GEOLOGIC SETTING of the OLYMPUS MACULAE, MARS. K. D. Seelos1, C

51st Lunar and Planetary Science Conference (2020) 2985.pdf

GEOLOGIC SETTING OF THE OLYMPUS MACULAE, MARS. K. D. Seelos1, C. E. Detelich1,2, K. D. Run- yon1, S. L. Murchie1, J. L. Bishop3, A. D. Rogers4, and K. E. Craft1. 1Johns Hopkins University Applied Physics La- boratory, 11100 Johns Hopkins Road, Laurel, MD 20723 ([email protected]); 2Univ. of Alaska, Anchorage, AK; 3 SETI Institute, Mountain View, CA; 4Stony Brook University, Stony Brook, NY.

Introduction: The Olympus Maculae are an arcu- controlled mosaics from the THermal EMission Imag- ate series of ten, ~2-20 km-diameter semicircular albe- ing System (THEMIS; daytime IR, nighttime IR, and do anomalies located in Lycus Sulci, the aureole ter- derived qualitative thermal inertia), and 6 m/pix visible rains northwest of Olympus Mons (Figure 1). These imagery acquired by the Context Camera (CTX). All features have no topographic expression and superpose data were supplied by the Planetary Data System other late Amazonian units [1, 2], including the aureole (PDS); CTX data were calibrated and processed by the terrains (unit Aa in [2]), Medusa Fossae Formation USGS Projection on the Web (POW) tool. materials (unit AHtu), and lava flows (unit Ave) (Fig- Descriptions and Stratigraphic Relationships: ure 1). While this region is an area of net dust accumu- Seven units were delineated and are shown in Figure 2: lation, detailed characterization [3,4] has shown that Ridged Terrain I and II. Two lobes of aureole ter- the maculae are conspicuous because of their relative rains overlap within in our study area. Thought to have lack of dust, and that the process of preferential dust formed as a result of massive underwater landslides removal is modern and active [5]. Current thinking is [6,7], these highly fractured landforms have a corrugat- that the anomalies are likely sustained by thermally- ed appearance with long, high relief linear ridges ex- driven atmospheric turbulence [3, 5]; however, it re- tending roughly perpendicular to the extensional direc- mains unclear exactly how the maculae were initiated. tion. In our map area, the ridges of the older of the two In this work, we map the local geology of the lobes are oriented primarily N-S and mapped as Olympus Maculae in order to provide spatial and strat- Ridged Terrain I. Ridged Terrain II corresponds to the igraphic context for these features and clues as to the younger lobe toward the north and consists of both the underlying mechanism(s) responsible for their origin. ridged terrain as well as long runout landslides origi- natig at the lobe’s marginal scarp. AMAZONIS Aeolian Terrain. Superposed on the Ridged Terrain Lycus PLANITIA units is a mantle of layered material consistent with Sulci Olympus Mons other regional occurrences of the Medusa Fossae For- “Medusa Fossae mation (MFF), most likely comprised of ash fall depos- Formation” Fig. 2 Pavonis its and ignibrites sourced to Apollinaris Patera and Mons perhaps other Tharsis volcanoes [8]. These materials Arsia were deposited after emplacement of Ridged Terrain II Apollinaris Mons Patera as well, although the marginal landslides may have AHv Amazonian formed later. Aa Global Geology lAv lAa from [2] The MFF materials are highly susceptible to aeoli- Late Amazonian Ave lAa Apron Unit an modification, resulting in extensive fields of both lAvf Amazonian Apron Fig. 2 Aa Unit linear and U-shaped yardangs [3]. Within the dust-free Amazonian Volcanic maculae themselves, dark sand ripples as well as cross- Ave Edi?ce AHtu Amazonian-Hesperian bedded layering is evident, indicating multiple cycles AHtu Transitional Unit AHtu of aeolian erosion and redeposition [3]. In a few areas, AHv Amazonian-Hesperian Volcanic Unit deflation of a previously more elevated surface is evi- Figure 1. Regional context showing the study area located denced by pedestal craters and highstanding U-shaped west of Olympus Mons on Lycus Sulci (the aureole ter- features that were likely former yardang troughs. rains). (Top) Viking color MDIM over MOLA shaded Volcanic Terrain. An extensive lava flow field en- relief. (Bottom) Amazonian global geologic units identi- croaches upon the eastern map area. The morphologic fied by [2] over MOLA shaded relief. texture of flow surfaces varies from smooth to crenu- Datasets and Methodology: Units were defined on lated with well-defined individual flow margins. The the basis of morphologic expression and albedo differ- flows clearly embay both Ridged Terrain I and II (Fig- ences (in the case of the maculae) at the scale of ap- ure 2). Volcanic Terrain tends to also superpose the proximately 1:50000. To do this we utilized multiple Aeolian Terrain unit, although there are a few instances global and local datasets including topography from where this relationship is ambiguous and concurrent the Mars Orbiter Laser Altimeter (MOLA), 100 m/pix deposition may be a possibility. 51st Lunar and Planetary Science Conference (2020) 2985.pdf

Figure 2. (Top) Geologic context Units for the Olympus Maculae (see Fig Crater 1). One of the characteristics of Ejecta the maculae is elevated thermal inertia, which appears bright in Crater the underlying THEMIS daytime IR basemap. (Lower left) En- Macula largement of map area that exem- plifies the stratigraphic relation- Volcanic ships observed between units. Terrain Embayment of both Ridged Ter- Aeolian rain I and II by the Volcanic Ter- Terrain rain (e.g., see white arrows in Ridged lower right frame) is clearly dis- Terrain II cernable and Macula terrain su-

Ridged perpose all other units. Craters Terrain I are rare. (Lower right) Visible CTX data for the same area. The maculae commonly have a cir- cumferentially zoned pattern and most appear darker than the sur- rounding dust-covered terrain. Ridged gafgfdg Terrain II However, the easternmost macula shown here (nicknamed the “Big Volcanic Island”) exhibits higher albedo material in the inner zone. Com- Terrain pact Reconnaissance Imaging Spectrometer for Mars (CRISM) vis- near infrared spectral analysis of this bright material is presented in a companion paper by Bishop et al., this conference. Ridged Terrain I exhibit no ejecta at all, although as few display pedestal morphology. The Volcanic Terrain and Ridged Terrains retain craters to a greater extent than other mapped units. Macula Discussion: The Olympus Maculae represent an intriguing series of very young features on the flank of Olympus Mons. Their presence appears largely inde- pendent from local geologic features (i.e., yardangs, “Big Island” ridges, or volcanic flows), and yet, the maculae may Crater Aeolian be fairly unique on Mars. This suggests that the prove- Terrain nance of the maculae may be sourced to subsurface activities and/or conditions, like structural controls or

hydrothermal circulation. Macula: As previously mentioned, the maculae are Acknowledgments: The authors gratefully defined based on their albedo and other anomalous acknowledge the MGS, Odyssey, and MRO mission characteristics, e.g., enhanced relative thermal inertia and instrument teams, USGS Astrogeology, and the land lack of topographic expression or any perceived PDS. This work was supported by NASA MDAP spatial relationship to other landforms. Emplacement grant number 80NSSC17K0451. post-dates that of both aureole Ridged Terrains and the Aeolian Terrain, with ongoing dust-clearing activity as References: [1] Morris and Tanaka (1994), USGS Map observed since the MY34 (2018) global dust event [5]. I-2327. [2] Tanaka et al. (2014), USGS Map I-3292. [3] De- Crater and Crater Ejecta: There are very few cra- telich et al. (2019) LPS 50, Abstract #1861. [4] Bishop et al. ters in the study area with none observed over 1 km in (2020) LPS 51 – this mtg. [5] Runyon et al. (2019), EPSC- diameter. This may be a result late Amazonian surface DPS Joint Mtg., Vol 13, Absract EPSC-DPS2019-721-1 [6] age but the apparent friable nature of the Aeolian Ter- De Blasio, F. B. (2011), Earth and Plan. Sci. Lett, 312, 126- rain materials as well as infilling by dust likely contrib- 139. [7] De Blasio, F. V. (2018), Icarus, 302, 44-61. [8] ute to obliteration. Similarly, a vast majority of craters Bradley et al. (2002), J. Geophys.Res, 107, E8 5058.