Making a Habitable Planet

Edwin (Ted) Bergin
University of Michigan

Today we stand on the cusp of characterizing potentially living worlds, or habitable planets, planets with orbits where liquid water would exist on a rocky surface. This concept of habitability implicitly assumes water in some form is present on rocky planets - but is it? More broadly are rocky planets generally "chemically habitable" - do they contain the elements needed for life, such as carbon and nitrogen, on their surfaces? In this talk, I will review what we know about the chemical habitability of forming planets. I will follow the most abundant volatile elements needed for life (C, H, O, N) from the vast cold and low pressure environs of the interstellar space to their presence on planetary surfaces of worlds such as our own. I will outline our current understanding of star and planetary birth while discussing the fate of primary carriers of life's elements, both volatile species (e.g. H2O, CO, CO2) and more refractory materials (e.g. silicates, aliphatic/aromatic hydrocarbons). Some of these carriers can be characterized via existing facilities (e.g. the Atacama Large Millimeter Array), planned facilities (the James Webb Space Telescope), while others require future options (such as the Origins Space Telescope). Finally, within a young planet, the ultimate fate of delivered material is not set until the hot young terrestrial world solidifies and core formation ceases - thus the process of planet formation and evolution itself influences whether a mature planet is habitable or uninhabitable. Looking forward, the astrochemical study of life's materials in space, and the astronomical characterization of terrestrial exoplanets, must be intimately linked to grounding knowledge from the planetary sciences. A fascinating interdisciplinary future awaits, where we will seek to ascertain the origin of our own biosphere and provide crucial chemical context for habitability.

Originally Broadcast 23 September 2020.

August Extravaganza!!

New, quick talks each week in the month of August from all areas of Astrochemistry and researchers from around the globe! Enjoy them on your own time, and we'll see you all back in September for our regularly scheduled programming.

Physicochemical Modelling: Source-Tailored or Generic?

Beatrice M. Kulterer
Univeristät Bern

Benzene on Ice and in the ISM

David Benoit
University of Hull

Matrix-Isolation FT-IR Spectroscopy of Nitrogen Based Heterocyclic Radicals

Mayank Saraswat
Indian Institute of Science Education and Research (IISER) Mohali, India

VUV Spectroscopy for Laboratory Astrophysics and Astrochemistry

Yu-Jung Chen
National Central University, Taiwan

Sketch Your Science!

Olivia Harper Wilkins
California Institute of Technology

Submit your art to Olivia's Sketch Your Science Contest at!

Pointing the Green Bank Telescope

Ellie White
Marshall University

My 15 Years of Astrochemistry

Zainab Awad
Cairo University

Laboratory Measurements of Gas-Phase Reaction Kinetics with CN radical at Low Temperatures

Divita Gupta
Univ. Rennes

Did Photons or Electrons Create Life?

Ella Mullikin
Wellesley College

Photodissociation of CS via Highly Excited Electronic States: Ab Initio and Experimental Study

Zhongxing Xu
University of California Davis

Black in Astrochem - A Round-Table Discussion

Ashley Walker, Ayanna Jones, Bryne Hadnott, Kathleen Rink, and Prof. William Jackson

This special session of Astrochemistry Discussions started with a keynote talk given by Dr. William M. Jackson, Distinguished Professor Emeritus at UC Davis, pioneer in the field of astrochemistry, and co-founder of the National Organization for the Professional Advancement of Black Chemists and Chemical Engineers (NOBCChe). A panel followed, moderated by Kathleen Muloma Rink and featuring Ashley Walker, Bryné Hadnott, and Ayanna Jones. This event was conceived of by Ashley Walker (@ThatAstroChic on Twitter), who founded #BlackinAstro week and is well known for highlighting Black astronomers and astrochemists. This is essential watching for all in academia, and most especially for non-Black mentors to Black students.

If you'd like to support the panelists who put their time and emotional labor into this event, their Venmo information is below. Donations to NOBCCHe can be made here and donations to African American Women in Physics can be made here.

Originally Broadcast 15 July 2020. Content Warning: Brief description of sexual assault.

Ashley Walker: Ashley-Walker-210
Ayanna Jones: onlyayanna

Bryné Hadnott: Bryne-Hadnott
Kathleen Muloma Rink: Kathleen-Rink-1

Using reaction rate theory (MESMER) to provide robust kinetic parameters for astrochemical modelling

Mark Blitz
University of Leeds

Experiments provide kinetic parameters, rate coefficients and product yields, over a limited range of temperatures and pressures. Often this experimental temperature range is not sufficient to cover the temperatures required for astrochemical modelling. Traditionally, simple analytical forms are used to represent rate coefficients, e.g.

k = A Tn exp(-Ea/RT).

However, this approach can result in extrapolated rate coefficients with an order of magnitude uncertainty, especially at very low temperatures.

A more robust approach is to use a theoretical model to fit to the experimental data before extrapolation. For a given reaction, the reaction rate theory code MESMER calculates rate parameters from the information provided by ab initio structure calculations. In addition, MESMER allows the theoretical parameters to be adjusted to best fit the data, and thus provide a robust description of the reaction. The reaction between the hydroxyl radical, OH, and formaldehyde, H2CO, will be used to illustrate how MESMER can provide robust rate coefficients down to 5 K.

Originally Broadcast 8 July 2020.

Bonus Questions & Answers - Mark's Talk

Q: Does MESMER give a range for the rate constant so as to incorporate an ‘error bar’?

A: Yes, you can give the errors for each of the rate constants. It is also possible to propagate the errors to give the MESMER rate constant error for a given T and p.

Q: Does MESMER allow for including and excluding sets of experiments? Some being more accurate than others?

A: It is up to you to assign the error for each rate constant. For measurement with problems you might want to give it a very large error, or not include it at all.

Q: Why is the same species labeled differently (and repeated) multiple times when defining the channels?

A: This was to check which channel the reaction was occurring along. You do not have to do this. By doing this I was able to show that at low T, the reaction was via the pre-reaction complex

Q: How do you think these changes reflect a thin porous ices? Do you expect larger cracks in the ice to influence the structure change?

A: We did study FEL irradiation as a function of ice thickness and found that desorption is more efficient at the surface of thick ices likely due to the bad conductivity of ASW. However changes are seen at all studied thicknesses, i.e. up to a few layers. Of course cracks and nucleation can help the crystallization process.

Q: Do you give any physical significance to any of the parameters in your equation or would you regard them as fitting parameters?

A: Are you referring to the MESMER fitting parameters, if so yes. If you are referring to final expression for k, then I do not.

Q: I think I missed what was "broken" about the 5 K simulation.

A: It was the species profile that was broken. This meant that MESMER did not converge properly, and it is best not to use the rate constant at 5 K.

Q: Which level of theory did you use to optimize minima and TS? how a poor description of van der Waals in the functional influences the rate?

A: The pre-reaction complex does have some very anharmonic vibrations and this can influence the result to some extent. The the fitted parameters can be skewed from the true answer. Need to describe the anharmonic motions as accurately as possible if claiming accuracy.

Q: Just as a matter of curiosity, how much off were the CCSD barrier results from the experimental fit?

A: About 1 kJ mol-1 adjustment of the TS barrier.

Q: Do we ever use the binding energy distributions as parameters in models? In other words, instead of using the average, one pulls from the distribution each time, in a more Monte Carlo approach?

A: It is possible to start the calculation off with different energy distributions. In the OH + acetylene + O2 MESMER example the fraction of O2 affects the products formed.

Q: Can MESMER consider also the 'jump' between two PES (e.g. triplet and singlet) when modelling rate constants?

A: Yes, if you look in the MESMER examples there is such an example.

Simulation of Reactivity on the Surface of Dust Grains: Pitfalls and Opportunities

Johannes Kästner
Universität Stuttgart

Gas-phase reaction rates can often be predicted with high accuracy because only a few atoms are involved. Compared to that, the simulation of reactions happening on the surface of a dust grain is much more involved, which often requires compromises concerning accuracy. They are very valuable, however, since the corresponding experiments also face the problem of many parameters that have to be controlled. I will give an overview of techniques to model adsorption and surface reactivity, their estimated accuracy, and their computational costs. It turns out that even an "active" surface, like ASW, often has rather little influence on the reaction barriers. Nevertheless, such surfaces influence the reactivity - but can be treated by a very efficient implicit surface model.

Originally Broadcast 8 July 2020.

Bonus Questions & Answers - Johannes's Talk

Q: Are laboratory experiments like TPD able to provide information about the distribution of binding energies or only a single desorption energy barrier?

A: I am no expert on that, but I talked to a few :-). TPD convolute the (real) binding energy distribution with the thermal desorption probability (which is time dependent). So in my understanding part of the broadening of the desorption time/temperature is due to this broad distribution.

Q: Is there a physical understanding of why those rate constants are so different?

A: In a first instance, because the barriers are quite different. In this particular case, the systems were similar, but not equal: there were different electron-withdrawing groups attached to the aldehyde.

Q: Please comment on the site dependency of the Binding energies.

A: For example different binding sites offer a different number of hydrogen bonds to the adsorbate. They also may lead to more or less buried adsorbates. All that influences the binding energy.

Infrared Resonant Vibrationally Induced Restructuring of Amorphous Solid Water

Sergio Ioppolo
Queen Mary University of London

Amorphous solid water (ASW) is abundantly present in the interstellar medium, where it forms a mantle on interstellar dust particles and it is the precursor for cometary ices. In space, ASW acts as substrate for interstellar surface chemistry leading to complex molecules and it is postulated to play a critical role in proton-transfer reactions. Although ASW is widely studied and is generally well characterized by different techniques, energetically induced structural changes, such as ion, electron and photon irradiation, in these materials are less well understood. Selective pumping of specific infrared (IR) vibrational modes can aid in understanding the role of vibrations in restructuring of hydrogen bonding networks. In this talk, I will present the first experimental results on hydrogen bonding changes in ASW induced by the intense, nearly monochromatic mid-IR free-electron laser (FEL) radiation of the FELIX-2 beamline at the FELIX Laboratory at the Radboud University in Nijmegen, the Netherlands. Experiments are complimented with Molecular Dynamics simulations to constrain the effect at the molecular level.

Co-Authors: Jennifer A. Noble, Herma M. Cuppen, Stephane Coussan, Britta Redlich

Originally Given 24 June 2020

Bonus Questions & Answers - Sergio's Talk

Answers below from both Sergio and Jennifer Noble.

Q: Do you have a feeling for how things would change not only based on the structure of the ice, but on the roughness or porosity of the surface, since you are investigating surface modes?

A: Yes, we also have a lot of data on compact versus porous water ice. We’re currently working on the analysis, but it does seem to modify the ice response, especially at the surface.

Q: Can you comment on what would happen if you irradiated the H2O combination band? What motivated your choice of band to irradiate?

A: If I recall well, we do see a similar effect when irradiatiating the combination mode. I did not discuss it due to the limited time. We extensively irradiated the MIR range at different frequencies, but here for simplicity I have shown only the main absorption peaks. We are working on a series of papers discussing all these different irradiations. So stay tuned :)

A: One of the biggest motivations to work at FELIX was the wide spectral range, allowing irradiation of vibrational modes from 2.7 microns through the MIR to the THz. In fact, we irradiated all major vibrational bands of water ice (amorphous and crystalline) to determine whether the effects that we observed were mode-dependent or frequency-dependent. The combination mode did give a similar, but less intense, response - maybe due to its low band strength.

Q: Do you think energy is used quickly to change the structure or do you expect there to be local hotspots that live for a short amount of time?

A: The ice should relax quite quickly. MD simulations show that it relaxes somehow between shots, but an overall local heating builds up in time. This is seen in the experiments as well but we are not very sensitive to that because the temperature sensore is far away from the ice, in the substrate. However the effect we see are not global heating.

Q: How do you think these changes reflect a thin porous ices? Do you expect larger cracks in the ice to influence the structure change?

A: We did study FEL irradiation as a function of ice thickness and found that desorption is more efficient at the surface of thick ices likely due to the bad conductivity of ASW. However changes are seen at all studied thicknesses, i.e. up to a few layers. Of course cracks and nucleation can help the crystallization process.

Q: To what extend will VUV be different from IR irradiation in changing the structure of water ice? Could VUV change crystalline back to ASW or do you expect the same behavior as with IR?

A: I forgot to add that VUV/electrons/ions do have enough energy to break molecular bonds disrupting the intermolecular bond network as well. We do not see nor expect that when using IR light. Therefore the final result should be different than selective IR irradiation of ices. However it is important to study the effect of IR photons to isolate and better understand how vibrational energy transfers and dissipate within the ice at the surface vs the bulk.

Q: Hi Sergio, thank you for your talk! Have you experiments led to any new insights on ozone ice chemistry? Have you found new production/destruction pathways?

A: We did not see any chemistry occurring in the pure water ice, but only restructuring. I plan to do experiments with radicals to basically investigate IR photon-induced thermal chemistry. Ozone would be a good candidate. I have not done that yet though.

Q: Is it possible to study ice mixtures on LISA?

A: Yes, absolutely. There is a gas mixing ramp that allows the preparation of gas phase mixtures at a wide range of concentrations. We can quantify the concentrations by a combination of partial pressures in the mixing ramp, and QMS & FTIR calibration experiments in the chamber.

Q: Low energy electrons also have a reasonably high cross-section for absorption of vibrational modes (Mason et al. 2003). Do you think that since a single Galactic Cosmic Ray can generate millions of low energy electrons, this could be a process that is responsible for generating crystallization in the ISM?

A: The answer is probably yes, but electrons will also induce chemistry in the ice, breaking molecular bonds. This effect will compete with crystallization and likely induce amorphyzation. Hence it is important to selectively study both these mechanisms.

Imaging the H2O and CO Snowlines Around Young Stars

Merel van't Hoff
University of Michigan

Planets form in disks of gas and dust around young stars. With the discovery of more than 4000 exoplanets and the ability to study circumstellar disks in great detail we can begin to address the question of how the composition of a planet is linked to its birth environment. Key aspects of circumstellar-disk chemistry are snowlines: radii at which molecular species freeze out from the gas onto dust grains. The temperature at which a molecule freezes out is species dependent. The radial temperature profile in the disk therefore results in radial gradients in the chemical composition, for example, the elemental C/O-ratio of both the gas and ice. The bulk composition of planets may therefore be regulated by their formation location with respect to major snowlines. In this talk I will focus on how we can determine the locations of the two most important snowlines, the H2O and CO snowlines, observationally.

Originally Given 24 June 2020

Bonus Questions & Answers - Merel's Talk

Q: I guess the same question goes for HCO+ and H2O. What kind of uncertainty is there?

A: An uncertainty/degeneracy that is more important for H2O/H13CO+ than for CO/N2H+, especially in disks, is the dust opacity which can create ring-shaped H13CO+ emission if the dust in the inner disk is optically thick. Since the CO snowline is at larger radii this is not so much a problem for N2H+.

Q: Are there other clever tracers like this for the other lines? Like N2? NH3?

A: N2H+ also works for N2, as long as the snow surface is high up in the disk such that there is a substantial drop in the N2H+ column density outside the N2 snowline. Qi et al. (2019) derived N2 snowlines for the three disks I showed. For NH3 or other snowlines, you would need a molecule whose chemistry is dominated by the presence of absence of gas-phase NH3. And then to be really able to use it, this molecule should be abundant and observable in disks. I am not aware of such molecule to trace the NH3 snowline.

Interstellar Comets: A New Window into the Diversity of Protoplanetary Disk Midplane Chemistry

Martin Cordiner
NASA Goddard

Comets contain a crucial record of the chemistry that occurred in the proto-Solar accretion disk during the epoch of formation of our planets. The recent discovery of interstellar comets provides a unique opportunity to measure the composition of planetary materials originating around other stars. Consequently, we are entering a new era of Galactic astronomy, with the ability to directly investigate the chemistry that occurred in the disk midplanes of extrasolar planetary systems. This talk will summarise our knowledge of the recently-detected interstellar objects 1I/'Oumumua and 2I/Borisov, focusing on our ALMA observations of gas-phase CO and HCN in 2I/Borisov. While the HCN abundance relative to water (~0.11%) was similar to that found in typical Solar System comets, the CO abundance (~68%) was among the highest observed in any comet within 2 au of the Sun, revealing a likely origin in a CO-enriched environment. The importance of this finding in the context our understanding of cometary and protoplanetary disk chemistry - in particular, the ice chemistry occurring close to the CO snowline - will be discussed.

Originally Given 24 June 2020

Bonus Questions & Answers - Martin's Talk

Q: How does the CO/H2O ratio you observed compare to the observations made with the HST? Does that work agree with yours on the interpretation of the origin of the high CO abundance?

A: The CO abundances observed with HST were about as high or higher than we found with ALMA. In fact, there is a clear trend in the HST data of increasing CO/H2O with time.

Q: Staying on theme with the previous talks, how might the structure of the ice alter the abundances that are being shed into the gas phase? Crystalline vs amorphous?

A: There is a lot we don’t know about the structure of cometary ice. Variations in outgassing rates with heliocentric distance could be taken as evidence for trapping of different volatiles in the ice matrix, so I think that’s something that should be looked at in the lab.

Cosmic Rays and Grain Chemistry in Star- and Planet-forming Regions

Christopher Shingledecker
Max Planck Institute for Extraterrestrial Physics

Interstellar matter is subjected to bombardment by several types of ionizing radiation including cosmic rays, stellar winds, x-rays, and gamma-rays. It is known that such radiation can have a significant physicochemical impact on interstellar environments, and a large body of experimental work has shown that the interaction between such energetic particles and low-temperatures ices can result in the formation of complex - even prebiotic - molecules. Even so, modeling the chemical effects of cosmic ray collisions with interstellar dust grain ice mantles has proven challenging due to the complexity and variety of the underlying physical processes. In this talk, recent work on this subject by us is reviewed and the possible applications to better understanding the chemistry in star and planet-forming regions is highlighted.

Originally Given 6 May 2020

The Effects of Cosmic Rays on Carbon Chemistry

Brandt Gaches
Universität zu Köln

Cosmic rays (CRs) are energetic charged particles accelerated in extreme environments. Galactic CRs are accelerated primarily through supernovae. However, it has been recently proposed that protostars may be able to accelerate CRs. These cosmic rays will substantially alter the carbon chemistry in molecular gas. I will present astrochemical calculations of the impact of different CR populations and physics on carbon chemistry. The calculations were performed on a modified version of the publicly available astrochemistry code, 3D-PDR, in which CR attenation has been included in-situ. As such, we're to directly study the impact of different external and internal CR spectra. We find embedded sources of CRs significantly impact the carbon chemistry in relation to the HI-to-Hw transition. The dense gas is heated above 50 K and bright in [CII] emission. Embedded CR sources allow for atomic carbon to exist in dense gas where traditional models have carbon locked into carbon monoxide. Finally, I will show that the CO-to-H2 conversion factor used in extragalactic studies is relatively stable to significant enhancements of the CR ionization rate. However, the CI-to-H2 conversion factor is sensitive to the CR ionization rate and external environment.

Originally Given 6 May 2020

Probing Galactic Cosmic Rays with Small Molecules

David Neufeld
Johns Hopkins University

In the century following their discovery by Victor Hess in 1912, cosmic rays have been recognized as an important constituent of the Galaxy. With a total energy density somewhat larger than that of starlight, cosmic rays are the dominant source of ionization for the cold neutral medium (CNM) within the Galactic ISM. In starless molecular cloud cores, they are also the dominant source of heating. Thus, cosmic rays play a central role in astrochemistry by initiating a rich ion-neutral chemistry that operates within the CNM, and the cosmic-ray ionization rate (CRIR) is a key parameter in models for the chemistry of the ISM. In this talk, I will discuss recent estimates for the cosmic-ray ionization rate in the Galactic disk, obtained by using a detailed model for the physics and chemistry of diffuse interstellar gas clouds to interpret previously-published measurements of the abundance of four molecular ions: ArH+, OH+, H2O+ and H3+. The CRIR estimates thereby obtained show a remarkably small dispersion from one interstellar cloud to another. At the Galactocentric distance of the Sun, the primary CRIR per H nucleus is ~ 2x10-16 s-1 in both diffuse atomic clouds and diffuse molecular clouds. I will also discuss a recently-selected SOFIA Joint Legacy Program, HyGAL, which (among other things) will greatly expand the number of sight-lines on which ArH+, OH+, and H2O+ have been observed.

Originally Given 6 May 2020

From Clouds to Planets, The Astrochemical Link

Paola Caselli
The Center for Astrochemical Studies, Max Planck Institute for Extraterrestrial Physics

All ingredients to make stars like our Sun and planets like our Earth are present in the dense (~100,000 H2 molecules per cc) and cold (~10 K) interstellar clouds. In these "stellar-system precursors" an active chemistry is already at work, as demonstrated by the presence of a rich variety of organic molecules in the gas phase and icy mantles encapsulating the sub-micrometer dust grains, the building blocks of planets. Here, I’ll present a journey from the earliest phases of star formation to protoplanetary disks, with links to our Solar System, highlighting the crucial role of astrochemistry as powerful diagnostic tool of the various steps present in the journey.

Originally Given 22 April 2020