Role of the Stratospheric Polar Freezing Belt in Denitrification

Geophys. Res. 102, 28673 (1997)]. In our approach and in earlier approaches that did not isolate the Tharsis load, the total load acting on the lithosphere consists of the topographic load plus the load necessary to account for the deformational response of the lithosphere to the imposed topographic load. Beneath Tharsis, the deformation is a depression. 11. This 95% compensation figure is an average over wavelengths corresponding to spherical harmonic degrees l # 10. 12. W. M. Folkner, C. F. Yoder, D. N. Yuan, E. M. Standish, R. A. Preston, Science 278, 1749 (1997). 13. The gravity mismatch could be remedied by introducing a degree of isostatic compensation into the topography induced by the Tharsis load (8). Nonetheless, spatial patterns of predicted gravity anomalies and topography and how well they correlate with patterns in the observed fields are valid with the present model because the planet responds to the load only at very long wavelengths (l # 10). [D. L. Turcotte, R. J. Willemann, W. F. Haxby, J. Norberry, J. Geophys. Res. 86, 3951 (1981)]. At such wavelengths, the load is supported by the resistance of the elastic shell to changes in its radius of curvature. This resistance is known as a “membrane stress. ” Because of membrane stress load support, plausible variations in the thickness and elastic properties of the lithospheric shell do not affect the spatial patterns of the modeled anomalies. 14. The trough may have been obscured by later deformation, evidence for which includes a series of ridges concentric to the Tharsis rise (5) [R. A. Schultz, K. L. Tanaka, J. Geophys. Res. 99, 8371 (1994)], and by early volcanic and sedimentary deposition at the edges of the Tharsis rise (15). 15. D. H. Scott, K. L. Tanaka, U.S. Geological Survey Map I-1802-A (1986). 16. On the basis of impact crater counts, Mars has been divided into three main epochs of time; from oldest to youngest, they are the Noachian, Hesperian, and Amazonian. Assignment of absolute ages to the epoch boundaries is dependent on models for the impact flux rate. Two specific models place the Noachian-Hesperian boundary at 3.8 and 3.5 Ga and the Hesperian-Amazonian boundary at 3.55 and 1.8 Ga (26). 17. R. C. Anderson et al., J. Geophys. Res., in press. 18. Recent modeling [W. B. Banerdt, M. P. Golombek, Lunar Planet. Sci. 31, abstr. 2038 (2000)] using gravity and topography fields derived from MGS observations, which are of higher degree and order than were available before, indicates that load support by bending and membrane stresses can explain the type, location, orientation, and strain of the majority of Tharsis structural features. 19. Early tectonic activity substantially predates many of the surface units of Tharsis (15), which typically are comparatively thin Hesperian and Amazonian deposits covering the earlier Noachian units [ J. B. Plescia, R. S. Saunders, Proc. Lunar Planet. Sci. Conf. 11, 2423 (1980)]. 20. M. H. Carr, Water on Mars (Oxford Univ. Press, New York, 1996). 21. iiii , J. Geophys. Res. 100, 7479 (1995). 22. Azimuths were estimated from a database (21) consisting of almost 8000 branch segments composing over 800 valley network systems located between 47.5°N and 47.5°S. The number of valley network systems used here was reduced to 520 by eliminating systems whose downstream directions are ambiguous and by excluding valley networks that flow into the Hellas impact basin, inasmuch as their directions cannot be predicted by the model. 23. The distribution of cos d lies between –1 and 1, with 1 meaning perfect agreement in orientation and –1 meaning that the vectors have exactly the opposite directions. 24. K. Chan, D. H. Rothman, Phys. Rev. E, in press. 25. B. M. Hynek, R. J. Phillips, Geology, in press. 26. K. L. Tanaka, D. H. Scott, R. Greeley, in Mars, H. H. Kieffer, B. M. Jakosky, C. W. Snyder, M. S. Matthews, Eds. (Univ. of Arizona Press, Tucson, AZ, 1992), pp. 345–382. 27. Mapped valley networks within Margaritifer Sinus (15, 25) that do not have azimuths explained by the model occur almost exclusively on the same mapped unit as those that fit the model, and hence are likely to be contemporaneous. Also indicative of an equivalent age is the observation that crater densities are, within error, identical on the terrains containing network azimuths that follow and do not follow longwavelength topography (25). 28. C. R. Chapman, K. L. Jones, Annu. Rev. Earth Planet. Sci. 5, 515 (1977). 29. R. A. Craddock, T. A. Maxwell, J. Geophys. Res. 98, 3453 (1993). 30. Horizontal layering to thicknesses of at least 8 km has been observed in the canyon walls of Valles Marineris and has been interpreted as volcanic in origin and mostly Noachian in age [A. S. McEwen, M. C. Malin, M. H. Carr, W. K. Hartmann, Nature 397, 584 (1999)]. From this observation, the volcanic volume in the Valles Marineris region is estimated at ;4 3 107 km3. Because this is only a small areal fraction on a flank of the Tharsis rise, the total volume of magmatic material associated with Tharsis overall must be approximately an order of magnitude larger, which is consistent with our model results. 31. T. M. Gerlach, E. J. Graber, Nature 313, 273 (1985). 32. H. Y. McSween Jr. et al., Nature 409, 487 (2001). 33. F. Forget, R. T. Pierrehumbert, Science 278, 1273 (1997). 34. D. A. Brain, B. M. Jakosky, J. Geophys. Res. 103, 22 (1998). 35. M. H. Carr, J. Geophys. Res. 104, 21 (1999). 36. We thank the MGS Radio Science and the Mars Orbiter Laser Altimeter (MOLA) investigations, which are supported by the NASA Mars Exploration Program.