Copyright: ESA/ATG medialab
ESA’s Herschel space observatory has discovered water vapour around
Ceres, the first unambiguous detection of water vapour around an object
in the asteroid belt.
With a diameter of 950 km, Ceres is the largest object in the asteroid
belt, which lies between the orbits of Mars and Jupiter. But unlike most
asteroids, Ceres is almost spherical and belongs to the category of
‘dwarf planets’, which also includes Pluto.
It is thought that Ceres is layered, perhaps with a rocky core and an
icy outer mantle. This is important, because the water-ice content of
the asteroid belt has significant implications for our understanding of
the evolution of the Solar System.
When the Solar System formed 4.6 billion years ago, it was too hot in
its central regions for water to have condensed at the locations of the
innermost planets, Mercury, Venus, Earth and Mars. Instead, it is
thought that water was delivered to these planets later during a
prolonged period of intense asteroid and comet impacts around 3.9
billion years ago.
While comets are well known to contain water ice, what about asteroids?
Water in the asteroid belt has been hinted at through the observation of
comet-like activity around some asteroids – the so-called Main Belt
Comet family – but no definitive detection of water vapour has ever been
made.
Now, using the HIFI instrument on Herschel to study Ceres, scientists
have collected data that point to water vapour being emitted from the
icy world’s surface.
“This is the first time that water has been detected in the asteroid
belt, and provides proof that Ceres has an icy surface and an
atmosphere,” says Michael Küppers of ESA’s European Space Astronomy
Centre in Spain, lead author of the paper published in Nature.
Although Herschel was not able to make a resolved image of Ceres, the
astronomers were able to derive the distribution of water sources on the
surface by observing variations in the water signal during the dwarf
planet’s 9-hour rotation period. Almost all of the water vapour was seen
to be coming from just two spots on the surface.
“We estimate that approximately 6 kg of water vapour is being produced
per second, requiring only a tiny fraction of Ceres to be covered by
water ice, which links nicely to the two localised surface features we
have observed,” says Laurence O’Rourke, Principal Investigator for the
Herschel asteroid and comet observation programme called MACH-11, and
second author on the Nature paper.
The most straightforward explanation of the water vapour production is
through sublimation, whereby ice is warmed and transforms directly into
gas, dragging the surface dust with it, and thus exposing fresh ice
underneath to sustain the process. Comets work in this fashion.
The two emitting regions are about 5% darker than the average on Ceres.
Able to absorb more sunlight, they are then likely the warmest regions,
resulting in a more efficient sublimation of small reservoirs of water
ice.
An alternative possibility is that geysers or icy volcanoes – cryovolcanism – play a role in the dwarf planet’s activity.
Much more detailed information on Ceres is expected soon, as NASA’s Dawn mission
is currently en route there for an arrival in early 2015. It will
provide close-up mapping of the surface and monitor how the water
activity is generated and varies with time.
“Herschel’s discovery of water vapour outgassing from Ceres gives us new
information on how water is distributed in the Solar System. Since
Ceres constitutes about one fifth of the total mass of asteroid belt,
this finding is important not only for the study of small Solar System
bodies in general, but also for learning more about the origin of water
on Earth,” says Göran Pilbratt, ESA’s Herschel Project Scientist.
“Localised sources of water vapour on dwarf planet (1) Ceres,” by M. Küppers et al. is published in Nature 23 January 2014.
Ceres was observed on four occasions between November 2011 and March
2013 initially as part of the MACH-11 (Measurements of 11 Asteroids and
Comets with Herschel) Guaranteed Time Programme, and complemented by two
additional Director’s Discretionary Time observations that confirmed
the tentative detection and measured the variation in water vapour as a
function of rotation period.
For further information, please contact:
Markus Bauer
ESA Science and Robotic Exploration Communication Officer
Tel: +31 71 565 6799
Mob: +31 61 594 3 954
Email: markus.bauer@esa.int
Michael Küppers
European Space Agency, ESAC
Email: michael.kueppers@sciops.esa.int
Laurence O’Rourke
Programme PI for MACH-11
European Space Agency, ESAC
Email: Laurence.O’Rourke@esa.int
Göran Pilbratt
ESA Herschel Project Scientist
Tel: +31 71 565 3621
Email: gpilbratt@rssd.esa.int
Source: ESA/Herschel
