THE Mars rover Opportunity discovered last week that surface water existed on early Mars not just for years but for aeons, making it more likely that life had enough time to get a foothold on the planet.
Within a few weeks of landing on Mars in January, Opportunity found evidence of a watery past on the surface of Mars, first in a small crater called Eagle near its landing site Meridiani Planum, and then in the much larger Endurance crater. In both places, the layered bedrock provided multiple lines of evidence, including voids left by dissolved salt crystals, and haematite spheres, showing that liquid water was once there. The latest discovery, reported at a NASA press conference on 16 July, makes it clear that surface water flowed on Mars for geological timescales.
After motoring down several metres into the stadium-sized Endurance crater, the rover has found what science team member Jack Farmer, of Arizona State University at Tempe has dubbed razorback 鈥� a ridge of thin, jagged vertical plates sticking up at the edge of a flat expanse of bedrock. Although these ridges are only a few centimetres high, and less than a centimetre wide, they are generating a lot of excitement. Farmer鈥檚 team suspects that the ridges formed when earlier layers of rock cracked, allowing mineral-laden water to percolate through. The mineral deposits left behind veins, or 鈥渇racture fill鈥�. These rocky veins were much harder than the surrounding material which eventually eroded away.
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It is these rocky veins that dramatically extend the timescale for water on Mars. To create them, liquid water must have flowed for a long time after a shallow lake or sea deposited the sediments that first produced the bedrock. The marine sediment would have needed time to be compacted into stone hard enough to crack and make fissures in which the veins formed. Farmer says that the cracks themselves may have been caused by the impact that created Endurance. And the water that deposited minerals in these cracks must have existed long after the impact.
Studying the razorback ridge at close range should help in figuring out the processes and timescales involved, Farmer says. 鈥淚f Mars cooperates, we may have some of those answers in the next few days.鈥�
On the other side of Mars, Opportunity鈥檚 twin, Spirit, has stumbled upon something new at its landing site in the Gusev crater. 鈥淪pirit has an outcrop under its wheels,鈥� says team member Matt Golombek of NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California. 鈥淎nd an outcrop is the currency for geologists.鈥� Studying it should reveal the history of the crater.
