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Identification of the Deepest Craters on Mars Based on the Preservation

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Identification of the Deepest Craters on Mars Based on the Preservation IDENTIFCATION OF THE DEEPEST CRATERS ON MARS BASED ON THE PRESERVATION OF PITTED IMPACT MELT-BEARING DEPOSITS. L. L. Tornabene1,2, V. Ling3, J. M. Boyce4, G. R. Osinski1,5, T. N. Harrison1, and A. S. McEwen6, 1Dept. of Earth Sciences & Centre for Planetary Science and Exploration, Western University, London, ON, N6A 5B7, Canada ([email protected]), 2SETI Institute, Mountain View, CA 94043, USA, 3Central Secondary School, London, ON, N6B 2P8, Canada, 4Hawaii Institute of Geophysics and Planetology, University of Hawaii, Honolulu, HI 96822, USA, 5Dept. Physics & Astronomy, Western University, London, ON, N6A 5B7, Canada, 6Lunar and Planetary Laborato- ry, University of Arizona, Tucson, AZ 85721, USA. Introduction: Crater-related pitted materials, were avoided. As such, the lowest elevation on the thought to be impact melt-rich deposits formed from floor off the central pit was used instead. Likewise, any volatile-rich substrates, have been observed in high- overprinting primary or secondary impact craters on resolution images of both the youngest and best- the host crater floors were avoided – again, using the preserved craters on both Mars and Vesta [1–3]. To next lowest elevation that clearly fell on “unmodified” date, 205 such craters have been identified on Mars host crater floor. After plotting all the dr vs. D values, ranging from 1–150 km in diameter, and are randomly outliers (i.e., extremely deep or shallow craters) were distributed between ~60°S and 60°N latitudes [1]. assessed carefully with respect to pre-existing topogra- Because pitted materials likely represent the prima- phy and post-impact modification. Craters with uneven ry crater-fill deposits, and show a strong correlation or complex background terrains, or craters that over- with crater preservation [1–3], we explore their use to printed other craters, specifically in the vicinity of the reassess depth-to-Diameter (d/D) scaling relationship rim or the floor were noted, adjusted if possible or not using Mars Orbiter Laser Altimeter (MOLA) data. used for the power law regression. Constraining this relationship for the youngest and best-preserved craters on Mars is useful as a tool in planetary studies. Crater d/D can be used to specifical- ly address the effect of target properties on crater mor- phology and the extent of erosion, degradation and deposition in various regions, or within specific craters on Mars [4–11]. Here we seek to measure the d/D ratio of as many pitted material-bearing craters (“PM craters” from here on) as possible to further test the assertion of Torna- bene et al. [1] that PM craters are amongst the freshest and best-preserved craters on Mars. We will also com- pare the d/D relationship with previously published Fig. 1. Scatter plot of dr/D of PM-bearing craters (gray cir- relationships [4–10]. If PM craters are indeed amongst cles), the dr/D data from [6] (gray diamonds) and of [10] the freshest craters on Mars, then their depths should (crosses) showing that PM-bearing craters are amongst the be comparable or deeper then the estimated depth val- deepest craters for their size on Mars. ues derived from these previous relationships. Of the 205 PM-bearing craters, 122 made the final Methods: We evaluated 205 simple and complex cut for analysis. The dr/D of PM-bearing craters >7 km PM craters from [1] with both the MOLA Mission in diameter are plotted in Fig. 1. Based on poor Experimental Gridded Data Record (MEGDR) and MOLA PEDR data-coverage, pre-existing slope and Precision Experimental Data Record (PEDR) data in topographic effects as described in our methods sec- the Java Mission And Remote Sensing (JMARS) soft- tion, 81 of the 205 craters were dropped from our re- ware package [12]. MOLA PEDR data were individu- gression analysis. The two largest craters (Hale and ally compiled as a shapefile for each crater using the Bakhuysen) were also dropped due to a lack of craters Planetary Data System (PDS) Geosciences Node’s in our sample population with diameters between 100 web-based MOLA PEDR Query applet [13]. Each and 150 km, resulting in 122 simple and complex (D shapefile was then read into JMARS and the PEDR ~2-100 km) craters that made the final cut. Also in- shots were carefully examined along with high- cluded in Fig. 1 are 2269 dr/D measurements of Mar- resolution visible images of each crater. Craters with tian craters by [6], and the 32,913 dr/D measurements poor MOLA PEDR coverage were discarded from our of [10] in this the diameter range ~7-450 km. analysis. Next, geometric binning of the PM-bearing crater Crater depth (dr) was measured as the difference in diameters to bin the dr/D data was used to derive a elevation between the maximum rim elevation and the power law regression for the deepest craters in our lowest floor elevation, while Diameter (D) values were sample population. This binning is based on a diame- used from the supplementary table in Tornabene et al. ter-bin size that increases with increasing diameter to [1]. Central pits with elevations below the crater floor ensure that a maximum of 4 craters are available in each bin. In this manner, 18 bins were created starting relationships provides a consistently deeper estimate with a 2-km bin size that increased to a maximum of for complex craters. These consistently higher d/Ds 20 km in size. A initial power-law regression analysis provide additional corroborative evidence that pitted of the deepest 10 complex craters and 8 simple craters materials are primary crater deposits. However, evi- for each bin indicated that there were 5 craters in each dence now suggests that the higher depth of some PM of the sub-populations that fall slightly below the best- craters may still be influenced by target properties well fit trend line. As such, they were subsequently discard- past the transition from simple to complex morpholo- ed to derive a maximum dr/D relationship for complex gies. As such, the scaling relationships derived here in and simple craters. For the start of the complex crater should be used as an upper limit for estimating crater relationship, we avoid craters 12 km or less, which is depth. based on the largest simple crater of ~11.8 km ob- Previously, painstaking d/D measurements involv- served by [7]. Simple craters appear to go up to ~9 km ing 1000s of craters (up to 6000 craters for [5]) were in our sample population. required to constrain the deepest craters on Mars, Results: The deepest PM crater dr/D relationships which were often assumed to be the freshest craters on are: Mars. Our results support that the presence of PMs 0.74 2 Simple: dr = 0.277 D (n = 5 r = 0.99) may be used as a criterion for identifying the best- 0.57 2 Complex: dr = 0.349 D (n = 8; r = 0.99) preserved non-polar (± 60°N) craters on Mars. Thus, and the average relationship for all complex craters PMs may provide a chronological marker and distinc- 0.53 2 >12 km is: dr = 0.341 D (n = 79; r = 0.92) tive morphologic criterion for evaluating crater preser- A comparison with previous of dr/D scaling rela- vation, and for use in a variety of mapping and geolog- tionships [5-10] shows that our deepest PM crater- ic studies of the Martian surface. derived relationship is deeper than previously reported Furthermore, Mars is a relatively active geological- ones, including [8]. The average PM-crater relation- ly, as such, the sample population of PM-bearing cra- ship for all complex craters is nearly comparable to the ters reported in [1] can be used to assess other crater- general relationship derived by [9]; however, a closer related scaling relationships, such as peak diameter, pit examination of these two shows that our PM-derived diameter, number of terraces, terrace spacing, ejecta relationship estimates deeper craters at diameters less blanket attributes. Continued studies of Martian PM- than ~32 km and shallower craters at diameters greater bearing craters are likely to provide additional insights than ~42 km. The deepest PM crater-derived relation- into the impact process as a geologic process (e.g., ship is for simple craters is greater than estimates made distribution and morphometry of various impactites) by [5], but complexities due to target properties in the and a reassessment of the strength properties of differ- strength-dominated regime [7-10] make the derivation ent hemispheres and regions as discussed by [6–10]. of a global relationship and comparisons difficult. Such studies, with respect to the study of well- While the culled sample population (n = 122) ap- preserved lunar craters, may be more relevant to un- pears to be randomly distributed (Fig. 2), the deepest derstanding terrestrial impact structures, which suffer sub-population of complex and simple craters (n = 13) from degradation, erosion and deposition via multiple, are not, clustering in the Isidis-Elysium-Hesperia re- active geologic processes. gion with one outlier on Olympus Mons. A closer in- References: [1] Tornabene L. L. et al. (2012) Icarus, spection reveals that these craters are within plains 220, 348-368. [2] Boyce J. M. et al. (2012) Icarus, 221, 262- materials interpreted to be lava flows [14]. This obser- 275. [3] Denevi B. W. et al. (2012) Science, doi: vation suggests that target-strength effects may contin- 10.1126/science.1225374. [4] Pike [5] Garvin J. B. et al. ue to play a role even within the gravity-dominated (2003) Mars 6 conf., Abstract #3277. [6] Boyce J. M. et al. regime that governs complex crater collapse.
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