Survival of organics in large impacts

Impact delivery of organics to Mars

E. Pierazzo, C.F. Chyba

Bull. A.A.S. 31(4), 1183, 1999.

31st Meeting of the Division for Planetary Sci., AAS (1999) Padua, Italy.

ABSTRACT

Apart from Earth, Mars is the most likely planet in our solar system to search for evidence of present or past life. In the case of Earth, in a recent study we concluded that impact delivery of organics, specifically amino acids, could have played an important role in the organic inventory of the early Earth. This was a result more optimistic with respect to organic impact survival than was found in much, though not all, previous work. Here we present the first results of an analogous study for Mars.
We carried out high resolution simulations of asteroid and comet impacts on the Martian surface, using the 2D finite differences hydrocode CSQ coupled to the ANEOS equation of state package. We modeled spherical asteroids (dunite and serpentine equation of state) and comets (ice equation of state) 2-km in diameter. Impact velocities range between 7.7 and 10 km/s for the asteroid simulations, and between 15 and 20 km/s for the comet simulations, covering both median and mean impact velocities for Mars-crossing asteroids and short-period comets respectively. Besides the lower impact velocities, the main difference with the terrestrial simulations are the lower gravity and lower surface temperature of Mars, as well as the absence of an ocean. A very thin atmosphere was included in the simulations for Mars, although it has been suggested that a thick atmosphere could have been present early in Martian history. Following the same procedure used in the study of impact delivery on Earth, temperature histories from Lagrangian tracers in the projectile were used in conjunction with known kinetic parameters for amino acids in the solid phase to calculate amino acid survivability during an impact event.
Analogous to our results for the Earth, the simulations suggest substantial survival for some amino acids in comet impacts on Mars. Asteroid impacts, however, do not seem to result in significant survival, even in the 7.7 km/s impact simulations.

COLOR FIGURES (To download GIF files click on the figures)

Temperature map of (a) peak shock temperature and (b) postshock (after 4 seconds) temperature inside the projectile and relative survival map i for a 1-km-radius comet impacting at 15.5 km/s on Mars. In the survival map, Asterisks indicate the initial position of Lagrangian tracers. The black-out region represents the part of the projectile (that is, 33.6% of the projectile volume) associated to tracers that reach escape velocity (left part of figure is a mirror image without black-out to show the complete survival map).
Estimated survival (and retained) of aspartic(glutamic) acid is 0.19(0.04)% of the initial concentration in the comet
(a surviving 0.53(0.23)% is lost to space).

3D HYDROCODE SIMULATIONS: OBLIQUE IMPACTS

Projectile volume reaching escape velocity for a comet 2-km in diameter impacting Mars surface at 15.5 km/s at various impact angles (measured from the surface), and relative survival for selected amino acids. Open symbols shows earlier estimates from the equivalent 2D hydrocode simulation (with CSQ).