Spot of bother (Image: JPL-Caltech/UCLA/MPS/DLR/IDA/NASA)

CERES is a dwarf planet with giant potential. As NASA’s Dawn spacecraft gears up for the first in-depth look at this tiny world, speculation is rife. Could Ceres be an overgrown comet? Host an ocean made of mud? Or even possess icy volcanoes that make it an unexpected host for life in the asteroid belt?

“When we complete our observations, we will show that Ceres is every bit a planet as its terrestrial neighbours Mars, Earth, Venus and Mercury.” That’s what Christopher Russell, who leads the Dawn mission, told the Lunar and Planetary Science Conference (LPSC) in The Woodlands, Texas, last week.

The first signs of excitement came earlier this month when Dawn spotted a mysterious bright spot just 1 pixel wide inside a crater as it pulled into orbit around the dwarf planet. There were suspicions that the spot could be a sign of water spewing into space, and now fresh views, presented for the first time at the LPSC, lend weight to the idea.


In Dawn’s latest pictures, the bright spot is visible even from the side, meaning it probably protrudes above the crater. “What is amazing is you can see this feature while the rim is very likely in front of the line of sight,” said Andreas Nathues, who is in charge of the mission’s camera. “We believe this could be some kind of outgassing.”

Remote observations using the Herschel space telescope show Ceres spitting water from somewhere on its surface, probably towards the equator. We think other icy bodies in the solar system, like Jupiter’s moon Europa and Saturn’s moon Enceladus, spew water in spectacular plumes from subsurface oceans (see diagram). If Ceres also has a buried sea, that could boost its chances of playing host to life – so astronomers are keen to track the plumes to their source.FIG-mg30143801.jpg

Images taken from dusk until dawn on Ceres seem to indicate something more transient, since the spot brightens throughout the day and completely disappears at night. So the patch could be a pocket of ice on the surface that releases gas as it warms up in the sun, which is similar to how a comet behaves. However, Nathues said only higher resolution data will confirm its true nature. This won’t come for a while, as Dawn is on the dark side of Ceres and won’t emerge until mid-April.

Dawn probe (Image: NASA/JPL-Caltech/Science Photo Library)

But a model of Ceres presented at the LPSC added a complication, suggesting comet-like behaviour is only possible at the dwarf planet’s poles, not the lower latitudes where the bright spot has been seen.

Timothy Titus of the US Geological Survey in Flagstaff, Arizona, presented a thermal model that considered where ice could remain stable on Ceres’s surface over the lifetime of the solar system, rather than boiling away more quickly. If Ceres is behaving like a comet, it must have ice patches that can survive for a long time until the sun’s heat reaches them as the dwarf planet moves into a warmer part of its orbit.

Titus found that long-lasting ice could only be possible in chilly latitudes above 40°. But the plumes spotted by Herschel seemed to come from nearer the equator. “The water ice is just not stable at the latitudes that the plumes are supposedly coming from,” Titus says.

What about cryovolcanism? In this scenario, ice and water are ejected from the surface by processes similar to those that drive magma volcanoes on Earth. Ceres doesn’t have enough muscle to drive these eruptions, according to a second model presented at the LPSC by David O’Brien of the Planetary Science Institute in Tucson, Arizona.

Let’s suppose that Ceres has a subsurface ocean covered by an icy shell. As the bottom of the shell freezes, it expands, putting pressure on the ocean and the shell itself. To create a cryovolcano, says O’Brien, the water pressure has to build up enough to shoot up through the shell before the ice cracks and relieves the pressure.

Since we don’t know exactly how deep the ice is on Ceres, O’Brien modelled a range of plausible depths. None recreated the conditions for spewing cryovolcanoes – the ice always cracked before enough pressure accumulated. In the best-case scenario, water reached about 90 per cent of the way to the surface.

Intriguingly, that might mean water could reach the surface from a deep crater, where there would be less ice to negotiate – perhaps even from a crater like the one where Dawn saw the bright spot. “Everybody wants to know what’s going on there,” says O’Brien. A cryovolcano could be producing enough of a plume to replenish the ice on the surface. So Ceres could be producing comet-like emissions, driven by a weak cryovolcano. “It’s sort of a midpoint between comets and cryovolcanic icy worlds,” says Titus.

Ceres could be producing comet-like emissions driven by a weak cryovolcano

There is more than one way to make a cryovolcano, though. Some models suggest the core of Ceres may be heated by radioactive isotopes left over from the dwarf planet’s formation. These could provide enough energy for punchier volcanism, perhaps producing larger plumes – and heat would be beneficial for any bacteria lurking below the surface. “Any place you’ve got the potential for liquid water, you’ve got the potential for life,” says Titus. “Ceres could be an extremely exciting astrobiological target.”

Any place you’ve got the potential for liquid water, you’ve got potential for life. Ceres could be a target

Even if water isn’t making it to the surface, cracks in the ice shell could give Dawn a way of looking deeper within Ceres. If there are gases dissolved in the ocean, cracks could release them for the spacecraft to sample, says O’Brien. “It’s like shaking up and opening a soda bottle.”

And there may be more than water there. Bryan Travis, also of the Planetary Society Institute, presented a model of Ceres’s interior that started out as a mix of ice and silicates, the minerals that make up the majority of Earth’s crust. Then the mix separated into a silicate core and an ice shell, with a mud ocean sandwiched between the two. Dawn won’t necessarily be able to see such mud directly, but Thomas Davison of Imperial College London has a way to distinguish between a mud and water ocean (see “Questions for Dawn“).

All of this is to come when Dawn swings back to the day side of Ceres next month. Researchers have already decided to base the names of regions on the surface, such as Yumyum, after harvest deities, but just what they’ll find there remains a mystery. “We really don’t know what to expect for Ceres,” says O’Brien. “It’s pretty much bound to surprise us.”

Leader: “Planet or not, Ceres rocks“

Questions for Dawn Is Ceres a jumped-up comet or a failed planet?

We think that comets are the building blocks left over from the formation of the rocky planets in our solar system. Ceres, the only dwarf planet in the inner solar system, may have failed to gather enough building blocks to become a proper planet, leaving it somewhere in-between. Does Ceres have an ocean?

Remote measurements of the dwarf planet’s density suggest it has a rocky core coated in an icy shell, but it’s still unclear whether liquid water lurks beneath the two. Even if Dawn doesn’t spot water at the surface, it might find other indicators, like patches of salty material left behind by water that has boiled away. Such signs can confirm the presence of an ocean. Is it water or mud?

To figure out whether the internal liquid is water or something sludgier, Dawn will have to take note of Ceres’s gravitational field. Asteroid impacts deform Ceres’s core, changing its internal density and thus its gravitational pull. Mud would dampen this effect, so as Dawn flies over Ceres, differences in its pull could reveal deformation in its core and in turn what kind of ocean it has. Why is Ceres different from Vesta?

Before reaching Ceres, Dawn spent a year hanging out at Vesta, the second largest object in the asteroid belt. Vesta isn’t round and has no water, but both objects are thought to be examples of the stuff that formed planets like Earth. One of Dawn’s goals is to figure out why these two bodies are so different, and how others like them built the rest of the inner solar system. Is there life on Ceres?

Dawn is not equipped to detect life directly, but it can search for signs of habitability, such as an ocean. It could also identify whether Ceres is heated internally by radioactive isotopes by looking for patches of cryovolcanoes on the surface. Both liquid water and heat raise the chances for life.

This article appeared in print under the headline “New dawn for Ceres”