Widespread surface weathering on early Mars: A case for a warmer and wetter climate John Carter, Damien Loizeau, Nicolas Mangold, Fraçois Poulet, Jean- Pierre Bibring. Presentation by Jesse Tarnas

Device Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) Spectral range of µm. Measures vibration bands of OH and H 2 O molecules. Measures spectral features (absorption bands, spectral shoulders) in bands within or near µm that allow researchers to distinguish between Al clays and Fe/Mg clays. Used HiRISE color channels to produce imagery of compositional differences in clay areas. < 50 cm/pixel resolution.

Spectra Halloysite: Al 2 Si 2 O 5 (OH) 4 Montmorillonite: (Na,Ca) 0.33 (Al,Mg) 2 (Si 4 O 10 )(OH) 2 * nH 2 O Vermiculite: (Mg,Fe +2,Fe +3 ) 3 [(Al,Si) 4 O 10 ](OH) 2 ·4H 2 O Nontronite: (CaO 0.5,Na) 0.3 Fe 3+ 2 (Si,Al) 4 O 10 (OH) 2 ·nH 2 O Rozenite: Fe 2+ SO 4 4(H 2 O) Clinoptilolite: (Na,K,Ca) 2-3 Al 3 (Al,Si) 2 Si 13 O 36 ·12H 2 O

Implications for Martian Climate Hydrated minerals are either remnants of the Martian past, or are currently being created via subsurface water transportation, or both. Al clays overlaying Fe/Mg clays give information about past climate. Dry and warm climate produce upper layer of smectite/kaolinite above Fe/Mg clays. Wet and tropical climates have additional upper layers of kaolines, smectities, and Al/Fe oxyhydroxides. This characteristic stratigraphy is due to preferential removal of Mg, then Fe through leaching. Widespread distribution of kaolinite minerals and Al-smectites indicates widespread wet climate.

Estimating Weathering Calculate weathering rates for basalt on Earth 8 mm/1000 years – Kohala Mountain, HI mm/1000 years – Vatnajokull, Iceland 0.03 mm/1000 years – Nasunogahara, Japan mm/1000 years – Yellowstone, USA. 0.4 mm/1000 years – Costa Rica Weathering sequences in this paper have thicknesses on the order of a few meters. Consistent with wet climates over < years when compared to Earth rates.

Terra Sirenum

The crater shows several to 10s of meters thick Al clay sequence. This thickness is similar to that of a neighboring crater. Suggests regional weathering event to increase leaching.

Arabia Terra

Arabia Terra/Mawrth Vallis High clay abundances at Mawrth Vallis are consistent with lasting surface weathering. Detection of allophanic material indicates cold and wet climates (snow) based on terrestrial analogues. Al 2 O 3 ·(SiO 2 ) ·(2.5-3)H 2 O When conditions change from cold and wet (snow) to warm and wet (rain), much of the allophanic material is replaced by kaolinites and smectites. Distribution of allophanic material vs kaolinites/smectities gives information regarding regional climate variations.

Why Surface Weathering? 1.Hydrated mineralogy is similar to terrestrial basaltic bedrock. 2.Al clays always stratigraphically above Fe/Mg clays. 3.Widespread on Mars, with similar ages dating to the middle to late Noachian. 4.Thickness of Mawrth Vallis clay sequence can be explained by hypothesis of alteration during sedimentary deposition. 5.Lack of unconformity between Al clay and Fe/Mg layers in several instances, which is consistent with terrestrial weathering studies. 6.Detection of clay sequences at topographic highs. 7.Lack of higher temperature/lower pH minerals, which are found elsewhere on Mars, argues against surface hydro-volcanic activity or sub- surface hydrothermal alteration.

Findings of Paper Found over 1000 new sites on Mars with spectral signatures of clay minerals. Differentiates between Al and Fe/Mg clays. Al clays include smectites and kaolinites. Detection rate of Al clays is artificially lowered because their absorption features are weaker. After calibration adjustments, authors estimate Al clay occurrence frequency of 56% at hydrated sites on Mars. Fe/Mg occurrence frequency is above 90% at hydrated sites on Mars. Also mapped hydrated silica, chlorites, serpentines, and carbonates.

Reference Carter, J., D. Loizeau, N. Mangold, F. Poulet, J.P. Bibring (2015) Widespread surface weathering on early Mars: A case for a warmer and wetter climate. Icarus, 248,