Cool messenger. Martian rocks indicate the temperature of the red planet has rarely risen above freezing for almost 4 billion years.

A Martian Deep Freeze

Ancient Mars, warm and wet? Maybe so, but it would have to be very ancient Mars. A new analysis of Martian meteorites claims the planet's average surface temperature has rarely risen above freezing for almost 4 billion years.

Geologists have learned a lot about Mars simply by studying meteorites blasted off the planet by asteroid impacts. If these meteorites land on Earth, researchers can date them by measuring the ratios of radioactive elements--the same way they do for homespun rocks. However, one dating method--comparing the ratio of potassium to argon--has a special quirk: because argon is a gas, when a rock gets hot enough, all the argon diffuses out, re-setting its clock.

Planetary scientists Benjamin Weiss of the Massachusetts Institute of Technology in Cambridge and David Shuster of Caltech in Pasadena, California realized they could use this quirk to glean clues about the ancient Martian climate. The researchers compared data on the age of two types of Martian meteorites, using four different isotope clocks. Astonishingly, the potassium/argon ages of the meteorites matched the other isotope clocks very closely, meaning almost none of the argon had diffused out of the rocks over time. This suggests that none of the Martian rocks could have been above freezing for more than a million years in the past billion, the researchers report 22 July in Science. One of the Martian meteorites they analyzed, much older than the others, appeared to have been at below-freezing temperatures for almost 4 billion years.

This frigid climate doesn't rule out liquid water on the surface, but it would have had to exist only briefly, or be salty enough that it would stay liquid at below-freezing temperatures, says Andrew Knoll, a geobiologist at Harvard University in Cambridge, Massachusetts. "If these guys are anywhere close to being right, it has implications for how we think about astrobiology," says Knoll. The abundance of argon in the meteorites means they were never cooked, even during the asteroid impact that sent them to Earth. That suggests, says Knoll, that it would have been possible for life to hitch a ride from Mars to Earth without being fried.

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