Forming Worlds
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Forming Worlds

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.

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!

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.

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).

Fig. 1: Left: Sketch illustrating the sequence of events during which we follow the fractionation of HO, CO, and CO in our work. Right: Volatile/rock mass ratios for: dust near the mid-plane of the disk (E), dust + pebbles (F), and primordial (G) and evolved (H) planetesimals as a function of radius of the disk in our fiducial model run. In (H), the dotted lines depict the initial conditions at t = 0. Before the formation of planetesimals in the protoplanetary disk the ratios of HO, CO, and CO evolve according to the local chemistry in the disk. Following planetesimal formation, however, the ices are “locked in” and the volatile content evolves depending on the level of outgassing from planetesimals.
Fig. 2: Scenario of disk- and planetesimal-based processing and delivery of carbon in terrestrial planetary systems. Dust grains and pebbles in the disk form with high C/H ratios, but become depleted in carbon over time from the combined effects of disk chemistry and outgassing from carbon-rich icy planetesimals. These planetesimals deliver their volatile abundances during planetary system formation to terrestrial worlds, and thus determine the climatic state and availability of carbon-based compounds on the surfaces of potentially habitable planets. Image sources: 0: Pat Rawlings / NASA, 1: Jessberger et al. (2001), 2: Washington University, Christie’s Images Ltd / Reuters / CSB Newspath, 3: Rixin Li, Quanta Magazine, 5: Thibaut Roger

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Tim Lichtenberg

Postdoctoral Fellow @ Atmospheric, Oceanic and Planetary Physics, University of Oxford. www.timlichtenberg.net