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NEWS FEATURE NEWS FEATURE News Feature: Space fossils

How missions may help scientists piece together the puzzle of the early .

Stephen Ornes Science Writer questions may even shed light on a mystery closer to home: the origin and evolution of life on Earth. On March 6, after a journey of nearly seven history, I talk to my oldest relatives. We are The Beginning and a half years, NASA’s Dawn spacecraft basically doing the same thing with Dawn. “You can’ttellthestoryoflifeonEarthunless reached its final destination: , a whopper We are interviewing the oldest intact survi- youunderstandwhatwasgoingoninsmall of an asteroid with a diameter about the size vors of the solar system.” bodies in the solar system,” says Bill Bottke, of Texas. Ceres contains nearly one-third of Ancient like Ceres are time cap- a planetary scientist at the Southwest Research the mass of the entire that lies sules waiting to be cracked open. Their origin Institute in Boulder, Colorado. Asteroids are between the orbits of Mars and . Ceres stories are encoded on their surfaces in craters, like evidence from a crime scene. Small bodies is so big that the International Astronomical dust, rocks, and ice. Decoding them could in the solar system are the “blood splatter, the Union has designated it a dwarf planet, the help answer questions about the formation of bone fragments,” he says, left behind when only one in the inner solar system. Dawn is the solar system itself. How did asteroids that planetary heavyweights wielded gravity like the first probe to come anywhere near Ceres. seemingly formed in disparate parts of the a weapon, flinging about the smaller denizens. Dawn’s lead scientist, Christopher Russell, solar system end up in the asteroid belt? Has Studying these remnants can tell us about the sees asteroid exploration as an ongoing quest the asteroid belt lost most of the mass that solar system’sviolentpast. of planetary genealogy. “I am interested in it had originally, as some models predict? According to most models, our solar sys- the history of the solar system just as I’m WhyisMarssmallerthanEarth?And tem formed in the aftermath of spectacular interested in the history of my own family,” why, soon after the birth of the solar system, smash-ups in a spinning cloud of dust and says Russell, a geophysicist and space scientist did space rocks smash into Earth and other debris that surrounded the . Grains of dust of the University of California, Los Angeles. planets during an event called the Late Heavy in the solar nebula collided and stuck together, “WhenIwanttolearnmoreaboutmyfamily’s Bombardment (LHB)? Answers to these forming about a kilometer in

NASA’s Dawn spacecraft captured these two views of Ceres on Feb. 12, 2015, from a distance of about 52,000 miles (83,000 kilometers). The Dawn spacecraft arrived at Ceres on March 6, 2015. Image courtesy of NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

www.pnas.org/cgi/doi/10.1073/pnas.1503841112 PNAS | March 31, 2015 | vol. 112 | no. 13 | 3849–3851 Downloaded by guest on September 28, 2021 It’s clear that objects in the asteroid belt don’t fit a pat description. So, how exactly did the small bodies in the solar system get mixed up? Jupiter, the Pitcher To answer the question, astronomers have turned to an unlikely resource: planets in other solar systems. They have identified hundreds of supersized gas giants, called “hot ,” that orbit their stars closer than Mercury orbits our sun. These discoveries suggest that planets may not stay put in the orbits where they first formed, as older theories suggested. “Things that start off stable can become unstable, and that may be a common thing,” Dawn first arrived at the asteroid Vesta in summer of 2011. An image analysis shows the says Bottke. Newer models of planetary for- asteroid’s topography at its northern and southern hemispheres. The colors indicate the dis- mation attribute the current configuration of our solar system to : the tance from each of the poles. Image courtesy of NASA/JPL-Caltech/UCAL/MPS/DLR/IDA/PSI. idea that gas giants lumbered through the solar system, greatly perturbing the orbits of diameter. These bodies grew into planetary January, using data from the Sloan Digital the smaller inhabitants as they traveled. One embryos and eventually accumulated enough Sky Survey, researchers from the Harvard- of those, the Nice model, proposed in 2005 heft to gravitationally capture more debris. Smithsonian Center for Astrophysics reported and named for the location of the observa- Within a span of a million years—possibly finding greater variation in size and distance tory in France, suggests that the gas giants orders-of-magnitude less—Jupiter, , from the sun among asteroids than had been formed much closer to the sun than they are , and swept up huge amounts predicted before. They likened the ancient today. Hundreds of millions of years later, of gas and swelled into giants. Planetesimals asteroid belt to a shaken snow globe (3). they migrated outward and scattered plane- near the sun collided and merged over tens “There’s sort of a continuum of asteroid tesimals and asteroids like billiard balls (4). of millions of years to become Mercury, types out there,” says Amy Mainzer, an as- Gravitational disturbances from Jupiter and Venus, Earth, and Mars. tronomer at NASA’s Jet Propulsion Labo- other gas giants have flung a big fraction of ’ However, these broad strokes leave many ratory in Pasadena, California. She directs the asteroid belt s mass out of the inner solar details murky, including the relatively dinky the NASA-funded Near-Earth Object Wide- system, according to the Nice model. size of Mars and the origin of the asteroids. “ Field Infrared Survey Explorer (NEOWISE) Jupiter is very efficient at throwing things “One of the biggest problems in asteroid sci- ” project, an asteroid-hunting mission that mea- out of the solar system, says Bottke of the ence is understanding how asteroids originally massive planet. The Nice model accounts for ” sures infrared radiation from space rocks. formed, says astronomer Kevin Walsh, a re- the mix of species and part of the missing search scientist also at the Southwest Research Mainzer says observations from NEOWISE support the idea of a blurry distinction be- mass in the asteroid belt. Institute. The Nice model also accounts for the tween comets and asteroids (the team plans LHB, during which asteroids and comets— The Asteroid Problem to release its latest round of data in March). attracted inwards by the gravity of Jupiter and Astronomers once viewed the asteroid belt as Astronomers used to think that comets were Saturn—plowed into the Earth and , as an orderly place where fragments of failed limited to the icy bodies that raced toward the well as Mercury, Venus, and Mars. Studies of planets orbited roughly as far from the sun as sun from the remote hinterlands of the Kuiper impact craters on the Earth and Moon sug- where they had formed (1). Over time, aster- Belt (which lies outside the orbit of Neptune) gest the LHB happened about four billion oids collided, broke apart, and fused to- and beyond, and asteroids in the asteroid belt gether. However, this neat view has suffered years ago and lasted hundreds of millions wererocksthatmoreorlessstayedintheir of years. from major challenges. By some estimates orbital place. However, those distinctions are (2), the asteroid belt lost 99.9% of its mass in TheLHB,someresearchersthink,may simplistic, says Mainzer. “Astronomers have the early years after the solar system formed, have even played a role in starting life on seen asteroids that behave like comets and perhaps because of planetary embryos in the Earth. In a study published in PNAS (5), comets that sometimes behave more like belt that flung smaller asteroids out of the physicists from the Czech Republic reported asteroids. Originally, we thought never the that the energy released by asteroids and solarsystemorbecauseofthemovements ” of Jupiter. In addition, different kinds of two should meet. With NEOWISE, Mainzer comets smashing into the Earth could have asteroids regularly commingle: The orbit of says they sometimes spot unusual dark helped forge genetic material. In laboratory —“ ”— Ceres—whose composition is similar to as- rocks like printer tone dark that ap- experiments to simulate the LHB, the scientists ’ teroids near Jupiter—takesitclosertothe proach the sun like comets but don t reflect fired a high-powered laser at formamide, a paths of some dry, rocky asteroids that swing light. “It’spossibletheseobjectsrepresent primitive chemical believed to be common in closer to Mars. some kind of dead comet,” she says, that Earth’satmospherebillionsofyearsago.The Such discoveries of rocks in peculiar pla- lost all their ice as they rattled inward from experiment produced nucleobases found in ces, first thought to be anomalous, have be- the . Occasionally, Mainzer says, thebackboneofRNA,orRNA,suggesting come commonplace in the last decade. Last asteroids assume comet-like orbits. that asteroids could have sparked life on Earth.

3850 | www.pnas.org/cgi/doi/10.1073/pnas.1503841112 Ornes Downloaded by guest on September 28, 2021 However, the Nice model, which was de- icy mantle beneath a dim clay crust, Vesta mantle, another sign that it formed under NEWS FEATURE veloped in the last 10 years, tells only part of is a bright and rocky world, riddled with different conditions than Vesta. However, the story. The Grand Tack model, proposed impact craters. Their differences suggest that right now, the notion that Ceres formed after in 2011 by Walsh et al., simulates the con- they formed under different circumstances— Vesta is just a hypothesis. “We believe Ceres ditions of the solar system leading up to the even in different places—yet they orbit at was a late starter, but we cannot prove that Nice model (6). According to the Grand similar distances from the sun. because we have no Ceres-identified mete- Tack,tensofmillionsofyearsbeforeEarth Dawn’s observations allowed Russell’steam orites,” says Russell. “Thus,wehavetovisit and the other inner planets formed, Jupiter to both map Vesta’s rough terrain and create Ceres to learn anything about it.” Dawn’s and Saturn migrated toward the sun—getting a “Vestan timescale,” a chronology of its four findings could go a long way toward showing as close as present-day Mars—and migrated major geological epochs. The team figured out that mixing of different asteroid types during outward again. The name of the model refers these epochs by dating three major high- the early phase of the solar system caused to a sailing maneuver, tacking, in which a boat energy impacts that created the largest craters Ceres and Vesta to end up in each other’s moves into the wind in a zigzag motion. on the asteroid (7). “Vesta’s existence and relative neighborhood. Computer simulations suggest that as Jupiter collisional history are consistent with giant And so, Dawn is headed toward Ceres. moved inward, its gravity hurled asteroids planet migration,” Russell says. After arriving, it will spend a few months toward the outer solar system. Then, as Jupiter And Dawn’s measurements of Vesta’s getting a high-altitude view before spiraling moved back outward its gravity repopulated mass, density, and gravitational field support down, closer and closer, to get an unprece- the asteroid belt with a mixture of rocks, in- the idea that Vesta is made of a rocky mantle dented view of a new world. Studies of the cluding some that had been there before and surrounding an iron core. Russell says Vesta alien world at different wavelengths will help some icy inhabitants from beyond Neptune. probably formed when the entire solar nebula researchers figure out the interior structure, “The Grand Tack introduces so much was suffused with aluminum-26, a radioac- map the minerals on its surface, and measure mixing of different populations [that] it gives tive isotope with a half-life of 700,000 years, its gravity. Those studies could help scientists us a comfortable explanation for how phys- likely left over from the nearby supernova discern Ceres’sbirthplaceandwhetheritwas ically different groups of asteroids could end that provided the oomph for the birth of the a rogue born beyond Neptune, drawn inward upinthesameasteroidbelt,” says Walsh. solar system. The Al-26 trapped inside Vesta by Jupiter’s travels. All these data will help Also, Jupiter’s in-and-out migration would provided enough heat to melt the interior to fine-tune our models of how the solar sys- have depleted the material needed for Mars iron, forming planet-like layers. In addition, tem formed. to accumulate and grow as big as Earth, minerals identified on the asteroid’ssurface Ceres won’t answer every open ques- giving Mars its distinctly smaller size. matched the composition of meteorites found tion. Understanding the birth of the solar But how accurate are the models? Long- on Earth, which scientists believe came system—and by extension, how life began haul missions to asteroids could offer some from the same powerful collisions that on Earth—will require even deeper knowl- indication. NASA’s planned OSIRIS-REx (Ori- carved out Vesta’s sprawling craters (8). Re- edge about the other worlds, big and small, gins Spectral Interpretation Resource Identifi- cent studies of Vesta, including analyses of says Russell, who also worked on the Cassini cation and Security-Regolith Explorer) mission, Dawn’s high-resolution data, point to a mission to Saturn and the European Space scheduled for an October 2016 launch, will scenario—supported by the Nice model— Agency’s recent Rosetta mission to a comet. visit an asteroid named Bennu. The space- in which these meteorites broke off Vesta Russell is already working on a new wish craft will study the asteroid from a distance during the high-energy collisions of the LHB. list: It includes visits to Hygiea, another large of three miles for six months before heading Argon-Argon dating suggests those mete- asteroid; Psyche, the iron asteroid that may down to the surface for a few seconds, long orites, and therefore Vesta, formed when the in fact be the exposed core of a protoplanet; enough to extend a robotic arm and collect solar system was only a few million years old. and Themis, which has ice on its surface. about enough material to fill a baseball. Astronomers think icy Ceres also formed in “There are a number of large asteroids Those few ounces, says Bottke, could reveal the system’s infancy, but later than Vesta or out there ready to share their secrets with where Bennu formed and whether or not it’s elsewhere in the system or both. Studies of us,” Russell says. In the next few months, the parent body for any meteorites found Ceres suggest it holds, in ice, more fresh Ceres will reveal some of its secrets to Dawn. on Earth. waterthanisfoundonEarth,asignthatit When the spacecraft runs out of fuel, it’ll had insufficient internal heat to melt the ice. continue to orbit Ceres, a small new sat- A Study in Contrasts Ceres likely has a rocky core beneath its icy ellite for a giant asteroid. Meanwhile, the Dawn mission has already visited one giant asteroid. In September 2012, the spacecraft wrapped up a 14-month stint 1 Gradie J, Tedesco E (1982) Compositional structure of the asteroid 5 Ferus M, et al. (2015) High-energy chemistry of formamide: A observing Vesta, which is the second-largest belt. Science 216(4553):1405–1407. unified mechanism of nucleobase formation. Proc Natl Acad Sci USA body in the asteroid belt, the brightest as- 2 Weidenschilling SJ (1977) The distribution of mass in the planetary 112(3):657–662. system and solar nebula. Astrophys Space Sci 51(1):153–158. 6 Walsh KJ, Morbidelli A, Raymond SN, O’Brien DP, Mandell AM teroid observable from Earth, and the densest 3 DeMeo FE, Carry B (2014) Solar System evolution from (2011) A low mass for Mars from Jupiter’s early gas-driven migration. object in the solar system. An estimated 5% compositional mapping of the asteroid belt. Nature 505(7485): Nature 475(7355):206–209. of the meteorites that land on Earth originated 629–634. 7 Williams DA, et al. (2014) The chronostratigraphy of protoplanet 4 Tsiganis K, Gomes R, Morbidelli A, Levison HF (2005) Origin of the Vesta. Icarus 244:158–165. from Vesta. Ceres and Vesta are a study in orbital architecture of the giant planets of the Solar System. Nature 8 Russell CT, et al. (2012) Dawn at Vesta: Testing the contrasts: whereas Ceres appears to hide an 435(7041):459–461. protoplanetary paradigm. Science 336(6082):684–686.

Ornes PNAS | March 31, 2015 | vol. 112 | no. 13 | 3851 Downloaded by guest on September 28, 2021