Mars has never been much warmer than it is now, reveals the first detailed analysis of the planetās long-term thermal history ā the current temperature on the equator is a bitter -58Ā°C on average. The study suggests liquid water could not have survived for long periods on the Red Planetās surface, lowering the chances that life could have taken hold on the frigid world.
Previous observations of channels and rocks altered by liquid water have pointed to periods when the surface temperature must have climbed above freezing. But this study adds to a growing body of evidence suggesting Mars has been piercingly cold during most of its 4.6-billion-year history.
David Shuster of the California Institute of Technology in Pasadena and Ben P Weiss of the Massachusetts Institute of Technology in Cambridge, both in the US, arrived at this conclusion after studying data from eight meteorites from Mars.
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They used previously published data on the ratio of two isotopes in the meteorites ā potassium 40 and argon 40 ā to compute the maximum temperatures reached over the rocksā lifetimes. And that is a very long time in the case of meteorite ALH84001, which in 1996 gained notoriety when some scientists announced it contained signs of past microbial life. The new analysis reveals the rock could not have risen above 0Ā°C for more than a million years since it formed, 3.5 billion years ago.
Gas leak
The method works because radioactive potassium 40, a solid, decays into the gas argon 40, which then diffuses very slowly out of the rock. But it does so in a temperature-dependent way so that, at higher temperatures it, leaks out faster.
The method is based on a very precise rock-dating technique. So by dividing each rockās age by the amount of argon it lost, the team could calculate the average annual rates of escape.
The new study suggests average temperatures were too low to keep bodies of liquid water on the surface for long periods of time. But it does allow for liquid water to exist on the surface over short periods. That is because the meteorites came from somewhere below the surface of Mars where they were not warmed by sunlight. So liquid water might have been able to flow on the planetās surface during the warmest part of the day.
Life without water?
The study also shows that these eight meteorites ā which represent about 20% of all known Mars meteorites ā were never subjected to temperatures high enough to sterilise them, even during the shock from the impact that ejected them from Mars. That proves that any living cells they carried could have survived the journey to Earth, Weiss told Ā鶹“«Ć½.
āBillions of tonnes of material have come from Mars to Earth over geological time,ā he adds, which āwould really seem to suggest the planets have not been biologically isolatedā.
But the study also casts doubt on whether there would be any life on the frigid world to make the trip. āMars may have just cooled off too quickly [for life to evolve],ā says Weiss.
Journal reference: Science (vol 309, p 594)
