Mars Balancing Factors

Mars is the most Earth-like of the other planets of the solar system with respect to possible environments for life, showing widespread evidence of both heat sources and water. The prospect that early Mars conditions may have been suitable for the origin of life is one of the main reasons for biological interest in Mars (e.g., McKay, 1997). Present knowledge on past Martian atmospheric and surface conditions support two possibilities: (1) a warm, wet early-Mars with a thick CO2-based atmosphere and a long-lasting greenhouse climate (e.g., Pollack, 1979; Pollack et al., 1987); (2) a cold, dry Mars, characterized by an almost endless winter broken by either volcanic or impact events causing subsurface water to melt (and evaporate) and run to the surface, for temporary periods of temperate climate (e.g., Segura et al., 2002). Both possibilities are conductive to liquid water being stable on the martian surface for some part of its history (although liquid water is not stable at the martian surface today). The organic material could have been endogenic in origin via a Miller-Urey synthesis in the atmosphere (although production would be reduced in a thin atmosphere), or in hydrothermal systems. However, overall endogenic production of organics on Mars may not have been as successful as on the Earth, especially in the case of a cold, dry Mars. Extraterrestrial delivery, thus, might have been very important for Mars.

An early study using 2D hydrocode simulations to investigate impact delivery on Mars (Pierazzo and Chyba, 1999b) indicated results similar to those for the Earth, that is a substantial survival for some amino acids in comet impacts on Mars with little or no survival in asteroidal impacts, even though the mean asteroidal impact velocity (9.3 km/s) is lower than that on the Earth (Pierazzo and Chyba, 1999b). However, Mars has a much lower escape velocity than the Earth, which increases the probability of projectile escaping after the impact. The 2D impact simulations showed that in vertical impacts significant escape of cometary material occurs even at the lowest impact velocity modeled, 12.5 km/s (Pierazzo and Chyba, 1999b). As found by Pierazzo and Melosh (2000), this effect should become even more important in oblique impacts, where a larger fraction of the initial impact energy is partitioned to the projectile.

To assess the importance of projectile escape with angle of impact, we have carried out 3D impact simulations with SOVA spanning different angles of impact, while keeping other input parameters constant (Pierazzo and Chyba, 2003). We model spherical comets 2 km in diameter impacting Mars' surface at 15.5 km/s (see Fig. 5.6). This corresponds to the median impact velocity for short-period comets on Mars according to the data from Olsson-Steel (1987). A very thin CO2 atmosphere was included in the simulations to model the present-day Martian atmosphere (although a thicker atmosphere could have been present early in Martian history, favoring retention of ejected material).

Around 1,000 Lagrangian tracers were regularly distributed inside the (model half-) projectile to follow its thermodynamic history during the impact. The Arrhenius equation (5.1) is integrated over time and over the projectile volume using the temperature histories from the projectile tracers. The fraction of amino acids successfully delivered to Mars in the impact event is then determined by integrating over the volume of the projectile that does not reach escape velocity.

The angle of impact affects significantly the postimpact dynamical evolution of the projectile. Table 5.4 and Figs. 5.4 and 5.5 summarize the results from this study compared to the earlier 2D simulation for the same cometary impact (Dpr=2 km; w¡mp=15.5 km/s) and normal incidence angle. In Table 5.4 the fraction of surviving amino acids contained in the fraction of the projectile that reaches escape velocity (and is considered lost) is shown in parentheses. Contrary to the earlier 2D numerical simulations, which showed a loss of 1/3 of the projectile, we find a much lower escape of projectile material in the 3D vertical impact simulation with SOVA. This discrepancy is a consequence of the switch from 2D to 3D numerical simulations (as discussed in the Hydrocode Simulations section). The lower impact temperature in the 3D simulation of a vertical impact (90°) results in a larger survival fraction for amino acids relative to the earlier 2D (CSQ) counterpart (Pierazzo and Chyba, 1999b). The 3D simulations also show that projectile escape increases rapidly with decreasing impact angle. For the most probable impact angle, 45°, almost half of the cometary material reaches Mars escape velocity, and well over 2/3 of the projectile escapes for impact angles below 30°.

Overall, Fig. 5.5 shows that the delivered fraction of surviving amino acid is significant for impact angles as low as 30° (which includes 75% of all cometary impacts on Mars). This is due to a balance between the increased loss of projectile material with decreasing impact angle and a weakening of the overall shock effects (e.g., Pierazzo and Melosh, 2000), which, in turn, increases the

Fig. 5.4. Projectile volume reaching escape velocity for a comet 2 km in diameter impacting Mars surface at 15.5 km/s at various impact angles. For comparison, the open diamond shows an earlier estimate from a 2D simulation (with CSQ).
Fig. 5.5. Survival for selected amino acids in impacts of a comet 2 km in diameter at 15.5 km/s as function of impact angle.

Table 5.4. Fraction of projectile that escapes from the target world (Vesc) and fraction (relative to the initial amount in the projectile) of amino acids surviving impact in 3D hydrocode simulations of comet (D=2 km) impacts on Mars and on the Moon, as a function of impact velocity (vimp) and impact angle. Results from an equivalent 2D simulation are also shown for comparison. In parentheses are surviving amino acids in the fraction of projectile that reached escape velocity.

Angle Vy

Ve.sc Glycine Aspartic acid Glutamic acid

Table 5.4. Fraction of projectile that escapes from the target world (Vesc) and fraction (relative to the initial amount in the projectile) of amino acids surviving impact in 3D hydrocode simulations of comet (D=2 km) impacts on Mars and on the Moon, as a function of impact velocity (vimp) and impact angle. Results from an equivalent 2D simulation are also shown for comparison. In parentheses are surviving amino acids in the fraction of projectile that reached escape velocity.

Angle Vy

Ve.sc Glycine Aspartic acid Glutamic acid

Mars 2D

15.5

33.6

0.11 (0.1)

0.5 (0.77)

0.65 (0.63)

90°

15.5

5.27

0.33 (0.09)

1.13 (0.19)

1.74 (0.35)

60°

15.5

18.5

0.52 (0.4)

1.1 (1.05)

2.0 (1.1)

45°

15.5

45.3

0.5 (2.1)

1.05 (5.0)

1.65 (5.3)

45°

12

35.2

1.9 (3.4)

1.4 (3.5)

4.8 (7.1)

45°

22

55.3

0.04 (0.5)

0.8 (3.9)

0.3 (2.1)

30°

15.5

69.4

0.3 (8.1)

0.9 (7.0)

1.0 (15.6)

15°

15.5

96.3

0.09 (38.9)

0.29 (6.7)

45°

20

69.9

0.02 (1.0)

0.42 (4.1)

0.16 (3.4)

overall amino acid survival (as discussed in Pierazzo and Chyba, 1999a). Below 30°, however, the fraction of surviving organics retained decreases rapidly. Grazing impacts, thus, are not a favorable condition for the delivery of a large amount of organics to Mars. According to our adopted escape criterion, in a 15° impact over 95% of the projectile quickly reaches escape velocity (Table 5.4), thus very little of the surviving organic material can be considered successfully delivered. On the other hand, in the case of lower impact angles (e.g., Fig. 5.6a), most of the projectile ends up moving downrange in the lower and denser part of the martian atmosphere, as shown in Fig. 5.6b. It is thus possible that some component of the projectile will be slowed down by the martian atmosphere before escape, and end up being retained on Mars. An accurate investigation of very low impact angles requires a more sophisticated and time-consuming modeling the hydrodynamic of two phases, which includes the interaction (momentum and heat exchange) of solid and molten particles with the postimpact gas flow (similar to the work presented in Stoffler et al., 2002).

2.00

# 'JA7

1: mm

0 10 20 30

Fig. 5.6. Outputs from 3D simulations (with SOVA) 2 s after impact for a comet 2 km in diameter impacting at 15.5 km/s at (a) 45° and (b) 15° from the surface. Different colors represent different materials: green = granite; yellow = atmosphere; gray = projectile. All colors are graded according to density variation. Tracers are represented as dots, colored according to peak shock pressure: red = 150-250 GPa; yellow = 100-150 GPa; green = 50-100 GPa; cyan = 30-50 GPa; blue = 18-30 GPa; magenta = 5-18 GPa; black = below 5 GPa.

Impact velocity affects impact delivery of organics in two ways: (1) by affecting the fraction of projectile escaping Mars gravity and (2) by affecting the impact energy, thus the intensity of the shock and the amount of postimpact internal energy available for expansion. We investigated the role of impact velocity for impacts on Mars by carrying out two more simulations of a 45° impact, bracketing the velocity range between 12 and 22 km/s, corresponding to 25% and 75% of the impacts on Mars, respectively (Olsson-Steel, 1987). The results are also shown in Table 5.4. The range in velocity results in an uncertainty of ±10% in the fraction of projectile escaping from Mars and affects the overall survival of organics. Over the range of velocities investigated, we find that the surviving fraction of amino acids delivered to the martian surface can vary by a factor of 2 for aspartic acid to about 50 for glycine.

A final integration of these results to estimate the overall delivery of amino acids to the Martian surface requires a more complete investigation of the role of impact velocity as well as of projectile size for varying impact angle.

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