Dynamic Desorption: A Tale of Cosmic Rays

Title: A Revised Description of the Cosmic Ray-Induced Desorption of Interstellar Ices

Authors: Olli Sipilä, Kedron Silsbee, Paola Caselli

First Writer’s Establishment: Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany

Standing: Accepted to the Astrophysical Journal, Accessible on ArXiv [Open Access]

Cosmic Ray Desorption

To first perceive the origins of life, we should first perceive the origins of life sustaining molecules. Complex Organic Molecules (COMs) are a typical group of carbon based mostly species which are typically thought-about to be the primary steps of sustaining molecules. COMs have been noticed within the gas-phase in house, however chemical fashions point out that COMs need to form in the ice layers of mud grains, slightly than within the fuel. A standard query astronomers search to reply is how do molecules, like COMs, go from the ice section to the fuel section?

Determine 1: From van Dishoeck (2014), Determine 5. On grain surfaces molecules and atoms can transfer round (diffuse), react, persist with the floor (accrete), and desorb into the gas-phase.

This course of, often called desorption (Determine 1), can happen a number of other ways, however for right now’s chunk we’ll concentrate on cosmic ray desorption. Cosmic rays are excessive vitality particles (principally hydrogen and helium) that may strike mud grains and deposit vitality. The deposited vitality is then transformed to thermal energy, thus heating the grain. Mud grains will then shed the warmth to ‘kick off’ molecules (like COMs) from the ice into the fuel section. The speed at which desorption happens might be immediately measured by the grain cooling time. The speed at which the grain cools is decided by the composition of the ice on the grain. If the ice is manufactured from species that strongly bind to the grain (often called a excessive binding energy), then it takes longer for the grain to chill. Oppositely, if the ice is manufactured from species that loosely bind to the grain (have low binding energies), then cooling and desorption happen extra shortly. 

Modeling Grain Heating

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Determine 2: (a): Two-phase chemical mannequin. For cosmic ray calculations, the ice is usually assumed to be CO. (b) Three-phase chemical mannequin consisting of two layers of ice, the place mixing between the mantle and core can happen. This permits for a various (and extra lifelike) ice combination.

Cosmic ray desorption was first modeled by Hasegawa & Herbst (1993), and their method and assumptions are nonetheless broadly adopted by computational astrochemists. This mannequin used a two-phase chemical mannequin consisting of a gas- and ice-phase (Determine 2a), with the idea that the outer layer of ice is totally manufactured from carbon monoxide (CO). Due to this assumption, the mannequin makes use of a single binding vitality, and due to this fact has a single grain cooling time, and a continuing desorption fee of 1 molecule each 10-5 seconds. 

At present’s paper challenges the idea that ice layers are dominantly CO and introduces a dynamic cool-down fee that’s depending on the ice composition. These fashions search to grasp if a more realistic representation of ice might have an effect on the speed at which molecules desorb to the fuel section, or if the beforehand held assumptions maintain. 

To check this, right now’s authors ran 4 completely different fashions together with a two-phase and three-phase fiducial (Determine 2b) mannequin based mostly on Hasegawa & Herbst (1993) and a two-phase and three-phase dynamic mannequin, with a variable grain cooling fee. The ice layers utilized in these fashions included species generally identified to exist within the ice layers on mud grains, not simply CO. For instance, their mannequin included carbon, nitrogen and oxygen, ammonia, methane, and water amongst different species. 

Cooling Down

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Determine 3: Adopted from Determine 1 in right now’s paper. This plot exhibits the completely different calm down charges (y-axis) for the 4 fashions over time (x-axis). Because the dynamic fashions have variable calm down charges, the calm down charges change over time. Be aware that the fiducial fashions (F2 and F3) have fixed cooling charges, the two-phase dynamic mannequin is quicker (D2), and the three-phase dynamic mannequin is slower (D3).

As anticipated, the fiducial fashions yielded a continuing cool-down fee of 10-5 sec. Nonetheless, the two-phase dynamic mannequin had sooner cooling charges, whereas the three-phase dynamic mannequin had slower cooling charges (Determine 3). The completely different cooling charges are mirrored within the desorption charges of various molecules, the place the three-phase fiducial mannequin usually had the slowest desorption charges (Determine 4).

The dynamic fashions yielded completely different outcomes from the fiducial fashions as a result of molecules and atoms frozen out should not ‘caught’ in place within the dynamic fashions. In actuality, species frozen on grains can migrate, which causes a dynamic and variable set of binding energies for use within the grain cooling fee calculation. Due to this, the dynamic fashions produce variable cooling charges in comparison with the fiducial mannequin, which assumes that the ice is stagnant.

The 2-phase fiducial mannequin yields sooner cooling occasions, doubtless as a result of species with low binding energies, like CO, dominate the ice layer. The three-phase fiducial mannequin, with two layers of ice, permits for species with greater binding energies, like ammonia, to decelerate the grain cooling fee. 

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Determine 4: Modified from Determine 5 from right now’s paper. This determine exhibits the desorption charges of the indicated species over time, for every of 4 fashions (outlined in Determine 3). Be aware how every species desorbs at completely different charges between every mannequin.

Whereas we observe a various variety of chemical species within the fuel section, nearly all of these species, reminiscent of COMs, type within the ice layers of mud grains. To finest perceive the previous of life sustaining molecules, we should first perceive how advanced and numerous ices are desorbed into the fuel. At present’s paper introduces a brand new, straightforward to use, dynamic cosmic ray desorption that’s doubtless extra correct to actual astronomical circumstances.

Particular due to Jason Hinkle for enhancing this chunk!
Credit score for Featured Picture:
Determine 1 from Cuppen, et al. (2017)

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About Abygail Waggoner

I’m a second yr chemistry graduate scholar on the College of Virginia and NSF graduate fellow. I research time variable chemistry in protoplanetary disks. After I’m not nerding out about house, I am nerding out about fantasy by studying or taking part in video games like dungeons and dragons.

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