the unique geomorphology and physical properties of the vestalia terra plateau abstract we produced a geologic map of the av 9 numisia quadrangle of asteroid vesta using dawn spacecraft data to serve as a tool to understand the geologic relations of surface features in this region these features include the plateau vestalia terra a hill named brumalia tholus and an unusual dark ribbon material crossing the majority of the map area stratigraphic relations suggest that vestalia terra is one of the oldest features on vesta despite a model crater age date similar to that of much of the surface of the asteroid cornelia numisia and drusilla craters reveal bright and dark material in their walls and both cornelia and numisia have smooth and pitted terrains on their floors suggestive of the release of volatiles during or shortly after the impacts that formed these craters cornelia fabia and teia craters have extensive bright ejecta lobes while diogenitic material has been identified in association with the bright teia and fabia ejecta hydroxyl has been detected in the dark material within cornelia numisia and drusilla three large pit crater chains appear in the map area with an orientation similar to the equatorial troughs that cut the majority of vesta analysis of these features has led to several interpretations of the geological history of the region vestalia terra appears to be mechanically stronger than the rest of vesta brumalia tholus may be the surface representation of a dike fed laccolith the dark ribbon feature is proposed to represent a long runout ejecta flow from drusilla crater

the origin of vesta s crust insights from spectroscopy of the vestoids high quality vnir spectra of 15 vestoids small asteroids that are believed to originate from vesta were collected and compared to laboratory spectra and compositional data for selected hed meteorites a combination of spectral parameters such as band centers and factors derived from modified gaussian model fits band centers band strengths calculation of the low to high ca pyroxene ratio were used to establish if each vestoid appeared most like eucrite or diogenite material or a mixture of the two howardite this resulted in the identification of the first asteroid with a ferroan diogenite composition 2511 patterson this asteroid can be used to constrain the size of diogenite magma chambers within the crust of vesta the vestoids indicate that both large scale homogeneous units 5 km and smaller scale heterogeneity 1 km exist on the surface of vesta as both monomineralogic eucrite or diogenite material alone and mixed both eucrite and diogenite spectra are observed the small scale of the variation observed within the vestoid population is predicted by the partial melting model which has multiple intrusions penetrating into the crust of vesta it is much more difficult to reconcile the observations here with the magma ocean model which would predict much more homogeneous layers on a large scale both at the surface and with depth

geologic mapping of the ac 2 coniraya quadrangle of ceres from nasa s dawn mission implications for a heterogeneously composed crust since its arrival at ceres dawns framing camera has been imaging the dwarf planet at different altitudes using 8 different filters based on these images global clear filter mosaics digital terrain models and global color mosaics were produced these datasets are basis for the derived photogeologic map of the ac2 coniraya quadrangle located between 0 and 90 eastern longitude and between 20 and 40 northern latitude crater size frequency distribution csfd measurements have been applied to derive the stratigraphic history of the identified geologic units in the ac2 coniraya quadrangle based on these investigations we found that the ac2 coniraya quadrangle shows evidence for a long geologic history lasting over 3 5 ga most of the identified geologic units are related to impact cratering at different periods of time while the oldest unit identified in the ac2 coniraya quadrangle the cratered terrain shows an absolute model age ama of 3 3 to 3 5 ga the youngest units including fresh impact craters and lobate flows show amas of 100 ma the large number of different sized impact craters excavated a variety of materials with different colors and albedo indicative of a heterogeneous crustal composition including water or volatiles pits identified at ikapati craters smooth deposits might further indicate the presence of volatiles in the subsurface furthermore our mapping of the ac2 coniraya quadrangle shows that possible h 2 o ice in the crust is heterogeneously distributed as lobate flows bright spots and pits are only found at certain locations related to relatively fresh impact craters

the geological nature of dark material on vesta and implications for the subsurface structure abstract deposits of dark material appear on vesta s surface as features of relatively low albedo in the visible wavelength range of dawn s camera and spectrometer mixed with the regolith and partially excavated by younger impacts the material is exposed as individual layered outcrops in crater walls or ejecta patches having been uncovered and broken up by the impact dark fans on crater walls and dark deposits on crater floors are the result of gravity driven mass wasting triggered by steep slopes and impact seismicity the fact that dark material is mixed with impact ejecta indicates that it has been processed together with the ejected material some small craters display continuous dark ejecta similar to lunar dark halo impact craters indicating that the impact excavated the material from beneath a higher albedo surface the asymmetric distribution of dark material in impact craters and ejecta suggests non continuous distribution in the local subsurface some positive relief dark edifices appear to be impact sculpted hills with dark material distributed over the hill slopes dark features inside and outside of craters are in some places arranged as linear outcrops along scarps or as dark streaks perpendicular to the local topography the spectral characteristics of the dark material resemble that of vesta s regolith dark material is distributed unevenly across vesta s surface with clusters of all types of dark material exposures on a local scale some craters expose or are associated with dark material while others in the immediate vicinity do not show evidence for dark material while the variety of surface exposures of dark material and their different geological correlations with surface features as well as their uneven distribution indicate a globally inhomogeneous distribution in the subsurface the dark material seems to be correlated with the rim and ejecta of the older veneneia south polar basin structure the origin of the dark material is still being debated however the geological analysis suggests that it is exogenic from carbon rich low velocity impactors rather than endogenic from freshly exposed mafic material or melt exposed or created by impacts

the leonard award address presented 1996 july 25 berlin germany the elemental composition of stony cosmic spherules five hundred stony cosmic spherules collected from deep sea sediments polar ice and the strato sphere have been analyzed for major and some minor element composition typical spherules are products of atmospheric melting of millimeter sized and smaller meteoroids the samples are small and modified by atmospheric entry but they are an important source of information on the composition of asteroids the spherules in this study were all analyzed in an identical manner and they provide a sampling of the solar system s asteroids that is both different and less biased than provided by studies of conventional meteorites volatile elements such as na and s are depleted due to atmospheric heating while siderophiles are depleted by less understood causes the refractory nonsiderophile elements appear not to have been significantly dis turbed during atmospheric melting and provide important clues on the elemental composition of millimeter sized meteoroids colliding with the earth typical spherules have cm like composition that is distinctively different than ordinary chondrites and most other meteorite types we assume that c type asteroids are the primary origin of spherules with this composition type s asteroids should also be an important source of the spherules and the analysis data provide constraints on their composition a minor fraction of the spher ules are melt products of precursor particles that did not have chondritic elemental compositions the most common of these are particles that are dominated by olivine the observed compositions of spherules are in consistent with the possibility that an appreciable fraction of the spherules are simply chondrules remelted during atmospheric entry

explosive volcanism and the compositions of cores of differentiated asteroids eleven iron meteorite groups show correlations between ni and siderophile trace elements that are predictable by distribution coefficients between liquid and solid metal in fractionally crystallizing metal magmas these meteorites are interpreted to be fragments of the fractionally crystallized cores of eleven differentiated asteroids many of these groups crystallized from s depleted magmas which we propose resulted from removal of the first partial melt the fe nifes cotectic melt by explosive pyroclastic volcanism of the type envisaged by wilson and keil 8 we show that these dense negatively buoyant melts can be driven to asteroidal surfaces due to the presence of excess pressure in the melt and the presence of buoyant bubbles of gas which decrease the density of the melt we also show that in typical asteroidal materials veins will form which grow into dikes and serve as pathways for migration of melt and gas to asteroidal surfaces since cotectic fe nifes melt consists of about 85 wt fes and 15 wt fe ni removal of small volumes of eutectic melts results in major loss of s but only minor loss of fe ni thus leaving sufficient fe ni to form sizeable asteroidal cores

vesta s shape and morphology vesta s surface is characterized by abundant impact craters some with preserved ejecta blankets large troughs extending around the equatorial region enigmatic dark material and widespread mass wasting but as yet an absence of volcanic features abundant steep slopes indicate that impact generated surface regolith is underlain by bedrock dawn observations confirm the large impact basin rheasilvia at vesta s south pole and reveal evidence for an earlier underlying large basin veneneia vesta s geology displays morphological features characteristic of the moon and terrestrial planets as well as those of other asteroids underscoring vesta s unique role as a transitional solar system body

volcanic eruptions and intrusions on the asteroid 4 vesta we use basic principles of fluid mechanics to explore the intrusive and eruptive consequences of early large scale silicate melting within the asteroid 4 vesta and relate our findings to the properties of the howardite eucrite diogenite meteorites surface lava flows on vesta would commonly have had widths ranging from a few hundred meters to a few kilometers and lengths lying between a few kilometers and at least several tens of kilometers flow thicknesses would have occupied a much narrower range lying between about 5 and 20 m these thicknesses imply cooling rates and plagioclase crystal growth rates which are extremely similar to those measured in the eucrite meteorites intrusions may have formed at very shallow depths as cooling dikes failed to erupt their contents completely to the surface or at the base of the crust where rising magmas would have been neutrally buoyant as the density structure of vesta s crust evolved deep intrusions would have had lateral extents up to 30 km and thicknesses up to 3 m shallow intrusions would have had vertical extents less than 10 km and widths of 1 m none of these intrusions and surface flows would have been thick enough or have formed frequently enough at a given location to have caused extensive thermal alteration of earlier eruptives lending support to the idea that burial and subsequent long term exposure to the rising geothermal gradient was the main mechanism of thermal metamorphism

clast sizes of ejecta from explosive eruptions on asteroids implications for the fate of the basaltic products of differentiation we use a simple model of the formation growth coalescence and migration of veins of basaltic melt generated by partial melting in chondritic asteroids to deduce the sizes of and pressures within the fluid filled dikes reaching the surfaces of such bodies the gas contents 1000 ppm of mainly co and n2 of the asteroids were high enough that bubbles of free gas trapped in the melt veins gave the basaltic melts significant buoyancy expansion of these gases as a dike opened to the vacuum at the surface led to fragmentation of the melts into liquid droplets which were transported upwards by the accelerating gases to the surface the sizes of these droplets and hence of the pyroclastic glass beads into which they cooled are calculated to lie in the range 30 m to 4 mm this range is essentially independent of the size or gas content of the asteroid parent and only weakly dependent on the internal pressure of the erupting fluid the fate of the pyroclasts however does depend on all of these factors at very low internal pressures significant separation of the gas and liquid in a rising dike may take place and not all of the liquid will be expelled from the dike when it opens to the surface for relatively large 100 km radius asteroids with relatively low 300 ppm gas contents the larger clasts are too heavy to be lifted from the level at which magma fragmentation takes place by the gas flow and so would also remain behind to form basaltic veins the apparent absence of basaltic veins in meteorites then implies both that internal pressures in near surface dikes were generally greater than 0 3 mpa and that low gas contents were not common finally as long as pyroclasts are lofted from the magma fragmentation level they will be accelerated to at least 90 of the final gas speed if this speed exceeds the escape speed from the asteroid as happens readily for high gas contents and small asteroids the pyroclasts will be expelled into space and lost from the meteorite record otherwise low gas contents or large asteroids they will eventually fall back to be incorporated into the surface regolith modifying the chemical and physical properties of meteorites subsequently derived from it

high temperature mass spectrometric degassing of enstatite chondrites implications for pyroclastic volcanism on the aubrite parent body wilson and keil 1991 argued that a few hundred ppm of expanding volatiles present in early partial basaltic melts on the aubrite parent body or on other differentiated asteroids 925 c principally co 1580 2830 ppm n2 110 430 ppm cl 120 450 ppm s see discussion for abundance are indigenous the precise host phases and host sites of the indigeneous volatiles are uncertain 2 2 n2 release appears to be associated with the melting of the fe ni sulfide cotectic at 950 c the amounts of gas released are not unlike those previously measured for iron meteorites and sulfides in irons but exceed by several orders of magnitude the equilibrium n2 solubility in metallic fe ni for solar nebula pressures this suggests that either partial pressures during n2 incorporation were much higher than solar values or n2 is trapped in lattice defects and microcracks possibly in sulfides 3 3 co release over a narrow temperature interval of 1025 1255 c is violent and results in pronounced vesiculation of the residue co may be trapped in microvesicles and cracks in silicates or less likely may form by a reduction reaction during heating between carbon graphite and the minor ferrosilite in the enstatite 4 4 cl release beginning at 950 eh s and 1025 c el s appears not to be associated with lawrencite which should sublime at t as low as 325 c in eh s some may be released from djerfisherite 5 5 gas release and textural studies suggest that metamorphic reheating of eh s and el s was to less than about 950 c el s did not form during metamorphic reheating of eh s by draining of cotectic fe ni sulfide liquid 6 6 sinoite present in some el6 s did not form by condensation from the solar nebula but during parent body metamorphism at t 950 c via the reaction of sio2 gaseous n2 and si in solid solution in the metallic fe ni 7 7 as little as 1 by volume partial melting in an asteroid of any size will generate excess pressures in the melt much greater 10 s of mpa than the tensile strength of silicate materials and will initiate formation of cracks this process will be enhanced by expansion of any exsolved or trapped gas phase in the asteroid and will lead to the formation of pathways cracks growing into dikes for melt migration 8 8 the pressure gradients implied when these pathways intersect the asteroid surface coupled with the observed amounts of indigenous volatiles released from eh s and el s are easily sufficient to drive the pyroclastic volcanic eruptions envisaged by wilson and keil 1991

space weathering on airless bodies resolving a mystery with lunar samples using new techniques to examine the products of space weathering of lunar soils we demonstrate that nanophase reduced iron npfe0 is produced on the surface of grains by a combination of vapor deposition and irradiation effects the optical properties of soils both measured and modeled are shown to be highly dependent on the cumulative amount of npfe0 which varies with different starting materials and the energetics of different parts of the solar system the measured properties of intermediate albedo asteroids the abundant s type asteroids in particular are shown to directly mimic the effects predicted for small amounts of npfe0 on grains of an ordinary chondrite regolith this measurement and characterization of space weathering products seems to remove a final obstacle hindering a link between the abundant ordinary chondrite meteorites and common asteroids

thermal and dynamical regimes of single and two phase magmatic flow in dikes finite element calculations of magma flow in dikelike channels with length to width ratios of 1000 1 or more have been used to investigate the coupling between thermal and dynamical regimes due to temperature dependent viscosity and dissipation steady state solutions with realistic thermal and dynamical parameter values have been obtained the models show that the onset of solidification on the boundaries of a basaltic or andesitic dike as predicted by idealized laminar flow models can be prevented or significantly delayed by a small amount of transverse flow induced by rising bubbles boundary roughness or turbulence this effect will reduce the critical initial widths of dikes estimated by bruce and huppert 1989 in the absence of transverse flow the bulk temperature of the magma may actually increase slightly with distance along the dike as a result of viscous dissipation even while solidification is occurring on the walls of the dike with converging or necking dike walls it is found that boundary temperatures fall to a minimum and then increase with distance along a dike even if viscous heating is neglected for viscous heating to offset significant rates of heat loss 2 kw m2 in a plane parallel 1 m wide basaltic dike an average flow velocity of 2 7 m s driven by a pressure gradient of 1 7 mpa km is required a 7 void fraction caused by exsolution of volatiles or chamber gas in the magma will produce this pressure gradient the ease of producing such a gradient by reducing the density of the magmatic column with addition of a gas phase makes it likely that flows of basaltic magma could be maintained in dikes tens of kilometers long furthermore a gas phase may be important for the propagation of the fracture prior to the initial injection of magma during dike emplacement rapid transport of more silicic magmas through dikes is inhibited by the requirement of large driving pressure gradients exceeding several hundred megapascals per kilometer however the pressure gradient can be substantially reduced if the magma is a heterogenous mixture of a predominantly silicic component with a more mafic component pipe flow experiments involving molten polymers that exhibit dynamical similarity to magmas strongly suggest that unmixing occurs when a two component magma mixture in which the components have different viscosities rises within a dike within a few dike widths of the inlet the less viscous mafic component encapsulates the more viscous silicic part effectively lubricating the passage of the more viscous component compositional variations in the obsidian dome volcano support the occurrence of this pressure reduction or self lubrication mechanism such a self lubrication process often may be necessary to permit very viscous magmas to reach earth s surface if so chemical and lithologic zoning would be anticipated as a common feature of near surface intrusions or lava flows that are characterized by a high silica and or high crystal content such as at deadman dome in long valley california

introduction the geologic mapping of vesta the purpose of this paper is to introduce the geologic mapping of vesta special issue section of icarus which includes several papers containing geologic maps of the surface of vesta made to support data analysis conducted by the dawn science team during the vesta encounter july 2011 september 2012 in this paper we briefly discuss pre dawn knowledge of vesta provide the goals of our geologic mapping campaign discuss the methodologies and materials used for geologic mapping review the global geologic context of vesta discuss the challenges of mapping the geology of vesta as a small airless body and describe the content of the papers in this special issue section we conclude with a discussion of lessons learned from our quadrangle based mapping effort and provide recommendations for conducting mapping campaigns as part of planetary spacecraft nominal missions

a comparison of volcanic eruption processes on earth moon mars io and venus the silicate planets and satellites display a wide range of physical chemical and atmospheric characteristics which may influence the nature of volcanism a major geological process common to the evolution of the surfaces of these bodies consideration of the process of magma ascent and eruption from first principles allows predictions to be made concerning volcanic eruption styles and expected landforms and deposits on each planetary body examination of actual land forms and deposits in light of these predictions leads to a better understanding of the nature of volcanic eruption processes and outlines outstanding problems

exploring exogenic sources for the olivine on asteroid 4 vesta abstract the detection of olivine on vesta is interesting because it may provide critical insights into planetary differentiation early in our solar system s history ground based and hubble space telescope hst observations of asteroid 4 vesta have suggested the presence of olivine on the surface these observations were reinforced by the discovery of olivine rich hed meteorites from vesta in recent years however analysis of data from nasa s dawn spacecraft has shown that this olivine bearing unit is actually impact melt in the ejecta of oppia crater the lack of widespread mantle olivine exposed during the formation of the 19 km deep rheasilvia basin on vesta s south pole further complicated this picture ammannito et al ammannito e et al 2013a nature 504 122 125 reported the discovery of local scale olivine rich units in the form of excavated material from the mantle using the visible and infrared spectrometer vir on dawn these sites are concentrated in the walls and ejecta of craters arruntia 10 5 km in diameter and bellicia 41 7 km in diameter located in the northern hemisphere 350 430 km from rheasilvia basin s rim here we explore alternative sources for the olivine in the northern hemisphere of vesta by reanalyzing the data from the vir instrument using laboratory spectral measurements of meteorites our rationale for using the published dataset was to bypass calibration issues and ensure a consistent dataset between the two studies our analysis of the vir data shows that while the interpretation of their spectra as an olivine rich unit is correct the nature and origin of that olivine could be more complicated we suggest that these olivine exposures could also be explained by the delivery of olivine rich exogenic material this hypothesis is supported by meteoritical evidence in the form of exogenic xenoliths containing significant amount of olivine in some of the hed meteorites from vesta previous laboratory work on heds show that potential sources of olivine on vesta could be different types of olivine rich meteorites either primitive achondrites acapulcoites lodranites ureilites ordinary chondrites h l ll pallasites or carbonaceous chondrites e g cv based on our spectral band parameters analysis the lack of correlation between the location of these olivine rich terrains and possible mantle excavating events and supported by observations of hed meteorites we propose that a probable source for the olivine seen in the northern hemisphere corresponds to remnants of impactors made of olivine rich meteorites the best curve matching results with laboratory spectra suggest these units are hed material mixed with either ordinary chondrites or with some olivine dominated meteorites such as r chondrites

consequences of explosive eruptions on small solar system bodies the case of the missing basalts on the aubrite parent body aubrite meteorites are composed of constituents which are almost certainly of igneous origin if they were generated by the melting and fractionation of enstatite chondrite like parental material as seems very likely then plagioclase rich basaltic complements to the aubrites should have formed however such materials are not known as individual meteorites and the compositions of two plagioclase silica clasts and one albite silica diopside anorthite clast probably an impact melt in the norton county aubrite breccia suggest that they are not the putative enstatite plagioclase basalts we propose a new mechanism that explains the absence of these materials showing that the expansion of even very small amounts of volatiles present in a melt approaching the surface of a small low gravity body will be enough to disrupt the melt into a spray of droplets moving faster than the local escape velocity this explosive volcanic process of melt removal requires larger melt volatile contents on larger bodies and data on the solubility of volatiles in basaltic melts suggest that the process was limited to bodies smaller than about 100 km in radius

asteroids dark and stormy weather can some of the ageing effects on asteroid surfaces be caused by an interplanetary rain of carbon rich solar system debris observations from the dawn space mission suggest that the answer is yes see letters p 79 p 83

compositional evidence of magmatic activity on vesta like the earth and other terrestrial planets the asteroid vesta has a basaltic crust and a large iron core its surface is dominated by impact craters of all sizes and has tectonic features the presence of basaltic compositions olivine howardite eucrite diogenite meteorites and models of vesta s formation suggests that volcanic and or magmatic activity could have occurred on vesta a global search for lobate structures did not find unequivocal evidence of volcanic features nevertheless several morphological properties of brumalia tholus on vestalia terra suggest that this topographic high most likely formed as a magmatic intrusion the presence of more orthopyroxene rich material relative to surrounding terrain in the ejecta of teia a fresh impact crater on the northern face of brumalia tholus supports the hypothesis of magmatic intrusions on vesta

metamorphic history of the eucritic crust of 4 vesta the petrologic properties of eucrites indicate that most formed as rapidly cooled lava flows that were subsequently metamorphosed at 800 c for periods of about 1 myr we have modeled the development of the eucritic crust of the eucrite parent body assumed to be asteroid 4 vesta by the eruption and subsequent burial of lava flows serial magmatism the burial of the flows causes them to reheat to metamorphic temperatures 800 c provided that the initial thermal gradient in the upper 5 km was steep as it would be if 26al was the heat source our model shows that most eucrites were metamorphosed for periods over 1 myr in a crust between 15 and 25 km thick if volcanism lasted only 1 to a few myr longer periods of volcanism do not result in high enough peak metamorphic temperatures to be consistent with eucrite data the hot lower crust in vesta provides an environment in which even small few meters dikes could have cooled slowly enough to produce cumulate eucrites and possibly diogenites

dynamics of a confined lava flow on kilauea volcano hawaii this paper presents a new method of analysing lava flow deposits which allows the velocity discharge rate and rheological properties of channelled moving lavas to be calculated the theory is applied to a lava flow which was erupted on kilauea in july 1974 this flow came from a line of fissures on the edge of the caldera and was confined to a pre existing gully within 50 m of leaving the vent the lava drained onto the floor of the caldera when the activity stopped but left wall and floor deposits which showed that the lava banked up as it flowed around each of the bends field surveys established the radius of curvature of each bend and the associated lava levels and these data together with related field and laboratory measurements are used to study the rheology of the lava the results show the flow to have been fast moving but still laminar with a mean velocity of just over 8 m s 1 the lava had a low or negligible yield strength and viscosities in the range 85 140 pa s an extension of the basic method is considered and the possibility of supercritical flow discussed

magma reservoirs and neutral buoyancy zones on venus implications for the formation and evolution of volcanic landforms we examine the production of magma reservoirs and neutral buoyancy zones nbzs on venus and the implications of their development for the formation and evolution of volcanic landforms the high atmospheric pressure on venus reduces volatile exsolution and generally serves to inhibit the formation of nbzs and shallow magma reservoirs for a range of common terrestrial magma volatile contents magma ascending and erupting near or below mean planetary radius mpr should not stall at shallow magma reservoirs such eruptions would be characterized by relatively high total volumes and effusion rates for the same range of volatile contents at 2 km above mpr about half of the cases result in direct ascent of magma to the surface and half in the production of neutral buoyancy zones in general neutral buoyancy zones and shallow magma reservoirs begin to appear as gas content increases and are nominally shallower on venus than on earth for a fixed volatile content nbzs become deeper with increasing elevation over the range of elevations treated here 1 km to 4 4 km depths differ by a factor of 2 4 these analyses reveal several factors that may help to account for the low height of volcanoes on venus larger primary reservoirs cause the wide dispersal of conduits building edifices models of the position of the shallow nbz reservoir during edifice growth show that for earth the magma chamber center remains at a constant depth below the growing edifice summit thus keeping pace with the increasing elevation while on venus the chamber center becomes deeper relative to the summit of the growing edifice because of the major change in atmospheric pressure as a function of altitude therefore neutral buoyancy zones and magma reservoirs on venus will remain in the prevolcano substrate longer and in many cases may not emerge into the edifice at all the lower rate of vertical migration implies that magma reservoirs would tend to stabilize undergo greater lateral growth and become larger on venus than earth the proportion of the available magma going into production of the edifice relative to that intruded into the substrate is smaller on venus than earth large reservoirs would encourage multiple and more widely dispersed source vents and large volumes for individual eruptions in large reservoirs positively buoyant materials are likely to be produced from differentiation substrate remelting and volatile exsolution nonbuoyant materials exsolving volatiles in a shallow reservoir will need higher gas bubble concentrations to produce eruptions than on earth and when this gas enriched melt emerges at the surface it is more likely to retain its bubbles than to undergo explosive disruption due to the high surface atmospheric pressure therefore there is the potential for the production of a range of erupted lavas that have very high gas bubble concentrations leading to anomalous more viscous rheological properties inhibition of disruption of volatile rich magma for both basaltic and more evolved compositions can lead to the production of 1 more bubble rich vesicular flows characterized by higher viscosity and greater thicknesses the venus equivalent of effusions that would have lost much of their volatiles in the fire fountaining process on earth 2 high volume high discharge rate extremely gas rich basaltic effusions that increase viscosity considerably upon surface extrusion and produce domes whose volumes are considerably higher than their dense rock equivalent and 3 more evolved compositions that are gas rich but do not undergo disruption upon effusion and produce domes and flows of high viscosity the venus equivalent of terrestrial ignimbrites on the basis of these analyses we predict that intrusions and dike emplacement should be common particularly around both shallow and deep magma reservoirs and the tops of dikes should be manifested as radial fractures there should be a range of diameters of circular features associated with the presence of both shallow nbz related and deep plumerelated magma reservoirs large volume lava outpourings should be favored at lower elevations and shallow reservoirs and edifices should be more common at intermediate elevations at the highest elevations magma reservoirs are predicted to be large and relatively deep with low associated edifices direct ascent of magma where neutral buoyancy zones are unlikely low elevations and the propensity for relatively larger magma reservoirs on venus when they do form are both factors which will tend to favor high volume eruptions independent of variations in magma temperature or chemistry individual volcanic features should be interpreted in terms of regional geologic setting and the dynamics of elevation changes commonly associated with various scales of mantle processes and magma emplacement in order to assess global crustal growth processes on venus

space weathering from mercury to the asteroid belt the variety of evidence bearing on the nature of space weathering is reviewed the effects of space weathering include spectral darkening reddening and subdued absorption bands and the distinctive magnetic electron spin resonance caused by single domain metallic iron particles ever since the apollo missions two paradigms have dominated the thinking of the planetary science community concerning space weathering 1 the optical effects are caused by impact vitrified glass in agglutinates and 2 the submicroscopic metallic iron results from the reduction of ferrous iron by the impact melting of minerals whose surfaces have been saturated with hydrogen from the solar wind however studies carried out since the apollo program showed that both of these paradigms are invalid a hypothesis first suggested by the author and his colleagues 26 years ago but not generally accepted at that time now appears to be essentially correct both the optical and magnetic effects are caused by metallic iron particles smaller than the wavelength in ubiquitous vapor deposited coatings on soil particle surfaces and inside agglutinates the vapor is generated by both solar wind sputtering and micrometeorite impact vaporization and injected preferentially downward into the porous regolith the iron is reduced by a physical process the selective loss of oxygen that occurs during deposition of the vapor and does not require heating melting or a reducing environment a mathematical theory that describes the optical effects of the submicroscopic iron quantitatively is derived and applied to the regoliths of the moon mercury and an s asteroid

geochronology geochemistry and geological implications of late carboniferous early permian mafic and felsic intrusive rocks from urad zhongqi western inner mongolia the mafic and felsic haertaolegai intrusions crop out in urad zhongqi western inner mongolia and are dominated by monzogranite porphyritic granite and gabbroic diorite intrusions we investigate the tectonic setting geochronology and anorogenic characteristics of the western inner mongolia through field investigation microscopic and geochemical analyses of samples from the haertaolegai bimodal intrusions and laser ablation multi collector inductively coupled plasma mass spectrometry la mc icp ms zircon u pb dating of gabbroic diorite and adakitic granites petrographic and geochemical studies of the bimodal intrusions indicate that the gabbroic diorites formed from a primary magma generated by the partial melting of lithospheric mantle material that had previously been modified by subduction related fluids the felsic rocks are high k calc alkaline and metaluminous have characteristics of adakitic rocks and were generated during the partial melting of juvenile crustal material zircon u pb dating indicates that the felsic portion of this pluton was emplaced during late carboniferous early permian time with the mafic portion of the pluton emplaced during early permian time the zircons of adamellites have hf t values and t dm2 ages of 1 0 to 2 7 and 1032 1128 ma respectively suggesting that they formed from magmas generated by partial melting of juvenile mesoproterozoic lower crust these data combined with the geology of the region indicate that the late carboniferous early permian bimodal intrusive rocks in western inner mongolia record a transitional period from collisional compression to post collisional extension these results indicate that the paleo asian ocean in western inner mongolia closed before 300 ma

partial melting and melt migration in the acapulcoite lodranite parent body we review petrologic evidence that the acapulcoites and lodranites formed by 1 vol to probably 20 vol whole rock partial melting of a chondritic precursor material at low degrees of partial melting only fe ni fes cotectic melting occurred migration distances for partial melts were short resulting in the formation of acapulcoites with essentially chondritic troilite and plagioclase contents but achondritic textures at high degrees of partial melting both fe ni fes and basaltic plagioclase pyroxene partial melts formed and the melts may have migrated out of the source rock the partial melt residues which are more or less depleted in fe ni fes and plagioclase are the lodranites melt migration was complex most acapulcoites which experienced relatively low degrees of partial melting lost little if any of the partial melt one acapulcoite lew 86220 represents a unique case in which fe ni fes and basaltic partial melts appear to have migrated from a lodranite source region into a cooler acapulcoite region where they were trapped in cases of the relatively high degrees of partial melting experienced by lodranites melts may have been partly selectively or totally removed from the rocks and fe ni fes and or basaltic partial melts may have been removed to different degrees and may in fact have been trapped on occasion in greater than chondritic proportions we model vein and dike formation and melt migration by calculating the excess pressures and vein and dike sizes for varying degrees of partial melting our calculations are broadly consistent with observations indicating that melt migration is inefficient at low degrees of partial melting and extremely efficient at high degrees of partial melting although the size of the acapulcoite lodranite parent body and the volatile contents of the chondritic precursor rocks are poorly constrained the lack of basaltic rocks in the world s meteorite collections complementary to the lodranites suggests that basaltic partial melts may have been accelerated off the body by explosive volcanism of the type envisioned by wilson and keil 1991 and ejected into space the diversity of rocks from the acapulcoite lodranite parent body may provide a basis for better understanding the diverse range of spectral subtypes recognized among the s type asteroids

the fate of pyroclasts produced in explosive eruptions on the asteroid 4 vesta we evaluate the consequences of explosive activity having taken place during volcanic eruptions on the differentiated asteroid 4 vesta which is the likely parent body of the howardite eucrite diogenite hed meteorites for a wide range of magma volatile contents we calculate the eruption speeds and subsequent trajectories of the pyroclastic magma droplets produced by considering the size distribution and eruption speeds of the droplets and the mass fluxes at which they are ejected we show that under all realistic circumstances the droplets will have formed lava fountains that were extremely optically dense as a result virtually all of the droplets will have reached the surface having suffered a negligible amount of radiative cooling and will have coalesced into lava ponds feeding lava flows typically 1 of the pyroclasts will have undergone enough cooling to allow them to accumulate into a recognizable fall deposit consisting of unwelded or partly welded volcanic glass beads this result is consistent with the apparent absence of identifiable pyroclastic material in the hed and other types of differentiated meteorites

did the paleo asian ocean between north china block and mongolia block exist during the late paleozoic first paleomagnetic evidence from central eastern inner mongolia china 1 the tectonic evolution of the paleo asian ocean between the north china block ncb and the mongolia block mob is a contentious issue and geodynamic models remain speculative in an effort to puzzle out this controversy a paleomagnetic study was carried out on the silurian to permian formations in central eastern inner mongolia china more than 680 sedimentary and volcanic samples were collected from 86 sites we have established titanium poor magnetite and hematite as the principal magnetic carriers anisotropy of the magnetic susceptibility measurements demonstrate negligible deformation of the majority of study rocks with sedimentary fabrics from primary magnetizations a late devonian and a permian pole are calculated for inner mongolia block imb at 46 8 n 349 1 e dp 14 6 dm 27 3 with n 3 and 48 7 n 3 7 e dp 5 2 dm 9 1 with n 6 respectively two stages of secondary magnetization are also identified probably due to early permian and early cretaceous magmatic events as preliminary results the comparison of our new paleomagnetic poles with available data from ncb mob and siberia indicates that 1 the paleolatitudes of imb ncb and mob are consistent between late devonian and permian suggesting pre late devonian closure of the paleo asian ocean and further evaluation of these three blocks as a single entity and 2 post permian intracontinental deformation was significant and characterized by block rotations which are due to strike slip faulting within the welded ncb imb mob block

the role of rift zone storage in controlling the site and timing of eruptions and intrusions of kilauea volcano hawaii repeated eruptive and intrusive activity on the east rift zone of kilauea volcano allows dikes to exist within the rift zone for extended periods of time leakage of magma from the summit storage region into the rift zone can cause inflation of a rift zone dike inflation raises the excess pressure at the dike centre and increases the stress intensity at the dike edges if the inflation is sufficiently large the stress intensity may exceed the fracture toughness of the country rocks and allow vertical or lateral dike emplacement to occur theoretical modelling can be used to predict the conditions under which vertical or lateral dike propagation and hence eruptive or intrusive activity may occur over typical ranges of dike half height and regional tension gradient vertical propagation and hence eruption are favored by high fracture toughness relatively low dike half heights and low regional tension gradients lateral propagation can occur when the fracture toughness is low when dike half height is higher and when the regional tension gradient is high the results of from the model constrain the fracture toughness within the east rift zone to be 100 110 mpa m and the regional tension gradient to be 1000 pa m

volcanism and the dynamics of open magma chambers the repeated eruption of many volcanoes is now accounted for by events taking place in a subterranean magma chamber which is open in the sense that the inflow of material to the chamber from below leads to a magma pressure p which exceeds the lithostatic pressure pl by an amount greater than the tensile strength of the chamber walls before the contents of the magma chamber have reached thermodynamic and geochemical equilibrium this is the condition which must be satisfied when an eruption occurs we have now derived a relationship giving the critical volume of added material v required to cause an eruption as a function of magma compressibility and the volume by which the chamber expands

a magma ocean on vesta core formation and petrogenesis of eucrites and diogenites abstract available evidence strongly suggests that the hed howardite eucrite diogenite meteorites are samples of asteroid 4 vesta abundances of the moderately siderophile elements ni co mo w and p in the hed mantle indicate that the parent body may have been completely molten during its early history during cooling of a chondritic composition magma ocean equilibrium crystallization is fostered by the suspension of crystals in a convecting magma ocean until the crystal fraction reaches a critical value near 0 80 when the convective system freezes and melts segregate from crystals by gravitational forces the extruded liquids are similar in composition to main group and stannern trend eucrites and the last pyroxenes to precipitate out of this ocean before convective lockup span the compositional range of the diogenites subsequent fractional crystallization of a main group eucrite liquid which has been isolated as a body of magma produces the nuevo laredo trend and the cumulate eucrites the predicted cumulate mineral compositions are in close agreement with phase compositions analyzed in the cumulate eucrites thus eucrites and diogenites are shown to have formed as part of a simple and continuous crystallization sequence starting with a magma ocean environment on an asteroidal size parent body that is consistent with vesta

pyroclast loss or retention during explosive volcanism on asteroids influence of asteroid size and gas content of melt we review the conditions under which explosive volcanism took place on early forming differentiated asteroids the pressure dependent solubility of typical asteroid volatiles in melts implies that the gas driving explosive volcanism on asteroids less than approximately 100 km in diameter was probably present mainly as a free phase capable of accumulating into large gas bodies and thus causing slug flow in melts approaching the surface in contrast in asteroids larger than approximately 100 km the gas was probably present largely as a dispersion of small bubbles we show that these gas distributions have implications for the size distribution of the pyroclastic droplets produced in explosive eruptions at the surface all pyroclastic melt droplets are accelerated by the expanding gases but their speeds lag the gas speed by a finite amount that is a function of the droplet size and density and the asteroid size and hence acceleration due to gravity we compute pyroclast speeds and by comparing them with escape velocities we identify the critical pyroclast diameter on a given size asteroid that distinguishes droplets lost to space from droplets that return to the surface identification of asteroidal pyroclasts and measurements of their sizes could throw light on the amounts of gas driving eruptions

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

rb sr and sm nd internal isochron ages of a subophitic basalt clast and a matrix sample from the y75011 eucrite the yamato polymict eucrite y75011 contains several coarse grained mesostasis rich basaltic clasts with subophitic texture the preservation of extensive mg fe chemical zoning in pyroxenes and the presence of a dark mesostasis shows that these clasts are representative of pristine lava like basalts extruded on the surface of the howardite eucrite diogenite hed parent body rubidium strontium analyses of a large clast y75011 84b yielded a precise internal isochron age of 4 60 0 05 b y for 87rb 0 0139 b y 1 or 4 50 0 05 b y for 87rb 0 0142 b y 1 the rb sr systematics of a matrix sample are nearly identical to those of the pristine clast showing that the matrix is dominantly comminuted mesostasis rich basalt the rb sr internal isochron age of the matrix is 4 56 0 06 b y for 87rb 0 0139 b y 1 or 4 46 0 06 b y for 87rb 0 0142 b y 1 these ages are within error limits of the rb sr whole rock age of chondrites and are in agreement with 206pb 207pb and sm nd ages of ordinary monomict eucrites for 87rb 0 01402 b y 1 based on a comparison of the rb sr and u th pb ages of chondrites minster et al 1982 the good precision obtained for the rb sr ages of both clast and matrix is due to the presence of a mesostasis phase in which the rb sr ratio is enriched 3 4 times above that of any eucrite phase previously analysed the initial 87sr 86sr ratios of clast and matrix are 0 69894 2 and 0 69896 3 respectively and are in satisfactory agreement with the babi value papanastassiou and wasserburg 1969 there is evidence of isotopic disturbance in both the rb sr and sm nd systems of the most magnetic separate analysed a sm nd internal isochron of 4 55 0 14 b y for the combined data from both clast and matrix exclusive of the most magnetic separate is in agreement with the more precise rb sr age the corresponding initial 143nd 144nd 0 50587 19 normalized to 148nd 144nd 0 24308 the clast data alone exclusive of the most magnetic separate define an isochron age of 4 52 0 16 b y and initial 143nd 144nd 0 50593 19 the isochron error limits quoted here for the sm nd data are 2 from the york 1966 program and include the factor s n 2 used in that program to compensate for geological error however this factor is not appropriate for the rb sr isochrons and has been omitted from the quoted error limits the initial 143nd 144nd of the pristine clast at 4 56 b y ago is 0 505877 25 in agreement within error limits with the chur value jacobsen and wasserburg 1984 clast y75011 84 represents the oldest pristine unambiguously basaltic lava as yet recovered from a solar system object rare earth element abundance measurements shimizu and masuda 1985 show it to be one of the most differentiated eucrites thus moderately evolved basaltic lavas were produced very early in the history of the hed parent body their preservation as clasts in the polymict eucrites suggests derivation from a different geologic setting on the parent body than that occupied by the more thermally metamorphosed monomict eucrites we suggest that the preservation of pristine clasts in polymict eucrites may be due to their impact excavation to the surface of the parent body away from heat sources concentrated within impact craters the monomict eucrites may be derived from the floors of craters where they were thermally metamorphosed by shock heating and by the presence of overlying impact melts and nearly molten breccias

the petrology and chronology of nwa 8009 impact melt breccia implication for early thermal and impact histories of vesta abstract studies of petrology mineralogy and geochronology of eucrites are keys to reconstruct the thermal and impact history of 4 vesta the proposed parent body for hed meteorites here we report the petrography mineralogy and geochemistry of nwa 8009 a newly found eucritic impact melt breccia and present sims u pb ages of zircon and phosphates nwa 8009 consists of coarse and fine grained lithic and mineral clasts set in fine grained recrystallized matrix it was derived from a protolith of monomict non cumulate eucrite evidence for intense shock metamorphism observed in nwa 8009 includes mosaicism deformed exsolution lamellae and partial melting of pyroxene melting and incipient flow of plagioclase planar fractures and granular textures of zircon these shock effects indicate nwa 8009 was subjected to an impact metamorphism with peak pressure of 50 60 gpa and post shock temperature of 1160 1200 c nwa 8009 is among the most intensely shocked heds reported yet after the impact the sample was buried near the surface in target rocks and experienced rapid cooling 23 c h and annealing resulting in recrystallization of the matrix and devitrification of plagioclase and silica glasses u pb isotopic system of apatite within plagioclase groundmass of lithic clasts is completely reset and constrains the timing of impact at 4143 61 ma providing a new robust impact age on vesta combined with the presence of synchronous impact resetting events especially those recorded by lu hf sm nd and pb pb isotopic systems we identified a period of high impacts flux at ca 4 1 4 2 ga on vesta this impact flux occurred coincident with the uptick at ca 4 1 4 2 ga in impact age spectra of the moon probably reflects widespread intense bombardment throughout the inner solar system at ca 4 1 4 2 ga based on evidence from zircon chemical zoning petrographic occurrences as well as the distinctive zr hf ratios we suggested that zircons in nwa 8009 have had a metamorphic instead of magmatic origin they mainly crystallized from melts produced by partial melting of mesostasis area due to reheating event during early global thermal metamorphism rather than by zr release from zr rich minerals the u pb isotopic system in zircons was not disturbed by subsequent impacts the weighted mean 207 pb 206 pb age of 4560 8 ma represents the timing of zircon growth during thermal metamorphism zircons from nwa 8009 and other eucrites may share a common origin during metamorphic growth events and constraining the global thermal metamorphism on vesta at ca 4 55 ga the main heat sources responsible for global metamorphism in basaltic crust of vesta might be heating from the hot interior especially heat flow related to magmatism rather than impact

magnesium oxide iron oxide mass balance constraints and a more detailed model for the relationship between eucrites and diogenites according to a currently popular model for petrogenesis on the howardite eucrite and diogenite hed parent asteroid the diogenites are not comagmatic with most eucrites but instead formed in separate orthopyroxenite dominated plutons this model can be tested for consistency with mass balance for mgo and feo assuming the overall diogenite diogenite eucrite ratio d of the parent asteroid is at least comparable to the average d for the eucrite diogenite dominated howardite regolith breccias average mg mgo mgo feo is much lower for eucrites especially noncumulate eucrites than for diogenites unless the diogenite parent magmas eventually produced a large proportion of low mg residual basalt and gabbro rbg the implied initial magma s mg is vastly higher than that of any noncumulate eucrite starting from a source previously depleted by putative primary eucrite genesis melt mg can be estimated as a function of the exchange reaction kd and degree of melting using several very conservative assumptions e g assuming that the total mgo feo concentration is nearly the same in the nascent melt as in the residual solids the degree of melting required to yield a melt with mg high enough to satisfy mass balance without implying an rbg component that accounts for 50 of all eucrites is an implausibly high 60 80 wt the separate orthopyroxenitic plutons sop model also seems inconsistent with the uniform density of melts across the diogenite eucrite compositional spectrum 2 77 2 82 g cm3 which implies that diogenitic magmas should have been as capable as eucrites of extruding to form lavas this difficulty cannot be reduced by simply assuming that later formed magmas were systematically both more plutonic and more mgo rich than earlier ones because the plutonic cumulate eucrites equilibrated with melts systematically lower in mg than noncumulate eucrites conceivably the bulk mg of the asteroid s silicate system was increased between primary melt eucrite genesis and sop diogenite genesis by graphite fueled reduction of feo however the graphite oxidation process generates a huge proportion of gas which would have enhanced the buoyancy of the nascent diogenite parent magmas thus exacerbating the difficulty of achieving the implied high degrees of partial melting to avoid these difficulties but still form most eucrites as rapidly cooled extrusives i propose the nerd noncumulate eucrites as extruded residua of diogenites model in this model the diogenites form as early cumulates from a large magma system probably a global magma ocean that yields a large proportion of eucritic melt as residuum this residual melt zone undergoes relatively little crystallization during a period when it is episodically tapped to produce extrusions dikes and sills of rapidly cooled noncumulate eucrites slight 5 10 porosity in the nascent eucritic crust keeps it marginally buoyant over the residual melt zone the common thermal metamorphism of noncumulate eucrites results from baking by superjacent flows during the episodic venting of the melt zone the nerd model s greatest advantage is that it does not require implausibly high degrees of localized melting in the mature stages of igneous evolution of the hed asteroid

distinctive space weathering on vesta from regolith mixing processes whereas space weathering of some airless bodies such as the moon occurs through the accumulation on regolith of nanophase metallic particles spectroscopic data show that space weathering of the asteroid vesta occurs through the small scale mixing of diverse surface components which gradually generates locally homogenized upper regolith between 16 july 2011 and 5 september 2012 nasa s space probe dawn was orbiting vesta a protoplanet thought to have survived virtually intact since an early phase of solar system formation in this issue of nature two groups report on the encounter carle pieters and co workers find that space weathering on vesta has followed a different course from that observed on the moon and on itokawa the asteroid sampled in an earth return mission on vesta weathering involved fine scale regolith soil mixing that has removed clear traces of recent impact deposits there are no signs of the nanophase metallic particle deposits seen on the moon and itokawa thomas mccord and co authors describe two main types of material on vesta s surface bright and dark the bright material may be uncontaminated indigenous vesta basaltic soil with the darker material derived from low albedo impactors dawn has now moved on and is due to rendezvous with the protoplanet ceres in february 2015 the surface of the asteroid vesta has prominent near infrared absorption bands characteristic of a range of pyroxenes confirming a direct link to the basaltic howardite eucrite diogenite class of meteorites1 2 3 processes active in the space environment produce space weathering products that substantially weaken or mask such diagnostic absorption on airless bodies observed elsewhere4 5 and it has long been a mystery why vesta s absorption bands are so strong analyses of soil samples from both the moon6 and the asteroid itokawa7 determined that nanophase metallic particles commonly nanophase iron accumulate on the rims of regolith grains with time accounting for an observed optical degradation these nanophase particles believed to be related to solar wind and micrometeoroid bombardment processes leave unique spectroscopic signatures that can be measured remotely8 9 10 but require sufficient spatial resolution to discern the geologic context and history of the surface which has not been achieved for vesta until now here we report that vesta shows its own form of space weathering which is quite different from that of other airless bodies visited no evidence is detected on vesta for accumulation of lunar like nanophase iron on regolith particles even though distinct material exposed at several fresh craters becomes gradually masked and fades into the background as the craters age instead spectroscopic data reveal that on vesta a locally homogenized upper regolith is generated with time through small scale mixing of diverse surface components

in situ pb pb dating of silica rich northwest africa nwa 6594 basaltic eucrite and its constraint on thermal history of the vestan crust

physical processes in volcanic eruptions mechanisms are modeled in terms of dynamic physico chemical processes in this respect volcanology shares with much of earth science in general the benefits that accrue from the application of the methods and concepts of other disciplines to its problems basic concepts from the fields of thermo dynamics fluid mechanics and rheology have been particularly impor tant in fostering new insights recent developments have been so numerous as to make it necessary to limit the scope of this review and thus we concentrate here on the basic physical processes involved in the rise of magma to the surface and the distribution of cruption products on thc ground or in the atmosphere the perspective of planetary volcanism wilson hcad 1983 underlines the fact that volcanoes can be regarded as probes of the composition and structure of a planetary interior as a function of position depth and time

fissure eruptions in tharsis mars implications for eruption conditions and magma sources the tharsis region has been the focus of many studies of volcanism on mars not only are the largest volcanoes on the planet located here but also many of the youngest volcanic features are found within tharsis comparatively little attention has been given to understanding the origin of the smaller volcanoes within tharsis and these smaller structures may provide important information on both the regional tectonics of the area and the availability of magma from the shallow or deep crust we have identified six separate but physically close vent systems in eastern tharsis just to the east of the volcano jovis tholus these vents are typically linear fissures a few to 20 km in length that have built small shields rising to 50 85 m above the level of the surrounding topography the maximum length of individual lava flows from these fissures is 30 km but more typical lengths are 15 20 km mapping of the vent complexes reveals that smooth material interpreted to be spatter from fire fountaining is located on the rims of some of the fissures this spatter then formed either short 5 km flows or merged close to the fissure to form the longer flows there appears to have been some temporal evolution of the flow field as the oldest parts of the basement at each center are built from a series of compound flows that cannot be subdivided into individual flows we find evidence of frozen lava ponds and channelized flows from central vents photoclinometric profiles are constructed across one of the longer flows to confirm an unusual attribute originally identified by mouginis mark and christensen mouginis mark p j christensen p r 2005 new observations of volcanic features on mars from the themis instrument j geophys res 110 e8 doi 10 1029 2005je002421 of the flows from the central vent complex they are all 5 m thick which is a factor of 8 15 thinner than flows elsewhere on mars to investigate the unusual eruption conditions limited total volume of the construct and of individual flows we model the lava rheology the duration of emplacement the subsurface conditions that may have led to these eruptions and the volatile content of the magma we find lava viscosities and yield strengths to be 100 pa s and 100 pa respectively eruption rates to be 5000 m3 s 1 flow speeds to be 1 2 m s 1 durations of emplacement of individual flow units to be 5 h and the equivalent magma water content to be 0 1 0 2 mass these eruption conditions are consistent with a wide range of possible depths of the magma reservoirs feeding the eruptions small vents such as the ones studied here have also been identified in other parts of eastern tharsis so that the eruptions described here may characterize a common style of volcanism on mars that can only be identified now that image spatial resolution in the range 1 20 m pixel is available it is therefore possible that additional searches of other volcanic areas syrtis planum elysium planitia and hesperia planum may also show greater diversity of activity than is currently accepted