Publications (62)539.13 Total impact
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Article: The origin of pits on 9P/Tempel 1 and the geologic signature of outbursts in Stardust-NExT images
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ABSTRACT: a b s t r a c t We consider the origin of ~380 quasi-circular depressions (pits) seen to be distributed in a broad band across the surface of 9P/Tempel 1 and show that possibly $96% may be due to outburst activity. Of the rest, <4%, are probably due to a mix of cryo-volcanic collapse events and collisional impacts with asteroidal material. We estimate the mass ejected during the June 14, 2005, mini-outburst on 9P to be in the range (6–30) Â 10 4 kg and find that the resulting pit should have a diameter in the range 10–30 m. Published locations of mini-outbursts are revised to account for changes in the nucleus shape, rotation rate, and rotation pole that have resulted from observations made during the Stardust-NExT mis-sion. Both of these locations are found to fall in, or on the edge of, the band of pits that encircles the nucleus. We have identified features in high-resolution images near one of these locations as the possible places of origin of the mini-outbursts. These features show close packing of multiple pits in the appropri-ate diameter range. We consider the distribution of pit diameters and show that the largest pits follow a power–law with exponent À2.24 ± 0.09. Using the June 14, 2005, mini-outburst and the Deep Impact crater to provide a cal-ibration, we establish empirical relationships between pit diameter, D, the total outburst energy, E, and the visual magnitude change, Dm abs , which is the visual amplitude of the outburst referenced to a standard ini-tial brightness. We find Log 10 D $ 0.107(±0.004)Dm abs + 1.3(±0.4) and Log 10 E $ 0.32(±0.01)Dm abs + 10.1(±1.2) where the uncertainties represent the range of values for the coefficient rather than formal error. We apply these approximate relationships to the mega-outburst on 17P/Holmes and predict that it left a pit-like scar on the surface with a diameter in the range 160–1300 m, that the total energy released was in the range 7 Â 10 12 –3 Â 10 15 J, and that between 6 Â 10 7 and 1.3 Â 10 11 kg of material was ejected from the surface. While these predictions are crude they encompass, particularly near the upper end of the range, the results on kinetic energy release and mass loss found by Reach et al.Icarus 02/2013; 222:477-486. · 3.38 Impact Factor -
Article: Shape, density, and geology of the nucleus of Comet 103P/Hartley 2
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ABSTRACT: a b s t r a c t Data from the Extrasolar Planet Observation and Deep Impact Extended Investigation (EPOXI) mission show Comet 103P/Hartley 2 is a bi-lobed, elongated, nearly axially symmetric comet 2.33 km in length. Surface features are primarily small mounds <40 m across, irregularly-shaped smooth areas on the two lobes, and a smooth but variegated region forming a ''waist'' between the two lobes. Assuming parts of the comet body approach the shape of an equipotential surface, the mean density of Hartley 2 is modeled to be 200–400 kg m À3 . Such a mean density suggests mass loss per orbit of >1%. The shape may be the evolutionary product of insolation, sublimation, and temporary deposition of materials controlled by the object's complex rotation.Icarus 02/2013; 222:550-558. · 3.38 Impact Factor -
Article: Comet Geology with Deep Impact Remote Sensing
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ABSTRACT: The Deep Impact mission will provide the highest resolution images yet of a comet nucleus. Our knowledge of the makeup and structure of cometary nuclei, and the processes shaping their surfaces, is extremely limited, thus use of the Deep Impact data to show the geological context of the cratering experiment is crucial. This article briefly discusses some of the geological issues of cometary nuclei.Space Science Reviews 04/2012; 117(1):193-205. · 3.61 Impact Factor -
Article: EPOXI at comet Hartley 2.
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ABSTRACT: Understanding how comets work--what drives their activity--is crucial to the use of comets in studying the early solar system. EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation) flew past comet 103P/Hartley 2, one with an unusually small but very active nucleus, taking both images and spectra. Unlike large, relatively inactive nuclei, this nucleus is outgassing primarily because of CO(2), which drags chunks of ice out of the nucleus. It also shows substantial differences in the relative abundance of volatiles from various parts of the nucleus.Science 06/2011; 332(6036):1396-400. · 31.20 Impact Factor -
Article: Imaging of Titan from the Cassini spacecraft.
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ABSTRACT: Titan, the largest moon of Saturn, is the only satellite in the Solar System with a substantial atmosphere. The atmosphere is poorly understood and obscures the surface, leading to intense speculation about Titan's nature. Here we present observations of Titan from the imaging science experiment onboard the Cassini spacecraft that address some of these issues. The images reveal intricate surface albedo features that suggest aeolian, tectonic and fluvial processes; they also show a few circular features that could be impact structures. These observations imply that substantial surface modification has occurred over Titan's history. We have not directly detected liquids on the surface to date. Convective clouds are found to be common near the south pole, and the motion of mid-latitude clouds consistently indicates eastward winds, from which we infer that the troposphere is rotating faster than the surface. A detached haze at an altitude of 500 km is 150-200 km higher than that observed by Voyager, and more tenuous haze layers are also resolved.Nature 04/2005; 434(7030):159-68. · 36.28 Impact Factor -
Article: Obituary: Fred Lawrence Whipple (1906-2004).
Nature 12/2004; 432(7013):31. · 36.28 Impact Factor -
Article: Cassini Imaging Science: Instrument Characteristics And Anticipated Scientific Investigations At Saturn
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ABSTRACT: The Cassini Imaging Science Subsystem (ISS) is the highest-resolution two-dimensional imaging device on the Cassini Orbiter and has been designed for investigations of the bodies and phenomena found within the Saturnian planetary system. It consists of two framing cameras: a narrow angle, reflecting telescope with a 2-m focal length and a square field of view (FOV) 0.35∘ across, and a wide-angle refractor with a 0.2-m focal length and a FOV 3.5∘ across. At the heart of each camera is a charged coupled device (CCD) detector consisting of a 1024 square array of pixels, each 12 μ on a side. The data system allows many options for data collection, including choices for on-chip summing, rapid imaging and data compression. Each camera is outfitted with a large number of spectral filters which, taken together, span the electromagnetic spectrum from 200 to 1100 nm. These were chosen to address a multitude of Saturn-system scientific objectives: sounding the three-dimensional cloud structure and meteorology of the Saturn and Titan atmospheres, capturing lightning on both bodies, imaging the surfaces of Saturn’s many icy satellites, determining the structure of its enormous ring system, searching for previously undiscovered Saturnian moons (within and exterior to the rings), peering through the hazy Titan atmosphere to its yet-unexplored surface, and in general searching for temporal variability throughout the system on a variety of time scales. The ISS is also the optical navigation instrument for the Cassini mission. We describe here the capabilities and characteristics of the Cassini ISS, determined from both ground calibration data and in-flight data taken during cruise, and the Saturn-system investigations that will be conducted with it. At the time of writing, Cassini is approaching Saturn and the images returned to Earth thus far are both breathtaking and promising.Space Science Reviews 10/2004; 115(1):363-497. · 3.61 Impact Factor -
Article: Cassini imaging of Jupiter's atmosphere, satellites, and rings.
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ABSTRACT: The Cassini Imaging Science Subsystem acquired about 26,000 images of the Jupiter system as the spacecraft encountered the giant planet en route to Saturn. We report findings on Jupiter's zonal winds, convective storms, low-latitude upper troposphere, polar stratosphere, and northern aurora. We also describe previously unseen emissions arising from Io and Europa in eclipse, a giant volcanic plume over Io's north pole, disk-resolved images of the satellite Himalia, circumstantial evidence for a causal relation between the satellites Metis and Adrastea and the main jovian ring, and information on the nature of the ring particles.Science 04/2003; 299(5612):1541-7. · 31.20 Impact Factor -
Article: The Comet Nucleus Tour (Contour); A NASA Discovery Mission
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ABSTRACT: In 1997, the COmet Nucleus TOUR (CONTOUR) was selected byNASA for a new start as part of the Discovery line. In this paper, we review the status of the mission, the mission timeline and the instruments to be flown. Detail is given of the science goals and how they are to be accomplished.Earth Moon and Planets 09/2000; 89(1):289-300. · 0.67 Impact Factor -
Article: Evidence for recent volcanism on Mars from crater counts
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ABSTRACT: Impact craters help characterize the age of a planetary surface, because they accumulate with time. They also provide useful constraints on the importance of surface erosion, as such processes will preferentially remove the smaller craters. Earlier studies of martian crater populations revealed that erosion and dust deposition are important processes on MarsNature 02/1999; 397(6720):586-589. · 36.28 Impact Factor -
Article: Search for temperature-related albedo changes in nightside and posteclipse images of Io
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ABSTRACT: Using an image-summing process that increases the visibility of Jupiterlit surface features in Voyager images, we have produced the best-ever violet-filter image of the nightside of Io and the best-ever nightside/dayside brightness ratio map of this jovian moon. The ratio map shows no convincing evidence, on either global or local scales, of diurnal temperature-dependent albedo variations. We have also taken an image-ratioing technique developed by O'Shaughnessy et al. (1989), which those authors applied to Voyager violet-filter observations of one Io eclipse reappearance, and extended it to two other, higher-resolution Voyager posteclipse imaging sequences. In none of three imaging sequences do we find any isolated surface regions that convincingly exhibit posteclipse temperature-related albedo variations. These negative results suggest that on Io, pure cyclo-octasulfur (S8), and transient nighttime or in-eclipse deposits of SO2 frost, are at best limited to isolated areas smaller than the resolution of the images in use (i.e., smaller than a few tens of kilometers in size). Such limits are consistent with (1) the negative results reported by the majority of telescopic observers who have searched for posteclipse brightening of Io, (2) indications that physical processes in the ionian surface environment will change any S8 into other allotropes of sulfur, and (3) suggestions that Io's atmosphere is too thin to allow the deposition of transient, optically thick SO2 frost layers at nighttime or during eclipse.03/1994; -
Article: Cometary Coma Chemical Composition (C4) Mission
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ABSTRACT: Cometary exploration remains of great importance to virtually all of space science. Because comets are presumed to be remnants of the early solar nebula, they are expected to provide fundamental knowledge as to the origin and development of the solar system as well as to be key to understanding of the source of volatiles and even life itself in the inner solar system. Clearly the time for a detailed study of the composition of these apparent messages from the past has come. A comet rendezvous mission, the Cometary Coma Chemical Composition (C4) Mission, is now being studied as a candidate for the new Discovery program. This mission is a highly-focussed and usefully-limited subset of the Cometary Rendezvous Asteroid Flyby (CRAF) Mission. The C4 mission will concentrate on measurements that will produce an understanding of the composition and physical makeup of a cometary nucleus. The core science goals of the C4 mission are 1) to determine the chemical, elemental, and isotopic composition of a cometary nucleus and 2) to characterize the chemical and isotopic nature of its atmosphere. A related goal is to obtain temporal information about the development of the cometary coma as a function of time and orbital position. The four short-period comets -- Tempel 1, Tempel 2, Churyumov-Gerasimenko, and Wirtanen -which all appear to have acceptable dust production rates, were identified as candidate targets. Mission opportunities have been identified beginning as early as 1998. Tempel I with a launch in 1999, however, remains the baseline comet for studies of and planning the C4 mission. The C4 mission incorporates two science instruments and two engineering instruments in the payload to obtain the desired measurements. The science instruments include an advanced version of the Cometary Ice and Dust Experiment (CIDEX), a mini-CIDEX with a sample collection system, an X-ray Fluorescence Spectrometer and a Pyrolysis-Gas Chromatograph, and a simplified version of the Neutral Gas and Ion Mass Spectrometer (NIGMS). Both of these instruments have substantial heritage as they are based on those developed for the CRAF Mission. The engineering instruments include a simplified Comet Dust Environmental Monitor (SCODEM) and a navigational Camera, NAVCAM. While neither of the instruments will be permitted to establish science requirements, it is anticipated that significant science return will be accomplished Radio science will also be included.02/1994; -
Article: The generation and use of numerical shape models for irregular Solar System objects
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ABSTRACT: We describe a procedure that allows the efficient generation of numerical shape models for irregular Solar System objects, where a numerical model is simply a table of evenly spaced body-centered latitudes and longitudes and their associated radii. This modeling technique uses a combination of data from limbs, terminators, and control points, and produces shape models that have some important advantages over analytical shape models. Accurate numerical shape models make it feasible to study irregular objects with a wide range of standard scientific analysis techniques. These applications include the determination of moments of inertia and surface gravity, the mapping of surface locations and structural orientations, photometric measurement and analysis, the reprojection and mosaicking of digital images, and the generation of albedo maps. The capabilities of our modeling procedure are illustrated through the development of an accurate numerical shape model for Phobos and the production of a global, high-resolution, high-pass-filtered digital image mosaic of this Martian moon. Other irregular objects that have been modeled, or are being modeled, include the asteroid Gaspra and the satellites Deimos, Amalthea, Epimetheus, Janus, Hyperion, and Proteus.06/1993; -
Article: Cratering on Gaspra
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ABSTRACT: The October 1991 Galileo flyby of Gaspra shows that its crater population is dominated by fresh craters several hundred meters in diameter and smaller. They appear to represent a production population because the spatial density is relatively low (few overlaps) and because fresh craters are very abundant; equilibrium could be attained at diameters near to or below the resolution limit of the best image. These craters are the first direct record of the population of main-belt asteroids some tens of meters in diameter. Craters primarily from the highest resolution, 'high phase' image, on which over 600 craters are visible in 90 sq. km were counted, measured, and classified; earlier counts were made on the lower resolution four-color images, which show an order of magnitude fewer craters because of the resolution limit. The population index (exponent of the differential power law approximately describing the crater sizes) has a very high negative value (-4.3 +/- 0.3, meaning that the log-log slope is 'steep'), appreciably steeper than the value of -3.5 thought to reflect collisional equilibrium according to theory.02/1993; -
Article: The Galileo Solid-State Imaging experiment
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ABSTRACT: The Solid State Imaging (SSI) experiment on the Galileo Orbiter spacecraft utilizes a high-resolution (1500 mm focal length) television camera with an 800 800 pixel virtual-phase, charge-coupled detector. It is designed to return images of Jupiter and its satellites that are characterized by a combination of sensitivity levels, spatial resolution, geometric fiedelity, and spectral range unmatched by imaging data obtained previously. The spectral range extends from approximately 375 to 1100 nm and only in the near ultra-violet region ( 350 nm) is the spectral coverage reduced from previous missions. The camera is approximately 100 times more sensitive than those used in the Voyager mission, and, because of the nature of the satellite encounters, will produce images with approximately 100 times the ground resolution (i.e., 50 m lp-1) on the Galilean satellites. We describe aspects of the detector including its sensitivity to energetic particle radiation and how the requirements for a large full-well capacity and long-term stability in operating voltages led to the choice of the virtual phase chip. The F/8.5 camera system can reach point sources of V(mag) 11 with S/N 10 and extended sources with surface brightness as low as 20 kR in its highest gain state and longest exposure mode. We describe the performance of the system as determined by ground calibration and the improvements that have been made to the telescope (same basic catadioptric design that was used in Mariner 10 and the Voyager high-resolution cameras) to reduce the scattered light reaching the detector. The images are linearly digitized 8-bits deep and, after flat-fielding, are cosmetically clean. Information preserving and non-preserving on-board data compression capabilities are outlined. A special summation mode, designed for use deep in the Jovian radiation belts, near Io, is also described. The detector is preflashed before each exposure to ensure the photometric linearity. The dynamic range is spread over 3 gain states and an exposure range from 4.17 ms to 51.2 s. A low-level of radial, third-order, geometric distortion has been measured in the raw images that is entirely due to the optical design. The distortion is of the pincushion type and amounts to about 1.2 pixels in the corners of the images. It is expected to be very stable.We discuss the measurement objectives of the SSI experiment in the Jupiter system and emphasize their relationships to those of other experiments in the Galileo project. We outline objectives for Jupiter atmospheric science, noting the relationship of SSI data to that to be returned by experiments on the atmospheric entry Probe. We also outline SSI objectives for satellite surfaces, ring structure, and darkside (e.g., aurorae, lightning, etc.) experiments. Proposed cruise measurement objectives that relate to encounters at Venus, Moon, Earth, Gaspra, and, possibly, Ida are also briefly outlined. The article concludes with a description of a fully distributed data analysis system (HIIPS) that SSI team members intend to use at their home institutions. We also list the nature of systematic data products that will become available to the scientific community. Finally, we append a short historical note outlining the responsibilities and roles of institutions and individuals that have been involved in the 14 year development of the SSI experiment so far.Space Science Reviews 04/1992; 60(1):413-455. · 3.61 Impact Factor -
Article: Large quasi-circular features beneath frost on triton.
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ABSTRACT: Specially processed Voyager 2 images of Neptune's largest moon, Triton, reveal three large quasi-circular features ranging in diameter from 280 to 935 kilometers within Triton's equatorial region. The largest of these features contains a central, irregularly shaped area of comparatively low albedo about 380 kilometers in diameter, surrounded by crudely concentric annuli of higher albedo materials. None of the features exhibit significant topographic expression, and all appear to be primarily albedo markings. The features are located within a broad equatorial band of anomalously transparent frost that renders them nearly invisible at the large phase angles (alpha > 90 degrees ) at which Voyager obtained its highest resolution coverage of Triton. The features can be discerned at smaller phase angles (alpha = 66 degrees ) at which the frost only partially masks underlying albedo contrasts. The origin of the features is uncertain but may have involved regional cryovolcanic activity.Science 02/1992; 255(5046):824-6. · 31.20 Impact Factor -
Article: Lunar Impact Basins and Crustal Heterogeneity: New Western Limb and Far Side Data from Galileo
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ABSTRACT: 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.Science 01/1992; 255(5044):570-576. · 31.20 Impact Factor -
Article: Search for glazed surfaces on Triton
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ABSTRACT: The paper summarizes arguments leading to suggestions that Triton's icy surface may be unusual in texture, sith special attention given to the hypothesis of the existence of glazed areas on Triton. Results are presented of a search for an evidence of specular reflection diagnostic of 'glazed' icy surfaces on Triton, using high-resolution Voyager 2 images of three regions on Triton: the South Polar Cap Mottled Unit, the Bright Fringe, and the Frost Band. No such evidence was found in these three different terrains.11/1991; -
Article: Images from galileo of the venus cloud deck.
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ABSTRACT: Images of Venus taken at 418 (violet) and 986 [near-infrared (NIR)] nanometers show that the morphology and motions of large-scale features change with depth in the cloud deck. Poleward meridional velocities, seen in both spectral regions, are much reduced in the NIR In the south polar region the markings in the two wavelength bands are strongly anticorrelated. The images follow the changing state of the upper cloud layer downwind of the subsolar point, and the zonal flow field shows a longitudinal periodicity that may be coupled to the formation of large-scale planetary waves. No optical lightning was detected.Science 10/1991; 253(5027):1531-6. · 31.20 Impact Factor -
Article: Voyager observations of Nereid
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ABSTRACT: The Nereid's size and basic photometric properties were determined by analyzing Voyager 2 observations of Nereid obtained over a 12-day interval at solar phase angles of 25-96 deg. It was found that Nereid's radius is 170 +/-25 km and its geometric albedo exceeds 0.16. No evidence was found of a rotation light curve. It is concluded that, while Nereid is simialr to some of the icy satellites of Uranus, it is quite distinct from the group of six small inner satellites of Neptune discovered by Voyager: these other satellites are much darker and have much higher phase coefficients.Journal of Geophysical Research 09/1991; 96:19253. · 3.02 Impact Factor
Top co-authors
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- Science (10)
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- Nature (2)
Institutions
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1980–2013
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Cornell University
- • Center for Radiophysics and Space Research (CRSR)
- • Department of Astronomy
New York City, NY, USA
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2004
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California Institute of Technology
- Jet Propulsion Laboratory
Pasadena, CA, USA -
New York University USA
New York City, NY, USA
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