Steven Soter

Radiation forces on small particles in the solar system

Abstract We present a new and more accurate expression for the radiation pressure and Poynting-Robertson drag forces; it is more complete than previous ones, which considered only perfectly absorbing particles or artificial scattering laws. Using a simple heuristic derivation, the equation of motion for a particle of mass m and geometrical cross section A, moving with velocity v through a radiation field of energy flux density S, is found to be (to terms of order v c ) m v = ( SA c )Q pr [(1 − r c ) S − v c ] , where Ŝ is a unit vector in the direction of the incident radiation, r is the particles radial velocity, and c is the speed of light; the radiation pressure efficiency factor Qpr ≡ Qabs + Qsca(1 − 〈cos α〉), where Qabs and Qsca are the efficiency factors for absorption and scattering, and 〈cos α〉 accounts for the asymmetry of the scattered radiation. This result is confirmed by a new formal derivation applying special relativistic transformations for the incoming and outgoing energy and momentum as seen in the particle and solar frames of reference. Qpr is evaluated from Mie theory for small spherical particles with measured optical properties, irradiated by the actual solar spectrum. Of the eight materials studied, only for iron, magnetite , and graphite grains does the radiation pressure force exceed gravity and then just for sizes around 0.1 μm; very small particles are not easily blown out of the solar system nor are they rapidly dragged into the Sun by the Poynting-Robertson effect. The solar wind counterpart of the Poynting-Robertson drag may be effective, however, for these particles. The orbital consequences of these radiation forces-including ejection from the solar system by relatively small radiation pressures-and of the Poynting-Robertson drag are considered both for heliocentric and planetocentric orbiting particles. We discuss the coupling between the dynamics of particles and their sizes (which diminish due to sputtering and sublimation). A qualitative derivation is given for the differential Doppler effect, which occurs because the light received by an orbiting particle is slightly red-shifted by the solar rotation velocity when coming from the eastern hemisphere of the Sun but blue-shifted when from the western hemisphere; the ratio of this force to the Poynting-Robertson force is ( R ⊙ r ) 2 [( w ⊙ n ) − 1] , where R⊙ and w⊙ are the solar radius and spin rate, and n is the particles mean motion. The Yarkovsky effect, caused by the asymmetry in the reradiated thermal emission of a rotating body, is also developed relying on new physical arguments. Throughout the paper, representative calculations use the physical and orbital properties of interplanetary dust, as known from various recent measurements.

Fluid ascent through the solid lithosphere and its relation to earthquakes

The Earth is continuously expelling gases and liquids from great depths—juvenile volatiles from the mantle and recycled metamorphic products. Some of these fluids ascend through liquid rock in volcanic processes, but others utilize fractures and faults as conduits through the solid lithosphere. The latter process may have a major influence on earthquakes, since fluids at near lithostatic pressures appear to be required to activate deep faults that would otherwise remain locked.Fluids can be driven upward through solid rock by buoyancy, but only if present in sufficient concentration to form large-scale domains occupying interconnected fracture porosity. A growing fluid domain becomes so mobilized only when it attains the critical vertical dimension required for hydrostatic instability. This dimension, depending on the ultimate compressive yield strength of the rock, may be as much as several kilometers.Any column of fluid ascending through fractures in the solid lithosphere from a prolific deep source must become organized into a vertical sequence of discrete domains, separated by fluid-pressure discontinuities. This is required because a continuous hydrostatic-fluid-pressure profile extending from an arbitrarily deep source to the surface cannot be permitted by the finite strength of rock. A vertically stacked sequence of domains allows the internal fluid-pressure profile to approximate the external rock-stress profile in a stepwise fashion. The pressure discontinuity below the base of the uppermost hydrostatic domain may be responsible for some occurrences of so-called anomalous geopressures. An ascending stream of fluid that percolates upward from a deep source through a column of domains must encounter a sequence of abrupt pressure decreases at the transitions between successive domains. If supercritical gases act as solvents, the dissolved substances may drop out of solution at such pressure discontinuities, resulting in a local concentration of minerals and other substances.At great depths, brittle fracture would normally be prevented by high pressure and temperature, with all excessive stress discharged by ductile flow. Rock strata invaded by an ascending fluid domain are weakened, however, because cracks generated or reactivated by the high-pressure fluid can support the overburden, greatly reducing internal friction. This reduction of strength may cause a previously stressed rock to fail, resulting in hydraulic shear fracture. Thus, earthquakes may be triggered by the buoyant migration of deep-source fluids.The actual timing of the failure that leads to such an earthquake may be determined by the relatively rapid inflation of a fluid domain and not by any significant increase in the probably much slower rate of regional tectonic strain. Many earthquake precursory phenomena may be secondary symptoms of an increase in pore-fluid pressure, and certain coseismic phenomena may result from the venting of high-pressure fluids when faults break the surface. Instabilities in the migration of such fluid domains may also contribute to or cause the eruption of mud volcanoes, magma volcanoes, and kimberlite pipes.

Atmospheric tides and the resonant rotation of Venus

Abstract The observed spin-orbit resonance of Venus, whereby the same side of Venus faces the Earth at each inferior conjunction, cannot be explained adequately by gravitational interaction with the Earth alone. The expected solar tidal drag on the solid body of Venus would easily overwhelm the Earths couple upon any reasonable permanent deformation of Venus. If there exists, however, a solar atmospheric tide, partly thermally induced and similar to that known on the Earth, its torque may counteract that due to the solar solid body tide at a particular rotation period. The small interaction with the Earth is then sufficient to lock the period to one of the resonances in the vicinity of that angular velocity.

Macroscopic seismic anomalies and submarine pockmarks in the Corinth–Patras rift, Greece

Prior to the Aigion earthquake (M 6.2) of 15 June 1995 in the western Gulf of Corinth, people in the area observed anomalous phenomena, including bubbling of the sea, extraordinary behavior of animals, earthquake lights, ground deformation, and the sound of wind immediately before the shock. Some precursory phenomena of this kind might be understood as symptoms of gas venting before an earthquake. Sonar observations made in 1988 showed a chain of submarine pockmarks tracing the Aigion Fault and evidently produced by the expulsion of fluids under pressure. T. Hasiotis et al. [Mar. Geol. 130, 333–344] reported that hot gas erupted from a field of giant submarine pockmarks in the neighboring Bay of Patras several hours prior to an earthquake in 1993. Automated monitoring of bottom water temperature in submarine pockmark fields may detect outgassing events prior to earthquakes in this region.

Cometary impact and the magnetization of the Moon

Abstract Collisions of comets with planetary bodies are capable of impressing patterns of magnetization onto them that match those observed for the Moon and possibly for Mercury. The ambient solar wind magnetic field is briefly but strongly enhanced as the large partially ionized cometary atmosphere is compressed against the planetary surface. Just at the time of peak field enhancement, the solid part of the comet collides with the surface and the compressed fields are permanently imprinted by shock magnetization.

Apollo 12 seismic signal: indication of a deep layer of powder.

The seismic signal caused by the Apollo 12 lunar module is interpreted in terms of propagation between source and receiver through a layer of powder in which sound velocity increases with depth. This increase, which is due to compaction, extends over several kilometers and leads to a concentration of seismic waves toward the surface. Computer simulations with the use of ray acoustics and on the assumption of a randomly undulating lunar surface approximate well the observed signal. Seismic amplitudes are greatly enhanced in such a medium compared to solid rock, so that the observed signal requires less power to be transmitted than previously estimated.

Atmospheric tides and the 4-day circulation on Venus

Abstract The high velocity retrograde circulation of the upper atmosphere of Venus may be an observable consequence of the solar couple upon a thermal semidiurnal atmospheric tide in the stratosphere. A couple of this type would help account for the resonant rotation of the planet.

Occupation horizons found in the search for the ancient Greek city of Helike

In 373 B.C. an earthquake and seismic sea wave destroyed and submerged Helike, the principal Greek city on the southwestern shore of the Gulf of Corinth. Our sonar survey of the seafloor in the area where ancient sources located Helike, southeast of Aigion, showed no evidence of a submerged city. We concluded that the site must now lie in the alluvial deposits of the adjacent coastal plain. Accordingly, we used bore hole drilling and geophysical techniques to look for buried occupation horizons on land. The bore hole cores yielded numerous ceramic fragments and remains of walls, ranging from near the surface to about 15 m deep, concentrated in an area of some 2 km2 on the upper part of the delta between the Selinous and Kerynites Rivers. Ceramic and organic samples from the cores yielded ages ranging from Byzantine to Early Helladic times. A shallow auger hole brought to light a superb fragment of an architectural terracotta statue from an Archaic building, ca. 600 B.C. Near the center of the ceramic-bearing area, we discovered by magnetometry a large Roman building and began its excavation. It may belong to a Roman settlement that Pausanias visited at the site of Classical Helike. The deeper layers of the excavation yielded black-glazed vase fragments from the 5th century B.C. and potsherds from Protogeometric and Mycenaean times. The overall results suggest that most of the Roman to Classical horizons lie within about 6 m of the surface, whereas Bronze Age horizons range down to 15 m. While we have yet to determine by excavation whether the occupation horizons include the center of a city, this area appears to be a strong candidate for the site of ancient Helike.

The equilibrium figures of Phobos and other small bodies

Abstract The shape of a close planetary satellite is distorted from a self-gravitating sphere into a triaxial ellipsoid maintained by tidal and centrifugal forces. Using the family of Roche ellipsoids calculated by Chandrasekhar, it should be possible in some cases to determine the density of an inner satellite by an accurate measurement of its shape alone. The equilibrium figure of Phobos is expected to be the most extreme of any satellite. The shape of Phobos as observed by Mariner 9 approaches but appears not to be a Roche ellipsoid, although the uncertainties of measurement remain too large to exclude the possibility. In any case, Phobos is so small that even the low mechanical strength of an impact-compressed regolith is sufficient to maintain substantial departures from the equipotential figure. If larger close satellites, particularly Amalthea, are found to be Roche ellipsoids, their densities can be estimated immediately from the data presented. Asteroids of size comparable to Phobos and Deimos appear to have more irregular shapes than the Martian satellites. This may reflect the absence of a deep regolith on those asteroids due to the low effective escape velocity for impact ejecta. For Phobos and Deimos, on the other hand, ejecta will tend to remain in orbit about Mars until swept up again by the satellite, contributing to a deeper equilibrium layer of debris.

Brontides: Natural Explosive Noises

Episodes of explosive noises of natural origin, or brontides, have been well documented, often in association with seismic activity and in a few cases as precursors to major earthquakes. Ground-to-air acoustic transmission from shallow earthquakes can account for many of these episodes, but not for all, and other causes, such as the sudden eruption of gas from high-pressure sources in the ground may at times have been responsible. Confusion with distant thunder or artillery at times of anomalous sound propagation complicates the analysis, and more recently the greatly increased frequency of artificial explosive noises and sonic booms has tended to mask the recognition of natural brontides.