Scientists believe that shortly after Earth was formed, it had a
glowing surface of molten rock extending down hundreds of miles. As
that surface cooled, a rigid crust was produced near the surface and
solidified slowly downward to complete the now-solid planet. Some
scientists have suggested that Earth lost all of its initial gases,
either during the molten stage or as a consequence of a massive
collision, and that the catastrophically expelled gases formed our
early atmosphere and oceans. Others contend that this early
�degassing� was incomplete, and that primordial gases still remain
sequestered at great depth to this day. Watson�s new results support
this latter theory.
�For the �deep-sequestration� theory to be correct, certain gases
would have to avoid escape to the atmosphere in the face of mantle
convection and volcanism,� Watson said. �Our data suggest that argon
does indeed stay trapped in the mantle even at extremely high
temperatures, making it difficult for the Earth to continuously purge
itself of argon produced by radioactive decay of potassium.�
Argon and other noble gases are tracer elements for scientists because
they are very stable and do not change over time, although certain
isotopes accumulate through radioactive decay. Unlike more promiscuous
elements such as carbon and oxygen, which are constantly bonding and
reacting with other elements, reliable argon and her sister noble
gases (helium, neon, krypton, and xenon) remain virtually unchanged
through the ages. Its steady personality makes argon an ideal marker
for understanding the dynamics of Earth�s interior.
�By measuring the behavior of argon in minerals, we can begin to
retrace the formation of Earth�s atmosphere and understand how and if
complete degassing has occurred,� Watson explained.
Watson�s team, which includes Rensselaer postdoctoral researcher Jay
B. Thomas and research professor Daniele J. Cherniak, developed reams
of data in support of their emerging belief that argon resides stably
in crystals and migrates slowly. �We realized from our initial results
that these ideas might cause a stir,� Watson said. �So we wanted to
make sure that we had substantial data supporting our case.�
The team heated magnesium silicate minerals found in Earth�s mantle,
which is the region of Earth sandwiched between the upper crust and
the central core, in an argon atmosphere. They used high temperature
to simulate the intense heat deep within the Earth to see whether and
how fast the argon atoms moved into the minerals. Argon was taken up
by the minerals in unexpectedly large quantities, but at a slow rate.
�The results show that argon could stay in the mantle even after being
exposed to extreme temperatures,� Watson said. �We can no longer
assume that a partly melted region of the mantle will be stripped of
all argon and, by extension, other noble gases.�
But there is some argon in our atmosphere -
slightly less than 1 percent. If it didn�t shoot through the
rocky mantle, how did it get into the atmosphere".
�We proposed that argon�s release to the atmosphere is through the
weathering of the upper crust and not the melting of the mantle,�
Watson said. �The oceanic crust is constantly being weathered by ocean
water and the continental crust is rich in potassium, which decays to
form argon.�
And what about the primordial argon that was trapped in the Earth
billions of years ago" �Some of it is probably still down there,�
Watson said.
Because Mars and Venus have mantle materials similar to those found on
Earth, the theory could be key for understanding their atmospheres as
well.
Watson and his team have already begun to test their theories on other
noble gases, and they foresee similar results. �We may need to start
reassessing our basic thinking on how the atmosphere and other
large-scale systems were formed,� he said.
The research was funded by the National Science Foundation.
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