Martian Polar Region Impact Craters: Geometric Properties from Mars Orbiter Laser Altimeter (Mola) Observations

First International Conference on Mars Polar Science 3037.pdf

MARTIAN POLAR REGION IMPACT CRATERS: GEOMETRIC PROPERTIES FROM MARS ORBITER LASER ALTIMETER (MOLA) OBSERVATIONS. J.B. Garvin1, S.E.H. Sakimoto2, J.J. Frawley3, and A. Matias4, 1NASA's GSFC, Code 921, Greenbelt MD 20771, USA ([email protected]), 2USRA at NASA's GSFC, Code 921, Greenbelt MD 20771, USA, 3Herring Bay Geophysics and Raytheon-STX, Dunkirk MD 20754, USA, 4University of Puerto Rico, Mayagüez and NASA's GSFC.

Introduction: The Mars Orbiter Laser Altimeter about -2.77. Subtle ramparts only tens of meters in (MOLA) instrument aboard the Mars Global Surveyor relief are resolved in the MOLA profiles. MGS) spacecraft [1] has so far observed approximately Large complex craters. For the large complex cra- 100 impact landforms in the north polar latitudes ters, a typical aspect ratio falls in the 0.023 to 0.029 (³ 60°N) of Mars [see 2, 6]. Correlation of the topogra- interval. They are not abundant in Mars’ northern phy with Viking Orbiter images indicate that many of plains, but MOLA has sampled each of the largest ones these are near-center profiles, and for some of the most at least once so far and for Korolev and Lomonosov, northern craters, multiple data passes have been ac- half a dozen passes each sampled either the cavity, the quired. The northern high latitudes of Mars may con- ejecta, or both. It appears that Korolev, like its coun- tain substantial ground-ice [3, 4, 5], and be topped terparts Lomonosov (C in Figure 1) and Mie, is ~2.4

with seasonal frost (largely CO2 with some water) km in depth, with a 800m rim, and interior terracing. forming each winter. We have analyzed various diag- However, given the limited MOLA crater floor cover- nostic crater topologic parameters for this high latitude age, it is difficult to ascertain whether the high albedo crater population with the objective of characterizing character of the crater floor is a mantling deposit that impact features in north polar terrains and we explore covers a central structure as is suggested for the two whether there is evidence of interaction with ground other high albedo craters discussed below. The ice, frost, dune movement, or other polar processes. Korolev ejecta blanket is well-expressed topographi- We find that there are substantial topographic varia- cally, with 300 m tall ramparts, and well-defined tions from the characteristics of mid-latitude craters in hummocks and lobes. The floors of Korolev, Lo- the polar craters that are not readily apparent from prior monosov, and Mie all lie within 100–200 m of a images. The transition from small simple craters to common elevation of -6500 m. This is the lowest ob- large complex craters is not well-defined, as was observed thus far on Mars, That the MOLA-derived floor served in the mid-latitude MOLA data (transition at 7- elevations are all within 100–200 m across thousands 8 km [6]). Additionally, there appear to be additional of kilometers may provide evidence of a widespread topographic complexities such as anomalously large upper weak layer approximately 2.2 to 2.5 km in central structures in many polar latitude impact fea- thickness overlying a more mechanically strong layer. tures. It is not yet clear if these are due to target- Ice-fill craters. For the craters with high-albedo in- induced differences in the formation of the crater, or teriors, there are two others in addition to Korolev post formation modifications from polar processes. outside the residual polar cap that have been sampled. Sampled Craters: In Mars’ northern high latitudes One of these is a 49 km feature at 77°N, 215°E (D in (³ 60°N), MOLA sampled numerous flow-lobe impact Figure 1). Viking Orbiter images show a fresh, po- craters, including a population of fresh-appearing lygonal rim, steep inner crater cavity walls, and an "pedestal" or "pancake" features which appear as indistinct ejecta blanket of highly variable albedo. The perched circular depressions at the center of a quasi- MOLA transects, on the other hand, illustrate consid- circular, positive-relief deposit [7]. MOLA also sam- erable topographic complexity. The crater is 2.4 km pled some small simple craters, dust-mantled craters, deep from its rim crest, with an aspect ratio (d/D) of several impact craters with high-albedo (frost-filled?) 0.052. The slope of the ejecta blanket, which is well- interior deposits (e.g., Korolev crater, at 73.2°N, defined topographically, is ~ 1.6 degrees, and the ejecta 162.8°E, A in Figure 1), several large complex craters, thickness function follows a -2.5 power-law, not unlike and a handful of odd impact features in or on the resid- fresh lunar craters [6]. The cavity is best approximated ual polar ice cap and polar layered terrain. by a polynomial with a power n of ~ 3. The southern Simple Craters. For the simple polar latitude cra- ejecta ramparts have 300 m of local relief, but are less ters on Mars, (e.g. the crater at 60°N, 352°E, B in distinct to the north with <100 m of relief. An enig- Figure 1), a typical aspect ratio (depth/Diameter, or matic central peak feature with D ~ 27 km and 670 m d/D) of 0.053 is observed, with a cavity cross-sectional in relief is an inverted "U" shape, in contrast to more shape that is well approximated by a paraboloid conical peaks in MOLA-sampled mid-latitude craters (n=1.95), and ejecta thickness function exponents of [6]. The central peak volume (if axisymmetric), is First International Conference on Mars Polar Science 3037.pdf

MARTIAN POLAR CRATER TOPOGRAPHY: J. B. Garvin et al.

~ 30% of that of the cavity. The anomalous volume distribution of the ejecta thickness function exponents and topology suggests that it could represent an ice- (see [6]) for polar region craters is more variable than mantled traditional central peak. It is not clear from the that reported in [6] for the mid-latitude craters, with an current image and topography coverage whether the ice average value near -2.2 (vs. -2.9 for mid-latitudes), but deposit is a remnant of a larger ablated deposit or ac- a standard deviation of 3.3 and a modal value of -0.5. cumulated from deposition of ice/CO2 atop a central In general, polar region craters do not follow the sim- deposit. The second possible ice-fill crater is at ple trends that MOLA observed for non-polar latitudes, 77.3°N, 90°E (E in Figure 1). The five available and there often are anomalously large central structures MOLA transects show a pronounced asymmetry in in near polar latitude impact features. Three- interior and rim elevations, a large interior deposit and dimensional polar crater ejecta blanket modeling is the floor near the level of the surrounding terrain. underway, with emphasis on polar process effects in Impact features on or near residual polar ice. The ejecta emplacement and modification. {We gratefully first of these is an impact feature at 81.6°N, 190E, (F acknowledge the support of the MGS Project, MOLA in Figure 1) that is mapped as part of the polar ice de- PI D.E. Smith, and Deputy PI M.T. Zuber. Greg posit outliers and is in well developed dunes in the Neumann provided essential support} Utopia Planitia polar erg field. Several MOLA passes References: [1] Smith D. E., et al. (1998) Science, were acquired for this enigmatic crater with indistinct 279, 1686–1692. [2] Garvin J. B. et al. (1998) EOS ejecta, which displays a d/D ratio of 0.052, and a cav- Trans. AGU., 79, S191. [3] Carr M. H., (1996) Water ity shape parameter n > 4.0, indicating it is very "U" on Mars, Oxford Univ. Press. 229pp. [4] Carr M. H., shaped. The central deposit’s volume occupies 72% of (1981) The Surface of Mars, Yale Univ. Press. 232pp. the cavity volume, which is larger than the volume of [5] Thomas, P. et al. (1992) in Mars, ed. H. H. Kieffer the ejecta, and 5-10 times that observed for similar et al., Univ. Arizona Press., 767-795. [6] Garvin J. B. diameter mid-latitude craters. The origin of this central and Frawley J. J. (1998) submitted to GRL. [7] Head deposit is unclear. It might be associated with a major J.W. and Roth R. (1976) In Papers Presented to deposition of ice over time, or be an ablation remnant Symp. on Planetary Cratering Mechanics, LPI, pp50- of a previously larger polar ice sheet that completely 52. [8] McGetchin T. R. (1973) Earth Planet. Sci. covered the crater. Its ejecta thickness function falls off Lett., 20, pp226-236. much more steeply than most martian craters, and the ejecta lack either the typical polar lo- bate morphology or ramparts. The Viking im- aging is poor and it is possible that dune mate- rials have encroached upon the distal ejecta. The second impact feature is at 81.3 °N, 255°E (G in Figure 1), and is on or near the residual ice margin. The third feature is a small crater in Chasma Boreale at 83°N, 312°E (H in Figure 1). Both of these latter features were poorly imaged by Viking, but both appear to have anomalous ejecta and cavity topography Discussion and Conclusions: The well- defined transition from small simple to large complex craters that was observed in the mid- latitudes MOLA data [1,2,6] is not observed in the polar craters. The transition may be delayed to larger crater sizes. Additionally, the complex craters have best-fitting d vs. D power law rela- tionship (d = kDx) with x = 0.79 instead of the mid-latitude value of x = 0.39, which suggests that polar complex craters tend to be deeper than their mid-latitude counterparts of the same Figure 1. Mars Polar Stereographic image from 55°N to diameter. However, a natural break in the topology of 90°N with a dozen inset examples of MOLA crater cross- crater cavities is still observed at high latitudes, such sections plotted with the lower left corner at the crater that simple craters have conical to paraboloidal shapes, center. Shaded relief base map is from the USGS while complex varieties are more "U" shaped. The 1:15,000,000 Topographic series, Map I-2160, 1991.