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 SPACE.com. [Our Solar System: A Photo Tour of the 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."
"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."
"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."
"We're certainly interested in looking at other bodies to see what they tell us about these early processes," Dale said.
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