Short-term variability on the surface of (1) Ceres

Abstract

Context. The dwarf planet (1) Ceres -next target of the NASA Dawn mission -is the largest body in the asteroid main belt. Although several observations of this body have been performed so far, the presence of surface water ice is still questioned. Aims. Our goal is to better understand the surface composition of Ceres and to constrain the presence of exposed water ice. Methods. We acquired new visible and near-infrared spectra at the Telescopio Nazionale Galileo (La Palma, Spain), and reanalyzed literature spectra in the 3-μm region. Results. We obtained the first rotationally resolved spectroscopic observations of Ceres at visible wavelengths. Visible spectra taken one month apart at almost the same planetocentric coordinates show a significant slope variation (up to 3%/10 3 Å). A faint absorption centered at 0.67 μm, possibly due to aqueous alteration, is detected in a subset of our spectra. The various explanations in the literature for the 3.06-μm feature can be interpreted as due to a variable amount of surface water ice at different epochs. Conclusions. The remarkable short-term temporal variability of the visible spectral slope and the changing shape of the 3.06-μm band can be hints of different amounts of water ice exposed on the surface of Ceres. This would agree with the recent detection by the Herschel Space Observatory of localized and transient sources of water vapor over this dwarf planet.

Rotationally resolved spectroscopy of Jupiter Trojans (624) Hektor and (911) Agamemnon

“…Table 1 reports the observational circumstances. We used the ESO-MIDAS software package and standard procedures to reduce the obtained data (see Perna et al 2015 for details). The wavelength calibration of DOLORES data was performed using Ar, Ne+Hg, and Kr lamps' spectral lines, while NIR spectra were calibrated through a look-up table available at the NICS website.…”

Grasping the Nature of Potentially Hazardous Asteroids

“…Data were reduced using standard procedures (bias and background subtraction, flat field correction, one-dimensional spectra extraction, atmospheric extinction correction-see Perna et al 2015 for more details). Wavelength calibration was generally obtained using emission lines from several lamps.…”

Surface Albedo and Spectral Variability of Ceres

“…Therefore, if the outgassing on Ceres were triggered by sporadic impact events similar to that considered for Scheila, then such events would not directly produce albedo changes with ejecta near the impact site detectable here. Perna et al (2015) suggested that Ceres displays short-term (months) visible spectral variability, and attributed it to "extended resurfacing processes such as cryovolcanism or cometary activity." Our interpretation of the ground-based visible wavelength spectral data shows an increasing spectral slope with increasing phase angle (Figure 1).…”

“…Bright spots within impact craters Dantu (125 km) and Occator (92 km) have been linked to intermittent and localized water vapor sources observed by the Herschel Space Observatory in 2011-2013 (Küppers et al 2014), and are, therefore, possibly related to the sublimation activity. Recent ground-based observations of Ceres also suggested short-term global spectral variability that was attributed to the changing amount of water ice on the surface (Perna et al 2015). These observations suggest a possibly changing surface caused by water outgassing on timescales of decades to months.…”

“…The spectral data archived in the Planetary Data System Small Bodies Node from four epochs (Vilas et al 1998;Bus & Binzel 2003;Lazzaro et al 2006), from Perna et al (2015) in two epochs, and from recent ground-based observations (Reddy et al 2015a) span nearly three decades, and are suitable for long-term spectral variability of Ceres (Figure 1(a)). The spectra of Ceres from various data sets all show a nearly linear shape between 0.53 and 0.85 μm, with increasing spectral slopes with solar phase angle and absorptions <0.54 μm and >0.9 μm.…”