A Chill on the Mars Vacation Plans

By David Whitehouse
Science Editor, BBC News website

A chemical study of Martian meteorites implies that the planet has always been cold and was rarely above freezing. Writing in Science, US researchers say they are able to determine the maximum temperature the rocks experienced.

There is no evidence they were ever warm, the team says, as the meteorites would have recorded near-surface conditions for four billion years. The water erosional features seen on Mars must have been made during very brief periods, they conclude.

Thermal history

Although the current average temperature at the Martian equator is about minus 55 Celsius, many scientists believe that the Red Planet was once warm enough for liquid water to have existed on its surface, and for life to possibly have evolved.

There is plentiful evidence that water has flowed on the surface. This includes the presence of deep canyons, dried up river beds and many examples of deposits left behind by running water.

But the recent analysis, by David Shuster of the California Institute of Technology, and Benjamin Weiss, of the Berkeley Geochronology Center, of meteorites blasted off the surface of Mars to Earth seems to paint a different picture.

The new work involves two of the seven known "nakhlite" meteorites (named after El Nakhla, Egypt, where the first such meteorite was found), and the celebrated ALH84001 meteorite that some scientists believe shows evidence of past microbial activity on Mars.

Using geochemical analysis techniques, the researchers reconstructed a "thermal history" for each of the meteorites to estimate the maximum long-term average temperatures to which they were subjected.

"We looked at meteorites in two ways," says Weiss. "First, we evaluated what the meteorites could have experienced during ejection from Mars, 11 to 15 million years ago."

Their conclusions were that ALH84001 could never have been heated to a temperature higher than 350 Celsius for even a brief period of time during the last 15 million years.

The nakhlites, which also show very little evidence of shock-damage, were unlikely to have been above the boiling point of water during their ejection from Mars 11 million years ago.

'Leaking' Argon

The other part of the research addressed the long-term thermal history of the rocks while they resided on Mars. The scientists did this by estimating the total amount of argon still remaining in the samples.

The gas argon is present in the meteorites as well as in many rocks on Earth as a consequence of the radioactive decay of potassium. A noble gas, argon is not very chemically reactive, and because the decay rate is precisely known it can be used to date rocks.

However, argon is also known to leak out of rocks at a temperature-dependent rate. The cooler the rock has been, the more argon will have been retained.

The researchers found that only a tiny fraction of the argon that was originally produced in the meteorite samples had been lost through the aeons, suggesting that the Martian surface has been in deep-freeze for most of the last four billion years.

"The small amount of argon loss that has apparently taken place in these meteorites is remarkable. Any way we look at it, these rocks have been cold for a very long time," says Shuster.

"The ALH84001 meteorite, in fact, couldn't have been above freezing for more than a million years during the last 3.5 billion years of history."

Water, water, everywhere?

This new line of research is a puzzle given the contrary evidence of running water on Mars.

"Our research doesn't mean that there weren't pockets of isolated water in geothermal springs for long periods of time, but suggests instead that there haven't been large areas of free-standing water for four billion years," says Shuster.

"Our results seem to imply that surface features indicating the presence and flow of liquid water formed over relatively short time periods."

In fact, the evidence shows that during the last four billion years, Mars has likely never been sufficiently warm for liquid water to have flowed on the surface for extended periods of time.

This implies that Mars has probably never had a hospitable environment for life to have evolved, unless biology got started during the first half-billion years of its existence, when the planet was probably warmer.

The study is bound to be controversial showing a disparity between those scientists who look at pictures of Mars to discern its history and those who study the only pieces of the planet we can examine in detail in the laboratory.