Search for Water in Martian Soil Using Global Neutron Mapping by the Russian HEND Instrument Onboard the US 2001 Mars Odyssey Spacecraft

We present the first results of the global neutron mapping of Mars by the Russian High-Energy Neutron Detector (HEND) onboard the US 2001 Mars Odyssey spacecraft. Global neutron maps of Mars in various spectral ranges allow the content of water ice and adsorbed and bound water in a near-surface layer of the planet 1 to 2 m in thickness to be estimated. Huge regions of permafrost with a high (several tens of percent by weight) content of water ice are shown to be present in the north and the south of Mars. The continuous observations of Mars for 12 months, from February 18, 2002, through February 8, 2003, are indicative of significant seasonal variations on Mars where the transition from northern winter to northern summer occurred.     

Search for short cosmic gamma-ray bursts using data from the APEX and LILAS experiments onboard the Phobos-2 interplanetary spacecraft

A statistical analysis of the spectral and temporal parameters for 546 triggering events on the APEX gamma-ray detector onboard the Phobos-2 spacecraft has revealed a group of 28 events that are probably short cosmic gamma-ray bursts (GRBs). The distribution of the full group of 74 events of the APEX experiment in duration parameter is bimodal in shape, which is in good agreement with the bimodal shape of the BATSE GRB distribution. A search for the detected group of short events using data from the LILAS X-ray and soft gamma-ray detector onboard the same spacecraft has yielded no positive result. A comparison of the APEX and LILAS data has led us to conclude that the short GRBs have a significantly reduced soft gamma-ray flux at energies <100 keV relative to the power law dN/dE=CE^?α with the average index α=2.62.     

Long-term observations of the evolution of the southern seasonal cap of Mars: Neutron measurements by the HEND instrument onboard the 2001 <Emphasis Type="Italic">Mars Odyssey</Emphasis> spacecraft

We present the results of five-year observations of the southern seasonal cap of Mars based on neutron spectroscopy of the surface fulfilled by the Russian HEND instrument onboard the NASA 2001 Mars Odyssey spacecraft. The numerical modeling of the observational data allowed us to reconstruct the curves of the variations of the total mass of the southern seasonal cap of Mars for different years (three Martian years) and to find the year-to-year variations of the seasonal cycle.     

Culmination of the flare activity of Group 10786 in July 2005: X-Ray observations from near-mars and near-earth orbits

A detailed study of two major solar flares that occurred in Group 10786 at the time of its disappearance behind the western limb is presented. The flares of July 14, 2005 were previously studied fairly poorly, as no RHESSI hard X-ray observations were available for themaxima of the twomost powerful of these flares. Observations carried out using the HEND equipment (on the Mars Odyssey spacecraft) developed at the Institute for Space Research in Moscow are used here to fill this gap. In the first flare, an intense, impulsive burst occurred at 07:23 UT, about 1.5 h after the onset of a weak, prolonged event. While processes in the neighborhood of the northern spot dominated in the flares of July 5–9, a powerful impulsive energy release on July 14 emerged when the flare process that originated in the North reached the southern spot. Our analysis of the flare activity of this medium-sized group reveals a gradual enhancement of the flare activity and a strong interaction between the acceleration above the magnetic-field neutral line and in the immediate vicinity of the spots. At the time of the culmination of the flare activity in the group on July 13 and 14, the pattern of nonstationary processes changes: fast coronal mass ejections form after a series of impulsive energy-release events. Spacecraft observations of the burst of July 14 after 11 UT at points separated in longitude (on RHESSI and Mars Odyssey) revealed clear anisotropy of the flare emission at energies exceeding 80 keV.     

Search for causes of the low epithermal neutron flux anomaly in the Arabia Terra region (Mars)

A geologic analysis of 274 images acquired by the high-resolution MOC camera onboard the Mars Global Surveyor spacecraft within the Arabia Terra low neutron flux anomaly (which is indicative of an anomalously high abundance of hydrogen: up to 16 wt % of the equivalent amount of water) was performed. Correlation between the enhanced abundance of equivalent water with the presence of dust on the surface was found. Since dust plays a key role in condensation of water from the atmosphere, we suppose that the anomalies could result from the retention of atmospheric moisture. To analyze this suggestion, we performed a theoretical modeling that allowed us to map the planetary-scale distributions of several meteorological parameters responsible for the atmospheric moisture condensation. Two antipodal regions coinciding rather well with the Arabia Terra anomaly and the geographically antipodal anomaly southwest of Olympus Mons were found in the maps. This suggests that the anomalies are rather recent than ancient formations. They were probably formed by a sink of moisture from the atmosphere in the areas where present meteorological conditions support this sink. Geological parameters, primarily the presence of dust, only promote this process. We cannot exclude the possibility that the Martian cryosphere, rather than the atmosphere, supplied the studied anomalies with moisture during their formation: the thermodynamic conditions in the anomaly areas could block the moisture flux from the Martian interior in the upper regolith layer. The moisture coming from the atmosphere or from the interior is likely held as chemically bound water entering into the structure of water-bearing minerals (probably, hydrated magnesium sulfates) directly from the vapor; or the moisture precipitates as frost, penetrates into microfissures, and then is bound in minerals. Probably, another geologic factor—the magnesium sulfate abundance—works in the Arabia Terra anomaly.     

Active neutron sensing of the Martian surface with the DAN experiment onboard the NASA “Curiosity” Mars rover: Two types of soil with different water content in the gale crater

This paper presents the water and chlorine content estimates on the bottom of the Martian crater Gale obtained by processing the data of active neutron sensing with the DAN experiment onboard theNASA “Curiosity”Mars rover at 412 spots along the 11-kilometer track. For 78% of the examined spots the water distribution in depth is found to be homogeneous with a mean content of 2.1±0.5% by mass (here and elsewhere variations correspond to the mean square deviations). For 22% of the examined spots the data require a two-layer model of water distribution down to the sensitivity limit of about 60 сm. The mean water content in upper layer of these spots is about 2?3% by mass, which is close to the content for spots with the homogeneous water distribution. In 8% of the examined spots the water content in the bottom layer at a depth of 27 ± 18 сm increases to 5.6 ± 2.7%. In 14% of the examined spots the water content in the bottom layer at a depth of 14 ± 7 сm decreases to 1.2 ± 0.5%. For interpretation of these results we conclude that the Gale crater has areas both with high and low water content, which correspond to distinct sedimentary layers from different past epochs, when sedimentation process took place underwater and in air correspondingly.     

Observation of the powerful solar flare of October 27, 2002 on the far side of the sun

Observations of the hard X-ray and radio event of October 27, 2002 are analyzed. This flare was observed from near-Martian orbit by the HEND instrument developed at the Space Research Institute of the Russian Academy of Sciences and installed on the Mars Odyssey satellite. Although this powerful flare was observed far over the eastern solar limb, the extended source associated with the flare was detected by RHESSI at energies up to about 60 keV. The eruptive event was observed in the radio at the Nobeyama Radio Observatory. The properties of the X-ray radiation are used to calculate the spectrum of the accelerated electrons responsible for the observed radiation, assuming that the target is thick for a Martian observer and thin for a terrestrial observer. The results are compared with the results of radio observations. The conditions for electron propagation in the corona are discussed.     

Dependence of the neutron and gamma-ray emission of Mercury on its surface composition and temperature

Results of computations of the energy spectrum of neutrons radiated by the planet Mercury, generated by the action of cosmic rays, are presented. The dependence of the neutron radiation on both the temperature and composition of the subsurface layers of the planetary regolith are investigated. The fluxes of γ-rays in lines of aluminum (²⁷Al, 7.7240 MeV), silicon (²⁸Si, 3.5390 MeV), calcium (⁴⁰Ca, 1.9427 MeV), and iron (⁵⁶Fe, 7.6312 and 7.6456 MeV) generated during neutron capture by these nuclei are also calculated. The intensity of the radiation in these lines depends on both the composition and temperature of the surface. This must be taken into account when interpreting measurements of γ-ray radiation from nuclear lines produced in neutron capture reactions on the substance of Mercury.     

The Medusae Fossae-Elysium region,Mars: geologic characteristics of the depression of epithermal-neutron flux based on HEND measurements onboard the Mars Odyssey spacecraft

We studied the geological structure and characteristics of fluxes of fast and epithermal neutrons from the area of Mars within 50°S–50° N and 120°–240° W, the Medusae Fossae-Elysium region. The study area does not show any anomalies in the energy spectrum of fast neutrons, indicating that a thin (10–20 cm) subsurface layer in the equatorial region is characterized by quite a uniform distribution of hydrogen-containing phases. At the same time, the studied region is characterized by a distinct depression in the epithermal-neutron flux, which is outlined by a 0.150–0.170 counts/s contour line and does not show any seasonal changes in its shape, depth, and latitudinal position. This suggests that the depression is not associated with the short-term (seasonal) variations of the temperature regime and that it represents an aggregation of hydrogen-containing phases with longer lifetimes. The position of the epithermal-neutron-flux depression does not correlate with regional geology, and none of the geological units can be responsible for its appearance. To a rather greater degree, the flux of epithermal neutrons correlates with the amount of dust material on the surface. This indicates that the dust material presumably played a major role in the formation of the neutron-flux depression within the Medusae Fossae-Elysium region. The depression of the epithermal-neutron flux presumably has a polygenic nature. It can be related both to the primary accumulation of water-bearing material and to the later blowing of this material by wind and transportation of the dust fraction with its simultaneous enrichment in hydrogen-containing phases (most likely, in water-bearing minerals).Translated from Astronomicheskii Vestnik, Vol. 39, No. 1, 2005, pp. 3–25. Original Russian Text Copyright © 2005 by Ivanov, Kozyrev, Litvak, Mitrofanov.