Natural Influx and Cross-Contamination
Solar system bodies are not isolated from each other. Earth, for example, encounters a suite of objects—interplanetary dust particles (IDPs) and small bodies commonly referred to as the interplanetary debris complex. These bodies range from fine dust that is delivered continuously to objects several tens of kilometers in size or larger, including comets and Earth-approaching asteroids, which impact very rarely. This chapter discusses the natural influx of extraterrestrial material to Earth and cross-contamination of planetary satellites and small solar system bodies.
To understand the potential dangers of returning samples from solar system bodies to Earth, it is vital to assess the material delivered to Earth by entirely natural processes. As a starting point, the task group assumed that the natural influx of objects to Earth is not hazardous, from the perspective of biological contamination. Much research supports the common perception that, except during very rare impact catastrophes (e.g., at the Cretaceous-Tertiary boundary), the evolution of life on our planet is explained by interactions among terrestrial species and changing environmental conditions without any external, cosmic biogenic agents. To the degree that sample return from missions to small bodies mimics even in a modest way the copious natural influx of interplanetary debris, it seems reasonable to assume that such samples are not potentially dangerous; this is the view that was adopted by the task group. For proposed missions to small solar system bodies that would return samples not introduced to Earth naturally, further analysis is needed before requirements for containment are considered.
NATURAL INFLUX TO EARTH
Most of the material that has struck Earth during the nearly 4 billion years since the Late Heavy Bombardment has come in the form of rare impacts by large bodies (comets and asteroids). These bodies range from 100 meters to tens of kilometers in diameter and strike Earth infrequently (i.e., less than once a century to as rarely as once every 100 million years for objects larger than 10 km). They contribute, on average, 10 times the amount of material that has been estimated for the more constant influx of material from smaller bodies (Ceplecha, 1997), which is estimated at 2 × 1010 g/yr. The rare impact of large bodies could have devastating consequences for life on Earth (Alvarez et al., 1980; Chapman and Morrison, 1994), potentially dwarfing the threat from any biological contamination. The material from these bodies has never been analyzed for the presence of life forms because such impacts have not been witnessed. Objects of this size are not considered further in this report.
The cross section of the interplanetary debris complex is dominated by the smallest particles.1 While the dominant mass is in objects exceeding 1014 kg, there are significant excesses in mass of particles ranging roughly
from 10-9 to 10-7 kg, 10-4 to 10-3 kg, and, especially 105 to 107 kg (Ceplecha, 1997). The processes that govern the production and losses of material within the interplanetary debris complex, including redistribution of material among the bodies and the planets that are sources of meteorites, depend on these characteristics of the size distribution.
Associated with the size distribution of particles and objects in the interplanetary debris complex are characteristic lifetimes against collisional destruction. In general, the smaller a particle, the shorter the time it lasts before being destroyed or physically removed from the system. In a biological context, an important criterion is whether material within a body is protected from solar and galactic cosmic rays. Generally speaking, the interiors of bodies larger than approximately 1 meter may be shielded for appreciable periods. However, in some cases (e.g., cometary dust liberated from a comet that is very near Earth) the lifetimes of very small particles before impact may be short enough to preclude sterilization of potential biological entities by radiation.
In general, the meteoroids and dust in the interplanetary debris complex are short-lived. Large objects move according to Keplerian and chaotic orbital dynamics, affected by perturbations from distant, massive bodies. Once in Earth-crossing orbits, they last for time periods (e.g., 106 to 107 years) that are very short compared with the age of the solar system before encountering a planet, impacting the Sun, or being ejected from the solar system primarily by Jupiter's gravity. Particles meters in size and smaller are pushed around by the solar wind and radiation forces (e.g., Yarkovsky effects) and are swept up by the Sun and the terrestrial planets or else driven beyond the terrestrial planet zone (Burns, 1987).
All of these short-lived objects must be resupplied from long-lived "parent bodies" that have survived in "storage locations." Large comets and asteroids, and still larger planets and planetary satellites, serve as parent bodies for such materials so long as they are away from regions with high impact rates or are in stable orbits that do not cross the orbits of other planets and thus serve as reservoirs of fresh material. Their smaller cousins may also serve as parent bodies provided that they have been stored in locations, such as the Oort Cloud, where the volume density and velocities of other objects are low enough that collisions are rare and less destructive. The chief locations of parent bodies that contribute to the interplanetary debris complex are the main asteroid belt, located between 2.2 and 3.2 AU from the Sun; the Trojan asteroids, at 5 AU; the Kuiper Belt (and associated Scattered Disk), ranging out several tens of astronomical units beyond Neptune's orbit; the Oort Cloud, a spherical halo of comets weakly gravitationally bound to the distant Sun and extending part way to the nearest stars; and the major planets and satellites of the solar system.
The smaller debris is resupplied from these parent bodies predominantly by exogenic and endogenic processes. First, hypervelocity impacts among the components of the interplanetary debris complex or impacts of asteroids and comets onto the surfaces of planets and satellites produce sprays of ejected material from the target bodies (dust, boulders, and so on), which launch the debris away from the impact point, some fraction of it often exceeding the target's escape velocity. Second, active processes on bodies (especially sublimation of ices near comet surfaces that carries away surficial grains, but also powerful volcanic processes on bodies like Io) drive fine material away from the parent object, thus incorporating it into the interplanetary debris complex.
This chapter assesses the parent objects and the modes of delivery to Earth of material produced by these processes, prior to its being removed from the interplanetary debris complex by processes of (primarily collisional) physical destruction or dynamical removal from the complex. Inasmuch as interplanetary debris encounters other planetary bodies and other bodies within the interplanetary debris complex, effectively "contaminating" them, material received on—or brought back to—Earth from one body may contain material originally derived from another body.
Analysis of lunar soils suggests that a small percentage are of extralunar origin, as small meteoroids become incorporated into the lunar regolith. In addition, meteorites have occasional (but not extremely rare) visible fragments of other meteorite types embedded within them, implying that relatively intact lithic fragments from one parent object can be delivered as a result of sampling a different parent body (Zolensky et al., 1996). Cross-contamination is not universal. It may be difficult or impossible to derive by natural processes materials contained deep within large bodies, located deep within the gravity fields of large planets, or associated with bodies far from Earth and in orbits that are not easily converted into Earth-crossing orbits. It may be that such materials do not reach "any" of the other bodies that reach Earth. These issues are addressed below.
PROCESSES OF DELIVERY FROM DIVERSE PARENT BODIES
The dominant sources of materials in the interplanetary debris complex near Earth are the inner half of the asteroid belt, the Kuiper Belt, and the Oort Cloud. Collisions among main-belt asteroids, at typical velocities of 5 km/s, generate ejecta that becomes part of the interplanetary debris complex. Large particles that cannot be moved by radiation forces can be delivered to Earth via chaotic dynamical processes after they are collisionally ejected into one of several commensurabilities (e.g., orbits having periods related to Jupiter's orbital period by simple fractions, like the 3:1 commensurability recognized as one of the so-called Kirkwood gaps in the distribution of asteroid semi major axes) and so-called secular resonances. The asteroidal size distribution is such that the production of most of the fragments is episodic and sporadic, as shown by spikes in histograms of cosmic-ray exposure that are used to estimate the ages of meteorites. Objects that are converted so as to have elongated planet-crossing orbits become short-lived, because if they do not strike a planet, their orbits are apt to be further perturbed so that they strike the Sun or are ejected from the solar system on a time scale of a few million years. If such an object's aphelion (the point on an elliptical orbit farthest from the Sun) remains in the asteroid belt, the object continues to suffer collisional fragmentation, further populating the smaller-mass ranges of the interplanetary debris complex. It is believed that most meteorites are derived from the asteroids (in the asteroid belt), as are their near-Earth-object relatives. Examination of meteorites reveals the degree to which the asteroidal rocks have been damaged by impacts. While small fractions of impact ejecta are melted or vaporized, and any biological entities in those portions consequently destroyed, most meteoritic material is relatively undamaged, having been derived from the periphery of impact craters or from ejecta from crater interiors that nevertheless escaped heavy shock or melting. It is doubted that meteorites are derived in any appreciable numbers from beyond 2.8 AU (the 5:2 commensurability with Jupiter).
Kuiper Belt comets may also be a population of collisional fragments (Davis and Farinella, 1996), like the asteroids, although this is not certain; Oort Cloud comets are much more likely to have escaped collisional fragmentation. During their short times in the inner solar system, comets from both storage locations become active as their volatiles are warmed by the sunlight from which they have been protected since formation. The activity liberates copious quantities of cometary dust, which manifests itself as a component of cometary comae and tails and as meteors in the night sky. It is very uncertain, however, if cometary processes liberate much larger fragments. Debris larger than radar wavelengths has been detected near some comets, but pieces of comets meters to tens of meters in size are generally unobservable, especially once they are devolatilized. Comets are observed to split, sometimes due to tidal forces (as in the case of Comet Shoemaker-Levy 9, whose fragments subsequently struck Jupiter in 1994) but often for no apparent reason, contributing to the perspective that comets are inherently weak. Little if any cometary material survives passage through Earth's atmosphere as meteorites, and it is difficult to calibrate the size distribution of cometary fragments from meteoric phenomena alone. Therefore, while most of the observational evidence about the contribution of cometary materials to Earth concerns cometary dust (and the rare impact by an intact comet nucleus), there is the possibility that significant cometary material is contributed by objects of intermediate sizes.
During the period that comets are near the Sun and active (i.e, generating comae and tails), their activity is far more important than are collisions with asteroids in liberating cometary materials. However, short-period comets (derived primarily from the Kuiper Belt) last long after their visible activity becomes dormant as they lose their volatiles, and they eventually "die." It is estimated that a few tens-of-percent of Earth-approaching asteroids are dead or dormant comets (Wetherill, 1988). These objects presumably suffer collisional interactions with main-belt asteroids, leading to cratering and fragmentation, just as small asteroids that originated in the asteroid belt do.
Materials from larger objects, like the Moon and Mars, can also be derived by impact cratering, despite the fact that such surface materials are trapped deep within the planetary gravity fields. Such high-velocity planetary ejecta can join the interplanetary debris complex and be subject to further collisions and dynamical evolution. Obviously, the forces required to excavate material from a planet at escape velocity might be expected to modify it physically (e.g., by shock heating, melting, vaporization) more than in the case of asteroids. But it is now understood, and demonstrated by the traits of lunar and martian meteorites, that modest-sized (meter-scale) fragments can be excavated with minimal physical damage. Conceivably, even larger fragments could be excavated,
portions of which might remain similarly undamaged. Most samples from planetary surfaces must be excavated in rare, large cratering events and do not result from the more frequent bombardment by smaller projectiles. Certainly this must be true for excavation of materials from the surfaces of planets with substantial atmospheres (e.g., Venus); given the long durations between such large impacts and the short survival time of any material excavated from Venus, it is very unlikely that any contemporaneous meteorites are being derived from Venus.
Earth itself could be a source of material migrating around interplanetary space; most tektites are derived from large impacts that produce craters (e.g., the Ries basin in Germany) exceeding 10 km in diameter; such events happen only once every few hundred thousand years. Still larger impacts are required to blow away a portion of Earth's atmosphere and eject major amounts of Earth material at escape velocity.
Although ions probably derived from Io have been detected away from Jupiter, it is very unlikely that meteorites can be derived from the galilean satellites. The difficulties of excavating materials from bodies as large as the Moon and Mars apply to them, as well, but are augmented by the fact that much greater velocities must be achieved to escape Jupiter's enormous gravity. Somewhat analogous difficulties exist for excavation from the satellites of Mars. Although the satellites are small, and cratering ejecta is readily lofted into Mars orbit, much larger velocities would have to be achieved for material from Phobos or Deimos to escape Mars's gravitational well and enter independent heliocentric orbits and join the interplanetary debris complex. If Phobos and Deimos are, as they may well be, gravelly or rubbly bodies, ejecta velocities may be too low to reach Mars escape velocity because loosely consolidated bodies absorb energy more efficiently than compact bodies through reduction of porosity. Thus, that energy is not available to accelerate the ejecta out of Mars's gravitational well.
The last stage of natural delivery of extraterrestrial materials to Earth is penetration through the atmosphere and impact or airborne settling onto the ground or ocean. The effects of such delivery depend on the impact velocity (and angle), the size of the projectile, and the nature of the projectile material. To objects sufficiently large and/or strong, Earth's atmosphere presents little resistance. Asteroids larger than roughly 100 m in diameter and comets larger than several hundred meters penetrate the atmosphere and strike the ground (or ocean) at hypervelocities, resulting in an explosion crater. Smaller objects may explode or burn up in the atmosphere. Iron objects are generally strong enough so that major chunks survive atmospheric penetration; most terrestrial impact craters the size of Meteor Crater and smaller are formed by iron projectiles. A fraction of interplanetary dust particles, preferentially those arriving at lower velocities (see Chapter 6), survive atmospheric penetration relatively intact and settle to Earth.
A typical meteorite that lands on Earth is the remnant of a somewhat larger preatmospheric mass, whose exterior has ablated away. Meteorites exhibit an external "fusion crust" of heated, melted material that did not quite ablate away. They land on Earth relatively gently, at terminal velocity. The interior of a meteorite is likely to be unaffected by its arrival on Earth; it is generally cold, there having been inadequate time for the brief heat pulse from its exterior to penetrate. Thus, in spite of the superficially violent mode of delivery, most interplanetary objects that strike Earth (both some small interplanetary dust particles and most meteorite-sized and larger projectiles) do so in a mode that would fail to sterilize or otherwise destroy many biological materials.
The question of interplanetary delivery of materials has recently been analyzed by Gladman et al. (1996). It is evident, based on current, evolving understanding of impact and dynamical processes, that there is appreciable mixing of materials among solar system bodies. In addition, there is considerable evidence for cross-contamination. Many meteorites, for example, contain sizeable (e.g., centimeter-scale) clasts (small, identifiable portions of a rock) of materials from foreign parent bodies; although such xenoliths constitute a tiny fraction (e.g., less than 1 percent) of meteorite mass (Zolensky et al., 1996), that would be enough to be important in the context of biological contamination. Similarly, a small percentage of the lunar regolith is believed to be composed of primarily carbonaceous chondritic material derived from infalling exogenic projectiles and dust. Martian and lunar meteorites, which constitute less than 0.2 percent of meteorites recovered from Antarctica, are mentioned above.
It is probable that cross-contamination among objects augments the variety of parent bodies represented in what reaches Earth by natural influx. For example, while outer-belt asteroids cannot be dynamically converted into Earth-crossing orbits, such objects in moderately eccentric orbits do collide with asteroids in the inner asteroid belt, which do communicate with Earth. One could imagine a chain of contamination providing an access route for materials from virtually any body—e.g., ejecta from Europa is encountered by a comet passing through the Jupiter system, which ultimately crashes on the Moon; then such material gets to Earth by excavation in a lunar cratering impact. Obviously, as in this example, the probabilities of occurrence of most multichain contamination routes drop geometrically toward zero, but cannot be totally ruled out in principle. However, to the degree that the apparent safety of the natural influx is relied upon to declare a body safe for sample return, it must be realized that the episodic nature of cross-contamination and delivery processes probably means that exactly zero percent (not just a very small percentage) of material impacting Earth during a finite time (e.g., a century) can be expected to come via such multichain routes. Only the tiniest particles might escape this generalization, and any biological materials are rapidly sterilized by radiation in such small particles.
Earth receives from other bodies in the solar system abundant material that is ejected from such bodies and delivered to the surface of Earth. Because of cross-contamination, small but significant fractions of the delivered materials could have been formed originally on bodies from which Earth does not receive meteorites directly. However, there are some bodies, as well as places on other bodies, from which material would be so difficult to obtain, or would arrive so infrequently, that it is unlikely that Earth has received samples during our lifetimes, even if such material is delivered on rare occasions. For some planetary satellites and small bodies, the uncertainty associated with cross-contamination reduced the degree of confidence in the inherent safety of a sample returned from such bodies.
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