Regolith depth growth on an icy body orbiting Saturn and evolution of bidirectional reflectance due to surface composition changes

Using a Markov chain model, we consider the regolith growth on a small body in orbit around Saturn, subject to meteoritic bombardment, and assuming all impact ejecta are re-collected. We calculate the growth of regolith and the fractional pollution, assuming an initial pure ice body and amorphous carbon as a pollutant. We extend the meteorite flux of Cuzzi and Estrada (Cuzzi, J., Estrada, P. [1998]. Icarus 132, 1-35) to larger sizes to consider the effect of disruption of the moonlet on other moonlets in the ensemble. This is a relatively small effect, completely negligible for moonlets of 1 m radius. For the given impact model, fractional pollution reaches 22% for 1 m bodies, but only 3% for 10 m bodies, 1.7% for 20 m bodies, and 1% for 30 m bodies after 4 byr. By considering an ensemble of moonlets, which have identical cross-sections for releasing and capturing ejecta, this analysis can be extended to a model of particles in Saturn’s rings, where the calculated spectra can be compared to observed ring spectra. The measured spectral reflectance of Saturn’s rings from Cassini observations therefore constrains the size and age of the ring particles. The comparison between 1 m, 10 m, 20 m, and 30 m particles confirms that for larger ring mass, the current rings would be less polluted; for the largest particles, we expect negligible changes in the UV spectrum after 4 byr of meteoritic bombardment. We consider two end members for mixing of the meteoritic material: areal and intimate. Given the uncertainties in the actual mixing of the meteoritic infall and in its composition (as a worst case, we assume the meteoritic material is 100% amorphous carbon, intimately mixed) initially pure ice 30 m ring particles would darken after 4 byr of exposure by 15%.     

Formation of radical species in photolyzed CH4:N2 ices

We report photochemical studies of thin cryogenic ice films composed of N₂, CH₄ and CO in ratios analogous to those on the surfaces of Neptune’s largest satellite, Triton, and on Pluto. Experiments were performed using a hydrogen discharge lamp, which provides an intense source of ultraviolet light to simulate the sunlight-induced photochemistry on these icy bodies. Characterization via infrared spectroscopy showed that C₂H₆ and C₂H₂, and HCO are formed by the dissociation of CH₄ into H, CH₂ and CH₃ and the subsequent reaction of these radicals within the ice. Other radical species, such as C₂, , CN, and CNN, are observed in the visible and ultraviolet regions of the spectrum. These species imply a rich chemistry based on formation of radicals from methane and their subsequent reaction with the N₂ matrix. We discuss the implications of the formation of these radicals for the chemical evolution of Triton and Pluto. Ultimately, this work suggests that , CN, HCO, and CNN may be found in significant quantities on the surfaces of Triton and Pluto and that new observations of these objects in the appropriate wavelength regions are warranted.     

A system for line profile studies at the 150-foot tower on Mount Wilson

We describe enhancements to the hardware and software for the 150-foot tower system on Mt. Wilson which make possible the acquisition of high precision line profile measurements. This system utilizes the 75-foot pit spectrograph with a photomultiplier detector system to scan line profiles repeatedly in order to study variations induced by the passage of waves vertically through the solar atmosphere. Oscillations of line profile parameters with an amplitude as low as 1.7 m s^–1 have been detected with this system using integrated sunlight. Phase relations between oscillations of different parts of the line profile are appropriate to upward energy transport. Consistent with the previous conclusion by Mein and Schmieder (1981), we find that the magnitude of the energy transport is compatible with the 5-min oscillations making an important contribution to the heating of the low chromosphere.The Mount Wilson Observatory is operated by the Mount Wilson Institute under agreement with the Carnegie Institution of Washington.     

New 1982 — 1990 photometry of λ Andromedae and its 11-year cycle

We present photoelectric photometry of λ And never before published, obtained between February 1982 and December 1990 at 29 different observatories. Then we combine it with all other photometry available to us (previously published, contained in the I.A.U. Commission 27 Archives, and obtained with the Vanderbilt 16-inch automatic telescope but not yet published), to yield a 14.8-year data base. Analysis reveals a long-term cycle in mean brightness, with a full range of 0^m.15 and a period of 11.4 ± 0.4 years. Because most of our new photometry was concentrated in the 1983-84 observing season, we analyze that one well-defined light curve with a two-spot model. Spot A keeps a 0^m.04 amplitude throughout four rotation cycles whereas the amplitude of spot B diminishes from 0^m.09 down almost to 0^m.03. The spot rotation periods were 55^d.9 ± 0^d.6 and 520^d.8 ± 1^d.0, respectively.     

Moonlets and clumps in Saturn's F ring

Cassini UVIS star occultations by the F ring detect 13 events ranging from 27 m to 9 km in width. We interpret these structures as likely temporary aggregations of multiple smaller objects, which result from the balance between fragmentation and accretion processes. One of these features was simultaneously observed by VIMS. There is evidence that this feature is elongated in azimuth. Some features show sharp edges. At least one F ring object is opaque and may be a “moonlet.” This possible moonlet provides evidence for larger objects embedded in Saturn's F ring, which were predicted as the sources of the F ring material by Cuzzi and Burns [Cuzzi, J.N., Burns, J.A., 1988. Icarus 74, 284-324], and as an outcome of tidally modified accretion by Barbara and Esposito [Barbara, J.M., Esposito, L.W., 2002. Icarus 160, 161-171]. We see too few events to confirm the bi-modal distribution which Barbara and Esposito [Barbara, J.M., Esposito, L.W., 2002. Icarus 160, 161-171] predict. These F ring structures and other youthful features detected by Cassini may result from ongoing destruction of small parent bodies in the rings and subsequent aggregation of the fragments. If so, the temporary aggregates are 10 times more abundant than the solid objects. If recycling by re-accretion is significant, the rings could be quite ancient, and likely to persist far into the future.     

Carbon dioxide segregation in 1:4 and 1:9 CO2:H2O ices

The mid-infrared spectra of mixed vapor deposited ices of CO₂ and H₂O were studied as a function of both deposition temperature and warming from 15 to 100 K. The spectra of ices deposited at 15 K show marked changes on warming beginning at 60 K. These changes are consistent with CO₂ segregating within the ice matrix into pure CO₂ domains. Ices deposited at 60 and 70 K show a greater degree of segregation, as high as 90% for 1:4 CO₂:H₂O ice mixtures deposited at 70 K. As the ice is warmed above 80 K, preferential sublimation of the segregated CO₂ is observed. The kinetics of the segregation process is also examined. The segregation of the CO₂ as the ice is warmed corresponds to temperatures at which the structure of the water ice matrix changes from the high density amorphous phase to the low density amorphous phase. We show how these microstructural changes in the ice have a profound effect on the photochemistry induced by ultraviolet irradiation. These experimental results provide a framework in which observations of CO₂ on the icy bodies of the outer Solar System can be considered.     

Scientific observations at total solar eclipses

The occasion of the longest totality of an eclipse in the 18 yr 11^1/3 d saros cycle leads to taking stock of the scientific value of ground-based eclipse observations in this space age. Though a number of space satellites from the U.S., Europe, Japan, and Russia study the Sun, scientists at eclipses can observe the solar chromosphere and corona at higher spatial resolution, at higher temporal resolution, and at higher spectral resolution than are possible aloft. Furthermore, eclipse expeditions can transport a wide variety of state-of-the-art equipment to the path of totality. Thus, for at least some years to come, solar eclipse observations will remain both scientifically valuable and cost-effective ways to study the outer solar atmosphere.     

Charged dust in the outer planetary magnetospheres

We assume that typical interplanetary grains are fragile, aggregates of the Brownlee type, and discuss the physical and dynamical processes associated with their entry into the Jovian magnetosphere. Limiting ourselves to the equatorial plane of the planet, we show that grains traversing the outer edge of the co-rotating magnetodisc (r35R _J ) are rapidly charged up to large negative potentials on both the day and the night sides. A parent grain of radiusR _g 20 is electrostatically disrupted when it attains a potential of about –220 V. While the eventual potential achieved by the smallest fragments (R _g 0.1 ) are controlled by the rapid field emission of electrons, those of the larger fragments (R _g 1 ) are set by the plasma and photoemission currents.All the negatively charged fragments are strongly attracted towards the planet by the (radial) corotational electric field and some are stably trapped. We suggest that the sudden enhancement by about 2 orders of magnitude of the interplanetary dust flux measured by Pioneer 10, at about 30R _J from Jupiter result from the combination of these two effects.The different brightness asymmetries between the leading and the trailing sides of the outer and inner Galilean satellites appear to be a natural consequence of the way the trajectories of these charged dust grains intersect these satellite surfaces. Finally, the similarity in the brightness asymmetries between the Jovian and Saturnian satellites, and our belief that they have a similar cause, leads us to the expectation that Saturn's magnetic momentM and spin , are parallel as in the case of Jupiter, with the limit of plasma co-rotation lying between the satellites Rhea and Iapetus.