Today we talk about asteroids with such familiarity that it's strange to imagine that the asteroid belt needed to be discovered, but it happened surprisingly recently. The first asteroid found was Ceres in 1801, by Giuseppe Piazzi, during the hunt for a missing planet suspected to exist between Mars and Jupiter. It wasn't called an asteroid at first, of course. For a while there, Ceres was considered a planet. (Note its Roman deity namesake, the goddess of agriculture, which is also where we get the word cereal.)
Then other such "planets" were discovered in Ceres's neighborhood—and with alarming regularity. After 50 years of too many planets, astronomers decided to classify this veritable planetary pestilence at the Martian-Jovian boundary as a new type of body: asteroid. In 2006, astronomers took another stab at the classification of Ceres, promoting it to dwarf planet with the same stroke of the pen that demoted Pluto.
Ceres is more than a big asteroid or small dwarf, however. The NASA spacecraft Dawn has been in orbit around Ceres since 2015, studying every square inch of it. What they've found is the Rosetta Stone for comparative planetology—an intriguing mix of Mars, asteroid, icy moon, and comet. Mental Floss spoke to Hanna Sizemore, a research scientist at the Planetary Science Institute and a guest investigator on the Dawn team. Here are a few things you ought to know about Ceres.
1. CERES BY THE NUMBERS.
Ceres accounts for one-third the mass of the asteroid belt, and is by far the largest object there. It has a radius of 295.9 miles, making it smaller than Earth's moon (whose radius is 1079 miles), and only about 2.8 percent of Earth's gravity. (That's enough, though, for you to walk around on, should you choose to visit.) The days on Ceres would fly by at 9 hours each; the years on Ceres would drag endlessly, at 4.6 Earth years. Relative to Earth, it would be a pretty cold place to live, with temperatures ranging from -225°F to -100°F.
There is no atmosphere on Ceres worth mentioning, so the view above the horizon would be pretty depressing: the infinite black loneliness of space. The view at the horizon and below wouldn't be much better. Picture the sort of asteroid you might land the Millennium Falcon on; that's what the surface looks like.
2. IT HAS SOMETHING FOR EVERYONE.
"Ceres is an interesting hybrid between a planet like Mars, which is a rocky body with a cryosphere [significant ice in the near-surface], and the icy satellites of Saturn," says Sizemore. "The outer surface of the planet has less ice than we expected and more dirt. As you go down, it seems like the ice content increases again, and as you go further in, there may (or may not) be a higher density core."
The chemistry of Ceres is more complex than was expected before Dawn arrived, and there are more nuances to the layered structure; it's not simply rigidly defined layers as you might find on Earth or Europa. Moreover, Dawn has found surface features suggestive of cryovolcanoes (ice volcanoes), as well as unexpected tectonic features. "It's got a little bit of everything. It's a mix between an icy satellite, a rocky body with a cryosphere, an asteroid—it's got things in common with comets, too. It's the hybrid body."
3. IT'S NOT A BAD PLACE TO LIVE …
"A lot of people are excited about Ceres from an astrobiological standpoint," says Sizemore. "You have a lot of water-rock interactions going on there. You have this extensively altered regolith. You have organics at the surface. That's a gold mine from an astrobiological perspective, this intimate mix of rock, water, and organics—the question is what bugs might grow, or what building blocks of life are there."
The data collected by Dawn's Visual and Infrared Spectrometer (VIR) suggest the organics are native to Ceres, formed under processes not yet fully known. (Scientists originally wondered if they were deposited by way of asteroid impacts.) To understand the nature of the compounds and how they formed, members of the planetary science community have begun discussing a prospective lander mission.
4. … BUT NOT SO GOOD THAT ALIENS LIVE THERE.
You might recall NASA's discovery a few years ago of two piercing, bewildering white spots on an exotic world? That was Ceres. The Keck II telescope in 2002 first revealed something unusual up there, but it wasn't until Dawn approached the then-unexplored world that things really got weird. Was it an ice mountain? An ice canyon? Salt? Some giant chunk of shiny metal? Or was it what everyone really hoped: technology from an intelligent alien race—perhaps a solar collector or beacon of some sort. (NASA even posted a poll for the public's guesses.)
I am sorry to report that the spots weren't built by aliens. Rather, according to a paper published last year in Nature, the spots are a type of salt, sodium carbonate, and constitute "the most concentrated known extraterrestrial occurrence of carbonate on kilometer-wide scales in the solar system." The spots are possibly the result of the crystallization of brines and altered material from the Ceres subsurface.
5. DAWN AND CERES MAY GIVE US MINING TOWNS ON THE ASTEROID BELT.
Any significant expansion of the human footprint beyond the lunar surface will require a process called in situ resource utilization, which involves the harvesting of resources on another celestial body and producing usable goods. (Expeditions during the Age of Discovery are analogous; explorers didn't fill ships with timber and then sail to the New World; they brought axes and used what they found when they arrived.) Lifting things from the Earth's surface is very expensive. Why launch barges of methane fuel to Mars, for example, when you can instead launch a single machine able to extract those elements from the Martian soil and manufacture the fuel there? With that in mind, Ceres might be the key to finding usable water for asteroid mining.
"An interesting feature we see on Ceres that we've previously seen on Mars and Vesta are little pits on smooth materials in fresh craters. They seem to be caused by the outgassing of ice vaporized during the impacts," says Sizemore. "It's starting to suggest a common indicator of volatile rich material at impact sites on asteroids." If volatiles, such as ice, are easily found and accessed on asteroids, the business case for mining them writes itself.
"At Ceres, there are actually surface exposures of ice, both at polar latitudes and at mid latitudes, and even at low latitudes we believe that ice is only meters deep. As we explore the asteroid belt more in the future, in situ resource utilization is going to be a big thing. Water is a really important resource even for hypothetical robotic missions, and we have a test case at Ceres to learn to quantify it," says Sizemore.
6. MUD OCEANS MEAN NO SHARKS.
It took 34 years from the first notion of an asteroid belt-specific exploration mission to NASA's Dawn spacecraft entering orbit around Ceres. (Notably, Ceres was the second stop on Dawn's journey, after a successful mission around Vesta. This makes Dawn the first and only spacecraft to orbit two bodies beyond Earth.)
Dawn is the only mission at Ceres. The next likely mission there will be a robotic lander or sample return, though such missions are only in the development stage. Unless mynocks start chewing on Dawn's power cables, causing NASA to send an exogorth-sensitive probe, it will likely be some time indeed before a Ceres lander reaches the launch pad.
It's a good thing, then, that Dawn is delivering the goods. Scientific instruments on the spacecraft have provided new insights on the Ceresian interior and talk of a Europa-like subsurface ocean has receded. Scientists now think Ceres has a "kind of a mud ocean, rather than a liquid water ocean comparable to our seas here on Earth, or what's under the ice shell on Europa," says Sizemore. "You have something quite dirty at the very outside shell, and as you go down, the water content increases, but it's probably a salty mud slurry." The thickness of the mud layer is still being determined by modelers.
"No sharks swimming in it," she adds. "No giant squids like on Europa Report."