Clay and sulfate-bearing terrains in Northern Meridiani Planum, Mars: constraining the characteristics of Mars&#8217; early climate at the Noachian-Hesperian boundary
Authors: Baschetti Beatrice , Massironi Matteo , Carli Cristian , Altieri Francesca , Frigeri Alessandro 
Published: September 2022

&lt;p&gt;&lt;strong&gt;Introduction:&amp;#160;&lt;/strong&gt;Despite the large amount of information from orbital and in situ missions, the characteristics and evolution of Mars&amp;#8217; early climate are still widely debated among the scientific community. Morphological evidence dating back to about 3-4 billion years ago, such as valley networks [1], and the presence of several hydrated minerals [2] seem to indicate that Mars once had a &amp;#8220;warm and wet&amp;#8221; climate with abundant water on the surface and possibly even an ocean in the northern lowlands [1-3]. These conditions eventually changed through time leading up to the hyperarid and cold planet we observe today.&amp;#160;&lt;/p&gt;
&lt;p&gt;The area of Meridiani Planum (MP), located SW of Arabia Terra, is well known for retaining multiple evidence of past aqueous activity [4] and a varied hydrated mineralogy [5]. The majority of the terrains exposed in MP formed at the Noachian-Hesperian boundary [6], that is between two important epochs of Mars: the Noachian (4.1-3.7 Ga), where most of the evidence for water is found, and the Hesperian (3.7-3.0 Ga), which is characterized by increasingly water-limited conditions. Constraining the potential water environment at the NH boundary is fundamental to understand Mars&amp;#8217; early climate and its evolution.&lt;/p&gt;
&lt;p&gt;In this study, we select&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;&lt;span lang=&quot;EN-US&quot;&gt;7 craters in Northern MP (figure 1) showing evidence of NH sediment infillings with hydrated materials (e.g., clays and sulfates) to assess in detail the stratigraphic sequence of the mineralogical&amp;#160;&lt;/span&gt;units and understand their origin and diagenetic history. The craters are roughly aligned SE to NW following the general slope of the area from the highlands to the lowlands. If Mars experienced a &amp;#8220;warm and wet&amp;#8221; period, MP would represent a transition zone between the subaerial alteration environment of the highlands and the subaqueous environment of the Martian ocean. Therefore, the area would be easily affected by climatic changes whose evidence should be retained in its sedimentary sequences.&lt;/p&gt;
&lt;p&gt;We show here some&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;&lt;em&gt;preliminary results&lt;/em&gt;&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;from 2 out of the 7 craters selected for the study:&lt;/p&gt;
&lt;p&gt;1) a 15-km-wide crater named Mikumi, centered at Lat. 2.45&amp;#176;N, Lon. 359.96&amp;#176;E;&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;&lt;br /&gt;2) an 18-km-wide unnamed crater centered at Lat. 4.25&amp;#176;N, Lon. 2.85&amp;#176;E.&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;&lt;img src=&quot;&quot; alt=&quot;&quot; width=&quot;834&quot; height=&quot;508&quot; /&gt;&lt;/strong&gt;&lt;/p&gt;
&lt;p&gt;&lt;em&gt;Figure 1:&lt;/em&gt;&lt;span class=&quot;apple-converted-space&quot;&gt;&lt;em&gt;&amp;#160;&lt;/em&gt;&lt;/span&gt;&lt;em&gt;&lt;span lang=&quot;EN-US&quot;&gt;THEMIS daytime image of Meridiani Planum. Selected craters are evidenced with points. Name (if given) and coordinates of the center for each crater are also indicated with a label. Most of the craters are unnamed. The arrows indicate the two craters described in this abstract.&lt;/span&gt;&lt;/em&gt;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;&lt;span lang=&quot;EN-US&quot;&gt;Datasets and methods:&amp;#160;&lt;/span&gt;&lt;/strong&gt;&lt;span lang=&quot;EN-US&quot;&gt;We investigate the mineralogy of the craters&amp;#8217; terrains through CRISM [7] hyperspectral data cubes mainly in the range 1.0-2.6 &amp;#956;m. This interval retains part of the key spectral features of primary rock-forming minerals, which constitute the Martian crust, and of most minerals produced by secondary processes such as aqueous circulation and alteration (e.g., clays and sulfates).&amp;#160;MOLA, THEMIS, CTX and HiRISE data are then used for morpho-stratigraphy and camera imaging.&lt;/span&gt;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;&lt;span lang=&quot;EN-US&quot;&gt;Results:&lt;/span&gt;&lt;/strong&gt;&lt;span class=&quot;apple-converted-space&quot;&gt;&lt;span lang=&quot;EN-US&quot;&gt;&amp;#160;&lt;/span&gt;&lt;/span&gt;&lt;span lang=&quot;EN-US&quot;&gt;Two types of&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;&lt;em&gt;clays&lt;/em&gt;&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;(smectites)&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;&lt;em&gt;with different Fe/Mg content&lt;/em&gt;,&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;&lt;em&gt;polyhydrated&lt;/em&gt;&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;sulfates and&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;&lt;em&gt;monohydrated sulfates&lt;/em&gt;&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;are observed in both craters (figure 2). Fe/Mg-clay spectra (e.g., nontronite and saponite) show absorptions at around 1.4, 1.9 and 2.3 &amp;#956;m with additional overtones at 2.4 &amp;#956;m. The exact position of the 2.3 &amp;#956;m feature depends on the relative Fe/Mg content in the clay mineral. We find some areas with clays richer in iron (e.g., nontronite), showing this feature centered at 2.305 &amp;#956;m, and clays richer in magnesium (e.g., saponite) where the absorption is centered at 2.310-2.315 &amp;#956;m. In Mikumi crater, polyhydrated sulfates (Mg sulfate) and monohydrated sulfates (kieserite) are detected&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;&lt;em&gt;stratigraphically below&lt;/em&gt;&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;the clay-bearing layer. For the unnamed crater, the stratigraphic relationship of the units is still to be investigated.&lt;/span&gt;&lt;/p&gt;
&lt;p&gt;&lt;img src=&quot;&quot; width=&quot;654&quot; height=&quot;685&quot; /&gt;&lt;/p&gt;
&lt;p&gt;&lt;em&gt;&lt;span lang=&quot;EN-US&quot;&gt;Figure 2: (top) CRISM spectra of detected mineralogy. Fe/Mg clays (smectites) are from Mikumi crater, mono and polyhydrated sulfates are from the Unnamed crater. The spectra are extracted from FRT0000BEF5 and FRT00009B5A TRR datasets respectively. (bottom) Laboratory spectra of clays and sulfates&amp;#160;from the CRISM spectral library [8]&amp;#160;(Mg Sulfate ID: CJB366;&amp;#160;Kieserite ID: F1CC15;&amp;#160;Saponite ID: LASA51;&amp;#160;Nontronite ID: NCJB26).&lt;/span&gt;&lt;/em&gt;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Discussion and conclusions:&lt;/strong&gt;&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;Clays and sulfates on Mars appear to have formed at different times and under different climatic conditions [9]. Clays are usually associated to Noachian terrains and may have formed under alkaline conditions in a &amp;#8220;warm and wet&amp;#8221; ancient Mars, whereas sulfates are typically associated to Hesperian surfaces and may have formed under a dryer and more acidic environment. Therefore, sulfates are expected to be found stratigraphically on top of clays, as they should have formed later in Mars&amp;#8217; history. However, this distinction between a clay-rich Noachian and a sulfate-rich Hesperian&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;&lt;em&gt;oversimplifies&lt;/em&gt;&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;the history of the aqueous chemistry and climate of Mars especially near the NH boundary.&lt;/p&gt;
&lt;p&gt;Our analysis suggests a stratigraphic sequence with&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;&lt;em&gt;interleaving clays and sulfates&lt;/em&gt;&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;at the NH boundary. &amp;#160;Similar stratigraphic sequences have also been observed in other areas of MP (Southern MP) by [5]. &amp;#160;In the case of [5], a clay-bearing layer is overlain by other sulfates, generating a sulfate/clay/sulfate stratigraphic sequence. All this argues is in favor of&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;&lt;em&gt;several distinct climatic episodes&lt;/em&gt;&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;pacing the NH climatic transition.&lt;/p&gt;
&lt;p&gt;The results obtained so far will be enriched by further analysis of these areas along with a thorough investigation of the remaining 5 craters selected.&amp;#160;&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Acknowledgments:&lt;/strong&gt;&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;Featured CRISM data were downloaded from the Planetary Data System (PDS). This project is partially supported by Europlanet RI20-24&amp;#160;GMAP project.&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;References:&lt;/strong&gt;&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;[1] M. H. Carr and J. W. Head (2010)&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;&lt;em&gt;Earth and Planet. Sci. Lett.&lt;/em&gt;,&lt;span class=&quot;apple-converted-space&quot;&gt;&lt;em&gt;&amp;#160;&lt;/em&gt;&lt;/span&gt;&lt;em&gt;293&lt;/em&gt;, 185-203. [2] B. L. Ehlmann and C. S. Edwards (2014)&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;&lt;em&gt;Annu. Rev. Earth Planet. Sci.&lt;/em&gt;,&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;&lt;em&gt;42&lt;/em&gt;, 291-315. [3] R. A. Craddock and A. D. Howard (2002) JGR, 107 (E11), 5111. [4] R. M. E. Williams et al. (2017)&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;&lt;em&gt;GRL, 44&lt;/em&gt;, 1669-1678. [5]&amp;#160;J. Flahaut et al. (2015)&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;&lt;em&gt;Icarus, 248&lt;/em&gt;, 269-288. [6] B. M. Hynek et al. (2002)&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;&lt;em&gt;JGR, 107 (E10),&lt;/em&gt;&lt;span class=&quot;apple-converted-space&quot;&gt;&lt;em&gt;&amp;#160;&lt;/em&gt;&lt;/span&gt;5088. [7] S. Murchie et al. (2007)&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;&lt;em&gt;JGR, 112 (E5)&lt;/em&gt;, E05S03. [8] C. E. Viviano-Beck et al. (2015)&amp;#160;MRO CRISM Type Spectra Library, NASA Planetary Data System.&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;&lt;em&gt;https://crismtypespectra.rsl.wustl.edu&lt;/em&gt;&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;[9] J. P. Bibring et al. (2006)&lt;span class=&quot;apple-converted-space&quot;&gt;&amp;#160;&lt;/span&gt;&lt;em&gt;Science, 312&lt;/em&gt;, 400-404.&lt;/p&gt;