A GLIMPSE of what Mars was like 4 billion years ago has added fuel to the fiery debate about whether the Red Planet once hosted life. By extracting gases from inside the famous Martian meteorite ALH84001, researchers have revealed tantalisingly mixed news for people who believe the meteorite contains alien fossils.
The new work suggests Mars once had a magnetic field about as strong as the Earth鈥檚. It also had a much thicker atmosphere then, but has been in a deep freeze ever since. 鈥淭he conclusions are supportive of our hypothesis, and certainly wouldn鈥檛 harm it,鈥� says David McKay from NASA鈥檚 Johnson Space Center, who first proposed that the meteorite contains signs of ancient magnetically orienting bacteria.
Benjamin Weiss, the geologist from the California Institute of Technology who led the research, agrees the magnetic field findings add weight to the fossil theory, but the chilly climate he has deduced for Mars is 鈥渘ot very encouraging from the life perspective鈥�.
Advertisement
At 4.5 billion years old, ALH84001 is the oldest bit of rock yet found on any planet. Most researchers agree that evidence within the meteorite suggests it was kicked off Mars 15 million years ago when some other body crashed into the planet, and that it landed on Earth 11,000 years ago (see Graphic).
It also appears that an earlier impact heated the rock 4 billion years ago, while it was still on Mars. If it has remained cold enough since then, then any magnetic alignment or gas in the rock today was literally set in stone when it cooled 4 billion years ago. But no one could be sure that the rock hadn鈥檛 been heated up again in the intervening period.
To find out, Weiss looked at the amount of argon-40 in the meteorite. This gas is produced at a steady pace by the decay of radioactive potassium-40, but rock spits it out when heated. Weiss determined that only 5 per cent of the expected amount of argon has been lost over the past 4 billion years, from which he calculated that the rock must have been no hotter than 500 掳C all that time, probably much colder. He speculates that it has probably stayed deep-frozen in conditions similar to Mars鈥檚 current 鈭�90 掳C to 鈭�40 掳C (Earth and Planetary Science Letters, vol 201, p 465).
The magnetic record in the rock is revealed by the alignment of an iron ore called magnetite. If Weiss is right, the rock has spent the past 4 billion years well below the temperatures at which magnetite realigns, meaning its magnetic record must date from 4 billion years ago too. Weiss determined that the strength of that ancient field was about the same as the Earth鈥檚 today (Earth and Planetary Science Letters, vol 201, p 449).
Mars doesn鈥檛 have a planetary field any more, but Weiss鈥檚 work supports the theory that it once did. That鈥檚 good news for those who believe these magnetite traces are part of the fossils of bacteria similar to bugs found here on Earth that orient to magnetic fields. 鈥淎 precondition for there being fossils is that there was a field,鈥� says Weiss.
The low temperature also makes it unlikely that the magnetite grains are simply the remains of minerals that have been heated to extreme temperatures 鈥� the only other explanation for the traces (快猫短视频, 19 May 2001, p 38). But Weiss adds that it is possible the magnetite was produced by heating earlier than 4 billion years ago.
He says the results also help explain some puzzling results about other gases trapped inside ALH84001, which are very different from the atmospheric gases on Mars today: it鈥檚 simply because they鈥檙e 4 billion years old, he explains. Since then, Mars must have lost a lot of its atmosphere. The earlier, thicker atmosphere, along with a magnetic field to protect against cosmic rays, could have made Mars a reasonably comfortable place for life.