Sand Flux Estimates and Aeolian-Driven Landscape Evolution on Mars

Lunar and Planetary Science XLVIII (2017) 2627.pdf

SAND FLUX ESTIMATES AND AEOLIAN-DRIVEN LANDSCAPE EVOLUTION ON MARS. M. Cho- jnacki1, A. C. Urso1, M. E. Banks2, L. L. Tornabene3 and N. T. Bridges4. 1Lunar and Planetary Lab, University of Arizona, Tucson, AZ ([email protected]); 2Planetary Science Institute, Tucson, AZ; 3Centre for Plane- tary Science & Exploration (CPSX), Western University; 4Johns Hopkins University, APL, Laurel, MD, USA.

Introduction and Motivation: The last decade of Mars years) producing units of m3 m-1 yr-1 in time units Mars exploration has revealed that the current (tenu- of Earth years [11]. See [7] for full methodology. ous) atmosphere is capable of moving surface sedi- Using sediment fluxes and the abrasion susceptibil- ment. Notably, low albedo, aeolian sand ripples and ity (Sa) of rocks (mass of sand needed to erode mass of dunes are actively migrating across the surface today, rock [12]) erosion rates were estimated. The method- which has implications for local wind regimes and ology and assumptions (e.g., Sa for basalt at impact landscape evolution [for a review see 1]. Indeed, aeoli- threshold, mean trajectory height, interdune fluxes, an processes have likely been the predominant geo- etc.) described in Bridges et al. [4] were used. Pressure morphic agent for most of Mars’ history, in contrast to estimates were done using an empirical expression as a the Earth where aqueous processes dominate. function of season and elevation [13]. [14] Global studies have shown geographic variations in Results: Thirty-six sand dune monitoring sites bedform activity status (active, migrating, no detec- across Mars are located within craters, canyons, fossae, tion) based on change detection using high resolution patera, basins, and extracrater terrain (Fig. 1). Average orbital images [1–3]. More detailed investigations have migration rates reach up to 2.1 m/yr for all dune fields quantified migration and volumetric sand fluxes, which (highest rates were detected in Endeavour crater [6]), are independent of dune size, but are limited to just a and the average across all sites was 0.5 m/yr (std. dev. few locations [4–7]. These measurements are im- +/-0.5 m/yr). These rates coupled with associated dune portant, as they relate to surface erosion and its poten- heights (1-90 meters tall) yield sand fluxes of 0.8-17.6 tial exposure of desirable units for in situ analysis [8]. m3 m-1 yr-1 (Fig. 2) – individual dunes may range up to Here, we investigate and quantify dune migration >25 m3 m-1 yr-1. The average and standard deviations trends across Mars, utilizing temporal image series and for all sites investigated are 6.5 +/-4.9 m3 m-1 yr-1. For topography. Related questions include: What are the context, these flux values are often an order of magni- spatial variations of aeolian bedform transport rates tude less than for dune fields on Earth, but larger than and volumetric sediment fluxes? How does the esti- earlier reports for Mars [4, 7]. Dunes fields with the mated abrasion (or erosion) rates vary for these sites highest fluxes are located in the North Polar Erg, Nili and how might they have changed over time? Patera/Fossae region, Mawrth Vallis (e.g., Fig. 3), and Data Sets and Methods: To assess aeolian activi- Hellespontus, whereas low and moderate fluxes are ty, we have utilized images acquired by the High Reso- located in Meridiani, Valles Marineris, and the South- lution Imaging Science Experiment (HiRISE) [9] cam- ern Highlands locations. In contrast, Banks et al. [3] era (0.25–1 m/pix). For image orthorectification and only found ripple movement or no detections for dune dune topography, Digital Terrain Models (DTMs) (at 1 sites south of 57°S latitude (Fig. 1). m post spacing) were constructed from HiRISE stereo Abrasion rates of local basaltic bedrock are esti- pairs [10]. Lee front advancements were recorded in mated to be 0.1–25 µm/yr for flat ground and 1–120 several locations per dune then averaged. Volumetric µm/yr for a vertical rock face. For example, swiftly sand fluxes of the dunes can be obtained using the migrating barchan dunes in McLaughlin crater (Fig. 3) product of the estimated height and the bedform dis- are estimated to be eroding the surface on the high end placement over the intervening time (typically 2–3 of this range (2–80 µm/yr). Fig. 1. Map of Mars showing locations of dune field monitoring sites with (average per site) crest flux estimates (units of m3 m-1 yr-1). Triangles indicate bedform activity status for ripples and dunes [3]. Dune field distribution is shown in red [14]. Base map is MOLA shaded relief with colorized elevation.

Lunar and Planetary Science XLVIII (2017) 2627.pdf

Discussion: Results demonstrate substantial geo- es we can expect at least an order of magnitude in- graphic heterogeneity in dune sediment fluxes across crease in abrasion rates. the planet and per site. High sand fluxes are observed As abrasion rates are only for basaltic target rock, at all the high northern latitude (>70°) sites. While composition is likely an important factor not accounted some dune slipface movement there is driven by sea- for in our estimates. The abrasion susceptibility of sonal CO2 frost, volume estimates suggest that this softer sedimentary terrains composed of clays and sul- process contributes no more than 20% of the local sand fates will allow for greater erosion. Additionally, re- movement [15]. In contrast, sites in the south (<57°S) cent studies have suggested certain geomorphic are often static [3], which is consistent with the hy- boundary conditions are more conducive to higher pothesized stabilization of high southern latitude dune fluxes and erosion rates (e.g., crater degradation state, fields following climate change [16]. Mid- and tropi- presence of rugged adjacent topography [7]). cal-latitude sites can vary substantially, but equatorial Aeolian-driven landscape evolution on Mars is sites tend to have low to moderate fluxes (<9 m3 m-1 yr- likely to be accentuated or muted given certain combi- 1). It is important to note, fluxes are not necessarily nations of sediment fluxes, atmospheric properties, correlated with migration rates. For example, dunes surrounding mesoscale topography, and terrain geolo- across Meridiani are some of the fastest on the planet gy. Future efforts will be employed in understanding (>1 m/yr), but because of their modest sizes have low these factors and the overall distribution of sand overall fluxes [6, 7]. Alternatively, slowly advancing movement across Mars. Syrtis Major barchans, dunes not initially detected as active using shorter baseline images [2], show moderate fluxes due to their larger sizes (>80 m). Monitoring site terrains are frequently found in the presence of streamlined landforms such as knobs and yardangs, often carved out of sedimentary units, where wind erosion and abrasion are likely formation agents. Estimated abrasion rates reported above are similar, but greater than earlier independent assessments. Sand abrasion will be greater during periods of higher atmospheric pressure as the threshold friction speed would be depressed along with the greater occurrence of high winds [17]. For example, high frequency sea- sonal monitoring of the Nili Patera dune field [5] showed the greatest ripple fluxes during perhelion (es- Figure 3. Displacement map of rapidly advancing timated local pressure of 6.3 mbar), while they were barchans (~1 m/yr) in McLaughlin crater. Red-yellow lowest during aphelion (5.3 mbar). This modest change areas indicate darkening (lee-front advancement), in pressure resulted in a three-fold increase in sand while purple-green show lightening (stoss-side ad- fluxes [5]. As atmospheric pressure rises during peri- vancement) over 4 Mars years. Scale bar is 250 m. ods of higher orbital obliquity so will sand fluxes and Acknowledgments: This research was supported in abrasion rates. For example, doubling the perhelion part by NASA MDAP Grants NNH14ZDA001N and the pressure to 12.6 mbar increase fluxes substantially, HiRISE/MRO mission. We would like to thank UofA assuming a linear relationship. With these greater flux- students and staff for assistance with DTM production. References: [1] Bridges N. et al. (2013) Aeolian Res., 9, 133–151. [2] Bridges N. et al. (2011) Geology, 40, 31– 34. [3] Banks M. et al. (This conference). [4] Bridges N. et al. (2012) Nature, 485, 339–342. [5] Ayoub F. et al. (2014) Nat Comm., 5, 10.1038/ncomms6096. [6] Cho- jnacki M. et al. (2015) Icarus, 251, 275–290. [7] Cho- jnacki M. et al. (in press.) Aeolian Res. [8] Williams J. and M. Rice (2016) GSA-Denver, 48, abs. 109-0. [9] McEwen A. et al. (2007) JGR Plan., 112, E05S02. [10] Kirk, R. et al. (2008) JGR Plan.,, 113, E00A24. [11] Ould Ahmedou D. et al. (2007) JGR Earth Surf., 112, F02016. [12] Greeley, R. (1982) JGR Sol. Earth, 113, E00A24. [13] Withers, P. (2012) Space Sci. Rev., 170, 837–860. [14] Hayward, R. (2014) Icarus, 230, 38–46. [15] Diniega Figure 2. Log-log plot of (average) migration rates vs. S. et al (This conference). [16] Fenton L. and R. Hayward heights for 36 dune fields monitoring sites across Mars (2010) Geomorp., 121, 98–121. [17] Armstrong J. and C. (Fig. 1). Diagonal lines are isopleths of sand flux. Leovy (2005), Icarus, 176, 57–74.