Size mattered,
astronomers say, when it came to whether or not material in our early solar
system stuck together to become todays terrestrial planets Mercury,
Venus, Earth and Mars. New models suggest that collisions between large objects
did not always result in those objects combining, as previously thought.
The Mariner spacecraft captured this image
of Mercurys surface during a 1974 flyby of the planet. The diameter of
Mercury is one-third the diameter of Earth, and some new models suggest that
in its late stage of formation, the planet may have lost much of its mantle
in an impact with a larger object. Image is courtesy of NASA.
Erik Asphaug, an astronomer at the University of California in Santa Cruz, suggests
in the Jan. 12 Nature that in some large collisions, the smaller of the
two objects escaped back into the solar system in what he calls a hit
and run event but not before the escaping body withstood violent
deformation from the encounter. Such changes, Asphaug and colleagues say, might
account for some of the unexplained compositions observed in local planets and
asteroids.
Before the planets achieved their current sizes and orbits, hundreds of moon-
and Mars-sized objects roamed the solar system, and over a period of 10 million
to 100 million years, the objects occasionally crossed paths. In past models,
researchers coded computer simulations to merge objects together at every collision,
Asphaug says, but problems exist for that scenario. For example, if all objects
that met were to have merged, the models show that planets would end up rotating
so fast that any planet would fling itself to death, Asphaug says.
Asphaug does not contest the widely accepted theory that planets formed from
accumulations of smaller planets. Direct hits, he says, would have stopped a
planet dead in its tracks, and resulted in a merge. But direct hits
between two large objects were likely pretty rare, he says, as most
collided at angles. About half of all collisions probably allowed the smaller
planet to escape, Asphaug says, although not without serious physical consequences.
To picture what a collision event would look like, Asphaug says to consider
what a human sitting on Mars would see if a large rogue object struck Earth.
A few hours before impact, Earths tidal forces would stretch the impactor
into a football shape and then into a cigar shape. Those forces could tear off
the impactors mantle, crust and atmosphere if it had one, and could also
melt it, reshape it and even tear it to pieces, all in a matter of hours
severely altering the would-be planet or even reducing it to cosmic debris.
Sudden pressure drops within the mantle of an impactor could also cause it to
explosively shed its volatiles, or de-gas, which would explain the
volcanic-like features found on many asteroids.
William Bottke, an astronomer at the Southwest Research Institute in Boulder,
Colo., calls Asphaugs model intriguing, and says that it may
help answer specific questions about the asteroid belt a region beyond
Mars containing asteroids and debris that never formed into planets. Iron-rich
asteroids that are abundant there, for example, could be the remnants of failed
planets, impactors from which the mantle and crust were stripped, leaving
mostly the iron core.
The process is not 100 percent efficient at stripping bodies to
their core, Bottke says. Asteroid 4 Vesta, for example, is a 500-kilometer intact
asteroid in the asteroid belt (located between Mars and Jupiter), complete with
a mantle and crust. Partial stripping might also account for Mercurys
thin mantle, if in fact it had been an impactor at some point in its history,
Asphaug says.
Also, Asphaugs model only applies to large bodies. Something tiny, like
the 10-kilometer asteroid that struck earth 65 million years ago (and likely
led to the extinction of the dinosaurs), will either directly impact a planet
and stick, miss the planet entirely, or graze past it, he says. But for larger
bodies, the model acts as a recipe for becoming more volatile-rich,
and smaller planets becoming more barren, Asphaug says. Thats
what we see with Mercury and Mars.
Researchers, Asphaug says, still need to look at debris stranded in the asteroid
belt, where he says he thinks there should be a solid record for
the hit and run model. Bottke agrees and says that Asphaugs model for
larger planets could be readily incorporated into his own research,
which, published in the Feb. 16 Nature, suggests that small bodies now
in the asteroid belt likely formed closer to the sun alongside the terrestrial
planets. Both models together, Bottke says, probably work in concert to
produce just the right mix of materials actually observed in the asteroid
belt.
Kathryn Hansen
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