Tweet
Bright 'hollows' on Mercury
are unique in solar system
It's the pits (Image: NASA/JHU APL/CIW)
Mercury is covered with pits that are unlike anything else in the solar
system, new observations from NASA's Messenger spacecraft show. They
may have been formed by processes still active today, and change our
view of the small rocky planet's history.
"The fact that these things are there at all is a big surprise," says David
Blewett of the Johns Hopkins University Applied Physics Laboratory in
Laurel, Maryland.
Messenger flew past Mercury three times before settling into orbit on 18
March. Those flybys – in 2008 and 2009 – revealed that some craters were
filled with unidentified bright stuff.
Now, after more than a Mercury year (88 Earth days) in orbit, Messenger's
high-resolution camera has revealed that the mysterious bright patches are
clusters of shallow, rimless, irregularly shaped pits.
Each such "hollow" is between tens of metres to a few kilometres across
and looks fresh and young – by planetary standards.
Spongy matrix
Other instruments on Messenger have found that Mercury's rocks contain a
lot more volatile elements than expected. Blewett thinks the hollows
formed when these elements – which are vaporised easily – were liberated
from the surface, leaving behind a spongy, fragile matrix of soil that then
collapsed.
Micrometeoroids and charged particles from the solar wind might have
vaporised volatiles in the rocks, triggering the collapses.
Volatiles might also have been concentrated in pockets by volcanic
eruptions that occurred during Mercury's long, frigid nights. In that case,
volatile volcanic gases from the eruptions could freeze solid and get buried
under lava flows. Later, a meteoroid impact could expose this material to
sunlight, causing it to evaporate and the surrounding rock to crumble.
Smack unlikely
In either case, a lot of volatile stuff seems to be hiding beneath the
surface of Mercury. That presents problems for theories that attempt to
explain the planet's high density – an enormous two-thirds of its mass is
made up of its metal core.
One theory argued that early on in the planet's life, the sun might have
vaporised part of its rocky exterior. Another suggested that a large
protoplanet smacked into it during its formation phase, blasting away its
outer layers. In both cases, the heating involved would have evaporated
volatile elements, allowing them to escape into space.
The fact that Messenger has found more volatiles than expected is
stumping theorists trying to model Mercury's past. "The people who think
about planetary-formation sorts of things are running back to their drawing
boards," Blewett says.
Bright 'hollows' on Mercury are unique in solar system - space - 29 September 2011 - New Scientist
are unique in solar system
It's the pits (Image: NASA/JHU APL/CIW)
Mercury is covered with pits that are unlike anything else in the solar
system, new observations from NASA's Messenger spacecraft show. They
may have been formed by processes still active today, and change our
view of the small rocky planet's history.
"The fact that these things are there at all is a big surprise," says David
Blewett of the Johns Hopkins University Applied Physics Laboratory in
Laurel, Maryland.
Messenger flew past Mercury three times before settling into orbit on 18
March. Those flybys – in 2008 and 2009 – revealed that some craters were
filled with unidentified bright stuff.
Now, after more than a Mercury year (88 Earth days) in orbit, Messenger's
high-resolution camera has revealed that the mysterious bright patches are
clusters of shallow, rimless, irregularly shaped pits.
Each such "hollow" is between tens of metres to a few kilometres across
and looks fresh and young – by planetary standards.
Spongy matrix
Other instruments on Messenger have found that Mercury's rocks contain a
lot more volatile elements than expected. Blewett thinks the hollows
formed when these elements – which are vaporised easily – were liberated
from the surface, leaving behind a spongy, fragile matrix of soil that then
collapsed.
Micrometeoroids and charged particles from the solar wind might have
vaporised volatiles in the rocks, triggering the collapses.
Volatiles might also have been concentrated in pockets by volcanic
eruptions that occurred during Mercury's long, frigid nights. In that case,
volatile volcanic gases from the eruptions could freeze solid and get buried
under lava flows. Later, a meteoroid impact could expose this material to
sunlight, causing it to evaporate and the surrounding rock to crumble.
Smack unlikely
In either case, a lot of volatile stuff seems to be hiding beneath the
surface of Mercury. That presents problems for theories that attempt to
explain the planet's high density – an enormous two-thirds of its mass is
made up of its metal core.
One theory argued that early on in the planet's life, the sun might have
vaporised part of its rocky exterior. Another suggested that a large
protoplanet smacked into it during its formation phase, blasting away its
outer layers. In both cases, the heating involved would have evaporated
volatile elements, allowing them to escape into space.
The fact that Messenger has found more volatiles than expected is
stumping theorists trying to model Mercury's past. "The people who think
about planetary-formation sorts of things are running back to their drawing
boards," Blewett says.
Bright 'hollows' on Mercury are unique in solar system - space - 29 September 2011 - New Scientist