Changes in blast zone albedo patterns around new martian impact craters
Introduction
Signs of past aeolian modification are ubiquitous on Mars: vast sand sheets, wind-scoured plains, and huge deposits of dust. Current aeolian processes are now being observed at the orbital (km) as well as the lander (m) scale (e.g., Bridges et al., 2013, Geissler et al., 2010, Silvestro et al., 2010). These processes are potentially a significant part of the martian dust cycle, with implications for the generation and recurrence of regional and global dust storms (e.g., Cantor, 2007). Here we investigate these processes by looking at the changes over time of blast zones (BZs), the albedo patterns that surround new impact sites. These BZs are the product of martian impact events detected in the past decade, with formation dates constrained by before-and-after images (Daubar et al., 2013, Malin et al., 2006). They are the freshest available examples of impact processes outside the laboratory, and for the purposes of aeolian studies, they are recently-modified surfaces in dusty areas of Mars with constrained dates for the initial disturbances. Sites where the surface has recently been disturbed are expected to show more physical changes; this has been demonstrated in areas disturbed by the wheels of the Mars Exploration Rover (MER) Opportunity rover (Chojnacki et al., 2014, Geissler et al., 2010, Johnson et al., 2012). Thus we examine the surroundings of these recently-formed craters for changes over time to improve our understanding of current aeolian processes on dusty surfaces such as the fallout of atmospheric dust, processes which are widespread on Mars.
The High Resolution Imaging Science Experiment (HiRISE) (McEwen et al., 2007) on the Mars Reconnaissance Orbiter (MRO) has now been monitoring many of these impact sites for over 3 Mars years. The rates and characteristics of changes at these locations will help shed light on the effects of the initial impact, the modification processes that have occurred since, and the ultimate fate of these features.
An additional motivation for this study is to determine whether the cratering rate measured by Daubar et al. (2013) might be incorrect because of under-counting new impacts due to fading. Here we examine the possibility that these impact sites are disappearing over short timescales. If the timescales of this process are sufficiently short, the martian cratering rate calculated by Daubar et al. (2013) may be artificially low due to new impacts not being discovered before they fade.
Section snippets
Descriptions of impact blast zones
Almost all known new impacts initially have an extended, usually lower, albedo pattern surrounding the craters (Malin et al., 2006, Daubar et al., 2013). Although most features are darker than their surroundings, there are examples of higher-albedo blast zones and sites with both higher- and lower-albedo components relative to pre-existing or adjacent surfaces. The prevalence of lower-albedo features surrounding new impacts is a direct result of the detection technique (Daubar et al., 2013):
Qualitative assessment of change
We assessed the degree of change at 106 new impact sites with repeat imaging. The entire set of ∼500 known new dated craters could not be examined for changes because not all of them have been imaged by HiRISE multiple times under suitable conditions. This subset of known impact sites has the required repeat imaging and meet the following criteria. We limited our study to sites equatorward of 50° north or south latitude and without exposed subsurface ice, because in polar regions different
Types of changes observed
We observed the following types of changes in BZs, in order of most to least common: (A) Diffuse low-albedo halos fade and approach surrounding albedo (Fig. 9). (B) Extended discrete rays and filamentary features fade, shorten, and disappear (Fig. 10). (C) Dark spots, most commonly smaller outlying ones, disappear completely (Fig. 11). (D) Dust devil tracks and wind streaks appear and disappear (these features are not caused by the impact event, but dust devil tracks are more visible over
Sites with undetectable change and possible darkening
Sites where the fading rate could not be measured (e.g., Fig. 22) could be cases where the albedo measurements are not reliable, or they could be the result of very subtle, extremely slow fading, such that the change in Arel is within the errors inherent in our method. We have observed examples of much older impacts, unconstrained with ‘before’ images, where the BZ has appeared to persist since Viking Orbiter images from the late 1970s (e.g., HiRISE observation ID PSP_002183_1970 at 17.03°N,
Conclusions
Many new martian impact blast zones have changed drastically over relatively short timescales. Surprisingly, however, approximately half of the new impact sites show little to no change, despite being freshly disturbed surfaces in areas with evidence of recent aeolian activity and other albedo changes. The qualitative amount of change observed is correlated with the amount of time elapsed, latitude, elevation, and effective crater diameter. From this we infer that processes responsible for BZ
Acknowledgments
We are grateful for the HiRISE operations staff for acquiring and processing the excellent data used in this study, and the CTX operations team for discovering candidate new impact sites. Our thanks go to Rod Heyd for answering questions about the detailed processing of HiRISE RDRs and Guy McArthur for help with the HiView software. Patricio Becerra also provided useful discussion. We appreciate the helpful comments from Moses Milazzo and two anonymous reviewers, especially one reviewer whose
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