A FROZEN sea just north of the sun-bathed equator, glaciers in the shadows of the planet鈥檚 tallest volcano, and lava fields just a few million years old 鈥� these dramatic images of a geologically active Mars are intriguing scientists who believed the planet was long dead.
Features just metres across are visible in colour images from the European Space Agency鈥檚 Mars Express spacecraft, which has been orbiting Mars since December 2003. 鈥淚 have been surprised by what I have seen,鈥� Gerhard Neukum, head of the camera team, told 快猫短视频 during the 1st Mars Express Science Conference in Noordjwik, the Netherlands, this week.
One image shows fragmented plate-like formations strikingly similar to the broken pack ice that forms at the edges of the Earth鈥檚 polar ice sheets. They lie over an area 800 by 900 kilometres, suggesting that they once formed an expanse of water the size of the North Sea, says John Murray of the Open University in Milton Keynes, UK, a member of the team that analysed the picture. The team estimates that the sea was originally about 45 metres deep.
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Murray thinks the water probably emerged from fissures in Mars鈥檚 crust hundreds of kilometres to the north, named Cerberus Fossae, and then flooded into the low-lying region. Michael Carr, an expert on Martian water at the US Geological Survey in Menlo Park, California, who was not part of the team, finds this theory plausible: 鈥淵ou can trace the valleys carved by water down to this area.鈥�
Plate-like features can form in frozen lava flows, but some on Mars are tens of kilometres across, much bigger than similar rocky structures on Earth. Also, the plates have crumpled where they鈥檝e collided with obstacles, and there are clear wakes behind such collisions. 鈥淪ome features are characteristic of ice on Earth and not at all characteristic of lava,鈥� Murray says.
No ice is visible directly because the surface of the plate-like areas is covered with dust. While it is possible there is no ice underneath, Murray thinks that is unlikely. The dust-covered areas are 鈥渧ery, very flat鈥�, he says, suggesting there is ice below. The dust layer would have played an essential role in preserving the frozen sea, as otherwise the ice would sublimate in the planet鈥檚 thin atmosphere.
One big surprise is the sea鈥檚 age. The scarcity of massive impact craters suggests it formed only 5 million years ago, long after the end of Mars鈥檚 warm, wet youth. Much of the water from that time escaped into space. But some may have seeped into the ground, from where it could have been released periodically by ruptures in the Red Planet鈥檚 crust, caused perhaps by convection in the planet鈥檚 molten interior, leading to flash floods.
Frozen seas are not the only new feature revealed by Mars Express. Images of the 25-kilometre-high volcano Olympus Mons suggest that Mars could still be volcanically active. 鈥淎 year ago I would have thought the volcano was dead,鈥� Neukum says. Now he is not so sure. The images indicate some lava flows on the volcano鈥檚 flanks that are just 2 million years old, raising the possibility it is still active today.
鈥淪ome lava flows on the volcano鈥檚 flanks are just 2 million years old, raising the possibility that it is still active鈥�
The spacecraft also found alcoves on the western side of the volcano from which arcs of debris fan out, as they do at the foot of glaciers on Earth. There are even hints that these areas may still be incubating glaciers, James Head of Brown University in Providence, Rhode Island, told the conference.
鈥淢ars Express has fundamentally changed ideas I鈥檝e had about Mars for 30 years,鈥� Murray says. He now thinks there is an outside chance that organisms that arose in an earlier volcanic time might be hibernating in the frozen sea.
NASA chose not to land its rovers on that region, believing they would touch down on spiky, uneven lava. So is it worth a visit? 鈥淵es, yes, yes,鈥� Murray says.
