Let’s build (some terrestrial planets)!

Title: Terrestrial planet formation from lost inner solar system material

Authors: Christoph Burkhardt, Fridolin Spitzer, Alessandro Morbidelli, Gerrit Budde, Jan H Render, Thomas S Kruijer, Thorsten Kleine

First author’s institution: Department of Planetology, University of Münster, Münster, Germany.

Status: Science [Accepted — Open Access]

When you wake up in the morning, what is the first thing you think of? It’s probably not “how the earth came to be”, but it could be! And if it is, it is a good, yet difficult question to ask. Even though we have lived on Earth for thousands of years, we still do not know exactly how Earth and similar rocky planets were formed.

For a time, the leading theory in planet formation predicted that these planets were formed from within the inner part of the solar system. Dust in this area of ​​the solar system accumulated to form planetary embryos that were approximately the size of the moon. Then, over time, these embryos collided and formed the planets we know so well (see the upper part of Fig. 1).

However, there is a new theory of planet formation involving the outer solar system. Instead of the planet’s embryos colliding with each other, small dusts (about the size of a millimeter) moved from the edge of the solar system inward, accumulating on the embryos to form our planets (see the lower part of Fig. 1).

Figure 1: Depiction of the two different types of planetary formation. Top part: Planetary embryos collide to form planets. Very few pebbles from the edge of the solar system are able to get to the inner solar system due to Jupiter’s location. Bottom part: While there are large, initial planetary embryos, they grow only with the growth of pebbles from the outer solar system.

Theoretically, both of these scenarios are equally plausible. So how can we determine which theory is correct for Earth? The key lies in studying nucleosynthetic isotope anomalies. “Nucleosynthetic isotope anomalies” are a fancy way of describing the fact that dust in the solar system is heterogeneous and therefore there will be different isotope abundance depending on where in the solar system you are. This leads to different compositions of planets formed by inner vs. external solar system material. To determine which formation model is the right one, we can simply compare the composition of rocky planets with inner and outer solar system objects to see which ones are more similar.

Study of the amount of Earth’s isotopes and comparison of them with inner and outer solar system objects has been done in the past, but the studies have been quite limited. They have mainly focused on comparing the amount of a single element instead of doing a multi-element analysis. Today’s authors make a major multi-element analysis that, spoiler alert, leads them to support a model in which planets are largely formed by inner solar system material.

Date, date, date!

There are four rocky planets in the solar system: Earth, Mars, Mercury and Venus. Today’s authors focus on two of these four: Earth and Mars. They compare the abundance of nine different isotopes on Earth and on Mars (they use 17 different Mars meteorites for their analysis) with those of objects known to originate from the inner and outer solar systems. But what inner and outer solar system objects can they compare them to? Carbonaceous and non-carbonaceous meteorites! Carbonaceous meteorites are a class of meteorites that are thought to form in the outer solar system, while non-carbonaceous meteorites are thought to form in the inner solar system. By comparing the composition of carbonaceous meteorites and non-carbonaceous meteorites with the composition of Earth and Mars, we should get a sense of where the materials for these rocky planets came from.

Let’s start analyzing!

To determine if the samples from Mars and Earth are more like non-carbonaceous meteorites (NC) or carbonaceous meteorites (CC), the authors compare the deviations from the ground-based default values ​​for the abundance of one isotope with the deviations from another isotope for all. four classes of objects (Earth, Mars, NC and CC).

As can be seen in FIG. 2, the samples from Earth and Mars closely follow the linear trend of the non-carbonaceous meteorites (the meteorites formed in the inner solar system) for most of the elements studied. If you look really closely, however, you will see that the Earth always lurks at the edge of the non-carbonaceous meteorite line, and sometimes even as much as jumps a bit away from the red line in the opposite direction of carbonaceous meteorites. (eg the green circle seems to be almost repelled by the purple circles / squares). What does this mean for our earth? Its composition has probably been enriched in a way that cannot be described by our current sample of non-carbon meteorites (the red circles / squares). If you happen to be interested in the small, rough details in it, the idea is that some of the elements that later formed the Earth were enriched by the slow neutron capture process (s-process), which led to lower ε94Mo and ε96Zr levels. The fact that this is not seen as strongly in the samples from Mars is probably due to Mars’ location farther from the Sun.

Figure 2: Deviations from the norm for different isotope occurrences as a function of each other. The ε notation notes the deviation from the terrestrial default value in parts per The non-carbonaceous meteorites (NC) are shown in red, with their linear tendency plotted as a red line. The carbon meteorites (CC) are shown in purple. Earth is shown in green (BSE), while Mars is shown in yellow (BSM).

However, not all isotope comparisons provide such strong evidence for a purely internal solar system model. As can be seen below in FIG. 3, for comparison of 95Mo and 94Mo, Earth and Mars appear to lie between the distribution of the non-carbonaceous meteorites and the carbonaceous meteorites. By running Monte Carlo simulations to reproduce the composition of Earth and Mars (and including all the different isotopes), the authors conclude that ~ 4% of the material in the planets should be attributed to material from the outer solar system.

Figure 3: Comparison of the deviation of the abundances of 95Mo and 94Mo. Earth and Mars lie between the lines of non-carbonaceous meteorites and the carbonaceous meteorites, suggesting that some outer solar system materials were involved in their formation.

This is pretty (chemically) dense, so let’s summarize

As can be seen in FIG. 2, the isotopic composition of Earth and Mars most closely resembles that of non-carbonaceous meteorites, suggesting that these rocky planets were formed from material in the inner solar system. However, the outer solar system was not completely absent. There is evidence of a small contribution (~ 4%) from materials in the external solar system (see Fig. 3). One explanation for this (which is the explanation put forward by today’s writers) is that Jupiter blocked most of the flow of pebbles from the outer solar system in reaching the inner solar system. Instead, only pebbles that had been properly scattered around Jupiter could reach the inner solar system and contribute to the formation of the rocky planets. Imagine if Jupiter had not been there – our Earth could be very, very different!

Astrobite edited by Macy Huston

Selected image credit: NASA / Lunar and Planetary Institute

About Alice Curtin

I’m a PhD student at McGill University studying fast radio eruptions and pulsars using the Canadian Hydrogen Mapping Experiment (CHIME). My work focuses mainly on characterizing radio frequency interference, investigating possible correlations between gamma-ray bursts and FRBs, and using pulsars as calibrators of future radio instruments. When I am not researching, I typically find myself teaching physics to elementary school students, spending time with friends, or doing something active outside.

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