Europa Impactor Loss

Strong evidence that a liquid water ocean exists beneath the surface of Jupiter's moon Europa (Pappalardo et al., 1999; Kivelson et al., 2000; Chyba and Phillips, 2006) fuels speculation about the possibility of life on that world and resulting plans for future spacecraft exploration (Chyba and Phillips, 2001; 2006). But too few biology-related characteristics of Europa are currently understood for firm conclusions to be drawn. We do not know how much organic material is present in its ocean. Even the inventory of elements needed for life (especially the "biogenic" elements C, H, N, O, P, S) in Europa's ocean is nearly entirely unknown. A bulk carbonaceous chondrite (CI) composition is often assumed (e.g., Kargel et al., 2000). However, Europa's formation conditions within the circum-Jovian nebula (Lunine and Stevenson, 1982; Prinn and Fegley, 1989) are poorly known. Thermochemical models predict that the most stable phase for carbon in the proto-Jovian nebula is CH4, in contrast to CO in the solar nebula (e.g., Prinn and Fegley, 1989). In the region where Europa formed the temperature may have been too high for CH4 to condense out (e.g., Lunine and Stevenson, 1982), pointing to a carbon-depleted Europa. Unless the bulk of proto-Europa originated from solid planetesimals that accreted in the solar nebula without undergoing further modification, it is difficult to feel confident in a bulk CI composition for Europa, at least for C. This suggests that Europa might have formed largely bereft of organic material.

We investigated the exogenous contribution to Europa's biogenic inventory through high-resolution 2D simulations with CSQ (coupled to ANEOS) to model cometary impacts on Europa (Pierazzo and Chyba, 2002). More than 90% of the craters on the Galilean satellites are estimated to be due to impact by Jupiter-family comets (Zahnle et al. 1998), with long period comets and Trojan asteroids making up the rest of impactors. The distribution of the cumulative impact velocity given by Zahnle et al. (1998) suggests that the median impact velocity of these comets on Europa is around 26.5 km/s. We modeled cometary projectiles 0.5 and 1 km in diameter, and impact velocities of 16, 21.5, 26.5, and 30.5 km/s. Based on Zahnle et al. (1998), these velocities correspond to the 10, 25, 50, and 75% levels (i.e., 10% of impacts occur at impact velocities below 16 km/s). The typical density of comets is unknown. Estimates of cometary nuclei densities in the literature have ranged between 0.2 and 1.2 g/cm3 (Rahe et al., 1994). To assess the effect of porosity, we used cometary densities of 1.1, 0.8, and 0.6 g/cm3, which correspond to porosities of 0, 27, and 45%, respectively. The latter value of 0.6 g/cm3 corresponds to the best estimate of comet Shoemaker-Levy 9's density, based on tidal physics (Asphaug and Benz, 1996).

Europa's surface was treated as pure nonporous ice with a surface temperature of 110 K and a temperature gradient of 32 K/km (equivalent to the linear temperature gradient of an ice shell about 5 km thick). Temperature gradients in Europa's ice shell depend not only on ice thickness but on tidal heating within the shell and whether the ice is convecting or conducting (Chyba et al., 1998); however, these uncertainties for the upper few kilometers of the ice (the depth relevant to the impacts simulated here) do not strongly affect the fate of the projectile, which is the focus of this study.

The velocity evolution recorded by the tracers allowed us to determine the percentage of the postimpact projectile that would exceed Europa's escape velocity (about 2 km/s); according to our adopted escape criterion, we treat all escaped material as gone from Europa forever, consistent with estimating a lower limit for accreted mass. While some of this material might be subsequently reaccreted by Europa, much of it could be lost because of ionization and subsequent acceleration in Jupiter's magnetic field. Because CSQ overestimates shock temperatures and underestimates shock pressures, the results of this study should be considered conservative.

The results of the various simulations, shown in Table 5.5 and Fig. 5.7, indicate that a large fraction of most impacting comets reaches escape velocity after colliding with Europa's surface. For a nonporous comet some fraction of the projectile is retained by Europa even at the highest impact velocity modeled, 30.5 km/s. However, for porous comets a significant fraction of the projectile is retained only for impact velocities below 16 km/s. Tracers from porous projectiles have a consistently higher velocity, suggesting that comet porosity strongly affects the fraction of material that escapes after the impact. Porosity affects the partitioning of the impact energy between target and impactor (extra PdV work must be done to close the pore in the projectile). The higher the projectile's porosity the more impact energy is partitioned to it rather than to the target, causing a hotter and faster expansion of the vaporized projectile material. (At the velocities of Jupiter-family comets on Europa nearly all of the comet is vaporized in the impact.) Overall, the retention of a nonnegligible amount of the impactor differs from the conclusions of Melosh and Vickery (1989) for planetary escape of impact-produced vapor. Their intentionally simple analytical model indicates that at typical Europan impact velocities (above about 5 km/s) all of the postimpact vapor plume, which for an icy impactor includes the vaporized projectile, should exceed escape velocity. Our hydrodynamic simulations suggest instead that significant retention,

Table 5.5. Fraction of projectile that escapes from Europa (Vesc) and fraction (relative to the initial amount in the projectile) of amino acids surviving impact in 2D hydrocode simulations of impacts of comets 1 km in diameter on Europa. ppr is the projectile density; vimp= impact velocity.

ppr vimp Ve.sc Glycine Aspartic acid Glutamic acid (g/cm3) (km/s) (%) (%) (%) (%)

1.1

16

15.9

0.86

0.83

3.50

1.1

21.5

44.9

0.02

0.54

0.27

1.1

26.5

69.4

5 x 10~4

0.31

0.02

1.1

30.5

80.1

10-5

0.13

0.002

0.8

16

46.9

0.76

0.75

2.36

0.8

21.5

88.7

0.001

0.12

0.02

0.8

26.5

100

-

-

-

0.6

16

73.3

0.83

0.45

0.61

0.6

21.5

100

-

-

-

at the 10% level, of impactor material occurs for impacts at the lower end of the velocity distribution of Jupiter-family comets on Europa.

Impact angle is another factor that influences the retention of impactor material. The dramatic increase in organic survivability with decreasing impact angle (Pierazzo and Chyba, 1999a) is counteracted by the increase in projectile material ejection velocity for decreasing impact angle. This effect is negligible for impact angles above 60°, and becomes significant below that. On Europa, where the escape velocity is very low, projectile escape dominates increased survival, as even in a vertical impact over 2/3 of the projectile escapes for median impact velocities. We thus extrapolated to Europan impacts by assuming that for impact angles below 60° all of the projectile will escape Europa's gravity, regardless of impact velocity. According to impact probability theory, 25% of randomly distributed impactors will strike within this angle of the normal (Shoemaker, 1962); the results from the 2D simulations can then be generalized to represent this fraction of near-vertical impacts.

Using the estimates of Zanhle et al. (1998; 1999) and Levison et al. (2000), we estimated an Europan average impactor flux of 5.5 x 107 g/year (see Pierazzo and Chyba, 2002). Holman and Wisdom (1993) argued that the comet

Fig. 5.7. Fraction of projectile material reaching Europa's escape velocity in 2D impact simulations (with CSQ) as a function of projectile bulk density (x-axis) and impact velocity (various lines/symbols) for a comet 1 km in diameter. From Pierazzo and Chyba (2002).

flux has decreased with time due to the depletion of the Kuiper belt. Integrating the current flux backwards in time with this assumption, the integrated flux over 4.4 Gyr is 8.2 x 1017 g, 3.4 times greater than if the flux had been constant (Pierazzo and Chyba, 2002). The total cometary mass accreted on Europa over 4.4 Gyr, Macc, assuming a given comet density is thus given by where F(0) = 0.25 is the fraction of comets impacting within 30° of the normal; F(< vi) is the fraction of comets impacting with velocities in that velocity bin (e.g., 10% for vi <16 km/s; 15% for 16 < vi < 21.5 km/s; and so on); and Fp is the fraction of the projectile that is retained on Europa, given in Table 5.5. For a 1.1 g/cm3 comet density, the total cometary mass accreted over 4.4 Gyr is: Macc = 8.2 x 1017 x 0.25 x [0.1 x 0.84 + 0.15 x 0.55 + 0.35 x 0.30] = 6.0 x 1016 g. This decreases to 1.4 x 1016 g for a 0.8 g/cm3 comet density, and down to 5 x 1015 g for a comet density of 0.6 g/ cm3.

The fraction of surviving amino acids delivered in the impact is determined by integrating Eq. (5.1) over the component of the projectile that does not exceed Europa's escape velocity. We obtain substantial survival fractions for some amino acids at all the impact speeds modeled. However, because a large

fraction of the projectile is lost to space, only part of the surviving organics will be deposited on the surface of Europa (Table 5.5). We also find that the survival of the retained amino acids increases from 30% for a nonporous comet 1 km in diameter to about 55% for a nonporous comet 0.5 km in diameter. However, when integrated over the mass of the projectile, the overall contribution to the Europan inventory is about the same for the two cases, indicating that smaller comet sizes are not any more favorable than the case examined. In the end, the total amount of amino acids delivered to Europa's surface is several orders of magnitude lower than that estimated for Earth. While some fraction of these organics may be cycled into the ocean, some may be lost by sputtering, but much may be mixed into the regolith by impact gardening (Chyba and Phillips, 2001; 2006).

Although delivery of complex organics is inefficient, cometary impact could still prove important for providing, over the age of the Solar System, an inventory of biogenic elements that may be scarce on Europa. We used mass abundances for C (16.8 wt%), N (4.3 wt%), and S (2.8 wt%) estimated for comet Halley for a dust/gas ratio of 0.8 (Delsemme, 1991). The (as yet unknown) cometary phosphorus abundance (taken to be 0.5 wt%) is estimated by scaling from the C abundance assuming a CI-chondrite P/C ratio (Lodders and Fegley, 1998). Cometary accretion by Europa over 4.4 Gyr should have provided between 0.9 and 10 Gt of C (where 1 Gt=1015 g), 0.2-3 Gt of N, 0.2-2 Gt of S, and 0.02-0.3 Gt of P (Pierazzo and Chyba, 2002). This material would be initially deposited on Europa's surface after the initially vaporized projectile condenses out. The young appearance of Europa's icy surface suggests a geological resurfacing timescale of about 50 Myr (Zahnle et al., 1998; Levison et al., 2000). If resurfacing leads to the biogenic elements accumulated on the surface being recycled to the ocean beneath, cometary delivery could provide the raw material to support a biosphere on Europa. (Direct delivery of projectile material, with its inventory of biogenic elements, to the Europan ocean is highly improbable even if the projectile could plunge through the moon's several km-thick icy surface - Turtle and Pierazzo, 2001 - since most of the projectile is vaporized by the impact and entrained in the expansion plume.) These values are about eight orders of magnitude lower than those given by an assumption of a CI bulk composition for Europa (Kargel et al., 2000). Alternatively, they may be compared to estimates of the prokaryotic biomass in the Earth's oceans today (Whitman et al., 1998), which contains 300 Gt of C, 70 Gt of N, and 8 Gt of P. Since Europa's ocean is expected to have a mass about twice that of Earth's oceans (Chyba and Phillips, 2001), this suggests that even if comets were the only source of biogenic elements to Europa's oceans, microorganism densities could in principle be as high as —1% that of the Earth's oceans, assuming otherwise favorable conditions.

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