If you could hop aboard NASA’s Dawn spacecraft and fly over the dwarf planet Ceres, it’s surface would look quite dark, about the same as an aged asphalt parking lot. But a closer look would reveal hundreds of bright spots. Ever since they were discovered in photos taken with the Dawn spacecraft, still in orbit about Ceres, they’ve been a hot topic. The dots and splats appear as bright as fresh concrete in comparison, a sure sign of youth, or geologically speaking, an active, evolving world.
“The mysterious bright spots on Ceres, which have captivated both the Dawn science team and us, reveal evidence of Ceres’ past subsurface ocean, and indicate that, far from being a dead world, Ceres is surprisingly active. Geological processes created these bright areas and may still be changing the face of Ceres today,” said Carol Raymond, deputy principal investigator of the Dawn mission, in a recent press release.
Since Dawn arrived in orbit at Ceres in March 2015, scientists have located more than 300 bright areas on Ceres. A new study by Nathan Stein (Caltech) in the journal Icarus, divides them into four categories
- Crater floors (the brightest). The most captivating spots spackle the floor of Occator Crater; the largest there is Cerealia Facula, in the center of the crater, consists of bright material covering a 6-mile-wide (10 km) depression or pit, within which sits a small dome. A collection of slightly less reflective and more diffuse features called Vinalia Faculae appears east of the center. Most of the bright material in Occator Crater is the salt called sodium carbonate, probably mixed with water in the past. Although Cerealia Facula is the brightest area on all of Ceres, it would resemble dirty snow to the human eye.
- Crater rims as material streaking down toward the floors. Impacting bodies likely exposed bright material that was already in the subsurface or had formed in a previous impact event.
- Crater ejecta. Bright material that was ejected when the craters formed.
- Ahuna Mons. This singular mountain gets its own category and marks the only location on the dwarf planet where bright material isn’t associated with craters. Scientist believe it’s a cryovolcano that formed by the gradual accumulation of thick, slowly flowing icy materials that welled up from beneath the surface.
Over hundreds of millions of years, bright material exposed by impacts has mixed with the dark material that forms most of Ceres’ surface, as well as debris ejected during impacts. Billions of years ago, when there was a lot more material was crashing into the asteroids and planets, Ceres would have been speckled with thousands of bright spots, many of which have since darkened from solar and cosmic radiation effects.
Why do the different bright areas of Occator seem so distinct from one another? The leading explanation is that in the recent past, the crater had a reservoir of salty water beneath it that was tapped into when the giant impact fractured and fissured the surface. In patchy Vinalia Faculae, water vapor, carbon dioxide, methane or ammonia trapped in the water may have caused it to spray or bubble out like carbonated water from those fractures. In Ceres’ near-zero atmosphere, the water would have fizzled away as vapor, leaving behind the pale salts.
The Bright Stuff at Ceres – NASA Dawn video
Cerealia Facula must have formed in a somewhat different process because it’s more elevated and brighter than Vinalia Faculae. The material at Cerealia may have been more like an icy lava, seeping up through the fractures and swelling into a dome. Intermittent phases of boiling, similar to what happened when Vinalia Faculae formed, may have occurred during this process, littering the surface with ice and salt particles that formed the Cerealia bright spot. I’m not expert but the roughly parallel cracks in Cerealia Faculae’s ice dome could indicate expansion from material pushing up from below.
As Dawn continues the final phase of its mission, in which it will descend to lower altitudes than ever before, we’ll learn more about how Ceres’ salt licks formed.