Charon's size and an upper limit on its atmosphere from a stellar occultation

Abstract

Pluto and its satellite, Charon (discovered in 1978; ref. 1), appear to form a double planet, rather than a hierarchical planet/satellite couple. Charon is about half Pluto's size and about one-eighth its mass. The precise radii of Pluto and Charon have remained uncertain, leading to large uncertainties on their densities. Although stellar occultations by Charon are in principle a powerful way of measuring its size, they are rare, as the satellite subtends less than 0.3 microradians (0.06 arcsec) on the sky. One occultation (in 1980) yielded a lower limit of 600 km for the satellite's radius, which was later refined to 601.5 km (ref. 4). Here we report observations from a multi-station stellar occultation by Charon, which we use to derive a radius, R(C) = 603.6 +/- 1.4 km (1sigma), and a density of rho = 1.71 +/- 0.08 g cm(-3). This occultation also provides upper limits of 110 and 15 (3sigma) nanobar for an atmosphere around Charon, assuming respectively a pure nitrogen or pure methane atmosphere.

Precise predictions of stellar occultations by Pluto, Charon, Nix, and Hydra for 2008–2015

“…Considering a surface temperature of 56 K rising up to 100 K above 20 km, a pure N 2 or CH 4 isothermal atmosphere leads to pressure limits of 15 μbar and 110 μbar respectively (Sicardy et al 2006). These low values are compatible with the expected volatile escape rates for Charon (Yelle & Elliot 1997).…”

“…The Charon radius ranges from 603.6 ± 1.4 km to 606.0 ± 1.5 km, as estimated by various authors from the 2005 July 11 stellar occultation (Sicardy et al 2006;Gulbis et al 2006;Person et al 2006). Pluto, Charon, Nix, and Hydra have estimated GM values of 870.3 ± 3.7, 101.4 ± 2.8, 0.039 ± 0.034 and 0.021 ± 0.042 km 3 s −2 respectively (Tholen et al 2008).…”

“…Furthermore, a stellar occultation observed in 1985 revealed Pluto s atmosphere (Brosch 1995), which was observed more extensively during another occultation in 1988 (Millis et al 1993). Also, besides the first stellar occultation recorded for Charon (Walker 1980), only two others were so far observed, one on 11 July 2005 (Sicardy et al 2006;Gulbis et al 2006) and another on 22 June 2008 (Sicardy et al 2010, in prep. ).…”

“…Another factor was the entering of Pluto in front of the projected Galactic plane, increasing the frequency of possible events. Successful examples of these new prediction methods are the stellar occultation campaigns of 2002 for Pluto (Sicardy et al 2003) and of 2005 for Charon (Sicardy et al 2006). Since 2004, our group has been engaged on a systematic effort to derive astrometric predictions for stellar occultations by Pluto and its satellites.…”

An exploration of Pluto’s environment through stellar occultations

“…Subsequent studies provided orbital elements and mass estimates for Charon, Nix and Hydra (Tholen et al 2008;Buie et al 2013). Occultation observations have provided Charon's radius at kilometric accuracy Sicardy et al 2006b;Person et al 2006), as well as constraints on Charon's orbital elements (Sicardy et al 2011).…”