R EPORTS Ϫ Ϫ 10 11 year 1) is less than the uncertainty in ly averageduniform surface pressure fieldover the 29. The authors gratefully acknowledge discussions (both the J rate given by (5), confirming that the global ocean. oral andvia e-mail) with J.-P. Boy, D. Bromwich, B. F. 2 18. Because atmospheric pressure trends over Antarctica Chao, Y. Chao, C. M. Cox, M. B. Dyurgerov, L.-L. Fu, M. climatic effects considered here account for are poorly constrained due to a lack of data and Ghil, E. Ivins, K. Mo, andY.Yung, andthe constructive the observed change in J2 slope within the calibration problems (19, 20), we extended the IB comments of the reviewers. The work of J.O.D., uncertainty of the observations. assumption to include the land surface of Antarctica S.L.M., andI.F. presents the results of one phase of In summary, our findings demonstrate in this study. research carriedout at the Jet Propulsion Laboratory, 19. K. M. Hines, D. H. Bromwich, G. J. Marshall, J. Clim. under contract with the National Aeronautics and that the J2 slope reversal observed in 1997– 13, 3940 (2000). Space Administration. O.D.V. is a postdoctoral re- 98 (5) was the result of dramatic changes in 20. G. J. Marshall, J. Clim. 15, 659 (2002). searcher of the National Fundfor Scientific Research, oceanic and glacial mass distribution at that 21. The model bottom pressure is directly proportional Belgium. This work was accomplishedduringO.D.V.’s to the mass-per-unit area above a given point. visit to JPL during Summer 2002. This is a contribu- time. Our modeling results show that an 22. P. Johnston, K. Lambeck, Geophys. J. Int. 136, 537 tion of the ECCO consortium funded by the National equatorward mass shift in the oceans contrib- (1999). Oceanographic Partnership Program. 23. R. Devoti et al., Geophys. Res. Lett. 28, 855 (2001). uted a substantial portion of the J2 increase Supporting Online Material during 1998 (Figs. 1B and 2B; figs. S1 and 24. R. Sabadini, B. L. A. Vermeersen, in Ice Sheets, Sea www.sciencemag.org/cgi/content/full/298/5600/1975/ S2), coincident with phase reversals in both Level and the Dynamic Earth, vol. 29 of Geodynamics DC1 Series, J. X. Mitrovica, B. L. A. Vermeersen, Eds. Materials andMethods ENSO and the PDO. The year 1998 also saw (American Geophysical Union, Washington, DC, SOM Text the warmest global mean surface temperature 2002). Figs. S1 andS2 on record (25), and we found that a concom- 25. J. Hansen et al., J. Geophys. Res. 104, 30,997 (1999). Table S1 26. J. Oerlemans, Ann. Glaciol. 31, 44 (2000). References andNotes itant surge in subpolar glacial melting (13, 27. H. J. Zwally et al., Science 297, 218 (2002). 26) can account for nearly all of the remain- 28. D. Adam, Nature 416, 10 (2002) 27 August 2002; 4 November 2002 ing nonlinear behavior in the J2 observations (Figs. 1C and 2C; Table 1). However, the dynamical links between these relatively rap- id mass shifts and concurrent climate anom- Environmental Effects of Large alies remain to be established. Further knowl- edge of Earth system processes, in particular Impacts on Mars polar ice sheet ablation (27), is needed to form a more comprehensive picture of ongo- Teresa L. Segura,1*† Owen B. Toon,1 AnthonyColaprete, 2 ing mass balance changes and their climatic Kevin Zahnle2 origins. New sources of geodetic data, such as the monthly time-variable gravity fields to The martian valley networks formed near the end of the period of heavy be supplied by the recently launched GRACE bombardment of the inner solar system, about 3.5 billion years ago. The largest mission (28), may soon revolutionize our impacts produced global blankets of very hot ejecta, ranging in thickness from ability to monitor and interpret these changes. meters to hundreds of meters. Our simulations indicated that the ejecta warmed the surface, keeping it above the freezing point of water for periods ranging from decades to millennia, depending on impactor size, and caused References and Notes 1. J. O. Dickey et al., Satellite Gravity and the Geosphere shallow subsurface or polar ice to evaporate or melt. Large impacts also injected (National Research Council, Washington, DC, 1997). steam into the atmosphere from the craters or from water innate to the 2. The definitionandmethodof calculation for J2 are impactors. From all sources, a typical 100-, 200-, or 250-kilometers asteroid in- given in the SOM Text. jected about 2, 9, or 16 meters, respectively, of precipitable water into the at- 3. C. F. Yoder et al., Nature 307, 757 (1983). 4. F. R. Stephenson, L. V. Morrison, Philos. Trans. R. Soc. mosphere, which eventually rained out at a rate of about 2 meters per year. The A 351, 165 (1995). rains from a large impact formed rivers and contributed to recharging aquifers. 5. C. M. Cox, B. F. Chao, Science 297, 831 (2002). 6. A. Cazenave, R. S. Nerem, Science 297, 783 (2002). The valley networks on Mars cut across the ers about 6 ϫ 1025 (4 ϫ 1026,9ϫ 1026)Jof 7. S. R. Hare, N. J. Mantua, Progr. Oceanogr. 47, 103 (2000). heavily cratered southern highlands, the old- energy to the planet and generates a crater 8. T/P andXBT datawere assimilatedinto the model est terrain on the planet, signifying that they ϳ600 (1000, 1300) km in diameter (fig. S1) run starting in 1993. Further details regarding the are contemporaneous with the period of and 3 ϫ 1018 (3 ϫ 1019,5ϫ 1019)kgof ECCO assimilation system are available at www. ecco-group.org. A brief summary is given in the SOM heavy cometary and asteroidal bombardment ejecta (6–8) (Fig. 1). Ejecta include vapor- Text. of Mars and of the rest of the inner solar ized and melted impactor and target materi- 9. E. Rignot, R. H. Thomas, Science 297, 1502 (2002). system (1, 2). There are about 25 visible als. About 20% of the ejecta are rock vapors 10. M. B. Dyurgerov, M. F. Meier, Arct. Alpine Res. 29, 379 craters with diameters between 600 and 4000 (6); most of the rest is melt (7). Only a few (1997). 11. ࿜࿜࿜࿜ , Arct. Alpine Res. 29, 392 (1997). km (fig. S1) (3). Many other large craters percent of the ejecta mass would escape from 12. ࿜࿜࿜࿜ , Proc. Natl. Acad. Sci. U.S.A. 97, 1406 may have been erased by resurfacing events Mars, given a 9 km/s impact velocity (6). In (2000). (4). Here we consider how impacts might the case of large impacts, the ejecta are hot 13. Annual volume changes in subpolar andmountain glaciers were obtainedfrom the NSIDC Web site have caused water to flow on Mars and create because of the large energy release and be- www-nsidc.colorado.edu/sotc/glacier_balance.html, the valley networks. cause of the low surface-to-volume ratio of courtesy of M. Dyurgerov, Institute of Arctic and An asteroid (5) with a diameter of 100 the ejecta, which inhibits cooling. The hot Alpine Research, University of Colorado, Boulder. 14. T. S. James, E. R. Ivins, J. Geophys. Res. 102, 605 (200, 250) km and traveling at 9 km/s deliv- ejecta are distributed globally both ballistical- (1997). ly and via the thermally expanding vapor 15. Glacial contributions to sea level change are not 1Program in Atmospheric andOceanic Sciences, Lab- cloud. For a time, the rock vapor is suspended included in the ocean model, which is constrained to oratory for Atmospheric andSpace Physics, University in the hot atmosphere because it is too warm have constant volume (Boussinesq approximation). of Colorado, Campus Box 392, Boulder, CO 80309– to condense immediately. Before computing the oceanic J2 contribution, the 0392, USA. 2NASA-Ames Research Center, MS 245-3, model bottom pressures are adjusted uniformly such Moffett Field, CA 94035, USA. There are several primary sources of wa- that the global ocean mass is time-invariant. ter. The impactor itself may deliver water. A 16. E. M. Kalnay et al., Bull. Am. Meteorol. Soc. 77, 437 (1996). *Present address: NASA-Ames Research Center, MS 17. The IB assumption accounts for oceanic compensa- 245-3, Moffett Field, CA 94035, USA. 100 (200, 250)-km asteroid that is 5% water tion of atmospheric loading by substituting a spatial- †To whom correspondence should be addressed. by mass (8) would deliver 40 (310, 620) cm www.sciencemag.org SCIENCE VOL 298 6 DECEMBER 2002 1977 R EPORTS of water (global equivalent thickness). The evaporate back into the atmosphere, nor did (Fig. 3). Parts of the martian regolith may be crater is important if the deep regolith of we include the radiative properties of clouds, kept above freezing for at least 2 years, 55 Mars is wet, and as much as 100 (360, 540) but their consideration in our model would years, and 200 years by the 100-, 200-, and cm of water could be liberated from the crater only lengthen the warm periods reported 250-km events, respectively (Fig. 3), and for (9). Because the ejecta are hot, most of this here. millennia for even larger impacts (Fig. 4). water directly enters the atmosphere as vapor. Below the fresh ejecta layer is the original The amount of water precipitated out of the A third possible source is vapor evaporated martian regolith, which we assumed contains atmosphere from vaporization of the impac- from exposed ice, such as the polar caps. If 20% water by mass under a 40-cm dry cap tor, target, and polar caps and melted below this ice were exposed, 30 (250, 500) cm of (14). Permafrost (up to 40% water) and sub- ground for asteroid impactors with different water would be liberated (10) (Fig.