Unexpected Changes of Bright Spots on Ceres Discovered
Astronomers have found unexpected changes in the bright spots on the dwarf planet Ceres. Although it appears as little more than a point of light from the Earth, very careful study of the light from the tiny world shows not only the changes expected as Ceres rotates, but also that the spots brighten during the day and also show other variations. These observations suggest that the material of the spots is volatile and evaporates in the warm glow of sunlight. The scientists, who used the HARPS spectrograph at the European Southern Observatory (ESO) 3.6-m telescope at La Silla in Chile, publish the new work in a paper in Monthly Notices of the Royal Astronomical Society.
About 945 km across, Ceres is the largest body in the asteroid belt between Mars and Jupiter, and the only such object classed as a dwarf planet. NASA’s Dawn spacecraft has been in orbit around Ceres for more than a year and has mapped its surface in great detail. One of the biggest surprises has been the discovery of very bright spots, only hinted at in earlier work, which reflect far more light than their much darker surroundings. The most prominent of these spots lie inside the crater Occator, and suggest that Ceres may be a much more active world than most of its asteroid neighbours.
The High Accuracy Radial velocity Planet Searcher (HARPS) spectrograph measures the Doppler shift of lines in the spectra of stars, a technique used to find planets in orbit around them. Its precision also makes it good for studying objects like Ceres. Data from HARPS have now not only detected the motion of the spots due to the rotation of Ceres about its axis, but also found unexpected additional variations suggesting that the material of the spots is volatile and evaporates in sunlight.
The lead author of the new study, Paolo Molaro, at the INAF–Trieste Astronomical Observatory, takes up the story: "As soon as the Dawn spacecraft revealed the mysterious bright spots on the surface of Ceres, I immediately thought of the possible measurable effects from Earth. As Ceres rotates the spots approach the Earth and then recede again, which affects the spectrum of the reflected sunlight arriving at Earth."
INAF–Catania Astrophysical Observatory in Italy, and a co-author of the study. "We did find the expected changes to the spectrum from the rotation of Ceres, but with considerable other variations from night to night."The team observed Ceres with HARPS for a little over two nights in July and August 2015. "The result was a surprise," adds Antonino Lanza, at the
The team concluded that the observed changes could be due to the presence of volatile substances that evaporate under the action of solar radiation. When the spots inside the Occator crater are on the side illuminated by the Sun they form plumes that reflect sunlight very effectively. These plumes then evaporate quickly, lose reflectivity and produce the observed changes. This effect, however, changes from night to night, giving rise to additional random patterns, on both short and longer timescales.
If this interpretation is confirmed Ceres would seem to be very different from Vesta and the other main belt asteroids. Despite being relatively isolated, it seems to be internally active. Ceres is known to be rich in water, but it is unclear whether this is related to the bright spots. The energy source that drives this continual leakage of material from the surface is also unknown.
Dawn is continuing to study Ceres and the behaviour of its mysterious spots. Observations from the ground with HARPS and other facilities will be able to continue even after the mission comes to an end.
Bright spots were also seen, with much less clarity, in earlier images of Ceres from the NASA/ESA Hubble Space Telescope taken in 2003 and 2004. It has been suggested that the highly reflective material in the spots might be freshly exposed water ice or hydrated magnesium sulphates.
The new research is published in "Daily variability of Ceres’ Albedo detected by means of radial velocities changes of the reflected sunlight", P. Molaro et al., Monthly Notices of the Royal Astronomical Society, Oxford University Press, in press.
The team is composed of P. Molaro (INAF-Osservatorio Astronomico di Trieste, Trieste, Italy), A. F. Lanza (INAF-Osservatorio Astrofisico di Catania, Catania, Italy), L. Monaco (Universidad Andres Bello, Santiago, Chile), F. Tosi (INAF-IAPS Istituto di Astrofisica e Planetologia Spaziali, Rome, Italy), G. Lo Curto (ESO, Garching, Germany), M. Fulle (INAF-Osservatorio Astronomico di Trieste, Trieste, Italy) and L. Pasquini (ESO, Garching, Germany).
Notes for editors
ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre European Extremely Large Telescope, the E-ELT, which will become "the world’s biggest eye on the sky".
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