the simud tiu valles hydrologic system a multidisciplinary study of a possible site for future mars on site exploration abstract when looking for traces of past life on mars we have to look primarily for places where water was present possibly for long time intervals the simud and tiu valles are two large outflow channels connected to the north with the chryse basin oxia palus quadrangle the area carved by water during the noachian early hesperian is characterized by a complex geological evolution the geomorphological analysis shows the presence of fluvial and alluvial structures interpreted as fluvial channels and terraces debris flow fronts and short lasting small water flows coexisting with maar diatremes and mud volcanoes several morphological features indicate a change in water flux direction after the main erosive phase during this period water originated from the masursky crater and flown southwards into the hydraotes chaos this phenomenon caused the studied area to become a depocenter where fine grained material deposition took place possibly in association with ponding water this setting is potentially quite valuable as traces of life may have been preserved the presence of water at various times over a period of about 1 ga in the area is corroborated by mineralogical analyses of different areas that indicate the possible presence of hydrated minerals mixtures such as sulfate bearing deposits given the uniqueness of the evolution of this region the long term interactions between fluvial volcanic and tectonic processes and its extremely favorable landing parameters elevation slope roughness rock distribution thermal inertia albedo etc we decided to propose this location as a possible landing site for the esa exomars 2018 the nasa mars 2020 and future on site missions

a closer look at water related geologic activity on mars water has supposedly marked the surface of mars and produced characteristic landforms to understand the history of water on mars we take a close look at key locations with the high resolution imaging science experiment on board the mars reconnaissance orbiter reaching fine spatial scales of 25 to 32 centimeters per pixel boulders ranging up to approximately 2 meters in diameter are ubiquitous in the middle to high latitudes which include deposits previously interpreted as finegrained ocean sediments or dusty snow bright gully deposits identify six locations with very recent activity but these lie on steep 20 degrees to 35 degrees slopes where dry mass wasting could occur thus we cannot confirm the reality of ancient oceans or water in active gullies but do see evidence of fluvial modification of geologically recent mid latitude gullies and equatorial impact craters

cone fields in sw elysium planitia hydrothermal venting on mars 1 we have studied a large cone field in sw elysium planitia that shows unique characteristics that have as yet not been described elsewhere on mars pitted cones and cone fields are quite common on mars and the elysium planitia region where they have been mainly interpreted as pseudocraters the cones in the study area differ from cones elsewhere on mars in morphology geologic setting and their assemblage with pitted ridges as well as their close association to aeolis planum medusae fossae formation mff material the cones appear almost exclusively along an extensive contact zone several hundred kilometers long where lava flows that emanated from the cerberus fossae flooded fine grained mff material of the aeolis planum complex particularly striking is the observation that several cones in the study area have developed on top of mesas and terraces of aeolis planum material that were embayed but not overflowed by lava and can thus not be explained as pseudocraters we propose that the striking assemblage of these structures was formed by a combination of pseudocrater formation and hydrothermal venting through mesas of aeolis planum mff material by a process similar to hydrothermal vent complexes on earth the voluminous cerberus fossae lava flows that flooded the study area covered large parts of aeolis planum material initiating the mobilization of volatiles and leading to hydrothermal activity along the lava mff contact furthermore our observations strongly support the hypothesis that the volatile content of the mff is or was high at least in that part of the mff constituting the aeolis planum complex

hirise observations of fractured mounds possible martian pingos 1 early images from the high resolution imaging science experiment hirise camera have revealed small fractured mounds in the martian mid latitudes hirise resolves fractures on the mound surfaces indicating uplift and shows that the mound surface material resembles that of the surrounding landscape analysis of mars orbiter camera moc images shows that in utopia planitia the mounds lie almost exclusively between 35 45 n this range coincides with the peak abundance latitudes of several landforms attributed to ground water or ice including gullies and suggests a ground ice related origin the best terrestrial analogues for the observed mound morphology are pingos although some differences are noted the presence of uncollapsed pingos would indicate the presence of near surface ground ice in the martian mid latitudes at depths greater than the 1 meter sampled by orbital spectrometers pingo formation may require near surface liquid water which is consistent with a shallow groundwater model for the origin of gullies

young late amazonian near surface ground ice features near the equator athabasca valles mars abstract a suite of four feature types in a 20 km 2 area near 10 n 204 w in athabasca valles is interpreted to have resulted from near surface ground ice these features include mounds conical forms with rimmed summit depressions flatter irregularly shaped forms with raised rims and polygonal terrain based on morphology size and analogy to terrestrial ground ice forms these athabascan features are interpreted as pingos collapsing pingos pingo scars and thermal contraction polygons respectively thermal infrared mapping spectrometer themis data and geological features in the area are consistent with a sedimentary substrate underlying these features these observations lead us to favor a ground ice interpretation although we do not rule out volcanic and especially glaciofluvial hypotheses the hypothesized ground ice that formed the mounds and rimmed features may have been emplaced via the deposition of saturated sediment during flooding an alternative scenario invokes magmatically cycled groundwater the ground ice implicit in the hypothesized thermal contraction polygons may have derived either from this flooding ground water or from atmospheric water vapor the lack of obvious flood modification of the mounds and rimmed features indicates that they formed after the most recent flood inundated the area analogy with terrestrial pingos suggests that ground ice may be still extant within the positive relief mounds as the water that flooded down athabasca valles emerged via a volcanotectonic fissure from a deep aquifer any extant pingo ice may contain evidence of a deep subsurface biosphere

response to comment on athabasca valles mars a lava draped channel system the recent geologic history of athabasca valles mars is controversial some studies report ice rich sediment in its channels whereas others find only lava data from the high resolution imaging science experiment camera now confirm that although certain features exhibit a superficial similarity to ice related landforms solidified lava coats the entire channel system

the eruption environment of multi tiered columnar basalt lava flows field studies on the eruption environment and prevailing cooling conditions of multi tiered lava flows of the type exemplified by the giant s causeway were carried out at localities on the columbia river plateau southern iceland on the islands of mull and staffa and around the giant s causeway ne ireland common features can be recognized in the volcanology of such flows in all the localities examined these include palaeotopographic evidence of river valleys giving conditions conducive to the ponding of flows and the damming of existing drainage evidence of high rainfall regimes or abundant surface water association of interlava lacustrine and fluviatile sediments association of hyaloclastite flow foot breccias within multi tiered basalt lava successions and evidence of large volume lava flows and high eruption rates examination of the petrography of the upper curvi columnar parts of the lava the entablature shows textural signs of quenching compared with the lower more regular columnar zone the colonnade these common field and petrographic features confirm the model of multi tiered flow formation by modification of the cooling regime in the upper part of the flow following inundation of the hot lava surface by displaced river drainage

catastrophic collapse at stratovolcanoes induced by gradual volcano spreading unlike ordinary mountains which are formed by slow uplift and erosion volcanoes are constructed rapidly as a consequence many are liable to massive flank failures leading to debris avalanches for example at mount st helens in 1980 such failures occur worldwide about once every 25 years ref 1 and even small ones can present a major hazard in particular if far reaching tsunamis are generated as at mayu yama in 1792 ref 2 previous work has tended to emphasize differences in eruption style associated with flank failure2 but here we focus on the fundamental structural causes of failure most volcanic failures are generated by magmatic intrusion and flank spreading3 we present evidence however that mombacho volcano in nicaragua experienced a previously unrecognized type of failure triggered by sub volcanic basement spreading notably collapses related to basement spreading do not require that the volcano be magma tically active and thus flank failure may pose a significant risk even at inactive volcanoes which are rarely monitored

stratigraphical and morphological evidence for pingo genesis in the cerberus plains rootless debris cones or pseudocraters occur in platy patterned ground throughout the cerberus plains of mars and are thought to represent the products of explosive magma ice interaction lanagan et al 2001 geophys res lett 28 2365 2368 fagents et al 2002 in smellie j l chapman m g eds volcano ice interaction on earth and mars in geol soc spec publ vol 202 pp 295 317 requiring lava and water interspersed they are central to theories of multiple magmatic and aqueous flood events burr et al 2002 icarus 159 53 73 berman d c hartmann w k 2002 icarus 159 1 17 and widespread sheet volcanism keszthelyi et al 2000 j geophys res 105 15027 15049 in the region during the late amazonian a region reported to have been occupied by water bodies ranging from lakes to oceans scott et al 1995 map of mars showing channels and possible paleolake basins usgs miscellaneous investigations series map i 2461 1 30 000 000 the nature of the platy substrate is the subject of debate with evidence given for lava keszthelyi et al 2000 j geophys res 105 15027 15049 plescia j b 2003 icarus 164 79 95 and ice brakenridge g r 1993 lunar planet sci xxiv part 1 175 176 rice et al 2002 lunar planet sci xxxiii abstract 2026 murray et al 2005 nature 434 352 355 the superposition relationships of cones and platy deposits in the channels of the athabasca valles precludes a magmatic origin indicating later formation as permafrost mounds or pingos with implications for geologically recent flood volcanism age constraints on young surfaces and recent climate change on mars

medusae fossae formation new perspectives from mars global surveyor 1 the nature and origin of the medusae fossae formation mff on mars has been debated since the return of the first viking images the mff s young age distinctive surface texture and lack of obvious source have prompted multiple hypotheses for its origin this study uses data from the mars global surveyor mgs mission to examine the mff at all available scales we discuss and quantify observations from mars orbiter laser altimeter mola topography and mars orbiter camera moc images to better constrain the origin of the mff topographic grid estimates yield a present extent of 2 1 106 km2 and a volume of 1 4 106 km3 however remnant yardang deposits observed far from the thicker lobes of mff material suggest that it may have once covered up to 5 106 km2 we do not find compelling evidence for extensive fluvial reworking of the mff however in several regions buried channels are apparent in the mff because the formation is draped over underlying topography layering is apparent at all scales from submeter to hundreds of meters with variable resistance to weathering continuity of layers appears to be local to regional but not likely formation wide yardangs form both parallel and bidirectional patterns with resistant layers and jointing probably influencing their orientations a comparative study of mff regional topography and surface expression indicates that the mff is quantitatively dissimilar to martian polar layered deposits the material is most likely a friable and irregularly consolidated air fall deposit of probable volcanic origin

heat transfer in volcano ice interactions on mars synthesis of environments and implications for processes and landforms we review new advances in volcano ice interactions on mars and focus additional attention on 1 recent analyses of the mechanisms of penetration of the cryosphere by dikes and sills 2 documentation of the glacial origin of huge fan shaped deposits on the northwest margins of the tharis montes and evidence for abundant volcano ice interactions during the later amazonian period of volcanic edifice construction and 3 the circumpolar hesperian aged dorsa argentea formation interpreted as an ice sheet and displaying marginal features channels lakes and eskers indicative of significant melting and interior features interpreted to be due to volcano ice interactions e g sub glacial volcanic edifices pits basins channels and eskers in this context we describe and analyse several stages and types of volcano ice interactions 1 magmatic interactions with ice rich parts of the cryosphere 2 subglacial volcanism represented by intrusion under and into the ice and formation of dikes and moberg like ridges intrusion of sills at the glacier volcano substrate interface and their evolution into subglacial lava flows formation of subglacial edifices marginal melting and channels 3 synglacial ice contact volcanism represented by flows banking up against glacier margins chilling and forming remnant ridges and 4 post glacial volcanism and interactions with ice deposits

subkilometer martian volcanoes properties and possible terrestrial analogs kilometer to subkilometer cones with large central craters are the most numerous volcanic structures in the northern plains of mars based on examination of more than 12 000 high resolution viking orbiter images most of these very small cones are located in parts of eastern acidalia and utopia isidis elysium planitia measurements of base and pit diameter for some 2000 well resolved features show small variations in the histogram of base diameters between and within regions but remarkable similarity in the distribution of crater cone diameter ratios this suggests that the small cones had a similar origin with only minor variations in local conditions the acidalia cones are generally larger more widely distributed and occur on a greater variety of terrains than those in utopia isidis elysium based on morphology distribution and the crater cone ratio histograms the smallest martian cones may be similar to icelandic pseudocraters somewhat larger steeper cones with smaller crater cone ratios which also occur in eastern acidalia may be analogs of terrestrial cinder cones

pyroclastic activity at home plate in gusev crater mars home plate is a layered plateau in gusev crater on mars it is composed of clastic rocks of moderately altered alkali basalt composition enriched in some highly volatile elements a coarse grained lower unit lies under a finer grained upper unit textural observations indicate that the lower strata were emplaced in an explosive event and geochemical considerations favor an explosive volcanic origin over an impact origin the lower unit likely represents accumulation of pyroclastic materials whereas the upper unit may represent eolian reworking of the same pyroclastic materials

ignimbrites of amazonis planitia region of mars a series of postulated ignimbrite units is mapped in the amazonis memnonia and aeolis quadrangles of mars the units cover about 2 2 106 km2 within a broad but discontinuous and irregular belt trending east west along the highland lowland boundary the ignimbrites overlie parts of the western and southern aureole materials of olympus mons but are embayed in places by the lava plains of the lowlands stratigraphic relations between the basalt flows from the tharsis montes region and the ignimbrites are not clearly defined crater counts suggest that the younger ignimbrites postdate the lava flows crater counts per square kilometer for the ignimbrites range from 7 29 1 95 10 4 to 6 36 2 01 10 5 for craters larger than 1 km in diameter the ignimbrite materials form thick 100 m extensive relatively flat sheets that are smooth to grooved or gently undulating grooved surfaces appear to be yardangs and in most places are not alined with prevailing wind directions the seven mapped ignimbrite units are characterized by morphologic expression stratigraphic position and crater counts similarities to ignimbrites in the pancake range of central nevada include 1 rounded patches of smooth high albedo nonwelded material superposed on jointed low albedo welded material 2 local complementary joint sets in welded materials and 3 thick flow sheets of great areal extent that follow but subdue underlying topography four major eruptive centers occur in areas where units are thickest and where a dominant nnw sse structural trend is expressed locally by unit margins elongate collapse features and normal faulting a minimum volume of 3 85 106 km3 for the deposits has been calculated from thickness estimates based on shadow measurements and crater rim height relations

evidence for igneous activity and implications for the origin of a fretted channel in southern ismenius lacus mars northern arabia terra mars is characterized by a complex landscape that includes fretted terrain fretted channels completely closed canyons shallow troughs and fracture bounded depressions the importance of ground ice in the initiation and development of these features and of large impact structures in controlling channel courses has long been recognized the development of the fretted channel systems including the integration of closed canyons craters and surface depressions into throughgoing channels has previously been attributed primarily to mass wasting processes the two channel systems in southern ismenius lacus quadrangle mamers valles and ismeniae fossae incorporate craters of probable impact origin but both also include irregular crater like features and scalloped channel reaches that have characteristics not consistent with an impact origin some of these are by analogy with morphologically similar features on earth and elsewhere on mars reasonably interpreted as of volcanic origin the detailed morphologies of a few diagnostic features associated with ismeniae fossae suggest a hydrovolcanic origin an inference supported by the probable presence of abundant ground ice or groundwater at the time of their formation the evidence described here for hydrovolcanism supports the inference that in addition to mass wasting processes explosive excavation intrusion driven sublimation subsurface erosion and erosion by running water may have played important roles in channel formation and evolution as well all of these processes are consistent with magma interacting with groundwater or ground ice

evidence from the mars express high resolution stereo camera for a frozen sea close to mars equator it is thought that the cerberus fossae fissures on mars were the source of both lava and water floods two to ten million years ago evidence for the resulting lava plains has been identified in eastern elysium but seas and lakes from these fissures and previous water flooding events were presumed to have evaporated and sublimed away here we present high resolution stereo camera images from the european space agency mars express spacecraft that indicate that such lakes may still exist we infer that the evidence is consistent with a frozen body of water with surface pack ice around 5 degrees north latitude and 150 degrees east longitude in southern elysium the frozen lake measures about 800 x 900 km in lateral extent and may be up to 45 metres deep similar in size and depth to the north sea from crater counts we determined its age to be 5 2 million years old if our interpretation is confirmed this is a place that might preserve evidence of primitive life if it has ever developed on mars

thermally distinct craters near hrad vallis elysium planitia mars abstract evidence of volcano ground ice interactions on mars can provide important constraints on the timing and distribution of martian volcanic processes and climate characteristics northwest of the elysium rise is hrad vallis a 370 m deep 800 km long sinuous valley that begins in a source region at 34 n 218 w flanking both sides of the source region is a lobate deposit that extends 50 km perpendicular from the source and is an average of 40 m thick previous studies have suggested the formation of the hrad vallis source region was the result of explosive magma ice interaction and that the lobate deposit is a mudflow here we use newly available mola moc and themis data to investigate the evidence supporting this hypothesis within the lobate deposit we have identified 12 craters with thermal infrared signatures and morphologies that are distinct from any other craters or depressions in the region the thermally distinct craters are distinguished by their cool interiors surrounded by warm ejecta in the nighttime themis ir data and warm interiors surrounded by cool ejecta in the daytime themis ir data the craters are typically 1100 1800 m in diameter one crater is 2300 m across and 30 40 m deep but may be up to 70 m the craters are typically circular and have central depressions several with interior dune fill surrounded by 1 to 6 concentric fracture sets the distribution of the craters and their morphology suggests that they are likely the result of the interaction between a hot mudflow and ground ice

emplacement of the youngest flood lava on mars a short turbulent story abstract recently acquired data from the high resolution imaging science experiment hirise context ctx imager and compact reconnaissance imaging spectrometer for mars crism onboard the mars reconnaissance orbiter mro spacecraft were used to investigate the emplacement of the youngest flood lava flow on mars careful mapping finds that the athabasca valles flood lava is the product of a single eruption and it covers 250 000 km 2 of western elysium planitia with an estimated 5000 7500 km 3 of mafic or ultramafic lava calculations utilizing topographic data enhanced with mro observations to refine the dimensions of the channel system show that this flood lava was emplaced turbulently over a period of only a few to several weeks this is the first well documented example of a turbulently emplaced flood lava anywhere in the solar system however mro data suggest that this same process may have operated in a number of martian channel systems the magnitude and dynamics of these lava floods are similar to the aqueous floods that are generally believed to have eroded the channels raising the intriguing possibility that mechanical erosion by lava could have played a role in their incision

identification of volcanic rootless cones ice mounds and impact craters on earth and mars using spatial distribution as a remote sensing tool 1 this study aims to quantify the spatial distribution of terrestrial volcanic rootless cones and ice mounds for the purpose of identifying analogous martian features using a nearest neighbor nn methodology we use the statistics r ratio of the mean nn distance to that expected from a random distribution and c a measure of departure from randomness we interpret r as a measure of clustering and as a diagnostic for discriminating feature types all terrestrial groups of rootless cones and ice mounds are clustered r 0 51 0 94 relative to a random distribution applying this same methodology to martian feature fields of unknown origin similarly yields r of 0 57 0 93 indicating that their spatial distributions are consistent with both ice mound or rootless cone origins but not impact craters each martian impact crater group has r 1 00 i e the craters are spaced at least as far apart as expected at random similar degrees of clustering preclude discrimination between rootless cones and ice mounds based solely on r values however the distribution of pairwise nn distances in each feature field shows marked differences between these two feature types in skewness and kurtosis terrestrial ice mounds skewness 1 17 1 99 kurtosis 0 80 4 91 tend to have more skewed and leptokurtic distributions than those of rootless cones skewness 0 54 1 35 kurtosis 0 53 1 13 thus nn analysis can be a powerful tool for distinguishing geological features such as rootless cones ice mounds and impact craters particularly when degradation or modification precludes identification based on morphology alone

evidence age and thickness of a frozen paleolake in utopia planitia mars abstract recent mapping studies west of elysium mons mars have pinpointed subice features that suggest the existence of a frozen paleolake in utopia planitia as recently as 1 8 billion years ago the subice features are interpreted to be hyaloclastic ridges and hills table mountains associated jokulhlaup deposits and fluvial channels photoclinometric studies of these features and of a basal scarp around the northwest flank of elysium mons interpreted to have been an ice sheet boundary indicate that the maximum thickness of ice within the basin may have been about 180 m this thickness of ice during a relatively late stage of martian geologic history would have important implications concerning the atmospheric the climatic and possibly the exobiologic history of the planet

modeling steam pressure under martian lava flows abstract rootless cones on mars are a valuable indicator of past interactions between lava and water however the details of the lava water interactions are not fully understood limiting the ability to use these features to infer new information about past water on mars we have developed a model for the pressurization of a dry layer of porous regolith by melting and boiling ground ice in the shallow subsurface this model builds on previous models of lava cooling and melting of subsurface ice we find that for reasonable regolith properties and ice depths of decimeters explosive pressures can be reached however the energy stored within such lags is insufficient to excavate thick flows unless they draw steam from a broader region than the local eruption site these results indicate that lag pressurization can drive rootless cone formation under favorable circumstances but in other instances molten fuel coolant interactions are probably required we use the model results to consider a range of scenarios for rootless cone formation in athabasca valles pressure buildup by melting and boiling ice under a desiccated lag is possible in some locations consistent with the expected distribution of ice implanted from atmospheric water vapor however it is uncertain whether such ice has existed in the vicinity of athabasca valles in recent history plausible alternative sources include surface snow or an aqueous flood shortly before the emplacement of the lava flow

geologic context of the mars radar stealth region in southwestern tharsis stealth is a region on mars that has no distinguishable radar return stealth was discovered in 1988 by muhleman et al 1991 in 3 5 cm earth based radar images and it was found to extend 2000 km along the martian equator west of arsia and pavonis montes stealth was proposed to be a deposit of unconsolidated granular material with a minimum thickness of several meters with the most likely explanation being that stealth is a volcanic tephra deposit muhleman et al 1991 1995 stealth overlies some of the youngest geologic units on mars including lava flows and a giant curvilinear ridged lobate unit on the west flank of arsia mons stealth also overlies part of a region of smooth and wind scoured materials called the medusae fossae formation examination of high resolution viking orbiter images suggests that stealth is the youngest of several discontinuous wind worked mantles of fine grained 1 cm sediment older deposits of fine sediment appear to be interbedded with lava flows and show evidence of aeolian deposition dunes and deposits in the lee of topographic obstacles and erosion yardangs and pedestal craters the geologic context supports a volcanic ash origin for stealth if stealth is composed of tephra then its extent and the nature of landforms in the region indicate that it resulted from multiple eruptions that occurred relatively late in martian history possible source vents include linear depressions flanked by constructive volcanic landforms that are located on the lower western slopes of arsia and pavonis montes

interior trough deposits on mars subice volcanoes widespread several kilometer thick successions of layered deposits occur as mounds that partly fill the troughs or chasmata that compose the valles marineris on mars like terrestrial subice volcanoes the layered deposits occur in a volcano tectonic setting within basins that may have held ponded water or ice on the basis of their dimensions morphologies and associated catastrophic floods and other geologic events as shown in viking and new mars global surveyor mgs data sets we suggest that the interior deposits are volcanic in origin and possibly generated by subice eruptions a tuya origin for the mounds can explain the lack of external sediment mound heights that can rival the plateau local flat topped mesas morphologically distinct mounds of different ages horizontal to steep dips fine grained materials indications of rare volcanic vents and lava flows and spectral composition the extremely diverse layering of west candor chasma and possible volcanic cones in melas may have formed by related subaerial eruptions consistent with the suggestion that interior deposits are eroding out of the wall rock some deposits could have been erupted from sites along the walls

pitted cones and domes on mars observations in acidalia planitia and cydonia mensae using moc themis and tes data 1 domes and cones with summit pits located in acidalia planitia and cydonia mensae were studied using moc and themis images and a tes derived thermal inertia map north of 40 5 n latitude the features have a dome like morphology and south of that latitude the morphology is more cone like layering is apparent in the summit craters of fresher looking southern cones and asymmetric aprons were observed in some instances some of the northern domes also display layering in their summit craters but asymmetric aprons were not observed the northern domes can also display multiple summit pits or no summit pits at all and can occur in association with higher albedo pancake features the northern domes are higher in albedo but have apparent thermal inertias that are lower than the surrounding plains the apparent thermal inertia values of the southern cones range from values comparable to the surrounding plains to slightly lower from the tes thermal inertia map we infer that the thermal inertia values of the pitted cones are between those of basaltic fine dust and sand while those of the surrounding plains are closer to that of basaltic sand while a unique interpretation of the origin of the pitted cones is not possible with the available data we do not find compelling evidence to suggest an origin related to either basaltic volcanism or ground ice instead an origin for these features through some combination of mud volcanism and evaporite deposition around geysers and or springs is most consistent with the observations

explosive volcanism in the tharsis region global evidence in the martian geologic record 1 a global equatorial set of layered deposits on mars has been reexamined with mars global surveyor data the stratigraphy morphology and erosional characteristics of units separated by thousands of kilometers are remarkably similar to each other and consistent with a widespread episodic subaerial source of fine grained material this study is the first to propose a genetic link between many units that have traditionally been considered on a local or regional level including the medusae fossae formation valles marineris interior deposits chaotic terrain terra meridiani and arabia materials mapping relationships suggest that all of these units are post noachian in age and formation may have continued until the recent past thinning of deposits with increasing distance from the tharsis rise suggests that much of these materials may be volcanic ash flows and air fall from explosive eruptions in the tharsis region analogous to those produced by terrestrial plinian eruptions contemporary martian winds would preferentially transport ash from an eruption plume near tharsis east or west along the equator depending on the season erosion has removed much of the original volcanic ash and only isolated layered stacks remain sometimes in topographic depressions that inhibit erosion modeling indicates that widespread dispersion of ash after an eruption is easily attainable in present or past atmospheric conditions consistent with the hypothesis that the far reaching deposits may be composed of ash from past explosive eruptions in the vicinity of tharsis

elysium utopia flows as mega lahars a model of dike intrusion cryosphere cracking and water sediment release 1 high resolution altimetry from the mars orbiter laser altimeter mola provides new data to test the interpretation that unusual flow like deposits in elysium utopia were emplaced as lahars mass flows fluidized by water that are induced by volcanism using several data products derived from mola altimetry we confirm that two major types of terrain dominate the region 1 lava flows and 2 deposits consistent with emplacement by debris flows along medial channels supporting the lahar hypothesis of previous workers associated terrain types are interpreted to be the result of modification of lavas by water and or ice associated with the debris outflows and by later climatically controlled deposition and removal of a volatile rich dust layer at high latitudes both lava flows and debris flows originate from northwest trending radial fossae on the flanks of the elysium rise the fossae are interpreted here to have been formed by laterally propagating dikes from elysium mons that have intersected or approached the surface fossae that are sources for only lava flows lie at higher elevations 3400 m and above than those that are the sources of lahars and channels 3100 m to 4300 m this discrepancy in source location suggests that the availability of groundwater responsible for lahar formation is elevation dependent and reflects a hydrostatic equilibrium distribution of water in the subsurface these observations at elysium allow a geologic test of the prevailing hypothesis on the configuration of the subsurface martian hydrosphere that an interconnected groundwater system at hydrostatic equilibrium may exist below a confining cryosphere if water at a certain elevation is under hydrostatic pressure and this cryosphere is disrupted the potential arises for groundwater to flow to the surface we propose the following flow emplacement model in which geologic activity in northwest elysium is consistent with the preceding hydrological model laterally propagating dikes from elysium approached or intersected the surface on the flanks of the rise resulting in effusion of lava flows and possibly pyroclastics disruption of the cryosphere by the dikes allowed groundwater to escape to the surface and form lahars by interaction with erupted and existing material at elevations at and below the maximum level of subsurface saturated ground this model provides a consistent explanation for the major structures and units both lava flows and lahars and their elevation and spatial relationships in the elysium utopia region this type of mega lahar with groundwater as its primary water source is rare on earth and occurs on mars due to martian climatic and hydrologic conditions and the presence of a global confining cryosphere this study supports the idea that groundwater outflow resulting from disruption of a confining cryosphere may be an important phenomenon in martian geology in particular general association of major outflow with major volcanic areas on mars is highly consistent with a model of disruption of a groundwater confining cryosphere by dikes expected to be intruded in these areas further geologic analyses of features and processes related to groundwater outflow will help constrain the degree and causes of variability within the martian hydrologic system

hydrovolcanic tuff rings and cones as indicators for phreatomagmatic explosive eruptions on mars hydrovolcanism is a common natural phenomenon on earth and should be common on mars too since its surface shows widespread evidence for volcanism and near surface water we investigate fields of pitted cones in the nephentes amenthes region at the southernmargin of the ancient impact basin utopia which were previously interpreted as mud volcanoes the cone fields contain pitted and breached cones with associated outgoing flow like landforms based on stratigraphic relations we determined ahesperian or youngermodel age we test the hypothesis of a hydro volcanic origin based on a detailed morphological and morphometrical analysis and an analysis of the regional context an igneous volcanic origin of these cones as hydrovolcanic edifices produced by phreatomagmatic eruptions is plausible several lines of evidence suggest the existence of subsurface water ice the pitted cones display well developed wide central craters with floor elevations below the preeruptive surface their morphometry and the overall appearance are analogous to terrestrial tuff cones and tuff rings mounds that are also observed in the same region resemble terrestrial lava domes the hydrovolcanic interaction between ascending magma and subsurface water and or water ice may explain the formation of the pitted cones although other scenarios such as mud volcanism cannot be ruled out together with the mounds the cones might represent effusive and explosive edifices of amonogenetic volcanic field composed of lava domes tuff rings tuff cones and possibly maars

mafic pyroclastic flows at tyrrhena patera mars constraints from observations and models abstract tyrrhena patera 22 s 104 e mars is a low relief 1800 m maximum relief central vent volcano whose flanks are dissected with broad 5 km wide flat floored channels the easily eroded nature of the shield materials combined with extremely low flank slopes 2 0 suggest that tyrrhena patera is composed of pyroclastic materials these shield deposits are observed to crop out 600 km from the volcano s summit with measured thicknesses of 100 150 m at these distances such dimensions strongly suggest emplacement via pyroclastic flows currently available data indicate that these deposits have a mafic composition application of models derived for terrestrial pyroclastic flow emplacement reveals that aside from environmental parameters atmospheric density gravity if all else is held equal a pyroclastic flow should travel a factor of 3 to 4 times farther on mars than on earth the observed tyrrhena patera deposits extend even farther suggesting that some emplacement mechanism may be enhancing pyroclastic flow runout distances on mars

arecibo radar imagery of mars the major volcanic provinces abstract we present earth based radar images of mars obtained with the upgraded arecibo s band 12 6 cm radar during the 2005 2012 oppositions the imaging was done using the same long code delay doppler technique as for the earlier pre upgrade imaging but at a much higher resolution 3 km and for some regions a more favorable sub earth latitude this has enabled us to make a more detailed and complete mapping of depolarized radar reflectivity a proxy for small scale surface roughness over the major volcanic provinces of tharsis elysium and amazonis we find that vast portions of these regions are covered by radar bright lava flows exhibiting circular polarization ratios close to unity a characteristic that is uncommon for terrestrial lavas and that is a likely indicator of multiple scattering from extremely blocky or otherwise highly disrupted flow surfaces all of the major volcanoes have radar bright features on their shields although the brightness distribution on olympus mons is very patchy and the summit plateau of pavonis mons is entirely radar dark the older minor shields paterae and tholi are largely or entirely radar dark which is consistent with mantling by dust or pyroclastic material other prominent radar dark features include the fan shaped deposits possibly glacial associated with the three major tharsis montes shields various units of the medusae fossae formation a region south and west of biblis patera where stealth deposits appear to obscure tharsis flows and a number of dark halo craters with radar absorbing ejecta blankets deposited atop surrounding bright flows several major bright features in tharsis are associated with off shield lava flows these include the olympus mons basal plains volcanic fields east and south of pavonis mons the daedalia planum flows south of arsia mons and a broad expanse of flows extending east from the tharsis montes to echus chasma the radar bright lava plains in elysium are concentrated mainly in cerberus and include the fluvio volcanic channels of athabasca valles grjota valles and marte valles as well as an enigmatic region at the southern tip of the cerberus basin some of the cerberus bright features correspond to the distinctive platy ridged flows identified in orbiter images the radar bright terrain in amazonis planitia comprises two distinct but contiguous sections a northern section formed of lavas and sediments debouched from marte valles and a southern section whose volcanics may derive in part from local sources this south amazonis region shows perhaps the most complex radar bright structure on mars and includes features that correspond to platy ridged flows similar to those in cerberus

combinations of processes responsible for martian impact crater layered ejecta structures emplacement 1 we utilized images and stereo derived topographic data acquired by the high resolution stereo camera hrsc and thermal emission imaging system themis images together with other data in order to study the geology of layered ejecta structures associated with relatively pristine martian impact craters the geomorphology and morphometric properties indicate their origin as complex combinations of a variety of impact processes the studied inner layered ejecta structures often exhibit ground hugging characteristics and many of them do not have topographic profiles expected from simple ballistic emplacement such profiles include ones that are plateau shaped or thickening outward we think that water rich fluidized flows driven by the momentum due to the impact and by gravity together with ballistic emplacement and vortex produced by the atmosphere ejecta curtain interaction were essential to the inner layered ejecta structure formation we hypothesize that the thinner outer layered ejecta structures were formed by various combinations of shockwave induced liquefaction of water rich near surface sediments ballistic emplacement of ejecta entraining water and strong winds expanding vapor vortex base surge related to the impact the contribution of each proposed layered ejecta structure formation mechanism should have been variable depending on the condition of the impact

lahars in the elysium region of mars photogeological studies of the elysium volcanic province mars show that its sinuous channels are part of a large deposit that was probably emplaced as a series of huge lahars some flows extend 1000 km from their sources the deposits are thought to be lahars on the basis of evidence that they were 1 gravity driven mass flow deposits lobate outlines steep snouts smooth medial channels and rough lateral deposits deposits narrow and widen in accord with topography and extend downslope 2 wet channeled surfaces draining features and 3 associated with volcanism the deposits and channels extend from a system of fractures which also fed lava flows heat associated with magmatism probably melted ground ice below the elysium volcanoes and formed a muddy slurry that issued out of regional fractures and spread over the adjoining plain the identification of these lahars adds to the evidence that mars has a substantial volatile element endowment

clustering within rootless cone groups on iceland and mars effect of nonrandom processes 1 this study aims at quantifying the spatial distribution of cones within rootless cone groups rcgs our data set consists of 1 seven icelandic rcgs identified through field investigations 2 seven candidate rcgs on mars identified through mars orbiter camera moc and thermal emission imaging system themis images and 3 four groups of impact craters on mars also identified through moc and themis images to determine if they can be remotely distinguished from the rcgs purely on the basis of their spatial distribution several independent statistical techniques are used including nearest neighbor analysis analysis of variance anova and linear alignment detection analysis our results indicate that the spatial distribution of each rcg is not random within a cone group cones preferentially form near existing cones the presence of at least one significant linear cone alignment in each rcg and strong linear alignments in some groups suggests that rootless cones form as the surface signature of preferred lava pathways an anova on mean nearest neighbor distances reveals the martian cone groups to be statistically indistinguishable from the icelandic rcgs supporting existing interpretations that they represent rootless cone groups a similar anova showed that the martian cone groups do not resemble the martian impact crater clusters studied the impact craters have significantly greater nearest neighbor distances and show no evidence of aggregation within a crater group

icelandic pseudocraters as analogs to some volcanic cones on mars pseudocraters are rootless vents formed by the interaction of lava flows with surface or near surface water this interaction can produce mild explosions and the accumulation of scoria and spatter into small constructs pseudocraters in several localities in iceland were examined in the field and compared to similar appearing features observed on mars the icelandic pseudocrater cones in this study range in size from 6 to 70 m in diameter have summit craters which range from 2 to 28 m in diameter many cones lack craters entirely and have flanks that are either concave up or convex up the size and spacing of icelandic pseudocraters might be a function of the availability of water in which larger closely spaced features result from efficient lava water interaction as suggested by the environments in which the features formed possible martian pseudocrater cones in amazonis planitia range in diameter from 30 to 180 m and have craters 12 to 80 m in diameter a numerical model for volcanic explosions was adapted to study the formation of pseudocraters under terrestrial and martian conditions the results suggest that explosions forming martian cones require significantly less water calculated masses are less by a factor of 4 to 16 than those forming icelandic pseudocraters despite their larger sizes this is attributed to the low gravity and atmospheric pressure in the mars environment and is consistent with the likely lower abundance of water which might be present as interstitial ice at shallow depths in the regolith locations of potential pseudocraters on mars at latitudes as low as 8 n imply the presence of crustal ice stores at the time of their formation

nearest neighbor analysis of small features on mars applications to tumuli and rootless cones 1 fields of low relief cones domes and mounds on mars have been interpreted as tumuli rootless cones and pingos the spatial distribution of such features can be characterized by their nearest neighbor nn distances the nn distributions can provide insight into the physical processes that affected field formation and can possibly provide a future diagnostic of the feature type present on mars the conventional approach for analyzing the observed nn distribution is based on the poisson probability distribution and uses one statistic c our simulations of spatial randomness show that there is an unrecognized bias in c that often leads to erroneous conclusions three alternative extensions of the poisson distribution are proposed here the first accounts for a minimum threshold on the size of features discerned the second considers the scavenging of the resources necessary to form neighboring features e g inflation pressurization and subsurface volatiles and the third models self limiting growth of the feature population three statistical tests are proposed to improve the conventional nn analysis we provide the critical regions needed to perform these tests for common sample sizes of these geologic features our methodology is applied to tumuli at mauna ulu hawaii and conjectured tumuli and rootless cones on mars each example exhibits a different aspect of spatial randomness these applications suggest avenues for further field and theoretical studies to develop this approach as a remote sensing diagnostic

phreatomagmatic explosive origin of hrad vallis mars 1 hrad vallis is a 370 m deep 800 km long depression located at 34 n 218 w in elysium planitia mars a distinctive deposit present on both sides of the source area of hrad vallis has a maximum width of 70 km along the strike of the depression a lower unit extends for nearly 100 km away from the depression on either side and an upper unit extends for 50 km viking and mars orbiter camera moc images show lobate deposit boundaries rheomorphic deformation and numerous craters 270 580 m in diameter within the deposit the deposit coincides with a broad topographic rise 30 km wide and 100 150 m high extending along the sides of the valley for at least 100 km along strike we propose that these features formed when a dike underlying the axis of the valley generated a shallow sill which in turn is partly responsible for the rise topography the remainder of the rise consists of a mud like deposit of ejecta from a large scale phreatomagmatic explosion due to violent mechanical and thermal mixing between the central part of the sill and ice rich rock layers in the upper few hundred meters of the crust in a fuel coolant type interaction the observed range and thickness of ejecta and the depth of the hrad depression imply explosion steam pressures up to 360 mpa ejecta speeds of 400 m s 1 and a sill at least 150 m thick intruded at a depth of 350 400 m below the original surface

high resolution imaging science experiment hirise images of volcanic terrains from the first 6 months of the mars reconnaissance orbiter primary science phase 1 in the first 6 months of the mars reconnaissance orbiter s primary science phase the high resolution imaging science experiment hirise camera has returned images sampling the diversity of volcanic terrains on mars while many of these features were noted in earlier imaging they are now seen with unprecedented clarity we find that some volcanic vents produced predominantly effusive products while others generated mostly pyroclastics flood lavas were emplaced in both turbulent and gentle eruptions producing roofed channels and inflation features however many areas on mars are too heavily mantled to allow meter scale volcanic features to be discerned in particular the major volcanic edifices are extensively mantled though it is possible that some of the mantle is pyroclastic material rather than atmospheric dust support imaging by the context imager ctx and topographic information derived from stereo imaging are both invaluable in interpreting the hirise data

volcano ground ice interactions in elysium planitia mars abstract the occurence within elysium planitia of meltwater deposits possible pseudocraters collapse features within troughs and outflow channels indicates that a layer of subsurface volatiles existed at the time of volcanic activity within this area the pseudocraters are interpreted to be indicators of near surface volatiles while meltwater deposits and the degree of preservation of trough walls and floors are thought to signify greater volatile depths a latitudinal variation in the distribution of these features indicates either that the depth to the volatile layer increased from less than about 50 m at 35 n to greater than 600 m at 24 n or that an ice wedge that existed at 35 n thinned to nonexistence at 24 n braided distributary channel systems within the chaotic terrain north of elysium planitia show that ephemeral lakes were repeatedly created and drained at this locality the existence of volatiles contemporaneous with volcanic activity permits a search to be made for explosively generated landforms predicted to exist by previous theoretical models morphological evidence for strombolian vulcanian and plinian eruptions is lacking within western elysium planitia there are no identifiable cinder cones pyroclastic flow deposits or mantled areas indicative of large airfall deposits at an image resolution of 50 150 m pixel however the pseudocraters indicate that small scale phreatomagmatic activity may have taken place

athabasca valles mars a lava draped channel system athabasca valles is a young outflow channel system on mars that may have been carved by catastrophic water floods however images acquired by the high resolution imaging science experiment camera onboard the mars reconnaissance orbiter spacecraft reveal that athabasca valles is now entirely draped by a thin layer of solidified lava the remnant of a once swollen river of molten rock the lava erupted from a fissure inundated the channels and drained downstream in geologically recent times purported ice features in athabasca valles and its distal basin cerberus palus are actually composed of this lava similar volcanic processes may have operated in other ostensibly fluvial channels which could explain in part why the landers sent to investigate sites of ancient flooding on mars have predominantly found lava at the surface instead

peperite a review of magma sediment mingling the study of peperite is important for understanding magma water interaction and explosive hydrovolcanic hazards this paper reviews the processes and products of peperite genesis peperite is common in arc related and other volcano sedimentary sequences where it can be voluminous and dispersed widely from the parent intrusions it also occurs in phreatomagmatic vent filling deposits and along contacts between sediment and intrusions lavas and hot volcaniclastic deposits in many environments peperite can often be described on the basis of juvenile clast morphology as blocky or fluidal but other shapes occur and mixtures of different clast shapes are also found magma is dominantly fragmented by quenching hydromagmatic explosions magma sediment density contrasts and mechanical stress as a consequence of inflation or movement of magma or lava magma fragmentation by fluid fluid shearing and surface tension effects is probably also important in fluidal peperite fluidisation of host sediment hydromagmatic explosions forceful intrusion of magma and sediment liquefaction and shear liquification are probably the most important mechanisms by which juvenile clasts and host sediment are mingled and dispersed factors which could influence fragmentation and mingling processes include magma host sediment and peperite rheologies magma injection velocity volatile content of magma total volumes of magma and sediment involved total volume of pore water heated presence or absence of shock waves confining pressure and the nature of local and regional stress fields sediment rheology may be affected by dewatering compaction cementation vesiculation fracturing fragmentation fluidisation liquefaction shear liquification and melting during magma intrusion and peperite formation the presence of peperite intraclasts within peperite and single juvenile clasts with both sub planar and fluidal margins imply that peperite formation can be a multi stage process that varies both spatially and temporally mingling of juvenile clast populations formed under different thermal and mechanical conditions complicates the interpretation of magma fragmentation and mingling mechanisms

cerberus fossae elysium mars a source for lava and water cerberus fossae a long fracture system in the southeastern part of elysium has acted as a conduit for the release of both lava and water onto the surface the southeastern portion of the fracture system localized volcanic vents having varying morphology in addition low shields occur elsewhere on the cerberus plains three locations where the release of water has occurred have been identified along the northwest athabasca and grjota vallis and southeast rahway vallis portions of the fossae water was released both catastrophically and noncatastrophically from these locations a fluvial system that extends more than 2500 km has formed beginning at the lower flank of the elysium rise across the cerberus plains and out through marte vallis into amazonis planitia the timing of the events is late amazonian

conductive cooling of lava columnar joint diameter and stria width as functions of cooling rate and thermal gradient abstract much evidence exists that column diameter and stria width of columnar joints in lava flows vary inversely with cooling rate t t and thermal gradient t x at the point of fracture as the flow cools some evidence suggests that lava flows cool conductively a conductive cooling model yields cooling rate and thermal gradient through space and time for cooling igneous bodies both for extrusive and intrusive settings thermal gradient and cooling rate are then linked to stria width and column diameter with faster cooling leading to narrower columns and steeper gradients to narrower striae this model shows large regions within a body where the ratio of thermal gradient to cooling rate is nearly constant and predicts that the ratio of stria width to column diameter is also nearly constant this prediction is consistent with field observations both in this and other works the model also suggests that if conductive cooling dominates then joint spacing i e column diameter should increase inward from the margins of the body as should stria width

lava water interaction and hydrothermal activity within the 2014 2015 holuhraun lava flow field iceland abstract lava that erupted during the 2014 2015 holuhraun eruption in iceland flowed into a proglacial river system resulting in aqueous cooling of the lava and an ephemeral hydrothermal system we carried out a monitoring study of this system from 2015 to 2018 to document the cooling of the lava over this time using thermocouple measurements and data logging sensors the heat loss rate from advection through this hydrothermal system in august 2015 was 5 5 108 w since eruption aqueous cooling likely accounted for 1 of the total heat loss from the lava this estimate excludes steam losses from fumaroles as well as any groundwater that was not released to the surface and thus is a lower bound near the terminus of the flow advection of heat by flowing water may have locally accounted for tens of percent of the total cooling of that part of the flow our data quantify the importance of water cooling for this lava flow and can be compared with models to better understand lava water interactions more generally we also provide detailed methods for simple low cost monitoring of similar instances in the future

widespread weathered glass on the surface of mars low albedo sediments cover 10 7 km 2 in the northern lowlands of mars but the composition and origin of these widespread deposits have remained ambiguous despite many previous investigations here we use near infrared spectra acquired by the mars express omega observatoire pour la mineralogie l9eau les glaces et l9activite imaging spectrometer to show that these sediments exhibit spectral characteristics that are consistent with both high abundances of iron bearing glass and silica enriched leached rinds on glass this interpretation is supported by observations of low albedo soil grains with possible rinds at the phoenix mars lander landing site in the northern lowlands by comparison with the extensive glass rich dune fields and sand sheets of iceland we propose an explosive volcanic origin for these glass rich sediments we also propose that the glassy remnant rinds on the sediments are the result of postdepositional alteration as these rinds are commonly formed in arid terrestrial volcanic environments during water limited moderately acidic leaching these weathered glass rich deposits in the northern lowlands are also colocated with the strongest concentrations of a major global compositional surface type previously identified in mid infrared spectra suggesting that they may be representative of global processes our results provide potential confirmation of models suggesting that explosive volcanism has been widespread on mars and also raise the possibilities that glass rich volcaniclastics are a major source of eolian sand on mars and that widespread surficial aqueous alteration has occurred under amazonian climatic conditions

ice rich terrain in gusev crater mars abstract the morphology of materials on the floor of gusev crater 14 s 175 w mars imply a history of volcanism and subsequent removal of an ice rich deposit fluid lava flows observed in the western portion of gusev crater paradoxically terminate in a steep thick 60 m flow front adjacent to hummocky terrain the hummocky terrain is morphologically similar to deglaciated terrain on earth generated when glacial debris are left behind after the glacier has retreated we propose the following scenario for the floor of gusev crater first ice rich material was deposited adjacent to thira crater second fluid lavas were emplaced and ponded against the ice rich deposits at some later time the ice within the deposit sublimated leaving hummocky terrain current age estimates for the gusev flows are hesperian suggesting that the ice removal occurred in the upper hesperian or more recently if this hypothesis is correct quench features glassy rinds columnar jointing should be observed at the lava flow margin the hummocky deposit should be poorly sorted angular debris

the morphology of lava flows in planetary environments predictions from analog experiments the rates of surface cooling and lateral spreading are evaluated for lava flows on the surface of earth venus mars the moon and io differences between the flow morphologies expected in these environments are then predicted under the assumption that the results of recent laboratory simulations of lavas using wax extruded beneath cold water fink and griffiths 1990 can be carried over these experiments involved the spreading of viscous liquid under gravity in the presence of a solidifying surface crust and revealed a set of four distinct surface morphologies transitions from one morphology to the next occurred in a well defined sequence when the relative rates of surface solidification and lateral spreading were varied in comparison with subaerial flows on earth the surface of lavas solidifies faster on venus where the dense atmosphere provides additional convective heat transfer lateral flow of lava is much slower under the smaller gravity of the moon hence for a given extrusion rate and viscosity solid crust is predicted to form closer to the vent on both the moon and venus than on earth equivalently faster extrusion rates than those on earth are required on the moon io and venus in order to produce a given surface morphology a comparison of observed structure flow rates and estimated viscosities for some well documented terrestrial lava flows shows reasonable agreement with the predictions of the model extrapolation to extraterrestrial lavas is then illustrated by constraining the effusion rate for a martian lava flow whose composition is assumed spectral information on the chemical composition of martian flows to be sought by the thermal emission spectrometer on the mars observer mission in conjunction with detailed morphologic observations from the mars observer camera could allow much more accurate estimates of effusion rates for martian lavas

discovery of columnar jointing on mars we report on the discovery of columnar jointing in marte valles mars these columnar lavas were discovered in the wall of a pristine 16 km diameter impact crater and exhibit the features of terrestrial columnar basalts there are discontinuous outcrops along the entire crater wall suggesting that the columnar rocks covered a surface area of at least 200 km 2 assuming that the rocks obliterated by the impact event were similarly jointed we also see columns in the walls of other fresh craters in the nearby volcanic plains of elysium planitia amazonis planitia which include marte vallis and in a well preserved crater in northeast hellas

quantification of submarine lava flow morphology through analog experiments we developed a technique for determining paleoeffusion rates and emplacement times for submarine lava flows using observations of flow morphology and estimates of eruption volume eruption temperature lava viscosity and preflow topography using laboratory simulations and correlating these results with sea floor observations four submarine lava flow morphologies are considered to be diagnostic of specific effusion rates jumbled folded and lineated sheets and striated pillows we applied this approach to the coaxial flow emplaced in less than 14 days on the juan de fuca ridge in early summer 1993 and we calculated an effusion rate of 100 m3 s giving an emplacement time of 10 days

olympus mons aureole deposits new evidence for a flank failure origin 1 the origin of the rough textured aureoles that surround the immense olympus mons volcano on mars is controversial we present data from the mars global surveyor and mars odyssey missions to demonstrate that at least two of the aureole lobes are derived from the volcano s flanks in large and probably catastrophic mass movement events leaving behind headwalls that constitute the basal scarp this evidence stems from the morphology and internal structure of aureole blocks which exhibit remnants of volcanic flow units on their surfaces our claim is supported by plausible reconstructions of the prefailure flanks structural analogs to known flank failure events at hawaiian volcanoes suggest that repeated cycles of flank growth and collapse at olympus mons allow generation of the observed aureoles from a protoedifice similar in size and shape to the present one

volcanic geology of tyrrhena patera mars tyrrhena patera is a large low relief volcano located in the southern cratered highlands of mars on a proposed ring fracture of the hellas basin the volcano is composed of four or possibly five principal units the oldest and most areally extensive of which are suggested to be ash on the basis of their morphology and erosional characteristics lava flows are observed in a flank flow unit extending from the central caldera to the southwest and apparently cap the summit region models derived from the study of terrestrial pyroclastic flows have been applied to the deposits at tyrrhena patera and indicate that the distribution and lengths of the observed deposits are consistent with an origin by the emplacement of gravity driven ash flows generated by hydromagmatic or magmatic explosive eruptions the hydromagmatic case requires considerable quantities of water 7 5 1016 kg however at volumetric water flow rates determined by other investigators 105 m3 s and calculated from the regional slopes in the hesperia planum region 106 m3 s the amounts necessary could be supplied relatively rapidly in geologic time tyrrhena patera represents the earliest central vent volcanism identified on mars and may reflect a transition from the flood style eruptions which dominated early martian history a hydromagmatic origin for tyrrhena patera is in agreement with suggested climatic changes on mars and may explain the absence of this style of volcanism in later eras alternatively a magmatic origin for tyrrhena patera implies an evolution in the types of magmas produced on mars from hesperian to amazonian time during which the prominent shields in the tharsis region formed

magma water interactions in subaqueous and emergent basaltic in the subaqueous growth and emergence of a basaltic volcano clasts are formed by one or a combination of 1 explosive release of magmatic volatiles 2 explosive expansion and collapse of steam formed at magma water contact surfaces 3 explosive expansion of steam following enclosure of water in magma or entrapment of water close to magma and 4 cooling contraction these processes named respectivelymagmatic explosivity contact surface steam explosivity bulk interaction steam explosivity andcooling contraction granulation can be enhanced by mutual interaction and feedback the first three explosive processes are limited at certain water depths hydrostatic pressures and become increasingly vigorous at shallower levels the depth of onset of magmatic explosivity depends largely on juvenile volatile content it is up to 200 m for tholeiitic magmas and up to 1 km for alkalic magmas at the depth where formation of clastic deposits becomes predominant over effusion of lavas magmatic explosivity is subordinate to steam explosivity as a clast forming process the upward transition to accumulation of dominantly clastic deposits is not simply related to the onset of substantial exsolution of magmatic volatiles and can occur without it contact surface explosivity commonly requires initiation by a vigorous impact between magma and water and although no certain depth limit is known likelihood of such explosivity decreases rapidly with depth clast generation by bulk interaction explosivity appears to be restricted to depths much shallower than that of the critical pressure of water which in sea water is at about 3 km cooling contraction granulation can occur in any depth of water but at shallow levels may be replaced by contact surface explosivity during continuous eruption under water tephra can be ejected and deposited within a cupola of steam such that rapid quenching does not occur emergent volcanoes are characterized by distinctive steam explosive activity that results primarily from a bulk interaction between rapidly ascending magma and a highly mobile slurry of clastic material water and steam the water gets into the vent by flooding across or through the top of the tephra pile and violent explosions cease when this access is sealed the eruptions during emergence of surtsey and capelinhos typify the distinctive explosive activity the style and controls of which are different from those of maar volcanoes

structures textures and cooling histories of columbia river basalt flows flood basalt flows of the columbia river basalt group commonly exhibit well developed colonnade and entablature structures formed during cooling of individual flows the colonnade refers to the well defined columnar structure of many flows which appears in the lowermost 10 to 30 and in places the uppermost 10 to 20 entablature refers to those parts of a flow that have smaller column diameters and more irregular pattern fractures than those of colonnade entablature commonly occupies the central 60 to 70 of a flow in some cases however the entablature is interrupted by colonnade in such cases the entablature and colonnade may be repeated once or twice within a single flow in other cases flows may lack an entablature entirely petrographic examination reveals a pronounced correlation between structural and textural features thin sections of colonnade basalt have relatively coarse cruciform or octahedral fe ti oxide crystals and 20 glassy mesostasis the central entablature basalt in contrast has much more mesostasis as much as 60 and has textural features such as feathery fe ti oxide crystals indicative of more rapid cooling these features are the reverse of what might be expected for normal conductive cooling because cooling rates at the flow margins should be greater than those in the flow interior we reconciled the observed basalt textures with thermal models by taking into account rainfall and deranged drainage during crystallization of the flow downward migration of water through cracks would cause convective cooling of underlying unsolidified lava this effect and its consequences have been modeled numerically in order to clarify the physical constraints on the mechanism by which the central part of the lava can be quenched we have found that a water infiltration front can explain the presence of a thick quenched flow interior provided that the infiltration front moves downward just behind the lava solidification front despite heavy rainfall on hawaii for example 250 cm yr at kilauea iki exposed prehistoric lava flows on that island exhibit only colonnades we deduce from this that heavy rainfall alone is inadequate to quench flow interiors extensive flooding by deranged drainage or perhaps extremely heavy rainfall 250 cm yr is apparently required to produce entablatures the model also can be used to explain the occurrence of repeated entablature and colonnade by considering the effects of intermittent inundation the imposition of boundary conditions that mimic the removal and reoccurrence of flooding yields cooling histories that are consistent with the development of multiple colonnades and entablatures

bimaterial effects in an earthquake cycle model using rate and state friction we have developed a computational framework to study earthquake cycles in 2 d plane strain and apply it to faults separating dissimilar material we consider a planar strike slip fault governed by rate and state friction where quasi dynamic events nucleate spontaneously due to remote tectonic loading we investigate the influence of material contrast over the course of many hundreds of years for the parameters we consider we find that the presence of bimaterial properties influences the earthquake nucleation site such that rupture in the preferred direction that is in the direction of particle motion of the side of the fault with lower shear wave velocity is favorable for large values of the critical slip distance dc events propagating in the preferred rupture direction occur for a wide range of material contrasts for smaller values of dc small events emerge even in the monomaterial case with material mismatch present some of these small events propagate in the nonpreferred direction made possible by a favorable stress distribution left on the fault from previous ruptures these results may shed light on our understanding of rupture directivity on large strike slip faults like the san andreas fault in california which occasionally host events rupturing in the nonpreferred direction

rootless cones on mars indicating the presence of shallow equatorial ground ice in recent times high resolution mars orbiter camera moc images have revealed the existence of clusters of small cones in the cerberus plains marte valles and amazonis planitia mars these cones are similar in both morphology and planar dimensions to the larger of icelandic rootless cones which form due to explosive interactions between surficial lavas and near surface groundwater impact crater size frequency relationships indicate that surfaces upon which the cones sit are no older than 10 ma if martian cones form in the same manner as terrestrial rootless cones then equatorial ground ice or ground water must have been present near the surface in geologically recent times

emplacement and inflation of pahoehoe sheet flows observations and measurements of active lava flows on kilauea volcano hawaii inflated pahoehoe sheet flows have a distinctive horizontal upper surface which can be several hundred meters across and are bounded by steep monoclinal uplifts the inflated sheet flows we studied ranged from 1 to 5 m in thickness but initially propagated as thin sheets of fluid pahoehoe lava generally 20 30 cm thick individual lobes originated at outbreaks from the inflated front of a prior sheet flow lobe and initially moved rapidly away from their source velocities slowed greatly within hours due to radial spreading and to depletion of lava stored within the source flow as the outward flow velocity decreases cooling promotes rapid crustal growth at first the crust behaves plastically as pahoehoe toes form after the crust attains a thickness of 2 5 cm it behaves more rigidly and develops enough strength to retain incoming lava thus increasing the hydrostatic head at the flow front the increased hydrostatic pressure is distributed evenly through the liquid lava core of the flow resulting in uniform uplift of the entire sheet flow lobe initial uplift rates are rapid flows thicken to 1 m in 1 2 hours but rates decline sharply as crustal thickness increases and as outbreaks occur from the margins of the inflating lobe one flow reached a final thickness of nearly 4 m after 350 hr inflation data define power law curves whereas crustal cooling follows square root of time relationships the combination of data can be used to construct simple models of inflated sheet flows as the flow advances preferred pathways develop in the older portions of the liquid cored flow these pathways can evolve into lavatube systems within a few weeks formation of lava tubes results in highly efficient delivery of lava at velocities of several kilometers per hour to a flow front that may be moving 1 2 orders of magnitude slower if advance of the sheet flow is terminated the tube remains filled with lava that crystallizes in situ rather than draining to form the cave like lava tubes commonly associated with pahoehoe flows inflated sheet flows from kilauea and mauna loa are morphologically similar to some thick icelandic and submarine sheet flows suggesting a similar mechanism of emplacement the planar sheet like geometry of flood basalt flows may also result from inflation of sequentially emplaced flow lobes rather than nearly instantaneous emplacement as literal floods of lava

thermal and near infrared analyses of central peaks of martian impact craters evidence for a heterogeneous martian crust central peaks of impact craters contain materials exhumed from depth and therefore investigation of these materials provides clues to subsurface geology and mineralogy a global spectral survey of central peaks of martian impact craters between 10 and 200 km diameter was completed using mars odyssey thermal emission imaging system themis data twenty six central peaks with distinctive spectral signatures from surrounding plains were identified and characterized with thermal infrared and visible near infrared data the distribution of spectrally distinct central peaks sdcps shows some degree of regional clustering with most craters found in western noachis terra tyrrhena terra within the northern rim of hellas basin and fewer in the northern lowlands with the exception of four craters in western noachis terra sdcps contain only one spectrally distinct unit at themis resolution 100 m pixel the maximum number of spectrally distinct units observed was three in jones and ostrov craters the western noachis terra sdcps may expose crustal stratigraphies of multiple igneous compositions or impact materials from argyre in the highlands most sdcp units are consistent with enrichments in olivine or pyroxene relative to surrounding plains suggesting olivine and pyroxene basaltic lithologies few are olivine and pyroxene poor no spatial trend in spectrally derived compositions of sdcps was observed three sdcps contain themis signatures consistent with high abundances of phyllosilicates which may contain the most phyllosilicate rich lithologies found in central peak associated materials globally

evidence of volcanic and glacial activity in chryse and acidalia planitiae mars abstract chryse and acidalia planitiae show numerous examples of enigmatic landforms previously interpreted to have been influenced by a water ice rich geologic history these landforms include giant polygons bounded by kilometer scale arcuate troughs bright pitted mounds and mesa like features to investigate the significance of the last we have analyzed in detail the region between 60 n 290 e and 10 n 360 e utilizing hirise high resolution imaging science experiment images as well as regional scale data for context the mesas may be analogous to terrestrial tuyas emergent sub ice volcanoes although definitive proof has not been identified we also report on a blocky unit and associated landforms drumlins eskers inverted valleys kettle holes consistent with ice emplaced volcanic or volcano sedimentary flows the spatial association between tuya like mesas ice emplaced flows and further possible evidence of volcanism deflated flow fronts volcanic vents columnar jointing rootless cones and an extensive fluid rich substratum giant polygons bright mounds rampart craters allows for the possibility of glaciovolcanic activity in the region landforms indicative of glacial activity on chryse acidalia suggest a paleoclimatic environment remarkably different from today s climate changes on mars driven by orbital obliquity changes or giant outflow channel activity could have resulted in ice sheet related landforms far from the current polar caps

structure and origin by injection of lava under surface crust of tumuli lava rises lava rise pits and lava inflation clefts in hawaii tumuli are positive topographic features that are common on hawaiian pahoehoe lava flow fields particularly on shallow slopes and 75 measured examples are presented here to document the size range tumuli form by up tilting of crustal plates without any crustal shortening and are thus distinguished from pressure ridges which are up buckled by laterally directed pressure the axial or star like systems of deep clefts that characterize tumuli are defined here as lava inflation clefts their tips advanced into red hot lava and they widened as uplift proceeded and while the lava crust was thickening flat surfaced uplifts formed like tumuli by injection of lava under a surface crust were previously called pressure plateaus but lava rise is proposed instead the pits that abound among lava rises previously attributed to collapse or subsidence are generally formed because the lava around them rose and the name lava rise pit is proposed unique examples of tumuli and lava rises from which lava drained out under a surface crust 1 5 to 2 5 m thick are described from kilauea caldera these examples show that in tumuli and lava rises the crust floats on considerable bodies of fluid lava and is able to do so because of its higher vesicle content the fluid lava loses many of its gas bubbles during residence beneath the crust the bulk densities of samples from tumuli show a general downward increase the form of the density profile is consistent with the relationship that for any given crustal thickness the density of fluid lava closely matched the average density of that crust suggesting that the lava was stably density stratified it is inferred that stable stratification was regulated by out flows of the more vesicular lava fractions loss of bubbles through the lava inflation clefts and entry of injected lava at its level of neutral buoyancy below the uppermost meter the downward decrease in vesicularity closely conforms with that expected by compression of a uniform mass of gas per unit mass of lava

candidate subglacial volcanoes in the south polar region of mars morphology morphometry and eruption conditions 1 a number of isolated mountains mapped in the south polar region of mars are documented using mars global surveyor mgs mars orbiter laser altimeter mola and mars orbiter camera moc data these mountains have average separation distances of 175 km they are typically 30 40 km in diameter and 1000 1500 m high and their bases fall near an elevation of 1200 m a significant number of the population are located on or very close to a 660 km long line extending toward the south pole the summits of a number of these mountains have unusual shapes flat topped flat topped with a cone and large summit craters relative to the summit diameter sinuous channels are found in association with the margins of several of the mountains several modes of origin are considered for these mountains including impact tectonic and volcanic these mountains occur in intimate association with the hesperian aged dorsa argentea formation a unit containing features and structures interpreted to be related to a thick areally extensive ice sheet on the basis of these observations and analyses it is concluded that the most plausible origin for these mountains is volcanic and that they represent extrusion of lava from vents many of which lay underneath an ice sheet the unusual shape of many of the mountains is consistent with construction of a volcanic edifice underneath an ice sheet and melting of adjacent ice rich material sometimes forming drainage channels the topography of these mountains suggests that the ice sheet averaged at least 1 4 km thick at the time of eruption these features appear to be associated with early hesperian aged ridged plains hr

formation of submarine lava channel textures insights from laboratory simulations 1 laboratory simulations using polyethylene glycol peg extruded at a constant rate and temperature into a tank with a uniform basal slope and filled with a cold sucrose solution generate channels that are defined by stationary levees and mobile flow interiors these laboratory channels consistently display the following surface textures in the channel smooth folded lineated and chaotic in the simulations we can observe specific local conditions including flow rate position within the channel and time that combine to develop each texture the textures in peg flows form due to relative differences in shear forces between the peg crust and the underlying liquid wax minimal shear forces form smooth crust whereas folded crust forms when the shear is sufficiently high to cause ductile deformation brittle deformation of solid peg creates a chaotic texture and lineated crust results from shear forces along the channel levee margin we observe similar textures in submarine lava channels with sources at or near the axial summit trough of the east pacific rise between 9 and 10 n we mapped the surface textures of nine submarine lava channels using high resolution digital images collected during camera tows these textural maps along with observations of the formation of similar features in analog flows reveal important information about the mechanisms occurring across the channel during emplacement including relative flow velocity and shear stress

channelized lava flows at the east pacific rise crest 9 10 n the importance of off axis lava transport in developing the architecture of young oceanic crust 1 submarine lava flows are the building blocks of young oceanic crust lava erupted at the ridge axis is transported across the ridge crest in a manner dictated by the rheology of the lava the characteristics of the eruption and the topography it encounters the resulting lava flows can vary dramatically in form and consequently in their impact on the physical characteristics of the seafloor and the architecture of the upper 50 500 m of the oceanic crust we have mapped and measured numerous submarine channelized lava flows at the east pacific rise epr crest 9 10 n that reflect the high effusion rate and high flow velocity end member of lava eruption and transport at mid ocean ridges channel systems composed of identifiable segments 50 1000 m in length extend up to 3 km from the axial summit trough ast and have widths of 10 50 m and depths of 2 3 m samples collected within the channels are n morb with mg indicating eruption from the ast we produce detailed maps of lava surface morphology across the channel surface from mosaics of digital images that show lineated or flat sheets at the channel center bounded by brecciated lava at the channel margins modeled velocity profiles across the channel surface allow us to determine flux through the channels from 0 4 to 4 7 103 m3 s and modeled shear rates help explain the surface morphology variation we suggest that channelized lava flows are a primary mechanism by which lava accumulates in the off axis region 1 3 km and produces the layer 2a thickening that is observed at fast and superfast spreading ridges in addition the rapid high volume flux eruptions necessary to produce channelized flows may act as an indicator of the local magma budget along the epr we find that high concentrations of channelized lava flows correlate with local across axis ridge morphology indicative of an elevated magma budget additionally in locations where channelized flows are located dominantly to the east or west of the ast the ridge crest is asymmetric and layer 2a appears to thicken over a greater distance from the ast toward the side of the ridge crest where the channels are located

origin of small pits in martian impact craters we propose a numerical model for the formation of the closely spaced pits found in the thin ejecta related deposits superposed on the floors interior terrace blocks and near rim ejecta blankets of well preserved martian impact craters our model predicts the explosive degassing of water from this pitted material which is assumed to originally be water bearing impact melt rich breccia at the time of deposition this process is analogous to what occurred in the fall out suevite deposits at the ries impact structure in germany at ries impact heating of water bearing target material resulted in the rapid degassing of its water and other volatiles the martian environment plays an important role in enhancing the effects of this degassing by increasing the flow speed of the escaping gas the high flow rate of gas through particulate materials such as suevite tends to quickly form segregation channels or vent pipes similar to those found in the ries deposits these pipes act as conduits for the efficient high speed escape of the gas and small clasts that it entrains escaping gas and entrained clasts abraded and eroded the conduit walls flaring them to form pits above a network of pipes

an assessment of evidence for pingos on mars using hirise pingos are small hills with cores of ice formed by injection and freezing of pressurized water the possibility of pingos on mars is of particular interest because of the associated implications for liquid water we have systematically searched for candidate pingos using images from the high resolution imaging science experiment hirise camera since pingos are expected to develop surface fractures due to extension of the frozen ground over the ice core we have searched for fractured features and identified a variety of mounds these features are confined to the martian mid latitudes in the bands where gullies are also most common the observed fractured mounds have a variety of morphologies and are likely of multiple origins isolated fractured mounds found on the floors of gullied craters in the southern hemisphere match the general morphologic characteristics of terrestrial pingos and are the best candidates for martian pingos but there is currently no direct evidence for presence of ice cores and it is difficult to produce the necessary water volumes so these features should still be interpreted with caution other fractured mounds appear more likely to be erosional remnants of an unusual mantling layer or possibly thermokarst structures flat topped mounds in utopia have some characteristics fracture pattern and latitudinal distribution consistent with pingos but differ in other aspects such as shape and setting while we do not rule out a pingo origin we prefer an erosional model for these enigmatic features

ultrahigh resolution topographic mapping of mars with mro hirise stereo images meter scale slopes of candidate phoenix landing sites 1 the objectives of this paper are twofold first to report our estimates of the meter to decameter scale topography and slopes of candidate landing sites for the phoenix mission based on analysis of mars global surveyor mgs mars orbiter camera moc images with a typical pixel scale of 3 m and mars reconnaissance orbiter mro high resolution imaging science experiment hirise images at 0 3 m pixel 1 and second to document in detail the geometric calibration software and procedures on which the photogrammetric analysis of hirise data is based a combination of optical design modeling laboratory observations star images and mars images form the basis for software in the u s geological survey integrated software for imagers and spectrometers isis 3 system that corrects the images for a variety of distortions with single pixel or subpixel accuracy corrected images are analyzed in the commercial photogrammetric software socet set bae systems yielding digital topographic models dtms with a grid spacing of 1 m 3 4 pixels that require minimal interactive editing photoclinometry yields dtms with single pixel grid spacing slopes from moc and hirise are comparable throughout the latitude zone of interest and compare favorably with those where past missions have landed successfully only the mars exploration rover mer b site in meridiani planum is smoother moc results at multiple locations have root mean square rms bidirectional slopes of 0 8 4 5 at baselines of 3 10 m hirise stereopairs one per final candidate site and one in the former site yield 1 8 2 8 slopes at 1 m baseline slopes at 1 m from photoclinometry are also in the range 2 3 after correction for image blur slopes exceeding the 16 phoenix safety limit are extremely rare

importance of aeolian processes in the origin of the north polar chasmata mars abstract a detailed examination of the location and orientation of sand dunes and other aeolian features within the north polar chasmata indicates that steep scarps strongly influence the direction and intensity of prevailing winds these steep scarps are present at the heads and along the margins of the north polar chasmata topographic profiles of the arcuate head scarps and equator facing wall of chasma boreale reveal unusually steep polar slopes ranging from 6 30 the relatively steep sloped 8 sinuous scarp at the head of two smaller chasmata located west of chasma boreale exhibits an obvious resistant cap forming unit scarp retreat is occurring in places where the cap unit is actively being undercut by descending slope winds low albedo surfaces lacking sand dunes or dust mantles are present at the base of the polar scarps a 100 300 m deep moat located at the base of the scarps corresponds with these surfaces and indicates an area of active aeolian scour from descending katabatic winds small local dust storms observed along the equator facing wall of chasma boreale imply that slope wind velocities in chasma boreale are sufficient to mobilize dust and sand sized particles in the polar layered deposits pld two amphitheater forms located above the cap forming unit of the sinuous scarp and west of chasma boreale may represent an early stage of polar scarp and chasma formation these two forms are developing within a younger section of polar layered materials the unusually steep scarps associated with the polar chasmata have developed where resistant layers are present in the pld offering resistance during the headward erosion and poleward retreat of the scarps steep slopes that formed under these circumstances enhance the flow of down scarp katabatic winds on the basis of these observations we reject the fluvial outflood hypothesis for the origin of the north polar chasmata and embrace a wind erosion model for their long term development in the aeolian model off pole katabatic winds progressively remove materials from the steep slopes below chasmata scarps undermining resistant layers at the tops of scarps and causing retreat by headward erosion assuming a minimum age for the onset of formation of chasma boreale 105 yr results in a maximum volumetric erosion rate of 0 35 km 3 yr 1 removal of this volume of material from the equator facing wall and head scarps of chasma would require a rate for scarp retreat of 0 5 m yr 1

ghosts of lithospheres past imaging an evolving lithospheric mantle in southern africa group ii 143 117 ma and group i kimberlites 108 74 ma intrude across the southwest boundary of the kaapvaal craton sampling the same volume of the subcontinental lithospheric mantle sclm in two time slices major and trace element analyses of 3699 peridotitic garnet xenocrysts were used to construct paleogeotherms for 17 kimberlite localities and to place each garnet at its depth of origin the ti contents of each garnet and the calculated x mg of its coexisting olivine were projected onto a southwest northeast section across the craton boundary and splines were used to interpolate between the virtual boreholes in each age group the sections show that the cratonic sclm extends at least 75 km southwest of the mapped craton boundary suggesting a dipping contact marked differences between the time slice sections show that between 117 and 108 ma the sclm on both sides of the craton boundary was heated and chemically refertilized by infiltrating asthenosphere derived melts thinning the depleted layer by 40 km the thermal and geochemical changes record a significant tectonothermal event that may be related to changes in the stress field associated with opening of the south atlantic or with mantle upwelling

widespread inflation and drainage of a pāhoehoe flow field the nesjahraun þingvellir iceland this study describes the emplacement of the nesjahraun a basaltic lava flow that entered the lake ingvallavatn sw iceland high resolution remotely sensed data were combined with fieldwork to map the flow field onshore the nesjahraun exhibits a variety of textures related to the widespread inflation and collapse of a p hoehoe flow field its emplacement is interpreted as follows initially the eruption produced sheet p hoehoe in the central part of the flow field the lava has a platy ridged surface which is similar to some other lava flows in iceland and on mars here the texture is interpreted to have formed by unsteady inflation of the brittle crust of stationary sheet p hoehoe causing it to break into separate plates the ridges of broken p hoehoe slabs formed as the plates of crust moved vertically past each other in a process similar to the formation of shatter rings upstream fresh lava overflowed repeatedly from channels and tubes covering the surface with shelly p hoehoe formation of a 250 m wide open channel through the flow field allowed the inflated central part of the flow to drain rapidly this phase produced a lava which eroded the channel walls carrying broken p hoehoe slabs lava balls and detaching large 200 m long rafts of compound shelly p hoehoe lava much of this channelized lava flowed into the lake leaving a network of drained channels and tubes in the upstream part of the flow as in other locations the platy ridged texture is associated with a low underlying slope and high eruption rate here its formation was possibly enhanced by lateral confinement hindered entry into the lake and an elevated vent location we suggest that formation of this type of platy ridged lava where the plates are smooth and the ridges are slabs of broken p hoehoe can occur without significant horizontal transport as the surface crust is broken into plates in situ this reconstruction of the emplacement of the nesjahraun also demonstrates that high resolution aerial survey data are extremely useful in the mapping and measurement of lithofacies distributions in large flow fields but that fieldwork is still necessary to obtain the detailed textural information necessary to interpret them

resurfacing history of the northern plains of mars based on geologic mapping of mars global surveyor data 1 geologic mapping of the northern plains of mars based on mars orbiter laser altimeter topography and viking and mars orbiter camera images reveals new insights into geologic processes and events in this region during the hesperian and amazonian periods we propose four successive stages of lowland resurfacing likely related to the activity of near surface volatiles commencing at the highland lowland boundary hlb and progressing to lower topographic levels as follows highest elevations indicated stage 1 upper boundary plains early hesperian 2 0 to 2 9 km stage 2 lower boundary plains and outflow channel dissection late hesperian 2 7 to 4 0 km stage 3 vastitas borealis formation vbf surface late hesperian to early amazonian 3 1 to 4 1 km and stage 4 local chaos zones early amazonian 3 8 to 5 0 km at acidalia mensa stage 2 and 3 levels may be lower 4 4 and 4 8 km respectively contractional ridges form the dominant structure in the plains and developed from near the end of the early hesperian to the early amazonian geomorphic evidence for a northern plains filling ocean during stage 2 is absent because one did not form or its evidence was destroyed by stage 3 resurfacing remnants of possible amazonian dust mantles occur on top of the vbf the north polar layered deposits appear to be made up of an up to kilometer thick lower sequence of sandy layers early to middle amazonian in age overlain by late amazonian ice rich dust layers both units appear to have outliers suggesting that they once were more extensive

evidence for recent climate change on mars from the identification of youthful near surface ground ice ground ice in the crust and soil may be one of the largest reservoirs of water on mars1 2 3 near surface ground ice is predicted to be stable at latitudes higher than 40 ref 4 where a number of geomorphologic features indicative of viscous creep and hence ground ice have been observed5 mid latitude soils have also been implicated as a water ice reservoir6 the capacity of which is predicted to vary on a 100 000 year timescale owing to orbitally driven variations in climate7 it is uncertain however whether near surface ground ice currently exists at these latitudes and how it is changing with time here we report observational evidence for a mid latitude reservoir of near surface water ice occupying the pore space of soils the thickness of the ice occupied soil reservoir 1 10 m and its distribution in the 30 to 60 latitude bands indicate a reservoir of 1 5 6 0 104 km3 equivalent to a global layer of water 10 40 cm thick we infer that the reservoir was created during the last phase of high orbital obliquity less than 100 000 years ago and is now being diminished

topographic evidence for geologically recent near polar volcanism on mars abstract the mars orbiter laser altimeter mola was used to measure the topography of several putative near polar martian volcanic craterforms we believe they were formed by effusive lava shield building eruptions and were not hydromagmatic events as previously suggested furthermore these craterforms appear to be geologically recent with ages between 1 and 20 million years and may provide new evidence of localized thermal anomalies in the upper martian crustal column

mapping rays and secondary craters from the martian crater zunil 1 zunil a 10 1 km rayed crater in elysium planitia mars produced more than 7 107 secondary craters 15 m in diameter we mapped zunil s rays from thermal ir themis nighttime images up to 1700 km from the primary crater and mapped bright and dark ejecta craters candidate secondaries up to 3600 km range ray segments were mapped up to 450 km east of zunil and up to 1700 km to the west both rays and bright ejecta craters are best detected over terrains with moderate thermal inertia which are abundant west of but not east of zunil nevertheless our interpretation is that zunil was created by a moderately oblique impact from the east zunil secondaries are abundant over all terrain types except the medusae fossae formation mff given the likely age of zunil 100 ma parts of the mff must be eroding at 0 08 m ma the size frequency distribution of the secondaries in zunil s rays and probable distant secondaries 750 1700 km to the west of zunil have a cumulative power law exponent near 5 whereas secondaries between the major rays have an exponent near 3 4 we modeled the size velocity relationship for zunil s ejected fragments it is consistent with the predictions of melosh 1984 for spallation of a strong surface layer and demonstrates that the inverse size velocity correlation continues up to at least 2 km s

compositional stratigraphy of clay bearing layered deposits at mawrth vallis mars phyllosilicates have previously been detected in layered outcrops in and around the martian outflow channel mawrth vallis crism spectra of these outcrops exhibit features diagnostic of kaolinite montmorillonite and fe mg rich smectites along with crystalline ferric oxide minerals such as hematite these minerals occur in distinct stratigraphic horizons implying changing environmental conditions and or a variable sediment source for these layered deposits similar stratigraphic sequences occur on both sides of the outflow channel and on its floor with al clay bearing layers typically overlying fe mg clay bearing layers this pattern combined with layer geometries measured using topographic data from hirise and hrsc suggests that the al clay bearing horizons at mawrth vallis postdate the outflow channel and may represent a later sedimentary or altered pyroclastic deposit that drapes the topography

the high resolution imaging science experiment hirise during mro s primary science phase psp the high resolution imaging science experiment hirise on the mars reconnaissance orbiter mro acquired 8 terapixels of data in 9137 images of mars between october 2006 and december 2008 covering 0 55 of the surface images are typically 5 6 km wide with 3 color coverage over the central 20 of the swath and their scales usually range from 25 to 60 cm pixel nine hundred and sixty stereo pairs were acquired and more than 50 digital terrain models dtms completed these data have led to some of the most significant science results new methods to measure and correct distortions due to pointing jitter facilitate topographic and change detection studies at sub meter scales recent results address noachian bedrock stratigraphy fluvially deposited fans in craters and in or near valles marineris groundwater flow in fractures and porous media quasi periodic layering in polar and non polar deposits tectonic history of west candor chasma geometry of clay rich deposits near and within mawrth vallis dynamics of flood lavas in the cerberus palus region evidence for pyroclastic deposits columnar jointing in lava flows recent collapse pits evidence for water in well preserved impact craters newly discovered large rayed craters and glacial and periglacial processes of particular interest are ongoing processes such as those driven by the wind impact cratering avalanches of dust and or frost relatively bright deposits on steep gullied slopes and the dynamic seasonal processes over polar regions hirise has acquired hundreds of large images of past present and potential future landing sites and has contributed to scientific and engineering studies of those sites warming the focal plane electronics prior to imaging has mitigated an instrument anomaly that produces bad data under cold operating conditions

hydrovolcanic features on mars preliminary observations from the first mars year of hirise imaging abstract we provide an overview of features indicative of the interaction between water and lava and or magma on mars as seen by the high resolution imaging science experiment hirise camera during the primary science phase of the mars reconnaissance orbiter mro mission the ability to confidently resolve meter scale features from orbit has been extremely useful in the study of the most pristine examples in particular hirise has allowed the documentation of previously undescribed features associated with phreatovolcanic cones formed by the interaction of lava and groundwater on rapidly emplaced flood lavas these include moats and wakes that indicate that the lava crust was thin and mobile respectively jaeger w l keszthelyi l p mcewen a s dundas c m russel p s 2007 science 317 1709 1711 hirise has also discovered entablature style jointing in lavas that is indicative of water cooling milazzo m p keszthelyi l p jaeger w l rosiek m mattson s verba c beyer r a geissler p e mcewen a s and the hirise team 2009 geology 37 171 174 other observations strongly support the idea of extensive volcanic mudflows lahars evidence for other forms of hydrovolcanism including glaciovolcanic interactions is more equivocal this is largely because most older and high latitude terrains have been extensively modified masking any earlier 1 10 m scale features much like terrestrial fieldwork the prerequisite for making full use of hirise s capabilities is finding good outcrops