Asteroid containing ices

Most of the thousands of meteorites in our collections are bits and pieces of rocky or metallic asteroids. Because we can analyze these meteorites in the laboratory, they play a pivotal role in cosmochemistry. In this chapter we will focus on the compositions of meteorites that were anhydrous, or nearly so. The hydrated carbonaceous chondrites, in particular the Cl and CM chondrites, which sample bodies that once contained ices and fluids, will be considered in Chapter 12. 

We now turn to ice-bearing planetesimals and asteroids that once contained ices. This will complete the inventory of chemically characterized small bodies, on which so much of cosmochemistry depends. 

Several classes of asteroids are also thought to contain ices presently, or contained them at some earlier time. The D- and F-class asteroids occur in the outmost main belt, and the C-, G-, B-, and F-class asteroids are concentrated within the central part of the belt. These asteroids probably formed near their present locations, in which case they represent icebearing planetesimals that accreted inside the orbit of Jupiter. A few asteroids exhibiting cometary activity also occur within the asteroid belt. 

Only one asteroid thought to have once contained ices has been imaged by spacecraft. A picture of the C-class asteroid 253 Mathilde was shown in the previous chapter (Fig. 11.1). This body has an extremely low albedo. 

The D- and P-class asteroids dominate the outer main belt and Trojan asteroids located in Jupiter s orbit. With only a few exceptions, the spectra of these asteroids show no 3 pm absorption bands (Jones et al., 1990). The D and P asteroids are thought to contain ice that has never been melted. However, it is also possible that D and P asteroids could contain hydrated silicates, and that the 3 pm feature is masked by an increasing abundance of elemental carbon with heliocentric distance. The unique carbonaceous chondrite Tagish Lake has a reflectance spectrum quite similar to D-class asteroids, and it has been hypothesized to be a sample of this class. However, Tagish Lake shows a significant 3 pm absorption. 

Asteroids that formed beyond the snowline represent rock and ice accreted inside the orbit of Jupiter. The most distant asteroids may still contain ices, but many asteroids have been heated. Melting of ice produced aqueous fluids, which reacted with chondritic minerals at low temperatures to form secondary minerals (phyllosilicates, carbonates, sulfates, oxides). The alteration minerals can be discerned in asteroid spectra and characterized by analyses of chondrites derived from these bodies. 

Earth and Mars clearly contain H2O. Venus s atmosphere is very dry, and composed mainly of CO2, but the high D/H ratio of the small amount of water present suggests Venus was once much wetter than today (Zahnle 1998). Mercury is perhaps too small and too close to the Sun to have acquired and retained water. Water may have been present in much of the material that accreted to form the Earth. Small amounts of water may have been adsorbed onto dust grains at 1 AU by physisorp-tion or chemisorption (Drake 2005). Once Jupiter formed, substantial amounts of water could have been delivered to the growing Earth in the form of planetesimals and planetary embryos from the Asteroid Belt (Morbidelli et al. 2000). It is also possible that Earth lay beyond the snowline at some point during the evolution of the solar nebula (Chiang et al. 2001) so that local planetesimals contained ice. 

Disk temperatures would have decreased rapidly with distance from the Sun as accretional energy release, optical depth, and solar radiation all declined. For example, some meteorite samples from main-belt asteroids contain hydrated silicates, formed by reactions between anhydrous rock and water ice. This implies that temperatures at 2-3 AU became low enough for ice to condense while the asteroids were forming. 

Water and carbon play critical roles in many of the Earth s chemical and physical cycles and yet their origin on the Earth is somewhat mysterious. Carbon and water could easily form solid compounds in the outer regions of the solar nebula, and accordingly the outer planets and many of their satellites contain abundant water and carbon. The type I carbonaceous chondrites, meteorites that presumably formed in the asteroid belt between the terrestrial and outer planets, contain up to 5% (m/m) carbon and up to 20% (m/m) water of hydration. Comets may contain up to 50% water ice and 25% carbon. The terrestrial planets are comparatively depleted in carbon and water by orders of magnitude. The concentration of water for the whole Earth is less that 0.1 wt% and carbon is less than 500 ppm. Actually, it is remarkable that the Earth contains any of these compounds at all. As an example of how depleted in carbon and water the Earth could have been, consider the moon, where indigenous carbon and water are undetectable. Looking at Fig. 2-4 it can be seen that no water- or carbon-bearing solids should have condensed by equilibrium processes at the temperatures and pressures that probably were typical in the zone of fhe solar... 

We have now set the stage for the next two chapters - anhydrous planetesimals and ice-bearing comets and asteroids. These objects contain the organic matter, noble gases, and sometimes ices that we have just learned about, and they provide us with the best record of primitive materials in the solar system. 

Some asteroids, thought to contain modest amounts of ices, might show cometary activity if they were to be perturbed into orbits that took them close to the Sun. Other asteroids originally accreted with ice components, which later melted. Such asteroids were then altered when fluids reacted with rock. Altered carbonaceous chondrites were discussed briefly in Chapter 6. Here we explore asteroid alteration in more detail. 

The aqueous fluids formed by melting of ices in asteroids reacted with minerals to produce a host of secondary phases. Laboratory studies provide information on the identities of these phases. They include hydrated minerals such as serpentines and clays, as well as a variety of carbonates, sulfates, oxides, sulfides, halides, and oxy-hydroxides, some of which are pictured in Figure 12.15. The alteration minerals in carbonaceous chondrites have been discussed extensively in the literature (Zolensky and McSween, 1988 Buseck and Hua, 1993 Brearley, 2004) and were most recently reviewed by Brearley (2006). In the case of Cl chondrites, the alteration is pervasive and almost no unaltered minerals remain. CM chondrites contain mixtures of heavily altered and partially altered materials. In CR2 and CV3oxb chondrites, matrix minerals have been moderately altered and chondrules show some effects of aqueous alteration. For other chondrite groups such as CO and LL3.0-3.1, the alteration is subtle and secondary minerals are uncommon. In some CV chondrites, a later thermal metamorphic overprint has dehydrated serpentine to form olivine. 

Laboratory experiments have shown that radiation processing of simulated presolar ices leads to more complex molecular species [25-27]. Hundreds of new compounds are synthesized, although the starting ices contain only a few simple common interstellar molecules. Many of the compounds formed in these experiments are also present in meteorites and cometary and asteroidal dust (interplanetary dust particles - IDPs), and some are presumably relevant to the origin of life, including amino acids [28,29], quinines [30], and amphiphilic material [31]. 

Figure 5.5 Winds in the solar nebula might be one of the possible processes responsible for the mixing of hot and cold components found in both meteorites and comets. Meteorites contain calcium-aluminum-rich inclusions (CAIs, formed at about 2000 K) and chondrules (formed at about 1650K), which may have been created near the proto-Sun and then blown (gray arrows) several astronomical units away, into the region of the asteroids between Mars and Jupiter, where they were embedded in a matrix of temperature-sensitive, carbon-based cold components. The hot component in comets, tiny grains of annealed silicate dust (olivine) is vaporized at about 1600 K, suggesting that it never reached the innermost region of the disk before it was transported (white arrows) out beyond the orbit of Pluto, where it was mixed with ices and some unheated silicate dust ( cold components). Vigorous convection in the accretion disk may have contributed to the transport of many materials and has been dramatically confirmed by the Stardust mission (Nuth 2001).

Although lapetus extreme hemispheric albedo contrast has been recognized essentially from the time of its discovery, the nature and origin of the dark material that dominates the leading hemisphere is still uncertain. Spectrally, the dark material resembles some low-albedo, red asteroids. This material is believed to be similar to hydrocarbon-rich materials produced in laboratory experiments, known as tholins, which are opaque, contain PAHs and are spectrally red (Cmikshank et al., 1991). Tholins are also candidates for at least some of the dark, non-ice constituents on Ganymede and Callisto. Unfortunately, spectra in the 3-5 p,m region where these material have some spectral features are not yet available for Saturn satellites. 

From a purely cosmochemical perspective, the present asteroid-comet terminology is a confusing and often misleading means of classifying primitive solar system bodies. An improvement would be to consider any primitive undifferentiated body that ever contained trace of higher amounts of ice to be an ice-bearing planetesimal or true comet. By this scheme, most of the planetesimals in the solar nebula were ice-bearing particles or tme comets. 

While many asteroids may have previously been comet-like, even active comets are somewhat asteroid like. The traditional concept of comets as dirty snowballs has been modified by some authors to consider them to be more like frosty rocks, because they contain more rock than ice. The dust gas production rate from the spectacularly active comet Hale-Bopp exceeded its gas production rate by a factor of 5 (Jewitt and Matthews, 1999). In considering the water contents of comets, it is interesting to consider that some comets may have ice abundances similar to the bound water content of hydrated silicate-rich asteroids. At least in some cases, comets and asteroids might have similar capacities for carrying water to other solar system bodies. 

The primitive carbonaceous meteorites, which include the hydrated CI and CM meteorites and mostly-anhydrous meteorites such as the Allende CV meteorite [69], reach Earth from the asteroid belt between Mars and Jupiter. Asteroid reflectance properties display a remarkably systematic distribution as a function of heliocentric distance for asteroids in this belt, and hence meteorite types, with the most primitive ones located farthest from the sun. Asteroid hydration occurred when internal heating melted (water) ice that had co-accreted with dust, chondrules and refractory inclusions in the solar nebula. These asteroids form the IR spectroscopic C-class with clays, carbon and organics at the surface similar to CI and CM meteorite parent bodies [70]. They and the Allende CV parent body, which apparently did not accrete (much) ice, are from the same zone of the asteroid belt. Even more primitive asteroids closer to Jupiter still contain co-accreted ices, organic materials and silicate dust. They define the IR spectroscopic primitive (P)-and dark (D)-class [70] bodies that include comet nuclei and many near-Earth asteroids [10]. 

Hydrocarbons are also found in outer space. Asteroids and comets contain a variety of organic compounds. Methane and other hydrocarbons are found in the atmospheres of Jupiter, Saturn, and Uranus. Saturn s moon Titan has a solid form of methane—water ice at its surface and an atmosphere rich in methane. Whether of terrestrial or celestial origin, we need to understand the properties of alkanes. We begin with a consideration of their shapes and how we name them. 

Besides the planets and their satellites, the Solar System harbors a large number of smaller objects, ranging from hundreds of kilometers in size down to dust particles. If they consist of solid material and have at least the size of small boulders, they are called asteroids. If they enter the atmosphere of Earth and reach the surface, they are named meteorites. If they are very small and bum up on entry, they are referred to as meteors. If the bodies contain a substantial fraction of ices and develop tails as they come closer to the Sun, they qualify as comets. However, the classifications are not very consistent for example, remnants of the dust tails of comets cause meteor showers, and older comets, once they have expended most of their volatile matter during many passes near the Sun, may not be distinguishable from asteroids. This section is devoted to comets and the next section (7.3) deals with asteroids. 

Fig. 4.4 A comparison of the reflectivity of Amalthea, Thebe, Callisto and Asteroids. Amalthea and Thebe have reflective spectra similar to those seen in regions of CaUisto where there is little water ice black line). The dip in the spectrum around 3 pm indicates the presence of water containing minerals. Copyright Subaru Telescope, National Astronomical Observatory of Japan (NAOJ)...

Eucrites are achondritic stony meteorites that originate from the surface of the asteroid 4 Vesta. Die meteorite Serra de Mag6, an eucrite, contains quartz veinlets. They are identical to crack-seaT quartz veinlets in terrestrial rocks, and are extraterrestrial and ancient because they pre-date a 4.40 Ga metamorphism. The quartz was likely deposited from liquid water solutions (as are terrestrial veins). Because there is no indication of internal (magmatic) water in the eucrite meteorites and thus in Vesta, the water from which the veinlet was deposited probably came from outside Vesta. By analogy with water ice deposits on the Moon and Mercury, Vesta and similar asteroids may have had (or now have) polar ice deposits, possibly remainders from comet impacts (Treiman et al., 2004 [339]). 

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