Free Astronomy Magazine January-February 2019

42 JANUARY-FEBRUARY 2019 ASTROBIOLOGY Simulations indicate that most peripheral rocks thrown into space reach tempera- tures below 100°C. On might conclude that the trauma of the fall to Earth seems im- possible to overcome, either for the piece of rock or for the organ- isms it might contain. However, various stud- ies have shown that entry into the atmos- phere occurs at speeds between 12 and 20 km/s, and the friction produced creates a melting crust around the meteoroid which prevents any penetration of the excessive heating beyond the first few millimeters. This would protect the hy- pothetical organisms from overheating dur- ing this critical atmospheric entry phase. Finally, the impact with the ground for something like a single-celled organism is less violent than we might imagine at first sight, since the speed has already fallen to a few tens of meters per second. In fact, a meteorite may even remain intact if it im- pacts a soft soil or the water. We do not know to what extent the models of this process are applicable to other plan- etary systems, also because the three phases of panspermia have always been modeled and simulated separately from each other, B etween 2008 and 2010, a colony of ex- tremophile bac- teria called Gloecapsa, taken from the cliffs of Beer, in Devon, UK (pictured left), was placed out- side the Interna- tional Space Station (below), only protected by the rocks that housed it. Re- turned to Earth after 553 days, the rock samples still contained a large number of viable bacteria. [NASA, Open University] The uncertainties about what may have happened elsewhere beyond our Solar Sys- tem have prompted researchers to focus on panspermia within “our home”, in particu- lar on the possibility that life has come to Earth from Mars (or vice versa). We know that in the first billion years, the Red Planet could host elementary forms of life. We also know that between 4.1 and 3.8 billion years ago, the rocky planets suffered an intense asteroid bombardment. Finally, we know that numerous meteorites originating from Martian rocks thrown into space during that bombing (and more recently too) have ended up plunging to Earth. The whole process has been scrutinized by scientists with sufficient precision, and al- though there is no evidence that terrestrial life descends from Martian organisms, the proximity between the orbits of the two planets allows the three phases of pansper- mia, i.e. initial expulsion, interplanetary travel and final fall, to be surmountable obstacles. Mathematical models and exper- iments in the laboratory and in low Earth orbit have shown that extremophile bacte- ria and spores are able to withstand violent accelerations and decelerations, as well as prolonged exposure to solar and cosmic ra- diation. It may seem strange that something survives the energy triggered by an asteroid collision on Mars, but we have to imagine that the land adjacent to the point of impact takes off from simple recoil and the only trauma organisms have to overcome is acceleration.