Did Life Begin from Space?
Mechanisms for Transport of Life to Earth
As has been seen, there are actually two types of objects that have been hypothesized to be potential life-bearers. One type is rocks ejected from other planets—these are meteorites that fall to Earth. The other type is comets. Both these sources of life have to be investigated to determine if they are capable of bearing organic material or even viable life forms to Earth.
Planetary Meteorites and Asteroids
Vaidya (2009) reported on the results from the Stardust spacecraft which flew through the tail of a comet and collected samples of the tail of comet 81P/Wild 2. The samples collected and returned to Earth dust. When analyzed, the cometary dust included an intriguing mixture of materials. Some of these had to have been formed at high temperatures, such as olivine, and calcium aluminum inclusions. Other materials found in the cometary tail had to be formed at very low temperatures (icy material which appeared to be formed at below 40 K…near absolute zero). Vaidya noted the panspermia hypothesis requires the presence of microorganisms in the solar nebula. The presence of both cold-formed and high-temperature formed materials in the same cometary dust implies that these elements were formed before the cometary head accreted, which in turn dates that formation to the earliest era of the solar system. If microorganisms in fact are present in such dust particles, Vaidya notes that they must also have been present in the earliest days of the solar system, before the planetary bodies formed. Which, in turn, implies that life was present in the materials that eventually made up the planetary bodies.
Comets come from two key areas in the solar system. A spherical Oort cloud that surrounds the sun, and the Kuiper Belt, which is similar to the asteroid belt, but which orbits the sun beyond Neptune’s orbit. The comets in these two regions have different origins and different compositions. Oort cloud comets are believed to have been created from similar materials as the giant planets (i.e., Jupiter, Saturn, Neptune, and Uranus). Kuiper Belt comets are much larger, including the planetoids such as Pluto. Because they originated from different sources, the assumption is that they may also have different materials in their composition, with many of the differences determined by the temperature at which they formed; this assumption has not yet been fully confirmed, however (Owen, 2008). Nonetheless, the basic building blocks of organic molecules, carbon, nitrogen, hydrogen, and oxygen, are all abundant in comets (and in meteors).
When analyzed relative to various planets, of particular interest are Earth and Mars. Looking at the ratio of deuterium (hydrogen atoms with an extra neutron) and regular hydrogen, it is possible to differentiate the water from comets and the water that came from the rocks that make up the planets. As it turns out, both Mars and Earth have similar ratios of deuterium to hydrogen, and those ratios are quite different from that of cometary water as known today. Thus, it seems unlikely that the bulk of water on Earth (or the bulk of water originally present on Mars) came from comets. This does not rule out any contribution of cometary water to the planets, only that it cannot be the majority of the water on Earth and Mars (Owen, 2008).
Scientists all over the world are interested in investigating this further, of course. As one example, the European Space Agency is considering a mission to retrieve a sample from a comet in 2014 and return that sample to Earth for detailed analysis (Kuppers et al., 2009).