Fossil Planet With lowlands, highlands, weird white spots, and even a pyramid, the largest object in the asteroid belt is unlike anything Dawn mission reveals dwarf planet else in the solar system. by Eric Betz IN THE BEGINNING, with a mass equivalent to just 4 percent of that contained in Earth’s Moon. What was OUR SOLAR SYSTEM left is what we still see today. One-third of that mass is held by a single WAS A VIOLENT PLACE. world, Ceres. At 590 miles (950 kilometers) NASA’s Dawn mission Radiation from neighboring massive across, it’s our solar system’s largest asteroid captured Ceres from stars bombarded our small part of a large and the only dwarf planet this side of Pluto. Ceres 8,400 miles (13,600 kilometers) away in molecular cloud — a many light-years-wide It’s also a relic of our violent origins. May as it spiraled body of gas and dust resembling the Eagle This icy body is the current focus of into ever-lower Nebula’s “Pillars of Creation” — as the NASA’s Dawn mission — a small spacecraft orbits. ALL IMAGES: NASA/ JPL-CALTECH/UCLA/MPS/DLR/IDA, whole expanse coalesced like a figure skater that’s powered its way across the inner solar EXCEPT WHERE NOTED pulling her limbs in tight for a spin. system since 2007 using unconventional Some 99.8 percent of the mass drew to ion propulsion. The engine allowed Dawn the center, forming our Sun. And out of the to become the first mission to ever orbit firmament 4.6 billion years ago, tiny bits of two extraterrestrial bodies. Astronomers dust, like particles in a smoke cloud, stuck spent 14 months studying the asteroid together to create ever-larger clumps. Vesta before embarking for Ceres in 2012. Grains grew into pebbles; pebbles “Ceres, and Vesta before it, are intact formed planetesimals. protoplanets — bodies that were growing But this process was still in its early days to planethood when their growth was when a supernova blast rang out, seeding stopped by external forces,” says Dawn radioactive elements across the still-forming Principal Investigator Chris Russell of the inner realm of planets, which trapped heat University of California, Los Angeles. inside any worlds already gathered together. “They have a record of the earliest days of Meanwhile, something pivotal took the solar system. They were there and expe- place between 2 and 3 astronomical units rienced it and have evolved little. ... We are from our young Sun (1 AU is the average returning to the scene of the crime to inter- Earth-Sun distance) in the present-day view the witnesses.” asteroid belt. As Jupiter, the largest planet, Now, these fossil planets are teaching took shape, it had catastrophic effects on a astronomers what our solar neighborhood group of planetesimals. was like when Earth saw its first sunrise. The gaseous giant perturbed the region Solar system models use bodies like and stopped its mass from ever coalescing Vesta and Ceres as building blocks for ter- to become a terrestrial planet. restrial planets. Ceres is likely similar to the Jupiter may have flung much of the planetesimals that brought Earth its oceans. mass to the solar system’s outer reaches, And our planet’s iron core might have leaving what’s now called the asteroid belt formed from a number of Vesta-like worlds. “Almost everything we see on Ceres was Eric Betz is an associate editor of Astronomy. unknown before we arrived,” Russell says. He’s on Twitter: @ericbetz. “Ceres had kept its secrets well.” © 2016 Kalmbach Publishing Co. This material may not be reproduced in any 44 ASTRONOMY • JANUARYform without 2016 permission from the publisher. www.Astronomy.com White spots explained At first, scientists speculated the white spots could be excavated water ice, salt, or clay. And determining which they were turned out to be tougher than expected. “We didn’t like the ice explanation, but we felt we were being driven to that expla- nation by how bright the surface was,” Russell says. At nearly 3 AU from the Sun, Ceres, unlike distant Pluto, is bombarded by sun- light, and that would cause any surface ice to quickly turn to gas via sublimation. But then as Dawn flew over the white Ceres’ most intriguing features are the dwarf planet’s mysterious white spots, which astrono- spots in its survey orbit, the spectra instru- mers now say are salt deposits. ment shut itself down. When that happens, all the data are dumped. Astronomers Ceres’ lone mountain vaults some 4 miles (6 kilo- would have to wait to find out. meters) above the surrounding surface, making “The question is whether Ceres is active Irrefutable evidence finally came as it taller than even Denali, North America’s high- like Pluto, or whether Ceres was once more Dawn descended into its science orbit and est summit. Occator Crater spans 50 miles (80 kilometers) from rim to rim and is home to Ceres’ brightest spots. active like Pluto and it lost its surface vola- sent back better images. Dawn’s photos After months of intense speculation, Dawn scientists now believe they understand what causes them. tiles because it’s closer to the Sun,” says showed that the white spots are actually far Dawn mission scientist Vishnu Reddy of bigger than expected. That meant salt, regions together like an aquifer,” Russell the Planetary Science Institute in Tucson. which reflects less light than water ice, was says. “Maybe you can’t get from one place to An active protoplanet? In fact, the resemblance between Ceres Reddy and a team of astronomers think the only likely solution. And when NASA another, but the chemistry is the same.” As Dawn neared Ceres in early 2015, and newly revealed Pluto is so strong that they may already have found some clues. In finally got spectral data, it confirmed salt’s He thinks spectra will eventually show something truly unexpected emerged in Dawn team members have been left scram- early 2014, the European Space Agency chemical fingerprints. The white material that the salt covering Ceres’ mountain is the its imagery — two weird white spots. The bling for answers. pointed its Herschel Space Observatory at has now been detected covering peaks and same stuff that covers the craters. bright areas, which shine almost like a cat’s Michael Bland is on the Dawn team and Ceres and caught water vapor streaming crater rims across the dwarf planet. The salt forms in the world’s interior, eyes when seen from afar, have remained an astronomer at the United States from two small regions. Reddy says that the “Definitely [the white spots] can’t be ice,” and depending on the acidity of water below the fossil planet’s most intriguing features. Geological Survey (USGS). He says he areas are now known to coincide with the Russell says. “We’ve got enough spectra the surface, models indicate that this salt Astronomers believe that unraveling their expected Ceres to have a smoother surface white spots. from them to see they don’t have the absorp- could be one most earthlings are familiar mystery could explain what’s happened to with fewer pristine craters. Instead, that Dust rains onto everything in the aster- tion bands that we would expect ice to have.” with: magnesium sulfate, also known as Collisions are common in the asteroid belt, and Ceres since its growth was stunted all those description better fits Pluto. oid belt, turning surfaces a darker shade of Epsom salt. The popular bath salt is found Ceres has the craters to prove it. What’s most sur- billions of years ago. “Pluto looks a lot like what I expected gray. So, the white spots are younger than Extraterrestrial bath salts? across our planet and on other worlds too. prising is that the dwarf planet has remained so Vesta’s relative abundance of radio- Ceres to look like, and Ceres looks like how the rest of the surface. But just how young That also better aligns with what astrono- intact throughout our solar system’s history. active aluminum-26 (Al-26), known from I expected Pluto to look,” Bland says. “It’s is anyone’s guess. mers are seeing elsewhere on the Texas-sized The great pyramid of Ceres meteorites commonly found on Earth and like someone switched them on us.” Astronomers don’t know how much dust body. While scientists believe Ceres’ interior Salt isn’t the only revelation at Ceres. The traced back to the asteroid, tells astrono- And while high-resolution data has only falls onto the world. If the white spots are is packed with water ice and even possibly dwarf planet hosts a lone mountain that And the peak is made from the same mers that the asteroid formed in the solar just begun streaming home from the Dawn truly young, it’s possible that the Herschel a liquid ocean, the surface is dry. However, vaults more than 21,000 feet (6,400 meters) material as the rest of the dwarf planet — system’s earliest days. Vesta was blasted by spacecraft, astronomers must now try to telescope caught some sort of icy eruption that doesn’t mean there’s never been ice at off the surface, a height greater than even not some alien substance. the supernova shock wave, and the radio- explain how oddball Ceres has evolved. from Ceres’ subsurface. the surface or even in the white spots. Denali, North America’s highest peak. The next best idea was that the moun- active isotopes generated heat in the plan- “Water vapor could temporarily freeze “The team is totally baffled by the tain is actually a volcano.