Herschel telescope detects water on dwarf planet in asteroid belt

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 periodically shoot out of Ceres as parts of its icy surface heat up slightly. Ceres is classified as a dwarf planet, a solar system body larger than an asteroid and smaller than a planet.

Herschel is a European Space Agency (ESA) mission with significant contributions from NASA.

“This is the first time that water vapor has been unequivocally detected on Ceres or any other object in the asteroid belt and provides evidence that Ceres has an icy surface and an atmosphere,” said Michael Küppers of ESA in Spain, main author of an article in the journal Nature.

The results come at a good time for NASA’s Dawn mission, which is on its way to Ceres now after spending more than a year in orbit around the large asteroid Vesta. Dawn is expected to arrive at Ceres in the spring of 2015, where she will take a closer look at its surface.

“We have a spacecraft en route to Ceres, so we don’t have to wait long to get more context on this intriguing result, straight from the source itself,” said Carol Raymond, Deputy Principal Investigator for Dawn at NASA. Jet Propulsion Laboratory in Pasadena, California. “Dawn will map the surface geology and chemistry in high resolution, revealing the processes that drive outgassing activity.”

During 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 due to its large size. It measures approximately 590 miles (950 kilometers) in diameter. When it was first 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 asteroid belt.

Scientists believe Ceres contains rock inside with a thick mantle of ice that, if it melted, would equal more fresh water than there is on the entire Earth. The materials that make up Ceres probably date back to the first million years of our solar system’s existence and accumulated before the planets were formed.

Until now, ice had been theorized on Ceres but had not been conclusively detected. It took Herschel’s far-infrared vision to finally see a clear spectral signature of water vapor. But Herschel didn’t see water vapor every time he looked. While the telescope observed water vapor four times, on one occasion there was no signature.

Here’s what scientists think is happening: As Ceres passes through the part of its orbit closest to the sun, part of its icy surface becomes hot enough for water vapor to escape in plumes at a rate approximately 6 kilograms (13 pounds) per second. When Ceres is in the coldest part of its orbit, no water escapes.

The strength of the signal also varied over hours, weeks, and months, due to plumes of water vapor spinning in and out of Herschel’s sights as the object rotated on its axis. This allowed scientists to locate the water source at two darker points on Ceres’ surface, previously seen by NASA’s Hubble Space Telescope and ground-based telescopes. Dark spots might be more likely to outgas because dark materials heat up faster than light materials. When the Dawn spacecraft arrives at Ceres, it will be able to study these features.

The results are somewhat unexpected because comets, the cooler cousins ​​of asteroids, are generally known to sprout jets and plumes, unlike objects in the asteroid belt.

“The lines are getting more and more blurred between comets and asteroids,” said JPL’s Seungwon Lee, who helped with the water vapor models with Paul von Allmen, also of JPL. “We previously knew of 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 examine 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.

Herschel is a mission of the European Space Agency, with scientific instruments provided by consortia of European institutes and with significant participation from NASA. While the observatory stopped making scientific observations in April 2013, after running out of coolant, as expected, scientists continue to analyze its data. NASA’s Herschel Project Office is based at JPL. JPL contributed technology to enable the mission of two of Herschel’s three science instruments. NASA’s Herschel Science Center, part of the California Institute of Technology’s Infrared Processing and Analysis Center in Pasadena, supports the American astronomy community.

Dawn’s mission is managed by JPL for NASA’s Science Mission Directorate in Washington. Dawn is a project of the Direction Discovery Program, operated by NASA’s Marshall Space Flight Center in Huntsville, Ala. UCLA is responsible for the overall science of the Dawn mission. Orbital Sciences Corp. in Dulles, Virginia, designed and built the spacecraft. The German Aerospace Center, the Max Planck Institute for Solar System Research, the Italian Space Agency and the Italian National Astrophysical Institute are international partners of the mission team. Caltech manages the JPL for NASA.

More information about Herschel is online at: http://www.esa.int/SPECIALS/herschel. More information on NASA’s role in Herschel is available at: http://www.nasa.gov/herschel. For more information on NASA’s Dawn mission, visit: http://www.nasa.gov/dawn.

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