Fourth Conference on Early Mars 2017 (LPI Contrib. No. 2014) 3033.pdf PROPERTIES, ORIGINS, AND PRESERVATION OF ANCIENT OLIVINE-BEARING BEDROCK: IMPLICATIONS FOR NOACHIAN PROCESSES ON MARS, A. D. Rogers1, J. C. Cowart1, J. W. Head2, N. H. Warner3, A. Palumbo2, and M. P. Golombek4, 1Stony Brook University, Stony Brook, NY, USA ([email protected]), 2Brown University, Providence, RI, USA, 3SUNY Geneseo, Geneseo, NY USA, 4Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA Introduction and significance: Many intercrater cratered terrains [10]. Even where Hesperian volcanic plains and degraded impact craters in the Noachian high- plains are in direct contact with older bedrock, a striking lands contain flat-lying, relatively high thermal inertia difference in TI and regolith cover is observed, such that (TI) surfaces (“bedrock”) that commonly show modest Hesperian plains are more mantled than the older subja- enrichments in olivine and/or pyroxene compared to sur- cent bedrock units [10] (Fig. 2). A candidate explanation rounding materials (Fig. 1). These surfaces have maxi- for this is that the Noachian bedrock was rapidly buried mum TI values above 500 J m-2 K-1 s-1/2 in THEMIS im- and then recently exposed [10], however an alternative ages; some units exhibit TI values well above 900 J m-2 explanation is that the Noachian bedrock units are me- K-1 s-1/2. These physicochemically distinctive units occur chanically weak/fine-grained materials that break up into in dozens of isolated exposures ranging from ~2x102 to fine-grained materials [e.g. 11,12] that are more easily ~3x104 km2 in area and exhibit morphologies indicating moved by wind. In contrast, competent Hesperian lavas reduced/minimal sediment mantling. The units lack evi- would have comminuted into blocky/coarser-grained, dence of fine-scale layering in HiRISE imagery, and also less-mobile materials. Additional examples of high TI lack evidence of aqueous alteration in spectral data [1-7]. (less mantled) units subjacent to Hesperian lavas are observed in Gusev crater [13] and NE Syrtis (Fig. 3). In those locations, significantly mantled Hesperian lavas directly abut older, less mantled rock units. In Gusev crater, the older units may represent olivine-bearing ba- saltic tephras, similar to the Algonquin-class rocks inves- tigated by the Spirit Rover [14]. At NE Syrtis, the older units contain sulfates and aqueously altered basaltic units Fig. 1. Global distribution of high TI intercrater plains exposures (white) [7]. Surfaces with TES TI values > [15]. Both of these rock types would likely be mechani- 325 J m-2 K-1 s-1/2 were delineated using the qualitative cally weak compared to unaltered basaltic lavas, such as THEMIS nighttime radiance mosaic. TI values quoted in those present in the Hesperian volcanic plains [e.g. 17]. text are derived from THEMIS nighttime imagery, from the warmest (least mantled) portions of the delineated bedrock unit. The origin(s) of these units is uncertain. In past stud- ies, an effusive volcanic origin was generally favored on the basis of the relatively high TI, distinctive composi- tion, and the difficulty of spatially concentrating olivine over such large scales through sediment transport and sorting [1-6,8]. However, the lack of volcanic morpholo- Fig. 2. Near the southern margin of Syrtis Major, light-toned gies (e.g. flow lobes, source vents), as well as the de- toned, basaltic bedrock (higher TI) underlies darker toned graded nature of these units and limited vertical exposure basaltic lavas (lower TI). (a)THEMIS TI over THEMIS makes their origin(s) uncertain. daytime radiance. (b)Portion of HiRISE image ESP_036579_ The widespread, distinctive nature of these units 1795. Light-toned bedrock contains isolated bedforms. Lows makes them significant; ascertaining their origins, within darker-toned Syrtis lavas are covered by sediment. preservation and early modification history is an im- 2. Quantitative analyses of bulk composition suggest portant aspect of understanding the geological processes that some olivine-bearing bedrock units are not signifi- and environments that were occurring on early Mars. In cantly compositionally distinct from surrounding low this work we present a set of new observations that sug- TI units. Early work examining Noachian bedrock fo- gest that some of these Noachian bedrock units may have cused on eastern Noachis Terra, where bedrock units are non-volcanic or non-effusive volcanic origins. most abundant and exhibit the highest TI values [2,4]. Observations: 1. The bedrock units have not fol- There, many bedrock surfaces are clearly compositional- lowed the same degradation and regolith development ly distinct (more mafic bulk mineralogy) from the sur- path as known volcanic plains. Hesperian volcanic rounding low TI plains, crater walls and ejecta, con- plains have developed a thick regolith [9, 16] and have a firmed by spectral extraction and comparison for some of notable lack of bedrock exposure compared to Noachian the regions. We have extended these analyses to bedrock Fourth Conference on Early Mars 2017 (LPI Contrib. No. 2014) 3033.pdf units in other highland locations, and find that some units likely require a volcanic (effusive or pyroclastic) or other are not spectrally distinct from surrounding materials explanation. One such hypothesis is described below. (e.g. [7], Fig. 4). The influence of large Noachian-aged impact crater- ing events [20-21] has recently been revisited and the timeline of events and geological effects updated in order to understand the climatic implications for early Mars [22-23]. For basin-scale impact events, the vaporized silicate portion of the expanding vapor plume should be globally and homogeneously distributed and is predicted to condense out at very high temperatures and fall as hot spherules to the surface; the deposit will initially be suf- ficiently hot that it is likely to behave as rheomorphic flows, filling low-lying areas before final cooling and Fig. 3. Examples where Hesperian lavas abut older, likely solidification. On the basis of this analysis, [22] suggest mechanically weak materials (see text). (a) Gusev (b) NE that an alternative interpretation to extrusive volcanism Syrtis. The differences in surface TI expression may result for Noachian-aged mafic silicate layers may be that of from differences in friability. exposed remnants of silicate condensate layers of Noa- chian large craters and basins, with mineralogies repre- senting crust and possible mantle contributions processed by the impact and vapor plume evolution, and subse- quent silicate condensate layer emplacement [22-23]. We are currently further assessing this as one of our hypothe- ses for the origin of these high TI Noachian surfaces. Concluding remarks: Intercrater plains bedrock units are variable in composition and morphology, and proba- Fig. 4. Example of olivine- bly have different origins [e.g. 8,22,24]. The preserva- bearing bedrock unit with tion and aeolian exposure history of these units is likely a minimal difference in bulk key factor in their spatial distribution and in giving rise composition from surround- ing low TI units. (a) CRISM to their relatively high TI values compared to average MSP OLV-INDEX (b) TES values for low-dust regions. A close examination of the 507 cm-1 index (an indicator properties of each exposure is underway [7]. of bulk mafic content [3]) (c) Acknowledgments: Thanks to Steve Ruff for useful THEMIS spectra from on and discussions. This work was supported by NASA MDAP off the bedrock unit. NNX14AM26G. Discussion: The observations discussed above indi- References: [1] Edwards et al., (2009), JGR 114, E11001 cate that some olivine-bearing bedrock units may repre- [2] Rogers et al. (2009) Icarus, 200 (2) 446-462 [3] Rogers and sent mechanically weak materials, such as sedimentary or Fergason (2011), JGR 116(E8), 1–24 [4] Rogers and Nazarian pyroclastic units. True thermal inertia values from these (2013), JGR 118, 1-19. [5] Ody et al. (2012), JGR, 118, 1-29. [6] Edwards, C.S, et al. (2014), Icarus, 228, 149-166 [7] Cow- units would help to address this question; for example, art and Rogers (2017), 48th LPSC, Abs. 1547 [8] Rogers and Mini-TES derived thermal inertia values from individual Edwards (2016), GSA abstracts, doi: 10.1130/abs/2016AM- blocks at Gusev crater showed that Adirondack class 286571 [9] Warner et al., (2017) Space Sci Rev (2017). (Hesperian) basalts exhibited TI values of ~1200 J m-2 K- doi:10.1007/s11214-017-0352-x [10] Rogers and Head (2017), 1 s-1/2, whereas Columbia Hills rocks exhibited values 48th LPSC, Abs. 1347 [11] Malin and Edgett (2000), Science, around ~600 J m-2 K-1 s-1/2 [18]. However, locating 290, 1927-1937 [12] Golombek et al. (2006), JGR, 111, THEMIS pixels with fully unmantled exposures is a E12S10 [13] Grant et al. (2006) GRL, 33, 2-7. [14] Ruff et al., challenge. Detailed thermal modeling using THEMIS (2014) Geology, 42, 359-362. [15] Ehlmann and Mustard images spanning multiple times of day are underway to (2012), GRL, 39, 1-7 [16] Golombek et al. (2006), JGR, 111, 1-27. [17] Thomson et al (2013), JGR, 118, 1233-1244 [18] constrain underlying bedrock TI of these units [19]. Fergason et al. (2016), JGR, 111, E02S21 [19] Ahern et al. Where the differences in bulk primary mineralogy (2017), 48th LPSC, Abs. 2337 [20] Segura et al. (2002) Sci- between the bedrock and surrounding low TI materials is ence, 298, 277-280 [21] Toon et al. (2010), Ann Revs., 38, minimal, a sedimentary origin remains a strong explana- 303-322 [22] Palumbo and Head (2017), Impact Cratering as a tion for these materials; however, lithification must have Cause of Climate Change, Surface Alteration, and Resurfacing occurred with minimal amounts of water, as aqueous During the Early History of Mars, M&PS, in revision [23] minerals are not observed in these units.