The planet Venus is like Earth in many ways. It has a similar size and mass, it is closer to us than any other planet, and it probably formed from the same sort of materials that formed Earth. For years scientists and science fiction writers dreamed of the exotic jungles and life forms that must inhabit Earth's twin sister.

David Grinspoon, a research scientist at the Southwest Research Institute in Boulder, Colorado, writes in his book, "Venus Revealed," that, through the Mariner 2 and other Venus missions, "we found our 'sister planet' to be chemically alien, as well as hot and dry to quite unearthly extremes.

With these revelations, the twin-sister imagery quickly disappeared, and the notion that 'Venus is hell' took hold."

Only 20 percent of the light that hits Venus makes it through the cloud cover, the other 80 percent of the Sun's light is reflected back into space. This doesn't make Venus a cold world, however, because the thick carbon dioxide atmosphere traps the planet's heat.

This greenhouse effect on Venus is often cited as a nightmare example of what could happen to Earth if we don't get our pollution under control.

In part 1 of this interview with Astrobiology Magazine editor Henry Bortman, Grinspoon explained how Venus evolved from a wet planet similar to Earth to the scorching hot, dried-out furnace of today. In part 2, Grinspoon discusses the possibility of life on Venus.

Astrobiology Magazine (AM): You've suggested, in contrast to the conventional view, that Venus may have held onto its water for perhaps as long as 2 billion years. What are the implications for habitability?

David Grinspoon (DG): For habitability, there are implications for Venus and there are implications for terrestrial planets in general. Venus almost certainly had liquid water when it was young. So the conditions for the origin of life, as conventionally defined, were satisfied there as much as on Earth and Mars.

We've been hearing a lot about how Mars may never have been warm, so perhaps Venus was more habitable in that sense than Mars.

It may have been Venus and Earth that were the two young habitable planets, perhaps even exchanging material through impact ejecta, like we hear more commonly described as a relationship between young Mars and young Earth.

It may in fact have been Venus and Earth that were enjoying this exchange.

Another intriguing thing about early Venus is that it may have had an oxygen-rich atmosphere. You had this massive loss of hydrogen to space from water, and what's left is all that oxygen.

We've heard a lot about the rise of oxygen being important in the development of complex life on Earth. Perhaps Venus was a warm, wet planet with an oxygenated atmosphere much earlier than Earth.

The problem in thinking about the habitability of Venus is that, in the conventional view, the water didn't last long. But if the water lasted for billions of years, that becomes much more interesting for the possibility of biological development.

Earth is going to lose its oceans in the future, just as Venus did in the past. How long planets retain their oceans is a function of distance from the sun, all other things being equal.

But clouds may allow planets to hold onto their oceans at closer distances to the sun than has been conventionally thought.

For habitable planets in general, when the planets are on the inner edge of what we think of as the habitable zone, clouds perhaps make it harder to lose oceans.

If planets on that inner edge retain their oceans longer, then there is more real estate of terrestrial planets in the galaxy that keep their oceans for biologically significant time scales.

AM: If there had been life on Venus, say for 2 or 3 billion years, would this resurfacing event have buried all the evidence?

DG: On a planet like Venus that's been recently geologically active compared to a planet like Mars, it's much harder to search for ancient life, just because an active planet buries its past. The very things that make Venus so geologically interesting also make it a real challenge to uncover its ancient history.

I believe the signs are probably there, they're just going to be harder to tease out. The way to do it is with future missions that are targeted at understanding this ancient history. Although 80 percent of Venus seems to have been resurfaced sometime in the last billion years, the other 15 percent or so was not.

There are these highland areas, called tesserae, which are clearly the oldest areas on Venus.

They're very rugged terrain, and have what looks like a long history of intense tectonic deformation. Those are the places I think you want to go to look for signs of the more ancient history on Venus.

I'm a strong advocate of new missions to Venus. We really have to go to the surface and dig in the rocks and drill to find out what is the mineralogy, and what is the history of the older areas in particular. Then we also can do new measurements of the atmosphere.

If we get very accurate measurements of the isotopes in the atmosphere, then I think we can start to piece together the evolutionary history of the atmosphere in a more complete way than has been done.

It's not going to be easy, because Venus is a hard place. It's a challenging place to explore on the surface, given the extreme conditions, and also because recent geological activity has destroyed the obvious signs of that older history.

But it's there in the rocks, just like on Earth. Earth has a relatively young surface. If you were studying the Earth only from space with orbital imagery, it would be very hard to know its ancient history.

AM: Our results from looking at ancient rocks on Earth are pretty ambiguous, though.

DG: Well, we do better than we would if we didn't have that ability. I'm not going to claim it would be easy. But I'd like to have the ability to do in situ experiments on the rocks of Venus, and eventually sample return, especially from the older areas, so we can study those rocks in Earth's laboratories.

It would be a challenging mission, but I've been on NASA panels that have studied these options, and there are designs for sample return missions from Venus.

By the way, one further implication for habitability bears mentioning. If Venus once had life, and there's no good reason to think that it couldn't have, then we can ask what happened to this life when the oceans disappeared.

One possibility is that it simply died out once its habitat vanished. But life is tenacious and highly adaptable. So I think that it is possible that Venusian life migrated to an atmospheric niche when the surface water dried up.

The clouds, after all, do contain water, mixed in with concentrated sulfuric acid. This is highly speculative, but I think it is possible that life could exist, even today, in the clouds of Venus.

We now know that life exists in clouds on Earth, and also that some terrestrial organisms can thrive in extremely acidic environments. Furthermore, the clouds of Venus are a much more stable and continuous niche than the comparatively ephemeral and wispy clouds of Earth.

So, from one point of view, the clouds of Earth are a more extreme environment for life than the clouds of Venus.

It seems like a long shot, but given our extreme ignorance about life elsewhere in the universe, let us not rule out an energetic, stable environment like the clouds of Venus until we've explored them much more fully.

AM: Are there any missions to Venus currently planned? If so, will they help answer the question about past or present life on Venus?

DG: The European Space Agency has a mission called Venus Express, which is going to be in an orbiter. It will not address the surface issues, but it will do some really interesting orbital science.

To get at these evolutionary questions that we are discussing here, though, you can't do it from an orbiter. You have to probe in the atmosphere for the isotopes, and you have to ultimately go to the surface, as forbidding as that is, to do these kinds of experiments.

The Decadal Survey of the NRC Commission called for a new mission to Venus to do surface and atmospheric in situ science. They called it VISE - Venus In Situ Explorer. It's one of NASA's top ranked goals for the next decade.

To send anything to the surface of Venus that's going to survive long enough to do measurements costs a lot, because you have to put it in this intense pressure vessel and you have to try to control the temperature.

Just an hour's worth of science on the surface of Venus costs more than a mission to do a month's worth on Mars.

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