Venus is slightly closer to the Sun than Earth, averaging about 0.718 AU with Earth being one AU (astronomical unit) away from the Sun. It appears so bright to us because its thick cloud layer reflects as much as 72% of the sunlight that strikes it. Here is a real-colour image of the planet.
(image processing by R.Nunes at http://astrosurf.com/nunes/)
Venus at first glance seems very similar to Earth. It's mass (0.81 that of Earth), density (5.204 g/cm3 compared to Earth’s 5.515 g/cm3) and surface gravity (0.9 g) are all very similar to Earth. Compare it with Earth in this NASA image of the solar system, with the planets in correct order of distance from the Sun at the bottom of the image and each planet enlarged to show detail at the top of the image. From left to right, for example, you can see Mercury, Venus, Earth and then Mars.
Venus was formed about the same time as Earth was in about the same region around our young Sun, drawing from the same pool of elements within the solar dust left over from the Sun’s formation. Yet with all these similarities, Venus is nothing like Earth. Its surface is more akin to Hell than anything. There is no surface water and almost no oxygen. The average surface temperature is over 460°C, hotter than the brutally baked surface of Mercury, and its atmosphere is so dense that the pressure on its surface is the same as it is nearly a kilometer beneath our ocean. A human on the surface would be instantly charred, crushed and suffocated. But what if you could survive in a special spacesuit? Winds of up to 300 km/h buffet the upper atmosphere, driven by convection, whereas surface winds travel only a few km/h. But don’t mistake them for gentle. With an atmosphere so dense, this breeze would blow you over and you could not walk. During your stay each single day would pass agonizingly slowly, once every 243 Earth days, about as long as one year lasts on Venus, with the Sun rising in the west rather than the East. Venus is the only planet in the solar system that spins clockwise and this may be because it suffered some kind of catastrophic event such as a collision. We will revisit this idea shortly. Each long day would pass much like the next as Venus experiences no seasonal changes thanks to its extremely dense and universal cloud deck. You would never see the Sun or the starry night sky, each dim day gradually eroding into darker night and back again, punctuated by frequent lightning. Venus, named after the goddess of love, will never be the future interplanetary romantic getaway some of us might like to imagine.
What happened to Venus? Why isn’t it more like Earth?
Venus has an extremely dry and dense carbon dioxide rich atmosphere. Two recent flybys by the European Space Agency’s Venus Express Orbiter have revealed some clues. In 2007, the probe discovered hydrogen and oxygen streaming off the night side of Venus in a ratio of 2:1, suggesting that what little water there is in the atmosphere is being ionized by UV radiation and blown away into space by solar wind. The rate of water loss suggests that Venus has lost enormous amounts of water over the eons. However, this doesn’t necessarily mean that Venus once has liquid water oceans, as Mars appears to have once had. Researcher Eric Chassefiére of France developed a computer model that suggests the water was mostly atmospheric and existed only when the planet was very young and its surface was still molten. The ionization and escape of water into space would have removed energy from the atmosphere and triggered the solidification of the surface. This doesn’t rule out the possibility that Venus was later bombarded with water-laden comets that contributed a new source of water and maybe even oceans, at least temporarily, to the planet. This modeling, based on Venus Express data, needs to be followed up with more data such as how water and volcanic activity together may have shaped a young Venus.
So much water would not be stripped away from Venus if it had a magnetosphere as robust as Earth’s. Venus’ lack of a strong magnetic field poses yet another mysterious difference between it and Earth. One would guess with all the similarities, Venus would have a similar magnetic field, yet it does not have a dynamo as Earth does. This could mean that either Venus doesn’t have a solid inner core or a molten outer core, or that its outer core isn’t cooling and creating any convective currents. It might mean that its entire core has already solidified. Or, perhaps its lack of any dynamo may have something to do with its very slow rotation. Venus, unlike Earth, also has no plate tectonic movement. It is possible that some kind of catastrophic event, alluded to above, may have shut down whatever plate tectonic movement that may have once been active and this could have reduced heat flux through the crust and caused the mantle to heat up, reducing the heat flux from the core. In this case, heat from the core is reheating the planet’s crust rather than driving a dynamo that in turn could generate a magnetic field. Venus does have a very weak induced magnetic field that derives from the rotation of ionized gases high in its atmosphere, which interact with the solar wind. However, it is not strong enough to prevent the escape of hydrogen and oxygen and possibly many other light gases from Venus’ atmosphere. If you have read the articles on Mars, Earth and Mercury, you may have noticed a trend among the rocky planets: Strong magnetosphere > significant atmosphere; weak or no magnetosphere > either no atmosphere or a thin exosphere. So why does Venus, with a very weak magnetosphere, have the densest atmosphere of all the rocky planets in the solar system? This is a significant scientific puzzle. We need to figure out through which mechanism Venus holds onto its dense CO2-rich atmosphere, and perhaps we need to re-explore the connection between magnetosphere and atmosphere.
A partial explanation comes from recent data coming mostly from the European Space Agency’s Venus Express Probe launched in 2005, and it has to do with the weak induced magnetic field around Venus. The Sun’s powerful magnetic field carried by the solar wind creates field lines that wrap around Venus. These field lines induce a magnetic field around Venus that is created by the charged atomic ions in its ionosphere (the ions are themselves the result of the Sun’s UV radiation striking gaseous atoms and molecules high up in Venus’ atmosphere). The induced magnetic field around Venus has a bow shock and a magnetopause. The bow shock slows down supersonic particles streaming from the Sun to subsonic speeds, while a space created between the magnetopause, located about 300 km from the surface, and the ionosphere, located about 250 km from the surface, creates a special magnetic barrier that prevents solar wind particles from penetrating deeper into the Venusian atmosphere. This diagram helps to explain this concept.
(copyright RuslikO, http://commons.wikimedia.org/wiki/File:Venusian_magnetosphere.jpg)
This barrier is not a perfect atmospheric seal. In fact, atmospheric gases continuously leak from even powerful magnetospheres like Earth’s. In Venus’ case, the magnetic barrier is much weaker, and light gases such as oxygen, helium and hydrogen continuously and significantly leak trough the magnetotail. The water loss, as mentioned, suggests that Venus once had significant water, if not in an ocean than in a moist atmosphere.
Venus as Run-Away Greenhouse Effect
A nagging and important question remians: If Earth and Venus were created by the same raw materials at the same time and existed in similar environments (similar distances from the Sun for example), then how can Venus have such a dramatically different atmosphere and no internal dynamo system? Should the planets not both have a similar crust, mantle and core makeup?
Some clues to this mystery unfold as we explore Venus’ atmosphere in more detail. The atmosphere on Venus is extremely hot and dense. It is enveloped in opaque sulfuric acid clouds. The atmosphere is almost all carbon dioxide (96.5 %) with 3.5 % nitrogen and trace amounts of sulfur dioxide, water, helium and other gases. These pie charts reveal Venus' atmospheric makeup.
These gases rapidly spin around the planet within the upper atmosphere at a rate of about 350 km/h. The atmosphere is so dense on Venus that the carbon dioxide at the surface is technically a supercritical fluid rather than a gas, giving it unusual properties such as the ability to effuse through solids like a gas and dissolve minerals like a liquid. Carbon dioxide is an effective greenhouse gas, trapping energy from sunlight, and it is this property that caused the surface of Venus to heat up to about 460°C, hot enough to melt lead, tin and zinc. With such thick cloud cover, this temperature doesn’t change much. During the night, which lasts about 58 days, it remains just as hot. Venus rains sulfuric acid high within its thick sulfur dioxide clouds but it never reaches the ground before it evaporates in the extreme heat. Unlike Earth, sulfur, which is released from volcanoes on both planets, does not become sequestered into solid compounds but rather circulates in the atmosphere in various compounds such as sulfur trioxide, sulfur dioxide and sulfuric acid.
Carbon also becomes sequestered and released in what is called the carbon cycle on Earth, where much of it is stored in limestone (CaCO3). It is released from limestone into the ocean and atmosphere (as well as into living things) whenever limestone is subducted or weathered. Atmospheric/oceanic carbon is then sequestered back into limestone when shell-bearing marine animals such as corals and shellfish die and leave their carbon-rich shells behind. The carbon cycle on Earth is made possible through both plate tectonics and surface liquid water, and it actually creates an elegant negative feedback loop that regulates Earth’s surface temperature. There is no such carbon cycle on Venus. For one thing Venus has no plate tectonics, in which giant plates of crust move, bump into one another and slide or subduct under one another. Subduction effectively traps massive amounts of carbon underground on Earth but Venus’ carbon, released from volcanoes as well as baked out of surface rocks, has nowhere to go but into the atmosphere. Venus is a planet of volcanoes. It has produced more volcanoes than any other planet in the solar system. Radar mapping of the surface reveals many volcanoes and lava plains. However, measurements of the density of impact craters on the surface reveal that volcanism may have quieted down about 500 million years ago, when a major resurfacing event may have occurred. This, along the mystery of Venus’ solidified crust, no (dynamo-creating) core convective activity, and its slow and backward rotation, suggests that something might have happened to Venus to take it on a path much different from that of Earth. What exactly happened to Venus, if anything, is still a matter of heated debate, as is the possible connection between such an event and the build-up of carbon dioxide in its atmosphere. New evidence from Europe’s Venus Express orbiter suggests that large plateaus on Venus may consist of granite, a rock that, on Earth, needs water and plate tectonics to form. Could these granite plateaus be the ancient remnants of oceans and could Venus have once had active plate tectonics? As previously mentioned, some researchers do not think that Venus ever had liquid oceans. Granite was detected by measuring small differences in surface infrared (heat) radiation off different rock formations. This was, of necessity, done from orbit, through the dense Venusian clouds and atmosphere, where infrared radiation could be scattered.
As you can see, the story of Venus is far from complete. Although Venus Express has garnered a great deal of invaluable data from the planet and other probes, such as two recent data-gathering flybys by NASA’s Messenger Mercury probe and Japan’s Akatsuki probe launched in 2010, may add to that data, more detailed research is clearly needed not only to further understand the geological and atmospheric processes on Venus but to further our understanding of these processes here on Earth as well.
NASA has proposed the Venus In-Situ Explorer, a mission that would land on the surface of Venus and study rock core samples, measuring their composition and mineralogy. This might help answer questions about early Venus and possible ancient plate tectonics and surface water. This mission obviously has many technical challenges in terms of constructing a probe able to withstand extreme surface conditions. A launch has been proposed for 2013 but current fiscal restraints will likely push this date significantly back. This is a NASA image of what it might look like as it approaches the surface of Venus.
As well, Europe is planning to send the Venus Entry Probe, to be launched at the same time, which would consist of a special balloon-like probe that will study the Venus’ atmospheric layers in detail.
In spite of Venus' hostility, some researchers, such as Geoffrey Landis of NASA's Glenn Research Center, believe that a case can be made for future colonization on Venus, not on its hellish surface but, instead, about 50 km up in its atmosphere. Here, air pressure drops to one atmosphere and the temperature ranges from 0°C to 50°C. The notion has several advantages: no bone loss due to low gravity living and Venus, being the closest planet to Earth, makes back and forth travel feasible. However, the problems are severe: almost no water and oxygen, the clouds are extremely corrosive, and the surface is practically unlivable. Human life there would have to consist of some kind of floating colony. Cases for colonization on Mars have also been made and in fact, NASA has created an interesting page full of external links exploring colonization, exploration and terraforming of Mars here. Both Mars and Venus come with their own difficulties in terms of human habitation and exploration. If you are curious about the habitability of other planets including those in other solar systems, I recommend keeping an eye out for a future article exploring what is called the Goldilocks Zone. This is a theoretical distance from a star in which a planet can maintain liquid water and sustain Earth-like life. In the meantime, I hope that these articles on the rocky planets thus far have given you an idea of just how much more we need to learn about planet dynamics, how rough indeed our outline is, of what constitutes a “habitable” planetary zone, and perhaps even a “habitable” planet.
Before you head off exploring, I recommend these Venus videos. I hope you enjoy them.
The Planet Venus (10 minutes, excellent info on all the exploratory missions to the planet)
All About Venus (6 minutes, a good introduction for beginners)
Venus: Death of a Planet (22 minutes, comprehensive)
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