Artist’s view of bright spots on Ceres imaged by the Dawn spacecraft
The bright spots on Ceres imaged by the Dawn spacecraft
The bright spots on Ceres imaged by the Dawn spacecraft
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Observations made using the HARPS
spectrograph at ESO’s La Silla Observatory in Chile have revealed
unexpected changes in the bright spots on the dwarf planet Ceres.
Although Ceres appears as little more than a point of light from the
Earth, very careful study of its light 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.
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, which reflect far more light than their much darker surroundings [1]. 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.
New and very precise observations using the HARPS spectrograph at the ESO 3.6-metre telescope
at La Silla, Chile, 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.”
Ceres spins every nine hours and calculations showed that the effects
due to the motion of the spots towards and away from the Earth caused
by this rotation would be very small, of order 20 kilometres per hour.
But this motion is big enough to be measurable via the Doppler effect with high-precision instruments such as HARPS.
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 INAF–Catania Astrophysical Observatory and 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 concluded that the observed changes could be due to the
presence of volatile substances that evaporate under the action of solar
radiation [2].
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 [3].
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 end of the space mission.
Notes
[1] 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.
[2] It has been suggested that the
highly reflective material in the spots on Ceres might be freshly
exposed water ice or hydrated magnesium sulphates.
[3] Many of the most internally active
bodies in the Solar System, such as the large satellites of Jupiter and
Saturn, are subjected to strong tidal effects due to their proximity to
the massive planets.
More Information
Contacts
Paolo Molaro
INAF-Osservatorio Astronomico di Trieste
Trieste, Italy
Email: molaro@inaf.oats.it
Richard Hook
ESO Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org
More Information
This research was presented in a paper entitled “Daily variability of
Ceres’ Albedo detected by means of radial velocities changes of the
reflected sunlight”, by P. Molaro et al., which appeared in the journal Monthly Notices of the Royal Astronomical Society.
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).
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”.
Links
Links
- Research paper
- Photos of the ESO 3.6-metre telescope and HARPS
Contacts
Paolo Molaro
INAF-Osservatorio Astronomico di Trieste
Trieste, Italy
Email: molaro@inaf.oats.it
Richard Hook
ESO Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org
Source: ESO
