Lithospheric loading by the northern polar cap on Mars

Cl. Johnson et al., Lithospheric loading by the northern polar cap on Mars, ICARUS, 144(2), 2000, pp. 313-328
Citations number
Categorie Soggetti
Space Sciences
Journal title
ISSN journal
0019-1035 → ACNP
Year of publication
313 - 328
SICI code
New topography data for the northern polar region on Mars, returned by the the Mars Orbiter Laser Altimeter (MOLA) during the aerobraking hiatus and s cience phasing orbits, allow characterization of the topography of the pres ent northern polar cap and its environs, Models for loading of an elastic s hell by an axisymmetric load approximating the present polar cap geometry i ndicate that the maximum deflection of the subice basement is in the range 1200 to 400 m for an elastic lithosphere of thickness 40 to 200 km overlain by a cap of pure H2O ice. Corresponding model cap volumes increase from 1. 5 to 1.8 x 10(6) km(3), as elastic lithosphere thickness decreases from 200 to 40 km. The presence of sediments in the polar cap increases the depth t o basement and resulting cap volume for a given value of elastic lithospher e thickness, One-dimensional heat flow calculations indicate that the tempe rature at the base of the cap may approach the melting point of cap materia l if the lithosphere underlying the cap is thin. The basal temperature is 1 70 K for a 200-km-thick lithosphere overlain by pure ice but is as great as 234 K for a 40-km-thick lithosphere overlain by a cap with a high sediment /ice ratio. Constraints on elastic lithosphere thickness are weak, but geol ogic mapping and MOLA data suggest that a flexurally derived circumpolar de pression filled with sediments is consistent with elastic lithosphere thick ness values in the range 60-120 km, Gravity and topography over the whole c ap are poorly correlated, possibly due to viscous relaxation of long-wavele ngth topography, but gravity and topography over the western portion of the main cap are consistent with an elastic lithosphere thickness of 120 km, f or a crustal thickness of 50 km, Both MOLA data and geological information suggest a formerly larger northern polar cap. The relationship of time scal es for changes in the polar cap volume and extent to time scales for viscou s relaxation of topography has important implications for investigations of even present polar cap topography. Viscoelastic calculations show that the Maxwell time for an Earth-like mantle viscosity for Mars (10(21) Pa-s) is 10(5) yr. The Maxwell time scales directly with the martian mantle viscosit y so that values as high as 10(7) yr are possible. Time scales for changes in polar cap volume are poorly constrained, but major changes in cap volume over periods of 10(6)-10(8) yr are consistent with current understanding o f polar cap processes. (C) 2000 academic Press.