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Minitti M.E.,Planetary Science Institute Tucson
Journal of Geophysical Research: Planets | Year: 2017

The Mars Science Laboratory Curiosity rover undertook comprehensive exploration of the Kimberley waypoint within Gale crater, Mars in order to understand its context within the larger geologic picture of Gale crater and its evidence for past Martian habitability. Coordinated observations from Curiosity's rich science payload revealed important insights into new Martian crustal compositions, the prevalence and diversity of sedimentary processes within Gale crater, and surface erosion rates. Exploration at the Kimberley, in part informed by a decade of orbital observations of Gale crater, underscored the critical synergy between landed and orbital observations and furthered understanding of complex geological processes on Mars. ©2017. American Geophysical Union.

Gallagher C.,University College Dublin | Balme M.,Open University Milton Keynes | Balme M.,Planetary Science Institute Tucson
Earth and Planetary Science Letters | Year: 2015

Although glacial landsystems produced under warm/wet based conditions are very common on Earth, even here, observations of subglacial landforms such as eskers emerging from extant glaciers are rare. This paper describes a system of sinuous ridges emerging from the in situ but now degraded piedmont terminus of a Late Amazonian-aged (~150 Ma) glacier-like form in the southern Phlegra Montes region of Mars. We believe this to be the first identification of martian eskers that can be directly linked to their parent glacier. Together with their contextual landform assemblage, the eskers are indicative of significant glacial meltwater production and subglacial routing. However, although the eskers are evidence of a wet-based regime, the confinement of the glacial system to a well-defined, regionally significant graben, and the absence of eskers elsewhere in the region, is interpreted as evidence of sub-glacial melting as a response to locally enhanced geothermal heat flux rather than climate-induced warming. These observations offer important new insights to the forcing of glacial dynamic and melting behaviour on Mars by factors other than climate. © 2015 Elsevier B.V.

Balme M.R.,Open University Milton Keynes | Balme M.R.,Planetary Science Institute Tucson | Gallagher C.J.,University College Dublin | Hauber E.,Institute For Planetenforschung
Progress in Physical Geography | Year: 2013

Liquid water is generally only meta-stable on Mars today; it quickly freezes, evaporates or boils in the cold, dry, thin atmosphere (surface pressure is about 200 times lower than on Earth). Nevertheless, there is morphological evidence that surface water was extensive in more ancient times, including the Noachian Epoch (∼4.1 Ga to ∼3.7 Ga bp), when large lakes existed and river-like channel networks were incised, and early in the Hesperian Epoch (∼3.7 Ga to ∼2.9 Ga bp), when megafloods carved enormous channels and smaller fluvial networks developed in association with crater-lakes. However, by the Amazonian Epoch (∼3.0 Ga to present), most surface morphogenesis associated with liquid water had ceased, with long periods of water sequestration as ice in the near-surface and polar regions. However, inferences from observations using imaging data with sub-metre pixel sizes indicate that periglacial landscapes, involving morphogenesis associated with ground-ice and/or surface-ice thaw and liquid flows, has been active within the last few million years. In this paper, three such landform assemblages are described: a high-latitude assemblage comprising features interpreted to be sorted clastic stripes, circles and polygons, non-sorted polygonally patterned ground, fluvial gullies, and solifluction lobes; a mid-latitude assemblage comprising gullies, patterned ground, debris-covered glaciers and hillslope stripes; and an equatorial assemblage of linked basins, patterned ground, possible pingos, and channel-and-scarp features interpreted to be retrogressive thaw-slumps. Hypotheses to explain these observations are explored, including recent climate change, and hydrated minerals in the regolith 'thawing' to form liquid brines at very low temperatures. The use of terrestrial analogue field sites is also discussed. © The Author(s) 2013.

Grindrod P.M.,University College London | Balme M.R.,Open University Milton Keynes | Balme M.R.,Planetary Science Institute Tucson
Geophysical Research Letters | Year: 2010

We describe a conceptual model of groundwater processes at Hebes Chasma, Mars, which can account for the distribution of hydrated minerals and their subsequent evolution. At Hebes Chasma, pressure gradients set up by the large central mound, Hebes Mensa, could cause groundwater to be sourced predominantly from beneath the central region, if such water were present. Evaporation of upwelling groundwater would cause monohydrates to form at or near the surface through efflorescence, and polyhydrates to form inside the central mound through subflorescence. This crystallization could lead to an excess pore pressure, causing large-scale weakening and subsequent collapse that can reveal the interior polyhydrated deposits. If evaporation is high compared to groundwater inflow, then increased crystallization would promote the formation of collapse zones. If evaporation is low compared to groundwater inflow then there would be a greater chance for water reaching the surface and the possible formation of karst landforms. © 2010 by the American Geophysical Union.

Quantin C.,Ecole Normale Superieure de Lyon | Popova O.,Institute for Dynamics of Geospheres Moscow Russia | Hartmann W.K.,Planetary Science Institute Tucson | Werner S.C.,University of Oslo
Journal of Geophysical Research E: Planets | Year: 2016

In response to questions that have been raised about formation and effects of secondary craters on crater chronometry techniques, we studied properties of the secondary crater field around the young Martian primary ray crater Gratteri (diameter 7km). The crater has an estimated age of 1 to 20Myr, based on counts of small craters on flat interior surface, consistent with a likely age for a young crater its size (Hartmann et al., 2010). The following are among our findings: (1) We identify an unusual class of craters we call "rampart secondaries" which may suggest low-angle impacts. (2) We measure size distributions of secondaries as a function of distance from Gratteri and used these data to reconstruct the mass-velocity distribution of ejecta blasted out of Gratteri. Our data suggest that crater density in rays tends to peak around 120-230km from Gratteri (roughly 20-30D) and reaches roughly 30-70 times the interray crater density. (3) Comparable total numbers of secondaries form inside rays and outside rays, and about half are concentrated in clusters in 2% of the area around Gratteri, with the others scattered over 98% of the area out to 400km away from Gratteri. (4) In the old Noachian plains around Gratteri, secondaries have minimal effect on crater chronometry. These results, along with recently reported direct measurements of the rate of formation of 10m to 20m primaries on Mars (Daubar et al., 2013), tend to negate suggestions that the numbers and/or clustering of secondaries destroy the effectiveness of crater counting as a chronometric tool. © 2016. American Geophysical Union. All Rights Reserved.

Gallagher C.,University College Dublin | Balme M.R.,Open University Milton Keynes | Balme M.R.,Planetary Science Institute Tucson | Conway S.J.,Open University Milton Keynes | And 3 more authors.
Icarus | Year: 2011

Self-organised patterns of stone stripes, polygons, circles and clastic solifluction lobes form by the sorting of clasts from fine-grained sediments in freeze-thaw cycles. We present new High Resolution Imaging Science Experiment (HiRISE) images of Mars which demonstrate that the slopes of high-latitude craters, including Heimdal crater - just 25. km east of the Phoenix Landing Site - are patterned by all of these landforms. The order of magnitude improvement in imaging data resolution afforded by HiRISE over previous datasets allows not only the reliable identification of these periglacial landforms but also shows that high-latitude fluviatile gullies both pre- and post-date periglacial patterned ground in several high-latitude settings on Mars. Because thaw is inherent to the sorting processes that create these periglacial landforms, and from the association of this landform assemblage with fluviatile gullies, we infer the action of liquid water in a fluvio-periglacial context. We conclude that these observations are evidence of the protracted, widespread action of thaw liquids on and within the martian regolith. Moreover, the size frequency statistics of superposed impact craters demonstrate that this freeze-thaw environment is, at least in Heimdal crater, less than a few million years old. Although the current martian climate does not favour prolonged thaw of water ice, observations of possible liquid droplets on the strut of the Phoenix Lander may imply significant freezing point depression of liquids sourced in the regolith, probably driven by the presence of perchlorates in the soil. Because perchlorates have eutectic temperatures below 240. K and can remain liquid at temperatures far below the freezing point of water we speculate that freeze-thaw involving perchlorate brines provides an alternative " low-temperature" hypothesis to the freeze-thaw of more pure water ice and might drive significant geomorphological work in some areas of Mars. Considering the proximity of Heimdal crater to the Phoenix Landing Site, the presence of such hydrated minerals might therefore explain the landforms described here. If this is the case then the geographical distribution of martian freeze-thaw landforms might reflect relatively high temperatures (but still below 273. K) and the locally elevated concentration of salts in the regolith. © 2010 Elsevier Inc.

Gallagher C.J.,University College Dublin | Balme M.R.,Open University Milton Keynes | Balme M.R.,Planetary Science Institute Tucson
Geological Society Special Publication | Year: 2011

The confirmation of near-surface ground ice and perchlorates at the Phoenix landing site suggest that high-latitude ground-ice thaw may be more easily achieved than previously envisaged, providing the potential to drive significant, distinctive morphogenesis. We describe the results of a survey of 23 High Resolution Imaging Science Experiment (HiRISE) images covering 337° of longitude between latitudes 59°N and 79°N in which such morphogenesis is apparent, confirming that thaw has been a regionally important morphological agent. Some of the strongest geomorphological indicators of cyclical ground-ice thaw described are assemblages of sorted landforms, including clastic patterned ground resulting from cryoturbation of ice-rich regolith and lobate forms reflecting solifluction. Also described are braided gully-fan systems sourced at thermokarst pits and channels that have evolved from enlarged thermal contraction cracks. Not only are these landforms indicative of thaw and flowing liquid but the incision of solifluction lobes by thermokarst gullies demonstrates that thaw has been responsible for polycyclic morphogenesis. The presence of these landforms across the high northern latitudes of Mars indicates that the regional importance of thaw has been underestimated. This in turn has important implications for the development of better climate models and the search for life on Mars. © The Geological Society of London 2011.

Barr A.C.,Planetary Science Institute Tucson
Journal of Geophysical Research E: Planets | Year: 2016

The Giant Impact is currently accepted as the leading theory for the formation of Earth's Moon. Successful scenarios for lunar origin should be able to explain the chemical composition of the Moon (volatile content and stable isotope ratios), the Moon's initial thermal state, and the system's bulk physical and dynamical properties. Hydrocode simulations of the impact have long been able to match the bulk properties, but recent, more detailed work on the evolution of the protolunar disk has yielded great insight into the origin of the Moon's chemistry and its early thermal history. Here I show that the community has constructed the elements of an end-to-end theory for lunar origin that matches the overwhelming majority of observational constraints. In spite of the great progress made in recent years, new samples of the Moon, clarification of processes in the impact-generated disk, and a broader exploration of impact parameter space could yield even more insights into this fundamental and uniquely challenging geophysical problem. ©2016. American Geophysical Union.

Tricarico P.,Planetary Science Institute Tucson
Geophysical Research Letters | Year: 2015

When comet C/2013 A1 (Siding Spring) passed nearby Mars in 2014, it offered an unprecedented opportunity to observe the interaction between the dust tail of the comet and the atmosphere of Mars. Here I provide an overview of a recent series of four articles reporting observations from three satellites fortuitously orbiting Mars at the appropriate time (MAVEN, MEX, and MRO). These observations reveal high-velocity ablation and ionization of metals from the comet, the diffusion and transport processes that operated in Mars' atmosphere, and the abundance of these metals in the comet. © 2015. American Geophysical Union. All Rights Reserved.

Smith A.W.,University of Southampton | Jackman C.M.,University of Southampton | Thomsen M.F.,Planetary Science Institute Tucson
Journal of Geophysical Research A: Space Physics | Year: 2016

Reconnection within planetary magnetotails is responsible for locally energizing particles and changing the magnetic topology. Its role in terms of global magnetospheric dynamics can involve changing the mass and flux content of the magnetosphere. We have identified reconnection related events in spacecraft magnetometer data recorded during Cassini's exploration of Saturn's magnetotail. The events are identified from deflections in the north-south component of the magnetic field, significant above a background level. Data were selected to provide full tail coverage, encompassing the dawn and dusk flanks as well as the deepest midnight orbits. Overall 2094 reconnection related events were identified, with an average rate of 5.0 events per day. The majority of events occur in clusters (within 3 h of other events). We examine changes in this rate in terms of local time and latitude coverage, taking seasonal effects into account. The observed reconnection rate peaks postmidnight with more infrequent but steady loss seen on the dusk flank. We estimate the mass loss from the event catalog and find it to be insufficient to balance the input from the moon Enceladus. Several reasons for this discrepancy are discussed. The reconnection X line location appears to be highly variable, though a statistical separation between events tailward and planetward of the X line is observed at a radial distance of between 20 and 30RS downtail. The small sample size at dawn prevents comprehensive statistical comparison with the dusk flank observations in terms of flux closure. © 2016. American Geophysical Union. All Rights Reserved.

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