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Venus Geology

The Hot Squeeze


venus surface

Radar surface map of Venus in false color. More pictures on page 1.

NASA image

The radar mapping of Venus's surface has laid bare its mysterious wrinkled crust and given us hints about the mantle beneath. We also have a bit of insight into the iron core from the planet's magnetic field—that is, from its absence. No magnetic field means the core is not stirring. For stirring, or convection, to occur there must be heat below and cold above, just like the pot of water boiling on your stove. But Venus is hot all the way up to the surface.

Moreover, Venus may not have a solid inner core as Earth does. On Earth, energy is released as iron freezes onto the inner core, stirring the liquid metal above it. If Venus lacks this source of heat below and is not strongly cooled from above, then its core must be too sluggish to generate a magnetic field.

Earth's geomagnetic field protects us from a great deal of high-energy radiation from the sun. On Venus, all of that same radiation—more of it, actually, since the sun is closer—slams onto the atmosphere. In fact the more we study Venus, using computer simulations, the more the atmosphere matters. It is a most unusual greenhouse.

The Deep Venus Greenhouse

Carbon dioxide (CO2) makes up 96 percent of Venus's dense atmosphere, retaining so much of the sun's energy that any ocean the planet once had was long ago boiled away. But two trace gases, water vapor and sulfur dioxide, have powerful effects too. The reason is that CO2 isn't a perfect greenhouse—it allows certain wavelengths of radiation through. Water vapor and sulfur gases plug those holes. These two gases are released during volcanic eruptions, and if there are enough eruptions Venus gets a really bad fever.

These volcanic gases are removed from the atmosphere over millions of years. Water vapor breaks down under the intense solar radiation into oxygen and hydrogen, and the hydrogen escapes into space. That's how Venus lost its water ages ago. The sulfur gases react with the rocks on Venus's surface and leave the atmosphere from the bottom. But until these processes are finished the temperature rises significantly, 100 degrees or more. After a few million years the extra heat penetrates the crust to the upper mantle, bringing it closer than ever to melting. This feedback encourages more eruptions.

Researchers look at the massive eruptions of 500 million years ago, which resurfaced the whole planet, and ask what the volcanic gases from that cataclysm did. Their models suggest that the pulse of greenhouse heat actually expanded the crust, warping it to create the wrinkle ridges covering most of the surface. As the atmosphere cooled, the crust shrank to open cracks elsewhere on Venus. Smaller eruptions have similar but lesser effects.

Venus's Crustal Turnover

One might ask next how such a big crustal turnover happened. As with other planetary puzzles there are two possible causes, one from below and one from above. On Earth, the largest continental plates tend to trap heat beneath them, eventually causing them to break up. If Venus is considered a "one-plate planet," then such a breakup from below could start with one large eruption. Reinforced by atmospheric feedback, the disturbance would spread to affect everything at once.

The alternative is that fresh greenhouse gases enter the atmosphere from above, from large comets. Simulations suggest that comet collisions would deposit water vapor without blasting the atmosphere away. Under that water vapor blanket, the rocks of Venus would roast and soften to the melting point, just as when heat builds up below. Either way, this strange world would give birth to its new face.

Is Venus Active?

Recent results from the Venus Express mission suggest an alternative to this picture. Researchers took a close look at several high mountains that, at least in their shapes, resemble large shields like the Hawaiian chain of volcanoes. Hawaii is considered the type example of a hotspot, and so it's natural to look for the same on Venus.

The way these high volcanoes interact with infrared light—their emissivity, to be exact—suggests that at their tops are fresh rocks. (See this explanation from the Venus Express site.) Given the chemistry that we expect between fresh basalt lava and the active atmosphere of Venus, and assuming a model of Venus in which volcanic resurfacing is constant rather than episodic, these rocks could be younger than a quarter-million years and possibly brand-new. That is a lot of assumptions, but when data is scant that's all you can do. The evidence for young lava is good, but is there enough of it to make it the rule or the exception? The answer is not yet nailed down.

PS: Early in the history of the solar system, Venus may well have contained oceans and a CO2-nitrogen atmosphere like just that of the early Earth. Thus life may have arisen there once. It's even conceivable that cosmic impacts carried pieces of Venus to our planet. After all, earthly meteorites include pieces of Mars.

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