Dissipation of tidal energy in the earth's mantle and the moon was calculated assuming a dissipation factor 1/Q constant throughout both bodies. In the mantle the dissipation varies from about 2 × 10−6/Q erg cm−3 sec−1 near the pole at the bottom of the mantle to about 0.02 × 10−6/Q erg cm−3 sec−1 near the surface. The effects of compressibility and inhomogeneity are less than 3%. In a homogeneous moon the dissipation varies from a maximum of about 0.03 × 10−6/Q erg cm−3 sec−1 near the center to a minimum of about 0.4 × 10−9/Q erg cm−3 sec−1 at the surface. A theory of orbital evolution is developed in which the disturbing function is expressed in a Fourier series with respect to time, so that the effects of variation of dissipation factor 1/Q, or lag angle ϵ, with amplitude and frequency can be examined. Comparisons with results of other authors are made.

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The nearly global radar imaging and altimetry measurements of the surface of Venus obtained by the Magellan spacecraft have revealed that deformational features of a wide variety of styles and spatial scales are nearly ubiquitous on the planet. Many areas of Venus record a superposition of different episodes of deformation and volcanism. This deformation is manifested both in areally distributed strain of modest magnitude, such as families of graben and wrinkle ridges at a few to a few tens of kilometers spacing in many plains regions, as well as in zones of concentrated lithospheric extension and shortening. The common coherence of strain patterns over hundreds of kilometers implies that even many local features reflect a crustal response to mantle dynamic processes. Ridge belts and mountain belts, which have characteristic widths and spacings of hundreds of kilometers, represent successive degrees of lithospheric shortening and crustal thickening. The mountain belts of Venus, as on Earth, show widespread evidence for lateral extension both during and following active crustal compression. Venus displays two principal geometrical variations on lithospheric extension: the quasi‐circular coronae (75–2600 km diameter) and broad rises with linear rift zones having dimensions of hundreds to thousands of kilometers. Both are sites of significant volcanic flux, but horizontal displacements may be limited to only a few tens of kilometers. Few large‐offset strike slip faults have been observed, but limited local horizontal shear is accommodated across many zones of crustal stretching or shortening. Several large‐scale tectonic features have extremely steep topographic slopes (in excess of 20°–30°) over a 10‐km horizontal scale; because of the tendency for such slopes to relax by ductile flow in the middle to lower crust, such regions are likely to be tectonically active. In general, the preserved record of global tectonics of Venus does not resemble oceanic plate tectonics on Earth, wherein large, rigid plates are separated by narrow zones of deformation along plate boundaries. Rather tectonic strain on Venus typically involves deformation distributed across broad zones tens to a few hundred kilometers wide separated by comparatively undeformed blocks having dimensions of hundreds of kilometers. These characteristics are shared with actively deforming continental regions on Earth. The styles and scales of tectonic deformation on Venus may be consequences of three differences from the Earth: (1) The absence of a hydrological cycle and significant erosion dictates that multiple episodes of deformation are typically well‐preserved. (2) A high surface temperature and thus a significantly shallower onset of ductile behavior in the middle to lower crust gives rise to a rich spectrum of smaller‐scale deformational features. (3) A strong coupling of mantle convection to the upper mantle portion of the lithosphere, probably because Venus lacks a mantle low‐viscosity zone, leads to crustal stress fields that are coherent over large...

Magellan radar images and altimetry data and Pioneer Venus gravity data of major highlands and lowlands on Venus are examined with the objective of relating these tectonic and volcanic landforms to convection within Venus' mantle. Two roughly circular lowlands, Atalanta and Lavinia planitiae, are bowl‐shaped depressions which contain compressional tectonic features and lack hotspot‐related features such as coronae and shield volcanoes. They are identified as surface expressions of young mantle coldspots or regions of mantle downwelling. Highlands can be divided into two distinct groups: volcanic rises and plateau‐shaped highlands. Volcanic rises are domical, circular to elongate regions characterized by volcanic construction; extensional tectonism; large, positive gravity and geoid anomalies centered on the highlands; and large apparent depths of compensation. Volcanic rises include Beta, Bell, Atla, and Western Eistla regiones and are identified as surface expressions of large mantle hotspots or plumes. Plateau‐shaped highlands are steep‐sided elevated regions that range from circular (Alpha Regio) to elongate or irregular (Ovda Regio, Tellus Regio) in shape. Their surfaces are dominated by complex ridged terrain, rather than the shield volcanoes and flows that dominate volcanic rises. Most plateau‐shaped highlands contain compressional structures, some of which commonly lie along their margins, and have small gravity and geoid anomalies and small apparent depths of compensation compared to volcanic rises. Plateau‐shaped highlands, which include those mentioned above and Western Ishtar Terra, Thetis Regio, and Phoebe Regio, are identified as surface expressions of mantle coldspots or regions of downwelling. The highland coldspot features are elevated as a consequence of crustal thickening; crustal thickening is absent or minor at lowland coldspots. The importance of mantle downwelling in the tectonic deformation of Venus suggests that its mantle, like the mantle of Earth, is strongly heated from within. The major Venusian hotspots attest to the existence of large mantle plumes carrying heat upward from the planet's core.

Improved estimates of impact energy partitioning are combined with models of planetesimal size distribution and planetary growth to infer the early thermal evolutions of the earth and the moon. The early stage partitioning of impact energy to kinetic energy is used to limit the contribution of impacts to heat transfer by turbulent mixing. The greatest uncertainty in the models is the portion of the impact energy partitioned to internal energy which is retained after material falls back in great impacts. Models of the earth with dynamically plausible growth times of the order of 50 m.y. are found to get hot enough for vaporization to occur if more than about 12% of the heat energy is retained upon impact. There is a slight positive correlation of heating with growth time, since a longer time implies larger planetesimals. Binary accretion models of the moon allowing for enhancement of velocities by the proximity of the earth do not get hot enough for appreciable melting unless a planetesimal mass distribution starting at a rather high value, ME/20 or more, is assumed. This melting occurs deeper than is inferred from petrological and thermotectonic data. Hence these results favor formation of the moon as a consequence of a great impact (or impacts) into the earth (Kaula, 1977).