Eleanor Greenspoon
University of California, Berkeley
Eleanor Greenspoon is a second year graduate student in the College of Chemistry at the University of California, Berkeley in an astrochemistry lab. Her work consists of building experimental methods to explore ice phase reactions in interstellar ice analogs.
Paper Title: Trapped Water on Silicates in the Laboratory and in Astrophysical Environments
Authors: Alexey Potapov, Cornelia Jäger, Harald Mutschke, and Thomas Henning
First Author’s Institution: Laboratory Astrophysics Group of the Max Planck Institute for Astronomy at the Friedrich Schiller University Jena, Institute of Solid State Physics, Helmholtzweg 3, 07743 Jena, Germany
Status: Published in ApJ [open access]
Astrochemists focus on the small-scale chemistry that supports the evolution of solar systems like ours over the course of millions of years. Part of why this evolution takes so long is that molecules are far less densely packed in planet-forming regions during early stages of evolution than they are in more mature planets. How does the chemistry look different when atomic/molecular collisions occur on the scale of months rather than fractions of seconds, like on Earth?
Perhaps surprisingly, ice and dust are VIPs (Very Important protostellar Structures). Studying how chemistry happens differently in the severe weather of a molecular cloud or protoplanetary disk hinges on the properties of ice and dust. Icy dust grains host the party for many relevant atoms to mix, mingle, and form complex and even prebiotic molecules. Note that ice and dust in these environments are different from ice and dust on earth: both can have an amorphous structure, for ice mostly water, and for dust mostly silicates or carbonaceous material. This disordered structure affects how atoms and molecules can move around and meet each other, especially when we consider the temperatures in early planet-forming regions (from a few 10s to 100 K!).
Today’s authors aimed to study in the lab how a mixture of silicate dust grains are able to trap water, and how this trapped water could impact ice chemistry later in stellar evolution. These were lab experiments, so accurate replicas of ice and dust in the interstellar medium needed to be formed, requiring some pretty cool machinery.
The authors use a technique called laser ablation, where a laser beam ejects material from a surface, to remove particles of magnesium and silicon, mix them with oxygen, and stick them to a substrate in a high vacuum chamber (~10-8 millibar) that can then be used for analysis. This mixture of MgSiO was prepared specially to replicate the makeup of dust particles in molecular clouds and planet-forming regions. They then leaked water vapor into the chamber where it stuck to the simulated dust surface, which had been cooled to 10K. This dust is especially porous, a lot like a sponge. The hypothesis of the authors was that this spongey dust can “soak up” water that comes in contact with it, trapping solid-phase water in its pores via a physical mechanism called physisorption.
After the authors formed the water ice, they began their two-step analysis process. The first step was a moderate temperature ramp from 10 to 300K. Infrared spectroscopy is the chosen technique to analyze interstellar ice analogues, as it can pick out features of different molecules due to their unique vibrations. It can then be used to compare to infrared telescope data, like that from the James Webb Space Telescope (JWST).
The second step of their analysis was a high temperature experiment, in a separate chamber from the initial analysis. This chamber is capable of achieving high pressures and high temperatures, which was needed for this experiment where the dust/ice mixture was heated from 300 to 700K. Infrared spectra were also taken during this heating process.
Some of the most interesting results from this experiment come from comparing the dust/ice mixture to pure water ice at a given temperature(Fig 3). Pure water ice sublimates to gas phase water between 160 and 180K, so the infrared spectrum looks essentially flat by 200K. By comparison, the mixed dust/ice still shows identifiable water features at 200K. This told the authors that some interaction between the dust analogs and the water allowed water ice to stick around far longer than it would have without the dust.
To understand what kinds of interactions lead to this behavior, the high temperature experiments became important. For a mechanical method (physisorption) to be the culprit, the trapped water must be released at a temperature below 700K. Chemisorption, a chemical bonding process, holds on to the trapped molecules much tighter than physisorption, leading to very high desorption temperatures. The water ice trapped in this experiment sublimated at 470K, strongly suggesting a mechanical trapping mechanism!
These lab results have interesting implications for astrochemistry in origin of life studies and planet formation. Presence of liquid water on the surface of a planet is a central criterion for a habitable planet, so understanding how water ends up on rocky, terrestrial planets like earth helps us understand our history as a planet and host for life. The water-trapping mechanism explored in these experiments could be part of the explanation for how solid water can stick around in planet-forming regions closer to the host star, in hotter environments than we would expect. Small, wet silicate grains become the building blocks of terrestrial planets, and those planets become host to liquid water.
A great deal of continued lab experiments and observations could shed further light on this subject, but utilizing James Webb is one of the most exciting fronts to be explored. New and upcoming JWST projects have been accepted that will be investigating both dust properties and the effects of trapped water on planet-formation. This will continue the crucial collaboration between lab astrochemists and the observational community!
Edited by Jessie Thwaites
Featured Image Credit: Adapted by Eleanor Greenspoon from Columbus Metropolitan Library, Public domain, via Wikimedia Commons, ALMA, via Wikimedia Commons; Dbc334 (first version); Jynto (second version), Public domain, via Wikimedia Commons; and ALMA, CC BY 4.0, via Wikimedia Commons
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