the third densest planet in the Solar System (behind Earth and Mercury). Its average radius is roughly 6,050 km (3759.3 mi), which is the equivalent of 0.95 Earths.
And when it comes to mass, the planet weighs in at a hefty 4.87 x 1024 kg, or 4,870,000,000 trillion metric tons. Not surprisingly, this is the equivalent of 0.815 Earths, making it the second most massive terrestrial planet in the Solar System. Combined with its density and size, this means that Venus also has comparable gravity to Earth – roughly 8.87 m/s², or 0.9 g.
Earth:
Like the other planets of the inner Solar System, Earth is also a terrestrial planet, composed of metals and silicate rocks differentiated between an iron core and a silicate mantle and crust. Of the terrestrial planets, it is the largest and densest, with an average radius of 6,371.0 km (3,958.8 mi) and a mean of density of 5.514 g/cm3.
The Earth’s layers, showing the Inner and Outer Core, the Mantle, and Crust. Credit: discovermagazine.com
And at 5.97 x 1024 kg (which works out to 5,970,000,000,000 trillion metric tons) Earth is the most massive of all the terrestrial planets. Combined with its size and density, Earth experiences the surface gravity that we are all familiar with – 9.8 m/s², or 1 g.
Mars:
Mars is the third largest terrestrial planet, and the second smallest planet in our Solar System. Like the others, it is composed of metals and silicate rocks that are differentiated between a iron core and a silicate mantle and crust. But while it is roughly half the size of Earth (with a mean diameter of 6792 km, or 4220.35 mi), it is only one-tenth as massive.
In short, Mars has a mass of 0.642 x1024 kg, which works out to 642,000,000 trillion metric tons, or roughly 0.11 the mass of Earth. Combined with its size and density – 3.9335 g/cm³ (which is roughly 0.71 times that of Earth’s) – Mars has a surface gravity of 3.711 m/s² (or 0.376 g).
Jupiter:
Jupiter is the largest planet in the Solar System. With a mean diameter of 142,984 km, it is big enough to fit all the other planets (except Saturn) inside itself, and big enough to fit Earth 11.8 times over. But with a mass of 1898 x 1024 kg (or 1,898,000,000,000 trillion metric tons), Jupiter is more massive than all the other planets in the Solar System combined – 2.5 times more massive, to be exact.
Jupiter’s structure and composition. (Image Credit: Kelvinsong CC by S.A. 3.0)
However, as a gas giant, it has a lower overall density than the terrestrial planets. It’s mean density is 1.326 g/cm, but this increases considerably the further one ventures towards the core. And though Jupiter does not have a true surface, if one were to position themselves within its atmosphere where the pressure is the same as Earth’s at sea level (1 bar), they would experience a gravitational pull of 24.79 m/s2 (2.528 g).
Saturn:
Saturn is the second largest of the gas giants; with a mean diameter of 120,536 km, it is just slightly smaller than Jupiter. However, it is significantly less massive than its Jovian cousin, with a mass of 569 x 1024 kg (or 569,000,000,000 trillion metric tons). Still, this makes Saturn the second most-massive planet in the Solar System, with 95 times the mass of Earth.
Much like Jupiter, Saturn has a low mean density due to its composition. In fact, with an average density of 0.687 g/cm³, Saturn is the only planet in the Solar System that is less dense than water (1 g/cm³). But of course, like all gas giants, its density increases considerably the further one ventures towards the core. Combined with its size and mass, Saturn has a “surface” gravity that is just slightly higher than Earth’s – 10.44 m/s², or 1.065 g.
Diagram of Saturn’s interior. Credit: Kelvinsong/Wikipedia Commons
Uranus:
With a mean diameter of 51,118 km, Uranus is the third largest planet in the Solar System. But with a mass of 86.8 x 1024 kg (86,800,000,000 trillion metric tons) it is the fourth most massive – which is 14.5 times the mass of Earth. This is due to its mean density of 1.271 g/cm3, which is about three quarters of what Neptune’s is. Between its size, mass, and density, Uranus’ gravity works out to 8.69 m/s2, which is 0.886 g.
Neptune:
Neptune is significantly larger than Earth; at 49,528 km, it is about four times Earth’s size. And with a mass of 102 x 1024 kg (or 102,000,000,000 trillion metric tons) it is also more massive – about 17 times more to be exact. This makes Neptune the third most massive planet in the Solar System; while its density is the greatest of any gas giant (1.638 g/cm3). Combined, this works out to a “surface” gravity of 11.15 m/s2 (1.14 g).
As you can see, the planets of the Solar System range considerably in terms of mass. But when you factor in their variations in density, you can see how a planets mass is not always proportionate to its size. In short, while some planets may be a few times larger than others, they are can have many, many times more mass.
This is an illustration to explain the dynamics of the ultra-relativistic third Van Allen radiation belt. Credit: Andy Kale
The dynamic relationship between Earth and the Sun two sides. The warmth from the Sun makes life on Earth possible, but the rest of the Sun’s intense energy pummels the Earth, and could destroy all life, given the chance. But thanks to our magnetosphere, we are safe.
The magnetosphere is our protective shield. It’s created by the rotation of the molten outer core of the Earth, composed largely of iron and nickel. It absorbs and deflects plasma from the solar wind. The interactions between the magnetosphere and the solar wind are what create the beautiful auroras at Earth’s poles.
Visualization of the solar wind encountering Earth’s magnetic “defenses” known as the magnetosphere. Clouds of southward-pointing plasma are able to peel back layers of the Sun-facing bubble and stack them into layers on the planet’s nightside (center, right). The layers can be squeezed tightly enough to reconnect and deliver solar electrons (yellow sparkles) directly into the upper atmosphere to create the aurora. Credit: JPL
In the inner regions of Earth’s magnetosphere are the Van Allen belts, named after their discoverer James Van Allen. They consist of charged particles, mostly from the Sun, and are held in place by the magnetosphere. Usually, there are two such belts.
The Van Allen radiation belts surrounding Earth. Image: NASA
But the output from the Sun is not stable. There are periods of intense energy output from the Sun, and when that happens, a third, transient belt can be created. Up until now, the nature of this third belt has been a puzzle. New research from the University of Alberta has shown how this phenomena can happen.
Researchers have shown how a so-called “space tsunami” can create this third belt. Intense ultra-low frequency plasma waves can transport the outer part of the radiation belt into interplanetary space, and create the third, transient belt.
The lead author for this study is physics professor Ian Mann from the University of Alberta, and former Canada Research Chair in Space Physics. “Remarkably, we observed huge plasma waves,” said Mann. “Rather like a space tsunami, they slosh the radiation belts around and very rapidly wash away the outer part of the belt, explaining the structure of the enigmatic third radiation belt.”
This new research also sheds light on how these “tsunamis” help reduce the threat of radiation to satellites during other space storms. “Space radiation poses a threat to the operation of the satellite infrastructure upon which our twenty-first century technological society relies,” adds Mann. “Understanding how such radiation is energized and lost is one of the biggest challenges for space research.”
It’s not just satellites that are at risk of radiation though. When solar wind is most active, it can create extremely energetic space storms. They in turn create intense radiation in the Van Allen belts, which drive electrical currents that could damage our power grids here on Earth. These types of storms have the potential to cause trillions of dollars worth of damage.
A better understanding of this space radiation, and an ability to forecast it, are turning out to be very important to our satellite operations, and to our exploration of space.
The Van Allen belts were discovered in 1958, and classified into an inner and an outer belt.
The Van Allen Belts around Earth. The inner red belt is mostly protons, and the outer blue belt is mostly electrons. Image Credit: NASA
In 2013, probes reported a third belt which had never before been seen. It lasted a few weeks, then vanished, and its cause was not known. Thanks to Mann and his team, we now know what was behind that third belt.
“We have discovered a very elegant explanation for the dynamics of the third belt,” says Mann. “Our results show a remarkable simplicity in belt response once the dominant processes are accurately specified.”
An understanding of the radiation in and around Earth and the Van Allen belts is of growing importance to us, as we expand our presence in space. Our technological society relies increasingly on satellite communications, and on GPS satellites. Radiation in the form of high-energy electrons can wreak havoc on satellites. In fact, this type of radiation is sometimes referred to as a satellite killer. Satellites require robust design to be protected from them.
Organizations like the United Nations Committee on the Peaceful Uses of Outer Space (COPUOS) and the International Living with a Star (ILWS) Program are attempts to address the threat that radiation poses to our system of satellites.
Planets in the Solar System. Image credit: NASA/JPL/IAU
The planets of our Solar System vary considerably in size and shape. Some planets are small enough that they are comparable in diameter to some of our larger moons – i.e. Mercury is smaller than Jupiter’s moon Ganymede and Saturn’s moon Titan. Meanwhile, others like Jupiter are so big that they are larger in diameter than most of the others combined.
In addition, some planets are wider at the equator than they are at the poles. This is due to a combination of the planets composition and their rotational speed. As a result, some planets are almost perfectly spherical while others are oblate spheroids (i.e. experience some flattening at the poles). Let us examine them one by one, shall we?
Mercury:
With a diameter of 4,879 km (3031.67 mi), Mercury is the smallest planet in our Solar System. In fact, Mercury is not much larger than Earth’s own Moon – which has a diameter of 3,474 km (2158.64 mi). At 5,268 km (3,273 mi) in diameter, Jupiter’s moon of Ganymede is also larger, as is Saturn’s moon Titan – which is 5,152 km (3201.34 mi) in diameter.
Mercury, as imaged by the MESSENGER spacecraft, revealing parts of the never seen by human eyes. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
As with the other planets in the inner Solar System (Venus, Earth, and Mars), Mercury is a terrestrial planet, which means it is composed primarily of metals and silicate rocks that are differentiated into an iron-rich core and a silicate mantle and crust.
Also, due to the fact that Mercury has a very slow sidereal rotational period, taking 58.646 days to complete a single rotation on its axis, Mercury experiences no flattening at the poles. This means that the planet is almost a perfect sphere and has the same diameter whether it is measured from pole to pole or around its equator.
Venus:
Venus is often referred to as Earth’s “sister planet“, and not without good reason. At 12,104 km (7521 mi) in diameter, it is almost the same size as Earth. But unlike Earth, Venus experiences no flattening at the poles, which means that it almost perfectly circular. As with Mercury, this is due to Venus’ slow sidereal rotation period, taking 243.025 days to rotate once on its axis.
The planet Venus, as imaged by the Magellan 10 mission. Credit: NASA/JPL
Earth:
With a mean diameter of 12,756 km (7926 mi), Earth is the largest terrestrial planet in the Solar System and the fifth largest planet overall. However, due to flattening at its poles (0.00335), Earth is not a perfect sphere, but an oblate spheroid. As a result, its polar diameter differs from its equatorial diameter, but only by about 41 km (25.5 mi)
In short, Earth measures 12713.6 km (7900 mi) in diameter from pole to pole, and 12756.2 km (7926.3 mi) around its equator. Once again, this is due to Earth’s sidereal rotational period, which takes a relatively short 23 hours, 58 minutes and 4.1 seconds to complete a single rotation on its axis.
Mars:
Mars is often referred to as “Earth’s twin”; and again, for good reason. Like Earth, Mars experiences flattening at its poles (0.00589), which is due to its relatively rapid sidereal rotational period (24 hours, 37 minutes and 22 seconds, or 1.025957 Earth days).
As a result, it experiences a bulge at its equator which leads to a variation of 40 km (25 mi) between its polar radius and equatorial radius. This works out to Mars having a mean diameter of 6779 km (4212.275 mi), varying between 6752.4 km (4195.75 mi) between its poles and 6792.4 km (4220.6 mi) at its equator.
Mosaic of the Valles Marineris hemisphere of Mars, similar to what one would see from orbital distance of 2500 km. Credit: NASA/JPL-Caltech
Jupiter:
Jupiter is the largest planet in the Solar System, measuring some 142,984 km (88,846 mi) in diameter. Again, this its mean diameter, since Jupiter experiences some rather significant flattening at the poles (0.06487). This is due to its rapid rotational period, with Jupiter taking just 9 hours 55 minutes and 30 seconds to complete a single rotation on its axis.
Combined with the fact that Jupiter is a gas giant, this means the planet experiences significant bulging at its equator. Basically, it varies in diameter from 133,708 km (83,082.3 mi) when measured from pole to pole, and 142,984 km (88,846 mi) when measured around the equator. This is a difference of 9276 km (5763.8 mi), one of the most pronounced in the Solar System.
Saturn:
With a mean diameter of 120,536 km (74897.6 mi), Saturn is the second largest planet in the Solar System. Like Jupiter, it experiences significant flattening at its poles (0.09796) due to its high rotational velocity (10 hours and 33 minutes) and the fact that it is a gas giant. This means that it varies in diameter from 108,728 km (67560.447 mi) when measured at the poles and 120,536 km (74,897.6 mi) when measured at the equator. This is a difference of almost 12,000 km, the greatest of all planets.
This portrait looking down on Saturn and its rings was created from images obtained by NASA’s Cassini spacecraft on Oct. 10, 2013. Credit: NASA/JPL-Caltech/Space Science Institute/G. Ugarkovic
Uranus:
Uranus has a mean diameter of 50,724 km (31,518.43 mi), making it the third largest planet in the Solar System. But due to its rapid rotational velocity – the planet takes 17 hours 14 minutes and 24 seconds to complete a single rotation – and its composition, the planet experiences a significant polar flattening (0.0229). This leads to a variation in diameter of 49,946 km (31,035 mi) at the poles and 51,118 km (31763.25 mi) at the equator – a difference of 1172 km (728.25 mi).
Neptune:
Lastly, there is Neptune, which has a mean diameter of 49,244 km (30598.8 mi). But like all the other gas giants, this varies due to its rapid rotational period (16 hours, 6 minutes and 36 seconds) and composition, and subsequent flattening at the poles (0.0171). As a result, the planet experiences a variation of 846 km (525.68 mi), measuring 48,682 km (30249.59 mi) at the poles and 49,528 km (30775.27 mi) at the equator.
In summary, the planets of our Solar System vary in diameter due to differences in their composition and the speed of their rotation. In short, terrestrial planets tend to be smaller than gas giants, and gas giants tend to spin faster than terrestrial worlds. Between these two factors, the worlds we know range between near-perfect spheres and flattened spheres.
