This chapter, along with the corresponding workshop session, deals with the bodies in our solar system that are thought to be attractive targets for life detection and outlines current or planned missions to sample or reconnoiter those bodies. It is here also that the question of public concerns about sample return, and National Aeronautics and Space Administration (NASA) responses to those concerns, are addressed. The principal public concern is whether samples that are returned from another planet might harbor unknown organisms that could pose a threat to life on Earth. Most scientists would agree that the risks are exceedingly small, yet no one can truthfully say that there is no risk. There are also concerns, ethical and scientific, associated with the possible introduction of terrestrial life to other habitable planetary objects. The introduction of terrestrial microorganisms could compromise life detection and possibly disrupt a native biology or prebiology, and is thus a fundamental consideration for mission design. Any sample return mission must therefore explicitly address both forward and back contamination, but all missions to targets that potentially harbor life must deal with the forward contamination issue. Unfortunately in the case of Mars, spacecraft of varying degrees of sterility have already impacted or landed on the surface. Europa's surface has not yet been reached, but the question of contamination already is affecting the design of the next mission to that object. The technology of spacecraft bioburden assessment is addressed in the contribution by Nealson (see Session 2) as well as in the technique papers presented in Session 3.
METEORITES AND COMETARY DEBRIS
It is useful to begin this section with a perspective on actual and perceived risk. Although we normally do not think of it as “sample return,” meteoritic infall has continuously delivered vast amounts of extraterrestrial material to Earth's surface throughout the 3.5 billion years of biological history, with estimates in the range of tens of tons per day (see the Session 2 paper by Brownlee). Typical extraterrestrial infall ranges in size from microscopic dust particles to ordinary meteorites and can be considered to represent a random sample return from asteroids and comets. On a time scale of centuries, much larger objects reach Earth, such as the Tunguska bolide that exploded in the atmosphere over Siberia in 1908. On a time scale of millions of years, kilometer-sized objects such as the one that formed the Chicxulub crater 65 million years ago (the physical effects of that impact altered the course of terrestrial evolution) have struck Earth. Despite this steady rain of material ranging from dust to asteroid-sized
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).
Europa, one of the four Galilean satellites of Jupiter, is approximately the size of Earth's Moon and is tidally heated by Jupiter (though to a lesser extent than the innermost Galilean moon, Io). Images of Europa's surface taken by NASA's Galileo spacecraft from 1995 through 2000 have revealed extraordinary vistas of a planet-spanning network of cracks in an otherwise smooth surface. Other evidence, most notably the Galileo magnetometer's mapping of an induced magnetic signature, is indicative of subsurface liquid water. A probable explanation for the cracks is that they represent breaks in a global ice layer overlying a liquid water ocean. The solid crust could be tens of kilometers or tens of meters thick, and no tighter constraint is available from the Galileo spacecraft. Such a europan ocean would be the only known example of globally extensive liquid water other than the oceans of Earth. It will require the Europa Orbiter mission, as currently planned by NASA, to demonstrate the existence of an ocean and identify promising sites for sampling oceanic material.
Given liquid water and a source of biogenic compounds, it is conceivable that microbial life could originate and flourish in deep europan seas (see the Session 2 paper by Chyba). Hydrothermal systems may also exist on Europa, similar to those on Earth that today support abundant and diverse communities of macro- and microbiota. The risk levels of sample returns from Europa are relatively high, and such samples will have to undergo some form of quarantine until stringent testing shows that no living organisms are present. The committee notes that there is a risk of forward contamination of Europa in the seeding of the subsurface global ocean with terrestrial microorganisms, a factor that is already an important consideration in mission design of the Europa Orbiter. The optimal orbit for that mission is polar, which allows complete imaging and radar coverage as well as geodetic mapping that will determine the time-varying shape of the moon and hence sense the presence of a subsurface ocean. However, because of the complex gravitational influences of both Europa and Jupiter on the spacecraft, such an orbit would lead to impact of the orbiter on the surface, whereas lower inclination orbits are more stable. Likewise, the desire to orbit very close to Europa must be measured against the consequences for the stability of the orbit. Extensive analysis of these and other risks connected with Europa exploration has been presented in a previous report of the Space Studies Board.2
Titan, the largest of Saturn's moons, is an extremely cold environment, with a surface temperature of 95 K (about −200 °C). It seems highly improbable that life could have originated under these conditions or that any living organism could survive. Instead, the aim of in situ studies or a sample return mission from Titan is to learn more about the remarkable organic chemistry that occurs in this moon's upper atmosphere and on its surface (see the Session 2 paper by C. McKay).
Atmospheric photochemical reactions lead to the synthesis of complex organic compounds called tholins that form the orange haze visible in spacecraft images of Titan. Tholins are solid polymers composed of the biogenic elements—carbon, hydrogen, oxygen, and nitrogen—and may provide clues to the kinds of organic synthetic reactions that produced complex organics during the origin of life on Earth. Tholins are themselves just one product of the photochemistry ongoing in the stratosphere of Titan. Much of the methane and nitrogen chemistry terminates with the condensation of lighter hydrocarbons and nitriles. The condensed aerosols, including tholins, fall to the surface. Some of these—ethane and other saturated hydrocarbons—would exist as liquids at the surface temperature of 95 K. Other photochemical products would be stable as solids, so that the surface of Titan may be a melange of liquid and solid hydrocarbons resting on a primarily water ice crust. Remote-sensing data taken from Earth suggest a complex and variegated surface.
The Cassini-Huygens mission will arrive at Saturn and deploy a probe into Titan's atmosphere in 2004. It will conduct a comprehensive survey, based on which decisions can be made as to what kinds of future missions (if any) should be mounted to Titan (e.g., sample return versus in situ analyses). Laboratory simulations are of demonstrated utility to the study of organic synthetic mechanisms and hence to replicating much of what is made in the atmosphere. The next step beyond Cassini-Huygens would likely be in situ analyses of the poorly understood surface. Sample return ought to be contingent on whether there are any products of organic chemistry
present in the atmosphere or on the surface of Titan that cannot be simulated readily in the laboratory or analyzed satisfactorily in situ. For the high-altitude organic haze, the answer appears to be no, but this conclusion might not hold for organic phases that have undergone longer-term chemistry on the surface. The risk associated with sample returns from Titan should be small, since organic compounds found on the surface exist under cryogenic conditions (95 K), and organics in the upper atmosphere are processed by free-radical chemistry that breaks down biopolymers. In any event, the extreme distance of Titan from Earth will limit organic analysis on the latter to in situ experiments for the foreseeable future.
There are practical and societal reasons for ensuring planetary protection for all interplanetary missions (see the Session 2 paper by Rummel). Although the probability that an extraterrestrial life form could be pathogenic to humans, or even viable at all in the terrestrial environment, is very low, it cannot be shown to be zero. During the past few years it has become well established that microorganisms inhabit environments thought at one time to be too extreme to harbor life. Some of these environments are associated with geothermal or hydrothermal systems and thus exhibit high temperatures, high concentrations of heavy metals and volatiles, and acidic pHs. Other inhabited environments include desert rocks and deep subsurface basaltic aquifers. These findings have expanded the number of extreme environments on Earth and other solar system bodies that might harbor organisms.
One of the recommendations of a previous National Research Council (NRC) report for handling Mars samples returned to Earth is that they should be contained and treated as though potentially hazardous until proven otherwise.3 This recommendation poses problems in both sample containment and the kinds of analyses to be used for life detection. What criteria will be used to declare whether a returned sample is hazardous or not? A recent NRC report attempts to define these criteria and outlines necessary containment procedures.4
Important issues related to forward contamination include compromising the search for life and the possibility that Earth life could grow elsewhere, a possibility of particular concern for missions to Europa. 5 The essential issue in forward contamination is ensuring that the landing vehicle is free of contamination by Earth microorganisms. For back contamination, the important issues are identifying biosignatures for extraterrestrial life and developing methods for detecting these biosignatures at low levels (see Chapter 3).
1. Space Studies Board, National Research Council, Evaluating the Biological Potential in Samples Returned from Planetary Satellites and Small Solar System Bodies, National Academy Press, Washington, D.C., 1998.
2. Space Studies Board, National Research Council, Preventing the Forward Contamination of Europa, National Academy Press, Washington, D.C., 2000.
3. Space Studies Board, National Research Council, Mars Sample Return: Issues and Recommendations, National Academy Press, Washington, D.C., 1997.
4. Space Studies Board, National Research Council, The Quarantine and Certification of Martian Samples, National Academy Press, Washington, D.C., 2002.
5. Space Studies Board, National Research Council, Preventing the Forward Contamination of Europa, National Academy Press, Washington, D.C., 2000.