M. Cartacci

Radar Soundings of the Subsurface of Mars

The martian subsurface has been probed to kilometer depths by the Mars Advanced Radar for Subsurface and Ionospheric Sounding instrument aboard the Mars Express orbiter. Signals penetrate the polar layered deposits, probably imaging the base of the deposits. Data from the northern lowlands of Chryse Planitia have revealed a shallowly buried quasi-circular structure about 250 kilometers in diameter that is interpreted to be an impact basin. In addition, a planar reflector associated with the basin structure may indicate the presence of a low-loss deposit that is more than 1 kilometer thick.

The organic-rich surface of comet 67P/Churyumov-Gerasimenko as seen by VIRTIS/Rosetta

The VIRTIS (Visible, Infrared and Thermal Imaging Spectrometer) instrument on board the Rosetta spacecraft has provided evidence of carbon-bearing compounds on the nucleus of the comet 67P/Churyumov-Gerasimenko. The very low reflectance of the nucleus (normal albedo of 0.060 ± 0.003 at 0.55 micrometers), the spectral slopes in visible and infrared ranges (5 to 25 and 1.5 to 5% kÅ−1), and the broad absorption feature in the 2.9-to-3.6–micrometer range present across the entire illuminated surface are compatible with opaque minerals associated with nonvolatile organic macromolecular materials: a complex mixture of various types of carbon-hydrogen and/or oxygen-hydrogen chemical groups, with little contribution of nitrogen-hydrogen groups. In active areas, the changes in spectral slope and absorption feature width may suggest small amounts of water-ice. However, no ice-rich patches are observed, indicating a generally dehydrated nature for the surface currently illuminated by the Sun.

Exposed water ice on the nucleus of comet 67P/Churyumov–Gerasimenko

Although water vapour is the main species observed in the coma of comet 67P/Churyumov–Gerasimenko and water is the major constituent of cometary nuclei, limited evidence for exposed water-ice regions on the surface of the nucleus has been found so far. The absence of large regions of exposed water ice seems a common finding on the surfaces of many of the comets observed so far. The nucleus of 67P/Churyumov–Gerasimenko appears to be fairly uniformly coated with dark, dehydrated, refractory and organic-rich material. Here we report the identification at infrared wavelengths of water ice on two debris falls in the Imhotep region of the nucleus. The ice has been exposed on the walls of elevated structures and at the base of the walls. A quantitative derivation of the abundance of ice in these regions indicates the presence of millimetre-sized pure water-ice grains, considerably larger than in all previous observations. Although micrometre-sized water-ice grains are the usual result of vapour recondensation in ice-free layers, the occurrence of millimetre-sized grains of pure ice as observed in the Imhotep debris falls is best explained by grain growth by vapour diffusion in ice-rich layers, or by sintering. As a consequence of these processes, the nucleus can develop an extended and complex coating in which the outer dehydrated crust is superimposed on layers enriched in water ice. The stratigraphy observed on 67P/Churyumov–Gerasimenko is therefore the result of evolutionary processes affecting the uppermost metres of the nucleus and does not necessarily require a global layering to have occurred at the time of the comet’s formation.

Total electron content in the Martian atmosphere: A critical assessment of the Mars Express MARSIS data sets

The total electron content (TEC) is one of the most useful parameters to evaluate the behavior of the Martian ionosphere because it contains information on the total amount of free electrons, the m ...

Seasonal exposure of carbon dioxide ice on the nucleus of comet 67P/Churyumov-Gerasimenko

Rosetta observes sublimating surface ices Comets are “dirty snowballs” made of ice and dust, but they are dark because the ice sublimates away, leaving some of the dust behind on the surface. The Rosetta spacecraft has provided a close-up view of the comet 67P/Churyumov-Gerasimenko as it passes through its closest point to the Sun (see the Perspective by Dello Russo). Filacchione et al. detected the spectral signature of solid CO2 (dry ice) in small patches on the surface of the nucleus as they emerged from local winter. By modeling how the CO2 sublimates, they constrain the composition of comets and how ices generate the gaseous coma and tail. Fornasier et al. studied images of the comet and discovered bright patches on the surface where ice was exposed, which disappeared as the ice sublimated. They also saw frost emerging from receding shadows. The surface of the comet was noticeably less red just after local dawn, indicating that icy material is removed by sunlight during the local day. Science, this issue p. 1563, p. 1566; see also p. 1536 Solid carbon dioxide on the surface of a comet nucleus has been found by the Rosetta spacecraft. Carbon dioxide (CO2) is one of the most abundant species in cometary nuclei, but because of its high volatility, CO2 ice is generally only found beneath the surface. We report the infrared spectroscopic identification of a CO2 ice–rich surface area located in the Anhur region of comet 67P/Churyumov-Gerasimenko. Spectral modeling shows that about 0.1% of the 80- by 60-meter area is CO2 ice. This exposed ice was observed a short time after the comet exited local winter; following the increased illumination, the CO2 ice completely disappeared over about 3 weeks. We estimate the mass of the sublimated CO2 ice and the depth of the eroded surface layer. We interpret the presence of CO2 ice as the result of the extreme seasonal changes induced by the rotation and orbit of the comet.

Solar cycle variations in the ionosphere of Mars as seen by multiple Mars Express datasets

The response of the Martian ionosphere to solar activity is analyzed by taking into account variations in a range of parameters during four phases of the solar cycle throughout 2005–2012. Multiple Mars Express data sets have been used (such as Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) in Active Ionospheric Sounding, MARSIS subsurface, and MaRS Radio Science), which currently cover more than 10 years of solar activity. The topside of the main ionospheric layer behavior is empirically modeled through the neutral scale height parameter, which describes the density distribution in altitude, and can be used as a dynamic monitor of the solar wind-Martian plasma interaction, as well as of the mediums temperature. The main peak, the total electron content, and the relationship between the solar wind dynamic pressure and the maximum thermal pressure of the ionosphere with the solar cycle are assessed. We conclude that the neutral scale height was different in each phase of the solar cycle, having a large variation with solar zenith angle during the moderate-ascending and high phases, while there is almost no variation during the moderate-descending and low phases. Between end-2007 and end-2009, an almost permanent absence of secondary layer resulted because of the low level of solar X-rays. Also, the ionosphere was more likely to be found in a more continuously magnetized state. The induced magnetic field from the solar wind, even if weak, could be strong enough to penetrate more than at other solar cycle phases.

MARSIS data inversion approach: Preliminary results

An approach to the inversion of the data available from the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument on Mars Express is described. The data inversion gives an estimation of the materials composing the different detected interfaces, including the impurity (inclusion) of the first layer, if any, and its percentage, by the evaluation of the values of the permittivity that would generate the observed radio echoes. The data inversion method is based on the analysis of the surface to subsurface power ratio and the relative time delay as measured by MARSIS. The constraints, due to the known geological history of the surface, the local temperature and the thermal condition of the observed zones and the results of other instruments on Mars Express and other missions to Mars, have to be considered to improve the validity of the utilized models and the obtained results that are given in parametric way.

MARSIS Data Inversion Approach

In this paper we describe an inversion approach in order to analyze data from the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument on Mars Express. The inversion process allows the dielectric constant of the subsurface material to be estimated provided the dielectric constant of the surface is known. In addition, if impurity are present, it is possible to estimate the dielectric constant of any inclusions as well as the percentage amount of material in the inclusions relative to the host material provided knowledge of the host material up to the depth where the interface has been detected is available. The data inversion method is based on the analysis of the surface to subsurface power ratio and the relative time delay as measured by MARSIS. The data inversion has been performed at several frequencies in order to estimate the frequency dependent parameters affecting the behavior of the radar echoes. It is necessary that the surface and subsurface interfaces have the same roughness in order to estimate the Subsurface Fresnel reflectivity. As a preliminary approach, only flat surface have been selected. MOLA (Mars Orbiter Laser Altimeter) has already provided detailed data on the visible Martian surface and a simulator, with a facet model, has been utilized to use MOLA data in order to verify the correct selection of the frames that will be used for the data inversion (absence of clutter echoes).

Mars ionosphere data inversion by MARSIS surface and subsurface signals analysis

According to the Mars Express mission , the MARSIS primary scientific objectives are to map the distribution of water, both liquid and solid, in the upper portions of the crust of Mars. Moreover three secondary objectives are defined for the MARSIS experiment: subsurface geologic probing, surface characterization, and ionosphere sounding.

Mars plasma system response to solar wind disturbances during solar minimum

This paper is a phenomenological description of the ionospheric plasma and induced magnetospheric boundary (IMB) response to two different types of upstream solar wind events impacting Mars in March 2008, at the solar minimum. A total of 16 Mars Express orbits corresponding to five consecutive days is evaluated. Solar TErrestrial RElations Observatory-B (STEREO-B) at 1 AU and Mars Express and Mars Odyssey at 1.644 AU detected the arrival of a small transient interplanetary coronal mass ejection (ICME-like) on the 6 and 7 of March, respectively. This is the first time that this kind of small solar structure is reported at Marss distance. In both cases, it was followed by a large increase in solar wind velocity that lasted for ~10 days. This scenario is simulated with the Wang-Sheeley-Arge (WSA) - ENLIL + Cone solar solar wind model. At Mars, the ICME-like event caused a strong compression of the magnetosheath and ionosphere, and the recovery lasted for ~3 orbits (~20 h). After that, the fast stream affected the upper ionosphere and the IMB, which radial and tangential motions in regions close to the subsolar point are analyzed. Moreover, a compression in the Martian plasma system is also observed, although weaker than after the ICME-like impact, and several magnetosheath plasma blobs in the upper ionosphere are detected by Mars Express. We conclude that, during solar minimum and at aphelion, small solar wind structures can create larger perturbations than previously expected in the Martian system.