Earth and Mars were formed from inner Solar System material
International research team investigated the isotopic composition of
rocky planets in the inner Solar System

Date:
December 22, 2021
Source:
University of Mu"nster
Summary:
Cosmochemists now present the most comprehensive comparison to date
of the isotopic composition of Earth, Mars and pristine building
material from the inner and outer Solar System.



FULL STORY ========================================================================== Earth and Mars were formed from material that largely originated in the
inner Solar System; only a few percent of the building blocks of these two planets originated beyond Jupiter's orbit. A group of researchers led by
the University of Mu"nster (Germany) report these findings today in the
journal Science Advances. They present the most comprehensive comparison
to date of the isotopic composition of Earth, Mars and pristine building material from the inner and outer Solar System. Some of this material
is today still found largely unaltered in meteorites. The results of
the study have far-reaching consequences for our understanding of the
process that formed the planets Mercury, Venus, Earth, and Mars. The
theory postulating that the four rocky planets grew to their present
size by accumulating millimeter-sized dust pebbles from the outer Solar
System is not tenable.


========================================================================== Approximately 4.6 billion years ago in the early days of our Solar System,
a disk of dust and gases orbited the young Sun. Two theories describe
how in the course of millions of years the inner rocky planets formed
from this original building material. According to the older theory,
the dust in the inner Solar System agglomerated to ever larger chunks
gradually reaching approximately the size of our Moon. Collisions of
these planetary embryos finally produced the inner planets Mercury,
Venus, Earth, and Mars. A newer theory, however, prefers a different
growth process: millimeter-sized dust "pebbles" migrated from the outer
Solar System towards the Sun. On their way, they were accreted onto the planetary embryos of the inner Solar System, and step by step enlarged
them to their present size.

Both theories are based on theoretical models and computer simulations
aimed at reconstructing the conditions and dynamics in the early Solar
System; both describe a possible path of planet formation. But which one
is right? Which process actually took place? To answer these questions, in their current study researchers from the University of Mu"nster (Germany),
the Observatoire de la Cote d'Azur (France), the California Institute
of Technology (USA), the Natural History Museum Berlin (Germany), and
the Free University of Berlin (Germany) determined the exact composition
of the rocky planets Earth and Mars. "We wanted to find out whether the building blocks of Earth and Mars originated in the outer or inner Solar System," says Dr. Christoph Burkhardt of the University of Mu"nster,
the study's first author. To this end, the isotopes of the rare metals titanium, zirconium and molybdenum found in minute traces in the outer, silicate-rich layers of both planets provide crucial clues. Isotopes
are different varieties of the same element, which differ only in the
weight of their atomic nucleus.

Meteorites as a reference Scientists assume that in the early Solar System these and other metal isotopes were not evenly distributed. Rather,
their abundance depended on the distance from the Sun. They therefore
hold valuable information about where in the early Solar System a certain body's building blocks originated.

As a reference for the original isotopic inventory of the outer and inner
Solar System, the researchers used two types of meteorites. These chunks
of rock generally found their way to Earth from the asteroid belt, the
region between the orbits of Mars and Jupiter. They are considered to be largely pristine material from the beginnings of the Solar System. While so-called carbonaceous chondrites, which can contain up to a few percent carbon, originated beyond Jupiter's orbit and only later relocated to
the asteroid belt due to influence of the growing gas giants, their
more carbon-depleted cousins, the non- carbonaceous chondrites, are true children of the inner Solar System.

The precise isotopic composition of Earth's accessible outer rock layers
and that of both types of meteorites have been studied for some time;
however, there have been no comparably comprehensive analyses of Martian
rocks. In their current study, the researchers now examined samples from
a total of 17 Martian meteorites, which can be assigned to six typical
types of Martian rock. In addition, the scientists for the first time investigated the abundances of three different metal isotopes.

The samples of Martian meteorites were first powdered and subjected
to complex chemical pretreatment. Using a multicollector plasma mass spectrometer at the Institute of Planetology at the University of
Mu"nster, the researchers were then able to detect tiny amounts of
titanium, zirconium, and molybdenum isotopes. They then performed
computer simulations to calculate the ratio in which building material
found today in carbonaceous and non-carbonaceous chondrites must have
been incorporated into Earth and Mars in order to reproduce their
measured compositions. In doing so, they considered two different
phases of accretion to account for the different history of the
titanium and zirconium isotopes as well as of the molybdenum isotopes, respectively. Unlike titanium and zirconium, molybdenum accumulates
mainly in the metallic planetary core. The tiny amounts still found
today in the silicate-rich outer layers can therefore only have been
added during the very last phase of the planet's growth.

The researchers' results show that the outer rock layers of Earth and
Mars have little in common with the carbonaceous chondrites of the outer
Solar System.

They account for only about four percent of both planets' original
building blocks. "If early Earth and Mars had mainly accreted dust
grains from the outer Solar System, this value should be almost ten
times higher," says Prof. Dr.

Thorsten Kleine of the University of Mu"nster, who is also director at
the Max Planck Institute for Solar System Research in Go"ttingen. "We
thus cannot confirm this theory of the formation of the inner planets,"
he adds.

Lost building material But the composition of Earth and Mars does not
exactly match the material of the non-carbonaceous chondrites either. The computer simulations suggest that another, different kind of building
material must also have been in play. "The isotopic composition of this
third type of building material as inferred by our computer simulations
implies it must have originated in the innermost region of the Solar
System," explains Christoph Burkhardt. Since bodies from such close
proximity to the Sun were almost never scattered into the asteroid
belt, this material was almost completely absorbed into the inner
planets and thus does not occur in meteorites. "It is, so to speak,
'lost building material' to which we no longer have direct access today,"
says Thorsten Kleine.

The surprising find does not change the consequences of the study for
theory of planet formation. "The fact that Earth and Mars apparently
contain mainly material from the inner Solar System fits well with planet formation from the collisions of large bodies in the inner Solar System," concludes Christoph Burkhardt.

========================================================================== Story Source: Materials provided by University_of_Mu"nster. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Christoph Burkhardt, Fridolin Spitzer, Alessandro Morbidelli, Gerrit
Budde, Jan H. Render, Thomas S. Kruijer, Thorsten
Kleine. Terrestrial planet formation from lost inner solar system
material. Science Advances, 2021; 7 (52) DOI: 10.1126/sciadv.abj7601 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/12/211222153019.htm

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