Early carbon depletion of terrestrial worlds from the outside-in
The form and availability of carbon compounds on the surface sensitively govern long-term climate and the chemistry of life, but curiously carbon is strongly depleted on the terrestrial planets of the Solar System. To better understand what separates potentially habitable from non-habitable worlds, we modelled disk and planetary processes that drive carbon sequestration and loss during planet formation. We found these to be a strong function of time: disk evolution and outgassing from planetesimals can lead to several orders of magnitude decrease in the carbon abundance on planetary system-wide scales, which limits the delivery of carbon and introduces a crucial parameter that influences the volatile inheritance of terrestrial worlds: timing.
This article narrates the research published in Lichtenberg & Krijt (Astrophysical Journal Letters, 2021). A video summary can be found here.
Carbon in and on the terrestrial planets
Carbon is perhaps the most essential of the elements of life. About 19% of your body is made of carbon and all bio-essential chemistry is driven by carbon-based molecules. Carbon — in the form of CO₂ — is also one of the main driving factors of planetary climate on terrestrial worlds due to its greenhouse effect. Generally, higher CO₂ abundances lead to a hotter climate, while declining CO₂ abundances lead to a colder climate. On Earth the CO₂ abundance is thought to be regulated by feedback with the interior through plate tectonics on million-year timescales (which is why injecting lots of it into the atmosphere on short timescales is an unwise move). But this feedback system is anything but a given: Venus is the best witness that in approximately similar formation conditions the temperature regulation on a potentially habitable planet can go terribly wrong!
This means the amount of carbon may sensitively control the climate state and availability of life-essential compounds on the surface of potentially habitable exoplanets. No carbon would mean either no or a fundamentally different climate control agent, and would make life as we know it impossible. However, the abundances of carbon on Earth curiously tell a very different story: carbon as an element is strongly depleted in the Earth relative to the amount of carbon in the interstellar medium, which is the material we think the planets form from during the making of a planetary system. Some carbon may be stuffed into the metal cores of terrestrial planets, but according to current high-pressure experiments this is not enough to explain the order-of-magnitude depletion we see on Earth! This means there are some fundamental physical or chemical mechanisms that operate during the formation of terrestrial planets that deplete the planet-forming building blocks of their carbon.
Cold dust and volatile carbon in planet-forming disks
Revolutionary astronomical observatories such as the Atacama Large Millimeter/submillimeter Array (ALMA) have made it possible to map the distributions of solids (especially “pebbles”, roughly ~mm-sized particles) and many carbon-bearing molecules (CO generally being the most abundant) in the cooler, outer regions of nearby protoplanetary disks. The amount of pebbles seems to be lower for older disks, suggesting dust and pebbles have either grown into planetesimals (10–100 km-size planetary building blocks that are invisible to ALMA) or even larger objects, or perhaps have migrated into the inner disk en masse, effectively serving as feedstock for planets growing closer to the star. The story for CO is equally interesting. In the coldest, darkest regions of protoplanetary disks (where temperatures dip below ~20K) CO molecules efficiently freeze out onto dust grain surfaces, increasing the amount of carbon and oxygen on their icy mantles. However, disks that are a few million years old seem to be missing more CO than can be explained through this freeze-out process. In the latest picture, ongoing chemical processing as well as the sequestration of ices on pebbles and larger objects are thought to contribute to this depletion. In this dynamic story, the composition of pebble-size particles in the disk interior -– the precursors of planetesimals and ultimately planets — is set not only by the local temperature and pressure, but affected by ongoing chemical processing and material transport. Before thinking about planets, however, there is another step we must consider.
From the Solar System, we have evidence for extensive geochemical alterations of the most primitive assemblages that make up the planets. For instance, iron meteorites in the inner Solar System show that their precursor planetesimals were substantially depleted in carbon from outgassing, and also more primitive, carbonaceous chondrites show evidence for geochemical changes in their elemental abundances *after* they were formed. This suggests that two mechanisms are at play: carbon is chemically altered in the disk during planet formation, and another time once the material accretes onto the first planetesimals, that further accrete then to form the planets.
Combining disk processes and planetesimal evolution
In this project we wanted to test how the relative influence of disk-based and planetesimal-based processing of volatile carbon carriers act together to potentially influence the total carbon abundances stored in planetary building blocks, and possibly reveal some systematic effects that we can use to probe the deviations between planetary systems in the planetary assemblage and therefore bulk abundances. We modeled the detailed physics and chemistry of the dust grains and volatile chemistry that operates in the disk. This alone can drive substantial fractionation between the various forms of carbon over time and space. Two major effects are at play: a reduction of CO ice in the coldest outer parts (as grain surface chemistry destroys CO molecules, leading to the formation of for example methanol), and, conversely, an accumulation of CO in the region straddling the CO iceline (caused by the large-scale inward migration of icy pebbles).
Going one step further, we then asked the question: what happens after these pebbles gravitationally collapse to form icy planetesimals in the outer parts of planetary systems? During planetesimal formation, the local assemblage of volatile ices gets locked in: no disk-based chemistry can alter their abundances anymore, because they are locked away into the interior of the planetesimal. After that, the chemical evolution of the planetesimal is subject to the interior evolution due to geophysics and geochemistry. Importantly, in the early Solar System this evolution was driven from the radioactive decay of aluminium-26, a very potent short-lived radionuclide. The decay of this isotope heats the material from the inside. The decay heat can be so strong that it literally melts rock-ice aggregates and creates internal lava worlds on the earliest planetesimals. This can lead to outgassing on planetesimals that accrete to form planets, and recent work has shown that it may have played a substantial role in setting the volatile inventory of the terrestrial planets of the Solar System!
Here, we were specifically interested in how the combined evolution of the disk and outgassing/loss of carbon-based compounds evolves with time. Because the half-life of aluminium-26 is short and can vary across planetary systems, we treated the initial abundances (and thus the internal heating) of planetesimals as a free parameter. As it turns out, if the planetesimals in the outer parts of the disk are heated to similar levels as the earliest planetary materials in the Solar System, a systematic depletion of carbon, specifically CO and CO₂, takes place in the outer disk. Depending on the heating level, the carbon abundances stored in CO and CO₂ are depleted by up to two orders of magnitude. This creates a substantially different level of carbon in the outer parts of the disk, and would be important information on the volatile chemistry and abundances that can be delivered to terrestrial planets later on (Fig. 1).
Comets and Kuiper belt objects in the outer Solar System presumably formed fairly late, largely escaping such vigorous internal heating and being cooked from the inside. However, with aluminum-26 content varying from system to system, and signs for early planet formation having been seen already in very young disks, this may not be the case everywhere and may substantially vary between systems.
On a statistical level, the carbon loss shrinks the overall variance of carbon in planetary building blocks in a system if planetesimals form relatively early and with lots of internal heating. On the other hand, if they don’t, then carbon chemistry and abundances are controlled by the temporal variations across the disk. This suggests that the amount of carbon that can be delivered from the outer parts of planetary systems to the inside is a strong function of time, not only of location! Early formation and strong internal heating imply depletion of carbon, which limits the total amount that can be delivered for instance via dynamical scattering, or accreted onto giant planets while they migrate from wide heliocentric orbits inside.
The strong dependence on formation time and internal geophysical evolution suggests that the carbon abundances of exoplanets should correlate with formation time (Fig. 2). If an exoplanet originates in a highly processed system where planetesimals formed early, the carbon abundances derived from volatile ices would be low. If, on the other hand, it formed primarily from planetesimals or pebbles that formed late and/or are not processed, the carbon abundances would be higher. This prediction could be tested via observations of volatile outgassing in debris disks. Some of these extrasolar planetesimal belts show signs of CO outgassing due to mutual collisions, making it possible to derive the abundances of carbon locked up in extrasolar planetesimal belts. Hence our prediction may be testable with these systems and further observations.
Ultimately, we hope that our work can give us new insights into the chronology and processes that determine the volatile abundances of planetary systems. A balanced carbon budget is a necessity for developing clement climates on potentially habitable worlds, and thus we need to develop observational tracers to tell apart systems with high and low carbon abundances. Our work suggests that the geophysical evolution of planetesimals during system formation and the time of accretion are first-order factors that may govern the fate of terrestrial worlds.