The Astrophysical Journal Letters, 817:L22 (7pp), 2016 February 1 doi:10.3847/2041-8205/817/2/L22 © 2016. The American Astronomical Society. All rights reserved. SURFACE ALBEDO AND SPECTRAL VARIABILITY OF CERES Jian-Yang Li (李荐扬)1, Vishnu Reddy1, Andreas Nathues2, Lucille Le Corre1, Matthew R. M. Izawa3,4, Edward A. Cloutis3, Mark V. Sykes1, Uri Carsenty5, Julie C. Castillo-Rogez6, Martin Hoffmann2, Ralf Jaumann5, Katrin Krohn5, Stefano Mottola5, Thomas H. Prettyman1, Michael Schaefer2, Paul Schenk7, Stefan E. Schröder5, David A. Williams8, David E. Smith9, Maria T. Zuber10, Alexander S. Konopliv6,RyanS.Park6, Carol A. Raymond6, and Christopher T. Russell11 1 Planetary Science Institute, 1700 E. Ft. Lowell Road, Suite 106, Tucson, AZ 85719, USA 2 Max Planck Institute for Solar System Research, Göttingen, Germany 3 University of Winnipeg, Winnipeg, Manitoba, Canada 4 Royal Ontario Museum, Toronto, Ontario, Canada 5 German Aerospace Center (DLR), Institute of Planetary Research, Berlin, Germany 6 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA 7 Lunar and Planetary Institute, Houston, TX 77058, USA 8 School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA 9 Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA 10 Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA 11 Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095, USA Received 2015 December 2; accepted 2016 January 13; published 2016 January 29 ABSTRACT Previous observations suggested that Ceres has active, but possibly sporadic, water outgassing as well as possibly varying spectral characteristics over a timescale of months. We used all available data of Ceres collected in the past three decades from the ground and the Hubble Space Telescope, as well as the newly acquired images by the Dawn Framing Camera, to search for spectral and albedo variability on Ceres, on both a global scale and in local regions, particularly the bright spots inside the Occator crater, over timescales of a few months to decades. Our analysis has placed an upper limit on the possible temporal albedo variation on Ceres. Sporadic water vapor venting, or any possibly ongoing activity on Ceres, is not significant enough to change the albedo or the area of the bright features in the Occator crater by >15%, or the global albedo by >3% over the various timescales that we searched. Recently reported spectral slope variations can be explained by changing Sun–Ceres–Earth geometry. The active area on Ceres is less than 1 km2, too small to cause global albedo and spectral variations detectable in our data. Impact ejecta due to impacting projectiles of tens of meters in size like those known to cause observable changes to the surface albedo on Asteroid Scheila cannot cause detectable albedo change on Ceres due to its relatively large size and strong gravity. The water vapor activity on Ceres is independent of Ceres’ heliocentric distance, ruling out the possibility of the comet-like sublimation process as a possible mechanism driving the activity. Key words: methods: observational – minor planets, asteroids: individual (1 Ceres) – space vehicles – techniques: image processing – techniques: photometric 1. INTRODUCTION and the most recent Dawn Framing Camera (FC) data collected during the approach to Ceres. Our search covered three Observations of the dwarf planet Ceres by NASAʼs Dawn different time frames: long-term in decades; mid-term in years, spacecraft have revealed a surface peppered with bright spots and short-term in months. associated with impact craters (Nathues et al. 2015). The bright spots are prominent albedo features on the otherwise uniform surface of Ceres in albedo and color. The compositional 2. ANALYSIS analysis reveals that these bright spots are likely made up of a mixture of water ice, salts, and dark background material, 2.1. Full-disk Spectral Variability suggesting a briny subsurface source. Bright spots within The spectral data archived in the Planetary Data System ( ) ( ) impact craters Dantu 125 km and Occator 92 km have been Small Bodies Node from four epochs (Vilas et al. 1998; Bus & linked to intermittent and localized water vapor sources Binzel 2003; Lazzaro et al. 2006), from Perna et al. (2015) in observed by the Herschel Space Observatory in 2011–2013 two epochs, and from recent ground-based observations (Küppers et al. 2014), and are, therefore, possibly related to the (Reddy et al. 2015a) span nearly three decades, and are sublimation activity. Recent ground-based observations of suitable for long-term spectral variability of Ceres Ceres also suggested short-term global spectral variability that (Figure 1(a)). The spectra of Ceres from various data sets all was attributed to the changing amount of water ice on the show a nearly linear shape between 0.53 and 0.85 μm, with surface (Perna et al. 2015). These observations suggest a increasing spectral slopes with solar phase angle and absorp- possibly changing surface caused by water outgassing on tions <0.54 μm and >0.9 μm. Therefore, we fit linear slopes to timescales of decades to months. We searched for evidence of all spectra between 0.54 and 0.85 μm to quantify the spectral surface albedo and spectral variations that might be evidence shape variation. Figure 1(b) suggests that the variations in slope for ongoing activity by comparing various historical data sets are ascribable to variations in phase angle, except for the Dawn 1 The Astrophysical Journal Letters, 817:L22 (7pp), 2016 February 1 Li (李荐扬) et al. Figure 1. (a) The visible spectra of Ceres taken at increasing phase angles (marked at the left of each spectrum) from bottom to top (vertically shifted for clarity). From bottom to top: Geo: Modeled geometric albedo spectrum from ground-based photometric observations (Reddy et al. 2015a); P01, P02, P03: Perna et al. (2015) spectra on 2012 December 18 and 19; V87: Vilas et al. (1998) spectrum in 1987; P04, P05, P06: Perna et al. (2015) spectra on 2013 January 18–20; SOS: S2OS3 (Lazzaro et al. 2006); RC1: Dawn RC1; SII: SMASS II (Bus & Binzel 2003); V92: Vilas et al. (1998) spectrum in 1992; and RC2: Dawn RC2. (b) The linear spectral slopes of Ceres’ spectra between 0.54 and 0.85 μm. The colors of symbols correspond to the colors in panel (a). The large error bar for the red symbol at 0°.8 phase angle, corresponding to the P02 spectrum, is due to the much higher noise in the spectrum compared to others. The black dashed line is a linear fit to the spectral slope with respect to phase angle, not including the slope measured from Dawn RC2 at a 44° phase angle. The spectral slope of Ceres increases nearly linearly from a 0° to 25° phase angle. 2 The Astrophysical Journal Letters, 817:L22 (7pp), 2016 February 1 Li (李荐扬) et al. Table 1 2.3. Occator Bright Spots Observation Conditions Extracted from the FC Color Data The HST/ACS/HRC data and Dawn FC data are separated Phase Average Reso- S/C Dis- Heliocentric by 12 years, and are both spatially resolved, suitable for Angle lution tance Distance searching for albedo changes on local scales on Ceres in a −1 (degree) (km pixel ) (km) (AU) medium time frame of years. We focus on the Occator crater RC1 17.2–17.6 7.68 82,151 2.8529 because the bright spots inside this crater are undoubtedly the RC2 42.7–45.3 4.24 45,359 2.8578 most prominent features on Ceres, and possibly related to the RC3 (Equa- 7.7–11.0 1.28 13,603 2.9051 water vapor activity (Nathues et al. 2015). Comparison of torial) absolute feature brightness between different data sets is extremely difficult, due primarily to the vastly different optical characteristics and absolute calibrations between HST and FC. RC2 observations at a 44° phase angle (described below), Our approach is to compare the Occator bright spots with a possibly indicating that the approximate linear relationship reference region, for which we chose the Haulani crater (32 km between spectral slope and phase angle stops somewhere diameter), because it is relatively easy to identify in HST between 20° and 40° phase angle. images. The underlying assumptions for our comparative Dawn FC observed Ceres for at least one full rotation of approach is that the reflectances of Occator bright spots and Ceres of 9.075 hr (Chamberlain et al. 2007) in three separate the Haulani crater did not change simultaneously, and this is epochs in 2015 February and April during the approach. These justified by the unchanging disk-integrated brightness of Ceres three observations are named Rotational Characterization over the same time period (Section 2.2). We used RC1 images (RC1, RC2, and RC3, see Table 1). The disk of Ceres did and RC3-equator images (Table 1) in this study. Note that the not fill the field of view of the FC during the RCs. Image Haulani crater is in fact the brightest feature on Ceres in HST calibration and spectral cube computation have been performed images (Thomas et al. 2005; Li et al. 2006), brighter than the according to Reddy et al. (2012a) and Nathues et al. (2014). Occator crater (Figure 2). The present study seeks an answer to The time difference between RC1 and RC2 is one week and the following question: Did the brightness of the Occator crater between RC2 and RC3 is 10 weeks, enabling us to explore change since the previous HST observations, or are the short-term (days/weeks/months) global spectral variability on different relative reflectance values of Haulani and Occator Ceres. To minimize dependence on the effect of the absolute an observing artifact from comparing the HST and Dawn calibration of the instrument, we calculated the relative spectra data sets? in each of the three RCs.