Environmental effects of large impacts on Mars

Science

Volumn 298, Issue 5600, 2002, Pages 1977-1980

  Segura, Teresa L ^a,b   Toon, Owen B ^a   Colaprete, Anthony ^b   Zahnle, Kevin ^b  

^a UNIVERSITY OF COLORADO   (United States)

^b NASA AMES RESEARCH CENTER   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

AQUIFERS; COMPUTER SIMULATION; ENVIRONMENTAL IMPACT; EXTRATERRESTRIAL ATMOSPHERES; FREEZING; ICE; MELTING; SOLAR SYSTEM; WATER;

IMPACTOR SIZES;

MARTIAN SURFACE ANALYSIS;

ENVIRONMENTAL EFFECT; IMPACT; MARS; PLANETARY ATMOSPHERE; VALLEY;

ARTICLE; ASTRONOMY; ATMOSPHERE; ENVIRONMENTAL CHANGE; EVAPORATION; FREEZING; GREENHOUSE EFFECT; MELTING POINT; PRECIPITATION; PRIORITY JOURNAL; RIVER; SIMULATION;

ATMOSPHERE; CARBON DIOXIDE; COMPUTER SIMULATION; EXOBIOLOGY; EXTRATERRESTRIAL ENVIRONMENT; ICE; LIFE; MARS; MINOR PLANETS; TEMPERATURE; WATER;

ASTEROIDEA;

EID: 0037032808     PISSN: 00368075     EISSN: None     Source Type: Journal    
DOI: 10.1126/science.1073586     Document Type: Article

References (32)

1. W. K. Hartmann et al., in Origin of the Earth and Moon, R. M. Canup, K. Righter. Eds. (Univ. of Arizona Press, Tucson, AZ, 2000), pp. 493-512. These models indicate that the heavy bombardment period lasted from about 3.8 to 4.3 billion years ago.
2. B. A. Cohen, T. Swindle, D. Kring, Science 290, 1754 (2000) and references therein. Studies of impact ages reveal that the late bombardment, whatever its duration, was felt through the inner solar system, Mars included.
3. K. L. Tanaka, D. Scott, R. Greeley, in Mars, H. H. Kieffer, B. Jakosky, C. Snyder, M. Matthews, Eds. (Univ. of Arizona Press, Tucson, AZ, 1992), pp. 345-382.
4. H. V. Frey, J. Roark, K. Shockey, E. Frey, S. Sakimoto, Geophys. Res. Lett. 29, 10.1029/2001GL013832 (2002).
5. 3. Such an object would produce a total (pulverized + global rock rain) ejecta layer of about 8 m. This is about as thick as the layer for an asteroid (Fig. 1) because most of it originates from the target material.
6. 6 J/kg for Qv (energy for complete vaporization) and Qv* (energy for incipient vaporization), respectively. These calculations are velocity-dependent; we have chosen an impacting velocity of 9 km/s.
7. E. Pierazzo, A. Vickery, H. Melosh, Icarus 127, 408 (1997).
8. K. Lodders, B. Fegley Jr., The Planetary Scientist's Companion (Oxford Univ. Press, New York, 1998). Types CM and CI carbonaceous chondrites contain 12 to 18% water, respectively.
9. 2 H, where we take the porosity of the regolith to decrease exponentially with depth with a scale height, H, of 2.8 km (29), and where the porosity at the top of the regolith, φ, is 0.2 and the regolith pore space is filled with water.
10. 11 erg/g is a characteristic latent heat of vaporization for rock. The current martian ice caps cover 1.7% of the planet to an average depth of 1.5 to 2 km (30).
11. K. Zahnle, Spec. Pap. Geol. Soc. Am. 247, 271 (1990).
12. 2 is assumed to be 150 mbar (13). Relative humidity at each level is calculated and as the injected water condenses and rains out, we replace it with a radiatively inert gas to maintain hydrostatic equilibrium. The 30-layer subsurface model solves explicitly for temperature using the diffusion equation. When the temperature in a layer exceeds 273 K, the water melted is calculated and tabulated until that layer is dry. Additional materials and methods are available as supporting material on Science Online.
13. 2 pressures of bars may have occurred on early Mars, Brain and Jakosky have argued that the pressure remaining near the end of heavy bombardment was in the range from 30 to 600 mbar, depending on sputtering and impact erosion. 150 mbar of carbon dioxide only warms Mars a few degrees above its temperature of thermal emission to space, which was about 198 K.
14. W. V. Boynton et al. [Science 297, 81 (2002)] used data from the Gamma-Ray Spectrometer onboard the Mars Odyssey to estimate that the current middle- and high-latitude martian regolith may be as much as 35% (± 15%) water by weight, with a 40-cm dry (1% water) cap. We have chosen the lower limit of 20% for our simulations, with a 40-cm (0% water) dry cap to represent global average conditions. We recognize that these numbers pull away from our otherwise conservative approach, because Boynton et al. found these numbers to be for the polar regions and not global. However, we note that a colder Noachian Mars probably had a much shallower equatorial ice depth than at present. It is possible that local conditions were much dryer or wetter than these conditions. Runs of our model with a dry (1%) regolith confirm that the water evaporated from the target material and melted in the subsurface (Fig. 4) scales linearly with the water fraction. These two values dominate the curve, so if 1% water, for example, were more correct, the total water precipitated and melted would be 20 times less than our numbers, but there are still plenty of large events in the crater (visible and buried) record to generate the required water for valley formation.
15. M. C. Malin, Geophys. Res. Lett. 13, 444 (1986).
16. ρ is the specific heat of water, λ is the permeability of rock, h is the height over which the temperature differences occur, η is the viscosity of water, and κ is the thermal conductivity of water. Our impact events have Rayleigh numbers of less than the threshold, so convection is not important. To fully estimate the importance of convection, we would need better information on the porosity of the martian regolith at depth, and so we have not included convection in our model. The most likely scenario for extensive groundwater flow would involve an impermeable capping layer, which would suppress such convection.
17. M. H. Carr, Water on Mars (Oxford Univ. Press, New York, 1996).
18. C. B. Leovy, J. C. Armstrong, in preparation.
19. M. H. Carr, M. C. Malin, Icarus 146. 366 (2000).
20. B. M. Jakosky, Icarus 55, 1 (1983).
21. M. P. Golombek, N. T. Bridges, J. Geophys. Res. Planets 105, 1841 (2000).
22. J. B. Pollack, J. Kasting, S. Richardson, K. Poliakoff, Icarus 71, 203 (1987).
23. F. Forget, R. T. Pierrehumbert, Science 278, 1273 (1997).
24. 2 ice clouds were considered, which is not enough to heat the planet above 273 K.
25. R. J. Phillips et al., Science 291, 2587 (2001).
26. J. F. Kasting, Icarus 91, 1 (1991).
27. P. R. Christiansen et al., Science 279, 1692 (1998).
28. M. C. Malin, K. S. Edgett, Science 290, 1927 (2000).
29. The energy supplied by an impactor, added to the energy from the Sun, places Mars into the runaway greenhouse regime of several steady-state models. Given a sufficiently rapid supply of groundwater to the atmosphere, a temporary runaway might have begun. The magnitude of such an event is beyond the scope of this work to determine.
30. D. E. Smith et al., Science 284, 1495 (1999).
31. K. Zahnie, J. Pollack, J. Kasting, Geochim. Cosmochim. Acta 54, 2577 (1990).
32. The authors thank the University of Colorado's Center for Astrobiology (grant NCC2-1052), the NASA Astrobiology Institute through Ames Research Center, and NASA (grant NAG5-6900) for support for this research.