Evidence for Chemical Heterogeneity in the Nucleus of C/1995 O1 (Hale–Bopp)

We present an analysis of OH, CN, and C₂ jets observed in thecoma of Comet Hale–Bopp on UT April 22, 23, and 25, 1997. Monte Carlomodels designed to simulate the gas jets were employed to analyze thenuclear active areas responsible for the observed coma gas jets. Ourresults indicate that four active areas are necessary to reproduce theCN and C₂ jets while five active areas are required to simulatethe OH jets. The additional OH active area must produce significantlevels of OH, but cannot emit measurable quantities of either carbonradical. This difference suggests that the nucleus of Comet Hale–Boppis chemically heterogeneous.     

Physical characteristics of Hayabusa target Asteroid 25143 Itokawa

In March 2001, the Hayabusa spacecraft target, Asteroid 25143 Itokawa, made its final close approach to Earth prior to the spacecraft's launch. We carried out an extensive observing campaign from January to September 2001 to better characterize this near-Earth asteroid. Global physical properties of the surface of Itokawa were characterized by analyzing its photometric properties and behavior. Results included here capitalize on analysis of broadband photometric observations taken with a number of telescopes, instruments, and observers. We employed a Hapke model to estimate the surface roughness, single particle scattering albedo, single particle scattering characteristics, phase integral, and geometric and bond albedo. We find that this asteroid has a higher geometric albedo than average main belt S-class asteroids; this is consistent with results from other observers. The broadband colors of Itokawa further support evidence that this is an atypical S-class asteroid. Broadband colors show spectral characteristics more typically found on large-diameter main-belt asteroids believed to be space-weathered, suggesting the surface of this small diameter, near-Earth asteroid could likewise be space-weathered.     

Gaseous Jets in Comet Hale-Bopp (1995 O1)

We report the identification of gas jets in comet Hale-Bopp in OH, NH, CN, C₂ and C₃. This is the first time OH and NH jets without an obvious optical dust jet counterpart have been identified in narrowband comet images. We also confirm the existence of CN jets as reported by Larson et al. (1997) and Mueller et al. (1998). Jet features can be seen in the March and April 1997 datasets, approximately a month before and after perihelion. Our results contribute to the understanding of both the chemical properties of the comet as well as the physical mechanisms necessary to produce these features. This revised version was published online in July 2006 with corrections to the Cover Date.     

Timescales of partial melting in the Himalayan middle crust: insight from the Leo Pargil dome, northwest India

The Leo Pargil dome (LPD) in northwest India exposes an interconnected network of pre-, syn-, and post-kinematic leucogranite dikes and sills that pervasively intrude amphibolite-facies metapelites of the mid-crustal Greater Himalayan sequence. Leucogranite bodies range from thin (5-cm-wide) locally derived sills to thick (2-m-wide) crosscutting dikes extending at least 100 m. Three-dimensional exposures elucidate crosscutting relations between different phases of melt injection and crystallization. Combined laser ablation inductively coupled plasma mass spectrometry U–Th/Pb geochronology and trace element analysis on well-characterized monazite grains from nineteen representative leucogranites yields a large, internally consistent data set of approximately 700 U–Th/Pb and 400 trace element analyses. Grain-scale variations in age correlate with trace element distributions and indicate semi-continuous crystallization of monazite from 30 to 18 Ma. The youngest U–Th/Pb ages in a given sample are consistent with the outcrop-scale crosscutting relations, whereas older ages within individual samples record inheritance from partially crystallized melt and source metapelites. U–Th/Pb isotopic and trace element data are incorporated into a model of melting within the LPD that involves (1) steady-state equilibrium batch melting of compositionally homogeneous metapelitic sources; (2) pulses of increased melt mobility lasting 1–2 m.y. resulting in segregation of melt from its source and amalgamation into mixed magmas; and (3) rapid emplacement and final crystallization of leucogranite bodies. Melt systems in the LPD evolved from locally derived, in situ melt in migmatitic source rocks into a vast network of dikes and sills in the overlying non-migmatitic host rocks.     

Factors Determining the Location of the Chlorophyll Maximum and the Fate of Algal Production within the Tidal Freshwater James River

Longitudinal variation in factors affecting phytoplankton production were analyzed to better understand the mechanisms that cause the formation of a chlorophyll maximum within the tidal freshwater James River. Phytoplankton production was two- to threefold higher in the region where persistent elevated chlorophyll concentrations occurred. Near this site, the morphology of the James transitions from a narrow, deep channel to a broad expanse with shallow areas adjoining the main channel. Shallower depths resulted in greater average irradiance within the water column and suggest that release from light limitation was the principal factor accounting for the location of the chlorophyll maximum. Grazing rates were low indicating that little of the algal production was directly consumed by zooplankton. Low exploitation by zooplankton was attributed to poor food quality due to high concentrations of non-algal particulate matter and potential presence of cyanobacteria. Metabolism data suggest that two thirds of net primary production was respired in the vicinity of the chlorophyll maximum and one third was exported via fluvial and tidal advection. Comparison of water column and ecosystem metabolism indicates that the bulk of respiration occurred within the sediments and that sedimentation was the dominant loss process for phytoplankton.     

How much material do the radar-bright craters at the Mercurian poles contain?

The depth-to-diameter (d/D) ratios were determined for 12 craters located near the Mercurian north pole that were identified by Harmon et al. (2001, Icarus 149) as having strong depolarized radar echos. We find that the mean d/D value of these radar-bright craters is the mean d/D value of the general population of non-radar-bright craters in the surrounding north polar region. Previous studies, however, show no difference between d/D values of Mercurian polar and equatorial crater populations, suggesting that no terrain softening which could modify crater structure exists at the Mercurian poles (Barlow et al., 1999, 194, Icarus 141). Thus, the change in d/D is governed by a change in crater depth, probably due to deposition of material inside the crater. The volume of infilling material, including volatiles, in the radar-bright craters is significantly greater than predicted by proposed mechanisms for the emplacement of either water ice or sulfur.     

Spectrophotometry of Kuiper Belt Objects 20000 Varuna, 2000 Eb173 and Centaur 10199 Chariklo

Due to the distance, faintness, and very recent discovery ofKuiper Belt Objects (KBOs) and Centaurs, very little is knownabout the physical characteristics of these basic buildingblocks of the solar system. New intermediate-band photometryobservations of KBOs and Centaurs suggest that absorption bandsexist in the visible portion of their spectra, which could offerinsights into the surface composition of these objects.     

Iapetus dark and bright material:: giving compositional interpretation some latitude

Narrowband reflectance spectra (0.53-1.0 μm) of Iapetus' leading and trailing sides were obtained in 2000 to test the presence of an absorption feature located near 0.67 μm seen in reflectance spectra of Iapetus' dark material and Hyperion's surface material. No feature was observed. The difference in reflectance across the UV/VIS/NIR spectral region, and the dependence of the presence or absence of this absorption feature on angular separation from the apex of Iapetus in its orbit, phase angle, and heliocentric distance (affecting temperature), were examined. A trend of increased reddening, and the presence of the absorption feature, correlate with an angular separation from the apex of ? approximately 10°. Spectral information is lost when the contribution of the bright water ice signal to the reflectance spectrum increases sufficiently. In order to optimize compositional studies of Iapetus, we encourage future ground-based and space-based spectral observations to maximize the concentration of dark material in the instrumental field of view.     

Chemical and physical properties of gas jets in comets: II. Modeling OH, CN and C2 jets in Comet C/1995 O1 (Hale-Bopp) one month after perihelion

We present an analysis of OH, CN, and C₂ jets observed in Comet Hale-Bopp during April 22-26, 1997. We conclude that an extended source, which peaks in productivity after a certain amount of time has passed after being released from the nucleus (8.5, 2.5, and 42.6×¹⁰⁴ s, respectively) is responsible for the observed coma jet morphology in all three species. Sub-micron organic grains are the favored explanation for the extended source. Our models indicate that this extended source produces approximately 40% of the OH, 50% of the C₂, and 75% of the CN. The balance for each species is created by a diffuse nuclear gas source. Compared with the nuclear gas source and normalized to the CN abundance, the composition of the extended source is depleted in OH by a factor of ∼6, and depleted in C₂ by a factor of ∼2. The existence of anti-sunward jets do not require production of radicals throughout the cometary night. Instead, our model demonstrates that active areas exposed to near-twilight conditions throughout the comet's rotational period can produce the observed anti-sunward morphology.     

Chemical and physical properties of gas jets in comets: I. Monte Carlo model of an inner cometary coma

We describe a 3-dimensional, time-dependent Monte Carlo model developed to analyze the chemical and physical nature of a cometary gas coma. Our model includes the necessary physics and chemistry to recreate the conditions applicable to Comet Hale-Bopp when the comet was near 1 AU from the Sun. Two base models were designed and are described here. The first is an isotropic model that emits particles (parents of the observed gases) from the entire nucleus; the second is a jet model that ejects parent particles solely from discrete active areas on the surface of the comet nucleus, resulting in coma jets. The two models are combined to produce the final model, which is compared with observations. The physical processes incorporated in both base models include: (1) isotropic ejection of daughter molecules (the observed gases) in the parent's frame of reference, (2) solar radiation pressure, (3) solar insolation effects, (4) collisions of daughter products with other molecules in the coma, and (5) acceleration of the gas in the coma. The observed daughter molecules are produced when a parent decays, which is represented by either an exponential decay distribution (photodissociation of the parent gas) or a triangular distribution (production from a grain extended source). Application of this model to the analysis the OH, C₂ and CN gas jets observed in the coma of Comet Hale-Bopp is the focus of the accompanying paper [Lederer, S.M., Campins, H., Osip, D.J., 2008. Icarus, in press (this issue)].