Astrobiology Revealed #11: Amber Young

On Disequilibrium Biosignatures in Exoplanet Atmospheres

by Aubrey Zerkle

This week we chatted with Dr Amber Young, civil servant science researcher at Goddard Space Flight Center, about her paper entitled “Inferring chemical disequilibrium biosignatures for Proterozoic Earth-like exoplanets.“ Amber discusses prospects for life detection from exoplanet atmospheres, why the Proterozoic Earth provides the best test case, and how the upcoming Habitable Worlds Observatory could be her dream mission! (This interview has been edited for length and clarity.)

We’ll start with your origin story - how did you first become interested in exoplanet research?

I actually didn't start out in the exoplanet realm. I was originally doing atmospheric modeling, and I've done the whole gambit. My geoscience research when I was an undergrad at Penn State was looking at early Earth and trying to constrain oxygen concentrations through micrometeorite detections. And then I expanded that into doing Mars research with Shawn Domagal-Goldman, using a photochemical model to understand methane in the Martian atmosphere. We were trying to incorporate SAM measurements from the Curiosity Rover and seeing whether we could reconcile the measurements with 1D photochemical modeling. I did that for my Master's thesis, when I was going to school at Fisk University in Nashville, Tennessee.

At the time, the NASA decadal surveys were in full swing, and there were two big [mission] concepts that were dealing with exoplanets, HabEx [Habitable Exoplanet Observatory] and LUVOIR [Large UV/Optical/IR Surveyor]. Shawn was working on both teams, on the science, technology and definition side, and he recruited me to do a summer project based on LUVOIR research. So, for my summer work I was investigating integration times for understanding modern Earth and how we would be able to observe it. I did a presentation on that work in a HabEx community meeting, and that was really my start in exoplanet science. I was interfacing with folks from the exoplanet community for the first time, and it was my first presentation in that kind of forum. That day I actually met my soon-to-be future Ph.D. advisor Tyler Robinson, who at the time was at Northern Arizona University. 

It was such a great and enthusiastic environment, because everybody was so hyped about the mission and very collaborative. It was very interdisciplinary work and I was just kind of enthralled with it. That really pushed me to want to pursue [exoplanet research] later in graduate school. I maintained connections with Ty and ended up going to Northern Arizona University to complete my Ph.D., which solidified my exoplanet presence in developing expertise in that area.

What is “chemical disequilibrium” and why is it a potential biosignature in exoplanet atmospheres?

The idea of chemical disequilibrium really hearkens back to some really old papers from the mid-60s, from Lovelock et al., showing that if you've got biology in an atmosphere, it's not just interacting within its own bubble. Organisms are interacting with the atmospheric environment through their metabolisms, like through their waste gases. On a fundamental level, if biology is pumping these gases into the atmosphere cumulatively, that's going to have a sizable effect on the composition and the atmospheric state. What biology does in producing these waste gases is shifting [the atmosphere] out of thermodynamic equilibrium. This shift that biology tends to have on its environment has been talked about a lot in the astrobiology community, and is something that really solidified itself as a potential biosignature. We want to understand how biology interacts with its environment on Earth, and whether or not it's similarly shifting its atmosphere into this disequilibrium state [on other planets]. 

In this paper, you focused your analyses on “Earth-like” exoplanets – can you explain what “Earth-like” means to an astronomer who’s thinking about habitability?

I love this question, because “Earth-like” can mean so many different things in the context of what we're trying to model and simulate. In terms of understanding Earth as a potential exoplanet analog, we wanted to stick as closely to Earth-like regimes as possible. So, assuming it's a terrestrial planet, and it's about the same size as Earth, and the same structural composition and atmospheric composition. In terms of the atmosphere, we’re talking our standard split of 78% nitrogen and 21% oxygen for a modern Earth-like planet. It’s got cloud properties, and the same Earth-to-Sun distance as well, so we’re modeling the stellar properties of the Sun as our star-like host for this model exoplanet system.

When does water come into play? A common theme in astrobiology is to “follow the water”, so at what point do you consider whether there’s water in the planetary system?

On the mission development side of things, some of my work is trying to go after the observational strategy for what to look for with the telescope to confidently search for biosignatures and evidence of life. I think from that perspective searching for water would be the preliminary step in identifying potentially promising exoplanet candidates.

There’s this whole idea of the habitable zone, which is a planet that's orbiting at the right distance from its host star to maintain the conditions suitable for life, in terms of temperature [that would allow for liquid water] and atmospheric stability. That idea of the habitable zone would be the first step. So you would ask, is it within the habitable zone of its host star? Yes! Okay, now let’s take it a step further and see if we can see water vapor in that planet’s atmosphere. Then, if we can say water is present, these potentially habitable planets are of interest. And we could then take another step forward in fidelity and complexity, and search for different biosignatures that we might expect. So from [the observational] perspective, habitability plays a critical role.

It plays into this [modeling] work as well, in that habitability is the precursor knowledge we’re assuming from the get-go. We have already determined [the exoplanet we’re modeling] is orbiting within the habitable zone region, because we’re mirroring an Earth-sun distance for our exoplanet. We presume there’s water in its atmosphere, and that oceans are present. That is all precursor knowledge going into the modeling. We’re just looking at Earth in different epochs of its evolution, that’s the only thing we’re modulating or changing in our models.

You tested your models with Earth during the Proterozoic era. What about the Proterozoic Earth, in particular, made it a good test case to look for disequilibrium biosignatures?

One of the reasons we were really excited about studying chemical disequilibrium for the Proterozoic Earth is that it's a very long-lasted period within Earth’s geological history, in comparison to modern Earth. In theory, if you were trying to do remote observations on Earth-like planets you’d have to get really lucky to find a modern Earth, because it presents such a small fraction of our overall history. In terms of serendipitous detection, we might be more likely to find something like an Archean Earth or a Proterozoic Earth, because both of those eras represent much longer periods of history. 

Another advantage the Proterozoic Earth has in terms of chemical disequilibrium is the actual chemical composition of the atmosphere itself, which was different from modern Earth. There was a little bit less oxygen in comparison with the present day. Oxygen in and of itself is a biosignature because it’s a gas being produced by photosynthetic organisms. But there was also elevated levels of methane during this particular era of Earth’s history. That turned out to help us a lot in these analyses because the chemical disequilibrium of Earth’s atmosphere is mainly controlled by the presence of oxygen and methane together as a gas pair. Methane is really the species of interest here, it’s the gas that’s being produced by methanogenic organisms. You would expect [methane] to get readily destroyed by oxygen in the atmosphere. But because there’s so much of it being produced by these metabolic processes, methane can be maintained within the atmosphere despite its really short atmospheric lifetime.

So, those were the main things that came into play when deciding to study the Proterozoic Earth. Since you’ve got more methane in the presence of some oxygen, we thought this would produce a more detectable biosignature. We figured it would actually be a really great test bed to see what the limits are and what observational requirements will be necessary to find the [disequilibrium] signal in the most optimistic of cases.      

As I understand it, part of what you were trying to do with your models was to test how observational uncertainties might affect astronomers’ ability to measure disequilibrium in planetary atmospheres. Can you briefly tell us what are the biggest uncertainties in these observations?

When we’re trying to constrain the uncertainty of chemical disequilibrium from an observation, mainly we're talking about potential noise sources from the observation itself. So we're looking at the systematics of the telescope, and the noise that’s inherent from the telescope observations. The thing that’s tricky about that though, is that in the context of HabWorlds [the Habitable Worlds Observatory] and direct imaging spectroscopy, none of those parameters are really set. We don’t have a full picture of what the telescope is going to look like yet. 

Another source of uncertainty that we couldn't necessarily incorporate into the model is that there can be multiple sources for chemical disequilibrium [besides just biology]. One main source that we put to the side for this first study is oceans. The species that can dissolve in our oceans and the way they interact with the atmosphere is another potential source of chemical disequilibrium. Actually, in previous papers from Josh Krissansen–Totton et al., 2017 and 2018, looking at Earth’s chemical disequilibrium with the contribution from the oceans actually increases the signal by a lot. For modern Earth, the [chemical disequilibrium] signal is in the thousands of joules per mole if you include the oceans. If you're just looking at the atmosphere alone, the signal for modern Earth is several joules per mole, in comparison. So the oceans are a sizeable portion. But it was really hard for us to figure out what species would be present and dissolved in the oceans of an exoplanet. There's a lot of uncertainty around whether or not we'd even be able to observe the presence of oceans on an Earth-like world directly. So, we focused on the atmosphere for now and just looked at the signal generated through the atmospheric species and any disequilibrium gas pairs within the atmosphere. But not knowing what chemical species could be in an exoplanet ocean is definitely another source of uncertainty to consider for future.

You concluded astronomers should be able to detect disequilibrium signatures if they have strong enough data, which is encouraging! But you also cautioned that other contextual information will be important in establishing a biological cause. In the paper you used the example that modern Mars has higher chemical disequilibrium than the Proterozoic Earth! What other contextual information would you recommend collecting to help identify false positives like these?

Right, modern Mars was a case study that we talked about in the discussion section of the paper as a point for comparison. Mars also has a chemical disequilibrium within its atmosphere, on the order of 100 or so joules per mole. In comparison to Proterozoic Earth, which is around 24 to 30 joules per mole in the atmosphere, it’s actually higher! That's really interesting for us, and definitely a realm in which we could explore potential false positives for a [biogenic] signal.

We think the disequilibrium on Mars is mainly produced through photochemical processes. The reason why we say that is because of the species that are in disequilibrium - it's CO2 and CO that are in disequilibrium within the Martian atmosphere. In that context, CO is a prime source of food for microorganisms and biology. We wouldn't expect that chemical disequilibrium to be sourced from biology, because it's all this free food microorganisms could be going after, and they're not. I think the contextual information [in the Mars example] is really understanding what chemical species are in disequilibrium and why they are present in the atmosphere. We need to ask if we would expect these species to be produced by biology versus some photolytic or other abiotic process instead.

So from a practical sense, what's the best way for the community to collect the observations required to recognize (as unambiguously as possible) a disequilibrium biosignature? In other words, how would you design your dream exoplanet DISEQ mission?

I feel like being affiliated with some of the conceptual studies that have gone into the decadel survey process, and being a part of that from such an early stage is really seeing the potential for what this new Habitable Worlds Observatory mission could be like. Nothing is really set in stone or stringently decided about the mission. That, to me is like “Oh I'm a kid in a candy store!” Just looking at all the different options and choices and really being a part of that decision process now is super exciting! Right now there are two established working groups for Habitable Worlds Observatory. Those groups have started up and we're starting to process the science cases that we want to drive the mission objectives, which will then drive the instrument requirements and the technological capability and feasibility. HabWorlds has tentative launch dates from the mid-2030s to early 2040s. But, it’s loose right now because we’re in the pre-phase A, or pre-pre-pre-pre-phase A stages of mission development.

HabWorlds could potentially be my dream mission, if we take into consideration all the different biosignatures that we want to search for and figure out a way to robustly characterize them. We can at least identify the potentially Earth-like candidates that are in the habitable zones of their host stars. From just having a sample size of those planets, we hope to get a better sense of whether the habitable zone even exists, in an observational sense. And then we can go after more detailed characterization on the back end of that, to search for things like chemical disequilibrium. The mission has already been developed in such a way that oxygen and methane could be the two big sources. Oxygen has a lot of great absorption features in the visible. And while methane is difficult to detect at modern Earth concentrations, it can be stronger for other eras of Earth’s history. If we’re looking at Archean Earth-like levels of methane, then we can see that [with this mission] in the visible and in the near-infrared.

I think there’s real potential with HabWorlds, in terms of what it can do and its capability. It just excites me to think about, in comparison, how well JWST is doing with exoplanet observations. It really wasn’t conceptualized to do that kind of science from the beginning, but it’s giving us all this great information on the back end that we probably would never have dreamed of! I’m excited to see what HabWorlds turns out to be, and I know it’s going to shock us and surprise us and give us even more than we anticipated! Or at least that’s what I would want my contribution to be, as part of the mission team - to really push the envelope!