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Dawn Mission Reveals Dwarf Planet Else in the Solar System

Dawn Mission Reveals Dwarf Planet Else in the Solar System

Fossil With lowlands, highlands, weird white spots, and even a pyramid, the largest object in the belt is unlike anything mission reveals else in the . by Eric Betz

IN THE BEGINNING, with a equivalent to just 4 percent of that contained in ’s . 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, . At 590 (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 . Ceres 8,400 miles (13,600 kilometers) away in — 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 . 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 . 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 , 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.” . As , 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 . 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 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 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 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 . But after scouring etesimal and vaporized its water. to form water ice that would sublimate and mountain at the present time, and it’s going the solar system for similar volcanoes, the But astronomers had to visit Ceres to leave the salt behind,” Reddy says. “This is to take more than just great pictures team has yet to find anything like it. fully understand its past. Unlike Vesta, the the story we have for now. We don’t have the because we just don’t see the sort of mani- ’ Pluto flyby on July 14 icy body didn’t create a family of rocky spatial and spectral resolution to rule out the festations that we would expect for moun- complicated things even more when it meteors to help tell its story. presence of water ice as a minor component.” tain growth,” Russell says. “We have to sort showed Ceres isn’t the only dwarf planet in However, current theory holds that Salt was expected on the dwarf planet, of think: What would you expect around a the solar system with mountains. Ceres was slightly farther out in the solar but its extent has surprised scientists. mountain on Earth?” Pluto has many ranges that sweep across system, so it took shape later — maybe only “On Ceres, it seems like the presence of So far, there’s nothing earthly about it. its complex surface, including in its light- a couple million years later. And planetary salts may be important for understanding At first, scientists wondered if something colored “heart,” informally known as scientists think that relatively short time even the basic morphology and what we see fell from space and created the odd moun- Tombaugh Regio. span could have made all the difference, on the surface,” Bland says. tain. Neighboring Vesta sports one of the Scientists say these mountains must be because Al-26 decays quickly. The Dawn team is now trying to under- highest mountains in the solar system, but made of water ice — a substance also abun- So, Vesta lost its water and was left with stand the salt’s source. Does Ceres have it’s surrounded by an obvious impact that dant on Ceres. But so far, astronomers differentiated layers of rock and iron like a many kinds of salts, or are the deposits con- ejected much of the asteroid’s south pole haven’t found any way to link the moun- terrestrial planet or Earth’s Moon. But snow- nected? Russell suspects the latter. into space, creating the body of meteorites tains on the two bodies. ball Ceres clumped together more like an icy Fresh topographic maps of Ceres’ hemispheres highlight elevation differences across the dwarf “The most obvious explanation to me commonly found on Earth. “The heights of [Pluto’s] mountains and outer moon or object — Pluto’s planet. Here high points are shown in red, and lower elevation features are blue. is that there is a fluvial connection down However, Ceres’ mountain is not sur- the shape of the mountains look very similar smaller cousins beyond ’s orbit. below the surface that connects all of these rounded by any obviously related impact. to the shape of our mountains,” Russell says.

46 ASTRONOMY • JANUARY 2016 WWW.ASTRONOMY.COM 47 “There is a story there, but we haven’t fig- ured out what the answer is. We believe Blended family these bodies formed in very different parts Ceres is known as our solar of the solar system. Is there a way they system’s largest asteroid, but could have been much closer together the watery sphere stands out from its rockier relatives. originally?” NASA/JPL-CALTECH/JAXA/ESA For now, the mission head says, the for- mation of Ceres’ mountain is the most common topic of conversation.

21 Lutetia Ceres’ thin Even though much of Ceres’ surface remains unexplained, astronomers already 243 Ida/Dactyl have named the most prominent features. Fresh topographic maps from the Dawn 433 Eros team highlight elevation changes as great 951 Gaspra as 9 miles (15km) between the crater bot-

2867 Šteins toms and mountaintops. Kerwan Crater, 1 Ceres named for the Hopi god of corn growing, is 5535 Annefrank Ceres’ largest confirmed crater at 175 miles 9969 Braille (280km) from rim to rim. That makes it roughly the same size as Earth’s largest 25143 Itokawa well-established impact site, South Africa’s 2-billion-year-old Vredefort Crater. Kerwan bottoms out some 3 miles (5km) below the surrounding surface. And while the other craters are significantly smaller, Dawn’s unconventional ion engine allowed it to they all look like those seen much farther slowly spiral into orbit around Ceres in March. out in the solar system. Dawn circles in “The craters we find on Ceres, in terms of their depth and diameter, are very simi- Instead of the pull from a massive host on Ceres lar to what we see on and Tethys, planet, Ceres — as well as Pluto — might two icy of Saturn that are about still have enough internal warmth to drive Departure the same size and density as Ceres. The its surface activity. And some astronomers September 2012 features are pretty consistent with an ice- have begun to question whether long-lived rich crust,” says Paul Schenk of the Lunar radioactive isotopes could still be driving and Planetary Institute in Houston. heat all these billions of years later. This backs up the idea that Ceres’ com- Titus suspects that Ceres’ surface is position has much more in common with more -like and that asteroid impacts well-known icy than it does with its excavate water ice, which sunlight evapo- asteroid belt neighbors. And yet the dwarf rates, leaving only the salt deposits.

Sun planet is also different from the icy moons. 15 km Whether plumes, impacts, or something

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o e September 2007 r r called central pit craters form when an believe this rocky mass supplied the family of commonly found as meteorites on Earth. possible for geological activity on such b e i C t E ar impact melts ice and it flows away at the small isolated bodies. th ’s o center, leaving a depression. This implies a And, in December, Dawn descends into rbit M mix of both rock and ice, but astronomers hasn’t confirmed the water vapor that the Dawn’s arrival, Bland, the other USGS its lowest orbit — just 233 miles (375km) ar s’ o are still trying to understand the ratio. Herschel telescope saw around Ceres. Yet astronomer, published a paper outlining above Ceres’ surface. From that vantage rbit “Ice seems to be an important compo- astronomers say they do believe the space how Ceres’ craters would relax if the dwarf point, astronomers can use the spacecraft’s Survey mission Mars begins nent, but the surface is very, very dry,” observatory’s results. planet had an internal ocean. Gamma Ray and Neutron Detector to map assist August 2011 Russell says. “Paradoxically, the surface USGS astronomer Timothy Titus is a So far, the crater edges all look pretty elements, including salt, and gain new February 2009 Vesta Arrival tells us [Ceres] must have been wet at some participating scientist in the Dawn mis- crisp. If there’s water, it’s not very close insights into the dwarf planet’s evolution. July 2011 time. It looks like the morphology of wet sion. His models show that water ice isn’t to the surface. Its primary mission wraps up in June 2016. Ceres areas on Earth.” stable within a few meters of the surface “The idea of cryovolcanism in the outer “[Dawn] has validated the origin of the Arrival March 2015 End of primary at Ceres’ equator. solar system is not new, and it’s been solar system that was deduced from the Path of Dawn mission The new icy dwarfs And that’s where the world’s white spots applied to a lot of the moons meteorites,” Russell says. “But that is not mission June 2016 And in a question similar to the one faced are. Instead, Titus says water could more because there you can get tidal heating,” the whole story. We had to explore Ceres to After launch in 2007, Dawn got a gravitational assist from Mars to reach orbit around Vesta, where the at Pluto, scientists are trying to determine easily flow in the polar regions. Before Titus says. “At Ceres, you don’t have that.” obtain its secrets.” spacecraft spent 14 months before departing for Ceres. Dawn is now the first human-made object to if Ceres has a liquid water ocean beneath LEARN MORE ABOUT DAWN’S FINDINGS AT VESTA BY VISITING www.Astronomy.com/toc. orbit two different extraterrestrial worlds. ASTRONOMY: ROEN KELLY, AFTER NASA its dirty surface. So far, the Dawn team

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