David Grinspoon

QUESTION: What are some of the things you do in comparative planetology?

GRINSPOON: Let's take the example of three planets: Mars, Venus and Earth. They are a trio of sibling worlds. Venus and Mars are our next of kin; they are the two most Earth-like planets that we know about. They're the only two other very Earth-like planets in our solar system, meaning they orbit close to the sun, they have rocky surfaces and thin atmospheres.

Due to these similarities, the study of Venus and Mars allows us to learn about the Earth's uniqueness in a way that no other kind of comparative study can. It will be a long time, if ever, before we get to study Earth-like planets orbiting around other stars, so really the study of Venus and Mars is the best opportunity that we have, and can imagine having anytime in the future, to understand the evolution of Earth-like planets. Through comparative planetology, we see the Earth in a wider context. We can understand which aspects of the evolution of our own planet seem inevitable through this study and which are due to chance. We can look at the surfaces, the atmospheres, the evolving climates of these worlds, and get a better handle on the Earth's uniqueness.

QUESTION: What kinds of features do you see on the surfaces of the planets?

GRINSPOON: Volcanoes are one feature. In observing the planets, we can see Olympus Mons, the largest volcano in the solar system, towering above the clouds of Mars. It is nearly twice as tall as Mt. Everest. We can see Matt Mons, a 25,000 foot high volcano on Venus. These are very similar volcanoes, and they're both similar to Puuo Oo in Hawaii. Puu Oo, like the others, is formed by running basaltic lava which spreads out in a shield shape, hence the name "shield volcano." The similarities in all these three volcanoes on these three different planets reflect the fact that these planets have common origins and have evolved out of common materials in nearby conditions in our solar system.

QUESTION: How does their volcanic activity compare?

GRINSPOON: We have now learned that Venus is the only other geologically active Earth-like planet in our solar system. All the other planets basically died out a long time ago, geologically speaking. So Venus presents us with an opportunity to study the ongoing evolution of a geologically active Earth-like planet. That's one of the most exciting things we've learned from the Magellan mission.

Now, Venus is not as active as Earth, but it's more active than anywhere else we know that's Earth-like. The reason you see so many volcanoes on Venus is partly due to the fact that there's virtually no erosion there. So on Venus you're seeing features, some of which are hundreds of millions of years old on the surface. On Earth we do not see any surface features nearly that old—you only see much more recent features.

QUESTION: How else do you compare the three planets?

GRINSPOON: The three planets also all formed impact craters, when asteroids or comets hit the surface at high speed from space. The differences between these craters reflect differences in the target planets. Looking at a crater on Mars, and you can see the margins of the crater look like mud slashes, and that's what they are. The object apparently landed in a surface that was either wet or had permafrost, frozen water underground, and then when the ejecta came out of the crater, it splashed out as mud.

Looking at images of craters on Venus, what you see coming out is a large flow of melted surface material running as lava across the surface, reflecting the high temperature of the Venusian environment which makes it easy for lava to melt with the energy of an impact.

One meteor craters still visible on earth is in North Arizona. This is an unusual crater for Earth because it's still there. Earth, with its highly erosive surface environment, wipes out its craters very fast. This particular crater, because it's in a desert environment, has survived relatively uneroded for the last 50,000 years or so.

Images taken from orbit around the three planets all show patterns on the surfaces caused by flowing liquid. River systems on Earth are clearly carved by liquid water, and the great similarity in form of channels of the well-known one on Mars suggests very strongly that it was also carved by liquid water in the ancient past on Mars, when water was more abundant and Mars was presumably a more Earth-like and wetter planet.

We've recently learned that Venus also has features that seem like river valleys, but in the case of Venus they cannot have been carved by liquid water because Venus is so hot and so dry today that we can't image that it was wet enough in the recent past for water to have carved these channels. So we think it was some sort of exotic lava that caused this river-like forms to flow on Venus.

The fact that we see these similar forms on all these planets with their somewhat different conditions tell us something about the way that nature chooses the same forms, the same shapes in different kinds of conditions, even working with different materials, to manifest flow on the surface of a planet.

QUESTION: Why would assume that it was water on Mars and not assume it was water on Venus?

GRINSPOON: The fact that we find river-like channels on Venus that are not carved by water is sobering to those of us who believe that the Martian channels are indeed carved by water, because one can ask the question, "Well, maybe the Martian ones were carved by lava." That's a very good question, but with Mars we have very detailed images, and the more we look in detail at the very specific forms that the channels take, the more it calls out to us and says, "This was really carved by water." Not only that, there's lot of other evidence on Mars for water in the past. The more we look at the kinds of soils and the nature of the atmosphere and the polar caps, it all adds up to tell us that some liquid, which we very much believe was water, did flow in abundance on Mars in the past.

QUESTION: What are the chances for life on Venus?

GRINSPOON: Early telescopic images of Venus showed that Venus and Earth are virtually identical in size, and that Venus is surrounded by clouds. The very first telescopic observers correctly deduced these facts. Since they assumed that the clouds were made out of water, like clouds on Earth are, this led to a very common view of Venus as being very Earth-like beneath the clouds. For hundreds of years it was imagined by scientists as well as science fiction writers that Venus was sort of a lush tropical Earth-like place with all kinds of life and basically a swamp planet. This view held sway until very recently when, with modern scientific telescopic observations and particularly with space craft investigations, we learned that those clouds in fact are not made out of water but out of concentrated sulfuric acid - battery acid — and that conditions on the surface of Venus are not at all Earth-like.

It's very, very hot, about 900 degrees, on the surface of Venus, and very high pressure, about 90 times the surface pressure on Earth. The atmosphere we found on Venus was not at all that lush, tropical fantasy Venus but it is in fact a hostile environment and would not be inhabitable by at least our kind of life.

One of the most dramatic things that we learned about Venus with Magellan is that something really extraordinary happened on Venus about 500 million years ago. It seems that Venus sort of turned itself inside out, that suddenly there were massive volcanic floods all over the planet which basically wiped out the previous surface of the planet. We know this because there are not many impact craters on Venus, but there's lots of evidence of great voluminous floods that all seem to have happened roughly around the same time.

For some reason Venus changed its character in a way that wiped clear the surface and probably also caused great havoc and great change to the climate of the planet, as these volcanic floods filled the air with greenhouse gases. This has perhaps disturbing implications because it tells us that Earth-like planets can undergo dramatic changes in both their surfaces and their climates in a rather recent geologic epoch, and this information could even mean that the Earth has future threatening changes of this magnitude in store for us.