The panspermia (“seeds everywhere”) theory says that life could originate anywhere in the universe where conditions are favorable, and that mechanisms exist for the movement of lifeforms through space. The abundant life on Earth need not have originated here. Scientific thinking about panspermia began to gain impetus in the 19th century after chemists reported finding organic compounds in samples of meteorites from space. The notion that these carbonaceous materials represented living matter inspired the German doctor H.E. Richter to propose a mechanism for panspermia, in which meteorites glancing a planet’s atmosphere at a very shallow angle could acquire atmospheric microorganisms before skipping back into space.
Richter’s idea of meteors as the transfer vehicles for life through space was expanded on by two of the leading physicists, von Helmholtz and William Thomson (Lord Kelvin). In 1871, each proposed a hypothesis that outlined many details of what has since become known as lithopanspermia based on cosmic impacts. Thomson proposed that space bodies hitting a living planet like Earth could blast life-bearing rocks into space, and that similar meteorites blasted off other living worlds may have inoculated the early Earth with life. In addition to meteorites, von Helmholtz included comets as possible vehicles and proposed an important test – that organisms arising from the donor and recipient planets would share a common ancestry.
In the mid-20th century, Hoyle and Wickramasinghe proposed a cyclical version of panspermia, in which they postulated that interstellar dust grains were actually viable microorganisms that were amplified in the “warm, wet interiors” of comets, then delivered to planets by cometary impacts. According to this theory, after further amplification on the planets, the resulting viable biological material was then returned to interstellar space to start the cycle again.
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Almost from the outset, the lithopanspermia hypothesis drew intense criticism – at the time it was thought that living organisms could not possibly survive ejection by impact, transit through space and entry onto another planet. These views held sway until the discovery on Earth and characterization of meteorites from Mars in the late 20th century.
Considerable experimental effort has been expended in constructing simulations of various aspects of lithopanspermia and measuring the survival of microorganisms to conditions approximating those during the process. Because of their intrinsic high resistance to a variety of environmental stresses and ease of cultivation, spores of Bacillus spp. (in particular B. subtilis) are the most widely-used model microorganism for lithopanspermia studies. However, various other model microorganisms have been utilized in such simulations, such as the soil bacterium Deinococcus radiodurans.
One of the main arguments against lithopanspermia is that the energy required to eject rocks from the surface of a planet into space would be so high as to melt or even vaporize the rock, rendering it sterile. However, since the late 1970s it has been recognized that some meteorites found on Earth are actually bits of crust derived from the Moon and Mars, and had never heated above 100°C.
Current results suggest that most terrestrial microbes tested to date would encounter severe difficulties surviving and growing in the present-day Martian surface environment. The logical extension of this reasoning is that Martian microorganisms might also have a difficult time prospering in today’s Earth environment. It must be kept in mind, however, that we have not yet completely defined the extreme limits of life on Earth. In addition, we have only just begun to scratch the surface of Mars in search of habitable conditions and evidence of past or present life. There is still a long journey ahead. Fancy being the first microbiologist on Mars?
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“NASA astrobiologist Richard Hoover thinks he’s found fossilized alien bacteria inside a meteorite. If he’s right, it’s world-shaking news. But that’s a very, very big if.
