A model ‘solar nebula’ is constructed by adding the solar complement of light elements to each planet, using recent models of planetary compositions. Uncertainties in this approach are estimated. The computed surface density varies approximately asr
−3/2. Mercury, Mars and the asteroid belt are anomalously low in mass, but processes exist which would preferentially remove matter from these regions. Planetary masses and compositions are generally consistent with a monotonic density distribution in the primordial solar nebula.

The distribution of mass in the planetary system and solar nebula

During the quadrennial term 1979–1982, major advances have been made in our knowledge of the rheology of the oceanic lithosphere by the skillful combination of experimental and theoretical rock mechanics, seismology and marine geophysics in increasingly sophisticated models for the flexure of the oceanic lithosphere at seamounts and island chains, along transform faults, and at subduction zones. The relative simplicity of plate bending geometry, thermal history, and mineralogical and chemical compositions of the oceanic plates in these settings make the geophysical observations very powerful constraints on the in situ rheology of the oceanic lithosphere. In the first part of this paper, I review the laboratory work on materials appropriate to the oceanic lithosphere with emphasis on contributions during the quadrennial period and the need for future work. The important results of flexure models incorporating realistic material properties are then summarized.

Rheology of the lithosphere

Microcracks in rocks: a review

An analysis is carried out of the hydrologic response of a water-rich Mars to climate change and to the physical and thermal evolution of its crust, with particular attention given to the potential role of the subsurface transport, assuming that the current models of insolation-driven change describe reasonably the atmospheric leg of the planet's long-term hydrologic cycle. Among the items considered are the thermal and hydrologic properties of the crust, the potential distribution of ground ice and ground water, the stability and replenishment of equatorial ground ice, basal melting and the polar mass balance, the thermal evolution of the early cryosphere, the recharge of the valley networks and outflow, and several processes that are likely to drive the large-scale vertical and horizontal transport of H2O within the crust. The results lead to the conclusion that subsurface transport has likely played an important role in the geomorphic evolution of the Martian surface and the long-term cycling of H2O between the atmosphere, polar caps, and near-surface crust.

A model for the hydrologic and climatic behavior of water on Mars

Magnetism and thermal evolution of the terrestrial planets

Direct observations of the closure of microcracks in rocks under increasing stress are reported. Uniaxial stresses up to 300 bars were applied to untreated and previously heated samples of Westerly granite and Frederick diabase by a small hydraulic press which fit entirely within a scanning electron microscope. Crack closure characteristics are found to depend on crack orientation, with cracks perpendicular to the applied stress closing and those parallel tending to open, as well as crack aspect ratio, crack intersection properties, stress concentrations and surface roughness. Uniaxial and hydrostatic stress measurements are found to be strongly dependent on fracture content as observed by SEM, and the observed hysteresis in strain measurements in the first stress cycles is also related to microscopic processes

Microcrack closure in rocks under stress: Direct observation

Mercury: Internal Structure and Thermal Evolution

The difference between the linear strain of a rock specimen and a reference sample of fused silica can be measured to a precision of 2 x 10/sup -6/. Compression curves of 2 samples, a granite with cracks produced by nonhydrostatic stress and a granodiorite with shock-induced cracks, contain linear segments. Compression curves that consist solely of such linear segments imply that the spectrum of crack aspect ratios alpha is sharply peaked about one or more values, whereas compression curves that contain both linear segments and smoothly varying segments imply that the crack spectrum consists of sharply peaked functions combined with smoothly varying functions. Therefore, some rocks contain sets of cracks with discrete values of alpha, and other rocks contain sets with both discrete values of alpha and continuous distributions of alpha.

Differential strain analysis: A new method for examining cracks in rocks

Linear strain of a rock sample as a function of hydrostatic pressure can be measured with a precision of 2 × 10−6. Such high-precision data for three orthogonal directions allow calculation of the distribution function for the porosity due to cracks closing completely at a given pressure. Such data for at least six independent directions yield the zero-pressure strain tensor due to cracks closing completely at a given pressure. The principal values and axes of this tensor distribution function provide information about the orientation of cracks as a function of closure pressure. In this manuscript we first develop the mathematical basis for the technique and then illustrate it with differential strain data for two samples, the Westerly (Rhode Island) granite and the Twin Sisters (Washington) dunite. Strain tensor calculations reveal that each of these samples has a different type of anisotropic crack distribution.

Robert W. Siegfried

Papers

Microcrack closure in rocks under stress: Direct observation

Mercury: Internal Structure and Thermal Evolution

Differential strain analysis: A new method for examining cracks in rocks

Characterization of oriented cracks with differential strain analysis

Microcracks in crustal igneous rocks: Physical properties