A new study of an old meteorite contradicts current thinking about how rocky planets like the Earth and Mars acquire volatile elements such as hydrogen, carbon, oxygen, nitrogen and noble gases as they form. The work is published June 16 in Science.
From: University of California - Davis
June 16, 2022 -- A basic
assumption about planet formation is that planets first collect these volatiles
from the nebula around a young star, said Sandrine Péron, a postdoctoral
scholar working with Professor Sujoy Mukhopadhyay in the Department of Earth
and Planetary Sciences, University of California, Davis.
Because the planet is a
ball of molten rock at this point, these elements initially dissolve into the
magma ocean and then degass back into the atmosphere. Later on, chondritic
meteorites crashing into the young planet deliver more volatile materials.
So scientists expect
that the volatile elements in the interior of the planet should reflect the
composition of the solar nebula, or a mixture of solar and meteoritic
volatiles, while the volatiles in the atmosphere would come mostly from
meteorites. These two sources -- solar vs. chondritic -- can be distinguished
by the ratios of isotopes of noble gases, in particular krypton.
Mars is of special
interest because it formed relatively quickly -- solidifying in about 4 million
years after the birth of the Solar System, while the Earth took 50 to 100
million years to form.
"We can
reconstruct the history of volatile delivery in the first few million years of
the Solar System," Péron said.
Meteorite from Mars'
interior
Some meteorites that
fall to Earth come from Mars. Most come from surface rocks that have been
exposed to Mars' atmosphere. The Chassigny meteorite, which fell to Earth in
north-eastern France in 1815, is rare and unusual because it is thought to
represent the interior of the planet.
By making extremely
careful measurements of minute quantities of krypton isotopes in samples of the
meteorite using a new method set up at the UC Davis Noble Gas Laboratory, the
researchers could deduce the origin of elements in the rock.
"Because of their
low abundance, krypton isotopes are challenging to measure," Péron said.
Surprisingly, the
krypton isotopes in the meteorite correspond to those from chondritic
meteorites, not the solar nebula. That means that meteorites were delivering
volatile elements to the forming planet much earlier than previously thought,
and in the presence of the nebula, reversing conventional thinking.
"The Martian
interior composition for krypton is nearly purely chondritic, but the
atmosphere is solar," Péron said. "It's very distinct."
The results show that
Mars' atmosphere cannot have formed purely by outgassing from the mantle, as
that would have given it a chondritic composition. The planet must have
acquired atmosphere from the solar nebula, after the magma ocean cooled, to
prevent substantial mixing between interior chondritic gases and atmospheric
solar gases.
The new results suggest
that Mars' growth was completed before the solar nebula was dissipated by
radiation from the Sun. But the irradiation should also have blown off the
nebular atmosphere on Mars, suggesting that atmospheric krypton must have
somehow been preserved, possibly trapped underground or in polar ice caps.
"However, that
would require Mars to have been cold in the immediate aftermath of its
accretion," Mukhopadhyay said. "While our study clearly points to the
chondritic gases in the Martian interior, it also raises some interesting
questions about the origin and composition of Mars' early atmosphere."
Péron and Mukhopadhyay
hope their study will stimulate further work on the topic.
Péron is now a
postdoctoral fellow at ETH Zürich, Switzerland.
https://www.sciencedaily.com/releases/2022/06/220616141516.htm
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