Reconstructing the Paleo-Climate and Hydrology of Gale Crater, Mars in the Late Noachian and Hesperian Epochs

Fourth Conference on Early Mars 2017 (LPI Contrib. No. 2014) 3069.pdf

RECONSTRUCTING THE PALEO-CLIMATE AND HYDROLOGY OF GALE CRATER, MARS IN THE LATE NOACHIAN AND HESPERIAN EPOCHS. D. G. Horvath1 and J. C. Andrews-Hanna2, 1Southwest Re- search Institute, Boulder, CO. 2Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ.

Introduction: The central sedimentary deposit in following the formation of Gale crater near the end of Gale crater, Aeolis Mons, may preserve one of the the Noachian [1]. By removing Aeolis Mons and best records of the early Martian climate during the reconstructing the pre-Aeolis Mons topography at late Noachian and early Hesperian, and the transition Gale, lake conditions during the deposition of the from wetter conditions when fluvial valleys and lakes basal units (Murray mudstones) of Aeolis Mons was were active [1, 2, 3] to drier conditions when most of modeled. Using the upper contact of the Murray lake the thick accumulations of sediments were forming bed mudstones as an indicator for lake level, we find [4]. Following these drier conditions, late stage hy- that an aridity index less than 6 is required (Fig. 1b), drology, after the formation of Aeolis Mons, has been at the transition between arid and semi-arid climates. identified from fluvial dissection of Aeolis Mons and Although the climate could have been wetter than the fan deposits on the Gale crater floor [5]. semi-arid conditions suggested by this model, drier Here we used numerical models of the surface and conditions cannot account for the extent and eleva- subsurface hydrology of Gale crater and its surround- tion of the lake bed deposits. This indicates that wet ings, constrained by the sedimentary record preserved conditions, and specifically a semi-arid or wetter cli- in Aeolis Mons (Fig. 1a) and late-stage lake levels, mate, were required to form the Murray mudstone inferred from fan deposits in Gale (Fig. 2a), to recon- lake deposits at the base of Aeolis Mons. A semi-arid struct the climate history of Gale crater. climate is consistent with previous estimates of the Model: In this study, we used the hydrological Noachian climate using the geomorphology of fluvial model of [8] with parameters appropriate for Mars networks [2]. over Gale crater. This model combines a finite- difference approximation of the groundwater flow equation to simulate subsurface hydrology with an analytical surface runoff model. The model was forced using evaporation potential (Ep) and precipitation (P) rates from Earth-based observations of ana- log climates provided by the North American Land Data Assimilation Systems (NLDAS). We focused on a semi-arid Great Plains and an arid Arizona climate. The model results are most sensitive to the mean an- Fig 1. a) Reconstructed topography at Gale crater nual ratio of Ep to P, referred to as the aridity index prior to the formation of Aeolis Mons (color map) (ϕ). The evaporation potential and precipitation rates showing the elevation contour (white outline) at the were then scaled to investigate a range of aridity in- upper contact of the Murray lake bed mudstone [1] dices from 1.5 to 33, representing climates on Earth and identified open and closed-basin lakes outside of ranging from sub-humid to hyper-arid. While a range Gale (“O” and “C” respectively; [6, 7]). b) A lake of annual precipitation was investigated, models (black) and hydraulic head (contour) map for an arid- shown here use an annual precipitation of 160 mm/yr. ity index of 6 corresponding to the highest elevation The total annual aquifer recharge and surface run- of the Murray mudstone. off were determined from the precipitation and evaporation potential using an Earth-based empirical rela- Early Hesperian climate at Gale crater: The tionship [9], which uses the aridity index to deter- majority of the thickness and volume of Aeolis Mons mine the fraction of precipitation that contributes to is comprised of the sulfate-cemented lower formation the surface and subsurface hydrology rather than [4]. Although MSL has yet to explore this unit in evaporating immediately back into the atmosphere. Gale, similarities to the deposits in Meridiani Planum This model assumed a megaregolith aquifer mod- [11] suggest a formation mechanism via groundwa- el adapted from [10], which has a vertically averaged ter-mediated cementation and alteration of aeolian aquifer permeability from the surface to 10 km depth sediments. The transition in deposits from mudstones of 3×10-13 m2. to cemented aeolian material cannot be explained Late Noachian climate at Gale crater: Lake and solely as a result of the infilling of the crater, and fluvial deposits observed by the Mars Science Labor- requires a climate change. Global and regional atory (MSL) making up the lowest stratigraphic unit groundwater models have been successfully used to of Aeolis Mons suggest persistent long-lived lakes reproduce the distribution and thickness of ground- Fourth Conference on Early Mars 2017 (LPI Contrib. No. 2014) 3069.pdf

water cemented sediments in both Meridiani and Gale, and have shown that the elevation of the top of the hydrated lower formation matches the predicted rise height of the water table during the infilling of the crater with sediments [12]. Thus, even if the an- hydrous upper formation were deposited in the same climate and hydrological environment as the lower formation, it would not be expected to be similarly cemented and altered. An origin for the Aeolis Mons lower formation by groundwater-mediated alteration and cementation implies that the crater was once filled with sediments to at least the top of the lower formation. A massive erosion event is then required to explain the present- day mound shape, similar to the widespread erosion of sedimentary deposits in Arabia Terra [13]. Aeolian erosion of the sediments likely required a drop in the water table to the level of the present-day crater floor Fig. 2. a) MOLA topography at Gale crater (color- or lower [14], and thus requires a change to a hyper- map) showing the inferred post-Aeolis Mons lake arid climate. stands in Gale (colored contours; [5]) and identified Gale crater lakes during the Early Hesperian: open and closed-basin lakes outside of Gale (“O” and Continued hydrology and lake formation during the “C” respectively; [6, 7]). Lakes (shown in black) and Hesperian, after the erosion of the crater deposits to hydraulic head maps (contours) are shown for differ- their current mound shape, is evidenced by fluvial ent aridity indices of b) 1.5 c) 3.5 and d) 9, and com- erosion of Aeolis Mons and well preserved fan de- pared to the highest lake stand inferred in Gale [5]. posits on the floor of Gale [5]. Based on lake level estimates from these fan deposits (Fig 1a), an aridity the deposits to their current mound shape, and period- index of 3.5 matches the highest observed lake stand ic returns to semi-arid conditions during the Hesperi- assuming a shape parameter of 1.6 and an annual an to reproduce the observed late-stage lake stands precipitation of 160 mm/yr (Fig. 2c). Climates in the [5]. These results indicate drastic climate changes sub-humid (ϕ=1.5) and arid (ϕ=9) regimes, predict over the lifetime of the hydrological system at Gale, lake levels that exceed the highest lake stand and fall suggesting a return to semi-arid conditions similar to short of the lowest observed lake stand in Gale crater the Noachian climate at some point during the Hespe- respectively (Fig. 2b, d). Depending on the assumed rian in order to produce lakes that correspond to the annual precipitation and amount of water that reaches observed late-stage lake stands. Furthermore, this the surface and subsurface hydrology, an aridity in- work has shown that with continued observation of dex range between 3 and 6, in the semi-arid climate lake bed deposits on Mars, hydrological modeling regime, matches the range of observed lake stands in can be used to further constrain the past climate and Gale crater during the Hesperian. At the assumed hydrology on Mars. annual precipitation of 160 mm/yr, this range of References: [1] Grotzinger, J. P. et al. (2015) Sci- aridity indices is comparable to high latitude, cold ence, 350, 6257. [2] Hynek, B. M. et al. (2010) deserts and steppe climates on Earth. These climates JGRP, 115, E09008. [3] Bibring, J. P. et al. (2006) are much wetter than is thought to characterize much Science, 312, 400-404. [4] Thomson, B. J. et al. of the Hesperian, requiring wetter interludes after the (2011) Icarus, 214, 413-432. [5] Palucis, M. C. et al. transition to arid conditions in the Hesperian. (2016) JGRP, 121, 472-496. [6] Goudge, T. A. et al. Conclusions: Using indicators of paleo-lake lev- (2012) Icarus, 219, 211-229. [7] Goudge, T. A. et al. els and the observed extent aqueous alteration we (2015) Icarus, 260, 346-367. [8] Horvath, D. G. et al. have used hydrological models to provide constraints (2016) Icarus, 277, 103-124. [9] Budyko, M. I. on the past climate at Gale crater. These results favor (1974) Climate and life, Academic Press., New York, a persistent semi-arid climate (ϕ<6) during the Late 508. [10] Hanna, J. C. & Phillips, R. (2005) JGR, Noachian to account for the lake bed mudstones at 110, E01004. [11] McLennan, S. M. et al. (2005) the base of Aeolis Mons, a transition to arid condi- Nature, 438, 1129-1131. [12] Andrews-Hanna, J. C. rd tions during the Early Hesperian and persistent et al. (2012) 3 Early Mars Conf., id. 7038. [13] Za- groundwater flow to Gale crater accounting for the brusky, K. et al. (2012) Icarus, 220, 311-330. [14] aqueously altered aeolian deposits in Aeolis Mons, a Andrews-Hanna, J. C. et al. (2010) JGR, 115, transition to hyper-arid conditions to allow erosion of E06002.