Image: 9,500 miles from Vesta
NASA's Dawn spacecraft obtained this image with its framing camera on July 17, 2011. It was taken from a distance of about 9,500 miles away from the protoplanet Vesta. Each pixel in the image corresponds to roughly 0.88 miles.
updated 4/5/2012 7:07:02 PM ET 2012-04-05T23:07:02

Shortly after Earth and Mars were born, they found themselves in a lengthy bout of cosmic bumper cars with comets and space rocks. A new study now suggests the asteroids of the inner solar system were also subjected to such impacts.

An international team of astronomers analyzed the chemical compositions of Vesta and several other asteroids and found "highly siderophile elements" — chemicals that bind tightly to iron  not only in the cores of the space rocks but in their mantles.

The presence of the iron-loving elements outside the core suggests they were deposited there by impacts with other bodies after the asteroids had formed, but still early in the history of the solar system, said study lead author Christopher Dale, a postdoctoral researcher at England's Durham University.

"This process of late accretion is recognized and well-accepted for Earth, the moon and Mars, but it wasn't clear if it was a process that was widespread," Dale told [ Our Solar System: A Photo Tour of the Planets ]

The detailed results of the study are published in the April 6 issue of the journal Science.

Building blocks of a planet
Scientists think planets are formed by a process known as core accretion. As giant disks of gas and dust are swirling around nascent stars, the grains of dust bond together to form objects called planetesimals that collide and stick together to make even larger clumps of material. Once these emerging bodies accumulate enough mass, their gravity begins to suck in gas and they eventually become planets.

Since siderophile elements bind tightly to iron, any that were present in the early stages of a planet's formation would have been pulled into the body's iron core. So the presence of these iron-loving elements in the mantles of Earth, the moon and Mars must have been delivered later, after the process of core formation ceased.

The asteroids in the inner solar system — including Vesta, which is large enough that many researchers call it a protoplanet – were done forming in less time than the planets, with their cores accreting at lower pressures and temperatures.

"Within the first few million years, but certainly 10 million years after the start of the solar system, these bodies had accreted and formed their cores," Dale said. For "a planetesimal like Vesta, during core formation at lower pressure and temperature, we'd expect almost all of the highly siderophile elements in the core. But that's not what we find."

Instead the researchers also found highly siderophile elements in the mantles, indicating that space rock impacts were not unique to larger planets and moons. The findings also suggest this process lasted longer than thought.

"It tells us that the process of accretion was certainly not a finite event; it continued for many millions of years," Dale said. "There also must have been lots of small or medium-size bodies present in the solar system for these collisions to have occurred over a range of time scales."

Shaking up the solar system
Astronomers think that about 600 million years after the solar system was formed (or about 4 billion years ago), a vast expanse of space beyond the orbit of Neptune, called the Kuiper Belt, was shaken up by the migration of the gas giants Jupiter and Saturn.

This gravitational disruption scattered comets and other icy bodies in the Kuiper Belt, flinging many into interstellar space but also throwing some onto orbital paths that wreaked havoc on the inner planets of the solar system.

This period, called the Late Heavy Bombardment, lasted until 3.8 billion years ago, during which time comets pummeled the side of the moon that faces Earth and created the contrasting light and dark patches on the lunar surface that are seen today.

Comets that hit Earth during the Late Heavy Bombardment were thought to have deposited water and carbon on the planet, astronomers have said.

But the results of the new study suggest that a lengthy round of impacts preceded the Late Heavy Bombardment. This would account for the highly siderophile elements seen on Earth, the moon, Mars and early asteroids.

"We're not relating the Late Heavy Bombardment to the increase in these highly siderophile elements," Dale said. "What this study shows is that the vast increase was probably prior to the Late Heavy Bombardment. I'm not sure that the amount of material in the Late Heavy Bombardment is great enough to explain highly siderophile elements on Earth, so much of the material was probably derived from fairly large impacts early on in its history."

The researchers plan to continue studying other bodies in the solar system to build upon these findings.

"We're certainly interested in looking at other bodies to see what they tell us about these early processes," Dale said.

You can follow staff writer Denise Chow on Twitter @denisechow. Follow for the latest in space science and exploration news on Twitter @Spacedotcomand on Facebook.

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Photos: Month in Space: January 2014

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  1. Southern stargazing

    Stars, galaxies and nebulas dot the skies over the European Southern Observatory's La Silla Paranal Observatory in Chile, in a picture released on Jan. 7. This image also shows three of the four movable units that feed light into the Very Large Telescope Interferometer, the world's most advanced optical instrument. Combining to form one larger telescope, they are greater than the sum of their parts: They reveal details that would otherwise be visible only through a telescope as large as the distance between them. (Y. Beletsky / ESO) Back to slideshow navigation
  2. A balloon's view

    Cameras captured the Grandville High School RoboDawgs' balloon floating through Earth's upper atmosphere during its ascent on Dec. 28, 2013. The Grandville RoboDawgs’ first winter balloon launch reached an estimated altitude of 130,000 feet, or about 25 miles, according to coaches Mike Evele and Doug Hepfer. It skyrocketed past the team’s previous 100,000-feet record set in June. The RoboDawgs started with just one robotics team in 1998, but they've grown to support more than 30 teams at public schools in Grandville, Mich. (Kyle Moroney / AP) Back to slideshow navigation
  3. Spacemen at work

    Russian cosmonauts Oleg Kotov, right, and Sergey Ryazanskiy perform maintenance on the International Space Station on Jan. 27. During the six-hour, eight-minute spacewalk, Kotov and Ryazanskiy completed the installation of a pair of high-fidelity cameras that experienced connectivity issues during a Dec. 27 spacewalk. The cosmonauts also retrieved scientific gear outside the station's Russian segment. (NASA) Back to slideshow navigation
  4. Special delivery

    The International Space Station's Canadian-built robotic arm moves toward Orbital Sciences Corp.'s Cygnus autonomous cargo craft as it approaches the station for a Jan. 12 delivery. The mountains below are the southwestern Alps. (NASA) Back to slideshow navigation
  5. Accidental art

    A piece of art? A time-lapse photo? A flickering light show? At first glance, this image looks nothing like the images we're used to seeing from the Hubble Space Telescope. But it's a genuine Hubble frame that was released on Jan. 27. Hubble's team suspects that the telescope's Fine Guidance System locked onto a bad guide star, potentially a double star or binary. This caused an error in the tracking system, resulting in a remarkable picture of brightly colored stellar streaks. The prominent red streaks are from stars in the globular cluster NGC 288. (NASA / ESA) Back to slideshow navigation
  6. Supersonic test flight

    A camera looking back over Virgin Galactic's SpaceShipTwo's fuselage shows the rocket burn with a Mojave Desert vista in the background during a test flight of the rocket plane on Jan. 10. Cameras were mounted on the exterior of SpaceShipTwo as well as its carrier airplane, WhiteKnightTwo, to monitor the rocket engine's performance. The test was aimed at setting the stage for honest-to-goodness flights into outer space later this year, and eventual commercial space tours.

    More about SpaceShipTwo on PhotoBlog (Virgin Galactic) Back to slideshow navigation
  7. Red lagoon

    The VLT Survey Telescope at the European Southern Observatory's Paranal Observatory in Chile captured this richly detailed new image of the Lagoon Nebula, released on Jan. 22. This giant cloud of gas and dust is creating intensely bright young stars, and is home to young stellar clusters. This image is a tiny part of just one of 11 public surveys of the sky now in progress using ESO telescopes. (ESO/VPHAS team) Back to slideshow navigation
  8. Fire on the mountain

    This image provided by NASA shows a satellite view of smoke from the Colby Fire, taken by the Multi-angle Imaging SpectroRadiometer aboard NASA's Terra spacecraft as it passed over Southern California on Jan. 16. The fire burned more than 1,863 acres and forced the evacuation of 3,700 people. (NASA via AP) Back to slideshow navigation
  9. Where stars are born

    An image captured by NASA's Spitzer Space Telescope shows the Orion Nebula, an immense stellar nursery some 1,500 light-years away. This false-color infrared view, released on Jan. 15, spans about 40 light-years across the region. The brightest portion of the nebula is centered on Orion's young, massive, hot stars, known as the Trapezium Cluster. But Spitzer also can detect stars still in the process of formation, seen here in red hues. (NASA / JPL-Caltech) Back to slideshow navigation
  10. Cygnus takes flight

    Orbital Sciences Corp.'s Antares rocket rises from NASA's Wallops Flight Facility on Wallops Island, Va, on Jan. 9. The rocket sent Orbital's Cygnus cargo capsule on its first official resupply mission to the International Space Station. (Chris Perry / NASA) Back to slideshow navigation
  11. A long, long time ago...

    This long-exposure picture from the Hubble Space Telescope, released Jan. 8, is the deepest image ever made of any cluster of galaxies. The cluster known as Abell 2744 appears in the foreground. It contains several hundred galaxies as they looked 3.5 billion years ago. Abell 2744 acts as a gravitational lens to warp space, brightening and magnifying images of nearly 3,000 distant background galaxies. The more distant galaxies appear as they did more than 12 billion years ago, not long after the Big Bang. (NASA / NASA via AFP - Getty Images) Back to slideshow navigation
  12. Frosty halo

    Sun dogs are bright spots that appear in the sky around the sun when light is refracted through ice crystals in the atmosphere. These sun dogs appeared on Jan. 5 amid brutally cold temperatures along Highway 83, north of Bismarck, N.D. The temperature was about 22 degrees below zero Fahrenheit, with a 50-below-zero wind chill.

    Slideshow: The Year in Space (Brian Peterson / The Bismarck Tribune via AP) Back to slideshow navigation
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