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Photogeologic principles can be used to suggest possible sequences of events that result in the present planetary surface. The most common method of evaluating the absolute age of a planetary surface remotely is to count the number of impact craters that have occurred after the surface formed, with the assumption that the craters occur in a spatially random fashion over time. Using additional assumptions, craters that have been partially modified by later geologic activity can be used to assess the time frames for an interpreted sequence of events. The total number of craters on Venus is low and the spatial distribution taken by itself is nearly indistinguishable from random. The overall implication is that the Venusian surface is much closer to Earth in its youthfulness than the other, smaller inner solar system bodies. There are differing interpretations of the extent to which volcanism and tectonics have modified the craters and of the regional and global sequences of geologic events. Consequently, a spectrum of global resurfacing views has emerged. These range from a planet that has evolved to have limited current volcanism and tectonics concentrated in a few zones to a planet with Earth-like levels of activity occurring everywhere at similar rates but in different ways. Analyses of the geologic record have provided observations that are challenging to reconcile with either of the endmember views. The interpretation of a global evolution with time in the nature of geologic activity relies on assumptions that have been challenged, but there are other observations of areally extensive short-lived features such as canali that are challenging to reconcile with a view of different regions evolving independently. Future data, especially high-resolution imaging and topography, can provide the details to resolve some of the issues. These different global-evolution viewpoints must tie to assessments of present-day volcanic and tectonic activity levels that can be made with the data from upcoming missions., Space Science Reviews, 219 (4), ISSN:1572-9672, ISSN:0038-6308

Geomorphic and geophysical evidence supports a debris-covered glacier origin for a suite of landforms at the mid-latitudes of Mars, including lobate debris aprons (LDA), lineated valley fill (LVF), and concentric crater fill (CCF). These large reservoirs of ice and their near-surface structure provide a rich record for understanding the planet's climate and history of global volatile exchange over the past billion years. LDA, LVF, and CCF are also potential sites for future robotic and human missions but the accessibility of glacial ice for direct sampling and in situ resource utilization depends largely on the geotechnical properties of the surface debris (“supraglacial debris”), including its thickness, grain sizes, and density structure. The physical properties of this supraglacial debris layer have been poorly constrained. We use images of morphology, digital elevation models, thermal inertia data, and radar sounding data to probe the near surface of LDA, LVF, and CCF in Deuteronilus Mensae in order to place constraints on the sources, grain sizes, thickness, and stratigraphy of supraglacial debris. We find evidence for at least a two-layer stratigraphy. Layered mantle consisting of atmospherically emplaced dust and ice superposes boulder-rich sediment sourced by rockfalls glacially transported downslope. High thermal inertia, boulder-rich termini and debris bands reminiscent of medial moraines are found throughout the study region, supporting a rockfall origin for at least a fraction of the debris exposed at the surface. This supraglacial debris layer would have thickened with time from sublimation of glacial ice and liberation of englacial sediment and dust. At present, the entire supraglacial debris package is a minimum of a few meters in thickness and is likely tens of meters in thickness in many locations, possibly thinning regionally at lower latitudes and locally thinning toward the headwalls. The lack of terracing or interior structures in craters formed within LDA, LVF, and CCF and the absence of near-surface reflectors in SHARAD radar data further suggest that no strong contrasts in permittivity or strength occur at the interface of the layers.

On Venus, relatively young deposits near volcanic and coronal summits with unique radar characteristics have been proposed to be emplaced by pyroclastic density currents (PDCs). The proposed units ...

In its polar layered deposits (PLD), Mars possesses a record of its recent climate, analogous to terrestrial ice sheets containing climate records on Earth. Each PLD is greater than 2 ​km thick and contains thousands of layers, each containing information on the climatic and atmospheric state during its deposition, creating a climate archive. With detailed measurements of layer composition, it may be possible to extract age, accumulation rates, atmospheric conditions, and surface activity at the time of deposition, among other important parameters; gaining the information would allow us to “read” the climate record. Because Mars has fewer complicating factors than Earth (e.g. oceans, biology, and human-modified climate), the planet offers a unique opportunity to study the history of a terrestrial planet’s climate, which in turn can teach us about our own planet and the thousands of terrestrial exoplanets waiting to be discovered. During a two-part workshop, the Keck Institute for Space Studies (KISS) hosted 38 Mars scientists and engineers who focused on determining the measurements needed to extract the climate record contained in the PLD. The group converged on four fundamental questions that must be answered with the goal of interpreting the climate record and finding its history based on the climate drivers. The group then proposed numerous measurements in order to answer these questions and detailed a sequence of missions and architecture to complete the measurements. In all, several missions are required, including an orbiter that can characterize the present climate and volatile reservoirs; a static reconnaissance lander capable of characterizing near surface atmospheric processes, annual accumulation, surface properties, and layer formation mechanism in the upper 50 ​cm of the PLD; a network of SmallSat landers focused on meteorology for ground truth of the low-altitude orbiter data; and finally, a second landed platform to access ~500 ​m of layers to measure layer variability through time. This mission architecture, with two landers, would meet the science goals and is designed to save costs compared to a single very capable landed mission. The rationale for this plan is presented below. In this paper we discuss numerous aspects, including our motivation, background of polar science, the climate science that drives polar layer formation, modeling of the atmosphere and climate to create hypotheses for what the layers mean, and terrestrial analogs to climatological studies. Finally, we present a list of measurements and missions required to answer the four major questions and read the climate record. 1. What are present and past fluxes of volatiles, dust, and other materials into and out of the polar regions? 2. How do orbital forcing and exchange with other reservoirs affect those fluxes? 3. What chemical and physical processes form and modify layers? 4. What is the timespan, completeness, and temporal resolution of the climate history recorded in the PLD?

The Radar Imager for Mars’ Subsurface Experiment (RIMFAX) is a Ground Penetrating Radar on the Mars 2020 mission’sPerseverancerover, which is planned to land near a deltaic landform in Jezero crater. RIMFAX will add a new dimension to rover investigations of Mars by providing the capability to image the shallow subsurface beneath the rover. The principal goals of the RIMFAX investigation are to image subsurface structure, and to provide information regarding subsurface composition. Data provided by RIMFAX will aid Perseverance’s mission to explore the ancient habitability of its field area and to select a set of promising geologic samples for analysis, caching, and eventual return to Earth. RIMFAX is a Frequency Modulated Continuous Wave (FMCW) radar, which transmits a signal swept through a range of frequencies, rather than a single wide-band pulse. The operating frequency range of 150–1200 MHz covers the typical frequencies of GPR used in geology. In general, the full bandwidth (with effective center frequency of 675 MHz) will be used for shallow imaging down to several meters, and a reduced bandwidth of the lower frequencies (center frequency 375 MHz) will be used for imaging deeper structures. The majority of data will be collected at regular distance intervals whenever the rover is driving, in each of the deep, shallow, and surface modes. Stationary measurements with extended integration times will improve depth range and SNR at select locations. The RIMFAX instrument consists of an electronic unit housed inside the rover body and an antenna mounted externally at the rear of the rover. Several instrument prototypes have been field tested in different geological settings, including glaciers, permafrost sediments, bioherme mound structures in limestone, and sedimentary features in sand dunes. Numerical modelling has provided a first assessment of RIMFAX’s imaging potential using parameters simulated for the Jezero crater landing site.

Lucus Planum, extending for a radius of approximately 500 km around 181° E, 5° S, is part of the Medusae Fossae Formation (MFF), a set of several discontinuous deposits of fine-grained, friable material straddling across the Martian highland-lowland boundary. The MFF has been variously hypothesized to consist of pyroclastic flows, pyroclastic airfall, paleopolar deposits, or atmospherically-deposited icy dust driven by climate cycles. MARSIS, a low-frequency subsurface-sounding radar carried by ESA's Mars Express, acquired 238 radar swaths across Lucus Planum, providing sufficient coverage for the study of its internal structure and dielectric properties. Subsurface reflections were found only in three areas, marked by a distinctive surface morphology, while the central part of Lucus Planum appears to be made of radar-attenuating material preventing the detection of basal echoes. The bulk dielectric properties of these areas were estimated and compared with those of volcanic rocks and ice-dust mixtures. Previous interpretations that east Lucus Planum and the deposits on the north-western flanks of Apollinaris Patera consist of high-porosity pyroclastic material are strongly supported by the new results. The north-western part of Lucus Planum is likely to be much less porous, although interpretations about the nature of the subsurface materials are not conclusive. The exact origin of the deposits cannot be constrained by radar data alone, but our results for east Lucus Planum are consistent with an overall pyroclastic origin, likely linked to Tharsis Hesperian and Amazonian activity.

The Mini-RF radar, launched on the Lunar Reconnaissance Orbiter, imaged the lunar surface using hybrid-polarimetric, transmitting one circular polarization and receiving linear H and V polarizations. Earth-based radar operating at the same frequency has acquired data of the same terrains using circular-polarized transmit waves and sampling circular polarizations. For lunar targets where the viewing geometry is nearly the same, the polarimetry derived from Mini-RF and the earth-based data should be very similar. However, we have discovered that there is a considerable difference in circular polarization ratio (CPR) values between the two data sets. We investigate possible causes for this discrepancy, including cross-talk between channels, sampling, and the ellipticity of the Mini-RF transmit wave. We find that none of these can reproduce the observed CPR differences, though a nonlinear block adaptive quantization function used to compress the data will significantly distort some other polarimetry products. A comparison between earth-based data sets acquired using two different sampling modes (sampling received linear polarizations and sampling circular polarizations) suggests that the CPR differences may be partially due to sampling the data in a different receive polarimetry bases.

We measured the length of the Venus sidereal day (LOD) from Earth-based radar observations collected from 1988 to 2017, using offsets in surface feature longitudes from a prediction based on a 243.0185d period derived from analysis of Magellan mission images over a 487-day interval. We derive a mean LOD over 29 years of 243.0212 ± 0.0006d. Our result is consistent with earlier estimates (but with smaller uncertainties), including those based on offsets between Venus Express infrared mapping data and Magellan topography that suggest a mean LOD of 243.0228 ± 0.002d over a 16-year interval. We cannot detect subtle, short-term oscillations in rate, but the derived value provides an excellent fit to observational data over a 29-year period that can be used for future landing-site planning.

SESAR (Space Exploration Synthetic Aperture Radar) is a next generation P-band beamforming radar instrument concept that will enable a new class of observations suitable to meet Decadal Survey science goals for planetary exploration. The radar operates at full polarimetry and fine (meter scale) resolution, and achieves beam agility through programmable transmit waveforms and digital beamforming on receive. The radar is based on a low power, lightweight design approach conceived to meet the stringent planetary instrument requirements. This instrument concept has the potential to provide unprecedented surface and near-subsurface measurements of planetary bodies including the Moon, Mars, and asteroids.

The Space Exploration Synthetic Aperture Radar (SESAR) is an advanced P-band beamforming radar instrument concept to enable a new class of observations suitable to meet multiple Decadal Survey science goals for planetary exploration. The radar is capable of providing unprecedented surface and near-subsurface measurements at full polarimetry and fine (meter scale) resolution, and achieves beam agility through programmable waveform generation and digital beamforming. The radar's highly flexible modular architecture employs a novel low power, lightweight design approach to meet stringent planetary instrument requirements, all while minimizing cost and development time.

We use orbital SHAllow RADar (SHARAD) sounder data to three-dimensionally visualize the subsurface structure of Elysium Planitia, the youngest volcanic province on Mars. Our results reveal an emplacement history consisting of multiple groups of overlapping lava flow units, originating from different sources. The uniquely complex “radar stratigraphy” of Elysium Planitia, relative to other volcanic regions, requires a distinct mechanism to generate the numerous reflectors observed in SHARAD data. Sedimentary deposits interbedded with successive batches of lava flows could account for the elaborate pattern of reflectors. We infer that widespread, rapidly emplaced material sourced from the enigmatic Medusae Fossae Formation (MFF) creates these sedimentary layers. This implies that episodes of atmospheric activity, perhaps linked with the obliquity of Mars, periodically erode and redeposit material from the MFF across a large region.

We combine Earth-based radar maps of Venus from the 1988 and 2012 inferior conjunctions, which had similar viewing geometries. Processing of both datasets with better image focusing and co-registration techniques, and summing over multiple looks, yields maps with 1–2 km spatial resolution and improved signal to noise ratio, especially in the weaker same-sense circular (SC) polarization. The SC maps are unique to Earth-based observations, and offer a different view of surface properties from orbital mapping using same-sense linear (HH or VV) polarization. Highland or tessera terrains on Venus, which may retain a record of crustal differentiation and processes occurring prior to the loss of water, are of great interest for future spacecraft landings. The Earth-based radar images reveal multiple examples of tessera mantling by impact “parabolas” or “haloes”, and can extend mapping of locally thick material from Magellan data by revealing thinner deposits over much larger areas. Of particular interest is an ejecta deposit from Stuart crater that we infer to mantle much of eastern Alpha Regio. Some radar-dark tessera occurrences may indicate sediments that are trapped for longer periods than in the plains. We suggest that such radar information is important for interpretation of orbital infrared data and selection of future tessera landing sites.

Radar-bright deposits on Venus that have diffuse margins suggest eruptions that distribute debris over large areas due to ground-hugging flows from plume collapse. We examine deposits in Eastern Eistla, Western Eistla, Phoebe, and Dione Regiones using Magellan data and Earth-based radar maps. The radar-bright units have no marginal lobes or other features consistent with viscous flow. Their morphology, radar echo strength, polarization properties, and microwave emissivity are consistent with mantling deposits comprised of few-cm or larger clasts. This debris traveled downhill up to ~100 km on modest slopes, and blanketed lava flows and tectonic features to depths of tens of cm to a few meters over areas up to 40×10(3) km(2). There is evidence for ongoing removal and exhumation of previously buried terrain. A newly identified occurrence is associated with a ridge belt south of Ushas Mons. We also note radar-bright streaks of coarse material west of Rona Chasma that reflect the last traces of a deposit mobilized by winds from the formation of Mirabeau crater. If the radar-bright units originate by collapse of eruption columns, with coarse fragmental material entrained and fluidized by hot gases, then their extent suggests large erupted volatile (CO(2) or H(2)O) amounts. We propose that these deposits reflect the early stage of renewed magmatic activity, with volatile-rich, disrupted magma escaping through vents in fractured regions of the upper crust. Rapidly eroding under Venus surface conditions, or buried by subsequent eruptions, these markers of recently renewed activity have disappeared from older regions.

The Space Exploration Synthetic Aperture Radar (SESAR) is an advanced P-band beamforming radar instrument concept to enable a new class of observations suitable to meet Decadal Survey science goals for planetary exploration. The radar operates at full polarimetry and fine (meter scale) resolution, and achieves beam agility through programmable waveform generation and digital beamforming. The radar architecture employs a novel low power, lightweight design approach to meet stringent planetary instrument requirements. This instrument concept has the potential to provide unprecedented surface and near-subsurface measurements applicable to multiple Decadal Survey Science Goals.

Venus should be the most Earth-like of all our planetary neighbours: its size, bulk composition and distance from the Sun are very similar to those of Earth. How and why did it all go wrong for Venus? What lessons can be learned about the life story of terrestrial planets in general, in this era of discovery of Earth-like exoplanets? Were the radically different evolutionary paths of Earth and Venus driven solely by distance from the Sun, or do internal dynamics, geological activity, volcanic outgassing and weathering also play an important part? EnVision is a proposed ESA Medium class mission designed to take Earth Observation technology to Venus to measure its current rate of geological activity, determine its geological history, and the origin and maintenance of its hostile atmosphere, to understand how Venus and Earth could have evolved so differently. EnVision will carry three instruments: the Venus Emission Mapper (VEM); the Subsurface Radar Sounder (SRS); and VenSAR, a world-leading European phased array synthetic aperture radar that is the subject of this article. VenSAR will obtain images at a range of spatial resolutions from 30 m regional coverage to 1 m images of selected areas; an improvement of two orders of magnitude on Magellan images; measure topography at 15 m resolution vertical and 60 m spatially from stereo and InSAR data; detect cm-scale change through differential InSAR, to characterise volcanic and tectonic activity, and estimate rates of weathering and surface alteration; and characterise of surface mechanical properties and weathering through multi-polar radar data. These data will be directly comparable with Earth Observation radar data, giving geoscientists unique access to an Earth-sized planet that has evolved on a radically different path to our own, offering new insights on the Earth-sized exoplanets across the galaxy.

The Tharsis region of Mars is covered in volcanic flows that can stretch for tens to hundreds of kilometers. Radar measurements of the dielectric properties of these flows can provide information regarding their composition and density. SHARAD (shallow radar), a sounding radar on the Mars Reconnaissance Orbiter, detects basal interfaces beneath flows in some areas of Tharsis northwest and west of Ascraeus Mons, with additional detections south of Pavonis Mons. Comparisons with 12.6 cm ground-based radar images suggest that SHARAD detects basal interfaces primarily in dust or regolith-mantled regions. We use SHARAD data to estimate the real relative permittivity of the flows by comparing the measured time delay of returns from the subsurface with altimetry measurements of the flow heights relative to the surrounding plains. In cases where the subsurface interface is visible at different depths, spanning tens of meters, it is also possible to measure the loss tangent (tan δ) of the material. The permittivity values calculated range from 7.6 to 11.6, with an average of 9.6, while the mean loss tangent values range from 7.8 × 10−3 to 2.9 × 10−2 with an average of 1.0 × 10−2. These permittivity and loss tangent estimates for the flows northwest of Ascraeus Mons, west of Ascraeus Mons, and south of Pavonis Mons are consistent with the lab-measured values for dense, low-titanium basalt.

We present a new empirical constraint on the rate of breakdown of large ejecta blocks on the Moon based on observations from the Lunar Reconnaissance Orbiter (LRO) Diviner thermal radiometer. We find that the rockiness of fresh crater ejecta can be quantified using the Diviner-derived rock abundance data set, and we present a strong inverse correlation between the 95 th percentile value of the ejecta rock abundance (RA 95/5 ) and crater age. For nine craters with published model ages derived from crater counts on their continuous ejecta, RA 95/5 decreases with crater age, as (age [m.y.]) −0.46 . This result implies shorter rock survival times than predicted based on downward extrapolation of 100 m crater size-frequency distributions, and represents a new empirical constraint on the rate of comminution of large rocks not previously analyzed experimentally or through direct observation. In addition, our result provides a new method for dating young lunar craters.

[1] We present a technique for estimating Mars topographic roughness on horizontal scales from about 10 m to 100 m using Shallow Radar (SHARAD) sounding data. Our results offer a view of surface properties complementary to Mars Orbiter Laser Altimeter (MOLA) pulse-width or baseline roughness maps and can be compared to SHARAD peak-echo properties to infer deviations from the average near-surface density. Latitudinal averaging of SHARAD-derived roughness over Arabia and Noachis Terrae shows good agreement with MOLA-derived roughness and provides clear evidence for latitude-dependent mantling deposits previously inferred from image data. In northwestern Gordii Dorsum, we find that bulk density in at least the upper few meters is significantly lower than in other units of the Medusae Fossae Formation. We observe the same behavior indicative of low near-surface density in wind-eroded crater fill in the southern highlands. Combining surface-properties analysis, subsurface sounding, and high-resolution optical images, we show that the Pavonis Mons fan-shaped deposit differs significantly from lobate debris aprons which SHARAD has shown to be ice-cored. There are no internal radar reflections from the smooth-facies portion of the Pavonis Mons fan-shaped deposit, and we suggest that these deposits are either quite thin or have little dielectric (i.e., density) contrast with the underlying terrain. Future application of these techniques can identify other low-density units across Mars, assist in the mapping of regional volatile-rich mantling units, and provide new constraints on the physical properties of the polar layered terrain.

We map the subsurface structure of Planum Boreum using sounding data from the Shallow Radar (SHARAD) instrument onboard the Mars Reconnaissance Orbiter. Radar coverage throughout the 1,000,000-km2 area reveals widespread reflections from basal and internal interfaces of the north polar layered deposits (NPLD). A dome-shaped zone of diffuse reflectivity up to 12 μs (∼1-km thick) underlies two-thirds of the NPLD, predominantly in the main lobe but also extending into the Gemina Lingula lobe across Chasma Boreale. We equate this zone with a basal unit identified in image data as Amazonian sand-rich layered deposits [Byrne, S., Murray, B.C., 2002. J. Geophys. Res. 107, 5044, 12 pp. doi:10.1029/2001JE001615; Fishbaugh, K.E., Head, J.W., 2005. Icarus 174, 444–474; Tanaka, K.L., Rodriguez, J.A.P., Skinner, J.A., Bourke, M.C., Fortezzo, C.M., Herkenhoff, K.E., Kolb, E.J., Okubo, C.H., 2008. Icarus 196, 318–358]. Elsewhere, the NPLD base is remarkably flat-lying and co-planar with the exposed surface of the surrounding Vastitas Borealis materials. Within the NPLD, we delineate and map four units based on the radar-layer packets of Phillips et al. [Phillips, R.J., and 26 colleagues, 2008. Science 320, 1182–1185] that extend throughout the deposits and a fifth unit confined to eastern Gemina Lingula. We estimate the volume of each internal unit and of the entire NPLD stack (821,000 km3), exclusive of the basal unit. Correlation of these units to models of insolation cycles and polar deposition [Laskar, J., Levrard, B., Mustard, J.F., 2002. Nature 419, 375–377; Levrard, B., Forget, F., Montmessin, F., Laskar, J., 2007. J. Geophys. Res. 112, E06012, 18 pp. doi:10.1029/2006JE002772] is consistent with the 4.2-Ma age of the oldest preserved NPLD obtained by Levrard et al. [Levrard, B., Forget, F., Montmessin, F., Laskar, J., 2007. J. Geophys. Res. 112, E06012, 18 pp. doi:10.1029/2006JE002772]. We suggest a dominant layering mechanism of dust–content variation during accumulation rather than one of lag production during periods of sublimation.

The SHARAD (shallow radar) sounding radar on the Mars Reconnaissance Orbiter detects subsurface reflections in the eastern and western parts of the Medusae Fossae Formation (MFF). The radar waves penetrate up to 580 m of the MFF and detect clear subsurface interfaces in two locations: west MFF between 150 and 155° E and east MFF between 209 and 213° E. Analysis of SHARAD radargrams suggests that the real part of the permittivity is ∼3.0, which falls within the range of permittivity values inferred from MARSIS data for thicker parts of the MFF. The SHARAD data cannot uniquely determine the composition of the MFF material, but the low permittivity implies that the upper few hundred meters of the MFF material has a high porosity. One possibility is that the MFF is comprised of low-density welded or interlocked pyroclastic deposits that are capable of sustaining the steep-sided yardangs and ridges seen in imagery. The SHARAD surface echo power across the MFF is low relative to typical martian plains, and completely disappears in parts of the east MFF that correspond to the radar-dark Stealth region. These areas are extremely rough at centimeter to meter scales, and the lack of echo power is most likely due to a combination of surface roughness and a low near-surface permittivity that reduces the echo strength from any locally flat regions. There is also no radar evidence for internal layering in any of the SHARAD data for the MFF, despite the fact that tens-of-meters scale layering is apparent in infrared and visible wavelength images of nearby areas. These interfaces may not be detected in SHARAD data if their permittivity contrasts are low, or if the layers are discontinuous. The lack of closely spaced internal radar reflectors suggests that the MFF is not an equatorial analog to the current martian polar deposits, which show clear evidence of multiple internal layers in SHARAD data.

The Shallow Radar (SHARAD) on the Mars Reconnaissance Orbiter has imaged the internal stratigraphy of the north polar layered deposits of Mars. Radar reflections within the deposits reveal a laterally continuous deposition of layers, which typically consist of four packets of finely spaced reflectors separated by homogeneous interpacket regions of nearly pure ice. The packet/interpacket structure can be explained by approximately million-year periodicities in Mars' obliquity or orbital eccentricity. The observed ∼100-meter maximum deflection of the underlying substrate in response to the ice load implies that the present-day thickness of an equilibrium elastic lithosphere is greater than 300 kilometers. Alternatively, the response to the load may be in a transient state controlled by mantle viscosity. Both scenarios probably require that Mars has a subchondritic abundance of heat-producing elements.

We describe new 70-cm wavelength radar images of the lunar near-side and limb regions obtained via a synthetic-aperture-radar patch-focusing reduction technique. The data are obtained by transmitting a circularly polarized pulsed waveform from the Arecibo telescope in Puerto Rico and receiving the echo in both senses of circular polarization with the Robert C. Byrd Green Bank Telescope in West Virginia. The resultant images in both polarizations have a spatial resolution as fine as 320 m 450 m near the lunar limb. The patch-focusing technique is a computationally efficient method for compensating for range migration and Doppler (azimuth) smearing over long coherence times, i.e., 983 s, which is needed to achieve the required Doppler resolution. Three to nine looks are averaged for speckle reduction and to improve the signal-to-noise ratio. At this long wavelength, the radar signal penetrates up to several tens of meters into the dry lunar surface materials, thus revealing details of the bulk loss properties and decimeter-scale rock abundance not evident in multispectral and other remote-sensing data. Application of the new radar images to the analysis of basalt flow complexes in Mare Serenitatis shows that the long-wavelength radar data are sensitive to differences in both flow age and composition, and may be particularly useful for studies of smaller deposits that do not have robust crater statistics. The new 70-cm lunar radar data are archived at the National Aeronautics and Space Administration Planetary Data System.

The equatorial Medusae Fossae Formation (MFF) is enigmatic and perhaps among the youngest geologic deposits on Mars. They are thought to be composed of volcanic ash, eolian sediments, or an ice-rich material analogous to polar layered deposits. The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument aboard the Mars Express Spacecraft has detected nadir echoes offset in time-delay from the surface return in orbits over MFF material. These echoes are interpreted to be from the subsurface interface between the MFF material and the underlying terrain. The delay time between the MFF surface and subsurface echoes is consistent with massive deposits emplaced on generally planar lowlands materials with a real dielectric constant of ∼2.9 ± 0.4. The real dielectric constant and the estimated dielectric losses are consistent with a substantial component of water ice. However, an anomalously low-density, ice-poor material cannot be ruled out. If ice-rich, the MFF must have a higher percentage of dust and sand than polar layered deposits. The volume of water in an ice-rich MFF deposit would be comparable to that of the south polar layered deposits.

We have compared radio images from the Australia Telescope Compact Array and X-ray images from ROSAT of an area about 560 pc in diameter around the very extended emission nebula N19 on the southwestern edge of the bright ridge of the Small Magellanic Cloud. These data have allowed us to identify tentatively seven supernova remnants in this region. This number is large enough to suggest that the area has recently undergone some starburst activity. N19 itself appears to contain two overlapping supernova remnants within the extended H II region.

The Mini-RF radar on the Lunar Reconnaissance Orbiter's spacecraft has revealed a great variety of crater ejecta flow and impact melt deposits, some of which were not observed in prior radar imaging. The craters Tycho and Glushko have long melt flows that exhibit variations in radar backscatter and circular polarization ratio along the flow. Comparison with optical imaging reveals that these changes are caused by features commonly seen in terrestrial lava flows, such as rafted plates, pressure ridges, and ponding. Small (less than 20 km) sized craters also show a large variety of features, including melt flows and ponds. Two craters have flow features that may be ejecta flows caused by entrained debris flowing across the surface rather than by melted rock. The circular polarization ratios (CPRs) of the impact melt flows are typically very high; even ponded areas have CPR values between 0.7-1.0. This high CPR suggests that deposits that appear smooth in optical imagery may be rough at centimeter- and decimeter- scales. In some places, ponds and flows are visible with no easily discernable source crater. These melt deposits may have come from oblique impacts that are capable of ejecting melted material farther downrange. They may also be associated with older, nearby craters that no longer have a radar-bright proximal ejecta blanket. The observed morphology of the lunar crater flows has implications for similar features observed on Venus. In particular, changes in backscatter along many of the ejecta flows are probably caused by features typical of lava flows.

Shallow Radar soundings from the Mars Reconnaissance Orbiter reveal a buried deposit of carbon dioxide (CO(2)) ice within the south polar layered deposits of Mars with a volume of 9500 to 12,500 cubic kilometers, about 30 times that previously estimated for the south pole residual cap. The deposit occurs within a stratigraphic unit that is uniquely marked by collapse features and other evidence of interior CO(2) volatile release. If released into the atmosphere at times of high obliquity, the CO(2) reservoir would increase the atmospheric mass by up to 80%, leading to more frequent and intense dust storms and to more regions where liquid water could persist without boiling.

[1] On 9 October 2009 the Lunar Crater Observation and Sensing Satellite (LCROSS) impacted Cabeus crater, located near the south pole of the Moon. Prior to that impact, the Mini-RF instruments on ISRO's Chandrayaan-1 and NASA's Lunar Reconnaissance Orbiter (LRO) obtained S band (12.6 cm) synthetic aperture radar images of the impact site at 150 and 30 m resolution, respectively. These observations show that the floor of Cabeus has a circular polarization ratio (CPR) comparable to or less than the average of nearby terrain in the southern lunar highlands. Furthermore

Shallow Radar soundings from the Mars Reconnaissance Orbiter reveal a buried deposit of carbon dioxide (CO2) ice within the south polar layered deposits of Mars with a volume of 9500 to 12,500 cubic kilometers, about 30 times that previously estimated for the south pole residual cap. The deposit occurs within a stratigraphic unit that is uniquely marked by collapse features and other evidence of interior CO2 volatile release. If released into the atmosphere at times of high obliquity, the CO2 reservoir would increase the atmospheric mass by up to 80%, leading to more frequent and intense dust storms and to more regions where liquid water could persist without boiling.

[1] We present new polarimetric radar data for the surface of the north pole of the Moon acquired with the Mini-SAR experiment onboard India's Chandrayaan-1 spacecraft. Between mid-February and mid-April, 2009, Mini-SAR mapped more than 95% of the areas polewards of 80° latitude at a resolution of 150 meters. The north polar region displays backscatter properties typical for the Moon, with circular polarization ratio (CPR) values in the range of 0.1–0.3, increasing to over 1.0 for young primary impact craters. These higher CPR values likely reflect surface roughness associated with these fresh features. In contrast, some craters in this region show elevated CPR in their interiors, but not exterior to their rims. Almost all of these features are in permanent sun shadow and correlate with proposed locations of polar ice modeled on the basis of Lunar Prospector neutron data. These relations are consistent with deposits of water ice in these craters.

[1] The SHARAD instrument on the Mars Reconnaissance Orbiter detects subsurface interfaces beneath lava flow fields northwest of Ascraeus Mons. The interfaces occur in two locations; a northern flow that originates south of Alba Patera, and a southern flow that originates at the rift zone between Ascraeus and Pavonis Montes. The northern flow has permittivity values, estimated from the time delay of echoes from the basal interface, between 6.2 and 17.3, with an average of 12.2. The southern flow has permittivity values of 7.0 to 14.0, with an average of 9.8. The average permittivity values for the northern and southern flows imply densities of 3.7 and 3.4 g cm ―3 , respectively. Loss tangent values for both flows range from 0.01 to 0.03. The measured bulk permittivity and loss tangent values are consistent with those of terrestrial and lunar basalts, and represent the first measurement of these properties for dense rock on Mars.

[1] Basaltic volcanism is widespread on the lunar nearside, and returned samples suggest that the mare-forming magmas had low viscosity that led to primarily sheet-like deposits. New 70-cm wavelength radar observations that probe several meters beneath the lunar surface reveal differences in mare backscatter properties not explained by age or compositional variations. We interpret areas of high backscatter and high circular polarization ratio in Maria Serenitatis, Imbrium, and Crisium as having an enhanced abundance of decimeter-scale subsurface rocks relative to typical mare-forming flows. The 3.5 b.y survival of these differences implies an initial platy, blocky, or ridged lava flow surface layer with thickness of at least 3–5 m. Such rugged morphology might arise from episodic changes in magma effusion rate, as observed for disrupted flood basalt surfaces on the Earth and Mars, very high flow velocities, or increased viscosity due to a number of factors. Significant information on lunar mare eruption conditions may thus be obtained from long-wavelength radar probing of the shallow subsurface.

[1] We use polarimetric radar observations to study the distribution, depth, and embedded rock abundance of nearside lunar pyroclastic deposits. Radar images were obtained for Mare Vaporum and the southern half of Mare Serenitatis; the imaged areas contain the large Rima Bode, Mare Vaporum, Sulpicius Gallus, and Taurus-Littrow pyroclastic deposits. Potential pyroclastic deposits at Rima Hyginus crater, the Tacquet Formation, and a dome in Mare Vaporum are also included. Data were acquired at S band (12.6 cm wavelength) using Arecibo Observatory and the Green Bank Telescope in a bistatic configuration. The S band images have resolutions between 20 and 100 m/pixel. The pyroclastic deposits appear dark to the radar and have low circular polarization ratios at S band wavelengths because they are smooth, easily penetrable by radar waves, and generally contain few embedded blocks. Changes in circular polarization ratio (CPR) across some of the pyroclastic deposits show areas with increased rock abundance as well as deposits that are shallower. Radar backscatter and CPR maps are used to identify fine-grained mantling deposits in cases where optical and near-infrared data are ambiguous about the presence of pyroclastics. The Tacquet Formation in southern Serenitatis, areas near Hyginus crater, and a dome in Mare Vaporum have lower-backscatter cross sections than would be expected for mare basalts of similar estimated titanium content. Combined with very low CPR values, this is strong evidence that these areas are covered in fine-grained pyroclastic mantling material.

[1] Amazonis Planitia has undergone alternating episodes of sedimentary and volcanic infilling, forming an interleaved sequence with an upper surface that is very smooth at the kilometer scale. Earlier work interprets the near-surface materials as either young, rough lava flows or ice-rich sediment layers, overlying a basement comprising the Vastitas Borealis Formation and earlier Hesperian plains. Sounding radar profiles across Amazonis Planitia from the Shallow Radar (SHARAD) instrument on the Mars Reconnaissance Orbiter reveal a subsurface dielectric interface that increases in depth toward the north along most orbital tracks. The maximum depth of detection is 100–170 m, depending upon the real dielectric permittivity of the materials, but the interface may persist at greater depth to the north if the reflected energy is attenuated below the SHARAD noise floor. The dielectric horizon likely marks the boundary between sedimentary material of the Vastitas Borealis Formation and underlying Hesperian volcanic plains. The SHARAD-detected interface follows the surface topography across at least one of the large wrinkle ridges in north central Amazonis Planitia. This conformality suggests that Vastitas Borealis sediments, at least in this region, were emplaced prior to compressional tectonic deformation. The change in radar echo strength with time delay is consistent with a loss tangent of 0.005–0.012 for the column of material between the surface and the reflector. These values are consistent with dry, moderate-density sediments or the lower end of the range of values measured for basalts. While a component of distributed ice in a higher-loss matrix cannot be ruled out, we do not find evidence for a dielectric horizon within the Vastitas Borealis Formation that might suggest an abrupt change from an upper dry layer to an ice-rich lower deposit.

[1] Quetzalpetlatl, a corona approximately 800 km in diameter, lies in Lada Terra centered at 68°S, 357°E. It is characterized by vast, radar-bright lava flows stretching to the southeast for over 1500 km, covering a total area of nearly 600,000 km2. Magellan SAR images covered most of the Quetzalpetlatl flow field, but 12.6 cm Arecibo radar data from March 2001 gives complete coverage of the region. With these new data, we examine the circular polarization properties of the Quetzalpetlatl flow field for comparison with other flows on Venus as well as terrestrial lava. Our analysis suggests that Quetzalpetlatl Corona is primarily a source for flows with 13-cm-scale roughness properties similar to terrestrial pahoehoe, and which appear to have traveled down a system of channels or tubes until reaching some unresolved topographic boundary. We also identify four new crater candidates.

International audience; Mars' polar regions are covered with ice-rich layered deposits that potentially contain a record of climate variations. The sounding radar SHARAD on the Mars Reconnaissance Orbiter mapped detailed subsurface stratigraphy in the Promethei Lingula region of the south polar plateau, Planum Australe. Radar reflections interpreted as layers are correlated across adjacent orbits and are continuous for up to 150 kilometers along spacecraft orbital tracks. The reflectors are often separated into discrete reflector sequences, and strong echoes are seen as deep as 1 kilometer. In some cases, the sequences are dipping with respect to each other, suggesting an interdepositional period of erosion. In Australe Sulci, layers are exhumed, indicating recent erosion.

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.

[1] We compare Arecibo dual-polarization radar image data for Venus to Magellan images and emissivity data to investigate the physical properties of volcanic deposits. Radar waves can easily penetrate smooth mantling layers such as ash, aeolian and crater-derived deposits. If a circularly polarized radar wave refracts into a surface that is smooth at wavelength scales, the vertical component of the wave will be preferentially transmitted, resulting in a net linear-polarized echo component. Arecibo polarimetry data were used to create maps of the degree of linear polarization in the radar echo. We find that some volcanic fields in plains regions on Venus are associated with enhanced linear polarization. These fields sometimes have nearby windstreaks which suggest fine-grained surface material, and we infer that the radar wave is penetrating into mantling deposits that are a few centimeters to ∼1 m thick. Enhanced linear polarization values are also correlated with specific lava flows. These lava flows have emissivity values of 0.80 to 0.84, similar to many other flows on Venus. The enhanced linear polarization may be produced by penetration of the radar wave into very smooth lava flows with internal air gaps. High-reflectivity, low-emissivity areas near the summits of Theia and Tepev Montes also have a linearly polarized echo component consistent with surface penetration by the radar wave. The cause of the high linear polarization in summit regions remains uncertain, but perhaps the radar wave is able to penetrate into high dielectric material in limited cases of very smooth surface texture.

Saturn's satellite Iapetus has the largest albedo asymmetry of any natural satellite, with the optical albedo of its trailing hemisphere as much as 10 times that of its leading hemisphere ([ 1 ][1]). The sharp contrast on this synchronously rotating satellite suggests that exogenic materials, such

[1] The distribution of surficial deposits in the vicinity of impact craters on Venus was studied using measurements of the polarization properties of the reflected radar wave. Subsurface scattering of an incident circularly polarized radar signal results in a linearly polarized component in the radar echo due to the differing transmission coefficients at a smooth (at wavelength scales) atmosphere-surface boundary for the horizontal (H) and vertical (V) linearly polarized components of the incident wave. Arecibo 12.6 cm wavelength radar observations in 1999 and 2001 provided images of the surface of Venus in the full Stokes polarization parameters of the reflected echo, from which images in the degree of linear polarization were derived. These images show that substantial areas of Sedna, Guinevere, and Lavinia Planitias return a radar echo with a significant degree of linear polarization, indicating that mantling deposits may be relatively widespread on the plains of Venus. The areas showing linear polarization enhancements are strongly correlated with topographic features, primarily impact craters, dome fields, and windblown deposits, including dune fields. A strong linearly polarized echo component (∼10–40% linear polarization) is found from regions near 45 impact craters, including 5 parabolas. These linear polarization features typically correspond to diffuse, higher backscatter cross-section (bright) regions in Magellan images. The linearly polarized component in these regions is attributed to subsurface echoes from a mantled substrate or from buried rocks.

Arecibo radar observations of Titan at 13-centimeter wavelength indicate that most of the echo power is in a diffusely scattered component but that a small specular component is present for about 75% of the subearth locations observed. These specular echoes have properties consistent with those expected for areas of liquid hydrocarbons. Knowledge of the areal extent and depth of any deposits of liquid hydrocarbons could strongly constrain the history of Titan's atmosphere and surface.

Lunar pyroclastic deposits reflect an explosive stage of the basaltic volcanism that filled impact basins across the nearside. These fine-grained mantling layers are of interest for their association with early mare volcanic processes, and as possible sources of volatiles and other species for lunar outposts. We present Earth-based radar images, at 12.6 and 70 cm wavelengths, of the pyroclastic deposit that blankets the Aristarchus Plateau. The 70 cm data reveal the outlines of a lava-flow complex that covers a significant portion of the plateau and appears to have formed by spillover of magma from the large sinuous rille Vallis Schroteri. The pyroclastics mantling these flows are heavily contaminated with rocks 10 cm and larger in diameter. The 12.6 cm data confirm that other areas are mantled by 20 m or less of material, and that there are numerous patches of 2 cm and larger rocks associated with ejecta from Aristarchus crater. Some of the radar-detected rocky debris is within the mantling material and is not evident in visible-wavelength images. The radar data identify thick, rock-poor areas of the pyroclastic deposit best suited for resource exploitation.