Protoplanetary discs in the laboratory: the fate of icy pebbles undergoing sublimation

Planet formation in protoplanetary discs is a process whereby the primitive solids that are initially of microscopic scale, must be converted into larger objects such as pebbles (mm-cm size), planetesimals, and eventually planets. It has been acknowledged that drifting pebbles play an important role in the core accretion scenario by triggering streaming instabilities or by aiding the growth of planetary cores. Moreover, ice lines of volatile species such as water seem to be promising sites for this process. At the water ice line, the higher surface energy of ice promotes coagulation and the sublimated vapor can diffuse outward in the disk and deposit onto pebbles, allowing fast growth [1][2]. These processes can then trigger streaming instabilities or core formation through gravitational collapse of small bodies. However, very little is known about the evolution of these objects’ cohesive properties and volatile content as they drift and encounter various conditions throughout the disc. Investigating the morphology, chemistry, and physical processes of pebbles close to the ice line is essential to have an insight into the overall evolution of small bodies in the early phases of protoplanetary discs. We are interested in studying the different outcomes of the sublimation of an icy pebble, to understand how its optical properties are changing over time and if the dust aggregates survive to the sublimation process. In the Laboratory for Outflow Studies of Sublimating Materials (LOSSy) at the University of Bern [3], we are researching optical and physical properties of ice-dust mixtures with relevance for protoplanetary discs and planet formation, with focus on the role of ice sublimation in changing these properties.

Two new methods for the preparation of ice-dust aggregate with mm-size have been developed. The first method (Pebble-A) uses an inclined superhydrophobic surface with dust on it; a mm-size droplet of distilled water rolls on it and collects the dust, then it falls into liquid nitrogen. The second method (Pebble-B) exploits the capillary forces between water and solid grains: a droplet impinges a dust bed and penetrates it forming a wet aggregate, which is then sunk in liquid nitrogen. PAs have generally ~50%wt of ice and the dust is mainly accumulated close to the surface of the pebble, while the core has more ice. PBs have lower amounts of ice (~15%wt) and the dust grains are connected through ice films. Different types of dust with relevance for protoplanetary discs are used, such as olivine, pyroxene, corundum, serpentine, CI and CR asteroids simulants [4]. Humic acid (HA) is used as an organic compound, simulating complex organics that can be found on comets.

The SCITEAS-2 (Simulation Chamber for Imaging the Temporal Evolution of Analogue Samples version 2.0) vacuum chamber provides a low-pressure and low-temperature environment for the sublimation of icy samples, and a hyperspectral measurement of the sample over time in the VIS and NIR [5]. The pressure inside the chamber is kept low (around 10^-6 mbar) through a turbomolecular vacuum pump, and a He-cryocooler guarantees a temperature at the base of the sample of  ~120 K. A fast sublimation of the icy pebbles is achieved by letting the temperature evolve freely up to room temperature, while the vacuum pump pumps out the vapor formed in the chamber. The overall process lasts around 20 hours.

Ice sublimation, gravity, grain size distribution of the dust, type of dust, ice content, and presence of organics concur all together to determine the sublimation outcome of the icy pebble. Is it going to disrupt to dust or will it maintain its shape? We list here some interesting preliminary observations:

- ice sublimation is detectable in the VIS and NIR reflectance spectra of the pebbles. Although the disappearance of the ice absorption bands provides information on ice content at the surface of the pebble only, the core could still contain ice;

- PAs seem to disrupt more easily with respect to PBs made of the same dust, due to the higher amount of ice that is embedded in the core, which pushes the particles away when sublimating;

- in PAs, the presence of HA mixed with mineral powders prevents disruption, which is observed in the absence of HA (Fig.1);

- in PBs made of pyroxene, different grain sizes result in different outcomes: PBs made of dust with grain sizes bigger than 100 microns disrupt, while smaller grain sizes are more efficient in maintaining the pebble intact, and are not affected by sublimation. Furthermore, a PB made of coarse pyroxene dust (100-300 microns) mixed with fine dust (<50 microns) can partially resist disruption, highlighting that fine dust plays a key role in cementing bigger dust particles.

A campaign of experiments is ongoing on PAs and PBs. The aim is to investigate the different physical processes that concur in the final sublimation outcome, for different dust properties. More experiments will study:

- The grain size distribution effects on the disruption. Different minerals are expected to have different size distributions that will avoid disruption during sublimation.

- The role of different organics in binding with mineral dust.

- The interaction of a gas stream at low pressure with a sublimated PB or PA, to understand their shear strength and determine their stokes number.

ACKNOWLEDGMENTS

The team from the University of Bern is supported by the Swiss National National Science Foundation and through the NCCR PlanetS.

REFERENCES

[1] Musiolik, G., & Wurm, G. (2019) The Astrophysical Journal, 873(1), 58.
[2] Ros, K., Johansen, A., Riipinen, I., & Schlesinger, D. (2019) Astronomy & Astrophysics, 629, A65.
[3] Pommerol, A., Jost, B., Poch, O., Yoldi, Z., Brouet, Y., Gracia-Berná, A., ... & Blum, J. (2019) Space science reviews, 215(5), 37.
[4] Britt, D. T., Cannon, K. M., Donaldson Hanna, K., Hogancamp, J., Poch, O., Beck, P., ... & Metzger, P. T. (2019) Meteoritics & Planetary Science, 54(9), 2067-2082.
[5] Pommerol, A., Jost, B., Poch, O., El-Maarry, M. R., Vuitel, B., & Thomas, N. (2015) Planetary and space science, 109, 106-122.