Survival

Once the oceans are vaporized, it takes little additional energy for the surface temperature to reach the melting point of rock. Yet there might remain possible refugia hidden deep below the surface, thermally insulated by several hundred meters of rock and sediment. The approximate depth of the extinc

3 In the usual runaway greenhouse, as applied to ancient Venus or future Earth, the planet absorbs more sunlight than the atmosphere can radiate, and so the oceans evaporate.

tion horizon can be estimated by considering the propagation of the thermal pulse into the Earth.

We present a model based on the work of Sleep and Zahnle (1997) to illustrate the propagation of the heat pulse from an ocean-boiling impact on the Earth. We consider a subsurface oceanic environment. The heat flow from below is 0.125 W m~2, approximately that through 16 Ma oceanic crust. We intend this to represent a relatively safe environment on the early Earth. The initial condition boils off the ocean and buries the original surface under 300 m of warm (300°C) ejecta. The surface temperature is set to 1300°C. Thereafter it is presumed to decrease linearly to 400° C over the next thousand years and then to 0°C over the next 3000 years.

Figure 7.6 shows the propagation of the thermal pulse into the crust. At 1,000 years temperatures have increased significantly down to 600-m depth.

Fig. 7.6. Propagation of the thermal pulse produced by an ocean-vaporizing impact into the oceanic crust at a point far away from the impact. Times are marked in thousands of years. The initial condition is a warm (300° C) blanket of impact ejecta and a 1300°C surface temperature; the surface temperature cools to 0°C over the next 4,000 years. The heavy dashed lines denote the background temperature gradient maintained by a heat flow of 0.125 mW/m2 through basalt. (Equivalent would be a heat flow of 0.04 mW/m2 and a thermal conductivity appropriate to deep sea sediment.) The lower line is the initial (pre-impact) condition, the upper line shows the asymptotic state when the thermal pulse has died away.

Fig. 7.6. Propagation of the thermal pulse produced by an ocean-vaporizing impact into the oceanic crust at a point far away from the impact. Times are marked in thousands of years. The initial condition is a warm (300° C) blanket of impact ejecta and a 1300°C surface temperature; the surface temperature cools to 0°C over the next 4,000 years. The heavy dashed lines denote the background temperature gradient maintained by a heat flow of 0.125 mW/m2 through basalt. (Equivalent would be a heat flow of 0.04 mW/m2 and a thermal conductivity appropriate to deep sea sediment.) The lower line is the initial (pre-impact) condition, the upper line shows the asymptotic state when the thermal pulse has died away.

The safest depths are around 1 km, where temperatures of around 90° C persist for 104 years. Only thermophiles could have survived.

After the surface temperature returns to normal (4,000 years in this example), the sterilized but clement surface is at first separated from the deeper habitable region by a region of high temperatures. At this time any mesophiles that survived can reclaim the surface. After 6,000 years committed ther-mophiles can cross the hot zone to reach the surface. The timescale is long enough for microbes to die selectively, but it is quite short for any biological innovations. We expect that microbial diversity remained low for a geologically significant time after such an impact.

Figure 7.7 generalizes the considerations addressed by Fig. 7.6 to different heat flows. The figure also indicates the corresponding age of the oceanic crust. We plot the maximum temperature reached after the impact at any depth for the minimal ocean-vaporizing impact. This gives a broader picture of the subsurface thermal effects. Only for low heat flows appropriate to old crust can mesophiles survive. They survive best at depths of around 2 km, where a 2 x 1035 erg impact has little effect on temperatures. In the range of heat flows appropriate for the early Earth (unknown; our estimate is shaded) thermophiles survive the heat pulse.

If photosynthetic organisms were to survive on the Earth, they would need to be mixed to the bottom of the ocean (by tsunamis, say) and rapidly buried by several hundreds of meters of ejecta. They must not get crushed, eaten, or poisoned in the process. They would also need to have been buried in the midst of abundant food, chemicals both oxidized and reduced, and yet not be able to access the food so quickly that they could eat all of it while buried for millenia. They must retain the latent ability to photosynthesize in the face of competition from creatures that had long ago discarded the useless skill. They would then need to be exhumed by erosion, but not so soon that they would be scalded by hot rain as the oceans rescondense. Finally, they would need to find a suitable environment where they could grow, presumably without the aid of those other creatures that had played important roles in the parent ecosystem. To meet any of these requirements would be lucky; to meet them all might be asking too much of luck.

Perhaps the best strategy for surviving such an impact is to go into orbit (Zahnle and Sleep, 1997; Sleep and Zahnle, 1998; Wells et al., 2003). Impacts can lift surface rocks into orbit essentially unshocked and unheated (Melosh, 1989). This process is known to occur, most famously among the SNC meteorites, which have travelled to the Earth from Mars. Rocks ejected from the Earth will mostly reach Earth-like orbits, and so will spend relatively little time in space before they are swept up again, typically tens of thousands of years or less (Wells et al., 2003). It is possible that passengers could survive the journey. It has also been noted that rocks can be exchanged between planets, although transit times are usually much longer and hence the voyage more dangerous (Melosh, 1989, Sleep and Zahnle, 1997, Mileikowsky et al., 2000).

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