S. P. Souza

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.

WAVES IN PLUTO'S UPPER ATMOSPHERE

Observations of the 2007 March 18 occultation of the star P445.3 (2UCAC 25823784; R = 15.3) by Pluto were obtained at high time resolution at five sites across the western United States and reduced to produce light curves for each station using standard aperture photometry. Global models of Pluto’s upper atmosphere are fitted simultaneously to all resulting light curves. The results of these model fits indicate that the structure of Pluto’s upper atmosphere is essentially unchanged since the previous occultation observed in 2006, leading to a well-constrained measurement of the atmospheric half-light radius at 1291 ± 5 km. These results also confirm that the significant increase in atmospheric pressure detected between 1988 and 2002 has ceased. Inversion of the Multiple Mirror Telescope Observatory light curves with unprecedented signal-to-noise ratios reveals significant oscillations in the number density, pressure, and temperature profiles of Pluto’s atmosphere. Detailed analysis of this highest resolution light curve indicates that these variations in Pluto’s upper atmospheric structure exhibit a previously unseen oscillatory structure with strong correlations of features among locations separated by almost 1200 km in Pluto’s atmosphere. Thus, we conclude that these variations are caused by some form of large-scale atmospheric waves. Interpreting these oscillations as Rossby (planetary) waves allows us to establish an upper limit of less than 3ms −1 for horizontal wind speeds in the sampled region (radius 1340–1460 km) of Pluto’s upper atmosphere.

CHARON'S RADIUS AND DENSITY FROM THE COMBINED DATA SETS OF THE 2005 JULY 11 OCCULTATION

The 2005 July 11 C313.2 stellar occultation by Charon was observed by three separate research groups, including our own, at observatories throughout South America. Here, the published timings from the three data sets have been combined to more accurately determine the mean radius of Charon: 606.0 ± 1.5 km. Our analysis indicates that a slight oblateness in the body (0.006 ± 0.003) best matches the data, with a confidence level of 86%. The oblateness has a pole position angle of 714 ± 104 and is consistent with Charons pole position angle of 67°. Charons mean radius corresponds to a bulk density of 1.63 ± 0.07 g cm-3, which is significantly less than Plutos (1.92 ± 0.12 g cm-3). This density differential favors an impact formation scenario for the system in which at least one of the impactors was differentiated. Finally, unexplained differences between chord timings measured at Cerro Pachon and the rest of the data set could be indicative of a depression as deep as 7 km on Charons limb.

THE STRUCTURE OF PLUTO'S ATMOSPHERE FROM THE 2002 AUGUST 21 STELLAR OCCULTATION

We have observed the 2002 August 21 occultation by Pluto of theR ¼ 15:7mag star P131.1, using 0.5 s cadence observations in integrated white light with the Williams College frame-transfer, rapid-readout CCD at the 2.24 m University of Hawaii telescope. We detected an occultation that lasted 5 minutes, 9:1 � 0:7 s between half-light points. The ‘‘kinks’’ in the ingress and egress parts of the curve that were apparent in 1988 had become much less pronounced by the time of the two 2002 occultations that were observed, indicating a major change in the structure ofPluto’satmosphere.AnalysisofourlightcurvesshowsthatthepressureinPluto’satmospherehasincreasedatall the altitudes that we probed. Essentially, the entire pressure scale has moved up in altitude, increasing by a factor of 2 since 1988. Spikes in our light curve reveal vertical structure in Pluto’s atmosphere at unprecedentedly high resolution.Wehaveconfirmationofourspikesatlowertimeresolutionaspartofobservationsoftheemersionmade at 1.4 s and 2.4 s cadence with the 3.67 m AEOS telescope on Maui.

POETS: Portable Occultation, Eclipse, and Transit System

Occultations of stars by small bodies in the outer solar system are opportunities to make high- resolution measurements of their geometries and orbital elements and to detect or probe their atmospheres. Such events are limited in space and time, so it is desirable to deploy highly capable camera systems on multiple fixed and/or portable telescopes anywhere in the world, potentially on short notice. Similar considerations apply to planetary transits and solar eclipses. We have designed a camera system called POETS (Portable Occultation, Eclipse, and Transit System), which is optimized for occultation and related observations, and have assembled five such systems. The core of this system is the Andor Technology DV-887 (now DU-897) frame-transfercamera, featuring a high frame rate, minimal dead time, high quantum efficiency, and low read noise. An electron- multiplying mode lowers effective read noise to below 1 e ! pixel ! 1 and is capable of photon counting. Each POETS includes a compact GPS timing system with microsecond accuracy, and a high-performance computer system capable of sustained fast frame rates. Each POETS is designed to be transportable as carry-on luggage and is adaptable to a wide variety of sites. POETS were deployed for the first time for the 2005 July 11 Charon occultation event, and they performed extremely well on telescopes with apertures from 0.6 to 6.5 m. Three POETS were subsequently deployed for the 2006 March 29 total solar eclipse, and five for the 2006 June 12 Pluto occultation.

PICO: Portable Instrument for Capturing Occultations

We describe a portable imaging photometer for the observation of stellar occultation events by Kuiper Belt objects (KBOs) and other small bodies. The system is referred to as the Portable Instrument for Capturing Occultations (PICO). It is designed to be transportable to remote observing sites by a single observer. A GPS timing system is used to trigger exposures of a Finger Lakes Instrumentation ML261E-25 camera to facilitate the combination of observational results from multiple sites. The system weighs a total of 11 kg when packed into its single rigid 55.1 × 35.8 × 22.6 cm container, meeting current airline size and weight limits for carry-on baggage. Twelve such systems have been constructed. Nine systems were deployed for observation of a stellar occultation by Kuiper Belt object 55636 in 2009 October. During the same month, one system was used to record a stellar occultation by minor planet 762 Pulcova.

Recent Stellar Occultation Observations Using High‐Speed, Portable Camera Systems

We have recently constructed six observing systems identified as POETS (Portable Occultation Eclipse and Transit System[1]). These systems are optimized for (i) high‐speed, high signal‐to‐noise observations at visible wavelengths and (ii) easy transport, to allow mounting on telescopes worldwide. The Andor iXon cameras have e2v CCD97 (frame transfer) sensors: a 512×512 array of 16‐micron pixels, back illuminated, with peak quantum efficiency >90%. The maximum readout rate is 32 full frames per second, while binning and subframing can increase the cadence to a few hundred frames per second. Read noise in conventional modes goes below 6 electrons per pixel. Further, an electron‐multiplying mode can effectively reduce the read noise to sub‐electron levels, at the expense of dynamic range. The cameras are operated via a desktop computer that contains a 3 GHz Pentium 4 processor, 2 GB memory, and a 10,000 rpm hard disk. Images are triggered from a GPS receiver and have an approximately 50 nanosecond timing unc...