Cryovolcanism helped shape dwarf planet Ceres

Icy volcanoes have erupted throughout the history of Ceres, but such continuous activity has not had the same extensive impact on the dwarf planet's surface as standard volcanism on Earth, says a new paper "Cryovolcanic Rates on Ceres Revealed by Topography" appearing in Nature Astronomy. Cryovolcanoes erupt liquid or gaseous volatiles such as water, ammonia or methane instead of spewing molten rock like seen on Earth. Salty water is likely the major component of cryolavas on Ceres.

Michael M. Sori of the University of Arizona Lunar and Planetary Laboratory is lead author, and Planetary Science Institute Senior Scientist Hanna G. Sizemore is second author.

Ceres, around which NASA's Dawn spacecraft continues to orbit, offers the best opportunity to test the significance of cryovolcanism on outer space system bodies, compared to regular volcanism on terrestrial planets like Earth.

"There was a great deal of interest in searching for cryovolcanoes on Ceres as soon as Dawn arrived there, because thermal models had predicted they might exist. Ahuna Mons was a great candidate right away. I carried out a global search that identified 31 other large domes, based on analysis of Dawn's Framing Camera images and topography data," Sizemore said.

"Making the case that they were volcanic was difficult because they were more ancient than Ahuna and the surfaces were heavily cratered. In this study, we were able to compare the shapes of the mountains to numerical models of how they should relax over time if they were made out of icy lava. That strengthened the case that they were volcanic features, and let us make comparisons to volcanism on other planets."

Finite Element Method models were used to analyze images from Dawn to show Ceres has experienced cryovolcanism throughout its geologic history, with an average surface extrusion rate of about 10,000 cubic meters per year, orders of magnitude lower than that of basaltic volcanism on the terrestrial planets.

"We measured the height and diameter of 22 domes, and from this calculated the aspect ratio and volume of each. We assumed that they all started out sharply peaked like the youngest mountain, Ahuna Mons. We then calculated the time it would take them to flatten to their current shape, using a numerical model of viscous relaxation," said Sizemore.

"This allowed us to assign approximate ages to the majority of the domes, cross check the model ages with other constraints, and approximate the rate at which the domes formed over the past 1 billion years. Having the volumes of the domes and the rate of formation on Ceres, we could then make direct comparisons to other worlds."

"Given how small Ceres is, and how quickly it cooled off after its formation, it would be exciting to identify only one or two possible cryovolcanoes on the surface. To identify a large population of features that may be cryovolcanoes would suggest a long history of volcanism extending up to nearly the present day, which is tremendously exciting," said Sizemore.

"Ceres is a little world that ought to be 'dead,' but these new results suggest it might not be. Seeing so much potential evidence for cryovolcanism on Ceres also lends more weight to discussions of cryovolcanic processes on larger icy moons in the outer solar system, where it's likely more vigorous."

Research Report: "Cryovolcanic Rates on Ceres Revealed by Topography," Michael M. Sori et al., 2018 Sep. 17, Nature Astronomy

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