The strange bright deposits inside Occator crater on Ceres are probably from cryovolcanic eruptions that are much younger than the crater itself.
Because 1 Ceres, the largest asteroid, is named for the Roman goddess of agriculture, it's no surprise that the International Astronomical Union opted to name craters on Ceres for deities from various cultures involved with agriculture and vegetation. For example, the prominent crater Occator (92 km across) honors the Roman agricultural deity who harrowed (tilled) fields. Seems like a boring job for a deity, don't you think?
But ever since the arrival of NASA's Dawn spacecraft in early 2015, Occator crater has been anything but boring. Bright areas at its center and on its floor, named Cerealia Facula and Vinalia Faculae, respectively, appear to be deposits of carbonate-rich salts — residue from briny flows that gurgled up from a fluid reservoir (perhaps a global ocean) deep in the asteroid's interior.
Occator was gouged into the landscape about 34 million years ago, but the whitish dome at its center is much younger — just 4 million years old. That's the conclusion of a new analysis published in this month's Astronomical Journal by Andreas Nathues (Max Planck Institute for Solar System Research) and nine colleagues.
Nathues and his team explored the evolution of Occator since its formation. Using crater counts to constrain ages, they found, for example, that much of the crater's interior is covered by debris from a landslide on the southeastern rim that tumbled to the floor roughly 9 million years ago.
But a lot of their attention is focused on the bright, high-standing dome, 3 km across and 400 m high, in the crater's center. This is not the classic "central peak" that many large craters get when they form, though there are vestiges of one of these nearby. Instead, the dome is sitting in the middle of a broad pit on the floor, about 11 km across, that's rimmed by fractures.
Now, the dome isn't really white, despite what images show — it's about 30% reflective. But it's far brighter than the surrounding terrain, which is only 2% to 4% reflective. Last year another Dawn team, using the spacecraft's infrared spectrometer, found distinct absorption bands in Cerealia Facula at 3.4 and 3.9 microns that are the signature of carbonates. The Nathues team used high-resolution images to identify a dozen small impacts in the dome, 80 to 300 m across. All the craters are bright, like their surroundings, so the carbonate deposit must be fairly thick.
The dome's relatively young age suggests that cold, briny eruptions, known as cryovolcanism, emerged from a liquid reservoir trapped between a muddy icy mantle and a silicate-rich core. Once the slushy stuff breached the surface, exposing it to the cold vacuum of space, the brine would have quickly frozen and its water would have rapidly boiled or sublimated away, leaving the salts behind as a solid residue.
Whether the salts now exist as a stiff layer or as a fine fluffy powder isn't known. Nathues tells Sky & Telescope that the Dawn project plans to examine the dome with an illumination phase angle of 0° — that is, with sunlight coming from directly behind the spacecraft. Observations made at this special geometry, achieved on April 29th, should constrain the grain sizes in the salt deposits.
Nor is it clear how often eruptions might have occurred. "A long-lasting process appears to be prevalent," the team concludes, "whereby periodically or episodically ascending bright material from a subsurface reservoir was deposited, expelled from fractures, and extruded onto the surface, forming the present-day central dome."
Meanwhile, investigators are less certain about the more diffuse deposits of Vinalia Faculae, in the crater's eastern half. There's no obvious infrared signature due to carbonates, for example, and the bright-hued topping must be relatively thin because two impact craters have punched through to reveal the dark material that covers the rest of crater's floor.
Last week Dawn celebrated its 2-year anniversary of arriving at Ceres. Who knows? If the spacecraft were to keep looking long enough, it might catch a cryovolcanic eruption in the act! But if it's going to do that, something had better happen soon — the mission is due to end on June 30th.
Read more about the case for Cerean cryvolcanism in press releases by the Max Planck Institute and by NASA's Jet Propulsion Laboratory. And catch up with all the activity of this unDawnted explorer by reading mission director Marc Rayman's periodic and highly entertaining postings.