Richard E. Young

Subsolidus convective cooling histories of terrestrial planets

Abstract The subsolidus convective cooling histories of terrestrial planets evolving from hot initial states are investigated with a simple analytic model which simulates the average heat transport in a vigorously convecting mantle devoid of internal heat sources. The temperature dependence of the effective viscosity of mantle rocks is the single most important factor controlling thermal history. It is responsible for the growth of the rigid lithosphere, a rheological and thermal boundary layer, and serves the function of a thermostat, regulating the rate of cooling by the negative feedback between viscosity and temperature. Except for a relatively short period of time when mantle temperature decreases rapidly during the early stages of cooling, a planet cools mainly by thickening its lithosphere; the underlying mantle temperature decreases relatively slowly. On one-plate planets, the growth of a rigid lithosphere involves an imbalance between the surface heat flux and the heat flow from the mantle; the former is always larger than the latter. Primordial heat can contribute substantially, e.g., as much as about a fourth or a third, to the present surface heat flux of a planet. For both these reasons, the radiogenic heat source content of a planet is likely to be overestimated by inferences from surface heat flow observations.

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.

Structure of the Atmosphere of Jupiter: Galileo Probe Measurements

Temperatures and pressures measured by the Galileo probe during parachute descent into Jupiters atmosphere essentially followed the dry adiabat between 0.41 and 24 bars, consistent with the absence of a deep water cloud and with the low water content found by the mass spectrometer. From 5 to 15 bars, lapse rates were slightly stable relative to the adiabat calculated for the observed H2/He ratio, which suggests that upward heat transport in that range is not attributable to simple radial convection. In the upper atmosphere, temperatures of >1000 kelvin at the 0.01-microbar level confirmed the hot exosphere that had been inferred from Voyager occultations. The thermal gradient increased sharply to 5 kelvin per kilometer at a reconstructed altitude of 350 kilometers, as was recently predicted. Densities at 1000 kilometers were 100 times those in the pre-encounter engineering model.

Finite-amplitude thermal convection in a spherical shell

The properties of finite-amplitude thermal convection for a Boussinesq fluid contained in a spherical shell are investigated. All nonlinear terms are retained in the equations, and both axisymmetric and non-axisymmetric solutions are studied. The velocity is expanded in terms of poloidal and toroidal vectors. Spherical surface harmonics resolve the horizontal structure of the flow, but finite differences are used in the vertical. With a few modifications, the transform method developed by Orszag (1970) is used to calculate the nonlinear terms, while Greens function techniques are applied to the poloidal equation and diffusion terms. Axisymmetric solutions become unstable to non-axisymmetric perturbations at values of the Rayleigh number that depend on Prandtl number and shell thickness. However, even when stable, axisymmetric solutions are not a preferred solution to the full equations; steady non-axisymmetric solutions are obtained for the same parameter values. Initial conditions determine the characteristics of the finite-amplitude solutions, including, in the cases of non-axisymmetry, whether or not a steady state is achieved. Transitions in horizontal flow structure can occur, accompanied by a transition in functional dependence of heat flux on Rayleigh number. The dominant modes in the solutions are usually the modes most unstable to the onset of convection, but not always.

Overview of VEGA Venus Balloon in Situ Meteorological Measurements

The VEGA balloons made in situ measurements of pressure, temperature, vertical wind velocity, ambient light, frequency of lightning, and cloud particle backscatter. Both balloons encountered highly variable atmospheric conditions, with periods of intense vertical winds occurring sporadically throughout their flights. Downward winds as large as 3.5 meters per second occasionally forced the balloons to descend as much as 2.5 kilometers below their equilibrium float altitudes. Large variations, in pressure, temperature, ambient light level, and cloud particle backscatter (VEGA-1 only) correlated well during these excursions, indicating that these properties were strong functions of altitude in those parts of the middle cloud layer sampled by the balloons.

Structure of Jupiter's upper atmosphere: Predictions for Galileo

The Voyager mission to the outer solar system discovered that the thermospheres of all the giant planets are remarkably hot. To date, no convincing explanation for this phenomenon has been offered; however, there are a number of recent observational results which provide new information on the thermal structure of Jupiters upper atmosphere that bear on this outstanding problem. We present an analysis of Jupiters thermal structure using constraints from H3+ emissions, Voyager UVS occultation data, ground-based stellar occultation data, and the properties of the Jovian UV dayglow. Although the initial, separate analysis of these data sets produced contradictory results, our reanalysis shows that the observations are consistent and that the temperature profile in Jupiters upper atmosphere is well constrained. We find that the data demand the presence of a large temperature gradient, of order 3–10 K/km, near a pressure of 0.3 μbar. Analysis of the temperature profile implies that an energy source of roughly 1 erg cm−2 s−1 is required to produce the high thermospheric temperature and that this energy must be deposited in the 0.1–1.0 μbar region. It is also necessary that this energy be deposited above the region where diffusive separation of CH4 occurs, so that the energy is not radiated away by CH4. We show that dissipation of gravity waves can supply the energy required and that this energy will be deposited in the proper region. Moreover, because the turbulent mixing caused by gravity waves determines the level at which diffusive separation of CH4 occurs, the location of the energy source (dissipation of waves) and the energy sink (radiation by CH4) are coupled. We show that the gravity waves will deposit their energy several scale heights above the CH4 layer; energy is carried downward by thermal conduction in the intervening region, causing the large temperature gradient. Thus dissipation of gravity waves appears to be a likely explanation for the high thermospheric temperature. Our arguments are general and should apply to Saturn, Uranus, and Neptune, as well as Jupiter. The model temperature profiles presented here and the relationship between the gravity wave flux and thermospheric temperature are directly testable by the Atmospheric Structure Instrument carried by the Galileo probe.

Modeling the quasi-biennial oscillation's effect on the winter stratospheric circulation

Abstract The influence of the equatorial quasi-biennial oscillation (QBO) on the winter middle atmosphere is modeled with a mechanistic global primitive equation model. The models polar vortex evolution is sensitive to the lower stratospheres tropical winds, with the polar vortex becoming more (less) disturbed as the lower stratospheric winds are more easterly (westerly). This agrees with the observed relationship between wintertime polar circulation strength and the phase of the QBO in the lower stratosphere. In these experiments it is the extratropical planetary Rossby waves that provide the tropical-extratropical coupling mechanism. More easterly tropical winds in the lower stratosphere act to confine the extratropical Rossby waves farther north and closer to the vortex at the QBO altitudes, weakening the vortex relative to the case of westerly QBO phase. While the QBO winds occur in the lower stratosphere, the anomaly in the polar vortex strength is strongest at higher levels.

Radiatively forced dispersion of the Mt. Pinatubo volcanic cloud and induced temperature perturbations in the stratosphere during the first few months following the eruption

A combined 3-dimensional circulation model and aerosol microphysical/transport model is used to simulate the dispersion of the Mt. Pinatubo volcanic cloud in the stratosphere for the first few months following the eruption. Radiative heating of the cloud due to upwelling infrared radiation from the troposphere is shown to be an important factor affecting the transport. Without cloud heating, cloud material stays mostly north of the equator, whereas with cloud heating, the cloud is transported southward across the equator within the first two weeks following the eruption. Generally the simulations agree with TOMS, AVHRR, and SAGE satellite observations showing the latitude distribution of cloud material to be between about 20°S and 30°N within the first few months. Temperature perturbations in the stratosphere induced by the aerosol heating are generally 1–4 K, in the range of those observed.

A three-dimensional model of dynamical processes in the Venus atmosphere

Abstract Three–dimensional calculations of the circulation of the Venus atmosphere have resulted in mean zonal winds in the same direction and of the same magnitude as those observed, i.e., retrograde with speeds ∼100 m s−1. The solutions exhibit other observed properties of the circulation: small horizontal temperature contrasts with the larger variations being between equator and pole, meridional velocities at mid and low latitudes less than 10 m s−1, and the existence of planetary waves which at certain times show vertical flow field contours in a horizontal Y configuration. The mechanism maintaining the large zonal winds is a nonlinear instability involving both the mean meridional circulation and planetary–scale eddies. The meridional circulation is the principal means by which zonal momentum is transported vertically. Planetary–scale eddies are the principal means by which potential energy is released, and they are also significant in transporting angular momentum horizontally. Planetary rotation pl...

Calculations of the Radiative and Dynamical State of the Venus Atmosphere

Abstract Modelling the atmosphere in accord with recent spacecraft and ground-based observations, we have carried out accurate, multiple scattering calculations to determine the solar energy deposition profile in the atmosphere of Venus. We find that most of the absorbed energy is deposited in the main cloud layer region, located at altitudes above 35 km, and that the ground receives approximately 3% of the energy absorbed in toto by Venus. Using these results we have computed vertical temperature profiles under conditions of pure radiative equilibrium and radiative-convective equilibrium. Since the latter results satisfactorily match the temperature structure determined from various spacecraft observations, we infer that the greenhouse effect can account for the high surface temperature. Aerosols make an important contribution to the infrared opacity in these calculations. Finally, we discuss preliminary three-dimensional calculations of the general circulation of the atmosphere that incorporate the resu...