Artificial meteorite landed by ESA: fossils survived
If life exists on Mars, could it have ever been transplanted to Earth via meteorite impact? Could the converse have occurred? For that matter, could sedimentary rocks containing Martian life – or fossil traces of life – survive an impact-assisted blastoff from Mars, an indefinite journey through space, then a fiery re-entry and landing on Earth? The European Space Agency has completed a study to try and address these questions.
Using a FOTON M3 capsule as a carrier vehicle, the ESA affixed samples of two sedimentary rocks and a basaltic rock to the capsule heat shield, where they would receive the full assault of re-entry. The mission – labeled STONE-6 – was launched last year from Baikonur Cosmodrome, and was brought back down on September 14th of this year. The sedimentary test subjects were a piece of 3.5 billion year old Pilbara sediment containing algal microfossils, and a piece of 370 million year old Orkney mudstone containing traces of kerogen and other organic biomarkers. The basalt sample was included as a control; all known Martian meteorites are fragments of basalt. The basalt sample didn’t survive re-entry on STONE-6, but the sedimentary samples did.
After impact back on Earth, the samples were immediately whisked away by ESA researchers to a clean lab for analysis. The results of the STONE-6 mission were presented on Sept 25th at the European Planetary Science Conference in Münster, Germany.
The good news is that both microfossils and chemical biomarkers for life survived in the sedimentary rock samples. STONE-6 hit Earth’s atmosphere at a lower velocity than a normal meteor (7.6 km/s, versus 12-15 km/s for natural meteors), so in many ways this experiment was a proof of concept and not a rigorous duplication of an actual meteor impact. Still, re-entry is never gentle, and the samples were exposed to temperatures over 1700º C during their brief transit of Earth’s atmosphere.
In a further test (though a fairly weak test) of the panspermia hypothesis, sedimentary rock samples were painted with a slurry of Chroococcidiopsis bacteria at the start of the experiment. None survived re-entry, though carbonized imprints of the bacteria were found on the sample surfaces. Both sedimentary rocks developed a fusion crust during descent, which is a common feature of natural meteorites. Fusion crusts form quickly during the heat of re-entry as the surface of a rock melts for a few seconds, but these crusts are typically only a millimeter or so thick (as were the STONE-6 rocks’ fusion crusts). Often the heat of re-entry doesn’t do much more damage than that to small bodies. Anecdotal instances of meteorites found just after landing have even reported the objects covered in frost… silicate rock has a fairly low thermal conductivity, so a hot, thin surface layer can quickly radiate its heat away while the interior remains near the temperature of space (~3 K).
The STONE-6 experiment demonstrates that microbes can’t survive exposure to 1700º C, which is not surprising. A more appropriate test of the panspermia concept would be to have a rock that contains living bacteria within pore spaces and fractures, which would afford a much more effective barrior to carbonization during the brief but intense fires of re-entry. Such bacteria may actually exist within the two samples used in the STONE-6 test, and if so their recovery would be illuminating.
Did life begin on Mars, only to be transplanted here by the impact of Martian meteorites onto Earth in the late Hadean Eon (ending about 3.8 billion years ago)? That hypothesis – or at least a hypothesis of life starting on either world and crossing over to the other at some point – is testable, if extant Martian microbes could be found and their biochemistry examined. If our two neighboring worlds independently developed life, the odds are astronomically remote that their two respective biochemistries would be identical. Conventional life would need to make use of self-replicating molecular complexes, and RNA templates are probably the best way of getting started… however, compositional differences between an early Martian ocean and that of Earth ought to yield slightly different organic products. Terrestrial life uses only 20 amino acids, for example, though many more can be produced in the lab. Martian life would probably not use exactly the same 20 we do, nor exactly the same 4 nucleotides that terrestrial RNA employs. RNA might be common to two independently evolved biologies, simply because RNA is a highly functional molecule. But even so, the fundamental biochemistry of Martian life ought to be uniquely identifiable as not sharing a common ancestor with terrestrial organisms.
Unless Mars-Earth transplantation occurred. In such a case, the basic biochemistries of life on both worlds should be very similar. If we find life on Mars and its ribosomal RNA looks like that of a primitive Earth microbe, the only question would then be whether life began here or there. Answering that question would require much more work.
Panspermia as an explanation for the origins of life on Earth is unsatisfying because it simply shifts the problem of abiogenesis to another planet somewhere else. Panspermia falls into the category of an extrordinary claim, and would require appropriately extrordinary evidence to be accepted. In the meantime, the null hypothesis should always be that life on Earth began on Earth, or at least on a plausible neighbor planet from which meteoric transport has already been demonstrated. At least 39 Martian meteorites have been found on Earth, so clearly that sort of thing does happen from time to time.