What’s causing Ceres’ white spots?
A series of mysterious white features lurk at the bottom of one of its most massive craters. Here’s what they could be, and how we’ll find out!
“One of the dreariest spots on life’s road is the point of conviction that nothing will ever again happen to you.” –Faith Baldwin
So it might’ve gone for Ceres, which for a brief, glorious moment was known as our Solar System’s eighth planet. You see, we knew about six planets — including the Earth — since ancient times, as they were all visible to the naked eye. With the Copernican revolution long behind us, followed by the discoveries of Kepler, Galileo and Newton, the structure of the Solar System was basically known. Throughout the 1600s and 1700s, better and better observations seemed only to vindicate our understanding of planetary motion, governed by Newton’s law of gravitation, and punctuated by the verified prediction of Halley’s comet’s return in 1758.
All was known and well within our Solar System.
Or so we thought.
At the dawn of the 19th century, the astronomy community was still reeling from William Herschel’s 1781 discovery of Uranus, the first planet discovered beyond the six (including Earth) naked-eye worlds orbiting the Sun known for millennia. While most of astronomy had consisted of star-watching, comet hunting and the cataloguing of deep-sky objects, there was a new pursuit just coming into its own: the hunt for new, permanent objects in our Solar System. It was on New Year’s Day, 1801, that Italian Astronomer Giuseppe Piazzi discovered something truly wonderful, that caused him to write the following:
“I have announced this star as a comet, but since it is not accompanied by any nebulosity and, further, since its movement is so slow and rather uniform, it has occurred to me several times that it might be something better than a comet. But I have been careful not to advance this supposition to the public.”
What he discovered wasn’t, in fact, a new planet, which is what Piazzi’s great hope actually was, but rather the first and largest object in what would turn out to be the asteroid belt: Ceres.
For over a decade, the best images we had of this tiny world — massive enough to be pulled into a sphere, but no bigger than the state of Texas — came from the Hubble Space Telescope, which even at closest approach was some 300,000,000 kilometers distant.
While some very clever image processing and the full suite of data was enough to give us some amazing views of this world that turned out to be just 900 km across, or barely half the diameter of our Moon, the great distances to Ceres were inherently (and severely) limiting.
NASA decided to change all of that with the launch of the Dawn spacecraft, designed to map and study two of the most massive and most intriguing objects in the asteroid belt: Ceres and Vesta, which is (just barely) the third-largest asteroid, losing out to Pallas by the tiniest of margins.
Dawn observed Vesta first, over a two year period from 2011–2012, constructing a mosaic and 3-D map of the world unlike anything else ever seen for an asteroid.
Then, it was on towards Ceres. Although you might think that the asteroid belt is roughly circular, some of the asteroids are barely out beyond the orbit of Mars, while others go almost as far away as Jupiter, some three times the distance from the Sun as Mars.
Between the two targets of the Dawn Mission, Ceres is farther out than Vesta, some 75 million km more distant from the Sun, and hence, colder. While both worlds were expected to be littered with craters, Vesta — at least on its “day” side — gets warm enough and has little enough gravity that any water-ice that would’ve formed on the surface gets sublimated almost instantly. Not only does it turn into vapor, but the surface gravity of Vesta is low enough that it’s insufficient to hold these gaseous, energetic water molecules onto its surface, and they escape away. The mere effect of sunlight is enough to give these particles, even at such great distances from the Sun, enough speed to overcome gravity’s pull.
Which is why Dawn’s latest discoveries — on Ceres here in 2015 — are so puzzling.
Sure, Ceres is significantly larger, more massive and somewhat farther away from the Sun than Vesta is. But is that really sufficient to explain these “white spots” at the bottom of what appears to be perhaps the largest crater on Ceres? Right now, there are three leading possible explanations:
- This is, in fact, water-ice. Frozen water at the bottom of this crater, quite surprisingly, remains stable, even in direct sunlight, even near the equator. This rocky, giant asteroid can stably hold onto this ice, even over billions of years.
- This is some other form of ice: perhaps frozen carbon dioxide (dry ice), which has a higher molecular weight than water does. In some ways, this would be even more surprising, since even though it’s more difficult for it to reach escape velocity, dry ice sublimates at a much lower temperature than water does.
- This is some solid, rock-like feature that simply has a different reflectivity (or albedo) than the rest of the asteroid. This could be intrinsic to Ceres (its version of bedrock), it could have been forced out of its interior (due to volcanism), or, quite possibly, it could have been from material brought to Ceres by an impact.
There’s a popular (conspiracy?) theory out there that this is aliens, or more specifically, that this is emitted light rather than reflected light. It’s a fun idea, but if that were truly the case, you wouldn’t see such tremendous brightness/albedo changes as the angle of the reflected sunlight changed relative to the spacecraft.
I’ve gone ahead and pulled various images out of the NASA mission, and you can see for yourself: this is reflection, not emission.
Whichever of these explanations proves correct — and the leading idea is presently the water-ice option — it’s important to recognize that Dawn is only beginning its mission at Ceres. Currently orbiting at a distance of 13,600 kilometers off of the asteroid’s surface, or some forty times more distant than the International Space Station is from Earth’s surface, Dawn will descend over the coming months, mapping out Ceres with a variety of instruments, and learning the elemental composition of this material.
Right now, we can already be sure of some things that it isn’t, but the coming months will shed so much light — at such improved resolution — on what’s really causing this most mysterious of features. There’s something reflecting the light here, something that’s unlike anything else on Ceres’ surface, and we’re about to find out what.
Sure, the images Dawn has taken so far are spectacular, but there’s so much more to come. The biggest surprise on this world, of a bright, reflected light at the bottom of its deepest crater, is going to be explained by the very device that discovered the anomaly in the first place.
In the most serendipitous of ways, this is a prime example of what science is all about: find a mystery, figure out exactly what you need to know in order to solve it, and then make exactly those measurements. Thanks to some amazing preparation by NASA’s Dawn team, we’re already equipped to solve a mystery that, up until it was already in orbit, we didn’t even know existed!
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