American Scientist

To the Editors:

I read Paul Byrne’s article “Unveiling Earth’s Wayward Twin” (January–February) with great interest. I learned a lot I had not known about the planet we once believed to be our near-twin and gained a better understanding of how studying Venus could help us learn more about planets outside our own Solar System.

Two facts that struck me are that Venus’s atmosphere is so much denser than Earth’s and that Venus has far fewer impact craters relative to other planets and moons. The author suggests that these facts show that Venus’s surface must have formed much more recently than that of, say, Mars. That’s certainly a possibility.

But might not Venus’s very dense atmosphere affect both the number and size distribution of impact craters? A dense atmosphere will burn up more incoming objects than a thin atmosphere, such as that of Mars, will. Small objects entering Venus’s thick atmosphere will be most likely to burn up completely, but even the surface effects of larger objects will be reduced. Perhaps Venus’s surface is old but little battered.

In any case, the author made a convincing case that Venus certainly deserves more study than it’s received so far given how different it is from Earth despite a few great similarities.

John Cushing
Bend, OR

Dr. Byrne responds:

It’s true that Venus’s atmosphere does a very effective job screening incoming asteroids and comets from hitting the surface. In fact, there are relatively few craters less than 25 kilometers in diameter, and none less than 3 kilometers, purely because of that thick atmosphere’s ability to shield the surface from impactors.

However, Venus doesn’t boast any of the really large impact basins that are so common on Mercury, the Moon, or Mars, say—none of the 500-kilometer-wide, 1,000-kilometer-wide, or even bigger basins such as Caloris on Mercury, Orientale on the Moon, and Hellas on Mars. Indeed, the largest impact feature on Venus is Mead crater, which is about 280 kilometers across; there are more than 40 basins larger than this on Mercury. Thus, although the atmosphere plays some role in the impact record (or lack thereof) on Venus, it’s far from the only factor responsible for the second planet’s relatively youthful average surface age.