There is another reason why life—at least microbial life—may be widely distributed: Planets may commonly be seeded by life from other, nearby planets. This may have happened on Earth; perhaps life arose on Mars or Venus and then seeded our Earth. If microorganisms, the primitive but nearly indestructible creatures at the low end of the cosmic IQ scale, exist on a given world, they must inevitably travel to its immediate neighbors. There is a natural "interplanetary transportation system" that distributes rocks between nearby planets. These rocks serve as natural spacecraft that are capable of carrying unwitting microbial stowaways from the surface of one planet, across hundreds of millions of miles of space, to neighboring planets. This process has nothing to do with the inclinations or technology of the inhabitants. It is an unavoidable act of nature. Each year, Earth is impacted by half a dozen 1-pound or larger rocks from Mars. These rocks were blasted off Mars by large impacts and found their way to Earth-crossing orbits, where they eventually collided with Earth. Nearly 10% of the rocks blasted into space by Mars end up on Earth. All planets are impacted by interplanetary objects large and small over their entire lifetimes, and the larger impacts actually eject rocks into space and into orbit about the sun.
A glance at the full Moon with binoculars shows long streaks, or rays, radiating from the crater Tycho, located near the bottom of the Moon as seen by observers in the Northern hemisphere. The rays are produced by the fallback of impact debris (rocks) ejected from the crater, which is 100 kilometers in diameter. The rays can be traced nearly across the full observable side of the Moon, and such long "airborne" flight is evidence that some ejecta were accelerated to near-orbital speed. Debris ejected to speeds higher than the escape speed (2.2 kilometers per second) did not fall back but flew into space. It has long been appreciated that material could be ejected from the Moon by impacts, but only in the past decade have we realized that whole rocks greater than 10 kilograms in mass could be ejected from terrestrial planets and not be severely modified by the process. it was formerly believed that the launch process would shock-melt or at least severely heat ejected material. There was little expectation that rocks capable of carrying living microbes from planet to planet would survive the great violence of the launch. The discovery of lunar rocks in Antarctica showed that this is possible.
There is also a rare class of meteorites called SNCs, or "Martian meteorites," that are widely believed to be from Mars. The first suggestion that these odd meteorites might be Martian was greeted with considerable skepticism. The discovery of lunar meteorites changed this by proving that there actually was an adequate natural launch mechanism. The lunar meteorites could be positively identified, because rocks retrieved by the Apollo program showed that lunar samples have distinctive properties that distinguish them from terrestrial rocks and normal meteorites derived from asteroids. Positive linking of the SNC meteorites with a Martian origin was a more complex process. It included showing that noble gas trapped in glass in the meteorite served as a telltale fingerprint that matched the composition of the Martian atmosphere, as measured by the Viking spacecraft that landed on Mars in 1976. The general properties of the SNC meteorites revealed that they were basalts formed on a large, geologically active body that was definitely neither Earth nor the Moon. Because the atmosphere of Venus is too thick and its surface too young, Venus was also ruled out as a source.
The astounding discovery that meteorites from the Moon and Mars reach Earth has profound implications for the transport of life from one planet to another. Over Earth's lifetime, billions of football-size Martian rocks have landed on its surface. Some were sterilized by the heat of launch or by their long transit time in space, but some were not. Some Martian ejecta are only gently heated and reach Earth in only a few months. This interplanetary shuttle is capable of carrying microbial life from planet to planet. Like plants releasing seeds into the wind, or palms dropping coconuts into the ocean, planets with life could seed their neighbors. Perhaps, then, life on nearby terrestrial planets might have common origins. The seeding process would be most efficient for planets that have small velocities of escape and thin atmospheres. In this regard, Mars is a better prospect than Earth or Venus. That is why it has been suggested that terrestrial life may have been seeded by Mars.
What about the transfer of microbes between stellar systems? Although microbes are killed by radiation in space some bacteria or viruses embedded in dust grains might be shielded sufficiently to survive. If so, they might possibly "seed" regions of a galaxy through the process known as Panspermia, as suggested by Fred Hoyle and his collaborators in the early 1980s.
once any planet in a particular planetary system is "infected" with life, natural processes may spread that life to other systems. Of course, this process can work only on organisms that can withstand the raw vacuum of outer space. Animal life cannot spread in this fashion.
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