Bolometric Bond Albedo and Thermal Inertia Maps of Mimas.

In 2011 a thermally anomalous region was discovered on Mimas, Saturn's innermost major icy satellite (Howett et al., 2011). The anomalous region is a lens-like shape located at low latitudes on Mimas' leading hemisphere. It manifests as a region with warmer nighttime temperatures, and cooler daytime ones than its surroundings. The thermally anomalous region is spatially correlated with a darkening in Mimas' IR/UV surface color (Schenk et al. 2011) and the region preferentially bombarded by high-energy electrons (Paranicas et al., 2012, 2014; Nordheim et al., 2017). We use data from Cassini's Composite Infrared Spectrometer (CIRS) to map Mimas' surface temperatures and its thermophysical properties. This provides a dramatic improvement on the work in Howett et al. (2011), where the values were determined at only two regions on Mimas (one inside, and another outside of the anomalous region). We use all spatially-resolved scans made by CIRS' focal plane 3 (FP3, 600 to 1100 cm-1) of Mimas' surface, which are largely daytime observations but do include one nighttime one. The resulting temperature maps confirm the presence and location of Mimas' previously discovered thermally anomalous region. No other thermally anomalous regions were discovered, although we note that the surface coverage is incomplete on Mimas' leading and anti-Saturn hemisphere. The thermal inertia map confirms that the anomalous region has a notably higher thermal inertia than its surroundings: 98±42 J m-2 K-1 s-1/2 inside of the anomaly, compared to 34±32 J m-2 K-1 s-1/2 outside. The albedo inside and outside of the anomalous region agrees within their uncertainty: 0.45±0.08 inside compared to 0.41±0.07 outside the anomaly. Interestingly the albedo appears brighter inside the anomaly region, which may not be surprising given this region does appear brighter at some UV wavelengths (0.338 µm, see Schenk et al., 2011). However, this result should be treated with caution because, as previously stated, statistically the albedo of these two regions is the same when their uncertainties are considered. These thermal inertia and albedo values determined here are consistent with those found by Howett et al. (2011), who determined the thermal inertia inside the anomaly to be 66±23 J m-2 K-1 s-1/2 and <16 J m-2 K-1 s-1/2 outside, with albedos that varied from 0.49 to 0.70.

Color, composition, and thermal environment of Kuiper Belt object (486958) Arrokoth.

The outer Solar System object (486958) Arrokoth (provisional designation 2014 MU69) has been largely undisturbed since its formation. We studied its surface composition using data collected by the New Horizons spacecraft. Methanol ice is present along with organic material, which may have formed through irradiation of simple molecules. Water ice was not detected. This composition indicates hydrogenation of carbon monoxide-rich ice and/or energetic processing of methane condensed on water ice grains in the cold, outer edge of the early Solar System. There are only small regional variations in color and spectra across the surface, which suggests that Arrokoth formed from a homogeneous or well-mixed reservoir of solids. Microwave thermal emission from the winter night side is consistent with a mean brightness temperature of 29 ± 5 kelvin.

The geology and geophysics of Kuiper Belt object (486958) Arrokoth.

The Cold Classical Kuiper Belt, a class of small bodies in undisturbed orbits beyond Neptune, is composed of primitive objects preserving information about Solar System formation. In January 2019, the New Horizons spacecraft flew past one of these objects, the 36-kilometer-long contact binary (486958) Arrokoth (provisional designation 2014 MU69). Images from the flyby show that Arrokoth has no detectable rings, and no satellites (larger than 180 meters in diameter) within a radius of 8000 kilometers. Arrokoth has a lightly cratered, smooth surface with complex geological features, unlike those on previously visited Solar System bodies. The density of impact craters indicates the surface dates from the formation of the Solar System. The two lobes of the contact binary have closely aligned poles and equators, constraining their accretion mechanism.

The solar nebula origin of (486958) Arrokoth, a primordial contact binary in the Kuiper Belt.

The New Horizons spacecraft's encounter with the cold classical Kuiper Belt object (486958) Arrokoth (provisional designation 2014 MU69) revealed a contact-binary planetesimal. We investigated how Arrokoth formed and found that it is the product of a gentle, low-speed merger in the early Solar System. Its two lenticular lobes suggest low-velocity accumulation of numerous smaller planetesimals within a gravitationally collapsing cloud of solid particles. The geometric alignment of the lobes indicates that they were a co-orbiting binary that experienced angular momentum loss and subsequent merger, possibly because of dynamical friction and collisions within the cloud or later gas drag. Arrokoth's contact-binary shape was preserved by the benign dynamical and collisional environment of the cold classical Kuiper Belt and therefore informs the accretion processes that operated in the early Solar System.

Initial results from the New Horizons exploration of 2014 MU69, a small Kuiper Belt object.

The Kuiper Belt is a distant region of the outer Solar System. On 1 January 2019, the New Horizons spacecraft flew close to (486958) 2014 MU69, a cold classical Kuiper Belt object approximately 30 kilometers in diameter. Such objects have never been substantially heated by the Sun and are therefore well preserved since their formation. We describe initial results from these encounter observations. MU69 is a bilobed contact binary with a flattened shape, discrete geological units, and noticeable albedo heterogeneity. However, there is little surface color or compositional heterogeneity. No evidence for satellites, rings or other dust structures, a gas coma, or solar wind interactions was detected. MU69's origin appears consistent with pebble cloud collapse followed by a low-velocity merger of its two lobes.

Impact craters on Pluto and Charon indicate a deficit of small Kuiper belt objects.

The flyby of Pluto and Charon by the New Horizons spacecraft provided high-resolution images of cratered surfaces embedded in the Kuiper belt, an extensive region of bodies orbiting beyond Neptune. Impact craters on Pluto and Charon were formed by collisions with other Kuiper belt objects (KBOs) with diameters from ~40 kilometers to ~300 meters, smaller than most KBOs observed directly by telescopes. We find a relative paucity of small craters ≲13 kilometers in diameter, which cannot be explained solely by geological resurfacing. This implies a deficit of small KBOs (≲1 to 2 kilometers in diameter). Some surfaces on Pluto and Charon are likely ≳4 billion years old, thus their crater records provide information on the size-frequency distribution of KBOs in the early Solar System.

Maps of Tethys' Thermophysical Properties.

On 11th April 2015 Cassini's Composite Infrared Spectrometer (CIRS) made a series of observations of Tethys' daytime anti-Saturn hemisphere over a nine-hour time period. During this time the sub-spacecraft position was remarkably stable (0.3° S to 3.9° S; 153.2° W to 221.8° W), and so these observations provide unprecedented coverage of diurnal temperature variations on Tethys' anti-Saturn hemisphere. In 2012 a thermal anomaly was discovered at low latitudes on Tethys' leading hemisphere; it appears cooler during the day and warmer at night than its surroundings (Howett et al., 2012) and is spatially correlated with a decrease in the IR3/UV3 visible color ratio (Schenk et al., 2011). The cause of this anomaly is believed to be surface alteration by high-energy electrons, which preferentially bombard low-latitudes of Tethys' leading hemisphere (Schenk et al., 2011; Howett et al., 2012; Paranicas et al. 2014; Schaible et al., 2017). The thermal anomaly was quickly dubbed "Pac-Man" due to its resemblance to the 1980s video game icon. We use these daytime 2015 CIRS data, along with two sets of nighttime CIRS observations of Tethys (from 27 June 2007 and 17 August 2015) to make maps of bolometric Bond albedo and thermal inertia variations across the anti-Saturn hemisphere of Tethys (including the edge of its Pac-Man region). These maps confirm the presence of the Pac-Man thermal anomaly and show that while Tethys' bolometric Bond albedo varies negligibly outside and inside the anomaly (0.69±0.02 inside, compared to 0.71±0.04 outside) the thermal inertia varies dramatically (29±10 J m-2 K-1 s-1/2 inside, compared to 9±4 J m-2 K-1 s-1/2 outside). These thermal inertias are in keeping with previously published values: 25±3 J m-2 K-1 s-1/2 inside, and 5±1 J m-2 K-1 s-1/2 outside the anomaly (Howett et al., 2012). A detailed analysis shows that on smaller spatial-scales the bolometric Bond albedo does vary: increasing to a peak value at 180° W. For longitudes between ~100° W and ~160° W the thermal inertia increases from northern to southern latitudes, while the reverse is true for bolometric Bond albedo. The thermal inertia on Tethys generally increases towards the center of its leading hemisphere but also displays other notable small-scale variations. These thermal inertia and bolometric Bond albedo variations are perhaps due to differences in competing surface modification by E ring grains and high-energy electrons which both bombard Tethys' leading hemisphere (but in different ways). A comparison between the observed temperatures and our best thermal model fits shows notable discrepancies in the morning warming curve, which may provide evidence of regional variations in surface roughness effects, perhaps again due to variations in surface alteration mechanisms.

Pluto's Interaction With Energetic Heliospheric Ions.

Pluto energies of a few kiloelectron volts and suprathermal ions with tens of kiloelectron volts and above. We measure this population using the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument on board the New Horizons spacecraft that flew by Pluto in 2015. Even though the measured ions have gyroradii larger than the size of Pluto and the cross section of its magnetosphere, we find that the boundary of the magnetosphere is depleting the energetic ion intensities by about an order of magnitude close to Pluto. The intensity is increasing exponentially with distance to Pluto and reaches nominal levels of the interplanetary medium at about 190R P distance. Inside the wake of Pluto, we observe oscillations of the ion intensities with a periodicity of about 0.2 hr. We show that these can be quantitatively explained by the electric field of an ultralow-frequency wave and discuss possible physical drivers for such a field. We find no evidence for the presence of plutogenic ions in the considered energy range.

The formation of Charon's red poles from seasonally cold-trapped volatiles.

A unique feature of Pluto's large satellite Charon is its dark red northern polar cap. Similar colours on Pluto's surface have been attributed to tholin-like organic macromolecules produced by energetic radiation processing of hydrocarbons. The polar location on Charon implicates the temperature extremes that result from Charon's high obliquity and long seasons in the production of this material. The escape of Pluto's atmosphere provides a potential feedstock for a complex chemistry. Gas from Pluto that is transiently cold-trapped and processed at Charon's winter pole was proposed as an explanation for the dark coloration on the basis of an image of Charon's northern hemisphere, but not modelled quantitatively. Here we report images of the southern hemisphere illuminated by Pluto-shine and also images taken during the approach phase that show the northern polar cap over a range of longitudes. We model the surface thermal environment on Charon and the supply and temporary cold-trapping of material escaping from Pluto, as well as the photolytic processing of this material into more complex and less volatile molecules while cold-trapped. The model results are consistent with the proposed mechanism for producing the observed colour pattern on Charon.

Convection in a volatile nitrogen-ice-rich layer drives Pluto's geological vigour.

The vast, deep, volatile-ice-filled basin informally named Sputnik Planum is central to Pluto's vigorous geological activity. Composed of molecular nitrogen, methane, and carbon monoxide ices, but dominated by nitrogen ice, this layer is organized into cells or polygons, typically about 10 to 40 kilometres across, that resemble the surface manifestation of solid-state convection. Here we report, on the basis of available rheological measurements, that solid layers of nitrogen ice with a thickness in excess of about one kilometre should undergo convection for estimated present-day heat-flow conditions on Pluto. More importantly, we show numerically that convective overturn in a several-kilometre-thick layer of solid nitrogen can explain the great lateral width of the cells. The temperature dependence of nitrogen-ice viscosity implies that the ice layer convects in the so-called sluggish lid regime, a unique convective mode not previously definitively observed in the Solar System. Average surface horizontal velocities of a few centimetres a year imply surface transport or renewal times of about 500,000 years, well under the ten-million-year upper-limit crater retention age for Sputnik Planum. Similar convective surface renewal may also occur on other dwarf planets in the Kuiper belt, which may help to explain the high albedos shown by some of these bodies.

The small satellites of Pluto as observed by New Horizons.

The New Horizons mission has provided resolved measurements of Pluto's moons Styx, Nix, Kerberos, and Hydra. All four are small, with equivalent spherical diameters of ~40 kilometers for Nix and Hydra and ~10 kilometers for Styx and Kerberos. They are also highly elongated, with maximum to minimum axis ratios of ~2. All four moons have high albedos (~50 to 90%) suggestive of a water-ice surface composition. Crater densities on Nix and Hydra imply surface ages of at least 4 billion years. The small moons rotate much faster than synchronous, with rotational poles clustered nearly orthogonal to the common pole directions of Pluto and Charon. These results reinforce the hypothesis that the small moons formed in the aftermath of a collision that produced the Pluto-Charon binary.

Surface compositions across Pluto and Charon.

The New Horizons spacecraft mapped colors and infrared spectra across the encounter hemispheres of Pluto and Charon. The volatile methane, carbon monoxide, and nitrogen ices that dominate Pluto's surface have complicated spatial distributions resulting from sublimation, condensation, and glacial flow acting over seasonal and geological time scales. Pluto's water ice "bedrock" was also mapped, with isolated outcrops occurring in a variety of settings. Pluto's surface exhibits complex regional color diversity associated with its distinct provinces. Charon's color pattern is simpler, dominated by neutral low latitudes and a reddish northern polar region. Charon's near-infrared spectra reveal highly localized areas with strong ammonia absorption tied to small craters with relatively fresh-appearing impact ejecta.

The Pluto system: Initial results from its exploration by New Horizons.

The Pluto system was recently explored by NASA's New Horizons spacecraft, making closest approach on 14 July 2015. Pluto's surface displays diverse landforms, terrain ages, albedos, colors, and composition gradients. Evidence is found for a water-ice crust, geologically young surface units, surface ice convection, wind streaks, volatile transport, and glacial flow. Pluto's atmosphere is highly extended, with trace hydrocarbons, a global haze layer, and a surface pressure near 10 microbars. Pluto's diverse surface geology and long-term activity raise fundamental questions about how small planets remain active many billions of years after formation. Pluto's large moon Charon displays tectonics and evidence for a heterogeneous crustal composition; its north pole displays puzzling dark terrain. Small satellites Hydra and Nix have higher albedos than expected.

Female neuregulin 1 heterozygous mice require repeated exposure to Δ9-tetrahydrocannabinol to alter sensorimotor gating function.

INTRODUCTION: The schizophrenia susceptibility gene neuregulin 1 (NRG1) confers vulnerability to the neurobehavioural eff ects of cannabinoids differently across sexes. Male but not female Nrg1 heterozygous (HET) mice display facilitation of prepulse inhibition (PPI) to acute Δ9-tetrahydrocannabinol (THC) exposure compared to WT controls. We aim to observe whether repeated administration of THC may overcome the acute insensitivity of female Nrg1 HET mice to THC exposure. METHODS: Female Nrg1 HET mice and WT controls were administered THC daily for 21 days, with PPI and anxiety-related behaviour in the light-dark test (LD) examined on the fi rst and last day of treatment and 21 days after cessation of dosing. RESULTS: Following repeated, but not acute THC exposure, female Nrg1 HET mice displayed THC-induced facilitation of PPI which was not observed in WT mice treated with THC. There were no residual eff ects of THC on PPI in either genotype when assessed 21 days following the final THC dose. An anxiogenic response to THC was evident following repeated, but not acute, administration in the LD test in both genotypes. DISCUSSION: These findings show that the acute insensitivity of female Nrg1 HET mice to THC-induced PPI facilitation may be overcome following repeated THC exposure.

Energetic particles in the jovian magnetotail.

When the solar wind hits Jupiter's magnetic field, it creates a long magnetotail trailing behind the planet that channels material out of the Jupiter system. The New Horizons spacecraft traversed the length of the jovian magnetotail to >2500 jovian radii (RJ; 1 RJ identical with 71,400 kilometers), observing a high-temperature, multispecies population of energetic particles. Velocity dispersions, anisotropies, and compositional variation seen in the deep-tail (greater, similar 500 RJ) with a approximately 3-day periodicity are similar to variations seen closer to Jupiter in Galileo data. The signatures suggest plasma streaming away from the planet and injection sites in the near-tail region (approximately 200 to 400 RJ) that could be related to magnetic reconnection events. The tail structure remains coherent at least until it reaches the magnetosheath at 1655 RJ.

Jupiter cloud composition, stratification, convection, and wave motion: a view from new horizons.

Several observations of Jupiter's atmosphere made by instruments on the New Horizons spacecraft have implications for the stability and dynamics of Jupiter's weather layer. Mesoscale waves, first seen by Voyager, have been observed at a spatial resolution of 11 to 45 kilometers. These waves have a 300-kilometer wavelength and phase velocities greater than the local zonal flow by 100 meters per second, much higher than predicted by models. Additionally, infrared spectral measurements over five successive Jupiter rotations at spatial resolutions of 200 to 140 kilometers have shown the development of transient ammonia ice clouds (lifetimes of 40 hours or less) in regions of strong atmospheric upwelling. Both of these phenomena serve as probes of atmospheric dynamics below the visible cloud tops.

Io's atmospheric response to eclipse: UV aurorae observations.

The New Horizons (NH) spacecraft observed Io's aurora in eclipse on four occasions during spring 2007. NH Alice ultraviolet spectroscopy and concurrent Hubble Space Telescope ultraviolet imaging in eclipse investigate the relative contribution of volcanoes to Io's atmosphere and its interaction with Jupiter's magnetosphere. Auroral brightness and morphology variations after eclipse ingress and egress reveal changes in the relative contribution of sublimation and volcanic sources to the atmosphere. Brightnesses viewed at different geometries are best explained by a dramatic difference between the dayside and nightside atmospheric density. Far-ultraviolet aurora morphology reveals the influence of plumes on Io's electrodynamic interaction with Jupiter's magnetosphere. Comparisons to detailed simulations of Io's aurora indicate that volcanoes supply 1 to 3% of the dayside atmosphere.

New horizons mapping of Europa and Ganymede.

The New Horizons spacecraft observed Jupiter's icy satellites Europa and Ganymede during its flyby in February and March 2007 at visible and infrared wavelengths. Infrared spectral images map H2O ice absorption and hydrated contaminants, bolstering the case for an exogenous source of Europa's "non-ice" surface material and filling large gaps in compositional maps of Ganymede's Jupiter-facing hemisphere. Visual wavelength images of Europa extend knowledge of its global pattern of arcuate troughs and show that its surface scatters light more isotropically than other icy satellites.

Io volcanism seen by new horizons: a major eruption of the Tvashtar volcano.

Jupiter's moon Io is known to host active volcanoes. In February and March 2007, the New Horizons spacecraft obtained a global snapshot of Io's volcanism. A 350-kilometer-high volcanic plume was seen to emanate from the Tvashtar volcano (62 degrees N, 122 degrees W), and its motion was observed. The plume's morphology and dynamics support nonballistic models of large Io plumes and also suggest that most visible plume particles condensed within the plume rather than being ejected from the source. In images taken in Jupiter eclipse, nonthermal visible-wavelength emission was seen from individual volcanoes near Io's sub-Jupiter and anti-Jupiter points. Near-infrared emission from the brightest volcanoes indicates minimum magma temperatures in the 1150- to 1335-kelvin range, consistent with basaltic composition.