flood volcanism in the northern high latitudes of mercury revealed by messenger messenger observations from mercury orbit reveal that a large contiguous expanse of smooth plains covers much of mercury s high northern latitudes and occupies more than 6 of the planet s surface area these plains are smooth embay other landforms are distinct in color show several flow features and partially or completely bury impact craters the sizes of which indicate plains thicknesses of more than 1 kilometer and multiple phases of emplacement these characteristics as well as associated features interpreted to have formed by thermal erosion indicate emplacement in a flood basalt style consistent with x ray spectrometric data indicating surface compositions intermediate between those of basalts and komatiites the plains formed after the caloris impact basin confirming that volcanism was a globally extensive process in mercury s post heavy bombardment era

caloris impact basin exterior geomorphology stratigraphy morphometry radial sculpture and smooth plains deposits the mariner 10 flybys revealed the eastern third of the caloris basin and demonstrated its importance as a chronostratigraphic marker for the geological history of mercury uncertain after that mission were the size of the basin the full distribution of materials deposited or modified during the impact process and the stratigraphic and age relationships between caloris and surrounding terrain images obtained during messenger s january 2008 flyby of mercury revealed that the caloris basin is 15 larger than previously estimated and moderately elliptical 1525 by 1315 km basin related sculpture and secondary craters are dispersed widely in areas surrounding the basin confirming the widespread significance of this event as a stratigraphic marker units mapped around caloris on the basis of mariner 10 data are generally recognized in regions observed for the first time by messenger including most facies of the caloris group the caloris montes odin formation and van eyck formation the only unit without obvious exposures in messenger images is the nervo formation which was originally interpreted as impact melt or fallback ejecta new measurements of the size frequency distribution of impact craters both within the caloris basin and on surrounding units particularly the odin formation support the hypothesis that the vast majority of circum caloris plains postdate the basin and are likely volcanic the messenger observations re emphasize the importance of understanding the exterior stratigraphy of the basin in order to utilize the caloris event as a discrete time horizon in the geological history of mercury

investigating the origin of candidate lava channels on mercury with messenger data theory and observations received 12 april 2012 revised 31 august 2012 accepted 1 november 2012 published 31 march 2013 1 volcanic plains identified on mercury are morphologically similar to lunar mare plains but lack constructional and erosional features that are prevalent on other terrestrial planetary bodies we analyzed images acquired by the messenger spacecraft to identify features on mercury that may have formed by lava erosion we used analytical models to estimate eruption flux erosion rate and eruption duration to characterize the formation of candidate erosional features and we compared results with analyses of similar features observed on earth the moon and mars results suggest that lava erupting at high effusion rates similar to those required to form the teepee butte member of the columbia river flood basalts 0 1 1 2 10 6 m 3 s 1 would have been necessary to form wide valleys 15km wide observed in mercury s northern hemisphere first by mechanical erosion to remove an upper regolith layer then by thermal erosion once a lower rigid layer was encountered alternatively results suggest that lava erupting at lower effusion rates similar to those predicted to have formed rima prinz on the moon 4400 m 3 s 1 would have been required to form via thermal erosion narrower channels 7km wide observed on mercury although these results indicate how erosion might have occurred on mercury the observed features may have formed by other processes including lava flooding terrain sculpted during the formation of the caloris basin in the case of the wide valleys or impact melt carving channels into impact ejecta in the case of the narrower channels citation hurwitz d m j w head p k byrne z xiao s c solomon m t zuber d e smith and g a neumann 2013 investigating the origin of candidate lava channels on mercury with messenger data theory and observations j geophys res planets 118 471 486 doi 10 1029 2012je004103

volcanism on mercury evidence from the first messenger flyby for extrusive and explosive activity and the volcanic origin of plains the first messenger flyby of mercury obtained images of 21 of the surface not seen by mariner 10 including the center and western half of the caloris basin and regions near the terminator that show details of the nature of smooth and intercrater plains these new data have helped to address and resolve a series of longstanding questions on the existence and nature of volcanism on mercury and the distribution of volcanic materials data from the mercury dual imaging system mdis on the messenger spacecraft have shown the following 1 numerous volcanic vents in the form of irregularly shaped rimless depressions are concentrated around the interior edge of the caloris basin 2 these vents appear to be sources for effusive volcanism that in one case built a shield in excess of 100 km in diameter and in some cases formed bright haloes around the vents that are interpreted to represent pyroclastic eruptions 3 lobate margins of plains units seen previously in mariner 10 data are documented in messenger images with more clarity and are often distinctive in morphology and color properties supporting the interpretation that these features are the edges of lava flow units 4 the interior of the caloris basin is filled with plains units spectrally distinctive from the rim deposits and comparison with the lunar imbrium basin and superposed impact crater stratigraphy provide evidence that these units are volcanic in origin detailed differences in the mineralogy of lava flow units so prominent in imbrium are not seen in the caloris interior 5 some of the smooth plains surrounding the exterior of the caloris basin show distinct differences in color and morphological properties supporting a volcanic origin 6 some smooth and intercrater plains units distant from the caloris basin show evidence of flooding and embayment relations unrelated to caloris ejecta emplacement local and regional geological and color relationships support a volcanic origin for these plains 7 large impact craters show a sequence of embayment of interior floor and exterior ejecta deposits that supports a volcanic origin for the embayment and filling processes 8 crater embayment and flooding relationships in selected areas suggest volcanic plains thicknesses of many hundreds of meters and local thicknesses inside impact craters of up to several kilometers 9 impact crater size frequency distributions for caloris exterior deposits including the facies of the caloris group and relatively high and low albedo smooth plains show that they are younger than plains interior to caloris and thus must be dominantly the product of post caloris volcanism these new data provide evidence that supports and confirms earlier hypotheses from mariner 10 data that volcanism was important in shaping the surface of mercury the emerging picture of the volcanic style of mercury is similar to that of the moon the other small one plate planetary body there are no major shield volcanoes e g comparable to tharsis montes on mars shallow magma reservoirs are rare and there is little evidence for surface deformation or long lived volcanic sources related to sites of upwelling mantle the close association of volcanic plains and surface deformation features suggests that future observations and analyses can help document the relation between the volcanic flux and the evolving state and magnitude of stress in the lithosphere of mercury

mercury s global contraction much greater than earlier estimates observations of compressional structures on mercury have fallen short of accommodating the global contraction that is required owing to cooling of the planet s interior mapping of folds and faults across mercury s surface using messenger spacecraft images reveals deformation consistent with a planet that has contracted radially as much as seven kilometres over its history

the roza member columbia river basalt group a gigantic pahoehoe lava flow field formed by endogenous processes we present studies on the physical volcanology of the 15 ma roza member of the wanapum formation in the columbia river basalt group the roza member represents a compound pahoehoe flood basalt lava flow field with an area of 40 300 km 2 and a volume of 1300 km 3 it consists of 4 major lava flows each composed of numerous decimeter to kilometer long pahoehoe lobes roza lavas feature a wide range of pahoehoe surface structures such as lava rise plateaus tumuli and surface breakouts and we illustrate that the lava morphology is inconsistent with previous proposals of rapid emplacement for these lavas an integral component of the roza flow field is the sheet lobe with internal structures identical to those of inflated pahoehoe sheet lobes from hawaii and iceland both at the same scale and at much larger scales we identify a three part division of the sheet lobes into basal crust lava core and lava crust which are interpreted as the equivalent to the bottom crust the liquid lava core and the surface crust of an active inflating pahoehoe lobe the upper lava crust grows continuously during lava emplacement and its growth rate can be approximated by conductive cooling this relationship is used to calculate the emplacement time for individual roza sheet lobes and to derive a first order estimate on the duration of the roza eruption the results indicate that the emplacement of individual roza lobes lasted for months to years and that the lava flow field was constructed over a period of at least 14 years we propose that the roza flows achieved great areal dimensions and thicknesses by inflation and endogenous growth as the lava flowed from vent to flow front it traveled under an insulating crust which maintained cooling rates of 0 1 c km and allowed for efficient transport of lava over distances up to 300 km

hollows on mercury messenger evidence for geologically recent volatile related activity high resolution images of mercury s surface from orbit reveal that many bright deposits within impact craters exhibit fresh appearing irregular shallow rimless depressions the depressions or hollows range from tens of meters to a few kilometers across and many have high reflectance interiors and halos the host rocks which are associated with crater central peaks peak rings floors and walls are interpreted to have been excavated from depth by the crater forming process the most likely formation mechanisms for the hollows involve recent loss of volatiles through some combination of sublimation space weathering outgassing or pyroclastic volcanism these features support the inference that mercury s interior contains higher abundances of volatile materials than predicted by most scenarios for the formation of the solar system s innermost planet

the apparent absence of kilometer sized pyroclastic volcanoes on mercury are we looking right spacecraft data reveal that volcanism was active on mercury evidence of large volume effusive and smaller scale explosive eruptions has been detected however only large 15 km volcanic features or vents have been found so far despite abundant high resolution imagery on other volcanic planets the size of volcanoes is anticorrelated with their frequency small volcanoes are much more numerous than large ones here we present results of a numerical model that predicts the shapes of ballistically emplaced volcanic edifices and hence can explain the lack of kilometer sized constructional explosive volcanoes on the surface of mercury we find that due to the absence of the atmosphere particles are spread on this planet over a larger area than is typical for earth or mars erupted volumes are likely insufficient to build edifices with slope angles that enable their easy recognition with currently available data or that could survive destruction by subsequent impact bombardment

evolution of the interior of mercury influenced by coupled magmatism mantle convection system and heat flux from the core to discuss mantle evolution in mercury i present two dimensional numerical models of magmatism in a convecting mantle thermal compositional and magmatic buoyancy drives convection of temperature dependent viscosity fluid in a rectangular box placed on the top of the core that is modeled as a heat bath of uniform temperature magmatism occurs as a permeable flow of basaltic magma generated by decompression melting through a matrix widespread magmatism caused by high initial temperature of the mantle and the core makes the mantle compositionally stratified within the first several hundred million years of the 4 5 gyr calculated history the stratified structure persists for 4 5 gyr when the reference mantle viscosity at 1573 k is higher than around 1020 pa s the planet thermally contracts by an amount comparable to the one suggested for mercury over the past 4 gyr mantle upwelling however generates magma only for the first 0 1 0 3 gyr at lower mantle viscosity in contrast a positive feedback between magmatism and mantle upwelling operates to cause episodic magmatism that continues for the first 0 3 0 8 gyr convective current stirs the mantle and eventually dissolves its stratified structure to enhance heat flow from the core and temporarily resurrect magmatism depending on the core size these models however predict larger contraction of the planet coupling between magmatism and mantle convection plays key roles in mantle evolution and the difficulty in numerically reproducing the history of magmatism of mercury without causing too large radial contraction of the planet warrants further exploration of this coupling

scratching the surface estimating dimensions of large igneous provinces a study of five major basaltic provinces including oceanic plateaus volcanic passive margins and continental flood basalts shows that they are voluminous constructions of extrusive igneous rock underlain by intrusive rock crustal thickness ranges from 20 to 40 km and lower crust is characterized by high 7 0 7 6 km s seismic velocities volumes and emplacement rates derived for two oceanic plateaus the ontong java and kerguelen broken ridge reveal short lived pulses of increased global crustal production and suggest an origin involving the lower mantle the ontong java rate of emplacement may have exceeded the contemporaneous global production rate of the entire mid ocean ridge system despite the importance off large igneous provinces in studies of mantle dynamics and the global environment scarce age and deep crustal data necessitate intensified efforts in seismic imaging and scientific drilling in a range of such features

analytical numerical modeling of komatiite lava emplacement and thermal erosion at perseverance western australia abstract we have applied a thermal fluid dynamic geochemical model to investigate the emplacement and erosional potential of archean komatiite flows at perseverance western australia perseverance has been proposed as a site of large scale thermal erosion by large volume komatiite eruption s resulting in a 100 150 m deep lava channel containing one of the world s largest komatiite hosted fe ni cu pge sulfide deposits using constraints based on field theoretical and geochemical data we have modeled the emplacement of a range of flow thicknesses over felsic tuffaceous substrates with various degrees of consolidation and water contents thermo mechanical erosion becomes more effective for substrates that are increasingly unconsolidated and water rich for thermo mechanical erosion to be responsible for the formation of the 100 m deep concave perseverance embayment and the highly contaminated 10 20 perseverance komatiites the most likely scenarios require emplacement of thick e g 10 30 m turbulent channelized liquidus or superheated komatiite lavas over a welded or unconsolidated submarine tuff flow distances must have been long tens to hundreds of kilometers and flow volumes must have been very high hundreds to thousands of km 3 lava channels and tubes 110 km long have not been observed on earth but are consistent with those formed by low viscosity lavas on venus the moon mars and io flow volumes are consistent with those in continental flood basalt eruptions and oceanic plateau large igneous provinces and may represent the initial outpourings of komatiite lavas from archean mantle plume activity

biogeochemical processes controlling oxygen and carbon isotopes of diatom silica in late glacial to holocene lacustrine rhythmites biogeochemical cycles and sedimentary records in lakes are related to climate controls on hydrology and catchment processes changes in the isotopic composition of the diatom frustules 18odiatom and 13cdiatom in lacustrine sediments can be used to reconstruct palaeoclimatic and palaeoenvironmental changes the lago chungara andean altiplano 18 15 s 69 10 w 4520 masl diatomaceous laminated sediments are made up of white and green multiannual rhythmites white laminae were formed during short term diatom super blooms and are composed almost exclusively of large sized cyclostephanos andinus these diatoms bloom during mixing events when recycled nutrients from the bottom waters are brought to the surface and or when nutrients are introduced from the catchment during periods of strong runoff conversely the green laminae are thought to have been deposited over several years and are composed of a mixture of diatoms mainly smaller valves of c andinus and discostella stelligera and organic matter these green laminae reflect the lake s hydrological recovery from a status favouring the diatom super blooms white laminae towards baseline conditions 18odiatom and 13cdiatom from 11 990 to 11 530 cal years bp allow us to reconstruct shifts in the precipitation evaporation ratio and changes in the lake water dissolved carbon concentration respectively 18odiatom values indicate that white laminae formation occurred mainly during low lake level stages whereas green laminae formation generally occurred during high lake level stages the isotope and chronostratigraphical data together suggest that white laminae deposition is caused by extraordinary environmental events el nino southern oscillation and changes in solar activity are the most likely climate forcing mechanisms that could trigger such events favouring hydrological changes at interannual to decadal scale this study demonstrates the potential for laminated lake sediments to document extreme pluriannual events

maskelynite in asteroidal lunar and planetary basaltic meteorites an indicator of shock pressure during impact ejection from their parent bodies abstract maskelynite is a diaplectic glass that forms from plagioclase at shock pressures of 20 30 gpa depending on the ca concentration the proportion of maskelynite rich samples in a basaltic meteorite group correlates with the parent body escape velocity and serves as a shock indicator of launching conditions for eucrites basalts widely presumed to be from vesta vesc 0 36 km s 1 5 of the samples are maskelynite rich for the moon vesc 2 38 km s 1 30 of basaltic meteorites are maskelynite rich for mars vesc 5 03 km s 1 93 of basaltic meteorites are maskelynite rich in contrast literature data show that maskelynite is rare 1 among mare basalts and basaltic fragments in apollo 11 12 15 and 17 soils which were never ejected from the moon angrites are unbrecciated basaltic meteorites that are maskelynite free they were ejected at low to moderate shock pressures from an asteroid smaller than vesta because most impacts that eject materials from a large 100 km parent body are barely energetic enough to do that a collision that has little more than the threshold energy required to eject a sample from vesta will not be able to eject identical samples from the moon or mars there must have been relatively few impacts if any that launched eucrites off their parent body that also imparted shock pressures of 20 30 gpa in the ejected rocks more energetic impacts were required to launch basalts off the moon and mars on average vesta ejecta were subjected to lower shock pressures than lunar ejecta and lunar ejecta were subjected to lower shock pressures than martian ejecta h and ll ordinary chondrites have low percentages of shock stage s5 maskelynite bearing samples 1 and 4 respectively probably reflecting shock processes experienced by these rocks on their parent asteroids in contrast l chondrites have a relatively high proportion of samples containing maskelynite 11 most likely a result of catastrophic parent body disruption 470 ma ago

volcanism on mercury evidence from the first messenger flyby the origin of plains on mercury whether by volcanic flooding or impact ejecta ponding has been controversial since the mariner 10 flybys 1974 75 high resolution images down to 150 meters per pixel obtained during the first messenger flyby show evidence for volcanic vents around the caloris basin inner margin and demonstrate that plains were emplaced sequentially inside and adjacent to numerous large impact craters to thicknesses in excess of several kilometers radial graben and a floor fractured crater may indicate intrusive activity these observations coupled with additional evidence from color images and impact crater size frequency distributions support a volcanic origin for several regions of plains and substantiate the important role of volcanism in the geological history of mercury

the evolution of mercury s crust a global perspective from messenger mapping the distribution and extent of major terrain types on a planet s surface helps to constrain the origin and evolution of its crust together messenger and mariner 10 observations of mercury now provide a near global look at the planet revealing lateral and vertical heterogeneities in the color and thus composition of mercury s crust smooth plains cover approximately 40 of the surface and evidence for the volcanic origin of large expanses of plains suggests that a substantial portion of the crust originated volcanically a low reflectance relatively blue component affects at least 15 of the surface and is concentrated in crater and basin ejecta its spectral characteristics and likely origin at depth are consistent with its apparent excavation from a lower crust or upper mantle enriched in iron and titanium bearing oxides

time and the rivers flowing fluvial geomorphology since 1960 fluvial geomorphology has been the largest single subdiscipline within geomorphology for many decades fluvial geomorphic expertise is integral to understanding and managing rivers and to developing strategies for sustainable development this paper provides an overview of some of the significant advances in fluvial geomorphology between 1960 and 2010 with respect to conceptual models fluvial features and environments being studied tools used by fluvial geomorphologists geomorphic specialty groups within professional societies journals in which fluvial geomorphic research is published and textbooks of fluvial geomorphology during this half century fluvial geomorphology broadened considerably in scope from a focus primarily on physical principles underlying process and form in lower gradient channels with limited grain size range to a more integrative view of rivers as ecosystems with nonlinear behavior and great diversity of gradient substrate composition and grain size the array of tools for making basic observations analyzing data and disseminating research results also expanded considerably during this period as did the diversity of the fluvial geomorphic community

geology of the caloris basin mercury a view from messenger the caloris basin the youngest known large impact basin on mercury is revealed in messenger images to be modified by volcanism and deformation in a manner distinct from that of lunar impact basins the morphology and spatial distribution of basin materials themselves closely match lunar counterparts evidence for a volcanic origin of the basin s interior plains includes embayed craters on the basin floor and diffuse deposits surrounding rimless depressions interpreted to be of pyroclastic origin unlike lunar maria the volcanic plains in caloris are higher in albedo than surrounding basin materials and lack spectral evidence for ferrous iron bearing silicates tectonic landforms contractional wrinkle ridges and extensional troughs have distributions and age relations different from their counterparts in and around lunar basins indicating a different stress history

thermal effects of late accretion to the crust and mantle of mercury abstract impact bombardment on mercury in the solar system s late accretion phase ca 4 4 3 8 ga caused considerable mechanical chemical and thermal reworking of its silicate reservoirs crust and mantle depending on the frequency size and velocity of such impactors effects included regional and global scale crustal melting and thermal perturbations of the mercurian mantle we use a 3d transient heating model to test the effects of two bombardment scenarios on early pre tolstojan mercury s mantle and crust results show that rare impacts by the largest 100 km diameter bodies deliver sufficient heat to the shallow mercurian mantle producing high temperature ultra magnesian komatiitic s s melts impact heating leading to effusive flood volcanism can account for the eponymous high magnesium region hmr observed during the mercury surface space environment geochemistry ranging messenger mission we find that late accretion to mercury induced volumetrically significant crustal melting 58 vol mantle heating and melt production which combined with extensive resurfacing 100 also explains why its oldest cratering record was effectively erased consistent with crater counting statistics

emplacement and tectonic deformation of smooth plains in the caloris basin mercury abstract messenger s first flyby of mercury provided the first complete view of the caloris basin and its interior smooth plains multispectral imaging shows that the interior plains are spectrally distinct from surrounding terrain with a reflectance about 15 20 higher and a steeper spectral slope than the global average the caloris basin rim massifs and hummocky material in contrast have lower reflectance and a shallower spectral slope than the global average and extend around the entire basin isolated relatively red patches along the margins of caloris each surround irregularly shaped scalloped edged rimless depressions that are interpreted to be volcanic vents some with associated pyroclastic deposits a nearly continuous annulus of smooth plains exterior to the basin displays spectral characteristics that contrast sharply with those of the basin interior smooth plains the exterior smooth plains are lower in reflectance and spectrally similar to the caloris rim massifs and the hummocky materials the rims and floors of larger impact craters in the interior plains expose low reflectance material there is widespread evidence of contractional and extensional tectonic deformation of the caloris interior smooth plains a set of linear radial graben pantheon fossae radiates outward from a zone near the basin center and some of these graben extend to and intersect a set of generally basin concentric graben distributed along the outer margin of the caloris basin floor the average areal extensional strain expressed by the mapped system of graben is 0 08 wrinkle ridges also deform the interior smooth plains and predate both basin radial and basin concentric graben the average areal contractional strain expressed by the mapped wrinkle ridges is 0 07 the density of wrinkle ridges on the western interior smooth plains is less than on the eastern interior plains in contrast to the more even distribution of graben a similar difference is seen between the density of wrinkle ridges on the western and eastern exterior smooth plains with far fewer wrinkle ridges per unit area on the western annular plains superposition relations spectral contrast and the occurrence of volcanic vents and possible pyroclastic deposits all support a volcanic origin for the caloris interior plains crater counts indicate that the caloris exterior plains and hummocky material are younger than the caloris rim and interior plains and are therefore also likely volcanic in origin the contrast in spectral properties between the interior and exterior plains indicates a difference in the composition of the two expanses of volcanic material models for extensional tectonic deformation of the caloris basin interior inferred from the distribution of graben viewed by mariner 10 must be reevaluated to account for the discovery of pantheon fossae

identification of mercurian volcanism resolution effects and implications for messenger the possibility of volcanism on mercury has been a topic of discussion since mariner 10 returned images of half the planet s surface showing widespread plains material these plains could be volcanic or lobate crater ejecta an assessment of the mechanics of the ascent and eruption of magma shows that it is possible to have widespread volcanism no volcanism on the surface whatsoever or some range in between it is difficult to distinguish between a lava flow and lobate crater ejecta based on morphology and morphometry no definite volcanic features have been identified on mercury however known lunar volcanic features cannot be identified in images with similar resolutions and viewing geometries as the mariner 10 dataset examination of high resolution low sun angle mariner 10 images reveals several features which are interpreted to be flow fronts it is unclear if these are volcanic flows or ejecta flows this analysis implies that a clear assessment of volcanism on mercury must wait for better data messenger will take images with viewing geometries and resolutions appropriate for the identification of such features

the tectonics of mercury the view after messenger s first flyby article i nfo during its first flyby of mercury messenger imaged many tectonic landforms most of which are contractional in nature and include lobate scarps high relief ridges and wrinkle ridges wrinkle ridges are found on the smooth plains that partially fill the interior and surround the exterior of the caloris basin and also on smooth plains that fill the interiors of smaller impact basins and larger craters messenger revealed a radial graben complex pantheon fossae nearlyco centered with the caloris basin pantheon fossae and a network of mostly basin concentric graben intheouterportionsofthecalorisbasininteriorformapatternofextensionnotseenelsewhereonmercury the first clear example of extensional faults outside of the caloris basin has been documented on smooth plains inside the peak ring of the relatively young raditladi basin a map of the distribution of tectonic landforms imaged by messenger and mariner 10 shows that lobate scarps are the most widespread type of feature estimates of the horizontal shortening associated with lobate scarps that crosscut and overthrust small impact craters imaged by messenger range from 1 to 3 km previously unrecognized lobate scarps detected in areas imaged by mariner 10 indicate thatearlierestimatesof contractional strainarelow anewestimate of theaverage arealcontractionalstrain 0 06 accommodated byscarps is at leastonethirdgreater thancomparableprevious estimates andcorresponds to a decrease in mercury s radius of at least 0 8 km since the end of heavy impact bombardment of the inner solar system these figures are lower bounds because of the likelihood that not all lobate scarps have been identified even in areas imaged to date crosscutting and embayment relations revealed by messenger suggest that lobate scarps formed before the end of smooth plains emplacement and continued to be active after the emplacement of the youngest smooth plains deposits relatively recent activity on lobate scarps may be the expression of slow but continuous radial contraction that accompanied cooling of mercury s interior and the growth of the planet s solid inner core published by elsevier b v

volcanism on mercury a new model for the history of magma ascent and eruption 1 we use the identification of volcanism on mercury together with lobate flow front like topography crustal composition information and data on the stress state and history of the lithosphere to derive a new model for the ascent and eruption of magma on mercury we find that extrusion is likely to be dominated by high effusion rate events initial emplacement of dikes in the crust and extrusions to the surface will result in a denser crust favoring even more extrusive volcanism this will be followed by cooling associated with planetary thermal evolution that will rapidly decrease the ability of magma to reach the surface resulting in a decline and termination of volcanism following the emplacement and deformation of regional smooth plains these predictions and the resulting history can be tested with observations from missions to mercury

lunar mare volcanism stratigraphy eruption conditions and the evolution of secondary crusts abstract lunar mare basalt deposits cover 17 of the lunar surface occur preferentially in topographic lows on the nearside and have a total volume estimated at 1 107 km3 analysis of returned samples and photogeologic and remote sensing studies shows that mare volcanism began prior to the end of heavy bombardment the period of cryptomare formation in pre nectarian times and continued until the copernican period a total duration approaching 3 5 4 ga stratigraphic analyses show that the flux was not constant but peaked in early lunar history during the imbrian period which spans the period 3 85 3 2 ga average volcanic output rate during this peak period was about 10 2 km 3 a very low relative to the present global terrestrial volcanic output rate and comparable to the present local output rates for individual volcanoes such as vesuvius and kilauea hawaii volcanic landforms indicate that many eruptions were of high volume and long duration some eruptions associated with sinuous rules may have lasted of the order of a year and emplaced 103 km3 of lava representing the equivalent in one year of about 70 000 years at the average flux shallow magma chambers were uncommon the nearsidefarside mare deposit asymmetry can be readily explained by differences in crustal thickness magma ascending from the mantle or from a buoyancy trap at the base of the crust should preferentially extrude to the surface on the nearside but should generally stall and cool in dikes in the farside crust extruding only in the deepest basins the occurrence of farside maria within craters whose diameter is generally near to or less than the thickness of the crust may be accounted for by the difference between local and regional compensation dikes that establish pathways to the surface on the nearside should have very high volumes comparable to the volumes associated with many observed flows and sinuous rilles an abundance of dikes should exist in the lower crust of the moon many more than those feeding surface eruptions the upper limit is 37 50 of the crust by volume the presence and abundance of such dike swarms have important implications for the interpretation of the average composition of the lunar crust and the composition of basin and crater ejecta the interplay between thermal contraction and differentiation leads to net cooling and ultimate contraction of the outer portions of the moon resulting in a regional horizontal compressive stress acting on the lunar crust in addition overall cooling and deepening of sources require the production of ever larger volumes of magma in order to reach the surface with time stresses became high enough so that few dikes could open to the surface causing eruptive activity to be severely diminished in the eratosthenian and perhaps to cease in the copernican period lower stress levels are required to terminate eruptive activity on the lunar farside consistent with the imbrian age of the farside maria and the nearside location of the youngest maria lunar mare deposits provide an example of the transition from primary crusts to secondary crusts and illustrate the significance of several factors in the evolution of secondary crusts such as crustal density variations in crustal thickness presence of impact basins state and magnitude of stress in the lithosphere and general thermal evolution these factors are also responsible for the extremely low volcanic flux even during periods of peak extrusion

emplacement and erosive effects of lava in south kasei valles mars abstract although it has generally been accepted that the martian outflow channels were carved by floods of water observations of large channels on venus and mercury demonstrate that lava flows can cause substantial erosion recent observations of large lava flows within outflow channels on mars have revived discussion of the hypothesis that the martian channels are also produced by lava an excellent example is found in south kasei valles skv where the most recent major event was emplacement of a large lava flow calculations using high resolution digital terrain models dtms demonstrate that this flow was locally turbulent similar to a previously described flood lava flow in athabasca valles the modeled peak local flux of approximately 106 m3 s 1 was approximately an order of magnitude lower than that in athabasca which may be due to distance from the vent fluxes close to 107 m3 s 1 are estimated in some reaches but these values are probably records of local surges caused by a dam breach event within the flow the skv lava was locally erosive and likely caused significant kilometer scale headwall retreat at several cataracts with tens to hundreds of meters of relief however in other places the net effect of the flow was unambiguously aggradational and these are more representative of most of the flow the larger outflow channels have lengths of thousands of kilometers and incision of a kilometer or more therefore lava flows comparable to the skv flow did not carve the major martian outflow channels although the skv flow was among the largest and highest flux lava flows known in the solar system

explosive volcanism on mercury analysis of vent and deposit morphology and modes of eruption abstract the messenger mission revealed for the first time conclusive evidence of explosive volcanism on mercury several previous works have cataloged the appearance and location of explosive volcanism on the planet using a variety of identifying characteristics including vent presence and deposit color as seen in multispectral image mosaics we present here a comprehensive catalog of vents of likely volcanic origin our classification scheme emphasizes vent morphology we have analyzed the morphologies of all vents in our catalog and recognize three main morphologies simple vent pit vent and vent with mound the majority of vents we identify are located within impact craters the spatial distribution of vents does not correlate with the locations of volcanic smooth plains deposits in contrast to the moon nor do vents correlate with the locations of large impact basins except for the caloris and tolstoj basins using the degradation state of the vent host crater as a proxy for maximum age we suggest that vent formation has been active through the mansurian and into the kuiperian periods although the majority of vents were likely formed much earlier in mercurian history the morphologies and locations of vents are used to investigate a set of plausible formation geometries we find that the most likely and most prevalent formation geometry is that of a dike stalled at depth which then explosively vents to the surface we compare the vent and deposit size of mercurian pyroclastic deposits with localized and regional lunar pyroclastic deposits and find a range of possible eruption energies and corresponding variations in eruption style localized lunar pyroclastic deposits and the majority of mercurian pyroclastic deposits show evidence for eruption that is consistent with the magmatic foam at the top of a dike reaching a critical gas volume fraction a subset of mercurian vents including the prominent copland rachmaninoff vent to the northeast of the rachmaninoff basin indicates eruption at enhanced gas volume fractions this subset of vents shows a similar eruptive behavior to the lunar orientale dark mantle ring deposit suggesting that the dikes that formed these vents and deposits on mercury underwent some form of additional volatile build up either through crustal volatile incorporation or magma convection within the dike there also exists a population of mercurian vents that no longer retain a visible associated pyroclastic deposit we hypothesize that the visible signature of the pyroclastic deposit has been lost through space weathering and regolith mixing processes together these results provide a comprehensive analysis of explosive volcanism on mercury and inform continued research on the thermal history of mercury and magma composition and evolution

recalibrated mariner 10 color mosaics implications for mercurian volcanism recalibration of mariner 10 color image data allows the identification of distinct color units on the mercurian surface we analyze these data in terms of opaque mineral abundance iron content and soil maturity and find color units consistent with the presence of volcanic deposits on mercury s surface additionally materials associated with some impact craters have been excavated from a layer interpreted to be deficient in opaque minerals within the crust possibly analogous to the lunar anorthosite crust these observations suggest that mercury has undergone complex differentiation like the other terrestrial planets and the earth s moon

paleo heat flows radioactive heat generation and the cooling and deformation history of mercury estimates of lithospheric strength for mercury based on the depth of thrust faults associated with large lobate scarps which were most probably formed previously to similar to 3 ga or on the effective elastic thickness of the lithosphere supporting a broad rise in the northern smooth plains whose formation is poorly constrained but posterior to 3 8 ga serve as a basis for the calculation of paleo heat flows referred to the time when these structures were formed the so obtained paleo heat flows can give information on the urey ratio ur the ratio between the total radioactive heat production and the total surface heat loss by imposing the condition ur 1 corresponding to a cooling mercury consistent with the observed widespread contraction we obtain an upper limit of 0 4 times the average surface value for the abundance of heat producing elements in the outer solid shell of mercury we also find that if the formation of the northern rise occurred in a time posterior to 3 ga then in that time the urey ratio was lower and the cooling more intense than when most of large lobate scarps were formed thus because largest lobate scarps deform older terrains suggesting more intense contraction early in the mercurian history we conclude that the northern rise was formed previously to similar to 3 ga if the age of other smooth plains large wavelength deformations is similar then tectonic activity in mercury would have been limited in the last 3 billion of years

the global population of large craters on mercury and comparison with the moon 1 we have compiled a near global catalog of impact craters on mercury for diameters 20 km from images obtained during the flybys of the mariner 10 and messenger spacecraft the observed variations in crater density suggest that the smooth plains within and around the caloris basin are the most prominent contiguous comparatively young regions on mercury no other comparably large area appears as young in more heavily cratered terrain even the most densely cratered regions on mercury are deficient in craters between 20 km and 100 km in diameter compared with the moon a result that extends an observation made from mariner 10 images of a smaller fraction of mercury s surface this deficit is interpreted to reflect crustal resurfacing of mercury early in its history most likely by volcanic processes for craters larger than 100 km in diameter the density of craters on mercury in a given size range is similar to that of the moon because such larger craters are less easily removed by volcanic resurfacing we interpret this similarity to be the result of impact saturation effects on both bodies

emplacement and cooling of komatiite lavas we propose that komatiite lavas were emplaced as turbulent flows accompanied by vigorous forced convection with cooling rates often in excess of hundreds of c h 1 they melted and assimilated up to 10 of the ground over which they flowed forming deep channels nickel sulphide mineralization may have resulted from incorporation of sulphur rich sediment after emplacement high cooling rates persisted resulting in spinifex textures due to compositional convection

fractal dimension of geologically constrained crater populations of mercury data gathered during the mariner10 and messenger missions are collated in this paper to classify craters into four geo chronological units constrained to the geological map produced after messenger s flybys from the global catalogue we classify craters constraining them to the geological information derived from the map we produce a size frequency distribution sfd finding that all crater classes show fractal behaviour with the number of craters inversely proportional to their diameter the exponent of the sfd i e the fractal dimension of each class shows a variation among classes we discuss this observation as possibly being caused by endogenic and or exogenic phenomena finally we produce an interpretative scenario where assuming a constant flux of impactors the slope variation could be representative of rheological changes in the target materials

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

basin formation on mercury caloris and the origin of its low reflectance material abstract the 1500 km diameter caloris basin is the largest most well preserved impact structure on mercury the interior of caloris contains an enigmatic geological unit at depth subsequently excavated by smaller impacts low reflectance material lrm lrm is interpreted to be the original basin floor material and be composed of melted and re crystallized mantle material in this work a comprehensive numerical modeling study of caloris basin formation is undertaken to provide insight into basin formation on mercury which may differ to that on the moon and to address the origin of the lrm analysis of the modeled basin attributes demonstrates that the caloris impact would have penetrated to depths approaching the core mantle boundary excavating mantle material to the surface and producing melt volumes on the order of 10 7 km 3 such a melt volume could differentiate forming a layer buried beneath later volcanism that could be the lrm in agreement with previous spectroscopic analyses furthermore lrm deposits at other smaller basins on mercury may also represent impact melted mantle material a comparison to lunar basin counterparts demonstrates that basin formation on mercury is similar to that on the moon this observation suggests that the observed difference in the number of large basins between the two bodies fewer on mercury is due to processes that modify basins i e greater impact melt volume and more extensive volcanism on mercury rather than first order impact process differences the greater average impact velocity and gravity exert a slight effect on some basin properties including an increased excavation depth to transient crater diameter ratio on mercury

evidence for rapid topographic evolution and crater degradation on mercury from simple crater morphometry examining the topography of impact craters and their evolution with time is useful for assessing how fast planetary surfaces evolve here new measurements of depth diameter d d ratios for 204 craters of 2 5 to 5 km in diameter superposed on mercury s smooth plains are reported the median d d is 0 13 much lower than expected for newly formed simple craters 0 21 in comparison lunar craters that postdate the maria are much less modified and the median crater in the same size range has a d d ratio that is nearly indistinguishable from the fresh value this difference in crater degradation is remarkable given that mercury s smooth plains and the lunar maria likely have ages that are comparable if not identical applying a topographic diffusion model these results imply that crater degradation is faster by a factor of approximately two on mercury than on the moon suggesting more rapid landform evolution on mercury at all scales

a thin dense crust for mercury abstract crustal thickness is a crucial geophysical parameter in understanding the geology and geochemistry of terrestrial planets recent development of mathematical techniques suggests that previous studies based on assumptions of isostasy overestimated crustal thickness on some of the solid bodies of the solar system leading to a need to revisit those analyses here i apply these techniques to mercury using messenger derived elemental abundances i calculate a map of grain density average 2974 89 kg m3 which shows that pratt isostasy is unlikely to be a major compensation mechanism of mercury s topography assuming airy isostasy i find the best fit value for mercury s mean crustal thickness is 26 11 km 25 lower than the most recently reported and previously thinnest number several geological implications follow from this relatively low value for crustal thickness including showing that the largest impacts very likely excavated mantle material onto mercury s surface the new results also show that mercury and the moon have a similar proportion of their rocky silicates composing their crusts and thus mercury is not uniquely efficient at crustal production amongst terrestrial bodies higher resolution topography and gravity data especially for the southern hemisphere will be necessary to refine mercury s crustal parameters further

intercrater plains on mercury insights into unit definition characterization and origin from messenger datasets abstract orbital observations by the mercury surface space environment geochemistry and ranging messenger spacecraft are used to re evaluate the nature and origin of the oldest mapped plains deposits on mercury the intercrater and intermediate plains units defined by mariner 10 investigators despite the large areal extent of these plains which comprise approximately one third of the planetary surface area viewed by mariner 10 their formation mechanism was not well constrained by mariner 10 imaging one hypothesis attributed plains formation to ponding of fluidized impact ejecta to create relatively smooth surfaces another hypothesis was that these plains are of volcanic origin to assess the origin of these older plains and the contribution of early volcanism to resurfacing on mercury we have used messenger data to analyze the morphology spectral properties impact crater statistics and topography of mariner 10 type areas of intercrater and intermediate plains on the basis of new criteria for the identification of intercrater and intermediate plains derived from these observations we have remapped 18 of the surface of mercury we find that the intercrater plains are a highly textured unit with an abundance of secondary craters whereas the intermediate plains are composed of both intercrater and smooth plains we suggest that the term intermediate plains not be used to map the surface of mercury henceforth but rather this unit should be subdivided into its constituent intercrater and smooth plains units we argue that a substantial percentage of the intercrater plains are composed of volcanic materials on the basis of 1 examples of areas where ejecta from a small number of superposed craters have transformed smooth plains deposits of volcanic origin into a unit indistinguishable from intercrater plains 2 the range in ages of intercrater plains deposits as interpreted from crater size frequency distributions and 3 the near global distribution of intercrater plains compared with the uneven distribution of impact basins and their associated ejecta deposits

reflectance and color variations on mercury regolith processes and compositional heterogeneity multispectral images of mercury obtained by the messenger spacecraft reveal that its surface has an overall relatively low reflectance with three large scale units identified on the basis of reflectance and slope 0 4 to 1 0 micrometer a higher reflectance relatively red material occurs as a distinct class of smooth plains that were likely emplaced volcanically a lower reflectance material with a lesser spectral slope may represent a distinct crustal component enriched in opaque minerals possibly more common at depth a spectrally intermediate terrain probably forms most of the upper crust three other spectrally distinct but spatially restricted units include fresh crater ejecta less affected by space weathering than other surface materials high reflectance deposits seen in some crater floors and moderately high reflectance relatively reddish material associated with rimless depressions

fault bound valley associated with the rembrandt basin on mercury the rembrandt basin is crosscut by the largest fault scarp on mercury enterprise rupes and a second scarp complex belgica rupes extends to the basin s rim topographic data derived from messenger orbital stereo images show that these tectonic landforms bound a broad relatively flat floored valley with a mean width of 400 km crosscutting relations suggest that the accumulation of structural relief likely postdates the formation and volcanic infilling of the rembrandt basin the valley floor bound by fault scarps of opposite vergence is significantly offset below the elevation of the back scarp terrains along with an offset section of rembrandt s rim the elevation differences are evidence that the valley floor was lowered as a result of the formation of bounding fault scarps the localization of the widely spaced thrust faults of enterprise and belgica rupis and the offset of the valley floor may be the result of long wavelength buckling of mercury s lithosphere

gravity field and internal structure of mercury from messenger radio tracking of the messenger spacecraft has provided a model of mercury s gravity field in the northern hemisphere several large gravity anomalies including candidate mass concentrations mascons exceed 100 milli galileos mgal mercury s northern hemisphere crust is thicker at low latitudes and thinner in the polar region and shows evidence for thinning beneath some impact basins the low degree gravity field combined with planetary spin parameters yields the moment of inertia c mr2 0 353 0 017 where m and r are mercury s mass and radius and a ratio of the moment of inertia of mercury s solid outer shell to that of the planet of cm c 0 452 0 035 a model for mercury s radial density distribution consistent with these results includes a solid silicate crust and mantle overlying a solid iron sulfide layer and an iron rich liquid outer core and perhaps a solid inner core

tectonic patterns of shortening landforms in mercury s northern smooth plains abstract mercury s northern smooth plains are volcanically emplaced units characterized by ghost craters volcanically buried impact basins and thrust fault related landforms we analyzed the thrust fault related landforms traditionally categorized as lobate scarps and wrinkle ridges within the northern plains in order to describe trends in how these landforms are organized and oriented and what style of deformation either thin or thick skinned their map patterns represent our analysis also establishes geologic constraints for which global processes may have produced stresses contributing to these tectonic patterns we mapped 4853 thrust fault related landforms in the northern plains at a map scale of 1 1 000 000 using three mercury surface space environment geochemistry and ranging messenger global monochrome mosaics these landforms described as curvi linear asymmetric ridges frequently occur in complex geometrical arrangements that are interpreted to share similar structural characteristics we called these arrangements compound landforms like prior studies we observed thrust faults to follow rims of buried craters we also observed 1 sigmoidal rises bounded by fault related landforms 2 v shaped rises composed of two landforms terminating at a single sharp point 3 broad arcuate rises of nearly equal width 4 parallel evenly spaced ridges and 5 landforms showing alternation in direction of tectonic transport along strike respectively we interpreted these landforms as transpressional uplifts faults with sharply juxtaposing ramps pop up structures fold and thrust belts and antithetic fault intersections by comparison with earth analogues and patterns produced in numerical and physical models our results suggest that deformation in the nsps is thin skinned orientation analysis showed that the northernmost landforms 90 70 n were predominantly oriented east west while most of the landforms between 50 and 30 n were oriented north south variations in orientation with latitude indicate that the growth of thrust fault related landforms was influenced by sources of stress other than global contraction if reorientation of the pole due to the formation of the caloris basin impact did occur the pattern of fault orientations indicates that geologic processes producing the pattern operated after reorientation

widespread effusive volcanism on mercury likely ended by about 3 5 ga crater size frequency analyses have shown that the largest volcanic plains deposits on mercury were emplaced around 3 7 ga as determined with recent model production function chronologies for impact crater formation on that planet to test the hypothesis that all major smooth plains on mercury were emplaced by about that time we determined crater size frequency distributions for the nine next largest deposits which we interpret also as volcanic our crater density measurements are consistent with those of the largest areas of smooth plains on the planet model ages based on recent crater production rate estimates for mercury imply that the main phase of plains volcanism on mercury had ended by 3 5 ga with only small scale volcanism enduring beyond that time cessation of widespread effusive volcanism is attributable to interior cooling and contraction of the innermost planet

cenomanian palaeotemperatures derived from the oxygen isotopic composition of brachiopods and belemnites evaluation of cretaceous palaeotemperature proxies sixty low magnesium calcite samples from cenomanian articulate brachiopods belemnites and oysters from the epicontinental shelf sea of europe were geochemically and microscopically studied in order to evaluate their preservation and potential as carriers of palaeoenvironmental information the sampled localities in northern spain germany and southern england cover the transition between subtropical 25 30 n and temperate 33 38 n climates mean cenomanian salinity adjusted palaeotemperatures of diagenetically unaltered terebratulid and rhynchonellid brachiopods are 25 5 1 3 c in spain and 19 4 1 9 c in southern england the resulting low to mid latitude meridional temperature gradient of 0 7 c per degree latitude is similar to the gradient today and suggests modern heat transport values for cenomanian low latitudes oxygen isotopic composition of unaltered specimens of the upper cenomanian belemnite praeactinocamax plenus mean 0 2 is enriched in 18o by 1 in comparison to brachiopod shells correspondingly upper cenomanian palaeotemperatures derived from belemnite calcite 13 0 1 3 c underestimate modelled mid latitude sea surface temperatures by 6 c since p plenus occurs as boreal pulse fauna during a short interval in the late cenomanian its heavy oxygen isotopic composition can be attributed either to migration from a cooler and or deeper water mass or to unknown vital effects