Wednesday, December 22, 2010

[Geology2] Our gold was a crash delivery from space, study finds

I love when that happens . . .

Our gold was a crash delivery from space, study finds

Gold prices seem otherworldly these daysâ€"which may be fitting in
light of a new study that said almost all our supplies of the coveted
metal probably crashed into Earth from space long ago. The study
suggests gold, platinum, palladium and related elements found in the
crusts and mantles of Earth, the Moon and Mars arrived as part of
impactors the size of small planets during the last phase of planet
formation in our solar system, some 4.5 billion years ago. The
collisions occurred within tens of millions of years of an even bigger
impact that produced our Moon, say the researchers, whose findings are
published in this week’s issue of the research journal Science.

Current understandings of Earth and planets of similar makeup suggest
our world should have “essentially no gold” that we can
get at, said University of Maryland Geology Professor Richard Walker,
one of the authors. That’s because gold and several other
precious metals are among a class of elements that are usually found in
combination with iron, which forms the core of Earth.
“Iron-loving elements are pulled into the planet cores as they
form,” Walker explained, so the yellow metal should have been
too deep for us to reach. Since this isn’t the case, Walker
said, something probably happened to bring these elements to Earth late
in its formation, after its layers finished separating. What scientists
didn’t know until now, he added, was whether this bombardment
occurred in big chunks over a relatively short time or as a
“rain” of smaller pieces of material over a longer time.

Walker and colleagues used computer models to assess what size objects
would best match the needed criteria. These included providing the right
amount of iron-loving metals to the Earth, Moon and Mars; being large
enough to breach the crusts and mantles of these bodies, creating local
molten rock ponds from their impact energy and efficiently mixing into
the mantle; and not being so large as to cause the cores to fragment and
re-form. That would cause the new gold to follow the old into the
unreachable depths.
The researchers found that they could best meet the criteria if the
impactors were few and massive. The largest Earth impactor should have
been about 1,500 to 2,000 miles wide (2,400 to 3,200 km), roughly
Pluto’s size; impactors hitting the Moon would have been around
a tenth as wide. “These impactors are thought to be large enough
to produce the observed enrichments in highly siderophile [iron-loving]
elements, but not so large that their fragmented cores joined with the
planet’s core,” said William Bottke of the Southwest
Research Institute in San Antonio, Texas, the lead author of the paper.
The team also maintains their predicted projectile sizes are consistent
with physical evidence such as the size distributions of today’s
asteroids and of ancient Martian impact scars.


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