Exploring Venus with Balloons: Science Objectives and Recent Technical Advances
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aerobots↗
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Grinspoon, David H.
Publications and source records attributed to Grinspoon, David H..
Evolutionary implications of a steady-state water abundance on Venus
In 1987, Grinspoon proposed that the data on hydrogen abundance, isotopic composition, and escape rate were consistent with the hypothesis that water on Venus might be in steady-state rather than monotonic decline since the dawn of time. This conclusion was partially based on a derived water lifetime against nonthermal escape of approximately 10(exp 8) years. Others have questioned this conclusion. De Bergh et al. found H2O lifetimes of greater than 10(exp 9) years. Donahue and Hodges derived H2O lifetimes of 0.4 - 5 x 10(exp 9) years. The most sophisticated analysis to date of near-IR radiation from Venus' nightside reveals a water mixing ratio of approximately 30 ppm. Recent re-analysis of Pioneer Venus Mass Spectrometer Data are consistent with a water abundance of 30 ppm. Hodges and Tinsley found an escape flux due to charge exchange with hot H(+) of 2.8 x 10(exp 7) cm(exp -2) s(exp -1). Gurwell and Yung estimated an escape flux of 3.5 x 10(exp 6) cm(exp -2) s(exp -1) from collisions with hot O produced by dissociative recombination of O2(+). Brace et al. estimated an escape flux of 5 x 10(exp 6) cm(exp -2) s(exp -1) from ion escape from the ionotail of Venus. The combined estimated escape flux from all of these processes is 3.7 x 10(exp 7) cm(exp -2) s(exp -1), suggesting a lifetime against escape for water of less than 10(exp 8) years. A recent estimate of H escape flux employing a different ionospheric model and using Pioneer Venus reentry data to estimate the response of the escape flux to the solar cycle finds a somewhat lower escape flux of 1.4 x 10(exp 7) cm(exp -2) s(exp -1), suggesting a water lifetime closer to 2 x 10(exp 8) years, significantly less than the age of the planet. Large uncertainties remain in these quantities, yet the data suggest that a source of water more recent than primordial sources is required and that a steady-state is likely. To obvious candidates for this source water are cometary impact and volcanic outgassing. Other aspects of this investigation are discussed.
Grinspoon, David H.↗
Monte Carlo modeling of the resurfacing of Venus
We have developed a three-dimensional model of venusian resurfacing that employs Monte Carlo simulations of both impact cratering and volcanism. The model simulates the production of craters on Venus by using the observed mass distributions of Earth- and Venus-crossing asteroids and comets. Lava flows are modeled by an energy minimization technique to simulate the effects of local topography on the shape and extent of flows. The model is run under a wide range of assumptions regarding the scale and time evolution of volcanism on Venus. Regions of the parameter space that result in impact crater distributions and modifications that are currently observed will be explored to place limits on the possible volcanic resurfacing history of Venus.
Bullock, M. A.↗
Venusian hydrology: Steady state reconsidered
In 1987, Grinspoon proposed that the data on hydrogen abundance, isotopic composition, and escape rate were consistent with the hypothesis that water on Venus might be in steady state rather than monotonic decline since the dawn of time. This conclusion was partially based on a derived water lifetime against nonthermal escape of approximately 10(exp 8) yr. De Bergh et al., preferring the earlier Pioneer Venus value of 200 ppm water to the significantly lower value detected by Bezard et al., found H2O lifetimes of greater than 10(exp 9) yr. Donahue and Hodges derived H2O lifetimes of 0.4-5 x 10 (exp 9) yr. Both these analyses used estimates of H escape flux between 0.4 x 10(exp 7) and 1 x 10(exp 7) cm(exp -2)s(exp -1) from Rodriguez et al. Yet in more recent Monte Carlo modeling, Hodges and Tinsley found an escape flux due to charge exchange with hot H(+) of 2.8 x 10(exp 7) cm(exp -2)s(exp -1). McElroy et al. estimated an escape flux of 8 x 10(exp 6) cm(exp -2)s(exp -1) from collisions with hot O produced by dissociative recombination of O2(+). Brace et al. estimated an escape flux of 5 x 10(exp 6) cm(exp -2)s(exp -1) from ion escape from the ionotail of Venus. The combined estimated escape flux from all these processes is approximately 4 x 10(exp 7) cm(exp -2)s(exp -1). The most sophisticated analysis to date of near-IR radiation from Venus' nightside reveals a water mixing ratio of approximately 30 ppm, suggesting a lifetime against escape for water of less than 10(exp 8) yr. Large uncertainties remain in these quantities, yet the data point toward a steady state. Further evaluation of these uncertainties, and new evolutionary modeling incorporating estimates of the outgassing rate from post-Magellan estimates of the volcanic resurfacing rate are presented.
Grinspoon, David H.↗
The faint young sun problem
It is shown that the faint young sun problem was most likely solved by an increase in atmospheric CO2 concentration in the earth's atmosphere brought about by the CO2 geochemical cycle. Because the loss process for atmospheric CO2 requires liquid water, and because the earth is continually resupplying atmospheric CO2 by carbonate metamorphism, the surface temperature should never have fallen below the point at which the ocean would freeze. Indeed, the early earth may have been quite warm if carbonate metamorphism was faster and if the continents were originally smaller, so that silicate weathering was inhibited.
Kasting, James F.↗
Vertical distribution of water in the atmosphere of Venus - A simple thermochemical explanation
Several lines of evidence concerning the vertical abundance profile of water in the atmosphere of Venus lead to strikingly unusual distributions (the water vapor abundance decreases sharply in the immediate vicinity of the surface) or to serious conflicts in the profiles (different IR bands suggest water abundances that are discrepant by a factor of 2.5 to 10). These data sets can be reconciled if (1) water molecules associate with carbon dioxide and sulfur trioxide to make gaseous carbonic acid and sulfuric acid in the lower atmosphere, and (2) the discrepant 0.94-micrometer water measurements are due to gaseous sulfuric acid, requiring it to be a somewhat stronger absorber than water vapor in this wavelength region. A mean total water abundance of 50 + or - 20 parts/million and a near-surface free water vapor abundance of 10 + or - 4 parts/million are derived.
Lewis, John S.↗
Cometary water on Venus - Implications of stochastic impacts
Attention is given to a Venus water abundance model, incorporating a stochastic cometary source and nonthermal hydrogen escape, that reproduces both the near-steady-state balance between escape loss and infall replenishment implied by Venus' short water lifetime, and the consistency of the observed deuterium-to-hydrogen ratio with a steady state. It is shown that the stochastic variability of each of these quantities is large. Water's quasi-steady state on Venus is judged to be mediated by comet impacts, leading to an obscuration of the early water history of Venus by the history of random impacts.
Grinspoon, David H.↗
Was Venus wet? Deuterium reconsidered
The ratio of deuterium to hydrogen on Venus has been accepted as proof of a wetter, more earth-like part on that planet. However, the present-day water abundance and the nonthermal hydrogen escape flux on Venus imply that hydrogen is in a steady state and that a hydrogen source, most likely cometary infall, is present. An alternative interpretation of the D/H ratio is offered, in which the measured value is consistent with a steady-state evolution over the age of the solar system. No past water excess is required to explain the isotopic data.
Grinspoon, David H.↗
Deuterium fractionation in the presolar nebula - Kinetic limitations on surface catalysis
A quantitative test is presented for the possibility that grain-based catalysis could shorten the equilibration times of low temperature equilibrium fractionation sufficiently to account for the D/H ratio values of the solar system even at nebular temperatures. It is found that under the highly idealized conditions in which the full cosmic abundance of Ni is available for catalysis in pure, 5-micron grains, the equilibration time constant becomes greater than the nebular lifetime at temperatures below 560 K. This lower limit is not, however, sufficiently low to permit strong fractionation.
Grinspoon, David H.↗