Venus and Giant Impacts

In a new paper in Earth and Planetary Science Letters John Huw Davies (Cardiff University) postulates that a giant impact in the late stages of planet formation is responsible for Venus’s hot and dry climate. The impact would have been between two planet-sized objects, not dissimilar to the impact believed to be responsible for the formation of the Earth’s Moon. That latter giant impact, now accepted as the standard model for the origin of the Moon, was off center and resulted in a disk around the proto-Earth and gave the Earth a rapid spin. Davies’ model would have a near-central impact that would leave Venus with virtually no rotation and vaporize any water it might have had at that point. The paper was reported on by USA Today via Space.com with the headline “Venus mysteries blamed on colossal collision”.

The “mysteries” that the headline refers to are (1) Venus’s slow, backwards rotation, and (2) its lack of water. Venus’s famously hot surface temperature (nearly 900 degrees Fahrenheit) is due to a crushing atmosphere of nearly pure Carbon Dioxide, famous back home for being a greenhouse gas produced by (among other things) burning organic matter such as oil, coal, and wood. Venus’s atmospheric pressure at the surface is 90 times that of Earth, and it is basically entirely a greenhouse gas. This gas prevents heat from radiating away from the surface of Venus freely into space. CO2 absorbs some of that radiant energy, trapping it in the atmosphere and making it hotter. A little greenhouse effect is nice to have and is responsible for Earth’s current comfortable temperatures. The standard model explains Venus’s high temperature and mystery number (2) above (lack of water) through a runaway greenhouse effect.

The runaway greenhouse effect occurs because like CO2, water is also a greenhouse gas. Take a planet with a lot of surface water (like the Earth) and heat it up a little, and you can drive water out of the oceans and into the atmosphere. In the atmosphere, it acts as a greenhouse gas and makes things a bit warmer which leads to more evaporation of water which makes it even hotter. Before you know it all the water is in the atmosphere and it’s hot as hell. CO2 dissolves in water (think Coke or Perrier), so without oceans there is a missing reservoir for CO2 and more of it ends up in the atmosphere. Also, as temperatures increase it drives CO2 out of rocks (think Tums or chalk (Calcium Carbonate)) making it hotter still. So why isn’t there a lot of water vapor in Venus’s atmosphere now? Solar radiation breaks water into Hydrogen and Oxygen atoms, and the light Hydrogen atoms can easily escape to space. A key piece of evidence in this story is that Deuterium (D), the isotope of Hydrogen that is twice as massive as vanilla Hydrogen (H), is far more abundant relative to H on Venus than it is on Earth. That is, D/H on Venus is much larger than D/H on Earth. Since D and H behave the same way chemically, the easiest way to explain a difference in that ratio is through thermal escape of H: the less massive H atoms have an easier time escaping Venus’s gravity than the more massive D atoms because at a given temperature, the H atoms will be moving faster than the D atoms. Thus, while both escape, the H escapes faster, and with less H, the ratio D/H gets big.

The runaway greenhouse model thus explains the D/H ratio, the lack of water, and the high temperatures on Venus quite nicely. The other mystery, Venus’s slow backwards (compared to most of the other planets) rotation, may not be a mystery after all. Certainly, giant impacts must have occurred and Venus may have suffered some whoppers. In fact, in addition to the formation of the Earth’s Moon, scientists have invoked giant late-stage impacts to explain Mercury’s high density (giant impact removes and vaporizes the outer layer of lower density material) and Uranus’s odd rotation (it is tipped over on its side relative to the other planets). These giant impacts undoubtedly have a significant, er, impact on the planet’s final rotation. However, some models of planet formation show that without giant impacts you end up with very slow rotation and the relatively rapid spin of Earth and Mars may be the mysterious ones. (And for the Earth, we know we can thank the Moon-forming impact and the Moon’s subsequent tidal evolution for our current 24 hour day.) Venus’s rotation is, almost by definition, a product of the angular momentum it received from all the impacts onto it during its formation (though a tidal interaction between the Sun and Venus’s atmosphere can change that significantly over the age of the solar system and can produce the slow retrograde rotation observed today).

The smoking gun for a giant impact onto the Earth is the Moon with its geochemical signatures of a terrestrial origin. The smoking gun for a giant impact onto Mercury is its large density, while for Uranus it is a tilted spin axis for which we have no other plausible explanation. For Venus, the case is much less clear. While there must have been major impacts late in its formation, neither its slow rotation nor its lack of water require it. In fact they are explained as a natural consequence of other mechanisms in the planet’s evolution. However, there is still much unknown about our sister planet. Its recent geological history remains somewhat controversial, with some evidence that it might have been volcanically active in the recent past. The problem is that it’s so hot there, and the atmosphere is opaque, so it is impossible to see the surface except with radar, and it is nearly impossible to put a spacecraft on the surface. Still, it will require dedicated spacecraft visits to Venus to untangle its recent history and clearly resolve the issue of how and why its climate diverged so dramatically from that of the Earth.

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