Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.u34b..05z&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #U34B-05
Other
1605 Abrupt/Rapid Climate Change (4901, 8408), 5225 Early Environment Of Earth, 5420 Impact Phenomena, Cratering (6022, 8136), 6225 Mars, 6295 Venus
Scientific paper
Cosmic impacts have an unsavory reputation in Earth science from their frequent use as dei ex machinis. But unwelcome as they may be, they do happen, and Earth, Venus, and Mars have all seen bigger impacts than the K/T event. The lunar crater record indicates that impacts were big and frequent when Earth was young. Impacts coeval with the lunar basins would have had drastic effects on Earth. The faint early Sun suggests that without an abundant greenhouse gas (e.g., 3 bars of CO2 would suffice), early Earth would have been very cold and icebound, a Hadean snowball. A Hadean snowball would differ from Paleoproterozoic and Neoproterozoic snowballs in that geothermal heat flow would surely have been high enough on a wide enough scale that the ice could not everywhere have been thick. If there were a Hadean snowball, impacts like those the late bombardment would have melted the ice, and so created transient "impact springs." Because Venus's atmosphere is very thick, only the biggest impacts can change it. Under current conditions the most interesting possibility is a big wet impact adding enough water to the atmosphere to ramp up the greenhouse effect. The effect of adding water to Venus's atmosphere has been discussed previously in the context of giant volcanoes. For Mars the chief effect of the late bombardment on the climate may be existential: impact erosion has been a leading hypothesis to explain why Mars has a thin atmosphere. A more controversial suggestion is that the evidence for warm wet climates on ancient Mars is really just another aspect of the impact cratering record. Impacts are capable of recycling and vaporizing buried martian volatiles --- water in particular --- but it remains to be shown that they do so often enough, or that the effects last long enough, to account for the apparent endurance of warm wet episodes.
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