Incoming views of the asteroid belt's largest body reveal spots where ice from the interior might be exposed on the surface.
NASA's Dawn spacecraft is just a few days from arriving at its second and final objective, asteroid 1 Ceres, on March 6th. Robert Mase, who manages the mission at the Jet Propulsion Laboratory in California, says Dawn will be captured into orbit at 4:20 a.m. (12:20 Universal Time). That's the first critical step in what promises to be an intensive, 16-month-long investigation.
But images recently captured by the spacecraft's German-built camera already have mission scientists scratching their collective heads.Tucked inside a 57-mile-wide crater are two tiny spots far brighter than their surroundings. Images show a few other bright spots on Ceres. It's not yet clear what they are — the spots were smaller than the camera's resolution when the camera took its snapshots — but there's already plenty of speculation about them.
With an average diameter of 590 miles (950 kilometers), Ceres is the largest denizen of the asteroid belt. In fact, by the International Astronomical Union's definition, it qualifies as a dwarf planet. Unlike most asteroids, however, Ceres is thought to contain 25% water by mass — likely existing as a deep ice mantle overlying a rock-and-metal core. The surface we see might be little more than a veneer of rock and dust.
These bright spots "appear at low latitudes and stand out against the dark surface," notes Carol Raymond, Dawn's deputy project scientist. The pair in the crater are "extremely surprising to the team." She says the most likely cause is either fresh exposures of ice from recent impacts or small eruptions of slushy ice (cryovolcanism) from the interior. However, she notes, there's no indication that the spots are high-standing piles of erupted material.
Adding to the mystery is the fact that ESA's Herschel space spacecraft detected water vapor around Ceres when it scrutinized the big body in 2011, 2012, and 2013. But the detection was intermittent — and it now seems that water vapor was present when the two spots were in view. This bright pairing is "unique in the solar system," Raymond says, an enigmatic feature that "really has us on the edge of our seats."
Learn more about the spacecraft and the scientific studies planned for Ceres at the Dawn website.