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Space Fossils NEWS FEATURE NEWS FEATURE News Feature: Space fossils How asteroid missions may help scientists piece together the puzzle of the early solar system. 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: Ceres, 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 asteroids like Ceres are time cap- a planetary scientist at the Southwest Research the mass of the entire asteroid belt that lies sules waiting to be cracked open. Their origin Institute in Boulder, Colorado. Asteroids are between the orbits of Mars and Jupiter. 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 sun. 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 planetesimals 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 Jupiters,” 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 planetary migration: 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, Saturn, from the sun among asteroids than had been formed much closer to the sun than they are Uranus, and Neptune 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 Moon, 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 Kuiper Belt. 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.
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