Posted on by Fraser Cain

Why is Venus So Hot?

You might have heard that Venus is the hottest planet in the Solar System. In fact, down at the surface of Venus it’s hot enough to melt lead. But why is Venus so hot?

Three words: runaway greenhouse effect. In many ways, Venus is a virtual twin of Earth. It has a similar size, mass and gravity as well as internal composition. But the one big difference is that Venus has a much thicker atmosphere. If you could stand on the surface of Venus, you would experience 93 times the atmospheric pressure we experience here on Earth; you’d have to dive down 1 km beneath the surface of the ocean to experience that kind of pressure. Furthermore, that atmosphere is made up almost entirely of carbon dioxide. As you’ve probably heard, carbon dioxide makes an excellent greenhouse gas, trapping heat from the Sun. The atmosphere of Venus allows the light from the Sun to pass through the clouds and down to the surface of the planet, which warms the rocks. But then the infrared heat from the warmed rocks is prevented from escaping by the clouds, and so the planet warmed up.

The average temperature on Venus is 735 kelvin, or 461° C. In fact, it’s that same temperature everywhere on Venus. It doesn’t matter if you’re at the pole, or at night, it’s always 735 kelvin.

It’s believed that plate tectonics on Venus stopped billions of years ago. And without plate tectonics burying carbon deep inside the planet, it was able to build up in the atmosphere. The carbon dioxide built up to the point that any oceans on Venus boiled away. And then the Sun’s solar wind carried the hydrogen atoms away from Venus, making it impossible to ever make liquid water again. The concentration of carbon dioxide just kept increasing until it was all in the atmosphere.

We’ve written many articles about Venus for Universe Today. Here’s an article about the atmosphere of Venus, and here’s an article about how to find Venus in the sky.

If you’d like more info on Venus, here’s a cool lecture about Venus and the greenhouse effect, and here’s more information on the runaway greenhouse effect on Venus.

We’ve also recorded an entire episode of Astronomy Cast just about Venus. Listen here, Episode 50: Venus.

Posted on by Fraser Cain

How Long is a Day on Venus?

The length of day on Venus is 243 Earth days. Read that again, it’s not a year, but the length of a single day. In fact, a year on Venus is only 224.7 days, so a day on Venus is longer than its year. And things get even stranger. Venus rotates backwards. All of the planets in the Solar System rotate counter-clockwise when you look at them from above. But Venus turns clockwise.

Of course it’s impossible to stand on the surface of Venus and survive. And even if you could, you wouldn’t be able to see the Sun through the dense clouds. But if you could stand on Venus and see the Sun, you’d see the Sun rise in the West, pass through the sky for 116.75 days and then set in the East. That’s the opposite of what we see here on Earth.

Why does Venus rotate backwards? Astronomers aren’t sure, but it’s possible that Venus suffered a massive impact from a large planetoid billions of years ago. This could have given the planet a kick that set it slowly tumbling, eventually flipping completely over so that it’s now upside-down.

We’ve written many articles about day length for Universe Today. Here’s an article about the length of day on Mercury, and here’s an article about the length of day for all the planets in the Solar System.

If you’d like more info on Venus, check out Hubblesite’s News Releases about Venus, and here’s a link to NASA’s Solar System Exploration Guide on Venus.

We’ve also recorded an episode of Astronomy Cast all about Venus. Listen here, Episode 50: Venus.

Posted on by Fraser Cain

Mercury Diagram

Interior of Mercury. Image credit: NASA

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Here’s a Mercury diagram, showing the interior of planet Mercury.

Mercury is the closest planet to the Sun, orbiting at an average distance of 57.9 million km from the Sun. It’s also the second densest planet in the Solar System, with an average distance of 5.427 grams per cubic centimeter. Based on this density, astronomers have some estimates about the interior structure of Mercury.

The center of Mercury is its metal core, similar to the Earth’s core. But in the case of Mercury, the core occupies 42% of the volume of Mercury, while the core of Earth is only 17%. And for some reason, the metal core of Mercury doesn’t create a magnetic field with the same intensity of Earth’s magnetic field. Mercury’s magnetosphere is only 1% as strong as Earth’s field.

Surrounding the core is Mercury’s mantle. This is a 500-700 km thick layer of rock, composed of silicates. And surrounding the mantle is Mercury’s crust. Based on observations made by Mariner 10 and Earth-based telescopes, astronomers think that Mercury’s crust is 100 – 300 kilometers thick. There are many large depressions in Mercury’s crust, and scientists think these formed as Mercury slowly cooled and contracted.

We’ve written many articles about Mercury for Universe Today. Here’s an article with a diagram of the Sun, and here’s a diagram of the Solar System.

If you’d like more info on Mercury, check out NASA’s Solar System Exploration Guide, and here’s a link to NASA’s MESSENGER Misson Page.

We’ve also recorded an episode of Astronomy Cast about Mercury. Listen here, Episode 49: Mercury.

Posted on by Fraser Cain

Length of Year on Mercury

Mosaic of Mercury. Credit: NASA / JHUAPL / CIW / mosaic by Jason Perry

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The length of year on Mercury is 87.969 days. In other words, it takes almost 88 Earth days for Mercury to complete one orbit around the Sun. Mercury completes just over 4 orbits for each year on Earth.

Mercury has the most eccentric of all the orbits of the planets. Its distance from the Sun varies between 46 million and 70 million kilometers. This means that the speed of its orbit varies dramatically depending on the point of its orbit. If you could stand on the surface of Mercury and watch the Sun, you would see the Sun rise in the morning go part way up in to the sky and then go backwards in the sky, and set again. And then it would rise again and this time it would go across the sky and set. Four days before the fastest point of its orbit around the Sun, Mercury’s orbital speed matches its rotational velocity so that the Sun appears to stop in the sky. Then it’s orbiting faster than it’s rotating for about 8 days and so the Sun appears to move backwards.

We’ve written several articles about the length of years for Universe Today. Here’s an article about the years of all the planets, and here’s an article about how long a year is on Mars.

If you’d like more info on Mercury, check out NASA’s Solar System Exploration Guide, and here’s a link to NASA’s MESSENGER Misson Page.

We’ve also recorded several episodes of Astronomy Cast about Mercury. Listen here, Episode 49: Mercury.

Posted on by Fraser Cain

Mercury Revolution

Mosaic of Mercury. Credit: NASA / JHUAPL / CIW / mosaic by Jason Perry

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In scientific terms, an orbital revolution is the amount of time it takes for one object to orbit completely around another. So a Mercury revolution is the amount of time it takes for Mercury to completely orbit the Sun and then come back to its initial position. Here on Earth, we call that a year.

Mercury’s revolution around the Sun takes 87.969 days. So, you could say that Mercury’s year lasts almost 88 days.

But if you were standing on the surface of Mercury, you wouldn’t experience that many days. That’s because Mercury rotates on its axis very slowly, taking almost 59 days to rotate once. The strange thing is that if you were standing on the surface of Mercury, you would experience something very different. You would see the sun rise halfway, and then go back down again, and then rise up again before setting. The whole process would take about 2 of Mercury’s years.

Remember, the revolution of Mercury is how long the planet takes to travel around the Sun. The rotation of Mercury is how long it takes to turn once on its axis.

We’ve written many articles about Mercury for Universe Today. Here’s an article about the gravity on Mercury, and here’s an article about the composition of Mercury.

If you’d like more info on Mercury, check out NASA’s Solar System Exploration Guide, and here’s a link to NASA’s MESSENGER Misson Page.

We’ve also recorded several episodes of Astronomy Cast about the Solar System. Listen here, Episode 49: Mercury.

Posted on by Fraser Cain

What is a Sun Dog?

A sun dog is an atmospheric phenomenon where you can see additional bright patches in the sky on either side of the Sun. Sometimes you just see bright spots, and sometimes you can actually see an arc or even a halo around the Sun. These are all related to sun dogs, and have to do with very specific atmospheric conditions. If you’ve ever seen a sun dog, you were very lucky, and they only occur rarely.

Sun dogs occur because of sunlight refracting through ice crystals in the atmosphere. The crystals cause the sunlight to bend at a minimum angle of 22°. All of the crystals are refracting the Sun’s rays, but we only see the ones which are bent towards our eyes. Because this is the minimum, the light looks more concentrated starting at 22° away from the Sun; about 40 times the size of the Sun in the sky. At this 22° point you can get arcs, a halo, or just bright spots in the sky.

They can occur at any time of the year and from any place on Earth; although, they’re easiest to see when the Sun is lower on the horizon. As the Sun rises, the sun dog can actually drift away from the 22° point. Eventually the Sun gets so high that the sun dog disappears entirely.

There are no set colors with sun dogs. The light from the Sun is being refracted equally by the ice crystals and so we don’t see the colors broken up as we do with a rainbow.

We’ve written several articles about the Sun for Universe Today. Here’s an article about a ring around the Sun, and here’s an article about rings around the Moon.

If you’d like more info on sun dogs, check out this site.

We’ve recorded several episodes of Astronomy Cast about the Sun. Listen here, Episode 30: The Sun, Spots and All.

Posted on by Fraser Cain

Green Flash Sunset

Green Flash in Santa Cruz, California. Image credit: Mila Zinkova

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Have you ever heard of a green flash sunset? You might think it’s a myth, but this is a real phenomenon that you can see if the conditions are just right. If you’re watching the Sun dip down on the horizon you might see a green dot appear just above the Sun for just a second. That’s a green flash sunset, and if you saw one, you’re a very lucky person.

Green flashes can occur at sunrise or sunset, and to see one, you need to have an unobstructed view to the horizon. They occur because the light from the Sun is refracted – or bent – as it passes through the Earth’s atmosphere, following the curvature of the Earth. Higher frequency light (bluer light) is bent more than lower frequency light. This is happening all the time, but we’re seeing all the colors of the light spectrum at the same time. But when the Sun is right at the horizon, the redder hues of the color spectrum are blocked by the horizon of the Earth, while the higher frequency wavelengths are still following the curve of the Earth. While the redder light is blocked, the green and blue light is still visible, so we see the green flash.

There are actually a few different kinds of green flashes that can occur. The most common example is an inferior-mirage flash, where a dot of green light appears on top of the Sun just as it’s gone below the horizon. But you can also get a situation where a portion of the Sun’s upper edge turns slightly green, or even a green beam of light appears above the Sun.

We’ve written a few articles about sunsets for Universe Today. Here’s an article about green flashes, and here are some cool pictures of sunsets seen from other worlds.

If you’d like more info on green flashes, check out this introduction to green flashes.

We’ve also recorded an episode of Astronomy Cast all about the Sun. Listen here, Episode 30: The Sun, Spots and All.

Posted on by Fraser Cain

Chromosphere

Plasma on the surface of the Sun. Image credit: Hinode

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The Sun may look like just a mass of incandescent gas (plasma, really), but it’s actually broken up into layers. The chromosphere is relatively thin region of the Sun that’s just above the photosphere.

The photosphere is the region of the Sun that we see. It measures an average temperature of almost 5,800 kelvin and produces the visible radiation. This is the point where photons generated inside the Sun can finally leap out into space. The chromosphere measures just 2,000 km, and it’s just outside the photosphere.

Even though it’s very thin, the chromosphere changes dramatically in density, from the top down to the photosphere, the density of the chromosphere increases by a factor of 5 million. The upper boundary of the chromosphere is the called the solar transition region, above which is known as the corona.

One surprising mystery is that the chromosphere is actually hotter than the photosphere. While the photosphere hovers around 5,800 kelvin, the temperature of the chromosphere varies between 4,500 K and 20,000 K. Even though it’s more distant from the center of the Sun, the chromosphere is hotter than the photosphere. Astronomers think turbulence in the Sun’s atmosphere might somehow cause this extra heating.

The chromosphere is difficult to see without special equipment because the light from the much brighter photosphere washes it out. It has a reddish color, but you can only really see it during a total solar eclipse.

One of the recognizable features of the chromosphere are spicules. These are fingers of gas that kind of look like grass growing on the surface of the Sun. These can rise up in the chromosphere and then disappear again within 10 minutes.

We’ve written several episodes about the Sun for Universe Today. Here’s an article about the Sun’s atmosphere, and here’s an article about how solar astronomers are getting better at predicting the solar wind.

If you’d like more info on the Sun, check out NASA’s Solar System Exploration Guide on the Sun, and here’s a link to the SOHO mission homepage, which has the latest images from the Sun.

We’ve also recorded an episode of Astronomy Cast just about the Sun. Listen here, Episode 30: The Sun, Spots and All.

Posted on by Fraser Cain

What is the Surface of Jupiter Like?

A true-color image of Jupiter taken by the Cassini spacecraft. The Galilean moon Europa casts a shadow on the planet's cloud tops. Credit: NASA/JPL/University of Arizona

Have you ever wondered what it might feel like to stand on Jupiter’s surface? Well, there’s a problem. Jupiter is made up almost entirely of hydrogen and helium, with some other trace gases. There is no firm surface on Jupiter, so if you tried to stand on the planet, you sink down and be crushed by the intense pressure inside the planet.

When we look at Jupiter, we’re actually seeing the outermost layer of its clouds. Jupiter upper atmosphere is made of up to 90% hydrogen, with 10% helium, and then other gases like ammonia. The bands and storms that we can see on the planet are all generated in the upper atmosphere. The cloud layer we can see is made of ammonia, and only extends down for about 50 km or so. The large storms like the Great Red Spot occur within this layer; although it’s thought they may dredge up material from deeper down inside the planet.

If you could stand on the surface of Jupiter, you would experience intense gravity. The gravity at Jupiter’s surface is 2.5 times the gravity on Earth. If you weighed 100 pounds on Jupiter, you’d weigh 250 pounds on Jupiter. Of course, there’s no actual surface, so you’d just sink into the planet if you tried to stand on it.

We’ve written many articles about Jupiter for Universe Today. Here’s an article about how Jupiter might have captured a comet as a temporary moon, and does Jupiter have a solid core?

If you’d like more info on Jupiter, check out Hubblesite’s News Releases about Jupiter, and here’s a link to NASA’s Solar System Exploration Guide to Jupiter.

We’ve also recorded an entire episode of Astronomy Cast all about Jupiter. Listen here, Episode 56: Jupiter.

Posted on by Fraser Cain

Planet Jupiter

Jupiter photo. Image credit: NASA/SSI

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Jupiter is the 5th planet in the Solar System, and by far the largest, containing 2.5 times the mass of the rest of the planets in the Solar System. Everything about planet Jupiter is big. It has the most moons in the Solar System, including the largest moon, Ganymede. It has the largest storm: the Great Red Spot; the most extreme gravity, and the biggest temperature extremes.

Because you can see Jupiter with the unaided eye, it’s impossible to say who actually discovered Jupiter. But it was Galileo Galilei who first turned his rudimentary telescope on Jupiter in 1610. Even with its dim optics, Galileo was able to make out the fact that Jupiter had 4 bright moons and bands across the planet. Since astronomers believed that everything orbited around the Earth, finding moons orbiting Jupiter threw the Earth-centered theory of the Universe into doubt. Even the smallest telescope will show you what Galileo saw.

Planet Jupiter orbits the Sun at an average distance of 779 million km (484 million miles), and it takes 4,333 Earth days to complete one orbit around the Sun; that’s almost 12 years. But Jupiter rotates once on its axis every 9 hours and 56 minutes. This high rotation speed has flattened out the planet, so that its equator is much further from the center of Jupiter than the poles. Jupiter’s also the largest plane in the Solar System, with an equatorial diameter of 142,984 kilometers (88,846 miles) – 11 times the diameter of Earth.

Jupiter has 318 times more mass than Earth, but it has a relatively low density; only 1/4 the density of Earth. It has such a low density because Jupiter is made up almost entirely of hydrogen with a little bit of helium. The upper atmosphere has tiny amounts of ammonia and other chemicals, which create the bands and clouds we see in photographs. The most familiar feature in Jupiter’s atmosphere is the planet’s Great Red Spot. This is a long-lived storm large enough to swallow up three planets the size of Earth.

It also has the largest number of moons in the Solar System: 63 at last count. The 4 largest moons are the Galilean moons, named after Galileo who discovered them. Ganymede measures 3,273 km across, and it’s the largest moon in the Solar System. Io orbits the closest of these moons and its undergoing almost constant volcanic activity because of tidal flexing being so close to Jupiter. Europa and Callisto probably have oceans of liquid water underneath thick shells of ice, and could be the home to exotic forms of life. Jupiter also has its own set of rings.

Seven spacecraft from Earth have made the journey to Jupiter: Pioneer 10, Pioneer-Saturn, Voyager 1, Voyager 2, Ulysses, Galileo, and New Horizons. Pioneer 10 was the first to reach the planet, making its flyby in 1972. The Galileo spacecraft actually went into orbit around Jupiter, to study the planet and its moons in great detail.

We’ve written many articles about Jupiter for Universe Today. Here’s an article about how Jupiter might protect us in the Solar System, and here’s an article about how you can see Jupiter in a telescope.

If you’d like more info on Jupiter, check out Hubblesite’s News Releases about Jupiter, and here’s a link to NASA’s Solar System Exploration Guide to Jupiter.

We’ve also recorded an episode of Astronomy Cast just about Jupiter. Listen here, Episode 56: Jupiter.

Reference:
NASA