Geological Study of a Section of Aeolis Mensae, a Possible Site Favorable for Life

Seventh International Conference on Mars 3262.pdf

GEOLOGICAL STUDY OF A SECTION OF AEOLIS MENSAE, A POSSIBLE SITE FAVORABLE FOR LIFE. S. Martínez-Alonso1,2 , M.T. Mellon2, A.S. McEwen3, and the HiRISE Team, 1Department of Geological 2 Sciences, University of Colorado at Boulder, CO, martinas at colorado.edu; Laboratory for Atmospheric and Space 3 Physics, University of Colorado at Boulder, CO; University of Arizona, Department of Planetary Sciences, Lunar and Planetary Lab, Tucson, AZ.

Introduction: Could life exist, or have existed in producing a large diversity of characteristic mineral the past, at Aeolis Mensae? High water and Cl con- species) are widely considered prime loci for the onset tents along a major structural boundary suggest that and development of organic activity. this locality has a strong potential as a favorable habi- - High water and Cl abundances, and complex sur- tat. This region is centered at a segment of the di- face composition could also be indicative of evaporitic chotomy boundary which separates Elysium Planitia deposits, which would constitute another type of envi- in the northern lowlands (where some of the most re- ronment favorable to the search for life, because of cent outflow/tectonic/volcanic activity in the planet high biological productivity and optimal conditions for has been documented) from the southern highlands. preservation of biosignatures. The fracture zones ob- The dichotomy boundary here consists of fretted ter- served may have acted as fluid conduits. rains developed along two distinct, strikingly linear - The water and/or Cl anomalies may be associated fracture zones which extend for approximately 2,000 with wind-transported materials (sedimentary and/or km. volcano-sedimentary) originating at the fractured zone This region coincides with anomalously high water or elsewhere. and Cl concentrations measured by the MO-GRS [1, These and alternate hypotheses are investigated 2]. These anomalies are elongated and either overlap through a comprehensive geological study of a section (water) or are adjacent and parallel to (Cl) the dichoto- of the Aeolis Mensae region (5oN 220oW to 15oS my boundary. This region also presents locally anoma- 200oW; ~1,200-by-1,200 km). lous, larger-than-average diversity of surface materi- Methodology: Evidence for the aforementioned als, as previously calculated from the global MGS- geologic processes operating in Aeolis Mensae is be- TES dataset [3]. ing sought through a study of the nature of surficial Many relevant geological processes may result in materials, surface composition (mineralogy, the concentration of a large diversity of surface mate- lithology), and morphology. rials in relatively small regions (e.g., hydrothermal ac- Thermophysical Analysis. TES thermal inertia and tivity, sedimentation, volcanism, erosion). The cause of the alignment of the dichotomy boundary in this re- albedo were analyzed to determine the physical prop- gion is not well understood; neither is its relationship erties of surface materials (particle size, rock/bedrock with, and the significance of, the anomalously high exposure, and degree of induration) and constrain the water and Cl abundances or the significance of the lo- character of the surface. Thermophysical classes were cally complex surface composition. determined by comparison to the global units defined The large surface composition diversity, positive by [4] and [5], based on thermal inertia and albedo water and Cl anomalies, and structural setting could be values. consistent with volcanic/hydrothermal or evaporitic Spectral Analysis. Data-derived spectral endmem- activity. Certain types of eolian deposits could explain bers were extracted from MGS-TES and MEx- the elemental abundances observed, but they would OMEGA data of the study area, and mapped utilizing produce low surface-composition diversity. spectral linear mixing models. Endmembers were Volcanic/hydrothermal and evaporitic environments identified by comparison to public spectral libraries [6, are of special interest because they are widely consid- 7, 8]. ered prime loci for the onset and development of or- Morphological Analysis. MGS-MOLA, MGS- ganic activity. MOC, and MRO-HiRISE data were analyzed to place We propose the following testable hypotheses to the compositional and thermophysical information in explain these observations: its geological context. The excellent spatial (up to 25 - The linear zone of fractured terrains, the water cm/pixel) and radiometric resolution of HiRISE allow and Cl anomalies, and the large diversity of surface for the study of surface's processes in unprecedented materials could be indicative of volcanic/hydrothermal detail. activity in the Aeolis Mensae region. Volcanic and hy- Discussion: The thermophysical analysis (Fig. 1) drothermal systems (regions where magmatic fluids indicates that the northern half of the study area (the rich in organogenic elements interact with wall rocks, lowlands), is -at TES scale- mostly covered by dust Seventh International Conference on Mars 3262.pdf

and duricrust. In the southern half (the highlands), The TES spectral data show dust signatures in the duricrust predominates; bedrock and/or rocks are dust- and duricrust-covered regions, and indicate that found in impact craters and elsewhere, possibly due to the bedrock/rocky regions in the highlands are consis- duricrust denudation. tent with Syrtis-type materials [9], widely interpreted as basaltic in composition. We have identified olivine- rich areas in the TES data coinciding with bedrock/rocky regions in the highlands, as well as in the floor of one of the troughs in the dichotomy boundary (Fig. 2). The olivine is forsteritic (Mg-rich) in composition. OMEGA data (Fig. 3) show the olivine is present where dust and/or duricrust has been removed by wind; thus, the olivine's extent may be larger than that mapped.

Figure 1. Spatial distribution of the thermophysical units derived from TES thermal inertia and albedo. Yellow: bright unconsolidated fines. Green: duricrust; some sand, rocks, bedrock. Red: sand, rocks, bedrock; some duricrust. Purple: rocks, bedrock, duricrust, po- lar ice.

Figure 3. (A) MNF color composite derived from OMEGA scene ORB0371_2. Orange areas are olivine- Figure 2. Minimum Noise Fraction (MNF, a PC-like rich; white asterisks show olivine seen in TES. (B) transformation) color composite derived from TES Another MNF color composite; in this case most of emissivity. According to their TES spectra, pink re- the complexity in the image can be explained by gions are basaltic; white asterisks indicate olivine-rich changes in overall reflectance which may be due to outcrops. White box indicates location of the OMEGA grain size and/or surface texture diversity in otherwise image shown in Fig. 3. spectrally homogeneous materials. Seventh International Conference on Mars 3262.pdf

The imaging data indicate that this is a geological- We find that the MFF outcrops transition spatially ly complex region. HiRISE data will be utilized to dis- to alignments of cones similar to terrestrial cinder criminate among several possible origins for the cones in scale and morphology (Fig. 4, 5, and 6); some basaltic and olivine-rich materials identified in the of the cones seem to be associated with fissures. The highlands: in situ volcanic or intrusive, impact materi- cones are located in regional troughs, topographically als, or sedimentary/volcano-sedimentary. Two main depressed areas which may be structurally controlled surface types are present in the lowlands: (1) very flat, (Fig. 4). Troughs, cones and fissures, and the MFF smooth terrains and (2) two conspicuous lobe-shaped, materials could be genetically related; thus, at least lo- layered deposits oriented NW-SE, part of the Medusae cally, the flute-like morphology of the MFF would not Fossae Formation (MFF) [10]. The morphology of the be due to erosion alone, but to the preferred orienta- former is consistent with fluid lava flows, as suggested tion of vents (cones and fissures) though which the by: layering, flow-like features, wrinkle ridges, quasi- circular features interpreted as impact craters covered by inflated lava flows, large bulges with central craters interpreted as volcanic shields, and platy/polygonal features. The MFF deposits have been previously in- terpreted as (among others) eolian and/or pyroclastic. Their flute-like morphology has been interpreted as yardangs, indicative of strong eolian erosion [11 and references therein]. HiRISE data [12] show the MFF deposits to be fairly massive, with some bedding, but lacking fine-scale layers and cross-bedding. Some of the layers are blocky and resistant enough to form cliffs. Locally, thin resistant layers (dikes or faults in- durated by fluids) diagonally cross the MFF deposits [12].

Figure 4. MOLA elevation (-4,200 to 2,700 m) stretched to increase contrast. The two large NW-SE lobate features are part of the Medusae Fossae Forma- tion (MFF). Red crosses: MOC images showing Figure 5. Subset of MOC image R1304097 (3 km aligned cones transitioning to MFF outcrops (encircled wide) showing spatial transition between MFF out- image is shown in Fig. 5). White crosses: cones crops (bottom) and aligned cones (top). Labels show aligned or in clusters. Green asterisks: olivine seen in location of the subsets of HiRISE image TES. White box: OMEGA image shown in Fig. 3. PSP_002411_1820 shown in Fig. 6. Seventh International Conference on Mars 3262.pdf

MFF materials were extruded. We estimate the MFF in this area to be at least 800 m thick (based on MOLA topography) and approximately 0.25x106 km3 in vol- ume, dimensions similar to those of the Deccan Traps or the Columbia basalts, both massive, layered terres- trial volcanic deposits, criss-crossed by dikes, and ex- truded along fissures. This interpretation contrasts with earlier studies [13] which advocated for Tharsis volcanism as the source of the MFF deposits, inferred to be volcano-sedimentary. Alternatively, the cones at Aeolis could be hydro- volcanic, formed when lava interacted explosively with water or ice contained within the substrate. HiRISE data of Athabasca Valles, approximately 900 km ENE of the study area, show numerous edifices (RMLs, or ring-mound landforms), of similar size as the Aeolis cones but contrasting morphology [14]. That study presents evidence showing that the RMLs (also locally aligned) are hydrovolcanic in origin. Their alignment could be caused by buried topogra- phy: steam would vent preferentially through the lava flow where the overburden pressure is the lowest (i.e., where the flow is the thinnest). Further analysis of available and newly acquired data will help discrimi- nating between these two models. No spectral or morphological evidence supporting an evaporitic origin for the materials in this region has been found thus far; a volcanic/hydrothermal origin is the most plausible. It is to be expected that diagnostic minerals exist under the prevalent dust/duricrust mantling; they may outcrop locally, where uncovered by impact cratering or wind denudation. Acknowledgments: This research is funded in part by a grant from NASA's Mars Data Analysis Program to S. Martínez-Alonso (MDAP04-0000-0065).

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