Dwarf Planet Ceres Sheds Light On Habitable Planets
The Texas-sized dwarf planet Ceres may be among the smallest Solar System bodies with evidence of potential habitable environments, according to a collection of seven new complementary scientific studies.
Now considered a “relict,” or former ocean world, Ceres orbits in the main asteroid belt between Mars and Jupiter and exhibits surface features related to hydrothermal activities on Earth and Mars associated with the rise of current and potential past habitable environments.
The findings from NASA’s Dawn mission, published Aug. 10 in the journals Nature Astronomy, Nature Geoscience, and Nature Communications, suggest these signature surface features could be worth seeking out on icy moons including the Galilean moons of Jupiter: Callisto, Europa, Ganymede and Io.
Bottom line, the possibility of habitable environments on planetary objects in this Solar System and others smaller and less warm than Earth and Mars cannot be ruled out.
Launched in September 2007, the solar-powered, ion-propelled Dawn became the first spacecraft to orbit two Solar System destinations: the large asteroid Vesta between July 2011 to September 2012 and the 600-mi. (970-km)-wide Ceres from March 2015 through October 2018 after two mission extensions.
During the five-month final phase, Dawn descended to an orbital altitude of 22 mi. over Ceres, which was designated a dwarf planet in 2006, for a high-resolution study of the 20 million-year-old, 57-mi.-wide Occator crater. The crater is home to intriguing bright surface features as well as surrounding low mounds and pits and thin sheets of sodium-carbonate minerals.
The source of the features can be attributed to large numbers of spring-like flows of chemically freeze-resistant water originating deep in the dwarf planet’s crust, according to Paul Schenk, staff scientist at the Universities Space Research Association’s Lunar and Planetary Institute. A member of the Dawn science team and lead author of “Impact Heat Driven Volatile Redistribution at Occator Crater on Ceres as a Comparative Planetary Process,” published in Nature Communications, he is a contributor to four of the seven new studies.
The high-resolution stereo imagery from Dawn’s close approach reveals surface features that formed as impact-induced, water-rich flows covering the crater floor refroze in a manner similar to the formation of ice-cored mounds of soil at the edges of glacial regions on Earth known as pingos. The features appear to have formed in the absence of cyrovolcanism, suggesting an alternative “cyro-hydrologic” process now evident beyond Mars and active in Ceres’ recent history, according to Schenk’s contributions to the research effort.
The features and the processes that formed them are strong candidates for habitable environments on the early Earth and are therefore of astrobiological significance, he believes.
In an accompanying Nature assessment, Julie Castillo-Rogez, NASA’s Dawn mission project scientist and a deputy principal investigator, also notes that data from the suite of studies suggests Dawn’s hydrothermal activity has been recent and likely due to both heat restored from the Occator impact and the presence of a persistent, sufficiently deep layer of liquid brine chemically resistant to freezing.
Dawn’s infrared spectrometer detected the presence of hydrohalite on a bright feature at Occator’s center, a compound comprised of sodium, chloride and water that is common in marine ice on Earth but not previously detected beyond Earth.
“It is a smoking gun for ongoing activity,” Castillo-Rogez says. “That material is unstable on Ceres’ surface, and hence must have been emplaced very recently.”
That surface activity is potentially supported by a deep and wide fracture network created by the forces imparted by the Occator impact, she reasons.
The findings justify further exploration of Ceres, perhaps even a sample return, to help explain the evolution of habitable planetary environments and organic matter, Castillo-Rogez said.
“I agree, the next step to consider for Ceres would be landing and if feasible, a sample return,” Schenk told Aerospace DAILY in an Aug. 10 phone interview.
“To better understand the processes we think we are seeing on Ceres, we need to go back to better define how abundant these brines were [and] how long the process continued. Because it is a very small body it provides additional context for interpreting what happened on Earth,” he explained, acknowledging a growing consensus that impact-related hydrothermal activities played a significant role in the development of either prebiotic or biotic activity on Earth.
As on Mars, wind and other sources of erosion on Earth have erased much of the past evidence of the prospect.
“The more we understand about this process on another planet, the more it helps us understand how it happened on Earth,” Schenk said. “It’s an indication that these kinds of environments are much more common than we thought.”
The focus of a follow-up mission and a sample return would be to determine the abundance of brine and other hydrothermal fluids and for how long they were a factor, not so much to seek out evidence for past microbial life in such a frigid environment, Schenk said.
Other published findings from the late stages of Dawn’s final months include:
•The latest cyrovolcanic activity on Ceres appears to have begun less than 9 million years ago and lasted for at least several million years.
• The most promising feature on Ceres on which to verify the presence of fluids in the very cold environment is at Cerealia Facula, a bright granular feature in Occator crater.
• Ceres sports a mechanically strong crust over a weak, fluid-rich upper mantle.