Pluto and Charon with HST I: Monitoring Global Change and Improved Surface Properties from Lightcurves

We present new lightcurve measurements of Pluto and Charon taken with the Advanced Camera for Surveys (ACS) High Resolution Camera (HRC) on the Hubble Space Telescope (HST). The observations were collected from June 2002 to June 2003 at twelve distinct sub-earth longitudes over a range of solar phase angle 0.36-1.74 degrees – a larger range than previously measured. The new measurements of Pluto show that the lightcurve amplitude has decreased since the mutual event season in the late 1980’s. We also show that the average brightness has increased in the F555W (Johnson V equivalent) passband while the brightness has decreased in the F435W (Johnson B equivalent) passband. These data thus indicate a substantial reddening of the reflected light from Pluto. We find a weighted mean (B − V ) = 0.9540± 0.0010 that is considerably higher than the long-standing value of (B−V ) = 0.868± 0.003 most recently measured in 1992-3. We also report on the discovery of a new rotational modulation of Pluto’s hemispherical color that ranges from 0.92 to 0.98 with the least red color at the longitude of maximum light and most red at minimum light. The phase coefficient of Pluto is nearly the same as measured in 1992-3 with a value of βB = 0.0392 ± 0.0064 and βV = 0.0355 ± 0.0045 mag/degree for the F435W and F555W data respectively. The Pluto phase curve is still very close to linear but a small but significant non-linearity is seen in the data. In contrast, the lightcurve of Charon is essentially the same as in 1992/3, albeit with much less noise. We confirm that Charon’s Pluto-facing hemisphere is 8% brighter than the hemisphere facing away from Pluto. The color of Charon is independent of longitude and has a mean weighted value of (B − V ) = 0.7315 ± 0.0013. The phase curve for Charon is now shown to be strongly non-linear and wavelength dependent. We present results for both Pluto and Charon that better constrain