Kilometers In Diameter Research Articles

Impact cratering on Titan II. Global melt, escaping ejecta, and aqueous alteration of surface organics

New three-dimensional hydrodynamic simulations of hypervelocity impacts into the crust of Titan were undertaken to determine the fraction of liquid water generated on the surface of Saturn's largest moon over its history and, hence, the potential for surface—modification of hydrocarbons and nitriles by exposure to liquid water. We model in detail an individual impact event in terms of ejecta produced and melt generated, and use this to estimate melt production over Titan's history, taking into account the total flux of the impactors and its decay over time. Our estimates show that a global melt layer at any time after the very beginning of Titan's history is improbable; but transient melting local to newly formed craters has occurred over large parts of the surface. Local maxima of the melt are connected with the largest impact events. We also calculate the amount of volatiles delivered at the impact with various impact velocities (from 3 km/s for possible Hyperion fragments to 11 km/s for Jupiter family comets) and their retention as a possible source of Titan's atmosphere. We find the probability of impact ejecta escaping Titan with its modern dense and thick atmosphere is rather low, and dispersal of Titan organics throughout the rest of the Solar System requires impactors tens of kilometers in diameter. Water ice melting and exposure of organics to liquid water has been widespread because of impacts, but burial or obscuration of craters by organic deposits or cryovolcanism is aided by viscous relaxation. The largest impactors may breach an ammonia–water mantle layer, creating a circular albedo contrast rather than a crater.

Geochemical signatures of Archean to Early Proterozoic Maria-scale oceanic impact basins

Early Precambrian impact ejecta units—consisting of vapor-condensate spherules (microkrystites), microtektites, rip-up clasts and fragmental tsunami deposits—display high siderophile element (Ni, Co, platinum group elements [PGEs]) abundances and, in some instances, high V and Cr levels. The data allow an indirect insight into the composition of crustal regions from which the ejecta were derived, including the contribution of extraterrestrial components and the fractionation history of impact ejected liquid and vapor plumes. The absence of shocked quartz grains in recorded early Precambrian ejecta and the largely ferromagnesian compositions of the microkrystite spherules, except where heavily altered, are consistent with impacts into mafic to ultramafic crust. The PGE data coupled with stratigraphic data are used to estimate the PGE flux and the size of projectiles. PGE patterns relative to chondritic values are mostly depleted in volatile, low-boiling-point species (Au, Pd) and enriched in refractory species (Ir, Ru, Rh), with consequently lower than chondritic Pd/Ir and Pd/Pt ratios, providing a useful tracer of microkrystite-rich impact ejecta units. First approximations of asteroid and crater sizes based on Ir mass balance and on spherule size frequencies suggest impact by 20–30 km asteroids, scaled to oceanic impact basins several hundred kilometers in diameter. A high ratio of sima to sial crust during the Archean is consistent with positive eNd, eHf, and low 87Sr/86Srinitial. The evidence suggests post–3.8 Ga geotectonic systems consisting of small sialic granite-greenstone nuclei surrounded by extensive sima crust in which transient Maria-scale impact basins formed during 3.47, 3.26– 3.24, 2.63, 2.56, 2.50–2.47 Ga and as yet unrecorded impact events.

Heating of the Lithosphere during Meteorite Cratering

The formation of thermal anomalies around the impact sites of large cosmic bodies on the Earth is studied. The parameters of thermal anomalies are compared for the impacts of bodies of various scales—from one to several hundred kilometers in diameter. The cooling time of the rocks under impact craters of various scales is estimated. The estimates obtained are used to model the input of heat by the impacts of small (less than ∼500 km in diameter) planetesimals late in the accretion of the Earth. The boundary conditions for calculating the thermal evolution of the early Earth are refined by simultaneously analyzing the sizes of impact thermal anomalies and the model size distributions of projectiles (the mass spectrum of planetesimals).

Large solar system planetoid discovered

A newly discovered planetoid, currently dubbed 2004 UN, could be confirmed as the largest such known body in the Kuiper Belt, scientists announced on 19 February.The planetoid, located about 4.4 billion miles from Earth, is estimated at about 1,400 kilometers in diameter, according to Michael Brown, associate professor of planetary astronomy at the California Institute of Technology.

Improving knowledge of impact cratering: Bringing together “Modelers” and “Observationalists”

Formation of a planetary‐scale impact crater hundreds of meters to hundreds of kilometers in diameter involves the interplay of multiphase processes occurring over size‐ and time‐scales that range over many orders of magnitude. To further complicate matters, a hypervelocity impact into geologic materials has never been observed at greater than laboratory scales. Understanding the formation process clearly is a very complicated problem that requires a highly interdisciplinary approach. Significant work has been done recently in several key areas of impact studies, but in many respects, there is a “disconnect” among groups employing different approaches. This pertains, in particular, to modeling versus observations.

Cratering on Titan: impact melt, ejecta, and the fate of surface organics

We conduct three-dimensional hydrodynamical simulations of hypervelocity impacts into the crust of Titan to determine the fraction of liquid water generated, under the reasonable assumption that the crust is largely water ice, and to track the fate of the organic-rich layer that is thought to overlie the ice over much of the surface. Impactors larger than a kilometer in diameter are only slightly affected by the atmosphere, while those well under that size are strongly decelerated and broken up before reaching the surface. Impact of a 2 km diameter icy projectile into the crust at velocities of 7 km per second or higher, and angles of impact between 30° and 45°, generate about 2–5% melt by volume within the crater. Our results for the amount of aqueous melt generated in impacts on Titan are broadly consistent with the analytic model developed by Thompson and Sagan (1992) although our numerical model allows us to more precisely quantify the fraction of melt, and fate of the organics, as a function of the impact parameters. While much of the organic surface layer is heavily shocked and ejected from the immediate region of the crater, a significant fraction located behind the oblique impact trajectory is only lightly shocked and is deposited in the liquid water at the crater base. Simple calculations suggest that the resulting aqueous organic phase may remain liquid for hundreds of years or longer, enough time for the synthesis of simple precursor molecules to the origin of life.

Impact cratering on porous asteroids

The increasing evidence that many or even most asteroids are rubble piles underscores the need to understand how porous structures respond to impact. Experiments are reported in which craters are formed in porous, crushable, silicate materials by impacts at 2 km/s. Target porosity ranged from 34 to 96%. The experiments were performed at elevated acceleration on a centrifuge to provide similarity conditions that reproduce the physics of the formation of asteroid craters as large as several tens of kilometers in diameter. Crater and ejecta blanket formation in these highly porous materials is found to be markedly different from that observed in typical dry soils of low or moderate porosity. In highly porous materials, the compaction of the target material introduces a new cratering mechanism. The ejection velocities are substantially lower than those for impacts in less porous materials. The experiments imply that, while small craters on porous asteroids should produce ejecta blankets in the usual fashion, large craters form without ejecta blankets. In large impacts, most of the ejected material never escapes the crater. However, a significant crater bowl remains because of the volume created by permanent compaction of the target material. Over time, multiple cratering events can significantly increase the global density of an asteroid.

Cold-based mountain glaciers on Mars: Western Arsia Mons

Surface environmental conditions on Mars are currently extremely cold and hyperarid, most equivalent to polar deserts on Earth. Coupling newly acquired Mars data with fieldbased observations regarding the flow, surface morphology, and depositional history of polar glaciers in Antarctica, we show that the multiple facies of an extensive fan-shaped deposit on the western flanks of Arsia Mons volcano are consistent with deposition from cold-based mountain glaciers. An outer ridged facies that consists of multiple laterally extensive, arcuate and parallel ridges, resting without disturbance on both well-preserved lava flows and an impact crater, is interpreted as drop moraines formed at the margin of an ablating and predominantly receding cold-based glacier. A knobby facies that consists of equidimensional knobs, each to several kilometers in diameter, is inward of the ridges; this facies is interpreted as a sublimation till derived from in situ downwasting of ashrich glacier ice. A third facies comprising distinctive convex-outward lobes with concentric parallel ridges and aspect ratios elongated downslope likely represents rock-glacier deposits, some of which may still be underlain by a core of glacier ice. Taken together, these surficial deposits show that the western flank of Arsia Mons was occupied by an extensive mountain glacial system accumulating on, and emerging from, the upper slopes of the volcano and spreading downslope to form a piedmont-like fan.

Tally of Jupiter's moons increases

With the International Astronomical Union's formal announcement on 16 May of 11 newly‐discovered moons orbiting Jupiter, the number of known moons around that planet now totals 39. This is the largest number orbiting any planet in the solar system, and nine moreEach of the newly‐found Jovian moons measures 2 to 4 kilometers in diameter, and they are irregular satellites with large, elongated, and inclined orbits.

SIGNIFICANCE OF MUD VOLCANISM

Mud volcanism and diapirism have puzzled geoscientists for ∼2 centuries. They have been described onshore and offshore in many places on Earth, and although they occur in various tectonic settings, the majority of the features known to date are located in compressional tectonic scenarios. This paper summarizes the main thrusts in mud volcano research as well as the various regions in which mud volcanism has been described. Mud volcanoes show variable geometry (up to tens of kilometers in diameter and several hundred meters in height) and a great diversity regarding the origin of the fluid and solid phases. Gas (predominantly methane), water, and mud may be mobilized at subbottom depth of only a few meters but, in places, can originate from several kilometers depth (with minor crustal or mantle input). The possible contribution of mud extrusion to global budgets, both from quiescent fluid emission and from the extrusive processes themselves, is important. In regions where mud volcanoes are abundant, such as the collision zones between Africa and Eurasia, fluid flux through mud extrusion exceeds the compaction‐driven pore fluid expulsion of the accretionary wedge. Also, quiescent degassing of mud volcanoes may contribute significantly to volatile budgets and, hence, to greenhouse climate.

Very-long-period seismic signals and caldera formation at Miyake Island, Japan.

Over a period of roughly 40 days, starting on 8 July 2000, a caldera structure 1.7 kilometers in diameter developed by means of gradual depression and expansion of the summit crater at Miyake Island, Japan. At the same time, very-long-period (VLP) seismic signals were observed once or twice a day. Source mechanism analyses of the VLP signals show that the moment tensor solutions are smooth step functions over a time scale of 50 seconds, with dominant volumetric change components. We developed a model to explain the caldera and the VLP signals, in which a vertical piston of solid materials in the conduit is intermittently sucked into the magma chamber by lateral magma outflow. This model offers potential for making quantitative estimations of the characteristic physical properties of magma systems.

Eocene cichlid fishes from Tanzania, East Africa

ABSTRACT A large collection of fishes from Tanzania, East Africa, includes representatives of four or five families of fishes. The majority of specimens are cichlids, with at least five species being represented, distinguished by osteological characters such as the shape of the opercular bones, frontals, and supraoccipital crest. These species are all named and described in the same genus based on shared scale and squamation characters. The site, at Mahenge in the Singida Region, was a crater lake about half a kilometer in diameter. It has been dated as Eocene, about 46 Ma, based on radiometric techniques. The cichlid species, being very closely related, may have constituted a species flock, which would indicate that cichlids had the ability to form flocks as early as the Eocene.

Tomographic imaging of isolated ground surfaces using radio ground waves and conjugate gradient methods

The ground surface of the Earth is usually mapped using microwave radar, and in particular synthetic aperture radar. A different technique is presented for the imaging of isolated ground surfaces, which may range from the order of a metre to a few kilometres in diameter, using radiowaves generated from antennas on the ground surface of the Earth. The technique is first formulated using groundwave propagation theory to solve the forward electromagnetic scattering problem of the isolated surface. Conjugate gradient methods are then used to solve the inverse problem to obtain the normalised surface impedance distribution in an iterative manner, with the establishment of a cost function of the measured and scattered fields around the surface. Numerical simulations at 0.5 MHz of two ground features, namely dry ground and dry–wet mixed ground surfaces confined within a diameter of 500 m and surrounded by seawater, show that the distributions of both real and imaginary parts of the normalised surface impedance can be well reconstructed using the technique. It is also shown that the Polak–Ribiere–Polyak conjugate gradient method gives a faster rate of convergence than the Fletcher–Reeves method. The simulation at 300 MHz of dry ground with a buried metallic object confined within a diameter of 50 cm shows that the metallic object and its position can be identified from the images of normalised surface impedance distribution. The study demonstrates a new technique for the tomographic imaging of isolated ground surfaces which may be useful in ground surface mapping, remote sensing and monitoring, and detection of buried metallic objects.

Bright Spot on a Centaur

PLANETARY SCIENCE Centaurs are primitive icy objects that are tens of kilometers in diameter that reside for a few million years in chaotic orbits between Jupiter and Neptune. Centaurs may originate in the Kuiper Belt, which is a zone of icy objects on the edge of the solar system beyond Neptune. They can provide clues to the composition of the original solar nebula, but observing these small distant objects is difficult, and there has been controversy about their size, shape, and composition. Kern et al. now present two long-exposure spectra from two regions of the Centaur 8405 Asbolus from the near-infrared camera and multiobject spectrograph (NICMOS) on the Hubble Space Telescope. They conclude that Asbolus is a spherical object that rotates with a period of about 4 hours (instead of the previously estimated 9 hours) and that has an impact crater on one side. The spectrum for this side shows absorption features that they attribute to an exposed deeper water-ice layer against a darkened weathered surface. The spectrum for the other side is featureless. Asbolus may have been ejected from the Kuiper Belt by a collision that produced the observed crater, and this collision may have increased its rotation rate. The chemical and structural heterogeneity of Asbolus may help explain discrepant observations of other centaurs, some of which were thought to be homogeneous objects. — LR Astrophys. J . 542 , L155 (2000).

Apparent Fractal Dimensions from Continental Scale Digital Elevation Models Using Variogram Methods

It is often assumed that real land surfaces demonstrate the statistically self‐affine scaling behaviour of fractional Brownian surfaces. Tests of this assumption against empirical data, however, show many deviations. Estimates of fractal properties vary between methods and over different scale ranges. So far, this empirical evidence has come from the analysis of variograms for DEMs representing areas up to tens of kilometres in diameter. Here we report results obtained by using variograms to analyse land surface DEMs at the continental scale, with a grid resolution of 30 arc seconds. Results reveal variogram curvature and breaks of slope, but also linear sections over distance lags of hundreds of kilometres. The estimated mean fractal dimension calculated from these sections is 2.66, substantially higher for all continents at these broad scales (around 200 km) than values calculated at the erosional landscape scale (around 200 m). Thus the land surface is not self‐affine, and it is not clear that it follows any simple multifractal model. At the longest wavelengths, patterns found in the variograms appear to be related to broad tectonic features of the Earth's surface. For the reader to assess their quality and generality, estimates of fractal dimension should always be accompanied by statements of the scale range covered and the goodness of fit to a log‐linear relationship.

NEAR at Eros: Imaging and Spectral Results

Eros is a very elongated (34 kilometers by 11 kilometers by 11 kilometers) asteroid, most of the surface of which is saturated with craters smaller than 1 kilometer in diameter. The largest crater is 5.5 kilometers across, but there is a 10-kilometer saddle-like depression with attributes of a large degraded crater. Surface lineations, both grooves and ridges, are prominent on Eros; some probably exploit planes of weakness produced by collisions on Eros and/or its parent body. Ejecta blocks (30 to 100 meters across) are abundant but not uniformly distributed over the surface. Albedo variations are restricted to the inner walls of certain craters and may be related to downslope movement of regolith. On scales of 200 meters to 1 kilometer, Eros is more bland in terms of color variations than Gaspra or Ida. Spectra (800 to 2500 nanometers) are consistent with an ordinary chondrite composition for which the measured mean density of 2.67 +/- 0.1 grams per cubic centimeter implies internal porosities ranging from about 10 to 30 percent.

Characteristics and origin of polygonal terrain in southern Utopia Planitia, Mars: Results from Mars Orbiter Laser Altimeter and Mars Orbiter Camera data

Giant polygons on Mars with several kilometers in diameter were first observed in Mariner 9 and Viking Orbiter images, and their origin, formation, and evolution have remained enigmatic since that time. New data obtained by the Mars Orbiter Laser Altimeter (MOLA) and the Mars Orbiter Camera (MOC) on board the Mars Global Surveyor spacecraft now permit analysis of topography and morphology of the polygonal terrain in unprecedented detail. MOLA data show that (1) giant polygonal terrain in Utopia is located at the lower slopes of a basin structure, that is the Utopia basin; (2) the onset of polygonal terrain in Utopia is close to the same elevation in several profiles and lies at about the same elevation as a terrace which was interpreted as a shore‐line of an ancient body of standing water within the Utopia basin proper; (3) polygonal terrain occurs over a wide range of elevations on regional slopes of ∼0.1°; (4) the depths of the troughs range from <5 m to 115 m, averaging about 30 m, and tend to be greater toward the center of the Utopia basin; (5) the mean width of polygonal troughs is of the order of 2 km, ranging from <0.5 to 7.5 km. Data from the Mars Orbiter Camera (MOC) indicate that (1) the troughs are generally broad graben‐like features with varying morphologies (i.e., terraces and lobateness) and dimensions (widths and depths); (2) trough morphology has been modified by aeolian processes; (3) polygonal troughs are weakly interconnected at MOC resolutions and there are no smaller polygons observed within the giant polygons; (4) circular depressions which are probably related to subsurface collapse sometimes occur at the bottoms of polygonal troughs; (5) small‐scale polygonal terrain on the ejecta blankets of young craters with fluidized ejecta in the investigated area exhibits different characteristics (i.e., smaller size, presence of rims, greater degree of interconnections) than giant polygons. In the Viking images we see that (1) old impact craters are increasingly buried with younger materials toward the basin center and that (2) a large number of superposed impact craters on polygonal terrain exhibit distinctive ejecta morphologies such as rampart ejecta blankets and single and double lobate ejecta blankets. MOLA and MOC observations are consistent with the formation of polygonal terrain in the area of a former standing body of water, but polygon size seems too large to be readily accounted for by desiccation or freezing processes alone. On the basis of the observed characteristics we propose that the giant polygons are primarily of tectonic origin, being caused by uplift of the floor of the Utopia basin. There are several candidates for the cause of uplift: (1) the removal of the load representing former standing bodies of water. Water thickness estimated for the Utopia basin is about 1 km. In this scenario, removal of this load (comparable to loads placed on terrestrial continental lithosphere by the Laurentide and Scandinavian ice sheets) by loss of the water or ice could cause uplift of the floor and polygon formation. (2) The freezing and expansion of residual water buried by sediments or in the near subsurface, or (3) a combination of these. From the discussion of alternative models in the light of the MOLA and MOC data we conclude that formation of giant polygons by tectonic uplift of the basin floor is most likely and deserves further investigation.

Identification of comet Hyakutake's extremely long ion tail from magnetic field signatures

Observations of the varying orientations of comet tails led to the suggestion of the existence of the solar wind--a continuous outflow of ionized material from the Sun. It is now well established that gas from comets is ionized by several processes and joins the solar wind, forming an ion (plasma) tail that points away from the Sun. The plasma environments of three comets have been measured in situ, but only in the upstream direction or less than 8,000 km downstream of the nucleus. Here we report a fortuitous crossing by a spacecraft of the plasma tail of comet Hyakutake (C/1996 B2), at a distance of more than 3.8 astronomical units (550 million kilometres) from its nucleus. This surpasses the tail length of 2 AU determined for the Great March Comet of 1843 (C/1843 D1). Our measurements reveal that, at this distance, the tail of comet Hyakutake was a structured entity at least 7 million kilometres in diameter.

Data highway to the stars

Connecting individual radio telescopes by microwave links has allowed radio astronomers to create instruments with resolving powers equivalent to a single telescope hundreds of kilometres in diameter. The author describes how optical fibre links could yield a further dramatic increase in our ability to explore the furthermost reaches of the universe.

Endemism and the pelagic larval duration of reef fishes in the eastern Pacific Ocean

We examined the relationship between pelagic larval duration (PLD) and species range for the wrasses and damselfishes of the eastern Pacific Ocean. This region and these species are par- ticularly appropriate for this analysis, since there is huge variation in the biogeographic range among these reef fishes, from endemics to islands a few kilometers in diameter to pan-Indo-Pacific ranges of 25 000 km or more. Furthermore, this variation in range occurs between pairs of congeners. We found no correlation between the mean (or maximum) PLDs measured from daily otolith increments and biogeographic range, for either family of reef fishes, and even after restricting the analysis to species with sample sizes over 50 individuals. A pairwise comparison of congeners showed, if anything, a sig- nificant inverse relationship between larval duration and range. The correlation between mean PLD and maximum PLD was very strong, even for large sample sizes (r = 0.987), indicating that mean PLD may be a good proxy measure for maximum PLD. We conclude that, at least within this region and within the limitations of this technique, larval duration appears not to determine species range.