ABSTRACT The detection of a gamma-ray burst (GRB) in the solar neighborhood would have very important implications for GRB phenomenology. The leading theories for cosmological GRBs would not be able ...to explain such events. The final bursts of evaporating primordial black holes (PBHs), however, would be a natural explanation for local GRBs. We present a novel technique that can constrain the distance to GRBs using detections from widely separated, non-imaging spacecraft. This method can determine the actual distance to the burst if it is local. We applied this method to constrain distances to a sample of 36 short-duration GRBs detected by the Interplanetary Network (IPN) that show observational properties that are expected from PBH evaporations. These bursts have minimum possible distances in the 1013-1018 cm (7-105 au) range, which are consistent with the expected PBH energetics and with a possible origin in the solar neighborhood, although none of the bursts can be unambiguously demonstrated to be local. Assuming that these bursts are real PBH events, we estimate lower limits on the PBH burst evaporation rate in the solar neighborhood.
Neutron spectroscopy data acquired by Mars Odyssey are analyzed to determine the abundance and depth of near‐surface water ice as a function of latitude in the southern hemisphere as well as the ...inventory of CO2 in the south polar residual cap. The surface is modeled as a semi‐infinite, water‐rich permafrost layer covered by desiccated material, which is consistent with theoretical models of ground ice stability. Latitude‐dependent parameters, water abundance and depth, are determined from zonally averaged neutron counting data. Spatial mixing of the output of neutrons from regions within the footprint of the spectrometer is modeled, and asymmetrical features such as the residual cap are included in the analysis. Absorption of thermal neutrons by major elements other than hydrogen is found to have a significant influence on the determination of water abundance. Poleward of −60°, the water‐rich layer contains 60% ± 10% water by weight (70% to 85% by volume) and is covered by less than 15 g/cm2 ± 5 g/cm2 of dry material. The volume fraction of water is generally higher than can be accommodated in the pore space of surface soils, which implies that water vapor diffusion processes alone cannot explain the observations. Alternatives for the formation of the water‐rich layer are discussed. Results of our analysis of the residual‐cap CO2 inventory support conclusions that the atmosphere is not buffered by a larger reservoir of surface CO2 at the poles and that Mars' total CO2 inventory is well represented by the present atmospheric mass.
The latest neutron spectrometer measurements with the Lunar Exploration Neutron Detector (LEND) onboard the Lunar Reconnaissance Orbiter (LRO) are presented. It covers more than 1 year of mapping ...phase starting on 15 September 2009. In our analyses we have created global maps showing regional variations in the flux of thermal (energy range < 0.015 eV) and fast neutrons (>0.5 MeV), and compared these fluxes to variances in soil elemental composition, and with previous results obtained by the Lunar Prospector Neutron Spectrometer (LPNS). We also processed data from LEND collimated detectors and derived a value for the collimated signal of epithermal neutrons based on the comparative analysis with the LEND omnidirectional detectors. Finally, we have compared our final (after the data reduction) global epithermal neutron map with LPNS data.
The OSIRIS‐REx Asteroid Sample Return Mission is the third mission in National Aeronautics and Space Administration (NASA)'s New Frontiers Program and is the first U.S. mission to return samples from ...an asteroid to Earth. The most important decision ahead of the OSIRIS‐REx team is the selection of a prime sample‐site on the surface of asteroid (101955) Bennu. Mission success hinges on identifying a site that is safe and has regolith that can readily be ingested by the spacecraft's sampling mechanism. To inform this mission‐critical decision, the surface of Bennu is mapped using the OSIRIS‐REx Camera Suite and the images are used to develop several foundational data products. Acquiring the necessary inputs to these data products requires observational strategies that are defined specifically to overcome the challenges associated with mapping a small irregular body. We present these strategies in the context of assessing candidate sample sites at Bennu according to a framework of decisions regarding the relative safety, sampleability, and scientific value across the asteroid's surface. To create data products that aid these assessments, we describe the best practices developed by the OSIRIS‐REx team for image‐based mapping of irregular small bodies. We emphasize the importance of using 3‐D shape models and the ability to work in body‐fixed rectangular coordinates when dealing with planetary surfaces that cannot be uniquely addressed by body‐fixed latitude and longitude.
Plain Language Summary
The OSIRIS‐REx Asteroid Sample Return Mission must map asteroid (101955) Bennu using the OSIRIS‐REx Camera Suite. Here we present the techniques that are established to accomplish this goal. Mapping helps us find the best place on the surface of Bennu from which to gather a sample. Because asteroids are small bodies with weak gravitational fields, maneuvering a spacecraft around them can be challenging. Considering these complexities, we have found ways to gather images of Bennu needed for creating maps. Additionally, due to the irregular shape of many asteroids, producing 2‐D maps in terms of latitude and longitude may be insufficient for describing their surface geography. To that end, we have developed software that is capable of creating and displaying image maps in 3‐D.
Key Points
The OSIRIS‐REx Asteroid Sample Return Mission performs image‐based mapping of (101955) Bennu to aid in the selection of a sample‐site
We develop observational strategies to perform mapping to address the challenges associated with surveying a small body
We identify pitfalls and best practices for mapping images of small bodies with large concavities, elongated axes, or overhanging terrain
The data from the collimated sensors of the LEND instrument are shown to be of exceptionally high quality. Counting uncertainties are about 0.3% relative and are shown to be the only significant ...source of random error, thus conclusions based on small differences in count rates are valid. By comparison with the topography of Shoemaker crater, the spatial resolution of the instrument is shown to be consistent with the design value of 5 km for the radius of the circle over which half the counts from the lunar surface would be determined. The observed epithermal‐neutron suppression factor due to the hydrogen deposit in Shoemaker crater of 0.25 ± 0.04 cps is consistent with the collimated field‐of‐view rate of 1.7 cps estimated by Mitrofanov et al. (2010a). The statistical significance of the neutron suppressed regions (NSRs) relative to the larger surrounding polar region is demonstrated, and it is shown that they are not closely related to the permanently shadowed regions. There is a significant increase in H content in the polar regions independent of the H content of the NSRs. The non‐NSR H content increases directly with latitude, and the rate of increase is virtually identical at both poles. There is little or no increase with latitude outside the polar region. Various mechanisms to explain this steep increase in the non‐NSR polar H with latitude are investigated, and it is suggested that thermal volatilization is responsible for the increase because it is minimized at the low surface temperatures close to the poles.
Key Points
LEND is well calibrated and has excellent spatial resolution
Even ignoring NSRs, H content increases with latitude at North and South Poles
Increase in H is due to thermal volatilization being ineffective at the poles
Neutron flux measurements by the Lunar Exploration Neutron Detector (LEND) on the Lunar Reconnaissance Orbiter (LRO) enable quantifying hydrogen-bearing volatiles in the lunar surface from orbit. ...Accurately determining hydrogen abundance requires discriminating between the instrument background detection rate and the population of lunar-sourced neutrons that are sensitive to surficial hydrogen. We have investigated the detection rate for lunar and non-lunar (spacecraft-sourced) neutrons in LEND by modeling maps of measured count rate in three LEND detector systems using linear combinations of maps compiled from LEND detectors and from the Lunar Prospector Neutron Spectrometer. We find that 30% of the global-average 24.926 ± 0.020 neutron counts per second (cps) detected by the LEND STN3 thermal-energy neutron sensor are lunar-sourced neutrons in the thermal energy range (E < 0.4 eV), 65% are lunar-sourced neutrons in the epithermal and fast energy range (E > 0.4 eV), and 5% are from spacecraft-sourced background signal. In the SETN epithermal neutron detector, 90% of the 10.622 ± 0.002 neutron detections per second are consistent with a lunar source of epithermal and fast neutrons combined (E > 0.4 eV), with 3% due to lunar-sourced thermal neutron leakage into the detector (E < 0.4 eV), and background signal accounting for 7% of total detections. Background signal due to spacecraft-derived neutrons is substantial in the CSETN collimated detector system, accounting for 57% of the global average detection rate of 5.082 ± 0.001 cps, greater than the 48% estimated from cruise-phase data. Lunar-sourced epithermal and fast neutrons account for 43% of detected neutrons, including neutrons in collimation as well as neutrons that penetrate the collimator wall to reach the detector. We estimate a lower limit of 17% of lunar-sourced neutrons detected by CSETN are epithermal neutrons in collimation (0.37 cps), with an upper limit estimate of 54 ± 11% of lunar-sourced neutrons received in collimation, or 1.2 ± 0.2 cps global average. The pole-to-equator contrast ratio in epithermal and high-energy epithermal neutron flux indicates that the average concentration of hydrogen in the polar regolith above 80° north or south latitude is ∼105 ppmw (parts per million by weight), or 0.095 ± 0.01 wt% water-equivalent hydrogen. Above 88° north or south, the concentration increases to ∼140 ppmw, or 0.13 ± 0.02 wt% water-equivalent hydrogen. The similar pattern of neutron flux suppression at both poles suggests that hydrogen concentration generally increases nearer the pole and is not closely associated with a specific feature such as Shackleton Crater at the lunar south pole that has no northern counterpart. Epithermal neutron flux decreases with increasing latitude outside the polar regions, consistent with surface hydration that increases with latitude if that hydration extends to ∼13–40 cm into the surface.
•Populations of neutrons detected by LEND on LRO constrain upper and lower limits on collimated detector capabilities.•Calibration to deduce hydrogen concentration in regolith.•Neutron suppression is consistent with increasing presence of H-bearing volatiles with increasing latitude.•Average concentration of hydrogen within 10° of both poles is ∼105 ppmw (0.095 ± 0.01 wt% water-equivalent).•Average concentration of hydrogen within 2° of both poles is ∼140 ppmw (0.13 ± 0.02 wt% water-equivalent).
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Widespread and global efforts to improve degraded coastal ecosystems, especially those experiencing hypoxia, warrant a renewed focus on understanding and quantifying restoration ...trajectories. We describe a whole-ecosystem experiment to manipulate dissolved oxygen concentrations using large-scale destratification aeration that was used to document the biogeochemical response of a small estuary to changes in oxygen availability. The experiment was successful in creating oxic and anoxic bottom water conditions at a spatial scale much larger than that encompassed by the aerators. After a period of anoxic conditions, the return of oxygenated bottom water by destratification resulted in rapid decreases in sediment phosphate fluxes, uptake of nitrate and nitrite, and an increase in simulated denitrification rates. Bottom water nutrient concentrations responded near-simultaneously to these changes to benthic–pelagic fluxes. The rapidity with which the ecosystem responded to increases in bottom water oxygen confirms the critical role of dissolved oxygen concentrations in modulating nutrient cycling. This result also provides insight into the likely response of hypoxic and anoxic estuaries to remediation of oxygen deficits at the sediment–water interface.
•We analyze Mars Science Laboratory Dynamic Albedo of Neutrons passive mode data.•Water equivalent hydrogen content varies along the MSL traverse.•DAN passive measurements are consistent with other ...MSL instruments.
The Dynamic Albedo of Neutrons (DAN) experiment on the Mars Science Laboratory (MSL) rover Curiosity is designed to detect neutrons to determine hydrogen abundance within the subsurface of Mars (Mitrofanov, I.G. et al. 2012. Space Sci. Rev. 170, 559–582. http://dx.doi.org/10.1007/s11214-012-9924-y; Litvak, M.L. et al. 2008. Astrobiology 8, 605–613. http://dx.doi.org/10.1089/ast.2007.0157). While DAN has a pulsed neutron generator for active measurements, in passive mode it only measures the leakage spectrum of neutrons produced by the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) and Galactic Cosmic Rays (GCR). DAN passive measurements provide better spatial coverage than the active measurements because they can be acquired while the rover is moving. Here we compare DAN passive-mode data to models of the instrument’s response to compositional differences in a homogeneous regolith in order to estimate the water equivalent hydrogen (WEH) content along the first 200 sols of Curiosity’s traverse in Gale Crater, Mars. WEH content is shown to vary greatly along the traverse. These estimates range from 0.5±0.1wt.% to 3.9±0.2wt.% for fixed locations (usually overnight stops) investigated by the rover and 0.6±0.2wt.% to 7.6±1.3wt.% for areas that the rover has traversed while continuously acquiring DAN passive data between fixed locations. Estimates of WEH abundances at fixed locations based on passive mode data are in broad agreement with those estimated at the same locations using active mode data. Localized (meter-scale) anomalies in estimated WEH values from traverse measurements have no particular surface expression observable in co-located images. However at a much larger scale, the hummocky plains and bedded fractured units are shown to be distinct compositional units based on the hydrogen content derived from DAN passive measurements. DAN passive WEH estimates are also shown to be consistent with geologic models inferred from other MSL instruments, which indicate that fluvial/lacustrine activity occurred at certain locations (e.g., Yellowknife Bay).
Five large rivers that discharge on the western North Atlantic continental shelf carry about 45% of the nitrogen (N) and 70% of the phosphorus (P) that others estimate to be the total flux of these ...elements from the entire North Atlantic watershed, including North, Central and South America, Europe, and Northwest Africa. We estimate that 61· 109 moles y-1 of N and 20· 109 moles y-1 of P from the large rivers are buried with sediments in their deltas, and that an equal amount of N and P from the large rivers is lost to the shelf through burial of river sediments that are deposited directly on the continental slope. The effective transport of active N and P from land to the shelf through the very large rivers is thus reduced to 292· 109 moles y-1 of N and 13· 109 moles y-1 of P. The remaining riverine fluxes from land must pass through estuaries. An analysis of annual total N and total P budgets for various estuaries around the North Atlantic revealed that the net fractional transport of these nutrients through estuaries to the continental shelf is inversely correlated with the log mean residence time of water in the system. This is consistent with numerous observations of nutrient retention and loss in temperate lakes. Denitrification is the major process responsible for removing N in most estuaries, and the fraction of total N input that is denitrified appears to be directly proportional to the log mean water residence time. In general, we estimate that estuarine processes retain and remove 30-65% of the total N and 10-55% of the total P that would otherwise pass into the coastal ocean. The resulting transport through estuaries to the shelf amounts to 172-335· 109 moles y-1 of N and 11-9· 109 moles y-1 of P. These values are similar to the effective contribution from the large rivers that discharge directly on the shelf. For the North Atlantic shelf as a whole, N fluxes from major rivers and estuaries exceed atmospheric deposition by a factor of 3.5-4.7, but this varies widely among regions of the shelf. For example, on the U.S. Atlantic shelf and on the northwest European shelf, atmospheric deposition of N may exceed estuarine exports. Denitrification in shelf sediments exceeds the combined N input from land and atmosphere by a factor of 1.4-2.2. This deficit must be met by a flux of N from the deeper ocean. Burial of organic matter fixed on the shelf removes only a small fraction of the total N and P input (2-12% of N from land and atmosphere; 1-17% of P), but it may be a significant loss for P in the North Sea and some other regions. The removal of N and P in fisheries landings is very small. The gross exchange of N and P between the shelf and the open ocean is much larger than inputs from land and, for the North Atlantic shelf as a whole, it may be much larger than the N and P removed through denitrification, burial, and fisheries. Overall, the North Atlantic continental shelf appears to remove some 700-950· 109 moles of N each year from the deep ocean and to transport somewhere between 18 and 30· 109 moles of P to the open sea. If the N and P associated with riverine sediments deposited on the continental slope are included in the total balance, the net flux of N to the shelf is reduced by 60· 109 moles y-1 and the P flux to the ocean is increased by 20· 109 moles y-1. These conclusions are quite tentative, however, because of large uncertainties in our estimates of some important terms in the shelf mass balance.
We report the concentrations of K, Th, and Fe on the Martian surface, as determined by the gamma ray spectrometer onboard the 2001 Mars Odyssey spacecraft. K and Th are not uniformly distributed on ...Mars. K ranges from 2000 to 6000 ppm; Th ranges from 0.2 to 1 ppm. The K/Th ratio varies from 3000 to 9000, but over 95% of the surface has K/Th between 4000 and 7000. Concentrations of K and Th are generally higher than those in basaltic Martian meteorites (K = 200–2600 ppm; Th = 0.1–0.7 ppm), indicating that Martian meteorites are not representative of the bulk crust. The average K/Th in the crust is 5300, consistent with the Wänke‐Dreibus model composition for bulk silicate Mars. Fe concentrations support the idea that bulk Mars is enriched in FeO compared to Earth. The differences in K/Th and FeO between Earth and Mars are consistent with the planets accreting from narrow feeding zones. The concentration of Th on Mars does not vary as much as it does on the Moon (where it ranges from 0.1 to 12 ppm), suggesting that the primary differentiation of Mars differed from that of the Moon. If the average Th concentration (0.6 ppm) of the surface is equal to the average of the entire crust, the crust cannot be thicker than about 118 km. If the crust is about 57 km thick, as suggested by geophysical studies, then about half the Th is concentrated in the crust.
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