Ceres’ bright spots significantly younger than crater they inhabit
Researchers who studied images of Ceres’ Occator Crater captured by the Dawn spacecraft’s scientific imaging system have determined that its bright spots, composed largely of carbonate salts, are significantly younger than the crater in which they sit.
The bright spots are also evidence that Ceres has experienced cryovolcanic outbursts over long periods of time, making Ceres the closest world to the Sun to have experienced cryovolcanism.
A science team at the Max Planck Institute for Solar System Research (MPS), which operates Dawn’s imaging instruments, analyzed images taken by the spacecraft between December 2015 and September 2016, the time period when it orbited just 233 miles (375 kilometers) above the dwarf planet’s surface.
At that close orbit, Dawn’s cameras were able to capture images with a resolution of just 115 feet (35 meters) per pixel. Detailed photos of Occator Crater, first observed when Dawn entered Ceres orbit in March 2015, show complex geological structures such as fractures, avalanches, and smaller craters within the large one.
Led by Andreas Nathues, Dawn Framing Camera lead investigator, researchers also studied measurements of the area taken by Dawn’s infrared VIR spectrometer.
Using the data, they determined the bright spots are approximately 4 million years old, making them 30 million years younger than Occator Crater.
The most reflective materials on Ceres’ surface, the bright spots initially puzzled astronomers who were uncertain about their composition.
With a diameter of 57 miles (92 kilometers), Occator Crater, located in Ceres’ northern hemisphere, contains a seven-mile (11-kilometer) wide pit in its center as well as steep slopes and jagged mountains at its edges that rise as high as 2,461 feet (750 meters).
A bright dome-shaped structure within the central pit rises 1,312 feet (400 meters). Fractures are visible along the structure of the 1.87-mile (three-km) wide dome.
“In these data, the origin and evolution of the crater as it presents itself today can be read more clearly than ever before,” Nathues said.
Based on the presence of the pit and the jagged ridges, researchers believe Occator Crater was created when an asteroid or comet impacted Ceres 34 million years ago.
The impact produced a central mountain within the crater that later collapsed and triggered cryovolcanic activity. Disruption caused by the impact allowed water as well as other dissolved gases, including methane and carbon dioxide, to form a vent system.
Surface fractures were created by eruptions of briny liquids through the vents, bringing with them the carbonate-rich materials that eventually formed the dome.
Scientists believe the dome formed over a long period of time and not as the result of a single event.
The deposits in Occator Crater are composed of carbonate-rich salts. In contrast, other bright spots scattered around the crater’s edges, thinner and less bright, are a mixture of carbonates and other, darker materials.
By counting and measuring the smaller craters within Occator Crater – produced by later impacts – scientists were able to determine the ages of both the bright spots and of Occator.
“The age and the appearance of the material surrounding the bright dome indicate that Cerealia Facula (the bright spots) was formed by a recurring, eruptive process, which also hurled material into more outward regions of the central pit,” Nathues said, noting that similar dome structures have been found on Jupiter’s moons Ganymede and Callisto.
The surface of the dome was likely created in the most recent eruptions within the crater.
Cryovolcanic activity may still be occurring in Occator Crater, though at a far less intense level than in the past. Images of the crater taken at specific angles reveal a haze scientists view as coming from the sublimation of water.
Earlier images captured by Dawn from higher orbits show regular changes in the crater’s brightness.
“The nature of the light scattering at the bottom of Occator differs fundamentally from that at other parts of the Ceres surface,” said MPS scientist Guneshwar Singh Thangjam. “The most likely explanation is that near the crater floor, an optically thin, semi-transparent haze is formed.”
Researchers think the haze formed by water coming from fractures in the crater floor sublimating in the presence of sunlight.
Dawn is now headed to a different orbital plane and a higher altitude of 12,400 miles (20,000 kilometers).
Video courtesy of JPL
Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.