Compositional X-ray image of the rim and margin of a ~4.6 billion year old calcium aluminum refractory inclusion (CAI) from the Allende carbonaceous chondrite. Core extending well beyond the field of view to the upper left consists of melilite, spinel and perovskite. Rim consists of a sequence of mono-mineral layers a few micrometers thick (hibonite, perovskite, spinel, melilite/sodalite, pyroxene, and olivine). A spinel-rich micro-inclusion appears to have been entrapped while the rim was forming. Credit: Lawrence Livermore National Laboratory |
Scientists
have found that calcium, aluminum-rich inclusions (CAIs), some of the oldest
objects in the solar system, formed far away from our sun and then later fell
back into the mid-plane of the solar system.
The
findings may lead to a greater understanding of how our solar system and
possibly other solar systems formed and evolved.
CAIs,
roughly millimeter to centimeter in size, are believed to have formed very
early in the evolution of the solar system and had contact with nebular gas,
either as solid condensates or as molten droplets. Relative to planetary
materials, CAIs are enriched with the lightest oxygen isotope and are believed
to record the oxygen composition of solar nebular gas where they grew. CAIs, at
4.57 billion years old, are millions of years older than more modern objects in
the solar system, such as planets, which formed about 10 to 50 million years
after CAIs.
Using
Lawrence Livermore’s NanoSIMS (nanometer-scale secondary-ion mass spectrometer),
LLNL scientists in conjunction with NASA
Johnson Space
Center; Univ.
of California, Berkeley;
and the Univ. of Chicago
measured the concentrations of oxygen isotopes found in the CAIs.
In
the recent research, the team studied a specific CAI found in a piece of the
Allende meteorite. Allende is the largest carbonaceous chondrite meteorite ever
found on Earth. It fell to the ground in 1969 over the Mexican state of Chihuahua and is notable
for possessing abundant CAIs.
Their
findings imply that CAIs formed from several oxygen reservoirs, likely located
in distinct regions of the solar nebula. CAIs travelled within the nebula by
lofting outward away from the sun and then later falling back into the
mid-plane of the solar system or by spiraling through shock waves around the
sun.
Through
oxygen isotopic analysis, the team found that rims surrounding the CAI show that
late in the CAI’s evolution, it was in a nebular environment distinct from
where it originated and closer in composition to the environment in which the
building materials of the terrestrial planets formed.
“Allende
is this very unusual meteorite with all these wonderful inclusions
(CAIs),” said Ian Hutcheon, one of the LLNL scientists on the team.
“The isotopic measurements indicate that this CAI was transported among
several different nebular oxygen isotopic reservoirs, arguably as it passed
through into various regions of the protoplanetary disk.”
A
protoplanetary disk is an area of dense gas surrounding a newly formed star. In
this case, the CAI formed when our star was quite young.
“It
is particularly interesting in understanding the formation and dynamics of our
solar system’s protoplanetary disk (and protoplanetary disks in general),”
said Justin Simon of NASA
Johnson Space
Center and lead author of
a paper appearing in Science.
The
new observations, “support early and short-lived fluctuations of the
environment in which CAIs formed, either due to transport of the CAIs
themselves to distinct regions of the solar nebula or because of varying gas
composition near the proto-sun”, Hutcheon said.
Other
Livermore researchers
include Jennifer Matzel, Erick Ramon, and Peter Weber.