M2020 Candidate Landing Site Data Sheets Mawrth Vallis 1 Mawrth Vallis Location (lat,lon): 24°N, 341°E Summary of observations and interpreted history, including unknowns: The Mawrth Vallis region contains extended outcrops of phyllosilicate-rich rocks. OMEGA and CRISM have detected Fe/Mg-smectites, Al-smectites, kaolinite, hydrated silica, and sulfates in association with light-toned exposures of Noachian bedrock. The Mawrth Vallis site would enable investigation of some of the most ancient outcrops of sedimentary and clay-bearing rocks on Mars. The clay-bearing units correspond to exposures of thick (>300 m), finely layered (layer thickness <<10 m) sedimentary rocks extending across a 300*300 km wide region. The origin of the layering is unknown: interpretations include subaqueous, fluvial and volcaniclastic deposits; the orbital facies does not allow a definitive interpretation. This unit is dominated by Fe/Mg smectites with local interbeds of sulfates. OMEGA and CRISM unmixing models suggest clay mineral abundances as high as 50 wt.%. It is unclear if the Fe/Mg-smectites are related to the global population of crustal Noachian Fe/Mg-smectites. Some portions of the Fe/Mg-unit exhibit large resistant filled fractures and halo-bounded veins that are interpreted to have formed due to fluid circulation. The close proximity of the large Oyama crater, which impacted into the Fe/Mg-unit, suggests that these may be impact hydrothermal deposits; however, low-T groundwater diagenesis cannot be ruled out based on orbital data. Al-rich clays, grading from Al-smectite and silica into kaolinite and possibly allophane, dominate the top 10-30m of the section. The Al-clays are interpreted to have formed during sub-aerial weathering (pedogenic leaching). However, significant mineralogical variability as well as features interpreted as inverted channels suggest that the surface supported aqueous environments. The kaolinite is concentrated near the top of the section, and may either have been formed due to (a) localized acid leaching in a “wetland” environment, (b) regional or global acid surface leaching, (c) long term, more neutral leaching (a laterite). Possible alunite detections at the top of the section support either scenario (a) or (b). Strong spectral signatures consistent with Fe(II)-bearing phyllosilicates associated with the kaolinite may support reducing, poorly drained conditions, but the spectral signature is non-unique. Two scenarios have been proposed for the origin of the Al-unit: (1) The Al-unit postdates the underlying Fe/Mg clays and the contact is an unconformity; (2) The Al-unit is the result of intense leaching of the pre-existing Fe/Mg-clays. The contact between the units is often also characterized by a spectral signature that is consistent with Fe(II)-bearing phyllosilicates, possibly indicating alteration by Fe-rich reducing groundwater. In some locations, close proximity to jarosite and copiapite may imply a strong Fe/S redox gradient, possibly due to oxidation at sub-aerial seeps (Farrand et al, 2014). The clays are capped by a regionally-extensive dark mesa-forming unit that exhibits pyroxene spectral signatures. This unit may either be a pyroclastic deposit or a mafic sandstone. From crater counts, the cap rock is 3.7 Gy old (Early Hesperian). The Al-unit predates this episode and is interpreted as being Late Noachian whereas the thicker layered deposits were deposited before (Middle Noachian or earlier).
TheMawrth Vallis region contains extended outcrops of phyllosilicate-rich rocks. OMEGA and CRISMhavedetectedFe/Mg-smectites,Al-smectites,kaolinite,hydratedsilica,andsulfatesinassociationwithlight-tonedexposuresofNoachianbedrock.TheMawrthVallissitewouldenableinvestigationofsomeofthemostancientoutcropsofsedimentaryandclay-bearingrocksonMars.
Theclay-bearingunitscorrespondtoexposuresofthick(>300m),finelylayered(layerthickness<<10m)sedimentaryrocksextendingacrossa300*300kmwideregion.Theoriginofthe layering isunknown:interpretationsincludesubaqueous,fluvialandvolcaniclasticdeposits;theorbitalfaciesdoesnotallowadefinitive interpretation.Thisunit isdominatedbyFe/Mgsmectiteswith local interbedsofsulfates.OMEGAandCRISMunmixingmodelssuggestclaymineralabundancesashighas50wt.%.ItisuncleariftheFe/Mg-smectitesarerelatedtotheglobalpopulationofcrustalNoachianFe/Mg-smectites.
SomeportionsoftheFe/Mg-unitexhibitlargeresistantfilledfracturesandhalo-boundedveinsthatareinterpreted to have formed due to fluid circulation. The close proximity of the large Oyama crater,which impacted into the Fe/Mg-unit, suggests that these may be impact hydrothermal deposits;however,low-Tgroundwaterdiagenesiscannotberuledoutbasedonorbitaldata.
Al-richclays,gradingfromAl-smectiteandsilicaintokaoliniteandpossiblyallophane,dominatethetop10-30m of the section. The Al-clays are interpreted to have formed during sub-aerial weathering(pedogenic leaching). However, significantmineralogical variability as well as features interpreted asinverted channels suggest that the surface supported aqueous environments. The kaolinite isconcentratednear the topof the section,andmayeitherhavebeen formeddue to (a) localizedacidleaching ina “wetland”environment, (b) regionalorglobalacid surface leaching, (c) long term,moreneutralleaching(alaterite).Possiblealunitedetectionsatthetopofthesectionsupporteitherscenario(a) or (b). Strong spectral signatures consistentwith Fe(II)-bearing phyllosilicates associatedwith thekaolinitemaysupportreducing,poorlydrainedconditions,butthespectralsignatureisnon-unique.
TwoscenarioshavebeenproposedfortheoriginoftheAl-unit:(1)TheAl-unitpostdatestheunderlyingFe/Mgclaysandthecontact isanunconformity;(2)TheAl-unit istheresultof intenseleachingofthepre-existing Fe/Mg-clays. The contact between the units is often also characterized by a spectralsignature that is consistentwith Fe(II)-bearing phyllosilicates, possibly indicating alteration by Fe-richreducinggroundwater. Insomelocations,closeproximityto jarositeandcopiapitemayimplyastrongFe/Sredoxgradient,possiblyduetooxidationatsub-aerialseeps(Farrandetal,2014).
Theclaysarecappedbyaregionally-extensivedarkmesa-formingunit thatexhibitspyroxenespectralsignatures.Thisunitmayeitherbeapyroclasticdepositoramaficsandstone.Fromcratercounts,thecap rock is3.7Gyold (EarlyHesperian).TheAl-unitpredates thisepisodeand is interpretedasbeingLateNoachianwhereasthethickerlayereddepositsweredepositedbefore(MiddleNoachianorearlier).
● Usemorphology,mineralogy,geochemistrytodetermineoriginoffracturefills● Evaluatehost-rockforsignsofimpactdisturbance● Searchfororganicandmorphologicalbiosignaturesinprecipitatedminerals● RIMFAX to seek reflections from fracture fills; if detectable, examine subsurface
geometry. Map subsurface layering on either side of the fracture fills.
● Analyzefacies/texture/mineralogytodetermineigneousvs.aeolianorigin● RIMFAXtoseek reflections from base of cap rock deposits; if detectable, examine
subsurface geometry and contact with surrounding layers. If possible, determine density of deposits, search for internal large blocks that could help determine origin.
