Artist's illustration of Quetzalpetlatl Corona on Venus displaying both active volcanism and a subduction zone. (Credit: NASA/JPL-Caltech/Peter Rubin)
While Earth and Venus are approximately the same size and both lose heat at about the same rate, the internal mechanisms that drive Earth’s geologic processes differ from its neighbor. It is these Venusian geologic processes that a team of researchers led by NASA’s Jet Propulsion Laboratory (JPL) and the California Institute of Technology hope to learn more about as they discuss both the cooling mechanisms of Venus and the potential processes behind it.
Venus and Earth have many similarities. Both are terrestrial planets, meaning that they are composed predominately of metal and silicate rock, which is differentiated between a metal core and a silicate mantle and crust. Both also orbit the Sun within its habitable zone (aka. “Goldilocks Zone“). Hence why Venus and Earth are often called “sister planets”.
However, Venus is also starkly different from Earth in a number of ways. It’s atmosphere, which is composed primarily of carbon dioxide and small amounts of nitrogen, is 92 times as dense as Earth’s. It is also the hottest planet in the Solar System, with temperatures hot enough to melt lead! And on top of all that, a year on Venus is much different than a year on Earth.
Orbital Period:
Venus orbits the Sun at an average distance of about 0.72 AU (108,000,000 km/67,000,000 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.
Earth and Venus’ orbit compared. Credit: Sky and Telescope
The planet’s orbital period is 224.65 days, which means 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.
Sidereal vs. Solar Day:
While a year on Venus lasts the equivalent of 224.65 Earth days, it only lasts the equivalent 1.92 days on Venus. This is due to the fact that Venus rotates quick slowly and in the opposite direction of its orbit. Because of this, a Solar Day – the time it takes for the Sun to rise, set, and return to the same place in the sky – takes 116.75 Earth days.
This means, in effect, that a single day on Venus lasts over half a year. In other words, in the space of just over a single Venusian year, the Sun will appear to have circled the heavens twice. In addition, to someone standing on the planet’s surface, the Sun would appear to rise in the west and set in the east.
Variations:
Because of its dense atmosphere and its highly circular rotation, Venus experiences very little in the way of temperature variations during the course of a year. Similarly, its axial tilt of 2.64° (compared to Earth’s 23.44°) is the second-lowest in the Solar System, behind Mercury’s extremely low tilt of 0.03.
This means that there is virtually no variation in Venus’ surface temperature between day and night, or the equator and the poles. All year long, the mean surface temperature of Venus is a scorching 735 K (462 °C/863.6 °F), with the only variations occurring as a result of elevation.
Yes, Venus is a truly hellish place. And unfortunately, that’s a year-round phenomena! The days are extremely hot, the nights extremely hot, and a day lasts over half as long as a year. So if you’re planning on vacationing somewhere, might we recommend somewhere a little less sunny and balmy?
A radar view of Venus taken by the Magellan spacecraft, with some gaps filled in by the Pioneer Venus orbiter. Credit: NASA/JPL
There are dozens upon dozens of moons in the Solar System, ranging from airless worlds like Earth’s Moon to those with an atmosphere (most notably, Saturn’s Titan). Jupiter and Saturn have many moons each, and even Mars has a couple of small asteroid-like ones. But what about Venus, the planet that for a while, astronomers thought about as Earth’s twin?
The answer is no moons at all. That’s right, Venus (and the planet Mercury) are the only two planets that don’t have a single natural moon orbiting them. Figuring out why is one question keeping astronomers busy as they study the Solar System.
Astronomers have three explanations about how planets get a moon or moons. Perhaps the moon was “captured” as it drifted by the planet, which is what some scientists think happened to Phobos and Deimos (near Mars). Maybe an object smashed into the planet and the fragments eventually coalesced into a moon, which is the leading theory for how Earth’s Moon came together. Or maybe moons arose from general accretion of matter as the solar system was formed, similar to how planets came together.
The International Space Station captured as it passed in front of the Moon on Dec. 6, 2013, as seen from Puerto Rico. Credit and copyright: Juan Gonzalez-Alicea.
Considering the amount of stuff flying around the Solar System early in its history, it’s quite surprising to some astronomers that Venus does not have a moon today. Perhaps, though, it had one in the distant past. In 2006, California Institute of Technology researchers Alex Alemi and David Stevenson presented at the American Astronomical Society’s division of planetary sciences meeting and said Venus could have been smacked by a large rock at least twice. (You can read the abstract here.)
“Most likely, Venus was slammed early on and gained a moon from the resulting debris. The satellite slowly spiraled away from the planet, due to tidal interactions, much the way our Moon is still slowly creeping away from Earth,” Sky and Telescope wrote of the research.
“However, after only about 10 million years Venus suffered another tremendous blow, according to the models. The second impact was opposite from the first in that it ‘reversed the planet’s spin,’ says Alemi. Venus’s new direction of rotation caused the body of the planet to absorb the moon’s orbital energy via tides, rather than adding to the moon’s orbital energy as before. So the moon spiraled inward until it collided and merged with Venus in a dramatic, fatal encounter.”
Venus as photographed by the Pioneer spacecraft in 1978. Some exoplanets may suffer the same fate as this scorched world. Credit: NASA/JPL/Caltech
There could be other explanations as well, however, which is part of why astronomers are so interested in revisiting this world. Figuring out the answer could teach us more about the solar system’s formation.
If you look to the morning sky – to the east that is, as the sun’s rising – you will notice a bright star in the firmament, one that should not be there. Theoretically, stars only come out at night and should be well on their way to bed by the time the sun rises, correct? Well, that’s because the Morning Star, as it’s known, isn’t a star at all, but the planet Venus. It is both the morning and evening star, the former when it appears in the east during sunrise and the latter when it appears in the west during sunset. Because of its unique nature and appearance in the sky, this “star” has figured prominently in the mythologies of many cultures.
In ancient Sumerian mythology, it was named Inanna (Babylonian Ishtar), the name given to the goddess of love and personification of womanhood. The Ancient Egyptians believed Venus to be two separate bodies and knew the morning star as Tioumoutiri and the evening star as Ouaiti. Likewise, believing Venus to be two bodies, the Ancient Greeks called the morning star Phosphoros (or Eosphoros) the “Bringer of Light” (or “Bringer of Dawn”) and the evening star they called Hesperos (“star of the evening”). By Hellenistic times, they had realized the two were the same planet, which they named after their goddess of love, Aphrodite. The Phoenicians, never ones to be left out where astronomy and mythology were concerned, named it Astarte, after their own goddess of fertility. In Iranian mythology, especially in Persian mythology, the planet usually corresponds to the goddess Anahita, and sometimes AredviSura, the goddesses of fertility and rivers respectively. Mirroring the ancient Greeks, they initially believed the planet to be two separate objects, but soon realized they were one.
The Romans, who derived much of their religious pantheon from the Greek tradition and near Eastern tradition, maintained this trend by naming the planet Venus after their goddess of love. Later, the name Lucifer, the “bringer of light”, would emerge as a Latinized form of Phosphoros (from which we also get the words phosphorus and phosphorescence). This would prove influential to Christians during the Middle Ages who used it to identify the devil. Medieval Christians thusly came to identify the Morningstar with evil, being somewhat more concerned with sin and vice than fertility and love! However, the identification of the Morningstar as a symbol of fertility and womanhood remains entrenched, best demonstrated by the fact that the astronomical symbol for Venus happens to be the same as the one used in biology for the female sex: a circle with a small cross beneath.
The Morningstar also figures prominently in the mythology of countless other cultures, including the Mayans, Aborigines, and Maasai people of Kenya. To all of these cultures, the Morningstar still serves as an important spiritual, agricultural and astrological role. To the Chinese, Japanese, Koreans and Vietnamese, she is known literally as the “metal star”, based on the Five Elements.
We have written many articles about the Morning Star for Universe Today. Here’s an article about how to find Venus in the sky, and here’s an article about the brightest planet.
[/caption]
Venus is one of the planets visible with the unaided eye. In fact, Venus is the brightest object in the sky, after the Sun and the Moon. So it’s impossible to know how was Venus discovered. The planet has been known about since prehistoric times. Perhaps a better question is, when did we realize that Venus was a planet?
Thousands of years ago, the Greek astronomers thought that the Earth was the center of the Universe, and everything revolved around us, including the Sun, the Moon, the planets and the stars. But in the in the 1500s, Nicolaus Copernicus developed his theories of a Sun-centered Solar System. Instead of the traditional idea, the Sun was at the center, and the Earth was just another planet like Venus and Mars.
This theory was given a tremendous amount of evidence when Galileo Galilei first turned his rudimentary telescope on Venus, showing that the planet went through phases, like the Moon. This meant that it orbited the Sun, and not the Earth. Galileo also discovered the 4 major moons orbiting Jupiter, demonstrating that not all objects in the Universe orbited the Earth.
So it was in the 16th and 17th centuries that astronomers really came to understand that both Venus and Earth were just planets orbiting the Sun.
But the story is a little stranger than that. Venus is actually rotating backwards compared to the rest of the planets in the Solar System. Seen from above the north pole, Venus is slowly rotating in a clockwise direction. Compare this to Earth and the rest of the planets, which rotate in a counter-clockwise direction.
And it gets even stranger, when you consider that a year on Venus only lasts 224.7 days. In other words, a day on Venus is actually longer than a year on Venus. If you could actually stand on the surface and see the Sun, you would see the Sun rise in the West, and pass through the sky over the course of 116.75 days and then set in the East. So a solar day on Venus is 116.75 days.
Astronomers aren’t sure why the length of day on Venus takes so long, and why Venus is rotating backwards. It’s possible that Venus was struck by a large object early on in its history, which flipped it over and caused its strange rotation.
We’ve written many articles about the day length of the planets, here’s an article about a day on Mars, and here’s an article about a day on Saturn.
[/caption]
How old is Venus? Scientists actually think that everything in the Solar System was formed at the same time, about 4.6 billion years ago.
Before that point, our entire Solar System was just a vast cloud of hydrogen, helium and other trace elements. Some event, like a nearby supernova, caused the cloud to collapse through its mutual gravity. As the ball collapsed down, it started to spin because of the conservation of momentum from all the atoms in the cloud. As it spun, it flattened out into a disk. The Sun formed out of a bulge in the middle, and the planets formed in the disk.
The planet started out as nothing more than dust, but then these dust particles collided together, forming larger grains, pebbles, rocks, boulders and eventually planetoids. For the first few millions years, the Solar System was a dangerous place with these planetoids constantly crashing into one another. Life wouldn’t stand a chance to survive.
Eventually the number of objects in the Solar System was cleared out; they were either swept up into the planets, or kicked out of the Solar System by gravity. And we were left with the planets we have today.
Astronomers know that everything in the Solar System (including Venus) is roughly 4.6 billion years old through radioactive dating of meteorites. They can tell that all the meteorites in the Solar System were formed at the same time because of the percentages of radioactive elements they contain. And they’re able to determine how much of these radioactive elements have decayed over time, to determine their age.
So, how old is Venus? 4.6 billion years old, just like everything else in the Solar System.
We’ve written many articles about the age of objects in the Universe. Here’s an article about the age of the Universe, and here’s an article about the age of the Milky Way.
The first color pictures taken of the surface of Venus by the Venera-13 space probe. The Venera 13 probe lasted only 127 minutes before succumbing to Venus's extreme surface environment. Part of building a longer-lasting Venus lander is figuring out how to power it. Credit: NASA
[/caption]
Venus has been know to humanity since we first looked up into the sky; it’s the brightest object in the night sky after the Sun and the Moon, so it’s pretty hard to miss. But Venus exploration really began with the invention of the telescope.
Although he didn’t invent the telescope, Galileo Galilei was the first to point it at the heavens and make detailed observations of what he saw. In 1610 he discovered that Venus goes through phases, like the Moon. This is because Venus is closer to the Sun than the Earth, and so we’re seeing different amounts of the planet illuminated by the Sun. This provided more evidence that the Solar System orbits around the Sun, and not the Earth.
But even with bigger and better telescopes, astronomers weren’t able to penetrate the thick clouds that shroud Venus and see the terrain below. They imagined a warm rainforest jungle world, but astronomers eventually worked out that Venus is really covered in a thick atmosphere of carbon dioxide, and the ground below is heated to hundreds of degrees.
The first spacecraft to arrive at Venus was NASA’s Mariner 2, which flew past Venus in 1962. It was followed by spacecraft from Russia, including several that actually landed on the surface of Venus, and survived up to a few hours in the horrendous heat. NASA’s Magellan spacecraft was equipped with a radar instrument that could pierce through the atmosphere of Venus and reveal the terrain below. It showed that Venus has evidence of volcanism, and impact craters, but no plate tectonics. This helps contribute to its runaway greenhouse effect.
The most recent spacecraft sent to Venus is the European Space Agency’s Venus Express. It arrived at Venus in 2006, and has been making continuous observations of the planet ever since.
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.
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.
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.
