<p>Geological Sciences professor John “Jack” Mustard and his former graduate student Bethany Ehlmann, now on the faculty at the California Institute of Technology, report in a paper published this week in <em>Nature</em> that in early Mars’s history, water was mostly underground, rather than on the surface. The finding could change thinking about where to search for life on the Red Planet. Science writer Richard Lewis asked Mustard about the paper and his decades-long study of Mars.</p>

Jack Mustard: “With the next rover, ... called Curiosity, we will make huge advances to learn more about Mars history.”
Jack Mustard “With the next rover, ... called Curiosity, we will make huge advances to learn more about Mars history.”
How did you conclude that water in early Mars history was mostly below ground?

The minerals we find are dominated by a particular kind of clay that is chemically almost identical to the volcanic rock that forms much or Mars. This means that while there was enough water to form the clay — important! — the water was not flowing vigorously as we would find in a surface environment. Under vigorous flow conditions we find a different type of clay, and that clay is chemically different from basalt — and by this study we find it is not commonly found on Mars.

But we see features that resemble rivers, lakes and sedimentary deposits all over Mars. Were these not created by water?

Absolutely these features were formed by water. The key is how much time did it take to form them? If it was a over a short time that the water flowed to carve the channels, then there was not enough time to form the minerals we see. Also, if it rained a lot, we would find the types of minerals formed when you have a lot of water seeping into soils and flowing over the landscape. We just don’t see a lot of those minerals.

If you are correct that water was mostly underground, what does that mean for the search for evidence of past life? Where should we be looking?

The places on Mars that were the most clement for life over the longest time were underground. So we should be looking for places where these underground environments have been exposed by Mars, such as in impact craters or deep canyons. We should be looking for places that have preserved the groupings of minerals that are typical of these sites.

In percentage terms, what do you think are the odds that there was past life on Mars?

Well, that is the $100,000 question, or should I say the billion-dollar question. I am definitely on the fence on this question, but definitely think we should be focusing on these ancient terrains that have clear evidence for past subsurface environments.

You’ve been studying Mars for some time. How well do we know the Red Planet’s history?

We know quite a bit about Mars’ history but there is so much we need to know. With the next rover, the Mars Science Laboratory called Curiosity, we will make huge advances to learn more about Mars history.

Should we send humans to explore Mars? If so, should that be the next human-led voyage into space?

It would be great to send humans! There is so much we can learn by sending humans, but the next voyage to space? I think that is premature given where the United States is now with space technology.