Dwarf planet Ceres was once muddy ocean, now surface is 90% ice: Study

A groundbreaking study has overturned previous assumptions about the dwarf planet Ceres, revealing it to be far icier than previously believed. Notably, Ceres has a heavily cratered and dimpled surface, which has led scientists to debate that the visible craters indicate it can’t be very icy.

The discovery that Ceres has a dirty ice crust was led by Ian Pamerleau, a Ph.D. student, and Mike Sori, an assistant professor in Purdue’s Department of Earth, Atmospheric, and Planetary Sciences, who published their findings in Nature Astronomy. Along with Jennifer Scully, a research scientist at JPL, they conducted computer simulations to study how craters on Ceres deform over billions of years.

According to the researchers, there is a significant amount of water ice near Ceres’ surface, which decreases in ice content with depth. It was previously believed that if Ceres were very icy, the craters would deform quickly over time, similar to glaciers on Earth or flowing honey.

However, their simulations show that ice can be much stronger under Ceres’ conditions than previously predicted, particularly when mixed with a small amount of solid rock.

Discovery reshapes understanding of its geological history

The team’s discovery contradicts the previous belief that Ceres was relatively dry. While it was commonly assumed that Ceres contained less than 30% ice, Sori’s team now estimates that the surface is closer to 90% ice.

They interpret this to mean that Ceres was once an “ocean world” similar to Europa but with a dirty, muddy ocean. Over time, as that muddy ocean froze, it formed an icy crust with some rocky material trapped within it.

Pamerleau explained that they utilized computer simulations to model how craters on Ceres relax over billions of years. He noted that even solid materials can flow over long timescales, with ice flowing more easily than rock. The deep bowls of craters create high stresses that eventually relax to a lower stress state, resulting in a shallower bowl shape through solid-state flow.

Following NASA’s Dawn mission, the conclusion was that the absence of relaxed, shallow craters suggested Ceres’ crust could not be very icy. However, the computer simulations introduced a new understanding of how ice can flow with minimal non-ice impurities, allowing for a highly ice-rich crust to remain mostly stable over billions of years.

A unique journey from launch to orbiting Ceres

On September 27, 2007, NASA launched the Dawn mission, which became the first and only spacecraft to orbit two extraterrestrial destinations: the protoplanet Vesta and Ceres. Although launched in 2007, Dawn did not reach Ceres until 2015, orbiting the dwarf planet until 2018.

According to Pamerleau, multiple observations made using data from the Dawn mission motivated the search for an ice-rich crust on Ceres that could resist crater relaxation. Different surface features, such as pits, domes, and landslides, suggest that the near subsurface of Ceres contains a significant amount of ice.

Additionally, spectrographic data indicates that ice likely exists beneath the regolith on the dwarf planet, while gravity data reveals a density value close to that of impure ice. The team also took a topographic profile of a complex crater on Ceres to construct the geometry for their simulations.

Sori explains that Ceres, being the largest asteroid, raised suspicions about its icy composition based on mass estimates from Earth, making it an ideal candidate for a spacecraft visit.