Astrobiology Magazine (AM): How did you discover the frozen sea on Mars?

John Murray (JM): What attracted me to the area were these broken plates on the surface. I'd been looking at Valles Marineris previously, and wondering how the collapsed features there had formed.

It took two hours to go through the entire image, because the images are huge, perhaps one or two hundred feet long, and you're looking at them on computer screens. They're 65 kilometers across at minimum, and they have a resolution down to 10 meters.

Some features looked as if they'd drifted, so I was looking for rotation, but there was none at all. Then I saw a huge block of rock several miles wide that had rotated.

Initially I thought it was the result of a vast flood that somehow had moved this rock and caused it to rotate. But then I realized the whole thing had a very flat surface, and that they were not really rocks at all but flat irregular plates colored slightly darker than the intervening areas.

I started looking at the image in more detail, and it suddenly struck me that it looked exactly like pack ice. Then I got together with my colleague Peter Muller and my student David Page, and we found seven or eight features that are common to pack ice in the Arctic and Antarctic.

At that stage, I hadn't realized that areas to the east of this region, where similar kinds of features are found, had already been interpreted as lava. But I am a volcanologist, and I have spent the last 36 years working on active volcanoes, spending a month or two every year looking at lava forming.

So I know what lava can and what it can't do. There were a number of things that indicated that these features on Mars could not be lava. The consensus among my colleagues is that this is ice.

AM: But speaking of volcanism, you say the ice is covered by volcanic ash and dust, and that has prevented it from subliming - turning directly from a solid into a gas.

JM: That's true, this ice has to be preserved by a covering because ice sublimes when it's exposed. Water ice is not stable under Mars atmospheric pressure. The obvious candidate for the covering is volcanic ash and dust. But we know there must be dust there anyway, because the ice is not bright white; it's not like Antarctica.

AM: But why a covering of volcanic dust versus the sands of Mars?

JM: When this event happened, there's evidence that the Cerberus Fossae erupted both volcanic ash and dust.

You can see the sequence of events. You can see where the water has come out of the Cerberus Fossae, these deep fissures on Mars, and flowed down for hundreds of kilometers.

The water filled a vast area the size of Lake Michigan, and the surface then froze. You had the volcanic ash falling on the ice when it first formed over this sea, and the ash protected the ice.

Then there was a drop in the water level - it's clear that the water level has dropped by about 20 meters since it first formed - and the surface ice broke up to form these icebergs or ice floes. The blocks drifted around and rotated, and then the whole thing froze solid again.

AM: How do you know the water level dropped? Do you see an erosion pattern in the surrounding soil caused by the receding water level?

JM: There are submerged craters where the ice has settled down on top of them as the water level dropped. You can see where the ice floe has become stuck on both the outside and the inside of the crater. Only the lowering of the water level would have caused ice to become draped along the outside of a crater and also down into it.

AM: What caused the water to flow out onto the surface of Mars? The volcanism?

JM: It might have been volcanism. The other possibility is this idea of Clifford and Parker, which has been around for years, of an ice-rich surface of Mars. It was a wacky idea when it first came out, but it does explain an awful lot of things.

The surface must be ice because of the cold temperatures, but as you go down into Mars, geothermal heating melts this ice into liquid. They've calculated that at a depth of several kilometers, you're supposed to get these pockets of liquid water.

So if you tectonically crack the ice-rich surface down to this water-rich layer, then, since Mars has virtually no atmospheric pressure, the water is going to shoot out, like opening a bottle of champagne.

That water will evaporate immediately because of the low pressure, but the volume of water would be so huge, the rate of this water discharge would be so high, that there's not enough time for most of it to boil or freeze. It just flows and erodes.

After it eroded the channel it would have spread out to form the vast lake, and then would have continued to evaporate until the top level began to freeze.

AM: Could the cracking of the crust down to that level have been caused by volcanism rather than tectonics?

JM: It could have been volcanism. There is the big Elysium volcano to the north, but that is really quite old. It is possible the cracking is some late manifestation associated with that, though it is actually quite far from the foot of Elysium, and there are very few signs of volcanic vents along most of the cracks.

AM: So these deep caverns on Mars are places where water has burst out of underground aquifers. Do you think there are more such places that haven't yet cracked open?

JM: There are a lot of places where you don't see signs of tectonic cracking. So presumably, beneath those, we'll find untapped areas of water.

AM: You found this frozen sea at the equator, which is significant because so far there's been no evidence for water at the equator.

JM: That's right. There's been some recent models looking at the tilt of Mars's axis, which at the moment is very similar to the Earth's. But that appears to be a coincidence, because throughout most of Mars's history, the axis was at 45 degrees, a far greater tilt.

As soon as you have a tilt of 45 degrees, you get a whole different climate, with more water ice and frost deposition at the equator. It's possible when this flood event occurred, it was during this period of frost deposition.

Gerhard Neukum dated this particular feature by counting the number of impact craters, and discovered it was only about 5 million years old. 5 million years sounds ancient to you and me, but in geological terms it's yesterday.

Now, as soon as you start to think about that, and that the kinds of things you require for life to form are water and carbon and an energy source and unlimited amounts of time - you have all those things here.

If you've got supplies of liquid water underground, then life may have been able to develop and sustain itself and reproduce. Underground life also would be protected from ultraviolet radiation and oxidation, things that will break up molecules at the surface.

Mars was warm and wet during its first billion years, and has probably had these vast underground water reserves ever since, from 3 or 4 billion years ago to at least 5 million years ago.

A million is an awful long time. There has not yet been a million days since the birth of Christ, for example. And we're not just talking about a million years, but thousands of millions of years of this water being there, under the surface.

That is plenty of time for life to develop, if it could. And so if life has developed, then I think it's highly likely it will be in this water, in this frozen sea. So we won't be looking for fossils, we'll be looking for the actual organisms themselves, frozen within the ice.

The paper Evidence from the Mars Express High Resolution Stereo Camera for a frozen sea close to Mars' equator by John B. Murray, Jan-Peter Muller, Gerhard Neukum, Stephanie C. Werner, Stephan van Gasselt, Ernst Hauber, Wojciech J. Markiewicz, James W. Head III, Bernard H. Foing, David Page, Karl L. Mitchell, Ganna Portyankina & the HRSC Co-Investigator Team is to be published in Nature on 17 March 2005

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