Environmental Effects of Large Impacts on Mars

The global killing mechanism on Mars, like on the Earth, is heat. However, the smaller size of Mars, its lack of a significant ocean, and its place in the solar system make for significant differences.

The lower gravity of Mars and the lower approach velocities of asteroids cause the typical collision velocity on Mars to be lower than that of the Earth. This changes the nature of typical impact ejecta. The lower impact velocity (~10/km/s vs. ~17 km/s) means that the hottest ejecta are forged at energies some three times lower than on the Earth; therefore, less rock vapor forms by the collision itself. Indeed at 10 km/s one expects little vaporization. A second reason is that the lower gravity means that the ejecta carry less specific energy, while the most energetic ejecta escape. For a high velocity impact the specific energy goes as the ratio of the square of the escape velocities. (In a low velocity impact the highest ejection velocities may be significantly less than escape velocity, in which case the event is more localized and the effects on the two planets more alike.) Much of the ejecta from the biggest impacts on Mars is ballistically and unevenly distributed as hot melt, rather than hy-drodynamically distributed more or less evently over the entire planet as rock vapor (Sleep and Zahnle, 1997). In a big impact there is not enough time for the ejecta to radiatively cool in transit, so that reentry (either aerobraking in the atmosphere or lithobraking at the surface) further heats already hot ejecta. Near the impact basin the molten ejecta behave like a large lava flow, ponding in low spots while leaving high spots sparsely covered. More distal ejecta can evaporate. For example, ejecta launched at half the martian escape velocity carry approximately 3 x 1010 ergs/g of kinetic energy, which corresponds to a 2,000 K temperature rise in an already hot material. Although relatively little of the ejecta evaporates, the rock vapors can take up much of the ejecta's energy at points far from the impact site.

From the perspective of life's survival there (given that there was life there), ancient Mars differed from the Earth in at least two important ways. One is that the small amounts of surface water on Mars provided little thermal buffering. An Imbrium-sized projectile would have left the planetary surface covered with on average ~10 m of molten rock. The surface temperature would have been briefly very high. We expect that photosynthetic and other obligate surface organisms would have perished.

A second difference is that molten ejecta, rather than a long-lasting steam atmosphere, governed the duration, depth, and magnitude of subsurface heating. Figure 7.8 follows the propagation into the surface of the heat pulse delivered by a 200-m blanket of molten ejecta. This is the global average generated by a Hellas caliber impact (in the real Hellas the ejecta blanket would be kilometers thick near the basin and on average ^100 m thick over many of the more distant provinces, and probably very patchy if rayed craters provide a useful guide). The Hellas impactor had about 1/3 the mass and released about 1/10 the energy of the hypothetical terrestrial ocean-evaporating event. For the illustration the ejecta pile is given an initial temperature of 1,300°C. The strongly heated region below the original surface is about 50% thicker than the ejecta layer itself. A background heat flow of 0.05 Wm~2 is assumed. This is equivalent to the heat flow through 100 Ma ocean crust on the Earth; one expects a similar heat flow on Mars through a basaltic crust of similar age. The ancient age of the martian meteorite ALH-84001 implies that stable regions like this have existed on Mars once the planet had aged 100 Myr.

Figures 7.9 and 7.10 show maximum temperatures underground as a function of heat flow for 200- and 500-m ejecta blankets. The figures are analogous to Fig. 7.7 for the Earth. The effects produced by 200 m of hot ejecta resemble

Fig. 7.8. Propagation of the thermal pulse produced by a Hellas-sized impact at a distal location on Mars. Times are marked in thousands of years. The initial condition is a hot (1,300°C) 200-m-thick blanket of impact ejecta and a 1,300°C surface temperature; the surface temperature cools to 0°C over the next 25 years. The heavy dashed lines denote the background temperature gradient maintained by a heat flow of 0.05 mW/m2 through basalt. The dashed lines have the same meaning as in Fig 7.6.

Fig. 7.8. Propagation of the thermal pulse produced by a Hellas-sized impact at a distal location on Mars. Times are marked in thousands of years. The initial condition is a hot (1,300°C) 200-m-thick blanket of impact ejecta and a 1,300°C surface temperature; the surface temperature cools to 0°C over the next 25 years. The heavy dashed lines denote the background temperature gradient maintained by a heat flow of 0.05 mW/m2 through basalt. The dashed lines have the same meaning as in Fig 7.6.

those produced by evaporating the oceans of the Earth while the 500-m case is much worse.

Differences between Mars and the Earth influence how Figs. 7.8-7.10 should be interpreted. Heat flow is generally lower on the Mars, roughly in proportion to surface gravity. Lower surface gravity on Mars also means that porosity extends commensurately deeper under the surface; this means that, other things being equal, habitable subsurface environments extend 2.6 times deeper than on the Earth. A third point is that Figs. 8-10 presume a 0°C surface before the impact, appropriate to a warm, wet, habitable early Mars. This is a self-consistent choice, given that we are considering the negative effects of impacts on the inhabitants of a habitable early Mars. But given the faint young Sun, a lower surface temperature may seem more likely. To first approximation, save near the surface itself, Figs. 8-10 can be rescaled by adding the original surface temperature in degrees C. A preimpact surface temperature of -60°C would change the "thermophile" fields in Figs. 9 and 10 into "mesophile" fields, and change the "mesophile" fields into "cryophile" fields. Whether a Mars with a -60 °C steady-state surface could have a living subsurface biosphere is a question one may justly ask.

Fig. 7.9. The highest temperature (Celsius) reached beneath the surface after a Hellas-scale impact on Mars. The crustal age corresponding to a heat flow is also given. In general, results resemble those generated by an ocean-vaporizing impact on the Earth, although it should be remembered that Mars is expected to be porous to greater than the Earth in inverse proportion to its gravity, so that a depth of 1 km on Mars is roughly comparable to a depth of 375 m on the Earth. Also, save near the surface, the temperature fields can be rescaled to higher or lower temperatures according to the steady-state surface temperature of Mars. A normally very cold surface (say -60°C) lowers the temperatures at depth by up to 60°C. Thus the martian subsurface can better protect mesophiles.

Fig. 7.9. The highest temperature (Celsius) reached beneath the surface after a Hellas-scale impact on Mars. The crustal age corresponding to a heat flow is also given. In general, results resemble those generated by an ocean-vaporizing impact on the Earth, although it should be remembered that Mars is expected to be porous to greater than the Earth in inverse proportion to its gravity, so that a depth of 1 km on Mars is roughly comparable to a depth of 375 m on the Earth. Also, save near the surface, the temperature fields can be rescaled to higher or lower temperatures according to the steady-state surface temperature of Mars. A normally very cold surface (say -60°C) lowers the temperatures at depth by up to 60°C. Thus the martian subsurface can better protect mesophiles.

Was this article helpful?

0 0

Post a comment