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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.

Cassini's Titan flyby The surface of Saturn's largest moon, Titan, is coated in a dense methane-rich atmosphere that prevents high-resolution imaging at visible wavelengths. During its first Titan flyby last October, the Cassini spacecraft's visual and infrared mapping spectrometer (VIMS) was able to reveal detailed surface structures, as reported in this issue. Notable features include a circular structure 30 km in diameter, thought to be a cryogenic dome. This may be volcanic, which could explain how the methane in Titan's atmosphere is replenished. Titan is the only satellite in our Solar System with a dense atmosphere. The surface pressure is 1.5 bar (ref. 1 ) and, similar to the Earth, N 2 is the main component of the atmosphere. Methane is the second most important component 2 , but it is photodissociated on a timescale of 10 7  years (ref. 3 ). This short timescale has led to the suggestion that Titan may possess a surface or subsurface reservoir of hydrocarbons 4 , 5 to replenish the atmosphere. Here we report near-infrared images of Titan obtained on 26 October 2004 by the Cassini spacecraft. The images show that a widespread methane ocean does not exist; subtle albedo variations instead suggest topographical variations, as would be expected for a more solid (perhaps icy) surface. We also find a circular structure ∼30 km in diameter that does not resemble any features seen on other icy satellites. We propose that the structure is a dome formed by upwelling icy plumes that release methane into Titan's atmosphere.

Phoebe a Kuiper-belt refugee? Phoebe, the outermost large satellite of Saturn, is of particular interest because its unusual orbit suggests that it was gravitationally captured by Saturn, having formed outside the solar nebula where Saturn itself formed. The Cassini–Huygens spacecraft encountered Phoebe on 11 June 2004, and imaging spectroscopy from Cassini was used to detect iron, bound water, trapped CO 2 , phyllosilicates, organics, nitriles and cyanide compounds on Phoebe. The presence of all these compounds makes Phoebe one of the most compositionally diverse objects in our Solar System, consistent with a surface of cometary origin incorporating primitive materials from the outer Solar System. Further evidence on Phoebe's past comes from density measurements made by two other instrument systems on Cassini. Phoebe's composition is distinctly different from the ice-rich material that formed the intermediate-sized saturnian satellites, and is consistent with formation from the same material out of which Pluto and Triton (archetypical Kuiper-belt objects) formed. The origin of Phoebe, which is the outermost large satellite of Saturn, is of particular interest because its inclined, retrograde orbit suggests that it was gravitationally captured by Saturn, having accreted outside the region of the solar nebula in which Saturn formed 1 . By contrast, Saturn's regular satellites (with prograde, low-inclination, circular orbits) probably accreted within the sub-nebula in which Saturn itself formed 2 . Here we report imaging spectroscopy of Phoebe resulting from the Cassini–Huygens spacecraft encounter on 11 June 2004. We mapped ferrous-iron-bearing minerals, bound water, trapped CO 2 , probable phyllosilicates, organics, nitriles and cyanide compounds. Detection of these compounds on Phoebe makes it one of the most compositionally diverse objects yet observed in our Solar System. It is likely that Phoebe's surface contains primitive materials from the outer Solar System, indicating a surface of cometary origin.

Spectra from Cassini's Visual and Infrared Mapping Spectrometer reveal that the horizontal structure, height, and optical depth of Titan's clouds are highly dynamic. Vigorous cloud centers are seen to rise from the middle to the upper troposphere within 30 minutes and dissipate within the next hour. Their development indicates that Titan's clouds evolve convectively; dissipate through rain; and, over the next several hours, waft downwind to achieve their great longitude extents. These and other characteristics suggest that temperate clouds originate from circulation-induced convergence, in addition to a forcing at the surface associated with Saturn's tides, geology, and/or surface composition.

We report on the characteristics of tidal variations in the Martian lower atmosphere (<45 km) using the Mars Express (MEX) Planetary Fourier Spectrometer (PFS) temperature data for about three Martian years (between the ends of MY26 and MY29). The PFS data, which widely cover local time, enable us to investigate diurnal variations in the atmospheric temperature at various altitudes. We focus on diurnal variations in the atmospheric temperature and on longitudinal temperature variability in a fixed local time frame. We find that the latitudinal and diurnal variations at 0.52 mbar (∼25 km) during the dust‐clear period (Ls = 30°–60°) are consistent with general characteristics presented by previous numerical simulations. The characteristics of the diurnal variations as a function of altitude in the tropics are also explained as results from the propagation of the migrating diurnal tide. The longitudinal temperature variability in the dayside (14.36–14.94 LT) equatorial regions (10°S–5°S) near the northern summer solstice (Ls= 76°–83°) in MY28 are investigated. The longitudinal temperature structure has two local maxima at 2.85 mbar (∼10 km) but is relatively uniform at 0.52 mbar. We find that the wave‐3 structure is apparent at 0.11 mbar (∼40 km) in the present case. This structure would be strongly dependent on activities of the atmospheric waves, e.g., the diurnal Kelvin wave 2 (DK2). Key Points Latitudinal and diurnal temperature variations are consistent with GCM results Diurnal variations in the tropics are explained by the migrating diurnal tide The wave‐3 structure caused by DK2 is dominated at altitude of ~40 km

In this study we present temperature profiles in the lower atmosphere of Mars from simultaneous observations performed by the Planetary Fourier Spectrometer (PFS) aboard the Mars Express spacecraft and the Miniature Thermal Emission Spectrometer (Mini‐TES) aboard the Mars Exploration Rovers. Thermal infrared spectra were collected in both the upward and downward looking geometries from the surface and from orbit, respectively. We used two sets of criteria to select PFS observations. These criteria took into account the location around the landing sites of the rovers, the local time (LT), and the solar longitude (Ls) corresponding to the Martian solar day (sol). The first set of criteria included PFS measurements carried out within ±1° in latitude and longitude, within 1 h in local time, and on the same sol. From the restricted set of measurements we conclude that the PFS data are consistent with the Mini‐TES data. The next set of criteria covered the area 5° × 5° around the landing sites, within 1 h in local time and within 9 sols. The latter criteria allow us to study the variation of parameters LT, distance, and Ls and their influence on changes of temperature profiles. This comparison for the group with relaxed criteria showed also that local time has strongest effect on temperature differences. The main purpose of this study is to confirm the validity of PFS temperature profiles close to the surface. Atmospheric temperatures below 5 km are retrieved from satellite measurements with a large uncertainty because of poor pieces of information in the wings of the CO2 absorption band at 667 cm−1. The Mini‐TES temperature profiles span atmospheric layers below 2 km. The good correspondence observed in a number of cases confirms the possibility of using PFS measurements to investigate the lower atmosphere.

The Cluster Ion Spectrometry (CIS) experiment is a comprehensive ionic plasma spectrometry package on-board the four Cluster spacecraft capable of obtaining full three-dimensional ion distributions with good time resolution (one spacecraft spin) with mass per charge composition determination. The requirements to cover the scientific objectives cannot be met with a single instrument. The CIS package therefore consists of two different instruments, a Hot Ion Analyser (HIA) and a time-of-flight ion COmposition and DIstribution Function analyser (CODIF), plus a sophisticated dual-processor-based instrument-control and Data-Processing System (DPS), which permits extensive on-board data-processing. Both analysers use symmetric optics resulting in continuous, uniform, and well-characterised phase space coverage. CODIF measures the distributions of the major ions (H^sup +^, He^sup +^, He^sup ++^, and O^sup +^) with energies from ~0 to 40 keV/e with medium (22.5°) angular resolution and two different sensitivities. HIA does not offer mass resolution but, also having two different sensitivities, increases the dynamic range, and has an angular resolution capability (5.6° × 5.6°) adequate for ion-beam and solar-wind measurements.[PUBLICATION ABSTRACT]

The recent identification of large deposits of sulphates by remote sensing and in situ observations has been considered evidence of the past presence of liquid water on Mars. Here we report the unambiguous detection of diverse phyllosilicates, a family of aqueous alteration products, on the basis of observations by the OMEGA imaging spectrometer on board the Mars Express spacecraft. These minerals are mainly associated with Noachian outcrops, which is consistent with an early active hydrological system, sustaining the long-term contact of igneous minerals with liquid water. We infer that the two main families of hydrated alteration products detected—phyllosilicates and sulphates—result from different formation processes. These occurred during two distinct climatic episodes: an early Noachian Mars, resulting in the formation of hydrated silicates, followed by a more acidic environment, in which sulphates formed. Ebb and flow The OMEGA spectrometer, orbiting on board Mars Express, is scanning the martian surface for signs of specific minerals. It has now detected a family of clays known as phyllosilicates, produced when volcanic basalt encounters water for long periods. The minerals are found mainly in rocky outcrops deposited early in martian history. The presence of a second family of sulphates suggests that there was a later, sporadically wet period, characterized by more acidic conditions.

The inventory of water and carbon dioxide reservoirs on Mars are important clues for understanding the geological, climatic and potentially exobiological evolution of the planet 1 . From the early mapping observation of the permanent ice caps on the martian poles 2 , 3 , the northern cap was believed to be mainly composed of water ice, whereas the southern cap was thought to be constituted of carbon dioxide ice. However, recent missions (NASA missions Mars Global Surveyor and Odyssey) have revealed surface structures 4 , altimetry profiles 5 , underlying buried hydrogen 6 , and temperatures of the south polar regions that are thermodynamically consistent with a mixture of surface water ice and carbon dioxide 7 . Here we present the first direct identification and mapping of both carbon dioxide and water ice in the martian high southern latitudes, at a resolution of 2 km, during the local summer, when the extent of the polar ice is at its minimum. We observe that this south polar cap contains perennial water ice in extended areas: as a small admixture to carbon dioxide in the bright regions; associated with dust, without carbon dioxide, at the edges of this bright cap; and, unexpectedly, in large areas tens of kilometres away from the bright cap.

The Cassini visual and infrared mapping spectrometer (VIMS) investigation is a multidisciplinary study of the Saturnian system. Visual and near-infrared imaging spectroscopy and high-speed spectrophotometry are the observational techniques. The scope of the investigation includes the rings, the surfaces of the icy satellites and Titan, and the atmospheres of Saturn and Titan. In this paper, we will elucidate the major scientific and measurement goals of the investigation, the major characteristics of the Cassini VIMS instrument, the instrument calibration, and operation, and the results of the recent Cassini flybys of Venus and the Earth-Moon system.