MOLA has traversed over 100 polar region impact craters during its operations as part of the Mars Global Surveyor Science Phasing Orbit period (April-Sept. 1998). Of those sampled, several frost-mantled craters were crossed, as well as examples of fluidized ejecta and pedestal craters. This figure is a map of the North Polar region of Mars north of 55° (N latitude) on which are displayed 12 "thumbnail" MOLA profiles of impact crater cross-sections. All thumbnail MOLA Profiles are at their own scales, so that some of the craters illustrated are relatively small (B), while others such as Korolev (crater just above the letter A in the figure) are 80 km in diameter. Craters designated with alphabetic letters are particularly interesting. For example, the craters indicated by D and F in the polar stereographic map, are frost-mantled and display anomalously large crater cavity interior deposits. A classic multilobed, fluidized-ejecta crater is shown to the lower right, just southeast of the margin of the martian north polar cap. Many of the impact features north of around 60 degrees North latitude on Mars have large rim relief and rapidly decaying ejecta deposits, relative to similar diameter craters sampled by MOLA in the equatorial and mid-latitudes. Enhanced pile-up of materials near the crater rim and anomalously large central cavity deposits (and uplifts) are characteristic of 30% of the impact features observed in the high northern latitudes. Some of the craters show evidence of nearly complete interior infill and subsequent loss of materials, as if these craters are now in the process of being exhumed. The larger craters at these high northern latitudes (Korolev, Lomonosov, etc.) also appear to "bottom out" at similar absolute elevations, suggesting that there may be a characteristic thickness of an upper crustal "weak layer" which is underlain by more competent, higher strength target materials. Results of MOLA analyses of these polar craters will be presented at the First International Conference on Mars Polar Science as well as at the Fall 1998 American Geophysical Union meeting. A research article is in final preparation for Meteoritics and Planetary Science (Garvin et al., Geometric Properties of Polar Region Impact Craters on Mars).

MOLA traversed a classic "pedestal" or pancake crater on Mars Global Surveyor orbit 219. Such features were first observed by the Viking Orbiters in the 1970's and have been difficult to interpret because their appearance suggests that large areas of the surrounding terrain has been stripped away. The debate continues as to the mechanism by which they appear to be perched much like a pancake with a hole in the middle in the martian northern high latitudes. MOLA topographic cross-sections such as this (and dozens of others now in hand) reveal the always elusive third dimension of these enigmatic landforms. This crater is approximately 24 km in diameter and 786 m deep (from its rim crest to its floor). The most interesting feature one sees is the 100 m tall "rampart" at the southern edge of the ejecta. Indeed, the ejecta is only slightly elevated above the surrounding northern plains (10's of meters), but the distal edge of the ejecta is distinctive, and notably different from the more subdued northern edge. The suggestion is that such pedestals form via a fluidization process that generates ramparts at the ejecta runout margin. Flank slopes on the ejecta blanket are 0.1° to 0.3°, less than that for more typical craters of this size. MOLA data allow one to see the subtleties of ejecta blanket topology and to provide constraints for models of ejecta emplacement.

On Mars Global Surveyor orbit 242, MOLA traversed this 49 km diameter impact feature in a region of Mars adjacent to the North Polar Cap. This frost-filled crater is located at 77.3°N, 214.5°E, and MOLA's cross-section is within about 10 km of the centerline of the feature. Viking era images of this relatively large and fresh-appearing crater indicated a smooth, homogeneous, and low relief crater floor, and a subdued rim. MOLA measurements show otherwise. The crater is 2.16 km deep, with a typical rim height of 1.2 km, and the slope of the continuous ejecta is approx. 1.3 degrees. The ejecta thickness decays from the rim crest in a fashion that suggests a relatively shallow fall-off with distance, unlike many other polar region martian impact features. Other than Korolev, an 80 km diameter crater at similar latitudes, this is the largest recognized impact crater adjacent to the North Polar Cap. Furthermore, it displays a 660 m tall central uplift structure that takes up about 30% of the cavity volume, which is more than a factor of 2 higher than that observed using MOLA data for most martian craters. In addition, the geometry of the cavity and the central peak are unusual, with a far greater tendency to be "U" shaped than is typical. One suggestion to explain the anomalous geometric properties of this feature could involve the exhumation of the crater from beneath a continuous mantle of polar ice deposits, preferentially leaving behind an ice mantle associated with the central uplift. If this were the case, then a cavity over 2 km in depth would have been filled and subsequent ablation required to produce the present-day topographic expression. Alternately, enhancement and oversteepening of the central uplift could have been associated with enhanced flow and fluidization in the floor rebound process; this could also explain the relatively large rim heights for this feature. Ramparts as high as 300 m are associated with this impact feature; such relief is also atypical of martian impact craters. Indeed, other frost-filled craters traversed by MOLA suggest extreme enhancement of central uplift deposits. The MOLA profile also indicates that the crater sites in a regional sag over 100 km in diameter, perhaps associated with the impact process at these latitudes.

Download a PDF image comparing Viking and MOLA images of craters.

Information presented here was provided by J.B. Garvin. More information on craters viewed by MOLA will be presented in an article by Garvin, J.B. et al., Geophys. Res. Lett., in press, 1998.

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