Amanda Bosh

Pluto's atmosphere

Abstract The stellar occultation by Pluto on June 9, 1988, was observed with a high-speed CCD photometer attached to the 0.9-m telescope aboard NASAs Kuiper Airborne Observatory (KAO). The occultation lightcurve, which probed two regions on the sunrise limb separated by about 200 km, reveals a clear upper atmosphere that overlies an extinction layer with an abrupt upper boundary. The observations demonstrate that the extinction layer extends along the portion of the sunrise limb bounded by the immersion and emersion regions, as well as long the corresponding portion of the sunset limb on the opposite side of the planet. In all, the total limb probed by the KAO data for extinction represents nearly half of Plutos circumference. Hence, the extinction layer may surround the entire planet. A model atmosphere is presented, from which is derived an occultation lightcurve that closely matches the data. In addition to the standard parameters describing the occultation curve by an isothermal atmosphere, our model includes the radius of the upper boundary of the extinction and the radius of unit observed optical depth as free parameters. Fits of this model to the immersion and emersion lightcurves show no significant differences in the derived atmospheric structure. A preliminary geometrical solution, based on three occultation chords, yields a half-light radius of 1214 ± 20 km. At this level, the mean scale height derived from the model fits to the KAO data is 59.7 ± 1.5 km. The corresponding ratio of temperature to mean molecular weight is 4.2 ± 0.4°K/amu, with the principal source of error arising from the uncertainty in the mass of Pluto. The extinction layer, whose upper boundary lies 25 km below the half-light level, has a minimum thickness of 46 km, a minimum vertical optical depth of 0.19, and a scale height of 33.4 ± 6.9 km. For a pure methane atmosphere, our results imply (for the clear atmosphere at the half-light level) a temperature of 67 ± 6°K, a number density of 8.3 × 10 13 cm −3 , and a pressure of 0.78 ωbar. Our occultation data are also consistent with a predominantly nitrogen atmosphere (such as that of Titan), in which case the temperature would be 117 ± 11°K. The substantially smaller scale height of the extinction layer may arise from properties of the “particles” causing the extinction or may indicate a lower temperature in this region. Since our analysis indicates that the extinction layer is optically thick at the limb of Pluto, determinations of Plutos radius by methods that use reflected light, such as speckle interferometry and observations of the mutual events, give results that refer to the “visible disk” of Pluto and not on the planets solid surface. Unit optical depth of the extinction layer (observed along the line of sight) lies at 1174 ± 20 km, a level consistent with the radius of Pluto derived from the mutual events (1142 ± 21 km). The mutual event radius is also consistent with the deepest level probed by the occultation: it lies at a radius of 1143 ± 20 km, which represents an upper limit on the surface radius. For a pure methane atmosphere, a surface pressure as low as 3 ωbar (the vapor pressure of methane at 50°K) would be consistent with the occultation data.

The recent expansion of Pluto's atmosphere.

Stellar occultations—the passing of a relatively nearby body in front of a background star—can be used to probe the atmosphere of the closer body with a spatial resolution of a few kilometres (ref. 1). Such observations can yield the scale height, temperature profile, and other information about the structure of the occulting atmosphere. Occultation data acquired for Plutos atmosphere in 1988 revealed a nearly isothermal atmosphere above a radius of ∼1,215 km. Below this level, the data could be interpreted as indicating either an extinction layer or the onset of a large thermal gradient, calling into question the fundamental structure of this atmosphere. Another question is to what extent Plutos atmosphere might be collapsing as it recedes from the Sun (passing perihelion in 1989 in its 248-year orbital period), owing to the extreme sensitivity of the equilibrium surface pressure to the surface temperature. Here we report observations at a variety of visible and infrared wavelengths of an occultation of a star by Pluto in August 2002. These data reveal evidence for extinction in Plutos atmosphere and show that it has indeed changed, having expanded rather than collapsed, since 1988.

Size and albedo of Kuiper belt object 55636 from a stellar occultation

The Kuiper belt is a collection of small bodies (Kuiper belt objects, KBOs) that lie beyond the orbit of Neptune and which are believed to have formed contemporaneously with the planets. Their small size and great distance make them difficult to study. KBO 55636 (2002 TX300) is a member of the water-ice-rich Haumea KBO collisional family. The Haumea family are among the most highly reflective objects in the Solar System. Dynamical calculations indicate that the collision that created KBO 55636 occurred at least 1 Gyr ago. Here we report observations of a multi-chord stellar occultation by KBO 55636, which occurred on 9 October 2009 ut. We find that it has a mean radius of 143 ± 5 km (assuming a circular solution). Allowing for possible elliptical shapes, we find a geometric albedo of in the V photometric band, which establishes that KBO 55636 is smaller than previously thought and that, like its parent body, it is highly reflective. The dynamical age implies either that KBO 55636 has an active resurfacing mechanism, or that fresh water-ice in the outer Solar System can persist for gigayear timescales.

Observations of Saturn's Inner Satellites During the May 1995 Ring-Plane Crossing

The 22 May 1995 Saturn ring-plane crossing was observed with the Hubble Space Telescope; the markedly reduced scattered light from the rings at this time allowed study of the small inner satellites of the Saturn system. Prometheus was further from its predicted location than expected based on uncertainties in the 1981 ephemerides propagated forward by 15 years. A body found orbiting near or within Saturns F ring is either an F-ring shepherd or a transient clump of dust within the F ring; given its approximate brightness, the clump theory is more likely.

Large Bodies in the Kuiper Belt

We present a survey for bright Kuiper Belt objects (KBOs) and Centaurs, conducted at the Kitt Peak National Observatory (KPNO) 0.9 m telescope with the KPNO 8K Mosaic CCD. The survey imaged 164 deg2 near opposition to a limiting red magnitude of 21.1. Three bright KBOs and one Centaur were found, the brightest KBO having red magnitude 19.7, about 700 km in diameter, assuming a dark Centaur-like 4% albedo. We estimate the power-law differential size distribution of the classical KBOs to have index q = 4.2, with the total number of classical KBOs with diameters larger than 100 km equal to 4.7 × 104. Additionally, we find that if there is a maximum object size in the Kuiper Belt, it must be larger than 1000 km in diameter. By extending our model to larger size bodies, we estimate that 30 Charon-sized and 3.2 Pluto-sized classical KBOs remain undiscovered.

The 2011 June 23 stellar occultation by Pluto: Airborne and ground observations

On 2011 June 23, stellar occultations by both Pluto (this work) and Charon (future analysis) were observed from numerous ground stations as well as the Stratospheric Observatory for Infrared Astronomy (SOFIA). This first airborne occultation observation since 1995 with the Kuiper Airborne Observatory resulted in the best occultation chords recorded for the event, in three visible wavelength bands. The data obtained from SOFIA are combined with chords obtained from the ground at the IRTF, the U.S. Naval Observatory Flagstaff Station, and Leeward Community College to give the detailed state of the Pluto-Charon system at the time of the event with a focus on Plutos atmosphere. The data show a return to the distinct upper and lower atmospheric regions with a knee or kink in the light curve separating them as was observed in 1988, rather than the smoothly transitioning bowl-shaped light curves of recent years. The upper atmosphere is analyzed by fitting a model to all of the light curves, resulting in a half-light radius of 1288 ± 1 km. The lower atmosphere is analyzed using two different methods to provide results under the differing assumptions of particulate haze and a strong thermal gradient as causes for the lower atmospheric diminution of flux. These results are compared with those from past occultations to provide a picture of Plutos evolving atmosphere. Regardless of which lower atmospheric structure is assumed, results indicate that this part of the atmosphere evolves on short timescales with results changing the light curve structures between 1988 and 2006, and then reverting these changes in 2011 though at significantly higher pressures. Throughout these changes, the upper atmosphere remains remarkably stable in structure, again except for the overall pressure changes. No evidence of onset of atmospheric collapse predicted by frost migration models is seen, and the atmosphere appears to be remaining at a stable pressure level, suggesting it should persist at this full level through New Horizons flyby in 2015.

An Occultation by Saturn's Rings on 1991 October 2-3 October 2-3 Observed with the Hubble Space Telescope

An occultation of the star GSC 6323-01396 (V = 11.9) by Saturns rings was observed with the High-Speed Photometer on the Hubble Space Telescope (HST) on 1991 October 2-3. This occultation occurred when Saturn was near a stationary point, so the apparent motion of Saturn relative to the star was dominated by the HST orbital motion (8 km/s). Data were recorded simultaneously at effective wavelengths of 3200 and 7500 A, with an integration time of 0.15 s. Fifteen segments of occultation data, totaling 6.8 h, were recorded in 13 successive orbits during the 20.0 h interval from UTC 1991 October 2, 19:35 until UTC 1991 October 3, 15:35. Occultations by 43 different features throughout the classical rings were unambiguously identified in the light curve, with a second occultation by 24 of them occurring due to spacecraft orbital parallax during this extremely slow event. Occultation times for features currently presumed circular were measured and employed in a geometrical model for the rings. This model, relating the observed occultation times to feature radii and longitudes, is presented here and is used in a least-squares fit for the pole direction and radius scale of Saturns ring system.

Stellar Occultation Observations of Saturn's North-Polar Temperature Structure

Abstract We have observed a stellar occultation of GSC5249-01240 by Saturns north polar region on November 20, 1995 from NASAs Infrared Telescope Facility (IRTF). This is the first recorded occultation by the polar region of a giant planet. The occulted region extends 88 km in vertical height and 660 km in horizontal length, over a region from 82.5° to 85° in planetocentric latitude and from 20° to 30° in planetocentric longitude. Based on isothermal model fits to the light curve, we find an equivalent isothermal temperature of 130 ± 10 K at a pressure level of 1.6 ± 0.1 μbar, which corresponds to a half-light latitude of 83.2 ± 0.2° and longitude of 24.1 ± 0.5°. Using numerical inversion procedures, we have retrieved the temperature profile of the occulted region, which suggests an increase in temperature (with radius) of 14.5 K between 6 and 10 μbar. We also find temperature fluctuations of 1 to 5 K along the path probed by the occultation; if the observed temperature gradients of these fluctuations apply to the vertical direction only, then this region is super-adiabatic. More likely, these thermal gradients are due to a combination of diffractive scintillations and horizontal temperature variations. Given that isothermal model fits and numerical inversions cannot separate individual contributions to observed temperature gradients, such as from vertical variations, horizontal variations, and scintillations, this occultation requires further study.

Photometric variability of Charon at 2.2 μm

Pluto-Charon images obtained on each of four nights at 2.2, 1.2, and 1.7 microns are presently fitted by a two-source image model in which the position of Charon and the ratio of its signal to that of Pluto are free parameters. At 2.2 microns, Charon is fainter than Pluto by magnitudes which, when combined with Pluto-Charon system photometry, yield apparent magnitudes of 15.01 + or - 0.08 for Charon at 0.06 lightcurve phase and 15.46 + or - 0.05 at lightcurve phase 0.42. In view of these results, Charon is variable in this filter bypass due to geometric albedo changes as a function of longitude.

HUBBLE SPACE TELESCOPE ASTROMETRIC OBSERVATIONS AND ORBITAL MEAN MOTION CORRECTIONS FOR THE INNER SATELLITES OF NEPTUNE

Six small inner satellites of Neptune were imaged in 1989 with Voyager 2. In 1997, we recovered the four outermost with the Hubble Space Telescope (HST) Wide Field Planetary Camera 2 for astrometric, dynamical, and photometric studies. The ring arcs were not detected in our images. Thirteen exposures were taken in each of three HST orbits: two orbits on July 3 and one on July 6. Exposures were taken in the BVI filters. Measurable images of Neptune and Triton were also obtained on the same PC1 frames with those of the faint satellites. We present here the astrometric observations of these four satellites relative to Neptune, as well as corrected orbital mean motions for them. Field distortions in the PC1 chip were corrected with both the Trauger et al. and the Anderson & King distortion models. Calibration of the scale and orientation was accomplished by comparing the measured positions of Neptune and Triton with an accurate JPL J2000 ephemeris. Separate calibrations were made for both distortion models. Small differences were detected in the calibrations, dependent on wavelength, saturation, and filter, and a small difference was found between the calibrations resulting from both distortion correction models. The resulting separation and position angle observations for the inner satellites were compared with the orbits of Owen et al. and corrections derived to their mean daily motions. A small but significant discrepancy was found for Proteus between the correction derived from the observations of separation and that from the position angles. This was shown not to be due to calibrational errors but, apparently, to the need for improvement of other orbital elements—at least for Proteus. Despite this anomaly, the mean motion accuracies were improved by almost 2 orders of magnitude as a result of the longer baseline since the Voyager observations. More HST observations of these satellites are recommended in order to improve their orbits further and for the investigation of satellite-ring interactions.