Rosetta mission image of the asteroid 21 Lutetia. Image: ESA |
On July 10, 2010, the European Space Agency’s Rosetta probe
flew by the asteroid 21 Lutetia, which at the time was the largest asteroid
ever to have been visited by a spacecraft. The fly-by occurred 282 million
miles from Earth; close-up images taken by the probe revealed cracks and
craters running across Lutetia’s surface, evidence of the asteroid’s long and
battered history.
Now an international team of researchers from France, Germany,
the Netherlands, and the United States
has analyzed Lutetia’s surface images, and found that underneath its cold and
cracked exterior, the asteroid may in fact have once harbored a molten-hot,
metallic core. The findings suggest that Lutetia, despite billions of years of
impacts, may have retained its original structure—a preserved remnant of the
very earliest days of the solar system.
The results are published in a series of three papers in the
journals Science and Planetary Space Science (PSS).
Benjamin Weiss, an associate professor of planetary sciences
in Massachusetts Institute of Technology’s (MIT’s) Department of Earth,
Atmospheric, and Planetary Sciences, says a melted core within Lutetia may
exemplify a “hidden diversity” within the greater asteroid belt.
“There might be many bodies that have cores and interesting
interiors that we never noticed, because they’re covered by unmelted surfaces,”
says Weiss, who is a coauthor on both Science papers and lead author for
the paper in PSS. “The asteroid belt may be more interesting than it
seems on the surface.”
More than a rubble pile
Most asteroids careening through the asteroid belt, between the orbits of Mars
and Jupiter, are scrambled versions of their former selves: essentially
mashed-up masses of rock and metal that have collided and cooled over billions
of years. These rocky conglomerations are relatively small and light, with
voids and cracks in their interiors that make them very porous. It had been
thought that the vast majority of these bodies never melted to form dense,
metallic cores, but instead are just primordial piles of space rocks and dust.
In contrast, the Rosetta team—led by Holger Sierks of the
Max-Planck Institute for Solar System Research and Martin Pätzold of the
Rheinisches Institut für Umweltforschung, both in Germany—found that Lutetia is
extremely dense. The team drew up a model of the asteroid’s shape, based on
images taken by the Rosetta probe. The researchers then calculated Lutetia’s
volume, mass, and finally its density, which they found, in collaboration with
the MIT team, to be greater than most meteorite samples measured on Earth.
The asteroid’s density would make sense if it were
completely solid, free of voids or cracks. However, Rosetta researchers
measured the asteroid’s surface craters and identified huge fractures
throughout, suggesting the asteroid is relatively porous, a finding that didn’t
quite square with the team’s density measurements—after all, the more porous an
object, the less dense it should be.
Weiss and his colleagues, including MIT professor Richard
Binzel and former MIT professor Linda T. Elkins-Tanton, now head of the
Carnegie Institution for Science’s Department of Terrestrial Magnetism, offered
a likely explanation for the discrepancy: Perhaps the space rock contains a
dense metallic core, with a once melted interior underneath its fractured
crust.
The direct observations from Lutetia may provide evidence
for a theory developed last year by Weiss, Elkins-Tanton, and MIT’s Maria
Zuber. The team studied samples of chondrites, meteorites on Earth that have
remained unchanged since their early formation. They found samples from the
meteorite Allende that were strongly magnetized, and theorized that such
magnetization most likely occurred in an asteroid with a melted, metallic core.
The theory was seen as a big shift from the traditional picture of most
asteroids as primordial, unmelted objects.
Planetary arrested development
If a metallic core does indeed exist, Lutetia would be the first asteroid known
to be partially differentiated: having a melted interior overlain by
progressively cooler layers. The asteroid would also represent a snapshot of
early planetary development.
As the solar system began to take form 4.5 billion years
ago, planets formed from collisions first of dust, then of larger chunks of
rock. Numerous chunks remained relatively small, cooling quickly to form
asteroids, while others grew with each collision, eventually reaching the size
of planets. These large bodies generated an immense amount of heat—but as a new
planet melted from the inside, it cooled from the outside, forming a crust
around a molten core.
According to Weiss, Lutetia is a case of arrested
development. The asteroid may have reached a size large enough to develop and
retain a melting core, and then simply avoided the larger collisions that
accelerated planet formation.
“The planets … don’t retain a record of these early
differentiation processes,” Weiss says. “So this asteroid may be a relic of the
first events of melting in a body.”
Erik Asphaug, a professor of planetary science at the University of California
at Santa Cruz,
studies “hit-and-run” collisions between early planetary bodies. He says the
work by Weiss and his colleagues is a solid step toward resolving how certain
asteroids like Lutetia may have evolved.
“We’ve had decades of cartoon speculation, and here’s
speculation that’s anchored in physical understanding of how the interiors of
these bodies would evolve,” says Asphaug, who was not involved in the research. “It’s like getting through the first 100 pages of a novel, and you don’t know
where it’s leading, but it feels like the beginnings of a coherent picture.”
Weiss says while the images and measurements of Lutetia are
intriguing evidence for a partially differentiated asteroid, a “smoking gun”
could be provided by a sample taken directly from an asteroid. Binzel and Weiss
are part of a NASA team that plans to launch a probe to an asteroid in 2016,
which will take a sample and return it to Earth.
Weiss says there are a number of hurdles he and his
colleagues will have to surmount before obtaining definitive evidence for a
molten core.
“The challenge is, the body has to be big,” Weiss says. “If
it’s not big, then it’s not going to retain a molten interior. The problem then
is, all the big bodies are not going to be easily excavated. So it’s sort of a
Catch-22.”
