Evidence for pervasive mud volcanism in Acidalia Planitia, MarsIntroductionGeologic settingData and methodologyDataMethodologyDistribution of the mounds – “Generalized Area of Occurrence”Estimates of abundance and spatial density of the moundsEstimates of mound diameter and heightDescription of the Acidalia moundsCharacteristicsOccurrence, abundance, and spatial densityIndications of flowAge of mounds in the plains of AcidaliaAge of mounds on the Acidalia Mensa platformMineralogyAlternative hypotheses for the origin of the Acidalia moundsAnalogs other than mud volcanoesImpact structuresPingosEvaporite or spring-related featuresMagmatic featuresMud volcano analogTerrestrial mud volcanoesAcidalia mounds and the mud volcano analogMud volcanism in AcidaliaDistinctive martian attributesFlow in Acidalia MensaFlow at polygon boundariesPotential venting of gas to the martian atmosphereUniqueness of Acidalia, habitability, and astrobiological potentialConclusionsAcknowledgmentsReferencesEvidence for pervasive mud volcanism in Acidalia Planitia, MarsDorothy Z. Oehler*, Carlton C. AllenAstromaterials Research and Exploration Science Directorate, NASA-Johnson Space Center, 2101 NASA Parkway, Houston, TX 77058, USAa r t i c l e i n f oArticle history:Received 29 August 2009Revised 16 March 2010Accepted 24 March 2010Available online 1 April 2010Keywords:Mars, SurfaceAstrobiologyabstractWe have mapped 18,000+ circular mounds in a portion of southern Acidalia Planitia using their sizes,shapes, and responses in Nighttime IR. We estimate that 40,000+ of these features could occur in the area,with a distribution generally corresponding to the southern half of the proposed Acidalia impact basin.The mounds have average diameters of about 1 km and relief up to 180 m and most overlie units mappedas Early Amazonian.High resolution images of mound surfaces show relatively smooth veneers, apron-like extensions ontothe plains, moats, and concentric circular crestal structures. Some images show lobate and flow-like fea-tures associated with the mounds. Albedo of the mounds is generally higher than that of the surroundingplains. Visible and near-infrared spectra suggest that the mounds and plains have subtle mineralogicaldifferences, with the mounds having enhanced coatings or possibly greater quantities of crystalline ferricoxides.Multiple analogs for these structures were assessed in light of new orbital data and regional mapping.Mud volcanism is the closest terrestrial analogy, though the process in Acidalia would have had distinctlymartian attributes. This interpretation is supported by the geologic setting of the Acidalia which sits atthe distal end of the Chryse–Acidalia embayment into which large quantities of sediments were depos-ited through the Hesperian outflow channels. In its distal position, Acidalia would have been a depocenterfor accumulation of mud and fluids from outflow sedimentation.Thus, the profusion of mounds in Acidalia is likely to be a consequence of this basin’s unique geologicsetting. Basinwide mud eruption may be attributable to overpressure (developed in response to rapidoutflow deposition) perhaps aided by regional triggers for fluid expulsion related to events such as tec-tonic or hydrothermal pulses, destabilization of clathrates, or sublimation of a frozen body of water. Sig-nificant release of gas may have been involved, and the extensive mud volcanism could have createdlong-lived conduits for upwelling groundwaters, providing potential habitats for an in situ microbiota.Mud volcanism transports minimally-altered materials from depth to the surface, and mud volcanoesin Acidalia, therefore, could provide access to samples from deep zones that would otherwise be inacces-sible. Since the distal setting of Acidalia also would favor concentration and preservation of potentially-present organic materials, samples brought to the surface by mud volcanism could include biosignaturesof possible past or even present life. Accordingly, the mounds of Acidalia may offer a new class of explo-ration target.! 2010 Elsevier Inc. All rights reserved.1. IntroductionThe potential for mud volcanism in the Northern Plains of Marshas been recognized for some time, with candidate mud volcanoesreported from Utopia, Isidis, northern Borealis, Scandia, and theChryse–Acidalia region (Davis and Tanaka, 1995; Tanaka, 1997,2005; Tanaka et al., 2000, 2003, 2008; Farrand et al., 2005; Kiteet al., 2007; Rodríguez et al., 2007; Skinner and Tanaka, 2007; Allenet al., 2009; Oehler and Allen, 2009; Skinner and Mazzini, 2009;McGowan, 2009; McGowan and McGill, 2010). We have focusedon the mud volcano-like mounds in Acidalia Planitia (Figs. 1–3),as these features are present in enormous numbers and may reflecta major event in the history of the martian lowlands.The Acidalia mounds (Figs. 2 and 3) were first noted in Vikingimagery (Allen, 1979; Frey et al., 1979; Frey and Jarosewich,1982) and have been variously interpreted as pseudocraters, cindercones, tuff cones, pingos, or ice-disintegration features (summa-rized in Tanaka (1997), Tanaka et al. (2005), and Farrand et al.(2005)). Recently, they have been compared to mud volcanoes (Ta-naka et al., 2003, 2005) or some combination of mud volcanoes andevaporite deposition around geysers and/or springs (Farrand et al.,2005). Our work uses new orbital data coupled with regional map-ping and basin analysis to evaluate the distribution of the moundsas well as the details of their surface geomorphology. New imagesfrom the Context Camera (CTX) and High Resolution Imaging0019-1035/$ - see front matter ! 2010 Elsevier Inc. All rights reserved.doi:10.1016/j.icarus.2010.03.031* Corresponding author.E-mail address: [email protected] (D.Z. Oehler).Icarus 208 (2010) 636–657Contents lists available at ScienceDirectIcarusjo u r n a l h o m e pa g e : w w w . e ls e v i e r . c o m/ l o c a t e/ i c a r u sScience Experiment (HiRISE) on Mars Reconnaissance Orbiter(MRO) have allowed identification of associated flow-like featuresand internal structure in some mounds. In addition, data from theCompact Reconnaissance Imaging Spectrometer for Mars (CRISM)on MRO have provided new insights into the mineralogy of thesestructures. Alternative hypotheses for the origin of the moundsare further assessed in light of these new data. Results are mostcompatible with a model for their formation involving basinwidemud volcanism.Mud volcanoes are terrestrial structures