objects over Earth's history, there is no evidence that organisms that might have been contained within such objects have engendered extinctions.
Might comets and asteroids be carriers of extraterrestrial life? It is highly unlikely that life could have begun and been sustained in the interior environment of a comet, where liquid water exists only for transient periods when the comet is at or near perihelion. Likewise, with the exception of a very early period in the solar system's history when liquid water might have existed on the parent bodies of today's asteroids and comets, the environments of meteorites and asteroids are also likely to be sterile because of a lack of liquid water. The strong association between life and the presence of liquid water is based on the known nature of cells and the absence of living organisms in places where liquid water (including interstitial liquid layers) is absent. For example, even after 4 billion years of evolution, no community adapted to grow in that environment has been found that can live in the relatively mild cold of the antarctic high desert regions in the absence of liquid water. These considerations can be used to argue that sample return of dust particles, comets, and asteroids poses a relatively low risk, similar to the known negligible risk of lunar sample returns.
As described by Brownlee, the Stardust spacecraft will, in 2004, collect dust from the coma of comet Wild-2 as the spacecraft passes just 150 km from the comet's nucleus. Together with previous collections of dust along the spacecraft's interplanetary trajectory, the sample container will be parachuted into the Utah desert in 2006. Acquisition of this material will represent the first collection of extraterrestrial samples by spacecraft since the U.S. and Soviet lunar missions of the 1960s and 1970s. In addition to the arguments presented above, the material collected by Stardust is thought to carry very negligible risk of contamination, since it will be collected by impact with an aerogel material and hence will be heated to sterilizing temperatures of 104 °C. The aerogel collection material melts around the dust, forming a protective glass layer.1
From a scientific point of view, the material collected by Stardust will be of very high scientific value given its documentable origin in a cometary coma. Comparison of the composition of this material with that of interplanetary dust particles (IDPs) that rain down naturally into our atmosphere will deepen our understanding of the origin of IDPs. The laboratory analysis of cometary material will place in context decades of remote sensing observations as well as the in situ analyses of the coma of comet Halley. Given that comets are leftover planetesimals from orbits at and beyond Jupiter, analysis of their dusty debris is of astrobiological interest in tracing the source regions of the organics from which life on Earth began some 4 billion years ago.
Sample return from Mars requires more serious consideration in regard to both contamination issues and appropriate site selection in the search for life (see the Session 2 paper by Nealson). Mars is a central focus of solar system exploration, because of the increasing evidence that liquid water was present and stable early in the planet's history. The discovery of relatively recent surface features that could be due to liquid water makes Mars even more interesting. Although the recent outflows would be extremely important sites for sampling purposes, they are on steep slopes and, for this reason, will not be initial targets due to limitations of existing Mars rover technology. More likely sites are in relatively flat terrain near what may be ancient seas that could have deposited sedimentary layers. Based on what we know of microbial populations on early Earth, such sediments might contain microfossils and other biosignatures of bacteria that either originated on or were delivered to Mars by impacts. There is even some prospect of extant microbial life on Mars in deep deposits of liquid water produced by geothermal activity beneath the surface. For this reason, of all sample return missions, those to Mars have the highest risk of back contamination, and until the problems are better understood, space agencies should plan on highly secure procedures to limit potential risks. However, such concerns should be tempered by the fact that more than a dozen SNC meteorites are actual samples of the martian crust delivered to Earth over millions of years as a result of crater-forming impacts on Mars. The best known of these—ALH84001—has been carefully studied by numerous investigators. There is no evidence that it contains viable life forms, irrespective of whether it bears evidence of extinct life (for the latter topic, see the papers in Session 3 by Kirschvink and in Session 4 by D. McKay).