Apollo 17 seismic profiling: probing the lunar crust

Apollo 17 seismic data are interpreted to determine the structure of the lunar crust to a depth of several kilometers. Seismic velocity increases in a marked stepwise manner beneath the Taurus-Littrow region at the Apollo 17 site. A thickness of about 1200 meters is indicated for the infilling mare basalts at Taurus-Littrow. The apparent velocity is high (about 4 kilomleters per second) in the material immediately underlying the basalts.     

Seismic data from man-made impacts on the moon

Unusually long reverberations were recorded from two lunar impacts by a seismic station installed on the lunar surface by the Apollo 12 astronauts. Seismic data from these impacts suggest that the lunar mare in the region of the Apollo 12 landing site consists of material with very low seismic velocities near the surface, with velocity increasing with depth to 5 to 6 kilometers per second (for compressional waves) at a depth of 20 kilometers. Absorption of seismic waves in this structure is extremely low relative to typical continental crustal materials on earth. It is unlikely that a major boundary similar to the crustmantle interface on earth exists in the outer 20 kilometers of the moon. A combination of dispersion and scattering of surface waves probably explains the lunar seismic reverberation. Scattering of these waves implies the presence of heterogeneity within the outer zone of the mare on a scale of from several hundred meters (or less) to several kilometers. Seismic signals from 160 events of natural origin have been recorded during the first 7 months of operation of the Apollo 12 seismic station. At least 26 of the natural events are small moonquakes. Many of the natural events are thought to be meteoroid impacts.     

Meteorite impact ejecta: dependence of mass and energy lost on planetary escape velocity

The calculated energy efficiency of mass ejection for iron and anorthosite objects striking an anorthosite planet at speeds of 5 to 45 kilometers per second decreases with increasing impact velocity at low escape velocities. At escape velocities of >10(5) and >2 x 10(4) centimeters per second, respectively, the slower impactors produce relatively less ejecta for a given impact energy. The impact velocities at which ejecta losses equal meteorite mass gains are found to be approximately 20, 35, and 45 kilometers per second for anorthosite objects and approximately 25, 35, and 40 kilometers per second for iron objects striking anorthosite surfaces for the gravity fields of the moon, Mercury, and Mars.     

Apollo 12 lunar module impact: laboratory simulation and possible downrange ballistic effects

Plastic pellets were fired into sand targets at a launch angle of 4 degrees and a velocity of 1.68 kilometers per second, the conditions of the Apollo 12 lunar module impact. Shallow elliptical or doublet craters were formed, similar to certain lunar craters. Analysis of the ejecta suggests (i) that lunar module debris skipped and, with some crater ejecta, reimpacted far downrange, but (ii) this ballistic rain does not account for the anomalous seismic signal.     

Lateral P-Velocity Gradients near Major Strike-Slip Faults in California

The P-wave velocity in shallow crystalline rock decreases systematically from a normal value of about 5.5 kilometers per second 20 kilometers or more from the Garlock and San Andreas faults to less than 3 kilometers per second at distances of less than 2 kilometers from these faults. This lateral velocity gradient closely resembles the shear stress profile. It is proposed that the velocity gradient results from increased fracturing nearer these major strike-slip faults and that this fracturing dominates the response of the shallow crust to tectonic stress.     

Oblique impact: a process for obtaining meteorite samples from other planets

Cratering flow calculations for a series of oblique to normal (10 degrees to 90 degrees ) impacts of silicate projectiles onto a silicate halfspace were carried out to determine whether or not the gas produced upon shock-vaporizing both projectile and target material would form a downstream jet that could entrain and propel SNC meteorites from the Martian surface. The difficult constraints that the impact origin hypothesis for SNC meteorites has to satisfy are that these meteorites are lightly to moderately shocked and yet have been accelerated to speeds in excess of the Martian escape velocity (more than 5 kilometers per second). Two-dimensional finite difference calculations were performed that show that at highly probable impact velocities (7.5 kilometers per second), vapor plume jets are produced at oblique impact angles of 25 degrees to 60 degrees and have speeds as great as 20 kilometers per second. These plumes flow nearly parallel to the planetary surface. It is shown that upon impact of projectiles having radii of 0.1 to 1 kilometer, the resulting vapor jets have densities of 0.1 to 1 gram per cubic centimeter. These jets can entrain Martian surface rocks and accelerate them to velocities greater than 5 kilometers per second. This mechanism may launch SNC meteorites to earth.     

Dipping low-velocity layer in the mid-lower mantle: evidence for geochemical heterogeneity

Data from western United States short-period seismic networks reveal a conversion from an S to a P wave within a low seismic velocity layer (greater than or equal to the 4 percent velocity difference compared to the surrounding mantle) in the mid-lower mantle (1400 to 1600 kilometers deep) east of the Mariana and Izu-Bonin subduction zones. The low-velocity layer (about 8 kilometers thick) dips 30 degrees to 40 degrees southward and is at least 500 kilometers by 300 kilometers. Its steep dip, large velocity contrast, and sharpness imply a chemical rather than a thermal origin. Ancient oceanic crust subducted into the lower mantle is a plausible candidate for the low-velocity layer because of its broad thin extent.     

The 25-km Discontinuity: Implications for Lunar History

The lunar velocity profile and laboratory data on terrestrial and lunar rocks are constraints on models of lunar history. They show that shock-induced microcracks are absent from the rocks present in the moon today at depths of 25 to 60 kilometers. All possible causes of this observation are examined, and the most likely explanations are that either the rocks at depths of 25 to 60 kilometers formed after the major impacts ceased or the microcracks have annealed at temperatures of about 600 degrees C over geologically long times.     

High-frequency pn phases observed in the pacific at great distances

Earlier observations of a seismic waveguide in the northwestern Pacific with a velocity of 8.3 kilometers per second to distances of approximately 30 degrees are complemented by suggestions of a possible waveguide with a velocity of 7.8 kilometers per second to distances well in excess of 30 degrees .     

Evidence for Alfvén waves in solar x-ray jets

Coronal magnetic fields are dynamic, and field lines may misalign, reassemble, and release energy by means of magnetic reconnection. Giant releases may generate solar flares and coronal mass ejections and, on a smaller scale, produce x-ray jets. Hinode observations of polar coronal holes reveal that x-ray jets have two distinct velocities: one near the Alfvén speed ( approximately 800 kilometers per second) and another near the sound speed (200 kilometers per second). Many more jets were seen than have been reported previously; we detected an average of 10 events per hour up to these speeds, whereas previous observations documented only a handful per day with lower average speeds of 200 kilometers per second. The x-ray jets are about 2 x 10(3) to 2 x 10(4) kilometers wide and 1 x 10(5) kilometers long and last from 100 to 2500 seconds. The large number of events, coupled with the high velocities of the apparent outflows, indicates that the jets may contribute to the high-speed solar wind.     

Determination of Venus Winds by Ground-Based Radio Tracking of the VEGA Balloons

A global array of 20 radio observatories was used to measure the three-dimensional position and velocity of the two meteorological balloons that were injected into the equatorial region of the Venus atmosphere near Venus midnight by the VEGA spacecraft on 11 and 15 June 1985. Initial analysis of only radial velocities indicates that each balloon was blown westward about 11,500 kilometers (8,000 kilometers on the night side) by zonal winds with a mean speed of about 70 meters per second. Excursions of the data from a model of constant zonal velocity were generally less than 3 meters per second; however, a much larger variation was evident near the end of the flight of the second balloon. Consistent systematic trends in the residuals for both balloons indicate the possibility of a solar-fixed atmospheric feature. Rapid variations in balloon velocity were often detected within a single transmission (330 seconds); however, they may represent not only atmospheric motions but also self-induced aerodynamic motions of the balloon.     

Missile inpacts as sources of seismic energy on the moon

Seismic signals recorded from impacts of missiles at the White Sands Missile Range are radically different from the signal recorded from the Apollo 12 lunar module impact. This implies that lunar structure to depths of at least 10 to 20 kilometers is quite different from the typical structure of the earth's crust. Results obtained from this study can be used to predict seismic wave amplitudes from future man-made lunar impacts. Seismic energy and crater dimensions from impacts are compared with measurements from chemical explosions.     

Astrophysical jets

Astrophysical jets are linear structures associated with stars and galaxies which span about seven orders of magnitude in size; the largest jets emanating from galaxies are about 100 times the size of our galaxy and are the largest single objects in the universe. Jets associated with stars are composed of ionized gas moving away from the star with velocities of a few hundred kilometers per second. Extragalactic jets are composed of relativistic particles, magnetic field, and probably additional amounts of cooler ionized plasma either originally ejected in the jet or entired by it out of the surrounding gaseous medium. The initial outflow velocity for extragalactic jets may be relativistic, and average outflow speeds of several thousand kilometers per second are likely. The energy flux carried by extragalactic jets may be in excess of 10(46) ergs per second, depending upon the nature of the jet. A definition of jet properties, deduced from their interaction with the ambient medium, can place essential constraints on models for the central power source in the parent galaxy or quasi-stellar object where they originate.     

The Large Crater Origin of SNC Meteorites

A large body of evidence strongly suggests that the shergottite, nakhlite, and Chassigny (SNC) meteorites are from Mars. Various mechanisms for the ejection of large rocks at martian escape velocity (5 kilometers per second) have been investigated, but none has proved wholly satisfactory. This article examines a number of possible ejection and cosmic-ray exposure histories to determine which is most plausible. For each possible history, the Melosh spallation model is used to estimate the size of the crater required to produce ejecta fragments of the required size with velocities >/=5 kilometers per second and to produce a total mass of solid ejecta consistent with the observed mass flux of SNC meteorites. Estimates of crater production rates on Mars are then used to evaluate the probability that sufficiently large craters have formed during the available time. The results indicate that the SNC meteorites were probably ejected from a very large crater (> 100 kilometers in diameter) about 200 million years ago, and that cosmic-ray exposure of the recovered meteorites was initiated after collisional fragmentation of the original ejecta in space at much later times (0.5 to 10 million years ago).     

Seismic trapped modes in the oroville and san andreas fault zones

Three-component borehole seismic profiling of the recently active Oroville, California, normal fault and microearthquake event recording with a near-fault three-component borehole seismometer on the San Andreas fault at Parkfield, California, have shown numerous instances of pronounced dispersive wave trains following the shear wave arrivals. These wave trains are interpreted as fault zone-trapped seismic modes. Parkfield earthquakes exciting trapped modes have been located as deep as 10 kilometers, as shallow as 4 kilometers, and extend 12 kilometers along the fault on either side of the recording station. Selected Oroville and Parkfield wave forms are modeled as the fundamental and first higher trapped SH modes of a narrow low-velocity layer at the fault. Modeling results suggest that the Oroville fault zone is 18 meters wide at depth and has a shear wave velocity of 1 kilometer per second, whereas at Parkfield, the fault gouge is 100 to 150 meters wide and has a shear wave velocity of 1.1 to 1.8 kilometers per second. These low-velocity layers are probably the rupture planes on which earthquakes occur.     

Seismic-wave attenuation associated with crustal faults in the new madrid seismic zone

The attenuation of upper crustal seismic waves that are refracted with a velocity of about 6 kilometers per second varies greatly among profiles in the area of the New Madrid seismic zone in the central Mississippi Valley. The waves that have the strongest attenuation pass through the seismic trend along the axis of the Reelfoot rift in the area of the Blytheville arch. Defocusing of the waves in a low-velocity zone and/or seismic scattering and absorption could cause the attenuation; these effects are most likely associated with the highly deformed rocks along the arch. Consequently, strong seismic-wave attenuation may be a useful criterion for identifying seismogenic fault zones.     

Sound velocity and compressibility for lunar rocks 17 and 46 and for glass spheres from the lunar soil

Four experiments on lunar materials are reported: (i) resonance on glass spheres from the soil; (ii) compressibility of rock 10017; (iii) sound velocities of rocks 10046 and 10017; (iv) sound velocity of the lunar fines. The data overlap and are mutually consistent. The glass beads and rock 10017 have mechanical properties which correspond to terrestrial materials. Results of (iv) are consistent with low seismic travel times in the lunar maria. Results of analysis of the microbreccia (10046) agreed with the soil during the first pressure cycle, but after overpressure the rock changed, and it then resembled rock 10017. Three models of the lunar surface were constructed giving density and velocity profiles.     

Size and age of the universe

The age of the universe based on abundances of isotopes is in the range 10 billion to 15 billion years. This is consistent with the age range 12 billion to 20 billion years calculated from the evolution of the oldest galactic stars. A third estimate of the age of the universe is based on the Hubble relation between the velocities of galaxies and their distances from us, where the inverse of the Hubble parameter H is a measure of the age of a uniformly expanding universe. Evidence that has been accumulating over the past few years indicates that the expansion of the universe may exhibit a rather large local perturbation due to the gravitational attraction of the Virgo supercluster. Different types of observations still produce conflicting evidence about the velocity with which the Local Group of galaxies (of which our Milky Way system is a member) is falling into the Virgo cluster. The results to date indicate that this velocity lies somewhere in the range 0 to 500 kilometers per second. The resulting ambiguity in the flow pattern for relatively nearby galaxies makes values of H derived from galaxies with radial velocities less than 2000 kilometers per second particularly uncertain, and this restricts determinations of H to distant galaxies, for which distances are particularly uncertain. The best that can be said at present is that H(-1) yields a maximum time scale in the range 10 billion to 20 billion years.     

Phase changes in the upper mantle

The C-region of the upper mantle has two transition regions 75 to 90 kilometers thick. In western North America these start at depths of 365 kilometers and 620 kilometers and involve velocity increases of about 9 to 10 percent. The locations of these transition regions, their general shape, and their thicknesses are consistent with, first, the transformation of magnesium-rich olivine to a spinel structure and, then, a further collapse of a material having approximately the properties of the component oxides. The velocity increases associated with each transition region are slightly less than predicted for the appropriate phase change. This can be interpreted in terms of an increasing fayalite content with depth. The location of the transition regions and the seismic velocities in their vicinity supply new information regarding the composition and temperature of the upper mantle. The depths of the transition regions are consistent with temperatures near 1500 degrees C at 365 kilometers and 1900 degrees C at 620 kilometers.     

Lunar impact basins and crustal heterogeneity: new Western limb and far side data from galileo

Multispectral images of the lunar western limb and far side obtained from Galileo reveal the compositional nature of several prominent lunar features and provide new information on lunar evolution. The data reveal that the ejecta from the Orientale impact basin (900 kilometers in diameter) lying outside the Cordillera Mountains was excavated from the crust, not the mantle, and covers pre-Orientale terrain that consisted of both highland materials and relatively large expanses of ancient mare basalts. The inside of the far side South Pole-Aitken basin (>2000 kilometers in diameter) has low albedo, red color, and a relatively high abundance of iron- and magnesium-rich materials. These features suggest that the impact may have penetrated into the deep crust or lunar mantle or that the basin contains ancient mare basalts that were later covered by highlands ejecta.