Bernard Foing, ESA Chief Scientist, provides on overview of the most notable discoveries made during Europe's first trip to the Red Planet.

In part one of this overview, Foing wades through the evidence for liquid water on Mars.

Mars is the little brother of the Earth, with different processes working on different scales.

Like Earth, Mars has tectonics, volcanics, erosion, and an atmosphere. We can study the cycle of water on Mars - water can be present as ice, we know it's in the atmosphere, and it has existed there in liquid form.

There was evidently liquid water on the surface of Mars in the first billion years of its history.

Although Earth and Mars formed from the same materials, the two planets evolved differently. Water in the atmosphere of Mars may have dissipated very early, and so Mars may have turned cold and dry after the first billion years.

One indication of this early loss of water comes from how the martian atmosphere interacts with the solar wind. An experiment on Mars Express shows that that this interaction is causing Mars to lose 100 tons of its atmosphere per day.

The Earth also loses part of its atmosphere every day to space, but Earth has a magnetic shield. The Earth's magnetosphere prevents particulates from the solar wind from impacting the Earth, and so for this reason, we don't eject as much of our atmosphere.

If there had been an extended ocean on Mars 3.5 billion years ago, it might have allowed the planet to maintain a dense atmosphere. The greenhouse on Mars would have kept water stable at the surface.

A close-up shot of carbonate pancakes in the meteorite Allan Hills 84001

But after Mars's own early magnetic shield disappeared, the atmosphere started to be lost due to interaction with the solar wind, and this dramatically accelerated the loss of water.

Today, the atmospheric pressure on Mars is very low, a hundred times lower than on Earth. At this low pressure, water on the surface just evaporates, and ice sublimates - goes directly from a solid to a gas.

But some network river features that we see here and there on the surface of Mars suggest that there have been some sporadic episodes of surface liquid water flows.

One way to determine if there was an ocean after the first billion years is to look for minerals like carbonates. The infrared OMEGA instrument on Mars Express can look at the signature of minerals.

The OMEGA team looked extensively for carbonates, but they could not find any. So perhaps there was no ocean in the last 3 billion years. Or, if there was one, the rocks that have been left have been covered by other soil layers. We still have to figure this out.

This should not be interpreted that there is no water on Mars at all today. There is water ice in the polar caps, and we think we see the accumulation of ice in the tropics and equator.

For instance, there is water ice deposited in high mountains at the equator - it's a bit like the snow cap of Mt. Kilimanjaro.

The transfer of ice from the poles to the equator seems to happen in a cycle of 5 million years.

We have found some features which suggest that 5 million years ago, there was an ice reservoir near Olympus Mons, the largest volcano on Mars, and also near Elysium, another big volcano.

This image, taken by the high resolution stereo camera (HRSC) onboard Mars Express shows the region in Elysium Planitia where underground ice packs and a frozen sea may exist. Image: ESA Desktop available - 1024x768

We believe we have evidence of recent volcanic activity on Mars. So these two discoveries - recent volcanic activity and recent water ice - means there were some eras where there could have been the melting of water, not only on the surface, but also in the subsurface.

This subsurface water melt could be resurfacing, and then collecting in some lower elevation.

It seems that we have found a place where, 5 million years ago, some of this melted water has deposited like a very flat ice mirror. The 3-D stereo images from Mars Express show that this frozen sea is extremely flat - a slope of only 5 millidegrees. It's flatter than a table.

It has been suggested that this frozen sea is nothing more than a solidified lava flow, but such lava flows tend to be much more bumpy.

We think the lake was deposited and then covered by a layer of ash, which, soon after the water froze, prevented the ice from sublimating away to the tenuous atmosphere.

This frozen sea is about the size of the North Sea, with the depth of 50 meters. There was a kind of liquid sea movement where the ice layer became fragmented and produced ice rafts.

This area has a huge potential for astrobiology and future exploration. But we still need to know how much of the ice has been preserved, and how much of it has been able to sublimate anyway through the ash layer.

We also need to know the thickness of the ash layer that covers it.

The MARSIS instrument on Mars Express will have the ability to investigate beneath the surface, down to a few kilometers in depth.

Since liquid water has a very strong radar signal, we might be able to get a vertical profile of how much ice and water there is in the overall subsurface of Mars.

Because MARSIS was designed to penetrate very deeply below the surface, we won't resolve very thin layers. So if the ice layer is only a few meters or a few tens of meters thick, MARSIS may not see it.

But NASA's Mars Reconnaissance Orbiter, to be launched this year, will have radar that can probe the near surface. So that will be a way to test our frozen sea.

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