Dwarf planet Ceres is located in the main asteroid
belt, between the orbits of Mars and Jupiter, as illustrated in this artist's
conception. Observations by the Herschel space observatory between 2011 and 2013
find that the dwarf planet has a thin water vapor atmosphere.
Image Credit: ESA/ATG medialab
Scientists using the Herschel space observatory have made the first
definitive detection of water vapor on the largest and roundest object in the
asteroid belt, Ceres.
Plumes of water vapor are thought to shoot up periodically from Ceres when
portions of its icy surface warm slightly. Ceres is classified as a dwarf
planet, a solar system body bigger than an asteroid and smaller than a
planet.
Herschel is a European Space Agency (ESA) mission with important NASA
contributions.
"This is the first time water vapor has been unequivocally detected on Ceres
or any other object in the asteroid belt and provides proof that Ceres has an
icy surface and an atmosphere," said Michael Küppers of ESA in Spain, lead
author of a paper in the journal Nature.
The results come at the right time for NASA's Dawn mission, which is on its
way to Ceres now after spending more than a year orbiting the large asteroid
Vesta. Dawn is scheduled to arrive at Ceres in the spring of 2015, where it will
take the closest look ever at its surface.
"We've got a spacecraft on the way to Ceres, so we don't have to wait long
before getting more context on this intriguing result, right from the source
itself," said Carol Raymond, the deputy principal investigator for Dawn at
NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif. "Dawn will map the
geology and chemistry of the surface in high-resolution, revealing the processes
that drive the outgassing activity."
For the last century, Ceres was known as the largest asteroid in our solar
system. But in 2006, the International Astronomical Union, the governing
organization responsible for naming planetary objects, reclassified Ceres as a
dwarf planet because of its large size. It is roughly 590 miles (950 kilometers)
in diameter. When it first was spotted in 1801, astronomers thought it was a
planet orbiting between Mars and Jupiter. Later, other cosmic bodies with
similar orbits were found, marking the discovery of our solar system's main belt
of asteroids.
Scientists believe Ceres contains rock in its interior with a thick mantle of
ice that, if melted, would amount to more fresh water than is present on all of
Earth. The materials making up Ceres likely date from the first few million
years of our solar system's existence and accumulated before the planets
formed.
Until now, ice had been theorized to exist on Ceres but had not been detected
conclusively. It took Herschel's far-infrared vision to see, finally, a clear
spectral signature of the water vapor. But Herschel did not see water vapor
every time it looked. While the telescope spied water vapor four different
times, on one occasion there was no signature.
Here is what scientists think is happening: when Ceres swings through the
part of its orbit that is closer to the sun, a portion of its icy surface
becomes warm enough to cause water vapor to escape in plumes at a rate of about
6 kilograms (13 pounds) per second. When Ceres is in the colder part of its
orbit, no water escapes.
The strength of the signal also varied over hours, weeks and months, because
of the water vapor plumes rotating in and out of Herschel's views as the object
spun on its axis. This enabled the scientists to localize the source of water to
two darker spots on the surface of Ceres, previously seen by NASA's Hubble Space
Telescope and ground-based telescopes. The dark spots might be more likely to
outgas because dark material warms faster than light material. When the Dawn
spacecraft arrives at Ceres, it will be able to investigate these features.
The results are somewhat unexpected because comets, the icier cousins of
asteroids, are known typically to sprout jets and plumes, while objects in the
asteroid belt are not.
"The lines are becoming more and more blurred between comets and asteroids,"
said Seungwon Lee of JPL, who helped with the water vapor models along with Paul
von Allmen, also of JPL. "We knew before about main belt asteroids that show
comet-like activity, but this is the first detection of water vapor in an
asteroid-like object."
The research is part of the Measurements of 11 Asteroids and Comets Using
Herschel (MACH-11) program, which used Herschel to look at small bodies that
have been or will be visited by spacecraft, including the targets of NASA's
previous Deep Impact mission and upcoming Origins Spectral Interpretation
Resource Identification Security Regolith Explorer (OSIRIS-Rex). Laurence O'
Rourke of the European Space Agency is the principal investigator of the MACH-11
program.
More information about Herschel is online at:
More information about NASA's role in Herschel is available at:
For more information about NASA's Dawn mission, visit:
