Ninth International Conference on 2019 (LPI Contrib. No. 2089) 6001.pdf

THE CASE FOR A WARM AND WET (SEMI-ARID) EARLY MARS. R. M. Ramirez1 and R. A. Craddock2, 1Earth-Life Science Institute, email: [email protected], 2The Smithsonian Institution, email: [email protected].

Introduction: The surface of Mars today is similar to The evidence: We will be reviewing both the geo- that of a dry desert, with cold mean surface tem- logical and climatological evidence and give reasons peratures ~70 K lower than Earth’s and an atmosphere why a warm and semi-arid climate best fits the observa- only ~ 1% thick. These conditions are very different tions to date. from those ~ 4 billion years ago, when the world had a more water-rich history, perhaps exhibiting a climate Icy hypotheses. Several scenarios assume that the more like that of our own planet. However, the climate baseline climate was very cold and icy. How- of early Mars has been a topic of intense debate for dec- ever, the majority of such cold and icy early Mars sce- ades. Although most investigators believe that the geol- narios acknowledge that at least transiently warm con- ogy, including the valley networks (Figure 1), indicates ditions had to be present (e.g. [3][4][5]) , although there the presence of surface water, disagreement has per- are exceptions [6]. sisted regarding how warm the surface must have been The challenge with nearly all of these transient and how long such conditions may have existed. Cli- warming mechanisms is their inability to generate the mate models that only include CO2 and H2O as - durations of warming and amounts of water required to house gases have been unable to generate warm surface form the modified craters or valleys. For instance, im- conditions given the faint young Sun. Some models sug- pact hypotheses struggle to have the long-lasting cli- gest that a continuously warm climate could have been matic impact that appears necessary to generate enough possible by supplementing this CO2-H2O warming with water to carve these immense fluvial features [1]. Plus, either secondary greenhouse gases or CO2 clouds. Oth- impact-induced cirrus clouds provide some warming, ers posit that Mars’ climate was cold most of the time, but it is insufficient to carve the valleys unless cloud but underwent periodic episodes of transient warming cover fractions are unrealistically high [7]. There are caused by external events. also timing issues, as the largest impactors hit the sur- Here, we review the geologic (and geochemi- face before the initiation of valley formation during a cal) evidence to date and show that a warm and semi- time period when the observed fluvial erosion was low arid climate is the most likely interpretation [1]. We also [8,9](Figure 2). stress here that such a climate does not imply warm and wet “Earth-like tropical climates”, but rather a season- ally warm periglacial climate, perhaps akin to the Great Basin Region during the Pleistocene [2]. We also dis- cuss results from both 1-D and 3-D models, and explain where they are both consistent and inconsistent with the evidence and each other [1]. Finally, we conclude with recommended future directions for atmospheric models, geologic observations, and upcoming missions (e.g. Mars 2020).

Figure 2: A schematic of the geologic evolution of Mars over time [1] Figure 1: The (a) versus a Martian den- dritic river system (b) (Arabia quadrangle; 12° N, 43° E). Scale bar is 60 km long [1]. Ninth International Conference on Mars 2019 (LPI Contrib. No. 2089) 6001.pdf

The icy highlands hypothesis [4], which relies on snow- warm solutions for pure CO2-H2O atmospheres is im- melt to form the valleys, suggests that weathered land- possible, predicting cold and icy climates with maxi- scapes would be localized to ice-covered areas and pre- mum mean surface temperatures hovering at ~225 – 230 dicted glacial melt paths. However, this is not observed. K [15,20]. Both suggest that atmospheres supplemented Instead, the widespread absence of small craters (< ~ with SO2 can produce significant warming although 1- 5km radius), and global distribution of modified craters D photochemical models find that SO2 would rain out imply a global process, likely rainfall, being a major before much warming is possible [21]. However, only agent of erosion [10]. The absence of glacial features in 1-D models have assessed the additional greenhouse ef- valley terrains [11](although there may have been ice in fect of CH4 or H2, finding stable warm climates may be other locations at high [12]), including perigla- possible [e.g., 20]. Both 1-D and 3-D models predict cial features, the apparent need for a hydrologic cycle that surface ice in small amounts only modestly impact [13] (e.g. crater), and various other geomorphic in- the planetary energy budget [14, 15]. One 3-D model dicators (explained in talk) challenge this hypothesis[1]. predicts little rainfall in Margaritifer Sinus and Arabia Another problem with all cold and icy scenar- Terra [22], regions showing fluvial features consistent ios is the higher greenhouse gas concentrations required with rainfall [e.g. 11,23]. I will discuss what these re- to warm icy worlds [14]. This is because ice is reflec- sults mean for upcoming missions (e.g. Mars 2020). tive. Clouds and high ice thermal inertia can also make warm conditions more difficult to achieve [15]. A warm and semi-arid climate. In spite of uncertainties, a warm and semi-arid climate best fits the evidence Geochemical evidence. This is considerably more am- [1,24]. It is also consistent with planetary water esti- biguous than geomorphic observations and remains in- mates (< 200 m global-equivalent)[25]. The climate conclusive as different interpretations are possible. Alt- could have been continuously or seasonally warm[1]. hough some studies suggest minimum aqueous altera- Unlike a perennially cold and icy scenario, thin surface tion in valley terrains, the sheer volume of the valley ice deposits would seasonally melt, as they do in some networks (e.g. Viking, MRO), suggesting large amounts terrains on Earth [2]. This may explain the lack of wet- of water, argue the opposite (Figure 2). The observed based glaciation features and avoids the ice problem in- sequence of Al- over Fe-bearing clays in some regions herent to cold and icy Mars models [1]. had been inferred to be consistent with a largely frozen References: [1] Ramirez, R.M. and R.A. Craddock [16]. However, this sequence is con- (2018) NatGeo 11, 230 – 237. [2] Matsubara Y. et al. sistent with a warm CO2-rich atmosphere with acid (2013) J. Geophys. Res. 118, 1365 - 1387 [3] R. Urata rain[17]. and O.B. Toon (2013) Icarus 226, 229 – 250. [4] R.D. The “missing carbonate” problem is consid- Wordsworth et al. (2013) Icarus 222, 1, 1-19. [5] Ba- ered a challenge for a warmer and wetter early Mars. talha, N.E. et al. (2015) EPSL 455, 7 – 13 [6] Fairen, The presence of trace carbonates in the present day soil, A.G. (2010) Icarus 208.1, 165 – 175 [7] Ramirez, R.M. coupled with trivial atmospheric escape rates over the and J.F. Kasting (2017) Icarus 281, 248 – 261. [8] past 4 Gyr, had been linked to low atmospheric CO2 Golombek, M.P. et al. (2006) J. Geophys. Res. 111 paleopressures [3]. However, 1) acid rain in a CO2-rich E12S10 [9] Hynek, B.M. et al. (2010) J. Geophys. Res. atmosphere would dissolve surface carbonates [17], 2) 115[10] Chapman, C.R. and K.L. (1977) AREPS carbonate grains in Martian rocks suggest that an under- 5, 515 – 540 [11] Davis, J.M. et al.(2016) Geology 44, ground reservoir exists [18], and 3) newer models from 847 - 850 [12] Bouquety, A. et al. (2019) Geomorph. MAVEN results find that atmospheric escape was more 334, 91 – 111. [13] Luo et al. (2017) Nat. Comm. 8, intense than once thought [19]. 15766 [14] Ramirez, R.M. (2017) Icarus 297, 71 - 82. [15] Forget, F. et al. (2013) Icarus 222, 81 – 99 [16] What do the climate models say? Both 1-D and 3-D cli- Ehlmann, B.L. et al. (2011) Nature 479, 53 – 60 [17] mate models are used to model Mars paleoclimate. One- Zolotov, M.Y. and M.V.Mironenko (2016) Icarus 275, dimensional models run much faster, and so a larger pa- 203 – 220 [18] H.Y. McSween (1994) Meteoritics 26, rameter space, including more complex greenhouse gas 757 – 779 [19]B.M. Jakosky et al. (2017) Science 355, chemistry and radiative transfer, can be considered. In 1408 – 1410 [20]Ramirez R.M. et al. (2014) Nat. Geosc. contrast, 3-D models calculate more processes self-con- 7, 59 - 63 [21] Tian et al. (2010) EPSL 295, 412 – 418 sistently (e.g. dynamics) than can be done in 1-D. Nev- [22] Wordsworth, R.D. et al. (2015) J. Geophys. Res. ertheless, both sets of models agree on many aspects of 120.6, 1201 – 1219 [23] Luo et a. (2002) J. Geophys. the early Martian climate. Res. 107, 5071 [24] Craddock, R.A. and A.D Howard Achieving warm solutions for early Mars is (2002) J. Geophys. Res. 107, 5111 [25] G.L. Villanueva difficult. Both 1-D and 3-D models find that achieving et al. (2015) Science 348, 218 - 221