Another Geyser World Erupts: Asteroid Ceres
The asteroid Ceres is popping off with water vapor on two sides. What’s that world coming to? One thing is sure: DAWN is coming to it.
A new paper in Nature provides evidence for localized water vapor emissions from Ceres, the largest asteroid or “dwarf planet” in the asteroid belt. Now that the DAWN spacecraft completed 14 months orbiting Vesta (second largest asteroid), it’s good to know that something exciting awaits when the spacecraft arrives at Ceres next spring. The geyser discovery, however, is a big surprise. Though it was known icy Ceres probably sublimates ice into space, the emissions are two orders of magnitude more than expected – an estimated 6 kg of water per second. (This compares with ~200 kg per second erupting from Enceladus, a moon of Saturn about half the diameter of Ceres.) The emissions, detected by the Herschel Space Observatory, may be sporadic.
Sid Perkins, writing about the discovery in the same issue of Nature, included an artist’s conception of the vapor geysers going off like teakettle plumes out of opposite sides of the asteroid. “Ceres spews intermittent plumes of water vapour, but their origin is unclear,” his subtitle reads. The eruptions appear to occur in regions that are darker in appearance. “The origin of the heat that turns the water or ice into vapour is as big a mystery as the source of the water itself,” he continued. Tidal flexing can’t explain the activity, as some planetologists have theorized for Io and Enceladus, “because the dwarf planet travels solo in its orbit within the asteroid belt.” Does the heat come from radioactive decay? Is it amplified sublimation? Next year’s arrival by DAWN will provide opportunity for a closer look.
Humberto Campins and Christine M. Comfort, also writing in the same issue of Nature, wrestled with another mystery about Ceres: why is it so different from Vesta? While Ceres is icy, Vesta is rocky and volcanic – yet both are neighbors in the asteroid belt. All the scientists could suggest is that they formed in different regions of the solar system and somehow migrated into their current positions, justifying the speculation with the growing belief that the “hot Jupiters” orbiting some stars formed far out in their planetary disks but then migrated inward. If our gas giants migrated, too, perturbations could have altered the asteroids’ original orbits. However, “the pieces of the puzzle of Solar System formation do not fit perfectly,” they admitted. The original paper offered these options:
The measured water production is two orders of magnitudes higher than is predicted from a model of sublimation maintained from water supplied from the interior of Ceres. In addition, the water activity is most probably not concentrated on polar regions, where water ice would be most stable. We propose two mechanisms for maintaining the observed water production on Ceres. The first is cometary-type sublimation of (near) surface ice. In this case the sublimating ice drags near-surface dust with it and in this way locally removes the surface layer and exposes fresh ice. Transport from the interior is not required. The second mechanism is geysers or cryovolcanoes, for which an interior heat source is needed. For Jupiter’s satellite Io and Saturn’s moon Enceladus the source of activity is dissipation of tidal forces from the planet. That can be excluded for Ceres, but some models suggest that a warm layer in the interior heated by long-lived radioisotopes may maintain cryovolcanism on Ceres at the present time (ref. 26 and references therein).
None of the articles dealt with the age question. How long could these emissions go on? A quick calculation shows about 190 million kg of water should erupt per year, assuming average flow at steady state. Over the assumed age of the solar system that would total over 8 x 1017 kg, about 0.1% of the total mass of the asteroid (9 x 1020 kg). The problem is much more pronounced at Enceladus, given its lower mass and higher eruption rate. Yet even that percentage should have darkened the surface and significantly depleted the crust, it would seem. Short-lived radioisotopes would long have decayed; the question remains what long-lived radioisotopes could reasonably be expected in accretion models to become entrapped within this icy body and not others.
For more on Ceres, see NASA’s Solar System Exploration site. Note: in the IAU’s evolving nomenclature, Ceres is classed as both an asteroid and a dwarf planet, the largest of the six dwarf planets named so far.
This will be an interesting phenomenon to watch come spring 2015 when DAWN gets a closer look. Whenever scientists have to invoke special (ad hoc) conditions to keep a theory intact, observers should be skeptical. (It’s called special pleading.) We’ve already seen the distraction tactic (sidestepping): in 2009, wacko astrobiologists, giddy with hydrobioscopy fever, speculated that life came from Ceres (3/05/09). In these new articles, too, the L-word life was ready to erupt onto the surface: “early impacts by asteroids and comets might have played a considerable part in the origin and evolution of life on our planet,” Campins and Comfort said. Perkins echoed that thought, saying asteroid impacts played “a considerable role in the origin and evolution of life.” OK, if it’s considerable, let’s consider it: asteroids pummel Earth, leading to fiery catastrophes that burn up must of Earth’s surface. Out of the ashes, life emerges. Sounds like the old Phoenix myth. That kind of role belongs in the theater, not the observatory.