NASA’s Dawn spacecraft isn’t the only one observing Ceres. Telescopes on the ground are also keeping a close eye on the largest object in the asteroid belt. And astronomers found something they didn’t expect.

The bright spots of Ceres are changing. That’s the word from scientists using the HARPS spectrograph on the ESO 3.6 meter telescope at La Silla, Chile.

HARPS instrument

The HARPS instrument

Paolo Molaro, lead author of the new study, talks about the initial reveal of the bright spots and what happened next. “As soon as the Dawn spacecraft revealed the mysterious bright spots on the surface of Ceres, I immediately thought of the possible measurable effects from Earth.”

Molaro and the team working with him had a plan. “As Ceres rotates the spots approach the Earth and then recede again, which affects the spectrum of the reflected sunlight arriving at Earth,” said Molaro. These effects would be incredibly small. But still large enough to be measurable via the Doppler effect with the HARPS instrument.

HARPS, or High Accuracy Radial velocity Planet Searcher, is actually designed to hunt for exoplanets. But Molaro and his team used the accurate instrument to observe Ceres during two summer nights last summer.

“The result was a surprise,” says Antonino Lanza, co-author of the study. “We did find the expected changes to the spectrum from the rotation of Ceres, but with considerable other variations from night to night.”

The team of astronomers saw Ceres’ bright spots brighten when sunlight hits the spots but then fade. As sunlight hit Occator crater, “plumes” form and reflect sunlight. But, they don’t last long. These plumes quickly evaporate and fade away.

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Astronomers noticed the effect wasn’t consistent. Notable variations were seen from night to night with the brightness from plumes lasting shorter and longer.

What do changing bright spots on Ceres mean?

First, let’s talk about what these bright spots are. The bright spots of Ceres aren’t limited to Occator crater. In fact, more than 130 bright areas have been discovered on the surface of Ceres by Dawn.

New research released last December points to a mixture of salt and water ice. Using images captured by Dawn’s framing camera, researchers believe the salt-rich spots we see today were left behind when water-ice sublimated following impacts. Asteroids hitting the surface of Ceres would have exposed a mixture of ice and salt. The ice would have instantly turned from solid to gas.

“The global nature of Ceres’ bright spots suggests that this world has a subsurface layer that contains briny water-ice,” said Andreas Nathues, lead author of the study.

Those are the basics. So, what does the new research mean for Ceres bright spots? If the research is confirmed, it hints at Ceres being internally active. It appears the plumes form often, and one of the only ways for that to happen is an internal process.

The plumes seen by ground observations also appear to confirm the hazes seen by Dawn in the Occator crater last year.

Ceres haze above Occator

Now what?

There’s still a lot scientists don’t know about the dwarf planet. Observations are hinting at internal activity on the dwarf planet, but proving that is going to take much more research. And if it is active, that opens up the biggest question of them all – how? There’s no massive body subjecting Ceres to strong tidal effects.

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Who knows what the ultimate answers will be. Ceres captivated the world with its bright spots. Being active might not be as sexy as mysterious bright spots on the surface, but it would be huge news for the astronomy world.

Image credits: ESA/NASA

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