Earth and Mars formed from material that originated largely in the inner solar system; only a small percentage originated beyond the orbit of Jupiter.
A group of researchers led by the University of Münster (Germany) reports these findings in the journal Science Advances. They present the most comprehensive comparison to date of the isotopic composition of Earth, Mars, and the pristine building material of the inner and outer solar system.
Some of this material is still largely unchanged in meteorites today. 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 that the four rocky planets grew to their current size by accumulating millimeter-sized dust pebbles from the outer solar system is not tenable, according to the authors.
About 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, over the course of millions of years, the rocky inner planets formed from this original building material.
According to the oldest theory, dust in the inner solar system clumped into larger and larger chunks that gradually grew to about the size of our moon.
The collisions of these planetary embryos eventually produced the inner planets Mercury, Venus, Earth, and Mars. However, a newer theory prefers a different growth process: "shingle" Millimeter-sized dust migrated from the outer solar system toward the sun. Along the way, they accumulated in the planetary embryos of the inner solar system and, step by step, enlarged them to their current size.
Both theories are based on theoretical models and computer simulations designed to reconstruct the conditions and dynamics of the early solar system; both describe a possible planetary formation path. But which one is correct? What process actually took place?
To answer these questions in their current study, the new study team determined the exact composition of the rocky planets Earth and Mars. "We wanted to find out if the building blocks of Earth and Mars originated in the outer or inner solar system."says Dr. Christoph Burkhardt of the University of Münster, first author of the study, in a statement.
To this end, isotopes of the rare metals titanium, zirconium and molybdenum found in minute traces in the silicate-rich outer layers of both planets provide crucial clues. Isotopes are different varieties of the same element, differing only in the weight of their atomic nucleus.
Scientists assume that in the early solar system these and other metal isotopes were not evenly distributed. Rather, its abundance depended on the distance from the sun. Thus, they contain valuable information about where the building blocks of a given body originated in the early solar system.
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 came to Earth from the asteroid belt, the region between the orbits of Mars and Jupiter. They are considered to be largely pristine material since the early solar system.
While the so-called carbonaceous chondrites, which can contain up to a small percentage of carbon, originated beyond the orbit of Jupiter and only later moved to the asteroid belt due to the influence of the rising gas giants, their more impoverished cousins. in carbon, the non-carbonaceous chondrites are true children of the inner solar system.
The precise isotopic composition of the accessible outer rock layers of the Earth and that of both types of meteorites has been studied for some time; however, there have been no complete comparable analyzes of Martian rocks. In their current study, the researchers examined samples from a total of 17 Martian meteorites, which can be assigned to six typical types of Martian rocks. In addition, scientists for the first time investigated the abundance of three different metal isotopes.
The Martian meteorite samples were first pulverized and subjected to complex chemical pretreatment. Using a multicollector plasma mass spectrometer at the Institute of Planetology at the University of Münster, the researchers were able to detect small amounts of isotopes of titanium, zirconium and molybdenum. They then ran computer simulations to calculate the proportion by which building material found today in carbonaceous and non-carbonaceous chondrites must have been incorporated into Earth and Mars to reproduce their measured compositions.
In doing so, they considered two different phases of accretion to explain the different history of titanium and zirconium isotopes, as well as molybdenum isotopes, respectively. Unlike titanium and zirconium, molybdenum accumulates primarily in the metallic planetary core.
Therefore, the small amounts that are still found today in the silicate-rich outer layers may only have been added during the 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 make up only about four percent of the original building blocks for both planets.
"If early Earth and Mars had accumulated mainly dust grains from the outer solar system, this value should be almost ten times higher."says Prof. Dr. Thorsten Kleine from the University of Münster, who is also director of the Max Planck Institute for Solar System Research in Göttingen. "Therefore, we cannot confirm this theory of the formation of the inner planets.", Add.
But the composition of Earth and Mars also does not exactly match the material of non-carbonaceous chondrites. Computer simulations suggest that a different type 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 that it must have originated in the innermost region of the solar system."explains Christoph Burkhardt. Since bodies so close to the sun were almost never scattered in the asteroid belt, this material was almost completely absorbed in the inner planets and is therefore not found in meteorites. "It is, so to speak, ‘lost building material’ to which we no longer have direct access today."says Thorsten Kleine.
The surprising finding does not change the consequences of the study for the theory of planet formation. "The fact that Earth and Mars apparently contain mostly inner solar system material fits well with planet formation from collisions of large bodies in the inner solar system."Christoph Burkhardt concludes.
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