https://doi.org/10.1130/G45793.1 Manuscript received 8 November 2018 Revised manuscript received 18 January 2019 Manuscript accepted 20 February 2019 © 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 11 March 2019 Boundary condition controls on the high-sand-flux regions of Mars Matthew Chojnacki1, Maria E. Banks2, Lori K. Fenton3, and Anna C. Urso1 1Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721, USA 2National Aeronautics and Space Administration (NASA) Goddard Space Flight Center, Greenbelt, Maryland 20771, USA 3Carl Sagan Center at the SETI (Search for Extra-Terrestrial Intelligence) Institute, Mountain View, California 94043, USA ABSTRACT DATA SETS AND METHODS Wind has been an enduring geologic agent throughout the history of Mars, but it is often To assess bed-form morphology and dynamics, unclear where and why sediment is mobile in the current epoch. We investigated whether we analyzed images acquired by the High Resolu- eolian bed-form (dune and ripple) transport rates are depressed or enhanced in some areas tion Imaging Science Experiment (HiRISE) cam- by local or regional boundary conditions (e.g., topography, sand supply/availability). Bed- era on the Mars Reconnaissance Orbiter (0.25–0.5 form heights, migration rates, and sand fluxes all span two to three orders of magnitude m/pixel; McEwen et al., 2007; see Table DR1 in across Mars, but we found that areas with the highest sand fluxes are concentrated in three the GSA Data Repository1). Digi tal terrain mod- regions: Syrtis Major, Hellespontus Montes, and the north polar erg. All regions are located els (1 m/pixel) and orthoimages of dune fields near prominent transition zones of topography (e.g., basins, polar caps) and thermophysical were constructed from HiRISE data using SOCET properties (e.g., albedo variations); these are not known to be critical terrestrial boundary SET® BAE Systems photogrammetry software conditions. The two regions adjacent to major impact basins (Hellas and Isidis Planitia) (Kirk et al., 2008). Volumetric sand fluxes q( crest) showed radially outward upslope winds driving sand movement, although seasonally revers- for dunes were obtained using the product of ing wind regimes were also observed. The northern polar dunes yielded the highest known the estimated height and lee crest displacement fluxes on the planet, driven by summer katabatic winds modulated by the seasonal CO2 cap over the intervening time between acquisition of retreat—processes not known to affect terrestrial dunes. In contrast, southern dune fields repeated HiRISE images (2–5 Mars years), mea- (<45°S) were less mobile, likely as a result of seasonal frost and ground ice suppressing sand sured in units of m3 m–1 yr–1 in time units of Earth availability. Results suggest that, unlike on Earth, large-scale topographic and thermophysi- years. Sand fluxes, migration rates, and heights cal variabilities play a leading role in driving sand fluxes on Mars. were collected for 54 dune fields (495 separate dunes). Ripple migration rates were integrated INTRODUCTION construction appears to be extremely limited on from prior work (Banks et al., 2015, 2018). For most of the history of Mars, eolian pro- Venus (based on orbital radar data; Greeley et al., To provide regional context for monitor- cesses have been one of the leading factors in 1995), whereas bed forms on Titan (a moon of ing sites, topographic and thermal properties landscape evolution, in contrast to Earth and Saturn) cover an estimated ~13% of the surface (albedo and daytime surface temperature) were early Mars, where aqueous processes prevailed. (from similar data; Le Gall et al., 2012). The examined from Mars Orbiter Laser Altimeter Recent orbital studies have revealed that winds numerous extraneous environmental factors on data (Smith et al., 2001) and Thermal Emission frequently transport fine-grained sediments planetary bodies with atmospheres (e.g., transport Spectrometer (on the Mars Global Surveyor) across the surface of Mars today, as evidenced capacity, sand supply, topography) operate non- data (Christensen et al., 1992), respectively. See by the mobility of eolian bed forms (dunes and linearly to produce self-organized bed forms that the GSA Data Repository, sections 1 and 2, for ripples; e.g., Silvestro et al., 2010; Chojnacki evolve in response to environmental conditions additional methodology and boundary condition et al., 2011, 2017, 2018; Bridges et al., 2011, (e.g., Kocurek and Lancaster, 1999; Ewing and characterization details, respectively. 2012; Ayoub et al., 2014; Banks et al., 2015, Kocurek, 2010). Recognition of these underly- 2018). The degree of contemporary sand mobil- ing boundary conditions can provide insight into RESULTS ity has implications for the Martian climate, dust landscape evolution on Mars because eolian pro- cycle, and landscape evolution. Moreover, spa- cesses have played a major role there for most of Global Results of Bed-Form Transport tial variations in bed-form transport parameters its history (Armstrong and Leovy, 2005). Parameters relate to external forcing factors (i.e., boundary The objective of this investigation was to Active eolian bed forms were detected across conditions), some of which may be unique to assess the boundary conditions that govern bed- Mars within craters, canyons, structural fossae, the red planet. form migration trends on Mars and whether those volcanic paterae, polar basins, and extracrater (e.g., Terrestrial antecedent conditions, such as conditions differ from ones on Earth. Recogniz- plains) terrain (Fig. 1). The average migration rate, near-surface water tables and vegetation, can ing these factors may also provide predictabil- typically for barchan or barchanoid dune morphol- critically influence local bed-form patterns and ity for other locations lacking sufficient data for ogies, across all sites investigated here was ~0.5 ± mobility. On Mars, factors like crater morphology monitoring. This was achieved by quantifying the 0.4 m/yr (1σ) for dunes ~2–120 m tall. Superposed and local surface roughness can influence sand dynamic activity of bed forms globally and evalu- sub-meter-tall ripples (~40 cm tall, where height fluxes (Chojnacki et al., 2017). Large-scale dune ating the regional factors that may play a role. estimates were from in situ observations; Ewing 1GSA Data Repository item 2019150, Figures DR1–DR5, Tables DR1 (list of images), Tables DR2 and DR3 (region summary tables), and Animations DR1–DR8, is available online at http:// www .geosociety .org /datarepository /2019/, or on request from editing@ geosociety .org. CITATION: Chojnacki, M., Banks, M.E., Fenton, L.K., and Urso, A.C., 2019, Boundary condition controls on the high-sand-flux regions of Mars: Geology, v. 47, p. 427–430, https:// doi .org /10 .1130 /G45793.1 Geological Society of America | GEOLOGY | Volume 47 | Number 5 | www.gsapubs.org 427 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/5/427/4680645/427.pdf by guest on 01 October 2021 Figure 1. Global trends in E conditions dictate many physical aspects of dune bed-form mobility. Top: N Polar cap morphology and activity, the three regions with Map of sand dune crest Abalos Undae Olympia Undae Sand Flux (m3 m–1 yr –1) remarkably high sand fluxes deserve further flux measurements for 0.8–5.0 54 dune fields (graduated consideration. 5.0–9.0 Arabia Terra Syrtis Major circles with low [red], 9.0–19.8 moderate [orange], and Mobile Valles Marineris Isidis basin Gale Syrtis Major high [green] sand flux No Detection Meridiani values), bed-form mobility Dunes across the low-albedo (~0.1) Syrtis detection status (trian- Hellespontus Major are found in a variety of geographical con- Hellas basin gles; Banks et al., 2015, Terra Cimmeria texts (e.g., fossae, paterae, plains, and craters), but 2018), and dune field dis- Terra Sirenum Noachis Terra their migration vectors appear to be strongly influ- tributions (red polygons; S Polar cap enced by the 4–5-km-deep Isidis basin to the east Hayward et al., 2014). Three high-sand-flux Dune Rate (m yr–1) Dune Height (m) Sand Flux (m3 m–1 yr –1) (Fig. 3; Table DR2). Westward sand transport at regions of Syrtis Major, n=495 n=495 n=495 this equatorial latitude is governed by the super- Hellespontus Montes, and 0.49 ± 0.43 19 ± 14 7.8 ± 6.4 position of the meridional (Hadley cell) overturn- the north polar erg (Mars) ing circulation and planetary rotation, but this flow are outlined in black and shown in more detail in is further enhanced by a strong thermal tide and Figure 3. Base map is Mars Orbiter Laser Altimeter (MOLA) shaded relief with colorized eleva- anabatic slope winds driven up and radially out- tion from +4 (red) to –5 km (blue). Bottom: Histograms of dune metrics with averages ± 1σ. ward from the dusty Isidis basin (albedo ~0.23) by daytime solar heating (e.g., Ayoub et al., 2014). et al., 2017) showed similar rates (~0.5 m/yr), west of the Isidis basin, Hellespontus west of the Dunes near Jezero crater and deep within Nili Fos- although examples of faster ripples have often been Hellas basin, and the circumpolar erg adjacent to sae are clearly migrating to the west-northwest reported (Bridges et al., 2012). The average qcrest the north polar cap (Figs. 3 and 4). Elevation pro- (Chojnacki et al., 2018), whereas sand dunes along for all dune fields ranged between 0.8 and 17.6 files several hundreds of kilometers in length from Nili and Meroe Paterae are translating toward the m3 m–1 yr–1 (Fig. 2), while the highest flux for an basins toward the higher-elevation terrains have west-southwest (Fig. 3; see Animations DR1 3 –1 –1 individual dune was 35 m m yr (average qcrest = 4–7 km in relief, except for the shorter (~2-km- and DR2 and Table DR1).