NASA planetary scientist David McKay is a member of the research team that announced in 1996 that a Martian meteorite known as ALH84001 appeared to contain traces of biological activity. In an interview, McKay discusses the continuing search for evidence of life on Mars.
On whether there is life today on Mars:
"I’m sort of operating under the theory that if life ever existed on Mars, it’s still there today, because life is so hardy and tenacious that even if the surface dries up, that life is going to go underground [and] follow the water.
"We now know that on Earth there’s a huge amount of living material underneath the surface of the Earth, living at depths of a mile, two miles — living in cracks in the rocks. Little bacteria where there’s a little bit of ground water.
"Well, I’m almost certain ... most people are fairly confident that there’s ground water on Mars at a depth of a kilometer, two kilometers, maybe five. But why would that ground water in the cracks of the rocks on Mars not harbor life? I think it would. And that’s just my speculation, but a lot of people would agree with that."
On the controversy over ALH84001, also known as the Allan Hills meteorite:
"No one is really criticizing our data that we got and published. They’re criticizing our hypothesis that these features we saw could be explained by life."
On researcher Gilbert Levin’s call for a fresh review of data from life-detection experiments on the 1976 Viking missions:
"He makes a lot of sense. He has some good ideas, and I think it is worth going back and looking carefully at all that Viking data which was dismissed as absolutely negative in terms of revealing life. Well, somebody needs to go back and look carefully in the light of what we know today, and see if perhaps some of it was misinterpreted."
Why scientists see better odds of finding life beyond Earth:
"The envelope is being stretched to these really small sizes, to the extreme environments of supercold. ...
"What is happening is there’s this incredible interest in expanding the envelope of life — where you find it, where it lives, where it can be kept freeze-dried and revived. And that makes us more and more optimistic that we’ll find life on Mars and Europa. And furthermore, we may find [that] life — if it’s not alive, if it’s not metabolizing right now — can somehow be revived and made to grow. Those are exciting ideas."
On whether the question about life on Mars would be answered in his lifetime:
"Yes, I think so. I’m still hopeful we’ll get it from the meteorites. But if we don’t, then we will have samples back by about 2007 — another nine years. And I hope I’m still around. But I think that’ll be really definitive.
"One of the things we have to do in the next eight or nine years ... we have to figure out how we’re really going to tell, without a shadow of a doubt, whether there’s life in those samples. We’re going to learn from the controversy over Allan Hills — and we already have learned — that we have to take some of these biomarkers like the magnetite and the organic compounds. We have to really understand which ones are formed by biology and which ones are formed by totally inorganic processes, and we have to be able to tell the difference.
"I think part of the controversy is not whether these features are in the meteorite. It’s a matter of interpreting these features as true biologic markers.
"And so that’s something that we can actually do without the samples. We can investigate these potential biologic markers and try to build up a database of the good ones and the ambiguous ones and the ones that are no good at all. And we hope to do that in the next five or eight years....
"[Once the samples come back,] we don’t want to spend five years arguing about whether this feature was caused by life or not. That’s just not a good use of those Mars samples. We want to have a series of yes-or-no questions that we can ask about those samples that will really give us the answer.
"One of the lessons we’ve learned from this controversy over the Martian meteorite is that we just don’t have those answers yet. We’ve really got to work on those answers.
Postscript: Since this report was first published, NASA has had to push back its schedule for bringing Martian samples back to Earth. A sample return mission cannot be done by 2007, as McKay had hoped.