The residual south polar cap of Mars (RSPC) is a region comprising tabular deposits of solid CO2 averaging a few m thick resting on layered, water-ice rich deposits a few km thick. Estimates of the ages of morphologic features in the RSPC have relied on average rates of change obtained from geographically limited samples. In this work we apply image coverage of the whole RSPC at 6 m/pixel spanning up to seven Mars years to determine how well ages of RSPC forms can be modelled from recent observations, and to explore implications of the calculated chronology. Topographic troughs separating CO2 layers of different ages and thickness, termed moats, are widespread, changing, and dateable features of the RSPC. Although they all develop by the same phenomena, we divide moats into two groups based on their linear dimensions and the span of relevant spacecraft imaging: 1) moats separating large areas of CO2 ice of different age and thickness, termed sinuous boundary moats, that imaging data show are related to post-1972 CO2 deposition. 2) moats separating the walls of pits from thinner, younger, interior mounds, termed pit moats; these are not resolved in early spacecraft images. Our survey of the entire summer RSPC spanning seven Mars years yields modelled formation times for both kinds of moats that cluster largely about Mars Years (MY) 11-13. A population of smaller moats is calculated to have formed near MY 26-28. These times closely follow planet-encircling dust events in MY 9, MY 12, and MY 28 (Earth years 1971, 1977, 2007). High-resolution images show that both pit and sinuous moats are initiated by slow downward erosion of debris left by retreating scarps of CO2 ice that are flanked by brighter, accumulating CO2 ice. We find that the most probable role of the dust events in moat formation is to clear thin CO2 layers from underlying water-ice-rich materials through thermal effects of the atmospheric dust content. The age results severely limit the magnitude of variations in moat scarp retreat rates during the last 25 MY. Pit moats have limited lifetimes because scarp retreat eventually removes the interior mound that defines the inner moat wall. Pits can reach much greater ages than moats; some pits formed prior to MY -100 (Earth year 1765).

Several large ridges, termed dorsa, stand over 800 m above their surroundings in a region centered on the trailing hemisphere of Saturn's icy moon Enceladus. In map view, these dorsa are linear to curvilinear, 20 km to more than 50 km long, and display near-orthogonal trends. They cross-cut (are younger than) most other geological features in the region. High-resolution limb profiles show the dorsa to be asymmetric in cross-sectioned profile and 5-6 km in width, and high-resolution images show striations along their crests. The structure and morphology of the dorsa suggest they are thrust blocks, possibly analogous to lobate scarps or wrinkle-ridges found on the terrestrial planets. The low slopes of their backlimbs and steeper forelimbs, suggest the dorsa were formed as wrinkle ridges or lobate scarps overlying thrust faults that penetrate 1-4 km deep to a detachment, most likely at the brittle-ductile transition (BDT). Their near-orthogonal trends are consistent with biaxial horizontal shortening. These relationships suggest that the central trailing hemisphere was recently subjected to a relatively high heat flow at the time that deformation occurred.

The small satellites are a diverse group of objects offering insights into the early formation of the Solar System and its collisional history. They can be subdivided into several groups: small inner moons, outer irregular moons, Martian moons, moons of asteroids, and moons of Centaurs/KBOs/TNOs. Many of these objects, especially the irregulars or their progenitors, are thought to be captured asteroids or Kuiper Belt Objects: a spacecraft mission that included them as targets would thus yield information on an object that came from elsewhere in the Solar System. Objects originating from the same reservoir as the irregular satellites may have brought both pre-biotic material and volatiles such as water to the inner Solar System. Flagship and New Frontiers class missions should plan trajectories and arrival times to capture spectroscopic and imaging observations of small satellites. It is also important to support ground-based and Earthorbiting observations of these objects, laboratory measurements of candidate surface materials, and curation facilities for organic materials and iceand gas-based materials. We place a high priority on a robust participating scientist program for JAXA’s MMX mission to Phobos and Deimos, as a similar mission was our highest priority New Frontiers class mission in the last Decadal Survey. A sample returned by this mission would include material from a primitive object and possibly the Martian surface. Scientific priorities for moons of asteroids and KBOs are covered in white papers for those objects

Properties of the small inner satellites of Saturn are updated using final Cassini image data. The equatorial ridges on Pan and Atlas are morphologically, structurally, and gravitationally distinct from their cores and represent distinct additions to, rather than modifications of, the precursor bodies. Exposures of lineated substrates on Pandora and Epimetheus show a relationship between regolith grooves and structures in underlying bedrock. Structures exposed on Epimetheus suggest formation at depth including filling or annealing of fractures. These structures, and sharp bedrock-regolith boundaries indicate a history far more complex than simple rubble-pile assembly. These exposures of bedrock also indicate past removal of the expected regolith from large fractions of these object's surfaces late in their cratering records.

We examine the surface brightnesses of Saturn's smaller satellites using a photometric model that explicitly accounts for their elongated shapes and thus facilitates comparisons among different moons. Analyses of Cassini imaging data with this model reveal that the moons Aegaeon, Methone, and Pallene are darker than one would expect given trends previously observed among the nearby mid-sized satellites. On the other hand, the trojan moons Calypso and Helene have substantially brighter surfaces than their co-orbital companions Tethys and Dione. These observations are inconsistent with the moons' surface brightnesses being entirely controlled by the local flux of E-ring particles, and therefore strongly imply that other phenomena are affecting their surface properties. The darkness of Aegaeon, Methone, and Pallene is correlated with the fluxes of high-energy protons, implying that high-energy radiation is responsible for darkening these small moons. Meanwhile, Prometheus and Pandora appear to be brightened by their interactions with the nearby dusty F ring, implying that enhanced dust fluxes are most likely responsible for Calypso's and Helene's excess brightness. However, there are no obvious structures in the E ring that would preferentially brighten these two moons, so there must either be something subtle in the E-ring particles' orbital properties that leads to asymmetries in the relevant fluxes, or something happened recently to temporarily increase these moons' brightnesses.

The Residual South Polar Cap (RSPC) of Mars is a thin covering of CO2 ice resting on water-ice rich deposits. As such, it is a likely indicator of the net effects of recent polar climate. This covering has had minor changes in outline for the period of spacecraft observation (Piqueux and Christensen, 2008a) and estimates of its recent mass balance suggest fractionally small changes in its volume (Thomas et al., 2016). Pit growth by scarp retreat (Malin et al., 2001; Byrne and Ingersoll, 2003a; Thomas et al., 2005, 2013) is a major, relatively easily measured component of the cap's mass balance. This scarp retreat is only the beginning of a process: fracturing, slumping, and sublimation (Byrne et al., 2008; Thomas et al., 2009) lead to production of a trail of debris. This study focuses on that debris using spacecraft imaging data. The rough, relatively dark debris forms ubiquitous ramps, typically tens of m wide, around scarps within pits or on perimeters of mesas. Much wider accumulations of debris, "debris fields," mimic distinctive scalloped outlines of mesas and are observed to originate by merging of expanding pits within a mesa. The subsequent evolution of debris fields includes repeated year-to-year local relief inversions that involve trapping and retention of seasonal ice in low areas that effectively reduce the vertical loss rates. Complete loss of the CO2 debris from a surface of water-ice rich Polar Layered Deposits (PLD) can initiate new net accumulation of CO2 ice. The longevity of some debris fields, essentially instances of slow downwasting, appear to facilitate relief inversion on large scales by allowing the surrounding areas to accumulate new CO2 ice and to thicken relative to the debris fields.

Saturn’s main ring system is associated with a set of small moons that either are embedded within it or interact with the rings to alter their shape and composition. Five close flybys of the moons Pan, Daphnis, Atlas, Pandora, and Epimetheus were performed between December 2016 and April 2017 during the ring-grazing orbits of the Cassini mission. Data on the moons’ morphology, structure, particle environment, and composition were returned, along with images in the ultraviolet and thermal infrared. We find that the optical properties of the moons’ surfaces are determined by two competing processes: contamination by a red material formed in Saturn’s main ring system and accretion of bright icy particles or water vapor from volcanic plumes originating on the moon Enceladus.

We point out some errors in the most recent report from the International Astronomical Union (IAU) Working Group on Cartographic Coordinates and Rotational Elements (Archinal et al. 2018).

Many objects in the solar system are suspected to have experienced reorientation of their spin axes. As their rotation rates are slow and their shapes are nearly spherical, the formation of mass anomalies, by either endogenic or exogenic processes, can change objects' moments of inertia. Therefore, the objects reorient to align their largest moment of inertia with their spin axis. Such a phenomenon is called True Polar Wander (TPW).Here we report the discovery of a global series of topographic lows on Saturn's satellite Enceladus that we interpret to show that this synchronously locked moon has undergone TPW by similar to 55 degrees about the tidal axis. We use improved topographic data from the spherical harmonic expansion of Cassini limb and stereogrammetric measurements to characterize regional topography over the surface of Enceladus. We identify a group of nearly antipodal basins orthogonal to a basin chain tracing a non-equatorial circum-global belt across Enceladus' surface. We argue that the belt and the antipodal regions are fossil remnants of earlier equator and poles, respectively. We argue that these lows arise from isostasic compensation and that their pattern reflects spatial variations in internal dynamics of the ice shell. Our hypothesis is consistent with a variety of geological features visible in Cassini images. (C) 2017 Elsevier Inc. All rights reserved.

An exceptional deposit covering an area of ∼7700km2, displaying morphology indicative of an originally fluid material, occurs near 42.2°N, 167.4°E on the lunar farside. The material occurs as smooth, flat deposits (here termed “ponds”) on the bottoms of many craters and in other topographic depressions, as well as a veneer across the majority of the region. Within this area, the ponded deposits and widespread-veneer have an estimated volume of ∼8km3; the veneer constitutes the great majority of this volume. This material appears to have flowed downslope across the surface, collecting in flat-surface accumulations. The surfaces of the ponds and veneer are only lightly cratered, indicating a young (i.e., late Copernican) age. Four possible modes of origin are investigated: basin ejecta, pyroclastic volcanism, effusive volcanism, and ballistically emplaced impact-melt. Volcanism and basin ejecta appear to be inconsistent with the observed morphology: an implausible number of vents are required for volcanism and the morphological properties do not resemble basin ejecta. We suggest that ballistically emplaced impact melt is most consistent with the observations. Possible source craters for impact melt, based on minimum required size (>20km diameter) and age, are at least 250km distant and cannot be definitively tied to the pond deposits. This discovery places important new constraints on our knowledge of the distribution of impact melt relative to the parent crater.

The Lunar Reconnaissance Orbiter Camera (LROC) consists of two imaging systems that provide synoptic and high resolution imaging of the lunar surface. The Wide Angle Camera (WAC) is a seven color push frame imager with a 90° field of view in monochrome mode and 60° field of view in color mode. From the nominal 50 km polar orbit, the WAC acquires images with a nadir pixel scale of 75 m for each visible band and 384 m for the two ultraviolet bands. The Narrow Angle Camera (NAC) consists of two identical cameras capable of acquiring images with a pixel scale of 0.5 to 1.0 m from a 50 km orbit. Each camera was geometrically calibrated prior to launch at Malin Space Science Systems in San Diego, California. Using thousands of images acquired since launch in June of 2009, improvements to the relative and absolute pointing of the twin NACs were made allowing images on the surface to be projected with an accuracy of 20 meters. Further registration of WAC and NAC images allowed the derivation of a new distortion model and pointing updates for the WAC, thus enabling sub-pixel accuracy in projected WAC images.

High-resolution ‘before and after’ imaging of the Moon is used to quantify the rate of crater production and provide insights into the cratering process.

Several planetary satellites apparently have subsurface seas that are of great interest for, among other reasons, their possible habitability. The geologically diverse saturnian satellite Enceladus vigorously vents liquid water and vapor from fractures within a south polar depression and thus must have a liquid reservoir or active melting. However, the extent and location of any subsurface liquid region is not directly observable. We use measurements of control points across the surface of Enceladus accumulated over seven years of spacecraft observations to determine the satellite's precise rotation state, finding a forced physical libration of 0.120 +/- 0.014 degrees (2 sigma). This value is too large to be consistent with Enceladus's core being rigidly connected to its surface, and thus implies the presence of a global ocean rather than a localized polar sea. The maintenance of a global ocean within Enceladus is problematic according to many thermal models and so may constrain satellite properties or require a surprisingly dissipative Saturn. (C) 2015 Elsevier Inc. All rights reserved.

Erosion of pits in the residual south polar cap (RSPC) of Mars concurrent with deposition and fluctuating cap boundaries raises questions about the mass balance and long term stability of the cap. Determining a mass balance by measurement of a net gain or loss of atmospheric CO2 by direct pressure measurements (Haberle, R.M. et al. [2014]. Secular climate change on Mars: An update using one Mars year of MSL pressure data. American Geophysical Union (Fall). Abstract 3947), although perhaps the most direct method, has so far given ambiguous results. Estimating volume changes from imaging data faces challenges, and has previously been attempted only in isolated areas of the cap. In this study we use 6 m/pixel Context Imager (CTX) data from Mars year 31 to map all the morphologic units of the RSPC, expand the measurement record of pit erosion rates, and use high resolution images to place limits on vertical changes in the surface of the residual cap. We find the mass balance in Mars years 9-31 to be -6 to +4 ke/male y, or roughly--0.039% to +0.026% of the mean atmospheric CO2 mass/male y. The indeterminate sign results chiefly from uncertainty in the amounts of deposition or erosion on the upper surfaces of deposits (as opposed to scarp retreat). Erosion and net deposition in this period appear to be controlled by summertime planetary scale dust events, the largest occurring in MY 9, another, smaller one in MY 28. The rates of erosion and the deposition observed since MY 9 appear to be consistent with the types of deposits and erosional behavior found in most of the residual cap. However, small areas (<10%) of the cap are distinguished by their greater thickness, polygonal troughs, and embayed contacts with thinner units. These deposits may require extended periods (>100 male y) of depositional and/or erosional conditions different from those occurring in the period since MY 9, although these environmental differences could be subtle. (C) 2016 Elsevier Inc. All rights reserved.