Image: Artist's conception, planet collision
Artist's conception showing a body about the size of Earth's moon slamming into a Mercury-size object. Such massive collisions may be relatively common in solar systems during the last stages of planet formation.
updated 12/9/2010 6:17:27 PM ET 2010-12-09T23:17:27

Gigantic collisions on Earth, the moon and Mars 4.5 billion years ago injected precious elements such as gold and platinum into the developing worlds, a new study suggests.

In the last days of planet formation, a body as big as Pluto likely slammed into Earth after the planet had been clobbered by a Mars-size object, researchers said. Mars and the moon absorbed smaller but still devastating blows, they added.

These traumatic cosmic crashes may have knocked Earth off its axis by 10 degrees. But they also delivered gold and other elements into the bodies' upper reaches, and possibly brought huge amounts of water to the moon, researchers said.

"We can use this information to tell us about the last great growth spurt of the Earth, moon and Mars," said study leader Bill Bottke of the Southwest Research Institute in Boulder, Colo. [ Photos: Impact Craters of the Solar System ]

The mystery of the plentiful platinum
Gold, platinum, palladium and other so-called "siderophile" elements have a strong affinity for iron. So they should have followed iron down into the cores of Earth, the moon and Mars as the bodies were forming, leaving a near void in their mantles and crusts, researchers said.

But siderophiles are found in these bodies' upper reaches in perplexing abundances.

"The abundance of these elements is actually surprisingly high," Bottke told "People have wondered, 'How can this be?' They've been arguing about it for decades."

One possibility is that the siderophiles were replenished shortly after core formation by collisions with planetesimals, the smaller building blocks of full-grown planets.

Bottke and his colleagues favor this explanation, and they marshal a variety of evidence in the new study to support it. They also put some numbers on how big the impacts likely were.

Modeling planet trauma
To account for present abundances of gold, platinum and the other siderophiles, researchers said, the impacts would need to deliver about 0.5 percent of Earth's mass to our planet's mantle, 10 times less mass than that to Mars and about 1,200 times less to our moon.

  1. Space news from
    1. KARE
      Teen's space mission fueled by social media

      Science editor Alan Boyle's blog: "Astronaut Abby" is at the controls of a social-media machine that is launching the 15-year-old from Minnesota to Kazakhstan this month for the liftoff of the International Space Station's next crew.

    2. Buzz Aldrin's vision for journey to Mars
    3. Giant black hole may be cooking up meals
    4. Watch a 'ring of fire' solar eclipse online

Using numerical models, the research team determined that this could happen if the impactors were dominated by a small number of huge space rocks.

"The populations that were hitting Earth, the moon and Mars were pretty top-heavy," Bottke said. "Most of the mass was in the big guys."

The results show that the largest Earth impactor was likely 1,500 to 2,000 miles wide. The biggest rocks to hit Mars and our moon were probably about 1,000 miles and 200 miles across, respectively, researchers said.

These big planetesimals may have delivered much of the moon's ample water along with the siderophiles, according to the study.

This bombardment came toward the end of our solar system's planet-formation period, around 4.5 billion years ago, researchers said. It likely took place within a few tens of millions of years of the most traumatic event in Earth's history: the collision with a Mars-size body that blasted a giant chunk off our planet, creating the moon.

Bottke and his colleagues published their findings online on Thursday in the journal Science.

Other evidence mounts
The results from the numerical model are buttressed by other evidence, researchers said.

For example, other models describing how planetesimals form and evolve suggest the biggest ones gobble up smaller ones very efficiently, leaving behind a top-heavy population. This same top-heaviness is seen in the asteroid belt, the last surviving population of planetesimals in the inner solar system, researchers said.

"The distribution of their sizes is actually remarkably consistent with what we'd expect based on our models," Bottke said.

A 1,000-mile-wide impactor is also about the right size to blast out Mars' enormous Borealis Basin if this ancient landform is indeed an impact crater, as some scientists think.

Taken together, the signs all point toward giant impacts pummeling the Earth, moon and Mars toward the end of planet formation, according to the researchers.

"We have a number of different lines of evidence to suggest that this is probably a real thing," Bottke said.

Is this violence typical?
The study paints a violent picture of the solar system's early days, with the inner planets being walloped heartily shortly after their birth. Such pummelings might be common in other newly forming solar systems, according to Bottke.

  1. Most popular

"Because we can reproduce some of these things in our models, I think that probably means we're fairly typical," he said. "But that's just a guess."

If our solar system is atypical if young planets in other solar systems were spared pummelings by massive impactors late in the formation process then alien planets might be different in significant ways, Bottke said. Their mantles, for example, might be lacking in siderophiles like gold and platinum, which we value for a variety of reasons.

In any event, Bottke said he hopes the study serves as a conversation starter, getting scientists thinking about the planet-formation process in a more general sense.

"If you can figure out how it worked here, you may be able to figure out how it worked elsewhere," Bottke said. "That's really exciting."

You can follow senior writer Mike Wall on Twitter: @michaeldwall.

© 2013 All rights reserved. More from

Gallery: The new solar system

Get the facts about dwarfs, giants, terrestrials and other denizens of our planetary realm.

Photos: Month in Space: January 2014

loading photos...
  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
  1. Editor's note:
    This image contains graphic content that some viewers may find disturbing.

    Click to view the image, or use the buttons above to navigate away.

  2. Editor's note:
    This image contains graphic content that some viewers may find disturbing.

    Click to view the image, or use the buttons above to navigate away.

  3. Editor's note:
    This image contains graphic content that some viewers may find disturbing.

    Click to view the image, or use the buttons above to navigate away.

  4. Editor's note:
    This image contains graphic content that some viewers may find disturbing.

    Click to view the image, or use the buttons above to navigate away.


Discussion comments


Most active discussions

  1. votes comments
  2. votes comments
  3. votes comments
  4. votes comments