The ghosts of nearly two dozen icy volcanoes haunt dwarf planet Ceres

Remains of eruptions suggest the world has been volcanically active for billions of years

Ceres' Ahuna Mons

NOT ONE OF A KIND  Scientists thought Ahuna Mons (shown in the center of this image constructed from surface maps taken by NASA’s Dawn spacecraft) was the only icy volcano on dwarf planet Ceres. Now it seems it’s just the youngest.

NASA’s Goddard Space Flight Center

Scientists have spotted the ghosts of nearly two dozen ice volcanoes on dwarf planet Ceres.

Found using topographic maps from NASA’s Dawn spacecraft, the slumped remains of once-grand cones suggest that Ceres has experienced continual eruptions for billions of years, the researchers report September 17 in Nature Astronomy.

When Dawn arrived at Ceres in 2015, scientists noticed just one cryovolcano, which spews water instead of magma: Ahuna Mons, a four-kilometer-high mountain that formed at most 240 million years ago (SN Online: 9/1/16). At the time, researchers wondered why Ceres didn’t seem to have any other volcanoes.

One possibility is that older volcanoes have been erased or altered over time. So planetary scientist Michael Sori of the University of Arizona in Tucson and colleagues ran computer simulations which showed that cryovolcanoes like Ahuna Mons slump and relax into shorter, wider domes over millions of years.

“If you pour a thick liquid like honey, you see it slowly spread and flow and flatten over time,” Sori says. “We think the same thing was going on on Ceres.”

Highs and lows

The sites of 22 ice volcanoes on dwarf planet Ceres are shown in this topographical map (black dots), created with data from NASA’s Dawn spacecraft.

Hover over the numbered dots for zoomed-in looks at three of the volcanoes.

M. Sori et al/Nature Astronomy 2018

Using that insight, the team picked out the remains of 21 ice volcanoes on the small world. The newly identified domes, plus Ahuna Mons, range from 16 to 86 kilometers wide and 1.1 to 4.4 kilometers tall. All but one show their age, being shallower, wider and shorter than Ahuna Mons. The exception, Yamor Mons, is similar to Ahuna Mons in size and steepness, but located near Ceres’ north pole. There, temperatures are cold enough — below –173° Celsius on average — to keep the ice rock hard, preventing it from spreading out.

Sori and colleagues estimate that the ghost volcanoes formed hundreds of millions to about 2 billion years before Ahuna Mons. Older volcanoes could exist, but are probably unrecognizable. The team calculates that the volcanoes should have spewed out an average of 10,000 cubic meters of water per year. That’s thousands of times less material than volcanoes on rocky planets, such as Earth or Venus, which erupt molten rock. So icy volcanism seems less important to Ceres’ history than rocky volcanism was to other worlds, the team concludes.

Whether that’s true of all cryovolcanic worlds, or just Ceres, remains to be seen. Other signs of ice volcanoes have been spotted in the solar system, including on Pluto and on icy moons Europa and Enceladus.

Lisa Grossman is the astronomy writer. She has a degree in astronomy from Cornell University and a graduate certificate in science writing from University of California, Santa Cruz. She lives near Boston.

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