Earth Significant Delivery

To assess the survival of amino acids in large impacts on the Earth, we carried out high-resolution 2D hydrocode simulations of asteroid and comet impacts with CSQ (Pierazzo and Chyba, 1999a). We model spherical asteroid and comet projectiles 2 to 10 km in diameter impacting both a continental crust and a 3-km-deep ocean. An impact velocity of 15 km/s, the median asteroid impact velocity with Earth, was used for asteroid impacts (Chyba, 1991). For comet simulations we used impact velocities of 15, 20, and 25 km/s. The latter is just above the median short-period comet impact velocity with Earth (Chyba, 1991). One hundred Lagrangian tracer particles were regularly distributed in half of the projectile to record the trajectories and thermo-dynamic histories of the projectile during the impact. The simulations show that organic survival is affected by various factors, such as projectile type and composition, size, impact velocity, and impact angle.

Projectile composition affects thermodynamic evolution, and therefore organic survival. Cometary material is easily shock vaporized, thus entering the expansion plume early in the impact event, whereas asteroidal material is mostly melted and remains at significantly higher temperatures for a longer time. As a result, in cometary material temperatures decrease to values well below 1,000 K relatively early, while asteroidal material experiences much higher temperatures (up to and above 2,000 K) for longer time (more than 4 s), resulting in virtually no amino acid survival.

To quantify the effect of impactor size on organic survival, we carried out comet impact simulations, using projectiles 2, 6, and 10 km in diameter at impact speeds of 20 km/s (Pierazzo and Chyba, 1999a). The shock experienced by the projectile does not differ much in the three cases. However, in a larger projectile the shock wave must travel farther to reach the rear of the projectile, where it is then reflected back as a rarefaction wave that unloads the material from the shocked state. At constant impact speed, the shock waves takes three to five times longer for the 6- and 10-km projectiles to reach the projectile's rear end than for the 2-km impactor. During this time the material is subject to extremely high temperatures, which are detrimental to amino acid survival (material near the rear of the projectile will experience a shorter shock pulse than material close to the impact point).

Impact velocity also affects organic survival: The higher the impact speed, the stronger the resulting shock, and relative pressures and temperatures experienced by the projectile. This, in turn, decreases the survival of organic matter. Figure 5.2 shows the surviving fraction of aspartic acid in a comet impacting at 20 km/s.

To investigate the effect of angle of impact, we took advantage of the scaling studies of Pierazzo and Melosh (2000). Through a series of high-resolution 3D hydrocode simulations, Pierazzo and Melosh (2000) concluded that projectile shock temperature scales with the sine of the angle of impact (0) to the 3/2 power, while the postshock temperature scales with the sine to the

0.8 power. A simple scaling of the projectile's temperature histories of the 2D simulations by (sinO)0'8 provides a conservative correction for impact angle (i.e., it overestimates the peak shock temperature for oblique impacts thus decreasing the probability of amino acid survival). Figure 5.3 shows the survival of selected amino acids for a 2-km-diameter comet impact at 20 km/s as function of 0. At 45°, the most probable angle of impact (Gilbert, 1893; Shoemaker, 1962), survival has increased over that for vertical impacts by a factor of 3 for aspartic acid, and by a factor of 5 for glutamic acid.

Fig. 5.3. Survival fraction for selected amino acids as a function of the angle of impact from the horizontal, using an ad hoc angle correction (see text). From Pierazzo and Chyba (1999a).

Among other factors that can influence the survival of organics in impacts are projectile inhomogeneities. Porosity is an important factor in generating inhomogeneities inside the impactor. For a porous material, extra work must be done to close the pores. Therefore, during an impact, more energy is partitioned to a porous projectile, causing higher peak and postshock temperatures. The simulation of a comet with 0.6 g/cm3 bulk density impacting at 25 km/s showed an increase of 50-75% in the average temperature of the projectile during the impact relative to the fully dense case. This, in turn, results in a decrease in survival of amino acids, which ranges between about 15% for asparagine and 40-50% for aspartic and glutamic acid.

Table 5.2. Fraction (relative to the initial amount in the projectile) of amino acids surviving impact in 2D hydrocode simulations of cometary impacts on the Earth. D = comet diameter; i>imp = impact velocity.

D l>imp (km) (km/s)

Glycine (%)

Aspartic acid (%)

Glutamic acid (%)

2

15

0.14

0.29

0.91

2

20

0.003

0.13

0.05

2

25

2 x 10-5

0.02

0.002

6

20

0

0

3 x 10-5

10

20

0

0

10~8

Table 5.2 shows the survival of few amino acids in comet impacts on the Earth. The conclusion that certain amino acids would survive even large cometary impacts is itself a remarkable result. To determine the quantitative importance of the potential cometary source, we compare resulting concentrations within the global ocean for certain amino acids resulting from Miller-Urey synthesis and cometary input. These concentrations are given by d[C]/dt = S/V - k[C] (5.2)

where [C] is the concentration (mol/l), S is the source term (mol/yr), V is the ocean volume (1.4 x 1021l), and k is the destruction rate (1/yr) for the amino acids. In the limit of large t, integration of Eq. (5.2) gives a steady-state value: [C];nf = S/(Vk). A variety of sinks can be considered (see discussion in Pierazzo and Chyba, 1999a). Following Stribling and Miller (1987), we use the circulation of the entire ocean through submarine vents at 300° C with a timescale of 107 years (Edmond et al., 1982) as the dominant sink for amino acids. Taking half the ocean to circulate through 300°C vents in 5 Myr gives k =1.4 x 10~8/yr.

In Pierazzo and Chyba (1999a), we used Schlesinger and Miller (1983) abundances of various amino acids for a [H2]/[CO2] =0.1 model atmosphere (it may be a value that greatly overestimates endogenous amino acid production, which falls off rapidly as [H2]/[CO2] drops below values for [H2]/[CO2] of ~0.01 on early Earth have been suggested; see, e.g., Kasting, 1993), as a source term for endogenous production of various amino acids on early Earth). Relative to glycine (=100), the abundance of alanine, aspartic acid, and glutamic acid are 7.0, 0.22, and 0.06, respectively. Integrating equation (5.2) yields values for [C] for aspartic acid, glutamic acid, and glycine

Table 5.3. Estimated oceanic amino acid concentrations from various sources (10-12 mol/l) 4 Gyr ago.

Source Glycine Aspartic acid Glutamic acid

Electrical discharge 2000 5 1 [H2]/[CO2] = 0.1 atm.

Steady-state cometary input 400 10 70

Low-angle 5-km-radius comet impact 30 0.7 4

shown in Table 5.3 (these are amino acids produced in detectable quantities in CO2 atmospheres that may be present in comets, by analogy with the Murchison meteorite, and have a nonnegligible survival in cometary impacts).

To determine the source term for cometary delivery to early Earth, we take initial concentrations in the Murchison meteorite for glycine, aspartic acid, and glutamic acid to be 37.8 (3 ppm by mass), 1.5, and 3.9 nmol/g, respectively (Cronin, 1976; see discussion in Pierazzo and Chyba, 1999a). These numbers are much lower than those listed in typical compilations of Murchison amino acid abundances (Cronin and Pizzarello, 1983; Shock and Schulte, 1990), because we make the conservative assumption that amino acid precursors in comets do not contribute to amino acids in the Earth's oceans. We multiply the Murchison amino acid concentrations by 10 to scale crudely for the fact that the overall organic fraction of comets is 10 times that found in carbonaceous chondrites (Chyba et al., 1990).

Following Chyba and Sagan (1992), we then determined the mass flux (based on the lunar record) of objects with masses between mmin and mmax impacting Earth 4 Gyr ago; mmin is equivalent to an asteroid 0.5 km in diameter (large enough to pass through the atmosphere unaffected; Chyba et al., 1993); mmax is given by an asteroid 6 km in diameter (above which our simulations show amino acid survival to be minimal). Finally, we make the assumption that only 10% of the mass flux is due to short-period comets. This assumption is consistent with previous modeling (Chyba et al., 1990) and current estimates (e.g., Kring and Cohen, 2002). The total cometary mass accreted by the Earth is thus 5 x 1011 g/year, conservatively assuming that about half of the mass from impacting comets would be lost from the atmosphere (Melosh and Vickery, 1989; Chyba, 1991). Integrated over 109 years, this rate would give 5 x 1020 g of cometary water delivered to the Earth, well below the upper limit of 1023 g now seemingly permitted by three observed cometary D/H ratios (Bockelee-Morvan et al., 1998). To determine the fraction of cometary amino acids that would survive impact, we integrated over comet radii (bins centered around the sizes simulated, therefore: 1-4 km, 4-8 km, 8-12 km), impact velocities (15, 20, and 25 km/s), and impact angles. Averages over impact angle were performed using the previously described temperature scaling with impact angle, weighted by impact angle probability (Gilbert, 1893; Shoemaker, 1962; see Pierazzo and Chyba, 1999a). Statistics for Earth impact velocities for short-period comets are poor, but suggest that 30% of impacts occur with velocities between 15 and 20 km/s and 30% of impacts occur at velocities between 20 and 25 km/s (Chyba, 1991). The surviving fraction of amino acids in each velocity bin was calculated by averaging survival for the velocity extrema of the bin, while impacts at velocities >25 km/s are taken to give zero contribution.

The estimated cometary contributions to oceanic concentrations, [C], are shown in Table 5.3, which compares the steady-state cometary input with Miller-Urey production rates. Concentrations are very low in both cases; to be credible for the origin of life, both cases must therefore appeal to concentration mechanisms in evaporating ponds or other special environments. Our results are based on the simple assumption that all surviving cometary amino acids are mixed into 1.4 x 1021 L of ocean water, giving a resulting amino acid molarity in the global ocean; therefore, regional or local instantaneous concentrations could be considerably higher. It is interesting in this context that Monnard et al. (2002) show that salt concentrations in Earth's oceans strongly interfere with some important prebiotic syntheses; they suggest that the origin of life on the Earth was therefore more likely to have occurred in freshwater environments than in the oceans.

Table 5.3 also displays results for the unlikely, but probably nevertheless realized, case of a single 5-km-radius comet striking the surface of the ocean at 20 km/s with an inclination of 5° to the horizontal (about 0.5% of all incident comets should have struck Earth at or below this inclination). This simulation of a special case is in the spirit of Clark's suggestion (Clark, 1988) that rare fortuitously low-angle cometary collisions may have been important for the origin of life. For this particular case, our simulations indeed show substantial amino acid survival, often in the tens-of-percent range; a single object may thereby itself deliver an abundance of certain amino acids comparable to that due to background production or delivery. Thus there may have been occasional periods of duration approximately 107 years with oceanic concentrations of certain amino acids above that of the steady-state cometary value.

These results suggest the possibility that the apparently extraterrestrial amino acid AIB found at K/T boundary sites could indeed have been directly delivered by the K/T impactor. The concentrations detected (Zhao and Bada, 1989) could be provided by a 10-km-diameter comet provided that much of the AIB present within the object survived impact. (This assumes cometary amino acid levels at least 10 times that of Murchison. Intriguingly, sample A91 suggests that some micrometeorites have AIB concentrations 100 times higher than Murchison; Brinton et al., 1998.) In the absence of thermal degradation parameters in the solid state (Rodante, 1992) for AIB, this problem cannot yet be addressed quantitatively. However, the results described here suggest that this scenario should be considered possible.

Additional endogenous sources for amino acids may also have existed. For an early atmosphere with CO2 about 100 times the present atmospheric level, and a methane flux equal to the modern abiotic hydrothermal flux (Chang, 1993), the HCN rainout rate via the production of N from solar Lyman alpha photolysis of N2 in the middle atmosphere could have been 1 x 10-2 nmol/cm2/yr, or about twice that produced by electrical discharge in a [H2]/[CO2]=0.1 atmosphere (Zahnle, 1986; Chang, 1993). Substantially higher HCN production would have occurred for correspondingly higher CH4 fluxes (Zahnle, 1986). It is clear that there are substantial uncertainties in estimates for both exogenous and endogenous sources of organics, as well as the dominant sinks. All of the likely mechanisms described here lead to extremely low global concentrations of amino acids, emphasizing the need for substantial concentration mechanisms or for altogether different approaches to the problem of prebiotic chemical synthesis.

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