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

lunar optical maturity investigations a possible recent impact crater and a magnetic anomaly 1 we have used maps of the omat optical maturity parameter along with other clementine uv vis image products to study two interesting lunar features related to albedo and optical maturity examination of the region of a small crater whose formation has been suggested to be a candidate for the flash photographed in 1953 by stuart demonstrates that the candidate crater is not unusually fresh compared to other small craters in the vicinity therefore it is unlikely that the formation of this impact crater generated the observed flash an area of unusually high albedo in the descartes highlands is also known to be a magnetic anomaly it has been proposed that the presence of the magnetic anomaly has prevented the solar wind from reaching the surface for billions of years and therefore that the area has not undergone normal space weathering and preserved a high albedo the high albedo is caused only by maturity differences with the surroundings not by an exotic composition the fact that the albedo anomaly has not darkened or reddened to the extent expected may be the result of the surface texture as revealed in 3 8 cm radar images we suggest that the fresh appearance is largely caused by overlapping ejecta from two nearby craters and continual exposure of immature material by the erosion of 1 to 10 cm sized fragments within the deposit the magnetic shielding mechanism if operative probably plays only a minor role in producing the descartes albedo anomaly

cycling of phosphorus maintains the production of microphytobenthic communities in carbonate sediments of a coral reef the phosphorus p cycle that maintains the production of microphytobenthic communities in carbonate sediments was investigated in the fringing shiraho coral reef system located off the east coast of ishigaki island japan production rates of the diatom dominated microphytobenthic communities and the p flux at the sediment water interface were determined by in situ benthic enclosure experiments the daily net production 6 9 24 mmol c m 22

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

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

evidence for young volcanism on mercury from the third messenger flyby during its first two flybys of mercury the messenger spacecraft acquired images confirming that pervasive volcanism occurred early in the planet s history messenger s third mercury flyby revealed a 290 kilometer diameter peak ring impact basin among the youngest basins yet seen having an inner floor filled with spectrally distinct smooth plains these plains are sparsely cratered postdate the formation of the basin apparently formed from material that once flowed across the surface and are therefore interpreted to be volcanic in origin an irregular depression surrounded by a halo of bright deposits northeast of the basin marks a candidate explosive volcanic vent larger than any previously identified on mercury volcanism on the planet thus spanned a considerable duration perhaps extending well into the second half of solar system history

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

local and regional lunar regolith characteristics at reiner gamma formation optical and spectroscopic properties from clementine and earth based data a detailed remote sensing survey of the reiner gamma formation rgf region by means of earth based telescopic and clementine multispectral imaging has been made in the uv visible near infrared domain the spectral mixture analysis reveals the existence of three basic end members relevant for modeling the observed spectral variations in the rgf vicinity these are mb mare background sws southwest swirl and rgs reiner gamma soil the first two components exhibit spectral characteristics consistent with a prevailing contribution of mature mare soils for the surroundings mb and of immature mare crater like soils rgs at rgf the third intermediate albedo component sws has general characteristics of a mature mare soil but with a redder continuum slope the reported observation can be modeled by a mechanism which would remove the finest fraction in the soil particle diameter 45 m at rgf and redistribute it in the vicinity with a laterally variable proportion and local accumulations such as at sws site according to the available set of in situ data documenting variations in the chemical composition in the distribution of particle sizes and in the degree of maturity with depth in the mare regolith the characteristics depicted at rgf are those of a subsurface soil layer from a depth of the order of 0 3 0 8 m in our view the simplest way to account for the whole body of information available from the present work lies in the proposition that in the area of rgf the uppermost layer of the regolith has been optically and mechanically modified by a process involving the fall of fragments of a low density cometary nucleus previously disrupted by tidal interaction in the earth moon system we recognize however that in the present state of knowledge one cannot rule out the hypothesized existence of a zone of seismically modified terrain peripheral to the imbrium or orientale basins just beneath the mare surface that would be the actual source of the rgf magnetic anomaly

photometric anomalies of the lunar surface studied with smart 1 amie data we present new results from the mapping of lunar photometric function parameters using images acquired by the spacecraft smart 1 european space agency the source data for selected lunar areas imaged by the amie camera of smart 1 and the data processing are described we interpret the behavior of photometric function in terms of lunar regolith properties our study reveals photometric anomalies on both small sub kilometer and large tens of kilometers scales we found the regolith mesoscale roughness of lunar swirls to be similar in mare marginis mare ingenii and the surrounding terrains unique photometric properties related to peculiarities of the millimeter scale regolith structure for the reiner gamma swirl are confirmed we identified several impact craters of subkilometer sizes as the source of photometric anomalies created by an increase in mesoscale roughness within the proximal crater ejecta zones the extended ray systems reveal differences in the photometric properties between proximal and distant ejecta blankets basaltic lava flows within mare imbrium and oceanus procellarum indicate higher regolith porosity for the redder soils due to differences in the chemical composition of lavas

scaling impact melting and crater dimensions implications for the lunar cratering record abstract the dimensions of large craters formed by impact are controlled to a large extent by gravity whereas the volume of impact melt created during the same event is essentially independent of gravity this differential scaling fosters size dependent changes in the dynamics of impact crater and basin formation as well as in the final morphologies of the resulting structures a variety of such effects can be observed in the lunar cratering record and some predictions can be made on the basis of calculations of impact melting and crater dimensions among them are the following 1 as event magnitude increases the volume of melt created relative to that of the crater will grow and more will be retained inside the rim of the crater or basin 2 the depth of melting will exceed the depth of excavation at diameters that essentially coincide with both the inflection in the depth diameter trend and the simple to complex transition 3 the volume of melt will exceed that of the transient cavity at a cavity diameter on the order of the diameter of the moon this would arguably correspond to a moon melting event 4 small lunar craters only rarely display exterior flows of impact melt because the relatively small volumes of melt created can become choked with clasts increasing the melt s viscosity and chilling it rapidly larger craters and basins should suffer little from such a process 5 deep melting near the projectile s axis of penetration during larger events will yield a progression in cen tral structure morphology with growing event magnitude this sequence should range from single peaks through multiple peaks to peak rings 6 the minimum depth of origin of central peak material should co incide with the maximum depth of melting the main central peak in a crater the size of tycho should have had a preimpact depth of close to 15 km

vesta s north pole quadrangle av 1 albana geologic map and the nature of the south polar basin antipodes as part of systematic global mapping of vesta using data returned by the dawn spacecraft we have produced a geologic map of the north pole quadrangle av 1 albana extensive seasonal shadows were present in the north polar region at the time of the dawn observations limiting the ability to map morphological features and employ color or spectral data for determination of composition the major recognizable units present include ancient cratered highlands and younger crater related units undivided ejecta and mass wasting material on crater floors the antipode of vesta s large southern impact basins rheasilvia and veneneia lie within or near the av 1 quadrangle therefore it is of particular interest to search for evidence of features of the kind that are found at basin antipodes on other planetary bodies albedo markings known as lunar swirls are correlated with basin antipodes and the presence of crustal magnetic anomalies on the moon but lighting conditions preclude recognition of such albedo features in images of the antipode of vesta s rheasilvia basin hilly and lineated terrain found at the antipodes of large basins on the moon and mercury is not present at the rheasilvia or veneneia antipodes we have identified small scale linear depressions that may be related to increased fracturing in the rheasilvia and veneneia antipodal areas consistent with impact induced stresses buczkowski d et al 2012b analysis of the large scale troughs on vesta and correlation to a model of giant impact into

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

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

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

lunar surface magnetic field concentrations antipodal to young large impact basins abstract we find that the major regions of strong surface magnetic fields detected by planetary electron reflection magnetometry perm measurements of the apollo 15 and 16 subsatellites are located near the antipodes of four young large ringed impact basins orientale imbrium serenitatis and crisium statistical analysis of the 26 impact basins whose antipodes fall within the 35 s to 35 n latitude band covered by perm measurements shows that the antipodal regions of older basins have median surface magnetic fields of 0 2 nt compared with 0 5 1 5 nt for the four youngest basins although the mechanisms for producing the surface magnetic field concentrations antipodal to impact basins are not well understood these results indicate a period of strong lunar magnetic fields between 3 85 and 3 6 aeons consistent with lunar sample paleomagnetic data the strong surface magnetic fields near the basin antipodes may have resulted from concentration of a preexisting ambient magnetic field at the antipode by the partially ionized vapor cloud produced in hypervelocity basin forming impacts and acquisition of shock remanent magnetization from the impact of secondary ejecta

initial mapping and interpretation of lunar crustal magnetic anomalies using lunar prospector magnetometer data maps of relatively strong crustal magnetic field anomalies detected at low altitudes with the magnetometer instrument on lunar prospector are presented on the lunar nearside relatively strong anomalies are mapped over the reiner gamma formation on western oceanus procellarum and over the rima sirsalis rille on the southwestern border of oceanus procellarum the main rima sirsalis anomaly does not correlate well with the rille itself but is centered over an imbrian aged smooth plains unit interpreted as primary or secondary basin ejecta the stronger reiner gamma anomalies correlate with the locations of both the main reiner gamma albedo marking and its northeastward extension both the rima sirsalis and the reiner gamma anomalies are extended in directions approximately radial to the center of the imbrium basin this alignment suggests that imbrium basin ejecta materials lying in many cases beneath the visible mare surface are the sources of the nearside anomalies if so then the albedo markings associated with the stronger refiner gamma anomalies may be consistent with a model involving magnetic shielding of freshly exposed mare materials from the solar wind ion bombardment two regions of extensive magnetic anomalies are mapped in regions centered on the ingenii basin on the south central farside and near the crater gerasimovic on the southeastern farside these regions are approximately antipodal to the imbrium and crisium basins respectively the imbrium antipode anomaly group is the most areally extensive on the moon while the largest anomaly in the crisium antipode group is the strongest detected by the lunar prospector magnetometer a consideration of the expected antipodal effects of basin forming impacts as well as a combination of sample data and orbital measurements on the nearside leads to the conclusion that the most probable sources of magnetic anomalies in these two regions are ejecta materials from the respective impacts in both regions the strongest individual anomalies correlate with swirl like albedo markings of the reiner gamma class visible on available orbital photography

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

mapping of crustal magnetic anomalies on the lunar near side by the lunar prospector electron reflectometer lunar prospector lp electron reflectometer measurements show that surface fields are generally weak in the large mare basalt filled impact basins on the near side but are stronger over highland terranes especially those lying antipodal to young large impact basins between the imbrium and nectaris basins many anomalies correlate with the cayley and descartes formations statistical analyses show that the most strongly magnetic nearside terranes are cayley type light plains terra materials and pre imbrian craters light plains and terrae include basin impact ejecta as a major component suggesting that magnetization effects from basin forming impacts were involved in their formation the magnetization of pre imbrian craters however may be evidence of early thermal remanence relatively strong small scale magnetic anomalies are present over the reiner gamma feature on western oceanus procellarum and over the rima sirsalis rille on the southwestern border of procellarum both apollo subsatellite and lp data show that the latter anomaly is nearly aligned with the rille though lp magnetometer and reflectometer data show that the anomaly peak is actually centered over a light plains unit this anomaly and the reiner gamma anomaly are approximately radially aligned with the center of imbrium suggesting an association with ejecta from this basin

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

origin of strong lunar magnetic anomalies further mapping and examinations of lroc imagery in regions antipodal to young large impact basins work at the university of arizona was supported under grant nnx12aj03g from the nasa lunar advanced science and exploration research laser program

the distribution and extent of lunar swirls abstract the mysterious high reflectance loops and ribbons known as swirls are not uncommon on the moon but are apparently unique to this body we mapped their distribution and extent using ultraviolet visible images from the lunar reconnaissance orbiter camera we find two main geographic groupings of swirls south pole aitken basin and marginis king and a host of smaller features including swirls near craters abel crozier dewar and dufay x all mapped swirls are associated with magnetic anomalies and swirls have magnetic field strengths shifted to higher values than their background though there is not a 1 1 correspondence between the locations of swirls and magnetic anomalies swirls are also found in regions with iron abundances shifted to higher than background values which could indicate that their formation is inhibited by low iron content the most distinguishing characteristic of swirls is a low 321 415 nm ratio coupled with moderate to high reflectance and swirls generally have high optical maturity omat parameter values stronger 1 m bands and shallower normalized continuum slopes than their surroundings consistent with a surface that has experienced less space weathering however some swirls cannot be discerned in omat or band depth images areas with low 321 415 nm ratios but non distinct visible near infrared properties could be related to the presence of fresh silicates or a glassy component that does not have a substantial abundance of embedded large submicroscopic iron grains i e a difference in the agglutinate fraction of the soil swirl color properties vary with distance from copernican and some eratosthenian craters their association with eratostheninan craters suggests fresh material may be preserved longer in swirls than in non swirl regions

photometric anomalies of the lunar surface results from clementine data 1 we mapped the photometric characteristics of the lunar surface for several small areas using clementine uvvis camera images the maps of the phase function steepness showed several anomalous sites several small fresh impact craters have anomalous halos in these maps the phase function within the halos is less steep than for the surrounding mare surface we interpret these halos to be due to geologically recent impact caused alteration of the equilibrium millimeter scale regolith structure this equilibrium structure is established through micrometeoritic bombardment at a geologically short timescale an anomaly of the same signature was found at the apollo 15 landing site we interpret it as being a result of the regolith structure alteration with the lander jets a unique photometric anomaly not correlated with albedo was found within the reiner gamma formation we suggest that this anomaly is genetically related to the formation which indicates its young age this favors the impact hypothesis for the nature of the reiner gamma formation our study showed that mapping of photometric characteristics is a new powerful tool in studies of the surfaces of atmosphereless bodies future photometric studies of the moon with existing and new data sets are promising for a search for traces of recent seismic events studies of the recent population of meteoroids in the inner solar system an advance in the understanding of swirls etc

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

probable swirls detected as photometric anomalies in oceanus procellarum images of the lunar nearside obtained by telescopes of maidanak observatory uzbekistan and simeiz observatory crimea ukraine equipped with canon cmos cameras and sony ccd linescan camera were used to study photometric properties of the lunar nearside in several spectral bands a wide range of lunar phase angles was covered and the method of phase ratios to assess the steepness of the phase function at different phase angles is applied we found several areas with photometric anomalies in the south west portion of the lunar disk that we refer to as oceanus procellarum anomalies the areas being unique on the lunar nearside do not obey the inverse correlation between albedo and phase curve slope demonstrating high phase curve slopes at intermediate albedo low sun images acquired with lunar orbiter iv and apollo 16 cameras do not reveal anomalous topography of the regions at least for scales larger than several tens of meters the areas also do not have any thermal inertia radar 70 and 3 8 cm magnetic or chemical mineral peculiarities on the other hand they exhibit a polarimetric signature that we interpret to be due to the presence of a porous regolith upper layer consisting of dust particles the anomalies may be interpreted as regions of very fresh shallow regolith disturbances caused by impacts of meteoroid swarms consisting of rather small impactors this origin is similar to one of the hypotheses for the origin of lunar swirls like the reiner formation the photometric difference between the shallow and pervasive reiner class swirls is that the latter appear to have a significant amount of immature soils in the upper surface layers

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

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

mercury s internal magnetic field constraints on large and small scale fields of crustal origin abstract messenger and mariner 10 observations of mercury s magnetic field suggest that small scale crustal magnetic fields if they exist are at the limit of resolution large scale crustal magnetic fields have also been suggested to exist at mercury originating from a relic of an internal dipole whose symmetry has been broken by latitudinal and longitudinal variations in surface temperature if this large scale magnetization is confined to a layer averaging 50 km in thickness it must be magnetized with an intensity of at least 2 9 a m fits to models constrained by such large scale insolation variations do not reveal the predicted signal and the absence of small scale features attributable to remanence further weakens the case for large scale magnetization our tests are hindered by the limited coverage to date and difficulty in isolating the internal magnetic field we conclude that the case for large and small scale remanence on mercury is weak but further measurements by messenger can decide the issue unequivocally across the terrestrial planets and the moon magnetization contrast and iron abundance in the crust show a positive correlation this correlation suggests that crustal iron content plays a determining role in the strength of crustal magnetization

correlations between magnetic anomalies and surface geology antipodal to lunar impact basins processing and mapping of the lunar prospector magnetometer data were supported by the nasa lunar data analysis program through a contract from the lunar research institute

solar wind as the origin of rapid reddening of asteroid surfaces a comparison of the laboratory reflectance spectra of meteorites with observations of asteroids revealed that the latter are much redder with the spectral difference explained by space weathering though the actual processes and timescales involved have remained controversial a recent study of young asteroid families concluded that they suffered only minimal space weathering here we report additional observations of those families revealing that space weathering must be a very rapid process the final colour of a silicate rich asteroid is acquired shortly after its birth within 10 6 years of undergoing a catastrophic collision this rapid timescale favours solar wind implantation as the main mechanism of space weathering as laboratory experiments have shown that it is the most rapid of several competing processes we further demonstrate the necessity to take account of composition when evaluating weathering effectiveness as both laboratory and asteroid data show an apparent dependence of weathering on olivine abundance the rapid colour change that we find implies that colour trends seen among asteroids are most probably due to compositional or surface particle size properties rather than to different relative ages apparently fresh surfaces most frequently seen among small near earth asteroids may be the result of tidal shaking that rejuvenates their surfaces during planetary encounters

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

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

age spot or youthful marking origin of reiner gamma 1 the highly magnetic field magnitudes of 50 nt at 18 km altitude reiner gamma albedo feature on the near side of the moon has been explained in terms of differential space weathering of an old feature or a recent cometary impact we investigated this feature using magnetometer data from lunar prospector the minimum magnetization necessary to explain the magnetic field observations varies from 100 a m for a 10 m thick layer to 1 a m for a 1 km thick layer magnetic sources appear to lie within a few km of the surface and be magnetized in a north south direction the strength of the magnetization appears spatially related to the albedo of the feature these constraints point towards an ancient origin for the magnetic field signal possibly due to basin impact ejecta and the origin of the albedo feature as a consequence of retarded ageing under the umbrella of the reiner gamma mini magnetosphere

correlation of a strong lunar magnetic anomaly with a high albedo region of the descartes mountains 1 mapping and model simulations of lunar prospector magnetometer measurements show that the source of the strongest known magnetic anomaly on the lunar near side 42 nanoteslas at 18 6 km altitude coincides approximately with a high albedo region of the descartes mountains centered 60 km south southeast of the apollo 16 landing site the descartes mountains represent primary ejecta from one or more basin forming events imbrium and or nectaris supporting the hypothesis that basin ejecta materials emplaced 3 8 gyr ago are the main sources of lunar magnetic anomalies the higher albedo of the surface at this location is consistent with a significant role for solar wind ions in the optical maturation or space weathering of the lunar surface

a serenitatis origin for the imbrian grooves and south pole aitken thorium anomaly the northwest corner of the moon s south pole aitken spa basin contains a high abundance of thorium and a unique imbrian aged geomorphologic unit that consists of grooves and mounds referred to here as the imbrian grooves because the location of these features are almost antipodal to the imbrium basin where high thorium ejecta and seismic energy are expected to have converged an imbrium origin for these units has long seemed certain by modeling the deposition of impact ejecta on the moon we have investigated whether the convergence of imbrium s ejecta at its antipode could be the origin of both the imbrian grooves and spa thorium anomaly as a result of the moon s rotation our results show that ejecta from this basin should converge more than 12 west of its antipode both the imbrian grooves and thorium anomaly within spa however reside slightly to the east of imbrium s antipode in an attempt to reconcile this disparity the effects of a putative oblique imbrium impact have been qualitatively investigated while this model can distribute ejecta in the general vicinity of the imbrian grooves the planform of our modeled antipodal ejecta is distinctly different from that which is observed as an alternative explanation for the origin of these features we find that the modeled distribution of ejecta from an oblique serenitatis impact is surprisingly similar to the planform of the imbrian grooves with the exception that it is offset directly to the east this eastward offset is likely to be an artifact of our not being able to properly include the effects of the moon s rotation in our oblique impact models we conclude that the imbrium grooves and spa thorium anomaly are most consistent with having an origin from the convergence of ejecta antipodal to the serenitatis basin if this conclusion can be substantiated once quantitative ejecta scaling relations for oblique impacts are determined then this implies that 1 the serenitatis target contained a high abundance of thorium and 2 the convergence of seismic energy at the antipodes of either the imbrium or serenitatis basin was not sufficient to cause substantial surface modification extrapolating this result to mercury suggests that the hilly and lineated terrain antipodal to the caloris basin was formed by the convergence of ejecta and not seismic waves

antipodal effects of lunar basin forming impacts initial 3d simulations and comparisons with observations abstract 3d simulations of basin scale lunar impacts are carried out to investigate a the origins of strong crustal magnetic fields and unusual terrain observed to occur in regions antipodal to young large basins and b the origin of enhanced magnetic and geochemical anomalies along the northwest periphery of the south pole aitken spa basin the simulations demonstrate that a basin forming impact produces a massive hot partially ionized cloud of vapor and melt that expands thermally around the moon converging near the basin antipode approximately 1 h after the impact for typical impact parameters in agreement with previous work analytic calculations of the interaction of this vapor melt cloud with an initial ambient magnetic field predict a substantial temporary increase in field intensity in the antipodal region the time of maximum field amplification coincides with a period when impacting ejecta also converge near the antipode the latter produce antipodal shock stresses within the range of 5 25 gpa where stable shock remanent magnetization srm of lunar soils has been found experimentally to occur calculated antipodal ejecta thicknesses are only marginally sufficient to explain the amplitudes of observed magnetic anomalies if mean magnetization intensities are comparable to those produced experimentally this suggests that pre existing ejecta materials which would also contain abundant metallic iron remanence carriers may be important anomaly sources a possibility that is consistent with enhanced magnetic anomalies observed peripheral to spa the latter anomalies may be produced by amplified secondary ejecta impact shock waves in the thick spa ejecta mantle occurring near the antipodes of the imbrium and serenitatis impacts together with converging seismic compressional waves these antipodal impact shocks may have produced especially deep fracture zones along the northwest edge of spa near the imbrium antipode allowing the ascent of magma with enhanced kreep concentrations

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

contour maps of lunar remanent magnetic fields the 2605 usable orbits of apollo 15 and 16 subsatellite magnetometer data have been reexamined for intervals suitable for analysis of crustal magnetic anomalies to minimize plasma related disturbances segments from 274 of these orbits were selected from times when the moon was either in a lobe of the geomagnetic tail or in the solar wind with the subsatellites in the lunar wake external field contributions which remained in the selected intervals were minimized by 1 quadratic detrending of individual orbit segments with lengths much greater than anomaly wavelengths and 2 two dimensional filtering with minimum passed wavelengths less than or equal to anomaly wavelengths improvements in coverage accuracy and resolution of previously published anomaly maps produced from these data are obtained in addition to improved maps of the reiner gamma and van de graaff aitken anomalies studied previously a third region of relatively high amplitude anomalies centered near the crater gerasimovich on the southeastern far side has been mapped both the van de graaff aitken region and the gerasimovich region are marked by the general occurrence of extensive groups of reiner gamma type swirls

characterization of lunar swirls at mare ingenii a model for space weathering at magnetic anomalies 1 analysis of spectra from the clementine ultraviolet visible and near infrared cameras of small immature craters and surface soils both on and adjacent to the lunar swirls at mare ingenii has yielded the following conclusions about space weathering at a magnetic anomaly 1 despite having spectral characteristics of immaturity the lunar swirls are not freshly exposed surfaces 2 the swirl surfaces are regions of retarded weathering while immediately adjacent regions experience accelerated weathering 3 weathering in the off swirl regions darkens and flattens the spectrum with little to no reddening which suggests that the production of larger 40 nm nanophase iron dominates in these locations as a result of charged particle sorting by the magnetic field preliminary analysis of two other lunar swirl regions reiner gamma and mare marginis is consistent with our observations at mare ingenii our results indicate that sputtering vapor deposition implanted solar wind hydrogen and agglutination share responsibility for creating the range in npfe0 particle sizes responsible for the spectral effects of space weathering

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

lunar swirls examining crustal magnetic anomalies and space weathering trends 1 we have used multispectral images from clementine and data from lunar prospector s magnetometer to conduct a survey of lunar crustal magnetic anomalies prominent lunar swirls and lesser known swirl markings to provide new information on the nature of swirls and their association with magnetic anomalies we find that all swirls and swirl like albedo patterns are associated with areas of magnetized crust but not all areas of magnetized crust are colocated with swirl like albedo anomalies all observed swirls exhibit spectral characteristics similar to immature material and generally have slightly lower feo values compared with their surroundings as determined with a multispectral iron mapping method we discuss these results in relation to the various hypotheses for swirl formation the comet impact hypothesis for lunar swirls would not predict a difference in the spectrally determined feo content between swirls and nearby ordinary surfaces the compositional difference could be explained as a consequence of 1 magnetic shielding of the surface from the solar wind which could produce anomalous space weathering little darkening with limited reddening and potentially alter the predictions of the multispectral iron mapping algorithm while the compositional contrast could be enhanced by delivery of lower feo ejecta from outside the swirl and 2 accumulation of fine plagioclase rich dust moving under the influence of electric fields induced by solar wind interactions with a magnetic anomaly therefore we cannot at present clearly distinguish between the solar wind shielding and electrostatic dust accumulation models for swirl formation we describe future measurements that could contribute to solution of the puzzle of swirl origin