In 2008, ESA created an artificial meteorite (STONE 6) and made it go through an atmospheric re-entry. The objective was to test the capacity to detect in sedimentary rocks possible fossil tracks which were submitted to the temperature conditions and shocks of an atmospheric re-entry. The endolithic micro-organisms (naturally living inside rocks) survival was also tested at the same time.
The Earth's plate tectonics is rubbing the tracks out
It is very difficult to find traces of the life's origin on Earth. In fact, the rocks which were formed during the first billion years after the Earth's formation (approximately 4.6 billion years) disappeared or were deeply modified by the Earth's plate tectonics.
The oldest rocks in which microfossils were found (silica elements with shape and size of current bacteria) are between 3.3 and 3.5 billion years old.
This presence shows that micro-organisms did develop and live in coastal areas, with shallow waters and under environmental conditions very far from the ones we currently experience.
The oceanic waters were very warm and high in salt, the atmosphere contained very little oxygen and the UV-radiations were very high.
The Martian lead
Similar conditions also existed in our neighbour, Mars featured the same ingredients than Earth. Many scientists believe that life, or the first elements, could also develop there.
Mars then experienced a very different evolution from the Earth's. The very old rocks are highly abundant and could be hosting traces of the early stages of life which could have appeared. In situ studies are already in progress with vehicles sent by NASA and, soon, Europe. Bringing back judiciously chosen samples could also be a great source of information.
While awaiting this sample return, scientists have on Earth a small set of Martian meteorites. This set, named SNC after the first found meteorites' names (Shergottie, Naklite, Chassignite), is now composed of 52 units.
These are basaltic rock samples, produced by a volcanic activity and in which there are little probability to find traces of life. The scientists believe that sedimentary rocks could have been ejected from Mars by huge impacts before ending up on Earth as meteorites.
An artificial meteorite
The Stone-6 experiment was thus designed to test the sediment-like sedimentary rocks' capacity to withstand a such atmospheric re-entry.
Rock samples were fixed into a Foton's heat shield to expose them to the highest temperature during the atmospheric re-entry.
A rock coming from the Pilbara region in Australia (Kity's Gap Chert), containing spherical (coccoids) and filamentous micro-organisms' fossils, of 3.3 billion years old, was selected.
Since it was too crumbly to be cut and "carved" in the desired shape, it was coarsely ground and the powder obtained, with grains of approximately 3 mm, was mixed with a specific cement for molding and obtaining a 2 cm thick sample with an appropriate shape (see next).
Before being fixed into the Foton spacecraft, the back of the sample was brushed with a Chroococcidiopsis (cyanobacteria) suspension. This bacteria contains a chlorophyll, the green pigment present in most plants. The Foton module was launch from Baïkonour (Kazakhstan) on September 14th, 2007 and recovered on September 26th after a 12-day flight.
The temperature on the heat shield increased to 2,000ºC during the atmospheric re-entry.
A white meteorite
Two of the three samples prepared for the experiment were recovered, protected and studied in the European laboratories involved in the experiment. The visual analysis of a cross section showed an approximately 0.8 mm thick glassy fusion crust, white and bright. Below, a heterogeneous cream-coloured layer which showed a heat alteration and then in a 5 mm thick layer, the cement was blackened from heat but the rock pieces had practically the same colour (see above).
The many microscopic and spectrographic analysis quantified the observed modifications and determined the temperature ranges in the different sample's areas. The samples' surfaces showed clear structure modifications and the microfossils were recognizable at approximately 7.5mm below the exposed surface. But the cyanobacteria on the back of the artificial rocks were charred (see next).
The microfossils were preserved, the cyanobacteria were not
Sedimentary rocks can thus withstand an atmospheric re-entry at an initial speed of approximately 7km/s. The alterations were quite easy to characterize by mineralogy and the possible traces of microfossils were preserved at approximately 7.5mm below the surface. Nevertheless, the living cyanobacteria were charred at approximately 1.5 cm below the surface. The sample lost roughly 1 cm of matter. So meteorite cannot carry photosynthetic organisms, even if they are included in a rock such as the ones existing on Earth. In fact, some micro-organisms can live up to 5 mm below the surface of a rock while using photosynthesis (photoendolithic organisms). The experiments showed that the deeper below the surface, the lower the temperature. With this experiment, it had been measured that it could reach 113ºC at approximately 5 cm below the surface. This temperature allows the survival of endolithic bacteria which draw their power from chemical reactions, without light (chemo-endolithic bacteria). On Earth, such species were found at several thousands meters depths.
Meteorites enter the atmosphere at a higher speed. The increase of the surface temperature is probably higher but shorter. And the orders of magnitude for reached temperature and removed quantity are representative. If this experiment showed that microfossils could remain recognizable after an atmospheric entry, it did not say anything about their ability to withstand the strong impact which would eject such rock into space. A first calculation assessed that during the impact, the temperature increase could reach 10,000ºC. Nevertheless, simulations also showed that an impact which would produce a 50 km diameter crater on Mars could eject 22 m diameter pieces of rocks giving them enough speed to escape the Martian attraction.
Matter exchanges did occur between Earth and Mars and Martian meteorites could arrive on Earth. Could those sedimentary rocks bring some living micro-organisms on Earth? Would they include traces of micro-organisms which could be still recognizable? However, sedimentary meteorites are difficult to distinguish from terrestrial rocks because of their white surface, unlike most meteorites which have heat discolourations that blacken them.