Interstellar

A dying massive star ends in a supernova explosion ejecting a large fraction of its mass into the interstellar medium. If this happens nearby, part of the ejecta might end on Solar System bodies and, in fact, radioactive ^{60}Fe has been detected on the Pacific ocean floor in about 2 Ma old layers. Here, we report on the detection of this isotope also in lunar samples, originating presumably from the same event. The concentration of the cosmic ray produced isotope ^{53}Mn, measured in the same samples, proves the supernova origin of the ^{60}Fe. From the ^{60}Fe concentrations found we deduce a reliable value for the local interstellar fluence in the range of 1×10^{8} at/cm^{2}. Thus, we obtain constraints on the recent and nearby supernova(e).

Comparison of water use for hydraulic fracturing for unconventional oil and gas versus conventional oil.

We compared water use for hydraulic fracturing (HF) for oil versus gas production within the Eagle Ford shale. We then compared HF water use for Eagle Ford oil with Bakken oil, both plays accounting for two-thirds of U.S. unconventional oil production in 2013. In the Eagle Ford, we found similar average water use in oil and gas zones per well (4.7-4.9 × 10(6) gallons [gal]/well). However, about twice as much water is used per unit of energy (water-to-oil ratio, WOR, vol water/vol oil) in the oil zone (WOR: 1.4) as in the gas zone (water-to-oil-equivalent-ratio, WOER: 0.6). We also found large differences in water use for oil between the two plays, with mean Bakken water use/well (2.0 × 10(6) gal/well) about half that in the Eagle Ford, and a third per energy unit. We attribute these variations mostly to geological differences. Water-to-oil ratios for these plays (0.6-1.4) will further decrease (0.2-0.4) based on estimated ultimate oil recovery of wells. These unconventional water-to-oil ratios (0.2-1.4) are within the lower range of those for U.S. conventional oil production (WOR: 0.1-5). Therefore, the U.S. is using more water because HF has expanded oil production, not because HF is using more water per unit of oil production.

Dawn at Vesta: testing the protoplanetary paradigm.

The Dawn spacecraft targeted 4 Vesta, believed to be a remnant intact protoplanet from the earliest epoch of solar system formation, based on analyses of howardite-eucrite-diogenite (HED) meteorites that indicate a differentiated parent body. Dawn observations reveal a giant basin at Vesta's south pole, whose excavation was sufficient to produce Vesta-family asteroids (Vestoids) and HED meteorites. The spatially resolved mineralogy of the surface reflects the composition of the HED meteorites, confirming the formation of Vesta's crust by melting of a chondritic parent body. Vesta's mass, volume, and gravitational field are consistent with a core having an average radius of 107 to 113 kilometers, indicating sufficient internal melting to segregate iron. Dawn's results confirm predictions that Vesta differentiated and support its identification as the parent body of the HEDs.

Ecological controls on water-cycle response to climate variability in deserts.

The impact of climate variability on the water cycle in desert ecosystems is controlled by biospheric feedback at interannual to millennial timescales. This paper describes a unique field dataset from weighing lysimeters beneath nonvegetated and vegetated systems that unequivocally demonstrates the role of vegetation dynamics in controlling water cycle response to interannual climate variability related to El Nino southern oscillation in the Mojave Desert. Extreme El Nino winter precipitation (2.3-2.5 times normal) typical of the U.S. Southwest would be expected to increase groundwater recharge, which is critical for water resources in semiarid and arid regions. However, lysimeter data indicate that rapid increases in vegetation productivity in response to elevated winter precipitation reduced soil water storage to half of that in a nonvegetated lysimeter, thereby precluding deep drainage below the root zone that would otherwise result in groundwater recharge. Vegetation dynamics have been controlling the water cycle in interdrainage desert areas throughout the U.S. Southwest, maintaining dry soil conditions and upward soil water flow since the last glacial period (10,000-15,000 yr ago), as shown by soil water chloride accumulations. Although measurements are specific to the U.S. Southwest, correlations between satellite-based vegetation productivity and elevated precipitation related to El Nino southern oscillation indicate this model may be applicable to desert basins globally. Understanding the two-way coupling between vegetation dynamics and the water cycle is critical for predicting how climate variability influences hydrology and water resources in water-limited landscapes.

Mars' south polar Ar enhancement: a tracer for south polar seasonal meridional mixing.

The gamma ray spectrometer on the Mars Odyssey spacecraft measured an enhancement of atmospheric argon over southern high latitudes during autumn followed by dissipation during winter and spring. Argon does not freeze at temperatures normal for southern winter (approximately 145 kelvin) and is left in the atmosphere, enriched relative to carbon dioxide (CO2), as the southern seasonal cap of CO2 frost accumulates. Calculations of seasonal transport of argon into and out of southern high latitudes point to meridional (north-south) mixing throughout southern winter and spring.

Global distribution of neutrons from Mars: results from Mars odyssey.

Global distributions of thermal, epithermal, and fast neutron fluxes have been mapped during late southern summer/northern winter using the Mars Odyssey Neutron Spectrometer. These fluxes are selectively sensitive to the vertical and lateral spatial distributions of H and CO2 in the uppermost meter of the martian surface. Poleward of +/-60 degrees latitude is terrain rich in hydrogen, probably H2O ice buried beneath tens of centimeter-thick hydrogen-poor soil. The central portion of the north polar cap is covered by a thick CO2 layer, as is the residual south polar cap. Portions of the low to middle latitudes indicate subsurface deposits of chemically and/or physically bound H2O and/or OH.

Distribution of hydrogen in the near surface of Mars: evidence for subsurface ice deposits.

Using the Gamma-Ray Spectrometer on the Mars Odyssey, we have identified two regions near the poles that are enriched in hydrogen. The data indicate the presence of a subsurface layer enriched in hydrogen overlain by a hydrogen-poor layer. The thickness of the upper layer decreases with decreasing distance to the pole, ranging from a column density of about 150 grams per square centimeter at -42 degrees latitude to about 40 grams per square centimeter at -77 degrees. The hydrogen-rich regions correlate with regions of predicted ice stability. We suggest that the host of the hydrogen in the subsurface layer is ice, which constitutes 35 +/- 15% of the layer by weight.

Induced radioactivity of LDEF materials and structural components.

We present an overview of the Long Duration Exposure Facility (LDEF) induced activation measurements. The LDEF, which was gravity-gradient stabilized, was exposed to the low Earth orbit (LEO) radiation environment over a 5.8 year period. Retrieved activation samples and structural components from the spacecraft were analyzed with low and ultra-low background HPGe gamma spectrometry at several national facilities. This allowed a very sensitive measurement of long-lived radionuclides produced by proton- and neutron-induced reactions in the time-dependent, non-isotropic LEO environment. A summary of major findings from this study is given that consists of directionally dependent activation, depth profiles, thermal neutron activation, and surface beryllium-7 deposition from the upper atmosphere. We also describe a database of these measurements that has been prepared for use in testing radiation environmental models and spacecraft design.

Cosmic-ray record in solar system matter.

The energetic nuclei in cosmic rays interact with meteoroids, the moon, planets, and other solar system matter. The nuclides and heavy nuclei tracks produced by the cosmic-ray particles in these targets contain a wealth of information about the history of the objects and temporal and spatial variations in the particle fluxes. Most lunar samples and meny meteorites have complex histories of cosmicray exposure from erosion, gardening, fragmentation, orbital changes, and other processes. There appear to be variations in the past fluxes of solar particles, and possibly also galactic cosmic rays, on time scales of 10(4) to 10(7) years.

Alpha Particles in Solar Cosmic Rays over the Last 80,000 Years.

Present-day (1967 to 1969) fluxes of alpha particles from solar cosmic rays, determined from satellite measurements, were used to calculate the production rates of cobalt-57, cobalt-58, and nickel-59 in lunar surface samples. Comparisons with the activities of nickel-59 (half-life, 8 x 10(4) years) measured in lunar samples indicate that the long-term and present-day fluxes of solar alpha particles are comparable within a factor of approximately 4.

Lunar surface radioactivity: preliminary results of the apollo 15 and apollo 16 gamma-ray spectrometer experiments.

Gamma-ray spectrometers on the Apollo 15 and Apollo 16 missions have been used to map the moon's radioactivity over 20 percent of its surface. The highest levels of natural radioactivity are found in Mare Imbrium and Oceanus Procellarum with contrastingly lower enhancements in the eastern maria. The ratio of potassium to uranium is higher on the far side than on the near side, although it is everywhere lower than commonly found on the earth.

Pattern of bombardment-produced radionuclides in rock 10017 and in lunar soil.

A large number of radionuclides have been measured as a function of depth in lunar rock 10017 and in bulk fines. Data are reported on (10)Be, (22)Na, (26)Al, (36)Cl, (49)V, (53)mn, (54)Mn (55)Fe, (56)Co, (57)Co, and (59)Ni and on upper limits for (46)Sc, (48)V, (51)Cr, and (60)Co. The results for several nuclides show striking evidence of excess surface production attributable to solar flare particles. Data for short-lived species, (56)Co, (57)CO, (54)Mn, (55)Fe, and (22)Na, appear consistent with fluxes from known recent events. Long-lived species demonstrate the existence of solar flare protons and alphas at least for the last 10(5) to 10(6) years, at fluxes comparable to those now observerved.