HR: 1330h
AN: P42A-0420    [PDF]
TI: Evidence for an Ocean in the Northern Lowland Plains of Mars Based on Crater Depth/Diameter Measurements
AU: * Boyce, J M
EM: jboyce@higp.hawaii.edu
AF: Hawaii Inst. Geophysics & Planetology, University Hawaii, Honolulu, HI 96822 United States
AU: Mouginis-Mark, P J
EM: pmm@higp.hawaii.edu
AF: Hawaii Inst. Geophysics & Planetology, University Hawaii, Honolulu, HI 96822 United States
AU: Garbeil, H
EM: harold@higp.hawaii.edu
AF: Hawaii Inst. Geophysics & Planetology, University Hawaii, Honolulu, HI 96822 United States
AB: It has been suggested that the complex history of Mars includes one or more ancient oceans and that the Vastitis Borealis Formation (VBF) is an ice-rich, fine-grained sedimentary deposit left as a remnant of these oceans. In many places in the northern lowland plains the VBF partially fills craters and mantles the surrounding terrain. Here we examine new crater depth (d) and diameter (D) measurements to test the hypothesis that VGF was deposited from a large body of water. The d and D for 2,102 craters (D ranging from 2 km to $>$100 km) were measured in the northern lowlands in Utopia, Arcadia, Acidalia, and North Pole regions, and in the northern highlands in the Tempe Terra, and Deuteronilus regions. Crater depths were measured in two ways, from the floor to the rim (d$_{R}$), and from the floor to the surface surrounding the crater (d$_{S}$). The data show that both types of d/D relationships (d$_{S}$/D and d$_{R}$/D) of craters in the northern plains are different than the same type of relationships for craters in the northern highlands or anywhere else on the planet. In particular, the d$_{S}$/D relationship shows that the floors of many craters in this region are at or near the same elevation as the surrounding terrain, unlike in all other regions where depth is a function of diameter. Examination of THEMIS and MOC images has revealed that craters with floors at or near the same elevations as the surrounding terrain are also the ones partially filled with VBF. This suggests that the VBF is the cause of the unique d$_{S}$/D relationship. In addition, the partially filled craters north of $45\deg$N are typically shallower than similar craters south of $45\deg$N (the latitude predicted for Mars' maximum obliquity). These relationships are consistent with those predicted for a terrain mantled by an ice-rich sedimentary mantle that is undergoing deflation by sublimation and with the hypothesis that VBF is a ice-rich deposit left by an ancient ocean.
DE: 5415 Erosion and weathering
DE: 5420 Impact phenomena (includes cratering)
SC: Planetary Sciences [P]
MN: 2003 Fall Meeting