Global distribution of near-surface hydrogen on Mars Feldman, W. C.; Prettyman, T. H.; Maurice, S. ...
Journal of Geophysical Research - Planets,
September 2004, Letnik:
109, Številka:
E9
Journal Article, Conference Proceeding
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Neutron data observed using the Neutron Spectrometer aboard 2001 Mars Odyssey provide a lower limit to the global inventory of Martian water‐equivalent hydrogen. Hydrogen‐rich deposits ranging ...between about 20% and 100% water‐equivalent by mass are found poleward of ±50° latitude, and less rich, but significant, deposits are found at near‐equatorial latitudes. The equatorial deposits between ±45° latitude range between 2% and 10% water‐equivalent hydrogen by mass and reach their maximum in two regions that straddle the 0‐km elevation contour. Higher water abundances, up to ∼11%, are required in subsurface regolith of some equatorial regions if the upper 10 g/cm2 of regolith is desiccated, as suggested on average by comparison of epithermal and fast neutron data. The hydrogen contents of surface soils in the latitude range between 50° and 80° north and south are equal within data uncertainties. A lower‐limit estimate of the global inventory of near surface hydrogen amounts to a global water layer about 14 cm thick if the reservoir sampled from orbit is assumed to be 1 m thick.
•LEND/LRO instrument neutron counting data sets have been analyzed to create high resolution maps of epithermal neutron flux at polar regions of the Moon.•The conversion from epithermal neutron flux ...to the H/H2O abundances is presented based on the simple subsurface models.•The polar lunar maps (poleward 70S/70N) of homogeneous hydrogen distribution are derived and discussed.
We present a method of conversion of the lunar neutron counting rate measured by the Lunar Reconnaissance Orbiter (LRO) Lunar Exploration Neutron Detector (LEND) instrument collimated neutron detectors, to water equivalent hydrogen (WEH) in the top ∼1m layer of lunar regolith. Polar maps of the Moon's inferred hydrogen abundance are presented and discussed.
We examine a sample of 2301 gamma-ray bursts, detected by Konus-Wind in the triggered mode between 1994 and 2017 and localized by the interplanetary network (IPN), for evidence of gravitational ...lensing. We utilize all the available gamma-ray burst (GRB) data: time histories, localizations, and energy spectra. We employ common IPN techniques to find and quantify similarities in the light curves of 2,646,150 burst pairs, and for the pairs with significant similarities, we examine their IPN localizations to determine whether they are consistent with a common origin. For pairs that are consistent, we derive and compare energy spectra, and compute a figure of merit that allows us to compare and rank burst pairs. We conduct both a blind search, between all possible burst pairs, and a targeted search, between pairs in which one burst has both a spectroscopic redshift and an identification of an intervening system, as measured by one or more lower spectroscopic redshifts. We identify six pairs in the blind search that could be taken as evidence for lensing, but none are compelling enough to claim a detection with good confidence. No candidates were detected in the targeted search. For our GRB sample, we set an upper limit to the optical depth to lensing of 0.0033, which is comparable to that of optical sources. We conclude that proposed scenarios in which a large fraction of the GRB population is lensed are extremely unlikely.
Estuarine and coastal marine sediment-water fluxes are considered to be important ecological features, but a global-scale assessment has yet to be developed. Goals of this work were to assemble a ...global-scale database of net sediment-water flux measurements, examine measurement techniques, characterize the geographic distribution and magnitude of sediment fluxes, explore the data for controls on sediment flux magnitude, and assess the importance of sediment fluxes in ecosystem-level metabolism and primary production. We examined 480 peer-reviewed sources and found sediment flux data for 167 estuarine and coastal systems. Most measurements were made in North America, Europe, and Australia. Fluxes varied widely among systems, some by several orders of magnitude. Inter-annual variability within sites was less than an order of magnitude but time series flux data to evaluate this were rare. However, limited time series data exhibited large and rapid responses to decreased external nutrient loading rates, climate change effects (possible temperature effects), and variability in trophic conditions. Comparative analyses indicated organic matter supply to sediments set the upper limits of flux magnitude, with other factors playing secondary roles. Two metrics were developed to assess ecosystem-level importance of sediment-water fluxes. Sediments represented 30% or more of depth-integrated rates of aerobic system respiration at depths of <10 m. An annual phytoplankton production data set was used to estimate N and P demand; sediments supplied an average of 15-32% of N and 17-100% of P demand and, in some cases, was as large or larger than external nutrient inputs. The percent of demand supplied by sediments was highest in temperate latitudes and lower in high and tropical latitudes.
Measurements of sediment–water fluxes of O
2
, NO
23
, NH
4
, and PO
4
and water column and sediment variables were conducted at 348 sites in Chesapeake Bay and Maryland Coastal Bays with most (~ ...76%) of the 1746 sets of measurements collected during warm seasons when these processes were most active. We performed a system-wide synthesis of these spatially extensive, long-term data to identify the primary controlling factors on sediment–water fluxes over seasonal and interannual time periods and assess the relative contribution of sediment–water fluxes to nutrient cycling across distinct regions of Chesapeake Bay and the Maryland Coastal Bays. Bay-wide spatial patterns revealed hotspots for sediment–water fluxes, and statistical models were able to explain 46% (O
2
), 23% (NH
4
), 25% (NO
23
), and 38% (PO
4
) of variability in fluxes, with solute-specific controlling variables including temperature, bottom water oxygen and nutrient concentration, and sediment organic matter. An analysis of long-term variations in fluxes at six locations in the Bay (12–17 year time series) exhibited only weak evidence of long-term trends, but interannual variability was related to both water column and sediment variables, depending on the solute of interest. Finally, we compared external loads of “new” total nitrogen and phosphorus (TN and TP) to system-wide sediment–water fluxes of NH
4
and PO
4
at 22 Bay tributary and Coastal Bays sites, finding that ~ 64% of sites had annual sediment recycling rates that exceeded annual external loading rates, revealing the importance of recycled nutrients in these shallow systems.
After 55 days of mapping by the High Energy Neutron Detector onboard Mars Odyssey, we found deficits of high-energy neutrons in the southern highlands and northern lowlands of Mars. These deficits ...indicate that hydrogen is concentrated in the subsurface. Modeling suggests that water ice-rich layers that are tens of centimeters in thickness provide one possible fit to the data.
The evidence for sulfate‐bearing strata, across Late‐Noachian to Amazonian eons, suggests a central role for sulfates in acidity and salinity of Martian paleofluids and the planet's habitability. ...However, details remain unclear owing to shallow sampling and the limited ability of visible/near‐infrared spectroscopy to distinguish among some sulfates. Using chemical data from the Mars Odyssey gamma ray spectrometer, including the sulfur map of Mars, we confirm the possibility of hydrous sulfates acting as key hydrates throughout the southern midlatitudinal soil at decimeter depths. An H2O:S molar ratio between 2.4 and 4.0 for 80% of the midlatitudes is also consistent with hydrous sulfate phases, including the many Fe sulfates hydrated in this range or mixtures of Ca and Mg sulfates. Nevertheless, hydrous Fe sulfates could explain our observations in a simpler manner relative to Ca/Mg mixtures. Furthermore, phyllosilicates, zeolites, amorphous phases, and H2O(s) do not seem to be strong candidates to explain the H‐S variations. Consequently, we speculate that sulfates, as the primary contributor of H2O in bulk soil, may influence modern aqueous processes including warm‐season slope lineae in the southern hemisphere.
Key Points
Sulfate subsurface abundance and hydration insight from gamma spectraH2O:S molar ratio between 2.4 and 4.0 for 80% of the midlatitudesRegional association of H2O with S in bulk soil at decimeter depth in the south
We report maps of the concentrations of H, Si, Cl, K, Fe, and Th as determined by the Gamma Ray Spectrometer (GRS) on board the 2001 Mars Odyssey Mission for ±∼45° latitudes. The procedures by which ...the spectra are processed to yield quantitative concentrations are described in detail. The concentrations of elements determined over the locations of the various Mars landers generally agree well with the lander values except for Fe, although the mean of the GRS Fe data agrees well with that of Martian meteorites. The water‐equivalent concentration of hydrogen by mass varies from about 1.5% to 7.5% (by mass) with the most enriched areas being near Apollinaris Patera and Arabia Terra. Cl shows a distribution similar to H over the surface except that the Cl content over Medusae Fossae is much greater than elsewhere. The map of Fe shows enrichment in the northern lowlands versus the southern highlands. Silicon shows only very modest variation over the surface with mass fractions ranging from 19% to 22% over most of the planet, though a significant depletion in Si is noted in a region west of Tharsis Montes and Olympus Mons where the Si content is as low as 18%. K and Th show a very similar pattern with depletions associated with young volcanic deposits and enrichments associated with the TES Surface Type‐2 material. It is noted that there appears to be no evidence of significant globally distributed thick dust deposits of uniform composition.
Simulated Thermal Evolved Gas Analyzer (TEGA) analyses have shown that a CO2 release detected between 400°C and 680°C by the Phoenix Lander's TEGA instrument may have been caused by a reaction ...between calcium carbonate and hydrated magnesium perchlorate. In our experiments a CO2release beginning at 385 ± 12°C was attributed to calcite reacting with water vapor and HCl gas from the dehydration and thermal decomposition of Mg‐perchlorate. The release of CO2 is consistent with the TEGA detection of CO2 released between 400 and 680°C, with the amount of CO2increasing linearly with added perchlorate. X‐ray diffraction (XRD) experiments confirmed CaCl2 formation from the reaction between calcite and HCl. These results have important implications for the Mars Science Laboratory (MSL) Curiosity rover. Heating soils may cause inorganic release of CO2; therefore, detection of organic fragments, not CO2 alone, should be used as definitive evidence for organics in Martian soils.
Key Points
Hydrated magnesium perchlorate decomposes to release HCl gas when heated
Evolved HCl reacts with carbonates to evolve carbon dioxide
Carbon dioxide releases are not proof of organic molecules in Martian soils
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