The Uninhabitable Habitable Zone

Life on the Earth depends critically on a handful of "biogenic" elements, such as carbon, hydrogen, nitrogen, oxygen, phosphorous, sulfur, and others. These elements are abundant in the outer Solar System, but comparatively rare in the region of the terrestrial planets. McKay (1991) has illustrated this graphically for the case of carbon. Figure 6.1 shows the ratio of carbon atoms to heavy atoms (i.e., all atoms heavier than helium) throughout the Solar System. The inner Solar System is extremely depleted in carbon, compared to solar abundances. Moving outward from the Sun, it is not until we reach the C-type asteroids in the main asteroid belt that the carbon abundance begins to approach that of the Sun.

Yet it is only at heliocentric distances less than the distance to the asteroid belt that planetary surface temperatures are high enough for the existence of surface liquid water (although subsurface liquid water oceans may be common

C.F. Chyba and K.P. Hand, Comets and Prebiotic Organic Molecules on Early Earth. In: P.J. Thomas et al., Comets and the Origin and Evolution of Life, 2nd ed., Adv. Astrobiol. Biogeophys., pp. 169-206 (2006)

DOI: 10.1007/10903490.6 © Springer-Verlag Berlin Heidelberg 2006

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Objects In order of distance from the Sun

Fig. 6.1. Ratio of carbon (C) atoms to heavy (Z) atoms (atoms heavier than He) throughout the Solar System. The relative abundances of C in the Sun and in life on the Earth are shown for comparison. From McKay (1991).

on icy moons). Liquid water is the sine qua non of life as we know it (Horowitz, 1986; Mazur, 1980) and those terrestrial environments in which liquid water is most rare are those closest to being sterile. (For a review of what is now known about life in extreme environments on the Earth, including liquid-water-poor environments, see National Research Council, 2006.)

6.1.1 The Habitable Zone and Liquid Water

The circumstellar habitable zone is defined as the volume of space around a star (or around a multiple-star system) within which an Earth-like planet could support surface liquid water. An Earth-like planet is one comparable in mass to the Earth and having similar surface inventories of CO2, H2O, and N2 (Kasting et al., 1993; Chyba et al., 2000). The zone's inner boundary is that distance at which a moist or runaway greenhouse effect occurs (as in the case of Venus in our Solar System). The outer boundary is that distance at which CO2 condenses out of an atmosphere and is no longer available as a greenhouse gas (in our Solar System, somewhere around the orbit of Mars). In the context of the solar habitable zone, the distribution of biogenic elements presents an apparent dilemma: the elements essential to life are comparatively rare exactly where the temperatures make surface liquid water possible. (For the prospects for life in liquid water subsurface oceans on icy moons, see, e.g., Chyba and Phillips, 2002).

The dilemma worsens only when Solar System formation models are taken into account. The problem is that many of the biogenic elements are among the most volatile elements. Delsemme (1997; and this volume) has reviewed progress in thermochemical equilibrium and chemical kinetics models of the accretion disk out of which our planetary system formed. Such models have consistently found temperatures around 1,000 K in the region of terrestrial accretion, too high for the Earth to be provided with its observed inventories of water, nitrogen, and carbon (Cameron, 1983; Lewis, 1974; Lewis et al., 1979; Prinn and Fegley, 1989). Delsemme finds similar temperatures by using the transition between volatile-poor S-type asteroids and comparatively volatile-rich C-type asteroids to peg the accretion disk's temperature to 450 ± 50 K at 2.6 AU, then extrapolating inward to the region of the terrestrial planets. At temperatures near 1,000 K, water, carbon, nitrogen, and other volatiles remain almost entirely in the gas phase over the range of pressures likely to have existed in the accretion disk. Volatiles are therefore almost entirely absent from the dust grains out of which the planet-forming planetesimals aggregate. From the point of view of the biogenic elements, the habitable zone would seem, initially at least, uninhabitable. However, Campins et al. (2004) argue that with the Earth's total water inventory (including very uncertain quantities of water in the mantle) amounting to less than 1% of the Earth's mass, only small amounts of water would need be present in Earth-region planetesimals for a significant contribution to the Earth's water inventory to have been made. We face the dilemma that small deviations from average conditions could be sufficient to account for the terrestrial volatiles; this makes confident modeling of the origin of the oceans or other volatiles difficult.

6.1.2 Are the Earth's Oceans Extraterrestrial?

There are several plausible sources of water for early Earth, of which comets are but one. Wetherill (1990, 1994) showed that the latter stages of planetary formation were marked by collisions of large planetary embryos scattered across considerable heliocentric distances, and this result is borne out by subsequent simulations (Morbidelli et al., 2000). Thus, the Earth may have acquired an initial complement of volatiles though early radial mixing. The later stages of outer planet formation should have led to the scattering of volatile-rich planetesimals from the Jupiter-Saturn region (both asteroids and comets; Morbidelli et al., 2000; Campins et al., 2004), and comets from the Uranus-Neptune region (Fernandez and Ip, 1983; Shoemaker and Wolfe, 1984) and Kuiper belt (Morbidelli et al., 2000). Hartmann (1987, 1990) has argued that spectral observations of solar system satellites, as well as the preponderance of CM clasts among the foreign fragments in polymict meteoritic breccias, provide evidence for an intense scattering of C-type asteroids during the first ~108 years of solar system history.

But it is unclear whether volatiles accreted by the Earth prior to approximately 4.4 Gyr ago would have been retained. The hypothesized Moon-forming impact may have stripped the Earth of whatever terrestrial atmosphere existed prior to that event (Cameron, 1986). Terrestrial water present prior to core formation around 4.45 (~108 years after the Earth's formation; Stevenson, 1983; Stevenson, 1990; Swindle et al., 1986; Tilton, 1988) should have been efficiently destroyed by reacting with metallic iron according to Fe + H2O^FeO + H2; large quantities of hydrogen produced in this way may have removed other degassed volatiles by hydrodynamic escape (Dreibus and Wanke, 1987, 1989). It is possible that only those volatiles accreted by the Earth subsequent to core formation would have contributed to the Earth's extant volatile inventory. Icy planetesimals scattered from the accretion regions of Uranus and Neptune, with scattering timescales of hundreds of Myr, may in this case have been more likely contributors to the present terrestrial volatile inventory than icy planetesimals scattered from the jovian and saturnian regions, whose scattering timescales were perhaps as short as tens of Myr (Fernandez, 1985; Ip, 1977).

Isolated zircon crystals eroded from 4.3-to 4.4-Gyr-old igneous rocks (Mo-jzsis et al., 2001; Wilde et al., 2001) suggest that a hydrosphere may have existed on the Earth as early as 4.4 Gyr ago. Zircons imply that liquid water was involved in the genesis of their source rock, and oxygen isotopes in the zircon grains reveal that they formed in a crust that was interacting with a hydrosphere (Mojzsis et al., 2001; Wilde et al., 2001; see also Campins et al., 2004).

Comets are more than 40% water by mass (Delsemme, 1992); only approximately 10% of the observed cratering record on the Moon needs to have been cometary in origin for the Earth to have acquired its entire oceanic inventory of 1.4 x 1021 kg water from comet impacts (Chyba, 1987; Chyba, 1990). Estimates of water in the Earth's crust suggest that the crustal inventory of water is small compared to that in the oceans; however, it is possible that much more water is present in the Earth's mantle (Campins et al., 2004). Alternatively, if the heavy bombardment responsible for the observed lunar cratering record were primarily due to CI carbonaceous chondrites, the Earth could have acquired its oceans entirely from an asteroidal source (Chyba 1991; Chyba et al., 1994). It now seems likely that there were, in fact, multiple source regions for the Earth's oceans (Morbidelli et al., 2000; Campins et al., 2004).

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