Comet or Asteroid? Herschel Space Observatory Detects Water Outgassing on Dwarf Planet Ceres

A team of astronomers using ESA's Herschel Space Observatory, have discovered clear signals of water vapour coming off Ceres. Image Credit: ESA/ATG medialab/Küppers et al.

A team of astronomers using ESA’s Herschel Space Observatory have discovered clear signals of water vapour coming off Ceres. Image Credit: ESA/ATG medialab/Küppers et al.

A new study published online in the weekly scientific journal Nature on Jan. 22 reports on observations made by the Herschel Space Observatory of water vapour seen coming out of the largest member of the asteroid belt: dwarf planet Ceres.

In the past, the Solar System seemed to be a tidy and well-organised place, with many families of objects neatly grouped together in different and distinct categories. There were planets and moons, comets and asteroids. But the nature of scientific discovery is the constant replacement of old knowledge with new, that most of the time leads to a major re-thinking and change of our established view of the Cosmos. Akin to this perpetual process of change, newer observations with modern ground- and space-based instruments have blurred the line between different families of objects. Such examples include the discovery of objects nearly as big as Pluto in the Kuiper Belt and the more recent discovery of a comet-like asteroid located in the main asteroid belt between Mars and Jupiter. Now, another member of the asteroid family, the 950 km-diameter dwarf planet Ceres, can be added to the list.

An enhanced image of Ceres from the Hubble Space Telescope's Advanced Camera for Surveys. Image Credit: NASA, ESA, J. Parker (Southwest Research Institute), P. Thomas (Cornell University), L. McFadden (University of Maryland, College Park), and M. Mutchler and Z. Levay (STScI)

An enhanced image of Ceres from the Hubble Space Telescope’s Advanced Camera for Surveys. Image Credit: NASA, ESA, J. Parker (Southwest Research Institute), P. Thomas (Cornell University), L. McFadden (University of Maryland, College Park), and M. Mutchler and Z. Levay (STScI)

Ceres is no stranger to identity changes. When it was discovered in 1801 by Italian astronomer Giuseppe Piazzi, it was initially considered to be the long-sought planet between the orbits of Mars and Jupiter. After the discovery of a similar object the following year named Pallas, noted astronomer William Herschel proposed that these objects be called “asteroids” because of their star-like appearance. By the mid-19th century, when discoveries of more similar objects started to accelerate, astronomers realised the true nature of these asteroids, demoting them from their planetary status. In a similar fashion, by the turn of the 21st century, the discovery of small objects beyond the orbit of Neptune with sizes and masses comparable to Pluto prompted the International Astronomical Union to another re-classification of minor bodies in the Solar System, with the addition of Ceres alongside Pluto to the newly adopted category of “dwarf planets.”

Further blurring the dividing line between asteroids and comets, a team of astronomers led by Michael Küppers, working at the European Space Agency’s Space Astronomy Centre near Madrid, Spain, just published its study on the results of its observations of Ceres with ESA’s Herschel Space Observatory.

Herschel, originally known as FIRST, was an infrared space observatory developed and launched by ESA as part of the agency’s Horizon 2000 Science Program, which included such notable missions like XMM-Newton, Rosetta, Hipparcos, and the joint NASA/ESA Ulysses, SOHO, Hubble Space Telescope, and Cassini-Huygens missions. With a diameter of 3.5 m, Herschel’s primary mirror was the largest ever built to date for a space telescope, operating from May 2009 to June 2013 from the L2 Earth-Sun Lagrange point, until eventually the observatory’s coolant supply of liquid helium ran out.

Küppers’ team made four observations of Ceres between November 2011 and March 2013, with Herschel’s Heterodyne Instrument for the Far Infrared, or “HIFI.” Their goal was to directly detect molecules of water on Ceres, whose presence was hinted at by previous observations of hydroxyl around the dwarf planet. Although HIFI wasn’t able to spatially resolve Ceres’ disk and produce any images, the instrument’s sensitivity on the far-infrared and submillimetre wavelengths from 55 to 672 µm, allowed it to obtain clear spectra of the dwarf planet. These observations revealed distinct absorption lines of water in three out of the four observations made between October 2012 and March 2013.

Furthermore, the observed water signatures on Ceres displayed daily and monthly variations, correlated to the dwarf planet’s 9-hour rotation around its axis and 4.6-year long orbit around the Sun. Ceres lies exactly on the Solar System’s “snow line,” or frost line—the distance from the Sun beyond which volatile compounds like water, ammonia, methane, hydrogen, nitrogen, and others become solid, freezing to become ice. Inside the frost line, closer to the Sun, such volatiles heated by solar radiation sublimate into space. The water signal was found to be present when Ceres was at the perihelion of its orbit, approximately 2.5 AU from the Sun, and absent when at aphelion, 2.9 AU away.

The water signals detected from Herschel, superimposed on a global topographical map of Ceres, showing their localised source. Image Credit: ESA

The water signals detected from Herschel, superimposed on a global topographical map of Ceres, showing their localised source. Image Credit: ESA

Besides being dependent on Ceres’ distance from the Sun, the water molecules weren’t observed globally around the dwarf planet, rather they seemed to originate from two distinct equatorial regions named “Piazzi” and “Region A,” which appear 5 percent darker in infrared observations.

To account for the observations, Küppers’ team run several computer models simulating the process of water ice outgassing from localised areas on Ceres, taking into account the dark areas on the dwarf planet that have been observed in infrared wavelengths. The computer simulations produced results that matched Herschel’s observations to a high degree.

“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 Küppers, the study’s lead author. “It’s not an atmosphere like Earth’s in that most of the water escapes into space, due to Ceres’s comparably low gravity. In that respect it is more similar to a comet. I do not expect a stable atmosphere.”

The Herschel observations and the presence of the darker equatorial regions were then found to be consistent with the hypothesis of localised underground water ice reservoirs on Ceres that find their way to the surface. When the dwarf planet reaches perihelion, the ice sublimes, creating an extremely tenuous exosphere of water vapor around Ceres. “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 observing campaign and co-author of the study.

What intrigues astronomers is the fact that this sublimation process of localised water ice on Ceres is common to comets as well. When comets lie far from the Sun beyond the Solar System’s frost line, they are nothing more than solid, dry objects, not unlike asteroids in appearance. When their orbits bring them adequately close to the Sun, solar radiation causes the ices that are hidden underground to heat up and begin to outgas, creating the spectacular views of bright comas and long, stretched tails of sublimation gas that comets are best known for. “The lines are becoming more and more blurred between comets and asteroids,” said Seungwon Lee of NASA’s Jet Propulsion Laboratory, who aided the team on their computer simulation studies. “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.”

Artist's concept of the Dawn spacecraft between Vesta (left) and Ceres (right). Image Credit: Background- William K. Hartmann, Courtesy of UCLA; image-NASA/MCREL - See more at: http://dawn.jpl.nasa.gov/multimedia/spacecraft.asp#sthash.aVVHcFns.dpuf

Artist’s concept of the Dawn spacecraft between Vesta (left) and Ceres (right). Image Credit: Background- William K. Hartmann, Courtesy of UCLA; image-NASA/MCREL

Although the study’s results better align with the comet-like hypothesis, the team nevertheless cautions that there is another process that could account for the observations as well. In their study, they propose that cryovolcanism could be considered as a viable mechanism producing the observed water signatures. Contrast to erupting molten rock and magma, cryovolcanoes give off volatile elements like water, in liquid or vapour form. Notable examples of cryovolcanic activity on the Solar System are the nitrogen-rich geysers observed on Neptune’s moon Triton by Voyager 2 in 1989 and the water-rich geysers of Saturn’s moon Enceladus, first seen by the Cassini spacecraft in 2005. Yet both of these moons’ interiors are tidally heated by their parent planets through tidal friction forces—something that does not happen on Ceres. One possible alternative energy source could be the decay of radioisotopes that may be present below the dwarf planet’s surface.

Could the water vapour observed on Ceres be an indication of an underground environment conducive to life? The idea that comets carried the seeds of life on Earth is a leading hypothesis within the scientific community. If the processes that create the water outgassing on Ceres are found to be similar to those on comets, then a more detailed study of the Solar System’s largest asteroid could have tremendous implications for our quest of the origins of life on Earth and the possibility of its existence elsewhere in the Cosmos, shedding more light to these fundamental enigmas.

Fortunately, the timing couldn’t have been better. After studying asteroid Vesta in great detail for more than a year during 2011-2012, NASA’s “Dawn” spacecraft is already en route to rendezvous with Ceres on February 2015. “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,” says Carol Raymond, deputy principal investigator of the “Dawn” mission at NASA’s JPL. “Dawn will map the geology and chemistry of the surface in high-resolution, revealing the processes that drive the outgassing activity.”

It turns out that 2015 will be the year of the dwarf planets. But contrary to what their classification implies, there’s nothing minor about them. Prior to New Horizons’ much anticipated first contact with Pluto at the edges of the Solar System, “Dawn” will be busy giving us the first detailed views of Ceres in another equally interesting and most exciting part of the Sun’s planetary family.

 

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