Cassini radar views the surface of Titan

The Cassini Titan Radar Mapper imaged about 1% of Titan's surface at a resolution of approximately 0.5 kilometer, and larger areas of the globe in lower resolution modes. The images reveal a complex surface, with areas of low relief and a variety of geologic features suggestive of dome-like volcanic constructs, flows, and sinuous channels. The surface appears to be young, with few impact craters. Scattering and dielectric properties are consistent with porous ice or organics. Dark patches in the radar images show high brightness temperatures and high emissivity and are consistent with frozen hydrocarbons.     

Radar mapping of mercury: full-disk images and polar anomalies

A random-code technique has been used at Arecibo to obtain delay-Doppler radar images of the full disk of Mercury. Anomalously bright features were found at the north and south poles. The north polar feature is oblong (4 degrees by 8 degrees ) and offset from the pole. The smaller south polar feature is mostly confined to the floor of the crater Chao Meng-Fu. The polar locations and radar properties of these features indicate that they may be produced by volume scattering in ice. The images also reveal a variety of more subdued reflectivity features ranging in size from hundreds to thousands of kilometers; some of these appear to have an impact origin.     

Radar glory from buried craters on icy moons

Three ice-covered moons of Jupiter, in comparison with rocky planets and Earth's moon, produce radar echoes of astounding strengths and bizarre polarizations. Scattering from buried craters can explain these and other anomalous properties of the echoes. The role of such craters is analogous to that of the water droplets that create the apparition known as "the glory," the optically bright region surrounding an observer's shadow on a cloud. Both situations involve the electromagnetic phenomenon of total internal reflection at a dielectric interface, operating in a geometry that strongly favors exact backscattering. Dim surface craters are transformed into bright glory holes by being buried under somewhat denser material, thereby increasing the intensity of their echoes by factors of hundreds. The dielectric interface thus formed at the crater walls nicely accounts for the unusual polarizations of the echoes.     

Cassini encounters Enceladus: background and the discovery of a south polar hot spot

The Cassini spacecraft completed three close flybys of Saturn's enigmatic moon Enceladus between February and July 2005. On the third and closest flyby, on 14 July 2005, multiple Cassini instruments detected evidence for ongoing endogenic activity in a region centered on Enceladus' south pole. The polar region is the source of a plume of gas and dust, which probably emanates from prominent warm troughs seen on the surface. Cassini's Composite Infrared Spectrometer (CIRS) detected 3 to 7 gigawatts of thermal emission from the south polar troughs at temperatures up to 145 kelvin or higher, making Enceladus only the third known solid planetary body-after Earth and Io-that is sufficiently geologically active for its internal heat to be detected by remote sensing. If the plume is generated by the sublimation of water ice and if the sublimation source is visible to CIRS, then sublimation temperatures of at least 180 kelvin are required.     

Flood volcanism in the northern high latitudes of Mercury revealed by MESSENGER

MESSENGER observations from Mercury orbit reveal that a large contiguous expanse of smooth plains covers much of Mercury's high northern latitudes and occupies more than 6% of the planet's surface area. These plains are smooth, embay other landforms, are distinct in color, show several flow features, and partially or completely bury impact craters, the sizes of which indicate plains thicknesses of more than 1 kilometer and multiple phases of emplacement. These characteristics, as well as associated features, interpreted to have formed by thermal erosion, indicate emplacement in a flood-basalt style, consistent with x-ray spectrometric data indicating surface compositions intermediate between those of basalts and komatiites. The plains formed after the Caloris impact basin, confirming that volcanism was a globally extensive process in Mercury's post-heavy bombardment era.     

Nitric acid trihydrate (NAT) in polar stratospheric clouds

A comprehensive investigation of polar stratospheric clouds was performed on 25 January 2000 with instruments onboard a balloon gondola flown from Kiruna, Sweden. Cloud layers were repeatedly encountered at altitudes between 20 and 24 kilometers over a wide range of atmospheric temperatures (185 to 197 kelvin). Particle composition analysis showed that a large fraction of the cloud layers was composed of nitric acid trihydrate (NAT) particles, containing water and nitric acid at a molar ratio of 3:1; this confirmed that these long-sought solid crystals exist well above ice formation temperatures. The presence of NAT particles enhances the potential for chlorine activation with subsequent ozone destruction in polar regions, particularly in early and late winter.     

Evidence that nitric acid increases relative humidity in low-temperature cirrus clouds

In situ measurements of the relative humidity with respect to ice (RHi) and of nitric acid (HNO3) were made in both natural and contrail cirrus clouds in the upper troposphere. At temperatures lower than 202 kelvin, RHi values show a sharp increase to average values of over 130% in both cloud types. These enhanced RHi values are attributed to the presence of a new class of HNO3-containing ice particles (Delta-ice). We propose that surface HNO3 molecules prevent the ice/vapor system from reaching equilibrium by a mechanism similar to that of freezing point depression by antifreeze proteins. Delta-ice represents a new link between global climate and natural and anthropogenic nitrogen oxide emissions. Including Delta-ice in climate models will alter simulated cirrus properties and the distribution of upper tropospheric water vapor.     

Diviner Lunar Radiometer observations of cold traps in the Moon's south polar region

Diviner Lunar Radiometer Experiment surface-temperature maps reveal the existence of widespread surface and near-surface cryogenic regions that extend beyond the boundaries of persistent shadow. The Lunar Crater Observation and Sensing Satellite (LCROSS) struck one of the coldest of these regions, where subsurface temperatures are estimated to be 38 kelvin. Large areas of the lunar polar regions are currently cold enough to cold-trap water ice as well as a range of both more volatile and less volatile species. The diverse mixture of water and high-volatility compounds detected in the LCROSS ejecta plume is strong evidence for the impact delivery and cold-trapping of volatiles derived from primitive outer solar system bodies.     

MESSENGER observations of the spatial distribution of planetary ions near Mercury

Global measurements by MESSENGER of the fluxes of heavy ions at Mercury, particularly sodium (Na(+)) and oxygen (O(+)), exhibit distinct maxima in the northern magnetic-cusp region, indicating that polar regions are important sources of Mercury's ionized exosphere, presumably through solar-wind sputtering near the poles. The observed fluxes of helium (He(+)) are more evenly distributed, indicating a more uniform source such as that expected from evaporation from a helium-saturated surface. In some regions near Mercury, especially the nightside equatorial region, the Na(+) pressure can be a substantial fraction of the proton pressure.     

Thermal structure of the venus atmosphere in the middle cloud layer

Thermal structure measurements obtained by the two VEGA balloons show the Venus middle cloud layer to be generally adiabatic. Temperatures measured by the two balloons at locations roughly symmetric about the equator differed by about 6.5 kelvins at a given pressure. The VEGA-2 temperatures were about 2.5 kelvins cooler and those of VEGA-1 about 4 kelvins warmer than temperatures measured by the Pioneer Venus Large Probe at these levels. Data taken by the VEGA-2 lander as it passed through the middle cloud agreed with those of the VEGA-2 balloon. Study of individual frames of the balloon data suggests the presence of multiple discrete air masses that are internally adiabatic but lie on slightly different adiabats. These adiabats, for a given balloon, can differ in temperature by as much as 1 kelvin at a given pressure.     

Composition and physical properties of Enceladus' surface

Observations of Saturn's satellite Enceladus using Cassini's Visual and Infrared Mapping Spectrometer instrument were obtained during three flybys of Enceladus in 2005. Enceladus' surface is composed mostly of nearly pure water ice except near its south pole, where there are light organics, CO2, and amorphous and crystalline water ice, particularly in the region dubbed the "tiger stripes." An upper limit of 5 precipitable nanometers is derived for CO in the atmospheric column above Enceladus, and 2% for NH3 in global surface deposits. Upper limits of 140 kelvin (for a filled pixel) are derived for the temperatures in the tiger stripes.     

Temperatures of the martian surface and atmosphere: viking observation of diurnal and geometric variations

Selected observations made with the Viking infrared thermal mapper after the first landing are reported. Atmospheric temperatures measured at the latitude of the Viking 2 landing site (48 degrees N) over most of a martian day reveal a diurnal variation of at least 15 K, with peak temperatures occurring near 2.2 hours after noon, implying significant absorption of sunlight in the lower 30 km of the atmosphere by entrained dust. The summit temperature of Arsia Mons varies by a factor of nearly two each day; large diurnal temperature variation is characteristic of the south Tharsis upland and implies the presence of low thermal inertia material. The thermal inertia of material on the floors of several typical large craters is found to be higher than for the surrounding terrain; this suggests that craters are somehow effective in sorting aeolian material. Brightness temperatures of the Viking 1 landing area decrease at large emission angles; the intensity of reflected sunlight shows a more complex dependence on geometry than expected, implying atmospheric as well as surface scattering.     

Hollows on Mercury: MESSENGER evidence for geologically recent volatile-related activity

High-resolution images of Mercury's surface from orbit reveal that many bright deposits within impact craters exhibit fresh-appearing, irregular, shallow, rimless depressions. The depressions, or hollows, range from tens of meters to a few kilometers across, and many have high-reflectance interiors and halos. The host rocks, which are associated with crater central peaks, peak rings, floors, and walls, are interpreted to have been excavated from depth by the crater-forming process. The most likely formation mechanisms for the hollows involve recent loss of volatiles through some combination of sublimation, space weathering, outgassing, or pyroclastic volcanism. These features support the inference that Mercury's interior contains higher abundances of volatile materials than predicted by most scenarios for the formation of the solar system's innermost planet.     

Energetics of hydrogen bond network rearrangements in liquid water

A strong temperature dependence of oxygen K-edge x-ray absorption fine structure features was observed for supercooled and normal liquid water droplets prepared from the breakup of a liquid microjet. Analysis of the data over the temperature range 251 to 288 kelvin (-22 degrees to +15 degrees C) yields a value of 1.5 +/- 0.5 kilocalories per mole for the average thermal energy required to effect an observable rearrangement between the fully coordinated ("ice-like") and distorted ("broken-donor") local hydrogen-bonding configurations responsible for the pre-edge and post-edge features, respectively. This energy equals the latent heat of melting of ice with hexagonal symmetry (ice Ih) and is consistent with the distribution of hydrogen bond strengths obtained for the "overstructured" ST2 model of water.     

Recalibrated Mariner 10 Color Mosaics: Implications for Mercurian Volcanism

Recalibration of Mariner 10 color image data allows the identification of distinct color units on the mercurian surface. We analyze these data in terms of opaque mineral abundance, iron content, and soil maturity and find color units consistent with the presence of volcanic deposits on Mercury's surface. Additionally, materials associated with some impact craters have been excavated from a layer interpreted to be deficient in opaque minerals within the crust, possibly analogous to the lunar anorthosite crust. These observations suggest that Mercury has undergone complex differentiation like the other terrestrial planets and the Earth's moon.     

Detection of water in the LCROSS ejecta plume

Several remote observations have indicated that water ice may be presented in permanently shadowed craters of the Moon. The Lunar Crater Observation and Sensing Satellite (LCROSS) mission was designed to provide direct evidence. On 9 October 2009, a spent Centaur rocket struck the persistently shadowed region within the lunar south pole crater Cabeus, ejecting debris, dust, and vapor. This material was observed by a second "shepherding" spacecraft, which carried nine instruments, including cameras, spectrometers, and a radiometer. Near-infrared absorbance attributed to water vapor and ice and ultraviolet emissions attributable to hydroxyl radicals support the presence of water in the debris. The maximum total water vapor and water ice within the instrument field of view was 155 ± 12 kilograms. Given the estimated total excavated mass of regolith that reached sunlight, and hence was observable, the concentration of water ice in the regolith at the LCROSS impact site is estimated to be 5.6 ± 2.9% by mass. In addition to water, spectral bands of a number of other volatile compounds were observed, including light hydrocarbons, sulfur-bearing species, and carbon dioxide.     

Protonic diffusion in high-pressure ice VII

Near ambient pressures, molecular diffusion dominates protonic diffusion in ice. Theoretical studies have predicted that protonic diffusion will dominate at high pressures in ice. We measured the protonic diffusion coefficient for the highest temperature molecular phase of ice VII at 400 kelvin over its entire stable pressure region. The values ranged from 10(-17) to 10(-15) square meters per second at pressures of 10 to 63 gigapascals. The diffusion coefficients extrapolated to high temperatures close to the ice VII melting curve were less by a factor of 10(2) to 10(3) than a superionic criterion of approximately 10(-8) square meters per second, at which protons would diffuse freely.     

Pressure-induced enhancement of tc above 150 k in hg-1223

The recently discovered homologous series HgBa(2)Can-1 Cun O2n+2+delta possesses remarkable properties. A superconducting transition temperature, T(c), as high as 133 kelvin has been measured in a multiphase Hg-Ba-Ca-Cu-O sample and found to be attributable to the Hg-1223 compound. Temperature-dependent electrical resistivity measurements under pressure on a (> 95%) pure Hg-1223 phase are reported. These data show that T(c) increases steadily with pressure at a rate of about 1 kelvin per gigapascal up to 15 gigapascals, then more slowly and reaches a T(c) = 150 kelvin, with the onset of the transition at 157 kelvin, for 23.5 gigapascals. This large pressure variation (as compared to the small effects observed in similar compounds with the optimal T(c)) strongly suggests that higher critical temperatures could be obtained at atmospheric pressure.     

Stable low-pressure hydrogen clusters stored in a binary clathrate hydrate

Thermodynamic, x-ray diffraction, and Raman and nuclear magnetic resonance spectroscopy measurements show that clusters of H2 can be stabilized and stored at low pressures in a sII binary clathrate hydrate. Clusters of H2 molecules occupy small water cages, whereas large water cages are singly occupied by tetrahydrofuran. The presence of this second guest component stabilizes the clathrate at pressures of 5 megapascals at 279.6 kelvin, versus 300 megapascals at 280 kelvin for pure H2 hydrate.     

Radar images of Mars

Full disk images of Mars have been obtained with the use of the Very Large Array (VLA) to map the radar reflected flux density. The transmitter system was the 70-m antenna of the Deep Space Network at Goldstone, California. The surface of Mars was illuminated with continuous wave radiation at a wavelength of 3,5 cm. The reflected energy was mapped in individual 12-minute snapshots with the VLA in its largest configuration; fringe spacings as small as 67 km were obtained. The images reveal near-surface features including a region in the Tharsis volcano area, over 2000 km in east-west extent, that displayed no echo to the very low level of the radar system noise. The feature, called Stealth, is interpreted as a deposit of dust or ash with a density less than about 0.5 gram per cubic centimeter and free of rocks larger than 1 cm across. The deposit must be several meters thick and may be much deeper. The strongest reflecting geological feature was the south polar ice cap, which was reduced in size to the residual south polar ice cap at the season of observation. The cap image is interpreted as arising from nearly pure CO(2) or H(2)O ice with a small amount of martian dust (less than 2 percent by volume) and a depth greater than 2 to 5 m. Only one anomalous reflecting feature was identified outside of the Tharsis region, although the Elysium region was poorly sampled in this experiment and the north pole was not visible from Earth.