David Harry Grinspoon

Mars’ Surface Radiation Environment Measured with the Mars Science Laboratory’s Curiosity Rover

The Radiation Assessment Detector (RAD) on the Mars Science Laboratory’s Curiosity rover began making detailed measurements of the cosmic ray and energetic particle radiation environment on the surface of Mars on 7 August 2012. We report and discuss measurements of the absorbed dose and dose equivalent from galactic cosmic rays and solar energetic particles on the martian surface for ~300 days of observations during the current solar maximum. These measurements provide insight into the radiation hazards associated with a human mission to the surface of Mars and provide an anchor point with which to model the subsurface radiation environment, with implications for microbial survival times of any possible extant or past life, as well as for the preservation of potential organic biosignatures of the ancient martian environment.

A Reappraisal of The Habitability of Planets around M Dwarf Stars

Stable, hydrogen-burning, M dwarf stars make up about 75% of all stars in the Galaxy. They are extremely long-lived, and because they are much smaller in mass than the Sun (between 0.5 and 0.08 M(Sun)), their temperature and stellar luminosity are low and peaked in the red. We have re-examined what is known at present about the potential for a terrestrial planet forming within, or migrating into, the classic liquid-surface-water habitable zone close to an M dwarf star. Observations of protoplanetary disks suggest that planet-building materials are common around M dwarfs, but N-body simulations differ in their estimations of the likelihood of potentially habitable, wet planets that reside within their habitable zones, which are only about one-fifth to 1/50th of the width of that for a G star. Particularly in light of the claimed detection of the planets with masses as small as 5.5 and 7.5 M(Earth) orbiting M stars, there seems no reason to exclude the possibility of terrestrial planets. Tidally locked synchronous rotation within the narrow habitable zone does not necessarily lead to atmospheric collapse, and active stellar flaring may not be as much of an evolutionarily disadvantageous factor as has previously been supposed. We conclude that M dwarf stars may indeed be viable hosts for planets on which the origin and evolution of life can occur. A number of planetary processes such as cessation of geothermal activity or thermal and nonthermal atmospheric loss processes may limit the duration of planetary habitability to periods far shorter than the extreme lifetime of the M dwarf star. Nevertheless, it makes sense to include M dwarf stars in programs that seek to find habitable worlds and evidence of life. This paper presents the summary conclusions of an interdisciplinary workshop (http://mstars.seti.org) sponsored by the NASA Astrobiology Institute and convened at the SETI Institute.

Probing Venus's cloud structure with Galileo NIMS

The spectral image cubes obtained by the Near-Infrared Mapping Spectrometer (NIMS) on Galileo as it flew by Venus have been analyzed to constrain the vertical structure of the clouds, the nature of the aerosol particles, and the location and particle properties of the opacity variations responsible for high contrast features observed in the near-infrared windows at 1.7 and 2.3 μm. A radiative transfer program was used to simulate mid-latitude curves of limb darkening at 3.7 μm. Best-fit models to these curves demonstrate that the upper clouds are dominated by mode 2 particles (r = 1.0 μm), with a contribution of ≈15% of opacity from mode 1 particles (r = 0.3 μm). The low-latitude upper cloud is well represented by a dual scale-height model, with a particle scale height of ≈1 km from an altitude of 61–63 km, and a scale height of ≈ 6 km above this, up to the level where τ = 1 at approximately 71 km. This model also successfully simulates limb-darkening curves at 11.5 μm from the Pioneer Venus Orbiter Infrared Radiometer. Successful simulations of correlation plots of 1.7 vs 2.3 μm intensities reveal that mode 3 particles (r = 3.65 μm) represent the dominant source of opacity in the lower and middle clouds, and that variation in total cloud opacity reflects chiefly the addition and removal of mode 3 particles near the cloud base. We find that the full spectrum of brightnesses at 1.7 and 2.3 μm implies that the total cloud optical depth varies from ≈ 25 to ≈ 40.

WATER IN THE DEEP ATMOSPHERE OF VENUS FROM HIGH-RESOLUTION SPECTRA OF THE NIGHT SIDE

High-resolution, near-infrared (1.09 to 2.5 μm) spectra of the night side of Venus have been obtained in 1990 and 1991 using the Fourier Transform Spectrometer at the 3.6-m Canada-France-Hawaii telescope. Absorptions due to H2O were detected in spectral windows near 2.3, 1.74, and 1.18 μm. Our analysis of these absorptions constrains the abundance of water vapor in three different altitude ranges located between the clouds and the surface: 30–40 km, 15–25 km and 0–15 km. A constant water vapor mixing ratio of 30±15 ppm below the clouds can fit the observations. These values are consistent with recent near-infrared studies of the night side of Venus at lower spectral resolution. The atmosphere of Venus appears to be dryer than originally suggested by the in-situ measurements made by the Pioneer Venus and Venera mass-spectrometers and gas-chromatographs.

The abundance of sulfur dioxide below the clouds of Venus

We present a new method for determining the abundance of sulfur dioxide below the clouds of Venus. Absorption by the 3ν3 band of SO2 near 2.45 µm has been detected in high-resolution spectra of the night side of Venus recorded at the Canada-France-Hawaii telescope in 1989 and 1991. The inferred SO2 abundance is 130±40 ppm at all observed locations and pertains to the 35–45 km region. These values are comparable to those measured by the Pioneer Venus and Venera 11/12 entry probes in 1978. This stability stands in contrast to the apparent massive decrease in SO2 observed at the cloud tops since these space missions. These results are consistent with laboratory and modelling studies of the SO2 destruction rates in the lower atmosphere of Venus. The new spectroscopic technique presented here allows a remote monitoring of the SO2 abundance below the clouds, a likely tracer of Venusian volcanism.

VARIATIONS IN VENUS CLOUD-PARTICLE PROPERTIES: A NEW VIEW OF VENUS'S CLOUD MORPHOLOGY AS OBSERVED BY THE GALILEO NEAR INFRARED MAPPING SPECTROMETER

Using Venus nightside data obtained by the Galileo Near-Infrared Mapping Spectrometer, we have studied the correlation of 1.74 and 2.30 μm radiation which is transmitted through the clouds. Since the scattering and absorption properties of the cloud particles are different at these two wavelengths, one can distinguish between abundance variations and variations in the properties of the cloud particles themselves. The correlation of intensities shows a clustering of data into five distinct branches. Using radiative transfer calculations, we interpret these branches as regions of distinct but different mixes of Mode 2′ and 3 particles. The data and calculations indicate large differences in these modal ratios, the active cloud regions varying in content from nearly pure Mode 2′ particles to almost wholly Mode 3. The spatial distribution of these branches shows large scale sizes and both hemispheric symmetries and asymmetries. High-latitude concentrations of large particles are seen in both hemispheres and there is banded structure of small particles seen in both the North and South which may be related. The mean particle size in the Northern Hemisphere is greater than found in the South. If these different branch regions are due to mixing of vertically stratified source regions (e.g. photochemical and condensation source mechanisms), then the mixing must be coherent over very large spatial scales.

A sulfur-based survival strategy for putative phototrophic life in the venusian atmosphere.

Several observations indicate that the cloud deck of the venusian atmosphere may provide a plausible refuge for microbial life. Having originated in a hot proto-ocean or been brought in by meteorites from Earth (or Mars), early life on Venus could have adapted to a dry, acidic atmospheric niche as the warming planet lost its oceans. The greatest obstacle for the survival of any organism in this niche may be high doses of ultraviolet (UV) radiation. Here we make the argument that such an organism may utilize sulfur allotropes present in the venusian atmosphere, particularly S(8), as a UV sunscreen, as an energy-converting pigment, or as a means for converting UV light to lower frequencies that can be used for photosynthesis. Thus, life could exist today in the clouds of Venus.

Search for spatial variations of the H2O abundance in the lower atmosphere of Venus from NIMS-Galileo

Abstract The spectroscopic data of the Near-Infrared Mapping Spectrometer (NIMS), recorded during the Galileo flyby of Venus, are analysed to retrieve the water vapour abundance variations in the lower atmosphere of Venus at night. The 1.18 μm spectral window, which probes altitude levels below 20 km, is used for this purpose. Constraints on the CO2 continuum and far-wing opacity from existing ground-based high-resolution observations are included in the modelling of the NIMS spectra. The NIMS measurements can be fitted with a water vapour mixing ratio of 30 ± 15 ppm, in agreement with analyses of ground-based nightside observations. The water vapour abundance shows no horizontal variations exceeding 20% over a wide latitude range (40°S, 50°N) on the nightside of Venus. Within the same selection of NIMS spectra, a large enhancement in the O2 fluorescence emission at 1.27 μm is observed at a latitude of 40°S, over a spatial area about 100 km wide.

Diurnal variations of energetic particle radiation at the surface of Mars as observed by the Mars Science Laboratory Radiation Assessment Detector

The Radiation Assessment Detector onboard the Mars Science Laboratory rover Curiosity is detecting the energetic particle radiation at the surface of Mars. Data collected over the first 350 Martian days of the nominal surface mission show a pronounced diurnal cycle in both the total dose rate and the neutral particle count rate. The diurnal variations detected by the Radiation Assessment Detector were neither anticipated nor previously considered in the literature. These cyclic variations in dose rate and count rate are shown to be the result of changes in atmospheric column mass driven by the atmospheric thermal tide that is characterized through pressure measurements obtained by the Rover Environmental Monitoring Station, also onboard the rover. In addition to bulk changes in the radiation environment, changes in atmospheric shielding forced by the thermal tide are shown to disproportionately affect heavy ions compared to H and He nuclei.

The upper clouds of Venus: determination of the scale height from NIMS-Galileo infrared data

Abstract The 3–5 μm thermal emission of the nightside of Venus, recorded by the NIMS instrument at the time of the Galileo flyby of Venus, is analysed to infer the properties of the upper cloud boundary. From the global maps of Venus at fixed wavelengths, the limb darkening of the flux is measured at several latitudes, within each infrared channel. By using the nominal Pioneer Venus thermal profile, these data give access to two parameters: the cloud deck temperature and the cloud scale height. It is verified independently, from the NIMS spectra, that this thermal profile is consistent with all the NIMS observations, and that the thermal structure does not vary significantly in the latitude range (25°S, 30°N). Within this range, the cloud scale height is found to be constant with latitude, and is H = 5.2 km, with an accuracy of about 15%, taking into account the various sources of theoretical and observational uncertainties. At higher latitudes, the temperature profile becomes more isothermal and the presented method to retrieve H is no longer valid.