Eat your heart out, Hubble! NASA's Dawn spacecraft is in the home stretch of its journey to Ceres and has snapped the best images yet of the dwarf planet. Grainy as they are, the new views of the 590-mile-wide world are already turning up unexpected features on the surface.
"What we expect at Ceres is to be surprised, so it's getting off to a good start," said deputy principal investigator Carol Raymond.
The images, taken 147,000 miles from Ceres on Jan. 25, are 30% higher-resolution than the images taken by NASA's Hubble Space Telescope in 2003 and 2004. They measure 43 pixels wide, a significant improvement over Dawn's images from earlier this month, which were 27 pixels across.
The images show significant brightness and darkness variations over the surface – particularly a bright spot gleaming in the northern hemisphere and darker spots in the southern hemisphere. While the scientists were aware of those major spots, they weren't expecting to see quite so much texture on the surface, said Raymond, a geophysicist at the Jet Propulsion Laboratory.
Ceres is fairly warm by ice-world standards; temperatures in its "tropical" zones are thought to range from 180 to 240 Kelvin (or minus-136 degrees Fahrenheit to minus-28 degrees Fahrenheit), Raymond said. Theoretically, the warm ice on Ceres' surface should flow and smooth out any bumps such as those from impact craters. But the brightness variations across the surface make it appear very rough, she said.
"We expected to see a fairly smooth surface," she added. "But our resolution right now is such that we can't resolve what exactly is going on."
By the way, even though that bright spot looks incredibly shiny, it's only bright compared to the rest of the very dim dwarf planet, Raymond pointed out.
"The fact that it's white in the image doesn't mean it's white," Raymond said. "Bright is a relative term."
Still, since brighter areas often indicate fresher material, it's possible that those spots are newly exposed spots that haven't t spent years getting darkened by cosmic rays, Raymond speculated. They do not appear to be consistent with pure water ice, she added. But as the spacecraft closes in and enters orbit March 6, the nature of these variations should soon become clear.
Ceres is one of a small cadre of icy "worlds" in our solar system that may have had a watery subsurface ocean in its past — and may still hide the remains of one, Raymond said. (Others in this category, including Saturn's moon Enceladus and Jupiter's moon Europa, have been caught squirting water out from cracks in their frozen surfaces.) Such worlds are interesting because they may hold water as well as the potential chemical ingredients for life.
"This is just starting to illuminate the fact that Ceres is one of these unique bodies that has astrobiological potential ... and it's just continued to become more intriguing as we've been marching inexorably closer," she added.
Ceres was not the first stop in Dawn's 3-billion-mile journey. The first was the protoplanet Vesta, which is vastly different from its fellow mega-asteroid, Ceres. Where Vesta is dry and lumpy, Ceres is icy and round, massive enough to have been pulled into a planet-like shape. Scientists want to find out why these two space-fossils from the early solar system ended up with such different geophysical life stories.
At least with Vesta, there were meteorites linked to the asteroid that planetary scientists can study, Raymond pointed out. For Ceres, there are no such space rocks found on Earth – so the researchers have somewhat less of an idea of what to expect.
"I am excited," Raymond said. "Just having had the wild ride at Vesta, I'm also just in awe of what's going to happen. It's going to be amazing."