David L. Shuster, now at the Berkeley Geochronology Center, and Benjamin P. Weiss of the Massachusetts Institute of Technology analyzed two of the seven so-called nakhlite meteorites from the Red Planet in addition to the well-studied ALH84001 Martian meteorite. They first calculated the maximum temperatures the samples were exposed to and determined that ALH84001 may have neared 340 degrees C when it was ejected from the planet and that the nakhlites could have reached about 100 degrees C when they were thrown off 11 million years ago. Next the team measured the amount of argon present in the samples and modeled how the chemical diffused out of the rocks--a property that is temperature dependent--over their entire history. "The small amount of argon loss that has apparently taken place in these meteorites is remarkable," Shuster says. "Any way we look at it, these rocks have been cold for a very long time." The thermal history that the meteorites reveal for Mars is remarkably different from that of our own planet. "On Earth, you couldn't find a single rock that has been below even room temperature for that long," he adds.
Although the findings indicate that the Red Planet has been in a near deep freeze for most of its history, they do not rule out the presence of liquid water entirely. "Our research doesn't mean that there weren't pockets of isolated water in geothermal springs for long periods of time," Shuster explains, "but suggests instead that there haven't been large areas of free standing water for four billion years." In addition, Weiss notes that if the liquid responsible for carving the observed gullies and trenches on the Red Planet was brine that was particularly salty or acidic--two properties that depress the melting point--then it could continue to flow at temperatures as low as around -50 degrees C.