A 'young' Martian lake would be at least half-billion years old, but Martian deltas might not seem as remote as the present day desert.
Nathalie Cabrol, who has spent many long hours poring over images of Mars taken from orbiting spacecraft, believes those images contain convincing evidence that lakes were present on Mars in the recent past.
"Recent" requires some definition when you're talking to geologists. In this case, it means some time in the past half a billion years or so, give or take a couple hundred million. There are no lakes present on the Martian surface today.
Cabrol, a member of the NASA Astrobiology Institute (NAI) and a principal investigator with the SETI Institute, works at NASA's Ames Research Center in Mountain View, CA. She has studied hundreds of images taken by the Viking spacecraft that orbited Mars in the mid-1970s. Cabrol has found nearly 200 locations on Mars where it appears that at some time in the past flowing water poured over the rims of Marian craters and pooled there, creating lakes. Most of these lakes occur within 20 degrees of the Martian equator.
Of these, Cabrol has identified some 20 lakes that she believes were formed within the past 400 million years, perhaps even as recently as 300 million years ago - a blink of the eye in geologic time. Some of them appear to have such pristine sedimentary deposits that they might be much younger. What makes the finding surprising - and controversial - is that until recently, many scientists believed that it had been 2 to 3 billion years since liquid water existed in any significant quantity on the surface of Mars.
On Earth, scientists determine the ages of rocks by measuring the quantities of different isotopes of the elements that make up the rock. When dealing with Mars, where there are no geologists available to perform such isotopic analyses, scientists use a very different technique to determine the likely age of events. They count craters.
Early in Mars's history, the planet was heavily bombarded by debris from space. About 3.5 billion years ago, there was a sharp drop-off in the rate of these impacts. Around 1.8 billion years ago, another sharp drop-off occurred.
So by counting the number and sizes of the craters within a given area on Mars, scientists can determine the approximate age of the features that appear there. The more heavily cratered the surface is, the older it is likely to be. The lakebeds that Cabrol believes are the youngest, then, are those whose sediments are the most pristine - those that haven't been pockmarked by subsequent meteor bombardment.
Cabrol uses lake sediments to tease out not only the ages of lakes, but other information about their histories as well. For example, by studying the shape formed by these deposits, she can determine whether a lake was created by a single massive outburst of water, by a series of successive flows, or by a long-term sustained flow. Many of the lakebeds Cabrol has detected appear to have complex delta structures that, she says, imply long-term flow.
"What I was seeing in the Martian lakes with deltas," Cabrol says, "was not consistent with a planet that didn't have some kind of water supply - and when I say some kind of water supply, I am not talking about two drops of water here. To build a delta you need between 10 thousand and 100 thousand years."
These deltaic structures also provide clues about whether the deposits formed at the surface of a lake, under water or, as Cabrol believes occurred in Gusev Crater, under ice. "These things have been studied for a long time on Earth, for example by oil companies. So we have a lot of information about delta morphologies and what they mean in terms of hydrological processes," says Cabrol.
If life once existed on Mars, argues Cabrol, ancient lakebeds would be a likely place to look for its signature. Lakebeds that show evidence of continuous or episodic flow over many thousands of years would be particularly good candidates.
One unexpected aspect of Cabrol's research is a connection recently discovered to a study by Bob Haberle into Mars's climatic history. In many of the same regions of Mars where Cabrol has found the greatest concentration of young lakebeds, Haberle, another member of the NAI who works at NASA Ames, has determined that liquid water could, in theory, exist under present-day conditions, without freezing or evaporating instantly.
Although no surface water is present in these regions today, Haberle is investigating the possibility that Mars's climate in the recent past could have delivered snow to these areas. That snow could then have melted, recharging the regional aquifer and contributing to the formation of the lakes whose remains Cabrol sees today.
The connection between Mars's past climate and the relatively young lakes detected by Cabrol is highly speculative. Haberle is currently working to understand better how changes in Mars's orbit in the past might have affected its climate. If he can show that Mars could have delivered snow to Cabrol's lakebeds in the recent past, it will lend credence to the idea that those lakebeds are, indeed, as youthful as Cabrol believes them to be.