carbon isotopes and event stratigraphy near the ordovician silurian boundary yichang south china abstract the organic carbon isotope data through the unweathered ordovician silurian boundary strata at the wangjiawan riverside section which is c 180 m southeast of the wangjiawan north section the gssp for the base of the hirnantian stage upper ordovician show that a positive 13 c org excursion begins just below the base of the hirnantian stage and peaks in the lower part of the n extraordinarius biozone this is followed by an interval of slightly reduced 13 c values and a second peak of 2 above pre hirnantian values which occurs in the lower part of the n persculptus biozone upper hirnantian the peaks in 13 c org values can be correlated well with episodes of glacial expansion described from africa and peri gondwanan europe evidence from sedimentological faunal and geochemical data from south china as well as the evidence of glacial sediments in north africa all suggests a short lived glaciation of 1 ma in the ordovician south pole region consisting of at least two major pulses the first phase of gradual glacial expansion began just before the hirnantian epoch and the second phase ended with rapid melting in the late hirnantian early n persculptus biozone

radar imagery of mercury s putative polar ice 1999 2005 arecibo results abstract we present an updated survey of mercury s putative polar ice deposits based on high resolution 1 5 km imaging with the upgraded arecibo s band radar during 1999 2005 the north pole has now been imaged over a full range of longitude aspects making it possible to distinguish ice free areas from radar shadowed areas and thus better map the distribution of radar bright ice the new imagery of the south pole though derived from only a single pair of dates in 2005 improves on the pre upgrade arecibo imagery and reveals many additional ice features some medium size craters located within 3 of the north pole show near complete ice coverage over their floors central peaks and southern interior rim walls and little or no ice on their northern rim walls while one large 90 km crater at 85 n shows a sharp ice cutoff line running across its central floor all of this is consistent with the estimated polar extent of permanent shading from direct sunlight some craters show ice in regions that though permanently shaded should be too warm to maintain unprotected surface ice owing to indirect heating by reflected and reradiated sunlight however the ice distribution in these craters is in good agreement with models invoking insulation by a thin dust mantle comparisons with goldstone x band radar imagery indicate a wavelength dependence that could be consistent with such a dust mantle more than a dozen small ice features have been found at latitudes between 67 and 75 all of this low latitude ice is probably sheltered in or under steep pole facing crater rim walls although since most is located in the mariner unimaged hemisphere confirmation must await imaging by the messenger orbiter these low latitude features are concentrated toward the cold longitudes possibly indicating a thermal segregation effect governed by indirect heating the radar imagery places the corrected locations of the north and south poles at 7 w 88 35 n and 90 w 88 7 s respectively on the original mariner based maps

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

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

mercurian impact craters implications for polar ground ice abstract recent radar observations of mercury have detected strong depolarized echoes from the north and south polar regions which have been interpreted by some as ice deposits in the floors of permanently shadowed impact craters we have used the experience from mars where subsurface ice lowers the depth to diameter ratio d d of impact craters to test for subsurface ice deposits on mercury this analysis determines the d d ratios for 170 impact craters in the borealis north polar tolstoj equatorial kuiper equatorial and bach south polar quadrangles of the planet possible effects from sun angle and terrain were eliminated to test whether d d differences could be detected at mariner 10 resolutions 1 km pixel we perform a similar analysis using mariner 9 images of mars which have similar resolutions we demonstrate that d d differences due to terrain softening can be detected between craters in the martian polar regions and the equatorial regions at the mariner 9 resolutions although our initial results indicate that the south polar bach quadrangle has a statistically lower d d than the north polar borealis or two equatorial tolstoj and kuiper quadrangles further investigation reveals that this finding is most likely the result of the filtering which was applied to the images of the bach quadrangle by jpl thus no unequivocal evidence exists that the possible ice deposits in craters at mercury s north and south poles are the exposed portions of more extensive subsurface ice caps combined with the temporal constraint imposed by the fact that the proposed ice deposits are found only in usgs class 4 craters this suggests a large rapidly emplaced exogenic source of water to mercury during the mansurian period we suggest that the source was multiple impacts from a fragmented comet or a comet shower

mars radar mapping strong backscatter from the elysium basin and outflow channel abstract radar reflectivity maps of mars obtained at arecibo in 1990 using a modified delay doppler technique have revealed strong depolarized echoes coming from the elysium flood basin and outflow channel as well as from elysium mons this result is consistent with recent studies which show that much of the basin channel floor is covered with lava flows

effects of orbital evolution on lunar ice stability 1 many regions near the lunar poles are currently cold enough that surface water ice would be stable against sublimation losses for billions of years however most of these environments are currently too cold to efficiently drive ice downward by thermal diffusion leaving impact burial as the primary means of protection from surface loss processes in this respect most of the present near surface thermal environments on the moon may actually be quite poor traps for water ice this was not always the case long term orbital changes have dramatically altered the lunar polar thermal environment we develop a simple model of the evolution of the lunar orbit and spin axis to examine the thermal environments available for volatile deposition and retention in the past our calculations show that some early lunar polar environments were in the right temperature regime to have collected subsurface ice if a supply were available however a high obliquity period which occurred when the moon was at about half its present distance from the earth would either have driven this ice out into space or deep into the lunar subsurface since that time as the lunar obliquity has slowly decreased to its present value environments have undergone their own thermal evolution and each of the current cold traps experienced a period when they were most efficient at thermally burying ice we examine the thermal history of a lunar polar crater to provide a framework for examining other processes effecting volatiles in the moon s near surface cold traps

mercury the enigmatic innermost planet abstract the planet mercury a difficult object for study by astronomical observation and spacecraft exploration alike poses multiple challenges to our general understanding of the inner planets mercury s anomalously high uncompressed density implies a metal fraction of 60 or more by mass an extreme outcome of planetary formational processes common to the inner solar system whether that outcome was the result of chemical gradients in the early solar nebula or removal by impact or vaporization of most of the silicate shell from a differentiated protoplanet can potentially be distinguished on the basis of the chemical composition of the present crust our understanding of the geological evolution of mercury and how it fits within the known histories of the other terrestrial planets is restricted by the limited coverage and resolution of imaging by the only spacecraft to have visited the planet the role of volcanism in mercury s geological history remains uncertain and the dominant tectonic structures are lobate scarps interpreted as recording an extended episode of planetary contraction issues that require global imaging to be fully examined that mercury has retained a global magnetic field when larger terrestrial planets have not stretches the limits of standard hydromagnetic dynamo theory and has led to proposals for a fossil field or for exotic dynamo scenarios hypotheses for field generation can be distinguished on the basis of the geometry of mercury s internal field and the existence and size of a fluid outer core on mercury can be ascertained from measurements of the planet s spin axis orientation and gravity field and the amplitude of mercury s forced librations the nature of mercury s polar deposits suggested to consist of volatile material cold trapped on the permanently shadowed floors of high latitude impact craters can be tested by remote sensing of the composition of mercury s surface and polar atmosphere the extremely dynamic exosphere which includes a number of species derived from mercury s surface offers a novel laboratory for exploring the nature of the complex and changing interactions among the solar wind a small magnetosphere and a solid planet recent ground based astronomical measurements and several new theoretical developments set the stage for the in depth exploration of mercury by two spacecraft missions within the coming decade

the migration of volatiles on the surfaces of mercury and the moon radar observations of mercury have provided the startling discovery of the probable existence of substantial deposits of ices in permanently shaded polar regions butler et al 1993 harmon et al 1994 this has renewed the old argument about the existence of such deposits in the polar regions of the moon it is most likely necessary for volatiles to be able to migrate to the polar regions of these bodies in order to build up such deposits this paper presents the results of the extension of a monte carlo simulation of the migration of molecules on the surface of mercury by butler et al 1993 the results of the simulations show that for typical conditions on mercury 5 15 of all h 2 o which is placed randomly on the surface migrates to stable permanently shaded polar regions for the moon the numbers are 20 50 the numbers for the migration of co 2 are 2 for mercury and 13 for the moon these percentages are similar to those previously calculated and support the idea of ice deposits in the polar regions of both mercury and the moon

craters hosting radar bright deposits in mercury s north polar region areas of persistent shadow determined from messenger images 1 radar bright features near mercury s poles were discovered in earth based radar images and proposed to be water ice present in permanently shadowed areas images from messenger s one year primary orbital mission provide the first nearly complete view of mercury s north polar region as well as multiple images of the surface under a range of illumination conditions we find that radar bright features near mercury s north pole are associated with locations persistently shadowed in messenger images within 10 of the pole almost all craters larger than 10 km in diameter host radar bright deposits there are several craters located near mercury s north pole with sufficiently large diameters to enable long lived water ice to be thermally stable at the surface within regions of permanent shadow craters located farther south also host radar bright deposits and show a preference for cold pole longitudes thermal models suggest that a thin insulating layer is required to cover these deposits if the radar bright material consists predominantly of long lived water ice many small 10 km diameter and low latitude extending southward to 66 n craters host radar bright material and water ice may not be thermally stable in these craters for 1 gy even beneath an insulating layer the correlation of radar bright features with persistently shadowed areas is consistent with the deposits being composed of water ice and future thermal modeling of small and low latitude craters has the potential to further constrain the nature source and timing of emplacement of the radar bright material

new images of mercury s surface from 210 to 290 w longitudes with implications for mercury s global asymmetry abstract ground based images of mercury in the sector 210 to 290 w 70 to 150 e longitude a region not imaged by mariner 10 show many impact craters and at least two large basins a millisecond exposure method was used at the abastumany and skinakas observatories under good atmospheric conditions enough images were obtained that when stacked increased the signal to noise ratio and gave rise to resolution that shows unprecedented detail of surface albedo and physical features we discuss images of a double rimmed basin whose inner portion extends 1000 km across with a total dimension of the outer eroded rim slightly more than 2000 km extending over 245 to 305 w 55 to 115 e longitude and from 40 n to 20 s this basin includes and extends west and north of the dark albedo feature known as solitudo criophori and was unofficially called skinakas after the observatory from which it was first imaged in this manuscript we call the feature basin s the inner basin extends from 263 to 287 w 73 to 97 e and from about 30 n to 5 s some regions within the basin have eroded circular rims that lack rayed structure or evidence of ejecta material these images also show large basins unusual clusters of circular rimmed features and darker albedo material between 210 and 330 w 30 to 150 e longitude implications of the huge basin for the geophysics of mercury are discussed many well defined impact craters have also been imaged an attempt has been made to restore information about relief of the 210 to 290 w 70 to 150 e longitude sector in addition regions on the order of 10 of latitude and 10 of longitude have been examined and reveal rayed craters in comparable detail to the arecibo radar imaging images at the same location where radar backscatter has indicated infilling of craters by volatiles or some other highly coherent backscattering material are also shown

mars mapping with delay doppler radar mars radar imaging results from arecibo 12 6 cm observations are presented the images were derived from delay doppler mapping using a coded long pulse technique to mitigate the effects of echo overspreading images of the depolarized echo are used to identify regions of high decimeter scale roughness some of the strongest echo features are located on the major shield volcanoes or on relatively young off shield flows such as the olympus and pavonis lava aprons the shields themselves have highly irregular radar signatures suggesting complex volcanic histories some mars radar features have twice the depolarized brightness of the roughest terrestrial lava flows apparently due to higher levels of multiple scattering from surfaces of spectacular roughness or from volume scattering low brightness smooth areas are associated with older surfaces such as fractured and highland terra as well as with terrain interpreted to be debris lobes ash flows and aureoles in particular a close connection was found between the 12 6 cm counterpart of the stealth feature and the medusae fossae formation postulated to be deep ignimbrite deposits marte valus is anomalous in being the only outflow channel showing strongly enhanced echoes which supports the idea that this channel and the elysium basin that it drained are filled with lava flows a weak radar feature was found for the south polar residual ice cap comparisons with goldstone 3 5 cm data show that the south polar enhancement is much weaker at 12 6 cm than at 3 5 cm indicating that the southern ice cap becomes optically thin at the longer wavelength a north polar enhancement has also been found which is comparable in strength to the 12 6 cm south polar feature

radar altimetry of mercury a preliminary analysis measurements of mercurian topography based on arecibo radar observations are presented the data which were obtained from 1978 to 1984 cover much of the equatorial zone of mercury between 12on and 5os latitude over thirty continuous altitude profiles were obtained each spanning from 20 to 90 degrees of longitude at a resolution of 0 15 o longitude by 2 5 o latitude radar depths for large craters support previous indications from imagery that mercurian craters are shallower than lunar craters of the same size one very large 800 km impact basin shows some distinct topographic structure although its relative shallowness suggests postimpact modification by isostatic relaxation or volcanic filling the plains of tir planitia appear topographically smooth to the radar these plains extend well into the hemisphere not imaged by mariner 10 possibly forming part of a large annulus of smooth plains around caloris basin the circum caloris smooth plains are strongly down bowed indicating subsidence under a load this and other similarities to lunar maria suggest a volcanic origin for these plains additional areas of topographically smooth terrain have been found in both the imaged and unimaged hemispheres several ridges scarps and fault zones have been identified in the altimetry three mapped arcuate scarps show heights of about 700 m and cross sectional widths of about 70 km one of these features is clearly a ridge while the other two scarps have a more ridge like appearance than is suggested by images one large scale topographic drop of 3 km correlates well with a mapped system of faults and intracrater scarps the equatorial zone of mercury shows 7 km of maximum relief although the typical elevation difference between highlands and lowlands is closer to 3 km three major highland areas are found the largest two of which are roughly antipodal and aligned within about 10 o of the hot poles of mercury the unimaged hemisphere possessing both large craters and topographically smooth areas does not appear to be markedly different in its topography from the imaged hemisphere no evidence has been found for another caloris type impact structure in the unimaged hemisphere

how thick are mercury s polar water ice deposits an estimate is made of the thickness of the radar bright deposits in craters near to mercury s north pole to construct an objective set of craters for this measurement an automated crater finding algorithm is developed and applied to a digital elevation model based on data from the mercury laser altimeter onboard the messenger spacecraft this produces a catalogue of 663 craters with diameters exceeding 4 km northwards of latitude 55 a subset of 12 larger well sampled and fresh polar craters are selected to search for correlations between topography and radar same sense backscatter cross section it is found that the typical excess height associated with the radar bright regions within these fresh polar craters is 50 35 m this puts an approximate upper limit on the total polar water ice deposits on mercury of 3 1015 kg

the thermal stability of water ice at the poles of mercury recent radar observations of mercury have revealed the presence of anomalous radar reflectivity and polarization features near its north and south poles thermal model calculations show that despite mercury s proximity to the sun the temperatures of flat low reflectivity surfaces at mercury s poles are not expected to exceed 167 kelvin the locations of the anomalous polar radar features appear to be correlated with the locations of large high latitude impact craters maximum surface temperatures in the permanently shadowed regions of these craters are expected to be significantly colder as low as 60 kelvin in the largest craters these results are consistent with the presence of water ice because at temperatures lower than 112 kelvin water ice should be stable to evaporation over time scales of billions of years

radar images of mars full disk images of mars have been obtained with the use of the very large array vla to map the radar reflected flux density the transmitter system was the 70 m antenna of the deep space network at goldstone california the surface of mars was illuminated with continuous wave radiation at a wavelength of 3 5 cm the reflected energy was mapped in individual 12 minute snapshots with the vla in its largest configuration fringe spacings as small as 67 km were obtained the images reveal near surface features including a region in the tharsis volcano area over 2000 km in east west extent that displayed no echo to the very low level of the radar system noise the feature called stealth is interpreted as a deposit of dust or ash with a density less than about 0 5 gram per cubic centimeter and free of rocks larger than 1 cm across the deposit must be several meters thick and may be much deeper the strongest reflecting geological feature was the south polar ice cap which was reduced in size to the residual south polar ice cap at the season of observation the cap image is interpreted as arising from nearly pure co 2 or h 2o ice with a small amount of martian dust less than 2 percent by volume and a depth greater than 2 to 5 m only one anomalous reflecting feature was identified outside of the tharsis region although the elysium region was poorly sampled in this experiment and the north pole was not visible from earth

circular polarization ratio characteristics of impact craters from mini rf observations and implications for ice detection at the polar regions of the moon 1 in an attempt to reduce the ambiguity on radar detection of water ice at the permanently shadowed regions near the lunar poles radar echo strength and circular polarization ratio cpr of impact craters are analyzed using the miniature radio frequency mini rf radar data from the lunar reconnaissance orbiter mission eight typical craters among over 70 craters are selected and classified into four categories based on their locations and cpr characteristics polar anomalous polar fresh nonpolar anomalous and nonpolar fresh the influences on cpr caused by surface slope rocks and dielectric constant are analyzed quantitatively using high resolution topography data and optical images a two component mixed model for cpr that consists of a normal surface and a rocky surface is developed to study the effect of rocks that are perched on lunar surface and buried in regolith our analyses show that inner wall of a typical bowl shaped crater can give rise to a change of about 30 in local incidence angle of radar wave which can further result in a cpr difference of about 0 2 there is a strong correlation between mini rf cpr and rock abundance that is obtained from high resolution optical images and predictions from the two component mixed model match well with the observed cprs and the estimated rock abundances statistical results show that there is almost no apparent difference in cpr characteristics between the polar and nonpolar anomalous craters or between the polar and nonpolar fresh craters the enhanced cpr in the interior of anomalous craters is most probably caused by rocks that are perched on lunar surface or buried in regolith instead of ice deposits as suggested in previous studies

thick ice deposits in shallow simple craters on the moon and mercury permanently shadowed regions near the poles of mercury and the moon may cold trap water ice for geologic time periods in past studies thick ice deposits have been detected on mercury but not on the moon despite their similar thermal environments here we report evidence for thick ice deposits inside permanently shadowed simple craters on both mercury and the moon we measure the depth diameter ratio of approximately 2 000 simple craters near the north pole of mercury using mercury laser altimeter data we find that these craters become distinctly shallower at higher latitudes where ice is known to have accumulated on their floors this shallowing corresponds to a maximum infill of around 50 m consistent with previous estimates a parallel investigation of approximately 12 000 lunar craters using lunar reconnaissance orbiter data reveals a similar morphological trend near the south pole of the moon which we conclude is also due to the presence of thick ice deposits we find that previously detected surface ice deposits in the south polar region of the moon are spatially correlated with shallow craters indicating that the surface ice may be exhumed or linked to the subsurface via diffusion the family of lunar craters that we identify are promising targets for future missions and may also help resolve the apparent discrepancy between the abundance of frozen volatiles on mercury and the moon ice deposits up to around 50 m thick infill some craters near the moon s south pole and mercury s north pole as inferred from the poleward shallowing of simple craters

imaging mercury s polar deposits during messenger s low altitude campaign images obtained during messenger s low altitude campaign in the final year of the mission provide the highest spatial resolution views of mercury s polar deposits images for distinct areas of permanent shadow within 35 north polar craters were successfully captured during the campaign all of these regions of permanent shadow were found to have low reflectance surfaces with well defined boundaries additionally brightness variations across the deposits correlate with variations in the biannual maximum surface temperature across the permanently shadowed regions supporting the conclusion that multiple volatile organic compounds are contained in mercury s polar deposits in addition to water ice a recent large impact event or ongoing bombardment by micrometeoroids could deliver water as well as many volatile organic compounds to mercury either scenario is consistent with the distinctive reflectance properties and well defined boundaries of mercury s polar deposits and the presence of volatiles in all available cold traps

a search for water ice at the lunar poles with clementine images abstract optical and near ir signatures of water ice on the moon s surface were sought in the permanently shadowed regions near its poles significant amounts of multiply scattered radiation partly illuminate primary shadows cast by craters and other features if there is water ice in the permanently shadowed regions of the moon s surface its spectral signature should appear in this multiply scattered light this investigation can be done most effectively with observations obtained by spacecraft because most selenocentric positions occupied by the earth will also be occupied by the sun at some point in time and because the lunar poles are seen only obliquely to a terrestrial observer images obtained by clementine are particularly well suited to this task because the spacecraft s polar orbit allowed images of the poles to be acquired on nearly every orbit resulting in literally thousands of images taken within a few degrees of each pole and because the filters on the ultraviolet visual camera uvvis and the near infrared camera nir occur at major absorption bands or within important continuum features of water ice approximately 5800 images obtained by the uvvis camera and 1800 images obtained by the nir camera were calibrated and combined into coadded mosaics to create multispectral maps of the lunar poles with the highest possible signal to noise unfortunately analysis of our uvvis mosaics indicates that any possible signal from multiply scattered light in primary shadows was overwhelmed by instrumental stray light for the nir camera we were able to determine the normalized reflectance of several regions that were identified by margot et al 1999 science284 1658 1660 as permanent shadows we have identified one permanently shadowed crater with a 1 5 m band spectral signature indicative of between 2 5 and 21 fractional coverage of h2o frost however the same region shows a 2 0 m spectral signature that is inconsistent with the presence of any water

high resolution radar imaging of mercury s north pole abstract the recently upgraded arecibo s band 12 6 cm radar was used to make delay doppler images of mercury s north polar region where earlier observations had shown strong echoes from putative ice deposits in craters the image resolution of 1 5 3 km is a substantial improvement over the 15 km resolution of the older arecibo images j k harmon et al 1994 nature 369 213 215 the new observations confirm all the original polar features and reveal many additional features including several at latitudes as low as 72 75 n and several from craters less than 10 km in diameter all of the new features located on the mariner imaged side of the planet can be matched with known craters or other shaded areas we find the north pole to be located 65 km from the original mariner based pole and 15 km from the new mariner based pole of m s robinson et al 1999 j geophys res 104 30 847 30 852 the improved resolution reveals fine structure in the radar features and their respective host craters including radar shadowing highlighting by central peaks and rim walls rim terracing and preferential concentration of radar bright deposits in shaded southern floor areas the radar features high brightness circular polarization inversion c 1 25 and confinement to regions permanently shaded from direct sunlight are all consistent with volume scattering from a cold trapped volatile such as clean water ice the sizes and locations of most of the features show good agreement with the thermal model of a r vasavada d a paige and s e wood 1999 icarus 141 179 193 for insulated buried water ice although the problems of explaining radar features in small craters and the rapid burial required at lower latitudes suggest that other factors may be suppressing ice loss after emplacement

radar mapping of mercury full disk images and polar anomalies a random code technique has been used at arecibo to obtain delay doppler radar images of the full disk of mercury anomalously bright features were found at the north and south poles the north polar feature is oblong 4 by 8 and offset from the pole the smaller south polar feature is mostly confined to the floor of the crater chao meng fu the polar locations and radar properties of these features indicate that they may be produced by volume scattering in ice the images also reveal a variety of more subdued reflectivity features ranging in size from hundreds to thousands of kilometers some of these appear to have an impact origin

age and timing of the permian mass extinctions u pb dating of closed system zircons the age and timing of the permian triassic mass extinction have been difficult to determine because zircon populations from the type sections are typically affected by pervasive lead loss and contamination by indistinguishable older xenocrysts zircons from nine ash beds within the shangsi and meishan sections china pretreated by annealing followed by partial attack with hydrofluoric acid result in suites of consistent and concordant uranium lead u pb ages eliminating the effects of lead loss the u pb age of the main pulse of the extinction is 252 6 0 2 million years synchronous with the siberian flood volcanism and it occurred within the quoted uncertainty

the neutron signature of mercury s volatile polar deposits neutron leakage flux spectra are simulated to explore whether the nature of volatile deposits residing within the permanently shaded craters near the poles of mercury can be identified using state of the art space neutron detection techniques deployed aboard a flyby mission to mercury the most sensitive method for discriminating between h2o ice rich and s rich deposits involves measurement of the flux of epithermal neutrons we conclude that a single flyby over crater chao meng fu at an altitude of 200 km is sufficient to detect a circular deposit rich in elemental sulfur at the 2 6 statistical significance level and that a similar deposit rich in h2o can be identified at the 7 8 level whereas identification of a s rich deposit is not unique but compelling because of auxiliary physical arguments that of an h2o deposit is unique

thermal stability of ice on ceres with rough topography the dwarf planet ceres may have an ice rich crust and subsurface ice exposed by impacts or endogenic activity would be subject to sublimation we model surface and subsurface temperatures on ceres to assess lifetimes of water ice and other volatiles topographic shadowing allows a small but non negligible fraction 0 4 of ceres surface to be perennially below the 110 k criterion for 1 gyr of stability these areas are found above 60 latitude other molecules ch3oh nh3 so2 co2 may be cold trapped in smaller abundances a model for the transport gravitational escape and photoionization of h2o molecules suggests net accumulation in the cold traps buried ice is stable within a meter for 1 gyr at latitudes higher than 50 an illuminated polar cap of water ice would be stable within a few degrees of the poles only if it maintained a high albedo 0 5 at present obliquity if the obliquity exceeded 5 in the geologically recent past then a putative polar cap would have been erased at latitudes 0 30 ice is stable under solar illumination only briefly 10 100 yr unless it has high albedo and thermal inertia in which case lifetimes of 104 yr are possible finally a small hemispheric asymmetry exists due to the timing of ceres perihelion passage which would lead to a detectable enhancement of ice in the northern hemisphere if the orbital elements vary slowly relative to the ice accumulation rate our model results are potentially testable during the dawn science mission

bright and dark polar deposits on mercury evidence for surface volatiles measurements of surface reflectance of permanently shadowed areas near mercury s north pole reveal regions of anomalously dark and bright deposits at 1064 nanometer wavelength these reflectance anomalies are concentrated on poleward facing slopes and are spatially collocated with areas of high radar backscatter postulated to be the result of near surface water ice correlation of observed reflectance with modeled temperatures indicates that the optically bright regions are consistent with surface water ice whereas dark regions are consistent with a surface layer of complex organic material that likely overlies buried ice and provides thermal insulation impacts of comets or volatile rich asteroids could have provided both dark and bright deposits

investigating mercury s south polar deposits arecibo radar observations and high resolution determination of illumination conditions there is strong evidence that mercury s polar deposits are water ice hosted in permanently shadowed regions in this study we present new arecibo radar observations of mercury s south pole which reveal numerous radar bright deposits and substantially increase the radar imaging coverage we also use images from messenger s full mission to determine the illumination conditions of mercury s south polar region at the same spatial resolution as the north polar region enabling comparisons between the two poles the area of radar bright deposits in mercury s south is roughly double that found in the north consistent with the larger permanently shadowed area in the older cratered terrain at the south relative to the younger smooth plains at the north radar bright features are strongly associated with regions of permanent shadow at both poles consistent with water ice being the dominant component of the deposits however both of mercury s polar regions show that roughly 50 of permanently shadowed regions lack radar bright deposits despite some of these locations having thermal environments that are conducive to the presence of water ice the observed uneven distribution of water ice among mercury s polar cold traps may suggest that the source of mercury s water ice was not a steady regular process but rather that the source was an episodic event such as a recent large impact on the innermost planet

near surface temperatures on mercury and the moon and the stability of polar ice deposits abstract in order to assess the thermal stability of polar ice deposits we present model calculated temperatures of flat surfaces and surfaces within bowl shaped and flat floored polar impact craters on mercury and the moon our model includes appropriate insolation cycles realistic crater shapes multiple scattering of sunlight and infrared radiation and depth and temperature dependent regolith thermophysical properties unshaded water ice deposits on the surface of either body are rapidly lost to thermal sublimation a subsurface water ice deposit is stable within 2 latitude of the moon s poles meter thick water ice deposits require billions of years to sublime if located in the permanently shaded portions of flat floored craters within 10 latitude of the poles of mercury and 13 latitude of the poles of the moon results for craters associated with radar features on mercury are consistent with the presence of stable water ice deposits if a thin regolith layer thermally insulates deposits at lower latitudes and within smaller craters a regolith cover would also reduce losses from diffusion ion sputtering impact vaporization and h ly and is implied independently by the radar observations permanently shaded areas near the moon s poles are generally colder than those near mercury s poles but the moon s obliquity history its orbit through earth s magnetospheric tail and its radar opaque regolith may limit the volume and radar detectability of ice deposits there

topography of the lunar south polar region implications for the size and location of permanently shaded areas we analyze clementine altimetry to constrain the size and location of proposed permanently shadowed regions in the vicinity of the lunar south pole long and short wavelength topography in the vicinity of the pole in combination with measurements of depths of well preserved craters and basins and the lunar topographic power spectrum have direct bearing on the nature of elevations in the south polar region a criterion based on geometric considerations and altimetry demonstrates that the existence of permanent shadowing is not very sensitive to the elevation of the south pole in addition permanent shadowing cannot be a consequence of large structures such as the south pole aitken basin and or a 300 km degraded polar basin perennially dark regions if they exist are most likely associated with craters or other axisymmetric features with diameters of at most 80 km centered at the pole for structures displaced 2 from the pole the maximum allowable diameter decreases to 30 km

the thickness of radar bright deposits in mercury s northern hemisphere from individual mercury laser altimeter tracks the discovery of mercury s radar bright deposits has expanded our understanding of volatiles in the solar system key to deciphering the history and origin of the radar bright deposits is an estimate of the volume of radar bright material that in turn requires a measure of the average thickness of the deposits in this study we investigate changes in topography across radar bright deposits hosted in flat floored complex craters using individual edited mercury laser altimeter mla tracks we compare the difference in heights of radar bright regions and non radar bright regions of the crater floor and the difference of similarly sized and located regions in non radar bright craters and show that the two populations cannot be distinguished the similarity of topography in these two sets of craters allows an upper limit of 15 m to be placed on the thickness of the radar bright deposits

no evidence for thick deposits of ice at the lunar south pole the rim of the shackleton crater at the lunar south pole is a candidate crash site for nasa s lcross probe lunar crater observation and sensing satellite due to launch in 2008 the plume of debris kicked up by the crash is to be analysed in the hope that it will reveal the water thought to be there suggestions of lunar ice date from 1996 when data from the clementine spacecraft gave some indications of the presence of ice on crater walls at the south pole now using high resolution radar imagery the radar scattering parameter thought indicative of thick ice deposits has been found also to match radar echoes from the rock strewn walls and ejecta of young impact craters at all lunar latitudes there is no evidence for thick ice though there could be grains of water ice spread more thinly through the lunar soil shackleton crater at the moon s south pole has been suggested as a possible site of concentrated deposits of water ice on the basis of modelling of bi static radar polarization properties and interpretations of earlier earth based radar images1 2 this suggestion and parallel assumptions about other topographic cold traps is a significant element in planning for future lunar landings hydrogen enhancements have been identified in the polar regions3 but these data do not identify the host species or its local distribution the earlier earth based radar data lack the resolution and coverage for detailed studies of the relationship between radar scattering properties cold traps in permanently shadowed areas and local terrain features such as the walls and ejecta of small craters here we present new 20 m resolution 13 cm wavelength radar images that show no evidence for concentrated deposits of water ice in shackleton crater or elsewhere at the south pole the polarization properties normally associated with reflections from icy surfaces in the solar system4 5 6 were found at all the observed latitudes and are strongly correlated with the rock strewn walls and ejecta of young craters including the inner wall of shackleton there is no correlation between the polarization properties and the degree of solar illumination if the hydrogen enhancement observed by the lunar prospector orbiter3 indicates the presence of water ice then our data are consistent with the ice being present only as disseminated grains in the lunar regolith

bistatic radar observations of the moon using mini rf on lro and the arecibo observatory the miniature radio frequency mini rf instrument aboard nasa s lunar reconnaissance orbiter lro is a hybrid dual polarized synthetic aperture radar sar that operated in concert with the arecibo observatory to collect bistatic radar data of the lunar nearside from 2012 to 2015 the purpose of this bistatic campaign was to characterize the radar scattering properties of the surface and near surface as a function of bistatic angle for a variety of lunar terrains and search for a coherent backscatter opposition effect indicative of the presence of water ice a variety of lunar terrain types were sampled over a range of incidence and bistatic angles including mare highland pyroclastic crater ejecta and crater floor materials responses consistent with an opposition effect were observed for the ejecta of several copernican aged craters and the floor of the south polar crater cabeus the responses of ejecta material varied by crater in a manner that suggests a relationship with crater age the response for cabeus was observed within the portion of its floor that is not in permanent shadow the character of the response differs from that of crater ejecta and appears unique with respect to all other lunar terrains observed analysis of data for this region suggests that the unique nature of the response may indicate the presence of near surface deposits of water ice

modeling radar scattering from icy lunar regoliths at 13 cm and 4 cm wavelengths 1 two orbital synthetic aperture radars sars the chandrayaan 1 mini sar 13 cm wavelength and the lunar reconnaissance orbiter lro mini rf 13 and 4 2 cm wavelengths have been imaging the lunar surface searching for ice deposits in the polar permanently shadowed areas to understand the radar signatures of lunar polar ices an empirical two component model with parametric variations of the specular and diffuse components was developed and validated this model estimates scattering differences associated with slopes surface roughness thin regolith over ice and patches of ice lunar radar backscatter cross sections for the average surface for the chandrayaan 1 and lro instruments are estimated from the radar cross sections from the moon at 3 8 23 and 68 cm wavelengths measured in the 1960s at the massachusetts institute of technology this modeling predicts that enhanced diffuse scattering from near surface ice can be separated from rocks if the scattering is characterized by both the high reflectivity and circular polarization ratios cprs like those observed on mercury mars and the galilean satellites scattering from near surface ices covered by a thin regolith can be separated from rocks if the enhancement is twice the average or more if however the lunar ice is dispersed throughout the regolith as ice filling pores then scattering differences might be too small to detect preliminary validation using lro radar data for a few polar and midlatitude craters indicate that the observed cprs are consistent with our models for different regolith ice and roughness conditions

lunar cold traps effects of double shielding abstract this paper deals with the problem of water permanence on the surface of the moon possible zones where water ice can survive are called cold traps k watson b c murray and h brown 1961 j geophys res 66 3033 3045 these are zones of the moon permanently obscured where the temperatures are low enough to preserve ice for billions of years in this work we developed a model for the topographic temperatures of complex craters whose shape was approximated by a capsized frustum of a circular right cone double shaded areas were simulated by embedding a small hemispherical crater in the shadowed part of the previous one their temperatures were calculated using the r r hodges jr 1980 proc lunar planet sci conf 11th 2463 2477 model first we verified that our results were in agreement with those of previous models our results confirm those obtained by j r salvail and f p fanale 1994 icarus 111 441 445 and in agreement with hodges 1980 we found that the lowest temperatures are reached by tycho like craters that are the larger and shallower among the examined cases when small craters are embedded in the shaded area of larger ones their temperatures are low enough to preserve other volatiles like co 2 hodges 1980 in particular if we consider double shaded areas in biot like craters the temperatures are lower than 103 k in a shell of almost 20 around the poles thus allowing the preservation of ices for geometrical reasons a hemispherical crater embedded in the bottom of a biot like crater cold remain in the shadowed area for latitude values lower than those reached by an analogous crater embedded in sosigene or tycho like craters therefore the latitudinal radius of polar frost caps could be greater than that predicted by previous models that did not consider double shaded areas however double shielding occurs in only a fraction of the secondary craters therefore in this case eventual deposits of ice would be of smaller dimensions compared with the case of primary shielding analysis of the clementine radar data s nozette c l lichtenberg p spudis r bonner w ort e malaret m robinson and e m shoemaker 1996 science 274 5292 5300 and the lunar prospector neutron spectrometer data seems to be consistent with the presence of water ice in very low concentrations across a significant number of craters thus confirming the old hypothesis of watson et al 1961

images of surface volatiles in mercury s polar craters acquired by the messenger spacecraft images acquired by nasa s messenger spacecraft have revealed the morphology of frozen volatiles in mercury s permanently shadowed polar craters and provide insight into the mode of emplacement and evolution of the polar deposits the images show extensive spatially continuous regions with distinctive reflectance properties a site within prokofiev crater identified as containing widespread surface water ice exhibits a cratered texture that resembles the neighboring sunlit surface except for its uniformly higher reflectance indicating that the surficial ice was emplaced after formation of the underlying craters in areas where water ice is inferred to be present but covered by a thin layer of dark organic rich volatile material regions with uniformly lower reflectance extend to the edges of the shadowed areas and terminate with sharp boundaries the sharp boundaries indicate that the volatile deposits at mercury s poles are geologically young relative to the time scale for lateral mixing by impacts and either are restored at the surface through an ongoing process or were delivered to the planet recently

a revised control network for mercury an improved control net for mercury has been completed by utilizing images acquired during the three mariner 10 flybys in 1974 1975 relative positional errors within the net are 1 km on average and absolute locations are estimated to be better than 25 km the analytical triangulation resulted in new values for focal lengths camera a 1493 6 mm camera b 1500 1 mm w0 329 548 and camera orientation angles for 811 images acquired during all three flybys of mercury

circular polarization radar properties of high altitude ice western kunlun shan and central andes full polarization synthetic aperture radar images of high altitude glaciers and ice fields in the western kunlun shan of tibet and the central andes near santiago chile made with the shuttle imaging radar display regions with circular polarization ratios c in excess of unity the mountainous topography at both locales allows reconstruction of the angular scattering behavior the behavior is very similar to that displayed by greenland s percolation zone glaciologic evidence from the tibetan site confirms ice pipes and lenses embedded in the firn which have been identified as the scatterers producing c 1 in greenland this demonstrates that the cold snow percolation facies radar signature exists outside of greenland the greenland radar behavior had been suggested as an analogue for that of the icy galilean satellites we like previous workers find that the near nadir backscatter is too specular to be considered analogous to that of the jovian moons we also present a crevasse region in tibet that exhibits c 1 and a possible detection of the ice substrate of a rock covered glacier in the andes

understanding the origin and evolution of water in the moon through lunar sample studies a paradigm shift has recently occurred in our knowledge and understanding of water in the lunar interior this has transpired principally through continued analysis of returned lunar samples using modern analytical instrumentation while these recent studies have undoubtedly measured indigenous water in lunar samples they have also highlighted our current limitations and some future challenges that need to be overcome in order to fully understand the origin distribution and evolution of water in the lunar interior another exciting recent development in the field of lunar science has been the unambiguous detection of water or water ice on the surface of the moon through instruments flown on a number of orbiting spacecraft missions considered together sample based studies and those from orbit strongly suggest that the moon is not an anhydrous planetary body as previously believed new observations and measurements support the possibility of a wet lunar interior and the presence of distinct reservoirs of water on the lunar surface furthermore an approach combining measurements of water abundance in lunar samples and its hydrogen isotopic composition has proved to be of vital importance to fingerprint and elucidate processes and source s involved in giving rise to the lunar water inventory a number of sources are likely to have contributed to the water inventory of the moon ranging from primordial water to meteorite derived water ice through to the water formed during the reaction of solar wind hydrogen with the lunar soil perhaps two of the most striking findings from these recent studies are the revelation that at least some portions of the lunar interior are as water rich as some mid ocean ridge basalt source regions on earth and that the water in the earth and the moon probably share a common origin

characterization of the morphometry of impact craters hosting polar deposits in mercury s north polar region 1 earth based radar images of mercury show radar bright material inside impact craters near the planet s poles a previous study indicated that the polar deposit hosting craters pdcs at mercury s north pole are shallower than craters that lack such deposits we use data acquired by the mercury laser altimeter on the messenger spacecraft during 11 months of orbital observations to revisit the depths of craters at high northern latitudes on mercury we measured the depth and diameter of 537 craters located poleward of 45 n evaluated the slopes of the northern and southern walls of 30 pdcs and assessed the floor roughness of 94 craters including nine pdcs we find that the pdcs appear to have a fresher crater morphology than the non pdcs and that the radar bright material has no detectable influence on crater depths wall slopes or floor roughness the statistical similarity of crater depth diameter relations for the pdc and non pdc populations places an upper limit on the thickness of the radar bright material 170 m for a crater 11 km in diameter that can be refined by future detailed analysis results of the current study are consistent with the view that the radar bright material constitutes a relatively thin layer emplaced preferentially in comparatively young craters

radar mapping of mercury s polar anomalies ground based radar observations of mercury have revealed unusually strong highly depolarized echoes from the north1 2 and south2 poles these anomalous echoes have been cited as evidence of polar ice deposits1 5 thermal studies3 5 suggest that the permanently shaded floors of large polar craters are cold enough to preserve water ice in a stable state over aeons in spite of mercury s proximity to the sun here we present high resolution radar maps of mercury s polar regions derived from delay doppler measurements we have resolved the north and south polar anomalies into numerous crater sized features and we have been able to identify the source craters for many of these features after making small corrections to the pole positions on the mariner 10 images the coincidence with crater locations together with other properties of the radar features are consistent with the polar ice hypothesis

evidence for water ice on the moon results for anomalous polar craters from the lro mini rf imaging radar 1 the mini rf radar instrument on the lunar reconnaissance orbiter spacecraft mapped both lunar poles in two different rf wavelengths complete mapping at 12 6 cm s band and partial mapping at 4 2 cm x band in two look directions removing much of the ambiguity of previous earth and spacecraft based radar mapping of the moon s polar regions the poles are typical highland terrain showing expected values of radar cross section albedo and circular polarization ratio cpr most fresh craters display high values of cpr in and outside the crater rim the pattern of these cpr distributions is consistent with high levels of wavelength scale surface roughness associated with the presence of block fields impact melt flows and fallback breccia a different class of polar crater exhibits high cpr only in their interiors interiors that are both permanently dark and very cold less than 100 k application of scattering models developed previously suggests that these anomalously high cpr deposits exhibit behavior consistent with the presence of water ice if this interpretation is correct then both poles may contain several hundred million tons of water in the form of relatively clean ice all within the upper couple of meters of the lunar surface the existence of significant water ice deposits enables both long term human habitation of the moon and the creation of a permanent cislunar space transportation system based upon the harvest and use of lunar propellant