Evidence of life could survive in Martian meteorites
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: September 24, 2008

An artificial meteorite designed by the European Space Agency and plunged through the Earth’s atmosphere has shown that traces of life, perhaps entombed within a Martian meteorite, could survive the high temperatures and pressures endured in such an ordeal.

The STONE-6 experiment, which comprised two samples of sedimentary rock and one sample of basalt from the Earth, were fixed to the heat shield of a FOTON M3 return capsule. After 12 days in orbit the capsule tumbled through the Earth’s atmosphere at 7.6 kilometres per second, slightly lower than normal meteorite velocities of 12-15 kilometres per second, forcing temperatures of at least 1700 degrees Celsius.

The STONE-6 experiment used a 3.5 billion year old volcanic sand containing microfossils (I), a mudstone (II) and a basalt (III). The sand is of particular relevance to Mars since it formed when environmental conditions on early Mars were similiar to those on Earth, i.e. when life could have existed. The backs of the samples were smeared with microorganisms. The sample holder shown here was fixed to the heat shield of the FOTON-3 re-entry vehicle. Image: Westall et al, 2008 (EPSC Abstract).

Although the basalt sample was lost during re-entry, a sample of 3.5 billion year old volcanic sand containing carbonaceous microfossils and a 370 million year sample of mudstone containing chemical biomarkers both survived. During re-entry about half of the volcanic sand sample rock melted away due to friction with the atmosphere, and formed a half-millimetre thick white coloured fusion crust, in stark contrast to the black fusion crusts that meteorite hunters are more familiar in recognising. Fusion crusts form once the meteorite has slowed down enough in the Earth’s atmosphere such that the melted surface of the rock can cool into a thin glassy coating. Approximately 30 percent of the mudstone also survived, and although the heat of entry resulted in mineralogical changes in both rocks, the microfossils survived at depth in the sample.

"The STONE-6 experiment shows that sedimentary Martian meteorites could reach Earth,” says Dr Frances Westell of the Centre of molecular biophysics in Orleans, France. “The fact that we haven't found any to date could mean that we need to change the way we hunt for meteorites. Most meteorites have been found in Antarctica, where their black fusion crust shows up clearly against the white snow. In this experiment we found that the sedimentary rocks developed a white crust or none at all. That means that we need to expand our search to white or light-coloured rocks."

The rocks also transported living organisms, a type of bacteria called Chroococcidiopsis, on the back of the rocks away from the edge exposed to the atmosphere. Unfortunately the heat of re-entry was so high, that even with a protective two centimetre-thick rock coating, the organisms were carbonised, leaving ‘pompeified’ remnants.

Martian meteorite ALH84001 was found in Antarctica in 1984. The characteristic thin black fusion crust is visible on the top of the meteorite in this image. Controversy surrounds this meteorite as to whether Martian microfossils were contained inside the rock, despite lying on the Earth for 13,000 years before being discovered. Image: NASA.

"The STONE-6 experiment suggests that, if Martian sedimentary meteorites carry traces of past life, these traces could be safely transported to Earth,” says Westell. “However, the results are more problematic when applied to Panspermia, a theory that proposes living cells could be transported between planets. STONE-6 showed at least two centimetres of rock is not sufficient to protect the organisms during entry."

The army of landers and orbiters probing Mars has gathered compelling evidence for water and sediments in the red planet’s early history. Potential traces of Martian life are more likely to be found in sediments that have been formed in water, however, of the 39 known Martian meteorites that have been collected on Earth, all are basaltic rocks, with no sedimentary samples found to date. This could be due to a bias in sampling however; if meteorite hunters widen their search to include white rocks or rocks with no fusion crusts, finding traces of life in Martian meteorites could become a reality.