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

explosive volcanic eruptions on mercury eruption conditions magma volatile content and implications for interior volatile abundances images obtained by the messenger spacecraft have revealed evidence for pyroclastic volcanism on mercury because of the importance of this inference for understanding the interior volatile inventory of mercury we focus on one of the best examples determined to date a shield volcano like feature just inside the southwestern rim of the caloris impact basin characterized by a near central irregularly shaped depression surrounded by a bright deposit interpreted to have a pyroclastic origin this candidate pyroclastic deposit has a mean radius of 24 km greater in size than the third largest lunar pyroclastic deposit when scaled to lunar gravity conditions from the extent of the candidate pyroclastic deposit we characterize the eruption parameters of the event that emplaced it including vent speed and candidate volatile content the minimum vent speed is 300 m s and the volatile content required to emplace the pyroclasts to this distance is hundreds to several thousands of parts per million ppm of the volatiles typically associated with pyroclastic eruptions on other bodies e g co co2 h2o so2 h2s for comparison measurements of the exsolution of volatiles h2o co2 s from basaltic eruptive episodes at kilauea volcano hawaii indicate values of 1300 6500 ppm for the terrestrial mantle source evidence for the presence of significant amounts of volatiles in partial melts derived from the interior of mercury is an unexpected result and provides a new constraint on models for the planet s formation and early evolution

pit floor craters on mercury evidence of near surface igneous activity abstract high resolution images obtained during the first mercury surface space environment geochemistry and ranging messenger flyby of mercury show evidence for igneous modification of the surface through the formation of pit craters on the floors of several impact craters pit craters are rimless steep sided depressions that are inferred to have formed by non impact processes among the arguments in favor of this interpretation are that the features lack evident rims exhibit no observable ejecta are irregularly shaped e g subcircular elliptical or arcuate with the long axis of the pit crater often concentric to the rim of the host impact crater and are superposed on smooth plains deposits within the craters pit craters documented to date do not exhibit signs of extrusive flows impact craters hosting pit craters termed pit floor craters vary in size from 55 to 120 km in diameter while the associated pit craters range in maximum horizontal dimension from 20 to almost 40 km pit crater radius and host crater radius are not well correlated for pit floor craters on mercury in contrast to the situation for otherwise similar features on mars on the basis of morphology structural association relative age and proximity to smooth plains for pit craters on mercury we test mechanisms of pit crater formation previously proposed for such features on other planetary bodies e g earth moon mars and venus we conclude that pit craters on mercury formed through collapse into an underlying drained magma chamber pit floor craters thus provide evidence for near surface magmatic activity on mercury and extend the range of evidence for magmatism beyond such surface expressions as smooth plains and pyroclastic deposits

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

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

upper mantle discontinuity structure in the region of the tonga subduction zone we study the mantle structure below the southwest pacific in order to examine the geometry of the tonga slab at depth and its interaction with the 410 and 660 km discontinuities hereafter called the 410 and the 660 we utilize data from stations of both the lau basin ocean bottom seismogram experiment and island stations of the southwest pacific seismic experiment the tectonic complexity of this region containing both the tonga subduction zone and the associated lau back arc spreading center make it an ideal area to investigate the upper mantle discontinuities using a high resolution technique such as common conversion point stacking of receiver functions we produce a high resolution image of the upper mantle near the tonga subduction zone to show the interaction between the discontinuities and the subducting slab our results show the 410 uplifted by 30 km near the tonga slab and the 660 depressed by 20 to 30 km as expected for thermally controlled olivine phase transitions

tertiary extension and tilting in the queen charlotte islands evidence from dyke swarms and their paleomagnetism the queen charlotte islands form the western margin of the tertiary queen charlotte basin which is situated on the western margin of the north american plate they contain seven major dyke swarms of late eocene to miocene age a period when the relative motions of the pacific and the north american plates in this region were dominantly dextral strike slip transform margin with intervals of highly oblique divergence and convergence within each swarm dykes have a systematic trend however trends vary from swarm to swarm indicating that the stress field varied a total of 678 cores 1352 specimens were collected from 129 dykes in six swarms over a distance of about 200 km magnetic stability is variable one hundred and one dykes yielded records of the paleofield data are also reported from an oligocene pluton 5 sites 27 cores 52 specimens and miocene lavas 8 sites 52 cores 101 specimens both normal and reversed magnetizations occur but irrespective of sign the mean directions of remanen

the new worldwide collapse caldera database ccdb a tool for studying and understanding caldera processes collapse calderas are one of the most important volcanic structures not only because of their hazard implications but also because of their high geothermal energy potential and their association with mineral deposits of economic interest the objective of this work is to describe a new general worldwide collapse caldera database ccdb in order to provide a useful and accessible tool for studying and understanding caldera collapse processes the principal aim of the ccdb is to update the current field based knowledge on calderas merging together the existing databases and complementing them with new examples found in the bibliography and leaving it open for the incorporation of new data from future studies this database does not include all the calderas of the world but it tries to be representative enough to promote further studies and analyses we have performed a comprehensive compilation of published field studies of collapse calderas including more than 200 references and their information has been summarized in a database linked to a geographical information system gis application thus it is possible to visualize the selected calderas on a world map and to filter them according to different features recorded in the database e g age structure the information recorded in the ccdb can be grouped in seven main information classes caldera features properties of the caldera forming deposits magmatic system geodynamic setting pre caldera volcanism caldera forming eruption sequence and post caldera activity additionally we have added two extra classes the first records the references consulted for each caldera the second allows users to introduce comments on the caldera sample such as possible controversies concerning the caldera origin a further purpose of this work is to construct the ccdb web page in this web page where registered users can acquire the current database version as well as to propose corrections or updates and to exchange information with other registered members also involved in the study of caldera collapse processes additionally the ccdb includes a formulary that will facilitate the incorporation of new calderas into the database

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

relative rates of optical maturation of regolith on mercury and the moon 1 quantifying relative differences in regolith optical maturation rates is critical to interpreting multispectral reflectance measurements of airless bodies in this study normalized reflectance measurements of crater ejecta blankets and rays are used as indicators of the relative state of regolith maturation on mercury and the moon as well as for a comparison of surface reflectance characterization of craters with high reflectance ejecta from lunar reconnaissance orbiter camera and mercury dual imaging system orbital mosaics indicates that the optical maturation rate is up to 4 times faster on mercury than on the moon observations also show that there are fewer immature craters per unit area on mercury than on the moon and suggest a younger average age for mercurian rayed craters than for lunar rayed craters a comparison of crustal reflectance of immature material yields a ratio of 1 9 0 4 for the average photometrically normalized reflectance of lunar material to that of mercurian material the difference in reflectance is attributed primarily to differences in composition between the lunar highlands and average surface material on mercury the new observations of immature craters per unit area and surface reflectance of immature materials are consistent with previous proposals that regolith maturation rates are faster on mercury than the moon and but here we quantify the relative rate from empirical observations for the first time

the distribution and origin of smooth plains on mercury 1 orbital images from the messenger spacecraft show that 27 of mercury s surface is covered by smooth plains the majority 65 of which are interpreted to be volcanic in origin most smooth plains share the spectral characteristics of mercury s northern smooth plains suggesting they also share their magnesian alkali basalt like composition a smaller fraction of smooth plains interpreted to be volcanic in nature have a lower reflectance and shallower spectral slope suggesting more ultramafic compositions an inference that implies high temperatures and high degrees of partial melting in magma source regions persisted through most of the duration of smooth plains formation the knobby and hummocky plains surrounding the caloris basin known as odin type plains occupy an additional 2 of mercury s surface the morphology of these plains and their color and stratigraphic relationships suggest that they formed as caloris ejecta although such an origin is in conflict with a straightforward interpretation of crater size frequency distributions if some fraction is volcanic this added area would substantially increase the abundance of relatively young effusive deposits inferred to have more mafic compositions smooth plains are widespread on mercury but they are more heavily concentrated in the north and in the hemisphere surrounding caloris no simple relationship between plains distribution and crustal thickness or radioactive element distribution is observed a likely volcanic origin for some older terrain on mercury suggests that the uneven distribution of smooth plains may indicate differences in the emplacement age of large scale volcanic deposits rather than differences in crustal formational process

ascent and eruption of a lunar high titanium magma as inferred from the petrology of the 74001 2 drill core abstract an analysis of the orange glasses and crystallized beads from the 68 cm deep 74001 2 core has been conducted to understand the processes occurring during ascent and eruption of the apollo 17 orange glass magma equilibrium between melt and metal blebs fe85ni14co1 within the core along with cr contents in olivine phenocrysts suggest there was an oxidation of c and a reduction of the melt at an o fugacity of iw 1 3 and 1320 c to form co gas at 200 bars or 4 km depth this was followed by development of more oxidized conditions during ascent also during ascent there was formation of euhedral homogeneous fo81 olivine crystals and spinel crystals with higher al and mg contents than the smaller spinels in the crystallized beads both the metal blebs and al rich spinels were trapped inside the fo81 olivine phenocrysts as they grew prior to eruption the composition of the orange glasses are homogeneous throughout the core except for a few distinct glasses at the top that appear to have been mixed in by micrometeorite reworking a few glassy melt inclusions of orange glass composition trapped in the fo81 phenocrysts contain 600 100 ppm s and 50 ppm cl compared to the 200 ppm s and 50 ppm cl in the orange glass melt when quenched these inclusions therefore document the addition of 400 ppm s to the co rich volcanic gas during the eruption the size and distribution of different volcanic beads in the apollo 17 deposit indicate a mode of eruption in which the orange glasses and partially crystallized beads formed further away from the volcanic vent where cooling rates were faster progressively larger and more numerous crystals in the black beads reflect slower cooling rates at higher optical densities in the volcanic plume the development of a brown texture in the orange glasses at the bottom of the core where the black beads dominate is interpreted to result from devitrification by subsolidus heating either as the orange glasses fell back through the hot plume or after deposition on the surface the change from domination by orange glasses to black beads in the core probably reflects a decrease in gas content over time which consequently would increase the plume optical density and favor slower cooling rates

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

mercury s hollows constraints on formation and composition from analysis of geological setting and spectral reflectance 1 landforms unique to mercury hollows are shallow flat floored irregular depressions notable for their relatively high reflectance and characteristic color here we document the range of geological settings in which hollows occur most are associated with impact structures simple bowl shaped craters to multiring basins and ranging from kuiperian to calorian in age hollows are found in the low reflectance material global color unit and in low reflectance blue plains but they appear to be absent from high reflectance red plains hollows may occur preferentially on equator or hot pole facing slopes implying that their formation is linked to solar heating evidence suggests that hollows form because of loss of volatile material we describe hypotheses for the origin of the volatiles and for how such loss proceeds intense space weathering and solar heating are likely contributors to the loss of volatiles contact heating by melts could promote the formation of hollows in some locations lunar ina type depressions differ from hollows on mercury in a number of characteristics so it is unclear if they represent a good analog we also use messenger multispectral images to characterize a variety of surfaces on mercury including hollows within a framework defined by laboratory spectra for analog minerals and lunar samples data from messenger s x ray spectrometer indicate that the planet s surface contains up to 4 sulfur we conclude that nanophase or microphase sulfide minerals could contribute to the low reflectance of the low reflectance material relative to average surface material hollows may owe their relatively high reflectance to destruction of the darkening agent sulfides the presence of alteration minerals and or physical differences in particle size texture or scattering behavior

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

hollows on mercury materials and mechanisms involved in their formation recent images of the surface of mercury have revealed an unusual and intriguing landform sub kilometre scale shallow flat floored steep sided rimless depressions typically surrounded by bright deposits and generally occurring in impact craters these hollows appear to form by the loss of a moderately volatile substance from the planet s surface and their fresh morphology and lack of superposed craters suggest that this process has continued until relatively recently and may be on going hypotheses to explain the volatile loss have included sublimation and space weathering and it has been suggested that hollow forming volatiles are endogenic and are exposed at the surface during impact cratering however detailed verification of these hypotheses has hitherto been lacking in this study we have conducted a comprehensive survey of all messenger images obtained up to the end of its fourth solar day in orbit in order to identify hollowed areas we have studied how their location relates to both exogenic processes insolation impact cratering and solar wind and endogenic processes explosive volcanism and flood lavas on local and regional scales we find that there is a weak correlation between hollow formation and insolation intensity suggesting formation may occur by an insolation related process such as sublimation the vast majority of hollow formation is in localised or regional low reflectance material within impact craters suggesting that this low reflectance material is a volatile bearing unit present below the surface that becomes exposed as a result of impacts in many cases hollow occurrence is consistent with formation in volatile bearing material exhumed and exposed during crater formation while in other cases volatiles may have accessed the surface later through re exposure and possibly in association with explosive volcanism hollows occur at the surface of thick flood lavas only where a lower reflectance substrate has been exhumed from beneath them indicating that this form of flood volcanism on mercury lacks significant concentrations of hollow forming volatiles

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

lunar regional dark mantle deposits geologic multispectral and modeling studies clementine five channel uv visible uvvis data have been used to study seven regional dark mantle deposits dmds on the moon the dmds were mapped in distribution to determine their extent and stratigraphic relationship to other geologic units based upon the spectral properties for each dmd the crystallization of the beads in each deposit was inferred and used to estimate cooling rates in the volcanic plumes that emplaced the deposits deposits with a high concentration of glasses reflect volcanic plumes that had low optical densities and high cooling rates whereas deposits dominated by crystallized beads indicate plumes with slower cooling rates due to higher optical densities spectral data from each of the regional dmds show that their glass crystallized bead ratio can be estimated based upon their 415 750 and 750 950 nm values and comparison to laboratory spectra for the beads patches of young dark mantle in the sinus aestuum dmd represent one extreme with the bluest color highest 415 750 and weakest glass band absorption lowest 750 950 of all the dmds at the other extreme is the aristarchus plateau dmd with the reddest color and strongest glass band absorption the other nearside dmds including taurus littrow sulpicius gallus rima bode and mare vaporum lie between these two extremes due to intermediate mixtures of the crystallized beads and glasses with other local soils the orientale ring dmd on the western limb is dominated by volcanic glasses and is spectrally similar to the localized dmds found in alphonsus crater we have identified a central vent for the orientale ring dmd and model the eruption as degassing of a near surface dike to produce a 20 km high umbrella shaped plume with ejection velocities of 360 m s and deposition of the glasses at an average radius of 80 km from the vent although we cannot identify the exact sources for the other regional dmds probably because they are buried beneath younger mare the eruptions most likely resulted from dikes breaching the surface and producing a volcanic plume dominated by larger greater than submillimeter hot clasts that formed mare and sinuous rules the small percent of clasts that form the volcanic beads are carried by the expanding gas cloud to large distances in some cases 100 km to produce the observed continuous regional dmds the lack of basalt samples that can be petrologically related to the volcanic glasses may be a result of their spatial separation with the basalts flowing into the basins while the beads are deposited both into the basins and on the adjacent highlands

moon near infrared spectral reflectance a first good look a positive identification of the minerals olivine plagioclase and several types of pyroxenes were made at several locations on the lunar surface by using remote measurements for example the crater aristarchus is found to have an average pyroxene composition of augite and plagioclase is obviously present a dark mantle deposit in the crater j herschel is at least partly composed of a mixture of 70o7o olivine and 30o7o pyroxene these determinations were possible because the reflectance spectra for 10 20 km diameter lunar areas have been measured for the first time in the ir spectral region 0 65 2 5 m with sufficient spectral resolution and photometric precision to define mineral electronic absorption bands the reflectance for all lunar regions observed over 100 to present continues to increase toward longer wavelengths to at least 2 5 m and several mineral absorption bands appear the telescopic spectra are of similar quality and contain features similar to laboratory spectra of lunar samples the absorption features in several spectra have been quantitatively analyzed using newly developed computer processing techniques including thermal flux removal and absorption band fitting to produce these mineral identifications detection and quantitative analysis of these absorptions provide a much improved basis for remotely determining and mapping surface mineralogy quantitatively from the ground or using airborne or spacecraft platforms

evolution of the rembrandt impact basin on mercury messenger s second mercury flyby revealed a 715 kilometer diameter impact basin the second largest well preserved basin scale impact structure known on the planet the rembrandt basin is comparable in age to the caloris basin is partially flooded by volcanic plains and displays a unique wheel and spoke like pattern of basin radial and basin concentric wrinkle ridges and graben stratigraphic relations indicate a multistaged infilling and deformational history involving successive or overlapping phases of contractional and extensional deformation the youngest deformation of the basin involved the formation of a 1000 kilometer long lobate scarp a product of the global cooling and contraction of mercury

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

return to mercury a global perspective on messenger s first mercury flyby in january 2008 the mercury surface space environment geochemistry and ranging messenger spacecraft became the first probe to fly past the planet mercury in 33 years the encounter revealed that mercury is a dynamic system its liquid iron rich outer core is coupled through a dominantly dipolar magnetic field to the surface exosphere and magnetosphere all of which interact with the solar wind messenger images confirm that lobate scarps are the dominant tectonic landform and record global contraction associated with cooling of the planet the history of contraction can be related to the history of volcanism and cratering and the total contractional strain is at least one third greater than inferred from mariner 10 images on the basis of measurements of thermal neutrons made during the flyby the average abundance of iron in mercury s surface material is less than 6 by weight

topography of lobate scarps on mercury new constraints on the planet s contraction lobate scarps are landforms on mercury that appear to have formed by thrust faulting and are thought to reflect global contraction due to cooling of the planet s interior topographic data for 10 mercurian lobate scarps derived from mariner 10 images using photoclinometry and digital stereoanalysis indicate a range in maximum relief of 0 1 to 1 5 km and a range in horizontal shortening of 0 3 to 3 2 km assuming fault plane dips of 25 previous estimates of cumulative compressional strain recorded in the lobate scarps suggest a decrease in mercury s radius of 1 to 2 km our estimate of compressional strain based on these new topographic measurements is 0 056 25 this suggests that mercury s radius decreased by 1 km

relation of the spectroscopic reflectance of olivine to mineral chemistry and some remote sensing implications using high resolution visible and near infrared diffuse spectral reflectance we have systematically investigated apparent wavelength shifts as a function of mineral chemistry in the fe mg olivine series from fo11 to fo91 the study also shows that trace amounts of nickel can be spectrally detected in the olivine structure we show that significant compositional information can only be extracted at relatively high resolution because the overall spectral characteristics of the olivines change only subtly as a function of the fe mg ratio significant spectral variation as a function of grain size is also demonstrated adding a further complication to the interpretation of remotely sensed data from olivine rich surfaces this laboratory study is expected to aid in the interpretation of remotely sensed data from both terrestrial and extraterrestrial bodies terrestrial applications may include the recognition of ultramafic ultrabasic and basaltic terrains which in themselves may have mineral potential among extraterrestrial applications the asteroids are obvious candidates for further examination if instrumentation can provide the necessary wavelength coverage resolution and signal to noise ratio so that spectra can be compared to those laboratory data discussed here some permutations of fe mg ni relations in olivines are discussed as they apply to the interpretation of asteroid surfaces and other extraterrestrial bodies

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

the partial volatilization of mercury during recent years my research on the primitive solar nebular has followed two main themes 1 very early in the development of the nebula conditions probably favored the occurence of major gaseous instabilities leading to the formation of giant gaseous protoplanets but the rapid rise of the external temperature soon evaporated the envelopes of these protoplanets possibly leaving behind precipitated solids which formed the cores and mantles of the terrestrial planets 2 models of the nebula indicate a later stage when conditions in the inner solar system became very hot at the position of mercury the temperature was probably in the range 2500 3500 k this leads to the hypothesis that the original protomercury was a body substantially more massive than the present planet and of normal composition but that when it was immersed in the high temperature field of the dissipating solar nebula most of the rocky mantle was vaporized and mixed into the solar nebula gases and carried away by them this hypothesis is investigated in the present paper for simplicity the vaporization of a mantle composed of enstatite mgsio3 was computed for a planet with 2 25 the mass of mercury at a temperature of 3000 k it is argued that the mantle could probably be largely removed in the available time of 3 104 years subsequent accretion would restore some magnesium silicates to the mantle of the planet

optical effects of space weathering the role of the finest fraction the optical properties of lunar softs are different than those of rocks from which they are derived as a consequence of lunar space weathering soils are darker and exhibit a distinctive red sloped continuum and weaker mineral absorption bands the accumulation of dark glass welded aggregates agglutinates has been thought to account for these optical effects of space weathering on lunar soils spectroscopic analyses of agglutinate separates and size fractions for a suite of lunar softs presented here indicate that the agglutinate paradigm is insufficient to fully account for lunar optical alteration it is the finest fraction of lunar soils 25 gm which constitute 25 wt that dominates the optical properties of the bulk soil unlike size fractions of most silicates for which the finest fraction is the brightest the lunar soil size fractions all have comparable albedos in the short wavelength visible in the near infrared however it is the finest fraction that exhibits the steep red continuum and weak absorption bands the properties of the finest fraction cannot be duplicated by preparing a fine fraction by grinding larger agglutinate rich soil particles these results suggest space weathering on airless bodies is dominated by surface correlated processes perhaps associated with the development of fine grained fe o on or near the surface of grains and the accumulation of the larger agglutinates is not necessarily required to account for lunar optical alteration

an analysis of the mariner 10 color ratio map of mercury abstract the results of a geological analysis of the mariner 10 orange uv color ratio man of mercury b hapke c christman b rava and j mosher proc lunar planet sci conf 11th 1980 pp 817 822 are given certain errors that occured in reproducing the published version of the 1980 map are pointed out the relationships between color and terrain are distinctly nonlunar there is no correlation between color boundaries and the smooth plains on mercury in contrast to the strong correlation between color and maria highlands contacts on the moon there are no large exposures of low albedo blue material that could be considered to be mercurian analogs of high fe ti lunar maria basalts on any part of mercury imaged by mariner 10 three lines of evidence imply that the crust is low in fe2 and ti4 rays and ejecta blankets are bluer than most areas on mercury the fe2 band in mercury s reflectance spectrum is very weak or nonexistent and the albedo contrasts are smaller than those on the moon there is no evidence in the spectral or albedo data that a lunar type of second wave of melting ever occured on mercury rather the observations are most consistent with the hypothesis that the smooth plains are extrusive landforms derived from local material possibly mobilized by the caloris event in several places correlations between color and topography can be explained if older redder higher fe materials underlie younger bluer lower fe surfaces there is some evidence of late fe rich pyroclastic like activity

topography of the northern hemisphere of mercury from messenger laser altimetry laser altimetry by the messenger spacecraft has yielded a topographic model of the northern hemisphere of mercury the dynamic range of elevations is considerably smaller than those of mars or the moon the most prominent feature is an extensive lowland at high northern latitudes that hosts the volcanic northern plains within this lowland is a broad topographic rise that experienced uplift after plains emplacement the interior of the 1500 km diameter caloris impact basin has been modified so that part of the basin floor now stands higher than the rim the elevated portion of the floor of caloris appears to be part of a quasi linear rise that extends for approximately half the planetary circumference at mid latitudes collectively these features imply that long wavelength changes to mercury s topography occurred after the earliest phases of the planet s geological history

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

global inventory and characterization of pyroclastic deposits on mercury new insights into pyroclastic activity from messenger orbital data we present new observations of pyroclastic deposits on the surface of mercury from data acquired during the orbital phase of the mercury surface space environment geochemistry and ranging messenger mission the global analysis of pyroclastic deposits brings the total number of such identified features from 40 to 51 some 90 of pyroclastic deposits are found within impact craters the locations of most pyroclastic deposits appear to be unrelated to regional smooth plains deposits except some deposits cluster around the margins of smooth plains similar to the relation between many lunar pyroclastic deposits and lunar maria a survey of the degradation state of the impact craters that host pyroclastic deposits suggests that pyroclastic activity occurred on mercury over a prolonged interval measurements of surface reflectance by messenger indicate that the pyroclastic deposits are spectrally distinct from their surrounding terrain with higher reflectance values redder i e steeper spectral slopes and a downturn at wavelengths shorter than 400 nm i e in the near ultraviolet region of the spectrum three possible causes for these distinctive characteristics include differences in transition metal content physical properties e g grain size or degree of space weathering from average surface material on mercury the strength of the near ultraviolet downturn varies among spectra of pyroclastic deposits and is correlated with reflectance at visible wavelengths we suggest that this interdeposit variability in reflectance spectra is the result of either variable amounts of mixing of the pyroclastic deposits with underlying material or inherent differences in chemical and physical properties among pyroclastic deposits

remote sensing of lunar pyroclastic mantling deposits abstract mantling deposits on the moon are considered to be pyroclastic units emplaced on the lunar surface as a result of explosive fire fountaining these pyroclastic units are characterized as having low albedos having smooth fine textured surfaces and consisting in part of homogeneous febearing volcanic glass and partially crystallized spheres mantling units exhibit low returns on depolarized 3 8 cm radar maps indicating an absence of surface scatterers in the 1 to 50 cm size range a number of reflectance spectra from several regional pyroclastic deposits are presented for the first time these data support a previous interpretation that mantling units have a unique spectral signature which is indicative of the presence of a significant fe bearing volcanic glass component the rima bode region is discussed as an example of an area in which several types of remote sensing data including 3 8 cm radar spectral reflectance and multispectral vidicon data were used to reconstruct the geologic events surrounding the emplacement of a regional pyroclastic mantling deposit the recognition of numerous varieties of volcanic glass samples especially relatively high albedo e g green yellow glasses suggests the existence of additional unrecognized mantling deposits with albedos higher than those studied to date on the basis of the remote sensing data summarized and presented five new areas have been identified which may represent higher albedo regional pyroclastic deposits

impact induced thermal effects in the lunar and mercurian regoliths while the moon and mercury are similar objects in a geomorphologic sense they also possess important differences particularly in the context of small scale impact phenomena not only is the surface of mercury notably hotter than that of the moon but the impact flux is also more intense at mercury due to higher impact velocities and a greater spatial density of micrometeoroids by extrapolating the terrestrial micrometeoroid flux to the moon and mercury it is found that the impact rate at mercury is 5 5 times greater and the mean impact velocity more than 60 percent higher than at the moon a model of impact melting and vaporization applied to the lunar and mercurian environments indicates that almost 14 times more impact melt and over 20 times more vapor are generated per unit time in the mercurian regolith although the surface temperature plays a role in determining the melt and vapor volumes the difference in impact velocity appears to be markedly more important average craters formed by identical projectiles under the two velocity distributions on the two planets should be very similar in size because differences in the kinetic energy of the average impactors will be offset by the differences in gravitational acceleration as a result variations in regolith mixing should depend more on the actual impact rates than on disparities in excavated volumes each average impact on mercury however will produce more than twice as much impact melt as its lunar counterpart introducing more glass into the mercurian regolith although substantial quantities of impact vapor are produced on both planets mixing of the regolith should occur quickly enough in each case to spread the deposited vapor over large surface areas the resulting vapor coatings should be negligibly small for most purposes a much larger agglutinate population should exist on mercury and individual agglutinates should be similar to those of the lunar highlands reflectance spectra and visual albedos indicate that the mercurian crust possesses relatively small quantities of fe perhaps similar to those of apollo 16 light and dark matrix breccias laboratory studies have demonstrated that as the magnetic fractions which are correlated with agglutinate abundances of soils derived from these breccias increase the differences in 0 565 mm visible light reflectivity between the soils diminish should the regional variations in fe content of the mercurian crust be similar to those of these apollo 16 samples then the intense agglutination environment could account for the lack of optical contrast across the mercurian surface at the same time the very large fraction of impact glass in the regolith would imply that most pyroxenes have been fused and incorporated into these glasses seriously attenuating the fe2 crystal field absorption near 0 9 mm supporting this picture are the most recently published spectral observations of mercury which provide little or no evidence of such an absorption feature