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

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 radar imaging evidence for polar ice the first unambiguous full disk radar mapping of mercury at 3 5 centimeter wavelength with the goldstone 70 meter antenna transmitting and 26 antennas of the very large array receiving has provided evidence for the presence of polar ice the radar experiments conducted on 8 and 23 august 1991 were designed to image the half of mercury not photographed by mariner 10 the orbital geometry allowed viewing beyond the north pole of mercury a highly reflective region was clearly visible on the north pole during both experiments this polar region has areas in which the circular polarization ratio pt was 1 0 to 1 4 values 0 1 are typical for terrestrial planets such high values of have hitherto been observed in radar observations only from icy regions of mars and icy outer planet satellites

remote sensing and geologic studies of the schiller schickard region of the moon near infrared reflectance spectra multispectral images and photogeologic data for the schiller schickard ss region were obtained and analyzed in order to determine the composition and origin of a variety of geologic units these include light plains deposits orientale related deposits mare units and dark haloed impact craters dhcs spectral data indicate that the pre orientale highland surface was dominated by noritic anorthosite near ir spectra show that dhcs in the region have excavated ancient 3 8 ga mare basalts from beneath highland bearing material emplaced by the orientale impact ancient mare basalts were widespread in the ss region prior to the orientale event and their distribution appears to have been controlled by the presence of several old impact basins including the schiller zucchius basin and a basin previously unrecognized both near ir spectra and multispectral images indicate that light plains and other orientale related units in the ss region contain major amounts of local pre orientale mare basalt the amounts of local material in these deposits approach but seldom exceed the maximum values predicted by the local mixing hypothesis of oberbeck and co workers

mercury s albedo from mariner 10 implications for the presence of ferrous iron mariner 10 clear filter 490 nm images of mercury were recalibrated and photometrically normalized to produce a mosaic of nearly an entire hemisphere of the planet albedo contrasts are slightly larger than seen in the lunar highlands excluding maria variegations indicative of compositional differences include diffuse low albedo units often overlain by smooth plains the high albedo smooth plains of borealis planitia and high albedo enigmatic crater floor deposits a higher level of contrast between immature crater ejecta and average mature material on mercury compared to the moon is consistent with a more intense space weathering environment on mercury that results in a more mature regolith immature lunar highlands materials are 1 5 times higher in reflectance than analogous immature mercurian materials immature materials of the same composition would have the same reflectance on both bodies thus this observation requires that mercury s crust contains a significant darkening agent either opaque minerals or ferrous iron bearing silicates in abundances significantly higher than those of the lunar highlands if the darkening agent is opaque minerals e g ilmenite or ulvospinel mercury s crust may contain significant ferrous iron and yet not exhibit a 1 m absorption band

mercury full disk radar images and the detection and stability of ice at the north pole the first full disk radar images of mercury were obtained on august 8 and 23 1991 these images were constructed using the very large array vla in socorro new mexico to receive and map radar flux at 3 5 cm x band which was continuously transmitted from the 70 m jet propulsion laboratory deep space network antenna at goldstone california approximately 77 of the surface was imaged at resolutions as good as 150 km about half of the hemisphere photographed by mariner 10 was imaged as well as most of the hemisphere which has not previously been photographed at the time of the observations the north pole was visible and the feature with the highest same sense ss circular reflectivity in the images is near the nominal polar position the peak ss reflectivity of this feature was 7 9 and the circular polarization ratio throughout much of the feature is 1 our best estimate of the size of the feature is that its diameter is 350 km we interpret the feature to be indicative of the presence of ices because of the signal strength and polarization characteristics the ices must be very clean and were thus probably deposited in a relatively short time period the most likely place to find these ices is in any permanently shadowed areas in the polar regions which would be very cold in comparison to absolute reflectivities of other icy bodies and regions in the solar system the reflectivity of the north polar feature is slightly depressed this is probably because the ices do not fill an observational resolution cell i e there is incomplete areal coverage or are covered by a layer of dust or soil which absorbs some of the incoming and outgoing radar energy or both a covering layer would also protect any ices from erosion by energetic sources other prominent features on the unphotographed hemisphere correspond to positions where atmospheric sodium enhancements have been measured from earth this may indicate that these locations are large basins similar to the caloris basin where an atmospheric potassium enhancement has been measured which may be the result of increased degassing in the disrupted surface and near surface direct comparison of the radar echoes from our features with those from the caloris basin are hard to make as caloris was not imaged by us in a favorable geometry features on the photographed hemisphere are smaller and mostly associated with craters and crater complexes most notably the kuiper crater and its environs

lunar dark haloed impact craters origin and implications for early mare volcanism remote sensing and photogeologic data were used to determine the composition and origin of lunar dark haloed craters near infrared reflectance spectra were obtained for numerous dark haloed impact craters located on either the ejecta blankets of large impact craters or imbrian and nectarian age light plains deposits spectral thermal radar and photogeologic data conclusively demonstrate that copernicus h and other dark haloed craters on the ejecta blanket of copernicus excavated mare basalts from beneath lighter surface deposits rich in highlands material analyses of reflectance spectra of dark haloed craters on light plains indicate that in every instance these craters exposed mare basalt which had previously been covered by varying thicknesses of highlands debris in the schiller schickard region a relatively thick highlands unit was emplaced as a result of the orientale impact event on the interior of balmer basin marelike basalt has been excavated from beneath much thinner highlands deposits emplaced by nearby craters mafic orbital geochemical anomalies are commonly associated with regions with abundant light plains deposits which exhibit dark haloed impact craters apparently some mare material has been incorporated into the regolith either by local mixing during the emplacement of the highlands debris or by subsequent vertical mixing the results of recent remote sensing photogeologic and lunar sample studies strongly indicate that mare volcanism was a significant process during much of the pre imbrian and may have been initiated as early as 4 2 4 3 ga these very early volcanic episodes contributed materials to the lunar surface which were incorporated into the upper portion of the highlands crust by subsequent impact mixing models concerning the extent and duration of mare volcanism as well as those involving the composition and thermal evolution of the lunar interior will have to be revised

ferrous oxide in mercury s crust and mantle mercury has widespread plains deposits proposed to be volcanic in origin in a mariner 10 color derived parameter image sensitive to feo and maturity these volcanic plains have a value equivalent to or slightly elevated above the hemispheric average thus implying feo equivalent to or slightly less than the hemispheric average 3 wt feo since feo has a solid liquid distribution coefficient 1 during partial melting we estimate the mantle of mercury to have an feo abundance equal to the lava flows this is consistent with models that predict mercury was assembled from planetesimals formed near the planet s current position this new estimate of mercury s bulk feo 3 wt is consistent with data for the other terrestrial planets that suggest there was a radial gradient in feo in the solar nebula

a mariner 10 color study of mercurian craters 1 we have used recalibrated mariner 10 color image data uv and orange to examine spectral trends associated with crater features on the incoming hemisphere of mercury spacecraft images of the moon galileo and clementine and lunar sample laboratory spectra provide a framework for interpreting the mercurian data two spectral parameter images were constructed one reveals opaque mineral abundance and the other provides a map of variations related to optical maturity and feo content the crater kuiper and its rays are bright mainly because they are fresh immature but also because kuiper has excavated material with a lower opaque content than the surroundings under the influence of space weathering kuiper will eventually lose its rays and evolve into a bright floored crater like lermontov lermontov and nearby smaller craters are examples of craters that are likely mature but remain bright because of the low opaque material exposed on their floors in this portion of the planet it appears that a surface layer with moderate opaque abundance overlies low opaque material found at depth on the basis of estimates of crater excavation depths we suggest that the moderate opaque surface layer is 3 4 km thick this two layer stratigraphy is reminiscent of the lunar highland crust and may have originated in similar magma ocean processes we also describe a feature that exposes dark material with a relatively high opaque content this feature could be a mercurian analog to a lunar dark halo impact crater or perhaps a pyroclastic deposit this implies that opaque rich possibly mafic magmas were generated in the interior and reached the surface through effusive or explosive activity

the formation of mercury s smooth plains there has been extensive debate about whether mercury s smooth plains are volcanic features or impact ejecta deposits we present new indirect evidence which supports a volcanic origin for two different smooth plains units in borealis planitia stratigraphic relations indicate at least two distinct stages of smooth plains formation at least one of these stages must have had a volcanic origin in the hilly and lineated terrain petrarch and several other anomalously shallow craters apparently have been volcanically filled areally extensive smooth plains volcanism evidently occurred at these two widely separated areas on mercury these results combined with work by other researchers on the circum caloris plains and the tolstoi basin show that smooth plains volcanism was a global process on mercury present data suggest to us that the smooth and intercrater plains may represent two distinct episodes of volcanic activity on mercury and that smooth plains volcanism may have been triggered by the caloris impact high resolution and multispectral imaging from a future mercury spacecraft could resolve many of the present uncertainties in our understanding of plains formation on mercury

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

a new technique for estimating the thickness of mare basalts in imbrium basin 1 the total volume of extrusive volcanism on the moon provides a basic thermal and geologic constraint and accurate volume assessments are contingent upon constraining lava flow depths here employing uv vis data from the clementine mission we estimate mare thickness values in the imbrium basin by analyzing ejecta from large 10 km diameter impact craters that penetrate or failed to penetrate through the mare mare thickness values are found to range from at least 2 km at the basin center to 1 5 2 0 km in the buried ring shelf annulus this corresponds to a basalt volume of 1 3 106 km3 almost a factor of two greater than the volume estimated from partially filled craters alone our results indicate that thickness measurements from penetrating craters combined with minimum estimates from partially filled and non penetrating craters provide a more complete picture of the spatial variation of basalt thickness values than could previously be obtained

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

the apparent lack of lunar like swirls on mercury implications for the formation of lunar swirls and for the agent of space weathering abstract images returned by the messenger spacecraft from the mercury flybys have been examined to search for anomalous high albedo markings similar to lunar swirls several features suggested to be swirls on the basis of mariner 10 imaging in the craters handel and lermontov are seen in higher resolution messenger images to lack the characteristic morphology of lunar swirls although antipodes of large impact basins on the moon are correlated with swirls the antipodes of the large impact basins on mercury appear to lack unusual albedo markings the antipodes of mercury s rembrandt beethoven and tolstoj basins do not have surface textures similar to the hilly and lineated terrain found at the caloris antipode possibly because these three impacts were too small to produce obvious surface disturbances at their antipodes mercury does have a class of unusual high reflectance features the bright crater floor deposits bcfds however the bcfds are spectral outliers not simply optically immature material which implies the presence of material with an unusual composition or physical state the bcfds are thus not analogs to the lunar swirls we suggest that the lack of lunar type swirls on mercury supports models for the formation of lunar swirls that invoke interaction between the solar wind and crustal magnetic anomalies i e the solar wind standoff model and the electrostatic dust transport model rather than those models of swirl formation that relate to cometary impact phenomena if the solar wind standoff hypothesis for lunar swirls is correct it implies that the primary agent responsible for the optical effects of space weathering on the moon is solar wind ion bombardment rather than micrometeoroid impact

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

evidence for intrusive activity on mercury from the first messenger flyby images from messenger s first flyby of mercury have shown convincing evidence for surface volcanism here we report on evidence in the new data for several features that are characterized by fractures and graben rare features on a planet dominated by contractional deformation that may be linked to intrusive activity these features include 1 a floor fractured crater interpreted to have been the site of laccolith like sill intrusions the feature is similar to some floor fractured craters on the moon and shows evidence for individual fractured dome like uplifts on the floor 2 a concentric complex of graben observed inside the peak ring on the floor of the 250 km diameter raditladi basin and associated with dark plains and possibly embayed by them the feature may represent an unusual type of floor fracturing associated with deeper intrusions and related ring dikes or cone sheets or the graben may instead be the product of non magmatic uplift of the basin floor 3 a large radial graben swarm pantheon fossae located near the center of the caloris basin thus far unique on mercury and characterized by hundreds of individual graben segments ranging from 5 km to 110 km in length in the nexus graben crosscut one another and produce a local polygonal pattern others curve away from the center as the nexus is approached two scales of graben length are observed the radius of the dense radially symmetric plexus of graben is 175 km and a few graben extend to greater radial distances to the north and southwest out to distances that intersect with a ring of generally concentric graben around the outer basin floor two width scales of graben are observed a large graben about 8 km wide emerges from the nexus and extends for 100 km most graben are less than half this width some graben walls appear cuspate with convex outward wall segments that resemble crater chain segments one crater chain with distinctive raised rims parallels nearby graben locally some graben appear in en echelon patterns and smaller graben sometimes show cross cutting superposition relationships abundant impact craters the most prominent being apollodorus and secondary crater clusters and chains are superposed on the graben system there is little evidence that craters greater than 5 km in diameter have been cut by a graben this relation implies that the graben swarm formed soon after the emplacement of the caloris floor plains these graben are interpreted to be the surface expression of a radial dike swarm emanating from a subsurface magma reservoir similar features in which the dikes contribute to a near surface stress field that favors radial graben are known on the earth venus and mars the location of pantheon fossae in the center of the caloris basin suggests that formation of the radial graben structure is linked to basin evolution

the scaling of complex craters the empirical relation between the transient crater diameter dg and final crater diameter dr of complex craters and basins is estimated using cumulative terrace widths central uplift diameters continuous ejecta radii and transient crater reconstructions determined from lunar and terrestrial impact structures the ratio dg dr is a power law function of dr decreasing uniformly from unity at the diameter of the simple complex crater morphology transition to about 0 5 for large multiring basins like imbrium on the moon the empirical constants in the dg dr relation are interpreted physically to mean that the position of the final rim relative to the transient crater and hence the extent of collapse is controlled or greatly influenced by the properties of the zone of dissociated material produced by the impact shock the continuity of the dg dr relation over the entire spectrum of morphologic types from complex craters to multiring basins implies that the rims of all these structures form in the same tectonic environment despite morphologic differences

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

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

exposure of spectrally distinct material by impact craters on mercury implications for global stratigraphy abstract messenger s mercury dual imaging system mdis obtained multispectral images for more than 80 of the surface of mercury during its first two flybys those images have confirmed that the surface of mercury exhibits subtle color variations some of which can be attributed to compositional differences in many areas impact craters are associated with material that is spectrally distinct from the surrounding surface these deposits can be located on the crater floor rim wall or central peak or in the ejecta deposit and represent material that originally resided at depth and was subsequently excavated during the cratering process the resulting craters make it possible to investigate the stratigraphy of mercury s upper crust studies of laboratory terrestrial and lunar craters provide a means to bound the depth of origin of spectrally distinct ejecta and central peak structures excavated red material rm with comparatively steep red spectral slope and low reflectance material lrm stand out prominently from the surrounding terrain in enhanced color images because they are spectral end members in mercury s compositional continuum newly imaged examples of rm were found to be spectrally similar to the relatively red high reflectance plains hrp suggesting that they may represent deposits of hrp like material that were subsequently covered by a thin layer 1 km thick of intermediate plains in one area craters with diameters ranging from 30 km to 130 km have excavated and incorporated rm into their rims suggesting that the underlying rm layer may be several kilometers thick lrm deposits are useful as stratigraphic markers due to their unique spectral properties some rm and lrm were excavated by pre tolstojan basins indicating a relatively old age 4 0 ga for the original emplacement of these deposits detailed examination of several small areas on mercury reveals the complex nature of the local stratigraphy including the possible presence of buried volcanic plains and supports sequential buildup of most of the upper 5 km of crust by volcanic flows with compositions spanning the range of material now visible on the surface distributed heterogeneously across the planet this emerging picture strongly suggests that the crust of mercury is characterized by a much more substantial component of early volcanism than represented by the phase of mare emplacement on earth s moon

basalt thickness in mare humorum the crater excavation method basalt thicknesses in mare basins have been determined using assumptions about the premare topography of partly buried craters and by comparison to the orientale basin differences in those assumptions have led to a factor of 4 difference in mare thickness estimates further knowledge of thickness is restricted to areas in which buried craters are present using mixing models applied to multispectral images acquired by the clementine spacecraft we have shown that craters in the mare sometimes excavate highland material from below the mare cover using such craters and assumptions about their depth of excavation we obtain independent estimates of basalt thickness our results are in agreement with the lowest of previous thickness estimates we derive a volume of 40 000 km 3 for basalts in mare humorum considerably less than the 110 000 km 3 from previous estimates we also confirm a diameter of 425 km for thc humorum multiring basin based on the clementine gridded global topography assuming an original morphology like orientale

middle ordovician conodonts from the buchans group central newfoundland and their significance for regional stratigraphy of the central volcanic belt discussion nowlan and thurlow have recently reported a conodont fauna of latest arenig early llanvirn age from the buchans group central newfoundland and have suggested that this demonstrates that all volcanogenic massive sulfide deposits and hence associated volcanism in the canadian appalachians are of caradocian or older ordovician age they go on to state that this requires substantial revisions of tectonic models for the northern appalachians we welcome this important contribution to the knowledge regarding the age of roberts arm volcanic belt which includes the buchans group dean 1977 but find their suggestion that current tectonic models for central newfoundland are inadequate or erroneous to be somewhat misleading perhaps this is the case if the tectonic models they are referring to are those for instance of dean 1977 1978 strong 1977 or dean and kean 1980 however their results strongly support the tectonic models set out by kidd et al 1977

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

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

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

multispectral images of mercury from the first messenger flyby analysis of global and regional color trends abstract we have analyzed messenger multispectral images of mercury in order to gain information on the nature of materials present on the planet s surface the quantitative analysis tools of lucey which are built on the hapke conceptual framework for the reflectance of a silicate regolith allow us to derive spectral parameter images that are sensitive to the abundance of ferrous iron in silicates the abundance of opaque phases and the state of maturity of the surface consistent with previous estimates we conclude that the silicate portion of mercury s surface is low in ferrous iron with a small range of variation the spectral changes that accompany regolith maturation on mercury are consistent with lunar style space weathering via the accumulation of nanophase iron the spectral character of mercury s major surface units high reflectance plains intermediate terrain and low reflectance material can be attributed to variations in the abundance of an opaque phase s a class of minor color unit the red spots occurs in at least three morphological varieties two of which are associated with deposits interpreted to be of pyroclastic origin the bright crater floor deposits a second type of minor unit have extremely shallow blue spectral slopes that are unrelated to the presence of immature material

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

nonuniform cratering of the terrestrial planets article history we estimate the impact flux and cratering rate as a function of latitude on the terrestrial planets using a model distribution of planet crossing asteroids and comets bottke w f morbidelli a jedicke r petit j m levison h f michel p metcalfe t s 2002 icarus 156 399 433 after determining the planetary impact probabilities as a function of the relative encounter velocity and encounter inclination the impact positions are calculated analytically assuming the projectiles follow hyperbolic paths during the encounter phase as the source of projectiles is not isotropic latitudinal variations of the impact flux are predicted the calculated ratio between the pole and equator is 1 05 for mercury 1 00 for venus 0 96 for the earth 0 90 for the moon and 1 14 for mars over its long term obliquity variation history by taking into account the latitudinal dependence of the impact velocity and impact angle and by using a crater scaling law that depends on the vertical component of the impact velocity the latitudinal variations of the cratering rate the number of craters with a given size formed per unit time and unit area is in general enhanced with respect to the equator the polar cratering rate is about 30 larger on mars and 10 on mercury whereas it is 10 less on the earth and 20 less on the moon the cratering rate is found to be uniform on venus the relative global impact fluxes on mercury venus the earth and mars are calculated with respect to the moon and we find values of 1 9 1 8 1 6 and 2 8 respectively our results show that the relative shape of the crater size frequency distribution does not noticeably depend upon latitude for any of the terrestrial bodies in this study nevertheless by neglecting the expected latitudinal variations of the cratering rate systematic errors of 20 30 in the age of planetary surfaces could exist between equatorial and polar regions when using the crater chronology method

non intrusive measurements of crater growth abstract an experimental technique to measure crater growth is presented whereby a high speed video captures profiles of a crater forming after impact obtained using a vertical laser sheet centered on the impact point unlike previous so called quarter space experiments where projectiles were launched along a transparent plexiglas sheet so that growth of half a crater could be viewed the use of the laser sheet permits viewing changes in crater shape without any physical interference to the cratering process this technique indicates that for low velocity impacts 300 m s into 220 m glass beads that are without cohesion and where the projectile is not disrupted craters initially grow somewhat proportionally but that later their depths remain essentially constant while their diameters continue to expand in addition these experiments indicate that as the impact velocity increases the rate of growth and the transient depth to diameter ratio at the end of ejecta excavation decreases these last two observations are probably due to the large time of penetration of the projectile which becomes a significant fraction of the time of crater formation this is contrary to the expectations for the scaling rules which assumes a point source very high curtain angles 45 are also seen and could be due to the low friction angle of the target significant crater modification which is rarely seen in quarter space experiments is also observed and appears to be controlled by the dynamic angle of repose of the target these latter observations indicate that differences in target friction angles may need to be considered when determining near rim ejecta mass distributions and large scale crater modification processes on the planets