Comparison between polar regions of Mars from HEND/Odyssey data

In this paper, we have analyzed neutron spectroscopy data gathered by the High Energy Neutron Detector (HEND) instrument onboard Mars Odyssey for comparison of polar regions. It is known that observation of the neutron albedo of Mars provides important information about the distribution of water-ice in subsurface layers and about peculiarities of the CO₂ seasonal cycle. It was found that there are large water-rich permafrost areas with contents of up to ∼50% water by mass fraction at both the north and south Mars polar regions. The water-ice layers at high northern latitudes are placed close to the surface, but in the south they are covered by a dry and relatively thick (10-20 cm) layer of soil. Analysis of temporal variations of neutron flux between summer and winter seasons allowed the estimation of the masses of the CO₂ deposits which seasonally condense at the polar regions. The total mass of the southern seasonal deposition was estimated as 6.3×¹⁰¹⁵ kg, which is larger than the total mass of the seasonal deposition at the north by 40-50%. These results are in good agreement with predictions from the NASA Ames Research Center General Circulation Model (GCM). But, the dynamics of the condensation and sublimation processes are not quite as consistent with these models: the peak accumulation of the condensed mass of CO₂ occurred 10-15 degrees of Ls later than is predicted by the GCM.     

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.     

Analysis of MESSENGER Gamma-Ray Spectrometer data from the Mercury flybys

During its three flybys of Mercury, the MESSENGER spacecraft made the first detection of gamma-ray emission from the planet's surface. With a closest approach distance of ∼200 km, the flybys provided an opportunity to measure elemental abundances of Mercury's near-equatorial regions, which will not be visited at low altitude during MESSENGER's orbital mission phase. Despite being limited by low planetary photon flux, sufficient counts were accumulated during the first two flybys to estimate bounds on abundances for some elements having relatively strong gamma-ray spectral peaks, including Si, Fe, Ti, K, and Th. Only for Si is the standard deviation σ sufficiently small to conclude that this element was detected with 99% confidence. Iron and potassium are detected at the 2−σ (95% confidence) level, whereas only upper bounds on Ti and Th can be determined. Relative to a Si abundance assumed to be 18 weight percent (wt%), 2−σ upper bounds have been estimated as 9.7 wt% for Fe, 7.0 wt% for Ti, 0.087 wt% for K, and 2.2 ppm for Th. The relatively low upper bound on K rules out some previously suggested models for surface composition for the regions sampled. Upper bounds on Fe/Si and Ti/Si ratios are generally consistent with Ti and Fe abundances estimated from the analysis of measurements by the MESSENGER Neutron Spectrometer during the flybys but are also permissive of much lower concentrations.     

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.     

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.     

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.     

Water distribution in Martian permafrost regions from joint analysis of HEND (Mars Odyssey) and MOLA (Mars Global Surveyor) data

We jointly analyze data from the High-Energy Neutron Detector (HEND) onboard the NASA Mars Odyssey spacecraft and data from the Mars Orbiter Laser Altimeter (MOLA) onboard the Mars Global Surveyor spacecraft. The former instrument measures the content of hydrogen (in the form of H₂O or OH) in the subsurface layer of soil and the latter instrument measures the surface albedo with respect to the flux of solar energy. We have checked the presence of a correlation between these two data sets in various Martian latitude bands. A significant correlation has been found between these data at latitudes poleward of 40° in the northern hemisphere and at latitudes 40°–60° in the southern hemisphere. This correlation is interpreted as evidence for the presence of stable water ice in these regions under a dry layer of soil whose thickness is determined by the condition for equilibrium between the condensation of water from the atmosphere and its sublimation when heated by solar radiation. For these regions, we have derived an empirical relation between the flux of absorbed solar radiation and the thickness of the top dry layer. It allows the burial depth of the water ice table to be predicted with a sub-kilometer resolution based on near-infrared albedo measurements. We have found no correlation in the southern hemisphere at latitudes >60°, although neutron data also suggest that water ice is present in this region under a layer of dry soil. We conclude that the thickness of the dry layer in this region does not correspond to the equilibrium condition between the water ice table and the atmosphere.     

Seasonal Neutron-Flux Variations in the Polar Caps of Mars as Revealed by the Russian HEND Instrument Onboard the NASA 2001 Mars Odyssey Spacecraft

We analyze the flux of epithermal neutrons from the Martian surface recorded by the Russian High-Energy Neutron Detector (HEND) from February 19 through December 19, 2002. The HEND was installed onboard the NASA 2001 Mars Odyssey spacecraft and is designed to measure neutron fluxes with energies above 1 eV. Over the period of observations, statistically significant variations in the flux of epithermal (10–100 keV) neutrons were found in the northern and southern polar caps. The largest neutron-flux variations were found at subpolar latitudes, where the relative difference between the summer and winter values can reach severalfold. This correlation becomes weaker with increasing distance from the poles. Thus, the relative change in the neutron flux near the 60° parallel is slightly more than 10%. We assume that the detected variations result from the global circulation of atmospheric carbon dioxide in subpolar Martian regions. To additionally test this assumption, we compared the HEND neutron measurements onboard 2001 Mars Odyssey and the seasonal variations in the CO₂-layer thickness as observed by the Mars Orbital Laser Altimeter (MOLA) onboard Mars Global Surveyor (MGS).     

Hard X-ray and gamma-ray flares on the Sun: Stereoscopic effects near the limb from observations on the 2001 <Emphasis Type="Italic">Mars Odyssey</Emphasis> spacecraft and near-Earth spacecraft

We present the first results of data on solar flares detected by the HEND instrument onboard the 2001 Mars Odyssey interplanetary spacecraft during its flight to Mars and in orbit around Mars. The instruments carried by the spacecraft, which was developed at the Space Research Institute of the Russian Academy of Sciences, included a scintillation detector with two crystals, enabling the detection of photons with energies from tens of keV to 2.5 MeV with high time resolution. Several dozen flares were detected on both the sides visible from the Earth and back side of the Sun, supplementing other available data in a number of cases. A joint analysis of the HEND data together with data obtained in near-Earth orbits enabled the detection of variations in the integrated fluxes of photons with energies exceeding 80 keV during observations of flares near the limb from various directions. Two events were analyzed in great detail: the setting of a region displaying frequent very short flares on May 20, 2001, and the rising of the group 10486, which displayed numerous flare phenomena on the limb followed by extremely high activity in October–November 2003. These variations appear in simultaneous observations of limb M flares made at angles differing by only 8°–10°. Analyses of observations of rising sources obtained on two spacecraft lead to similar results. This indicates that the vast majority of emission at energies exceeding 80 keV arises at altitudes of no higher than seven to ten thousand kilometers. We briefly consider the powerful solar-disk gamma-ray flare of August 25, 2001. In this case, there are some differences in the behavior of the hard radiation in the decay phase for observations made at angles differing by 25°, which is most likely due to differences in the instrumental responses to radiation with this spectrum. The absence of hard radiation at great heights in the region of the “cusp” places some constraints on our picture of the physical processes occurring in powerful solar flares.