Introduction

1. Characterizing the present and past aeolian transport environments in Meridiani PlanumL. K. Fenton1, T. I. Michaels1 , M. Chojnacki2 1SETI Institute, Mountain View, CA, USA, 2Lunar and Planetary Lab, Tucson, AZ, USA Fig. 1 Fig. 1. The 2°x 2° study area, centered on Opportunity’s traverse (yellow), showing the locations of LDDs and TARs. Introduction • Meridiani Planum is well sampled by both in situ data from MER Opportunity and orbiting spacecraft data, making it well suited to an in-depth study of surface features. • There is a known, possibly climate-related, shift in wind regime over time: • Ancient: Easterly winds dominate plains ripples [1] and ripple streaks [2], active ~50-200 ka [3]. • Today: LDDs in Endeavour crater are dominated by a NW wind [4]; dark sand streaks on the plains are formed by a SE wind [5]. • The timing of, duration of, and wind patterns driving sand saltation on Mars are typically poorly constrained. • The patterns of aeolian activity within recent climate shifts on Mars is not well understood. • Meridiani Planum presents an environment with a diverse array of bedforms, each of which is driven by unique transport conditions: large dark dunes (LDDs), transverse aeolian ridges (TARs), plains ripples, and both bright and dark wind streaks (see Fig. 1). We used ArcGIS 10.1 to map locations of LDDs and TARs, orientations of bright, dark, and ripple/TAR wind streaks LDD crestlines, TAR crestlines, and selected crestlines of plains ripples. HiRISE images were used where they are available; where not, CTX images were used. We present three significant results from a comprehensive study of aeolian features (see each column below). 1. Wind Streaks 2. Crater Gradation 3. Recently active TARs An unnamed, 0.84 km diameter crater ~50 km west of Endeavour crater (see Fig. 1) was dated to ~200 ka by [3]. Plains ripples ~9 km south have partially migrated over its secondaries [3]. No plains ripples have reformed within ~3 km of the crater. However, there are 3 TAR fields near the crater that, although surrounded by secondaries and ejecta, appear undisturbed (see Fig. 7). Part of one TAR field may be buried by ejecta (see Fig. 7b), suggesting that the TARs predated the impact. Thus TARs in Meridiani Planum may have formed prior to 200 ka, but they have been active than since the impact – perhaps more active than the plains ripples. Both bright and dark streaks are present throughout the study area (see Fig. 2). Bright streaks are oriented both NW and SE, whereas dark streaks are mainly oriented NW (see Fig. 3). Vostok crater is a ~40 m diameter crater in Meridiani Planum that [9] assigned an age of ~10 Ma (see Fig. 4). It is nearly filled with rippled sediment, and its rim has no topographic relief. Opportunity traversed by many similar craters, including several with apparent positive relief (see Fig. 5). A vertical accuracy of ~35 cm implies a minimum relief of >~1 m to be clearly identified on a HiRISE DTM, placing an upper limit on the height of these deposits. We propose that they are inverted craters (see Fig. 6). Fig. 4. Vostok crater Fig. 2. Examples of bright and dark streaks in Meridiani Planum Fig. 3. a) Bright and b) dark streak orientations Bright streaks are thought to be made of dust transported some distance [e.g., 6], but the Victoria crater dark streak is composed of dark basaltic sand sourced from within the crater [7]. This implies that the main sand transport direction is by SE winds toward the NW. However, this contrasts with LDD morphology and migration: toward the SE by NW winds. Differences in crater morphology [8] or atmospheric conditions (e.g., Froude number) may determine which wind is more effective at transporting sand in different locations in Meridiani Planum. Fig. 7. Fresh TARS surrounded by ejecta and secondary craters from an impact crater dated to ~200 ka. TARs in Meridiani Planum may be the first examples of this type of bedform with activity that can be dated. They may be more active than the Meridiani plains ripples. Fig. 5. a) HiRISE ESP_018846_1755, b) HiRISE DTM, c) Pancam anaglyph, and d) Navcam mosaic showing possible inverted craters. Neither dunes nor wind streaks alone characterize major sand-moving winds in Meridiani Planum. This may be the case elsewhere on Mars, complicating determination of wind regimes from aeolian features. Assuming crater fill is protected from erosion, and the plains erosion rate is 1-10 nm/yr [10], inverted craters with heights of 1 m have ages of 0.1-1 Ga. References [1] Sullivan et al. (2005) Nature, 7047, doi:10.1038/nature03641. [2] Silvestro et al. (2014) this mtg., 3 posters down. [3] Golombek et al. (2010) JGR, 115, doi:10.1029/2010JE003628. [4] Chojnacki et al. (2011) JGR, 116, doi:10.1029/2010JE003675. [5] Greeley and Thompson (2003) JGR, 108, doi:10.1029/2003JE002110. [6] Yen et al. (2005) Nature, 7047, doi:10.1038/nature03637. [7] Geissler et al. (2008) JGR, 113, doi:10.1029/2008JE003102. [8] Kienenberger and Greeley (2012) PSS, 68, doi:10.1016/j.pss.2011.03.003. [9] Golombek et al. (2012) LPS XLIV,Abst. #2267. [10] Golombek et al. (2006) JGR, 111, doi:10.1029/2006JE002754. Meridiani: windswept plains and shifting sands engineer landscapes Fig. 6. Sequence of events leading to inverted craters.