Interplanetary Seeding

Some scientists believe that microbial life may be distributed across terrestrial planets by interplanetary rocks. Rocks are capable of carrying microbes from the surface of one planet, across hundreds of millions of miles of space, to neighboring planets. Each year, Earth is impacted by half a dozen half-kilogram or larger rocks from Mars. These rocks were blasted off Mars by large impacts and found their way to orbits that cross Earth's path, where they eventually collided with Earth as meteorites. Nearly 10 percent of the rocks blasted into space from 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.

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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 （impact material）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 （ejected materials） 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 relatively recent past 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 the 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（Moon）rocks in Antarctica showed that this is possible.

There is also 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, which brought astronauts to the Moon, showed that lunar samples have distinctive properties that distinguish them from terrestrial rocks and normal meteorites derived from asteroids （rocky objects that orbit the Sun and are smaller than planets). Positive linking of the SNC meteorites with a Martian origin was a more complex process. It included showing that gas trapped in glass in the meteorite 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 （dark-colored volcanic rocks） 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 few months.This interplanetary transporter 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, nearby terrestrial planets might contain microbial life with common origins. The seeding process would be most efficient for planets that have small velocities of escape （the minimum speed needed for an object to break free from a planet's gravitational attraction） and thin atmospheres. In this regard, Mars would be a more likely source than Earth or Venus.