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Lab Work on ‘Super-Earth’ Atmospheres

How we do laboratory work on exoplanet atmospheres is an interesting challenge. We’ve worked up models of the early Earth’s atmosphere and conducted well-known experiments on them. Still within our own system, we’ve looked at worlds like Mars and Titan and, with a good read on their atmospheric chemistry, can reproduce an atmosphere within the laboratory with a fair degree of accuracy.

In the realm of exoplanets, we’re in the early stages of atmosphere characterization. We’re getting good results from transmission spectroscopy, which analyzes the light from a star as it filters through a planetary atmosphere during a transit. But thus far, the method has mostly been applied to gas giants. Getting down to the realm of rocky worlds is the next step, one that will be aided by space-based assets like the James Webb Space Telescope. Can lab work also help?

Probing the Atmosphere of a ‘Super-Earth’

Worlds smaller than gas giants are plentiful. Indeed, ‘super-Earths’ are the most common planets we’ve found outside our own Solar System. Larger than the Earth but smaller than Neptune, they present us with a challenge because we have no nearby examples to help us project what we might find. That leaves us with computer modeling to simulate possible targets of observation and, in the lab, experimentation to see which mixture produces what result.

At Johns Hopkins University, Sarah Hörst has been conducting experimental work that varies possible exoplanet atmospheres, working with different levels of carbon dioxide, hydrogen and water vapor, along with helium, carbon monoxide, methane and nitrogen. Hörst and team adjust the percentages of these gases, which they mix in a chamber and heat. The gaseous mixture is passed through a plasma discharge that initiates chemical reactions within the chamber.

The research team used JHU’s Planetary Haze Research chamber (PHAZER) to conduct the experiments. A key issue is how to choose atmospheric compositions that would be likely to be found on super-Earths, as the paper on this work explains:

Atmospheres in chemical equilibrium under a variety of expected super-Earth and mini-Neptune conditions can contain abundant H2O, CO, CO2, N2, H2 and/or CH4, various combinations of which may have a distinct complement of photochemically produced hazes, such as ‘tholins’ and complex organics in the low-temperature, H2-rich cases, and sulphuric acid in the high-metallicity, CO2/H2O-rich cases. Warm atmospheres outgassed from a silicate composition can also be dominated by H2O and CO2. We therefore chose to focus on a representative sample of gas mixtures that are based on equilibrium compositions for 100×, 1,000× and 10,000× solar metallicity over a range of temperatures from 300–600 K at an atmospheric pressure of 1 mbar.

Image: This is Figure 2 in the paper. Caption: Due to the large variety of gases used for the experiments, this schematic provides a general idea of the setup. The details varied depending on the gases used, with attention paid to the solubility of gases in liquid water, condensation temperatures and gas purity. Credit: Sarah Hörst/JHU.

At issue is the question of haze, solid particles suspended in gas that can make it difficult to gauge the spectral fingerprints that identify individual gases. You might recall the clear upper atmosphere scientists found at the ‘hot Saturn’ WASP-39b (see Probing a ‘Hot Saturn’). Using transmission spectroscopy on this world, much larger than a super-Earth, Hannah Wakeford’s team at STscI found clear evidence of water vapor, and a surprising amount of it.

It was the fact that WASP-39b’s upper atmosphere is apparently free of clouds that allowed such detailed study of the atmospheric constituents. When we’re dealing with planets with haze, our ability to read these signs is more problematic. Learning more about the kinds of atmospheres likely to be hazy should help us refine our target list for future observatories.

Hörst’s laboratory work probes the production of haze, as the scientist explains:

“The energy breaks up the gas molecules that we start with. They react with each other and make new things and sometimes they’ll make a solid particle [creating haze] and sometimes they won’t,” Hörst said. “The fundamental question for this paper was: Which of these gas mixtures – which of these atmospheres – will we expect to be hazy?”

Two of the atmospheres in which water was dominant turned out to produce a large amount of haze, an indication that haze is not solely the result of interactions in methane chemistry. From the paper:

The two experiments with the highest production rates had the two highest CH4 concentrations, but the one with the third highest production rate (10,000× at 600 K) had no CH4 at all, demonstrating that there are multiple pathways for organic haze formation and that CH4 is not necessarily required. In the case of the experiment with no CH4, the gas mixture had CO, which provided a source of carbon in place of CH4. However, it is important to note that the production rates are not simply a function of carbon abundance, C/O, C/H or C/N ratios in the initial gas mixtures. This result also demonstrates the need for experimental investigations to develop a robust theory of haze formation in planetary atmospheres.

The researchers found a wide variation in particle color as a function of metallicity. The color of particles produced in the haze turns out to have an effect on the amount of heat it traps. Such findings may have implications for astrobiology, when we consider that primitive layers of haze could shield life in its early stages, preventing energetic photons from reaching the surface.

This work is in its early stages, as the paper makes clear:

Although models of atmospheric photochemistry and haze optical properties provide good first estimates, they are incomplete and biased due to the relatively small phase space spanned by the Solar System atmospheres on which they are based. Laboratory production of exoplanet hazes is a crucial next step in our ability to properly characterize these planetary atmospheres. These experimental simulations of atmospheric chemistry and haze formation relevant to super-Earth and mini-Neptune atmospheres show that atmospheric characterization efforts for cool (T <  800 K) super-Earth- and mini-Neptune-type exoplanets will encounter planets with a wide variety of haze production rates.

The paper also reminds us that hazes will have an effect on reflected light, which will have a bearing on future direct imaging of exoplanets. Lab work like this is part of building the toolsets we’ll need for probing rocky worlds around nearby stars in search of biosignatures. My assumption is that in the early going, we are going to see a lot of ambiguous results, with atmospheres with potential biosignatures being likewise capable of interpretation through abiotic means. Homing in on the most likely targets and understanding the chemistry at play will give us the best chance for success when looking at worlds so unlike any in our own system.

The paper is Hörst et al., “Haze production rates in super-Earth and mini-Neptune atmosphere experiments,” Nature Astronomy 5 March 2018 (abstract).


{ 17 comments… add one }
  • Geoffrey Hillend March 9, 2018, 17:12

    I don’t think haze will be a problem in the long term. There will be no doubt a lot of haze in the atmosphere if we use Venus as an example. Haze might be temperature dependent. Titan has a lot of haze due to the effects of cold temperature on the atmospheric chemistry. Venus has a lot of haze due to a larger amount of solar light it receives and large amount of atmosphere. The Earth has haze some times and cloud cover so when we observe an exoplanet over a long time, we should be able to get a good signature.. Near infra red and infrared goes through haze. I could be wrong, but I intuitively think that the James Web space telescope should be able to good signature right away and differentiate between planets inside and outside the life belt.

  • Geoffrey Hillend March 9, 2018, 19:40

    Earths atmosphere is opaque to some infra red radiation since water vapor absorbs in the near infra red and infra red. Maybe infra red does not go through hazes but that is o.k. since there should be spectral absorption bands in the infra red as a result where that light is missing from the spectrum since it has been absorbed. These spectra will show that water vapor is present in the atmosphere. We should be able to differentiate between hazes with a different composition such as Co2, water vapor etc.

  • Geoffrey Hillend March 9, 2018, 23:17

    It depends on how thick the haze is. On Titan it can block the visible light. We still knew that there was little free oxygen and no water on Venus through the telescope in the 1950’s before any space probes visited it. I have to agree that there will be bio signatures detected especially water. It’s hard to rule out the ambiguity since we will have more Earth like targets with environmental conditions potentially more favorable than our other inner planets.

  • Geoffrey Hillend March 9, 2018, 23:58

    It’s the spectroscope through the telescope which allowed us to know there was no free oxygen and no water in Venus atmosphere in 1950’s. If we have no CH4, but only O2, H20, N2, Co2 that would be ambiguous. If there is no CH4, N2, or O2 that would certainly be a conservative view for M dwarf exoplanets, but I would not be surprised if it turns out to be correct.

  • Alex Tolley March 10, 2018, 11:42

    Some observations of the paper:

    I note that this paper restricts the atmospheres to those in equilibrium for tractable reasons. As the Figure 1 of the paper showing the 9 tested atmospheres, none have the gas mixtures that we hope to find when searching for life during any eon.

    The gas mixtures are subjected to energy to create the hazes, but the impact on spectra is not investigated [at this time?] which is likely to be important in our initial investigations of atmospheres.

    The impact of haze is, as the paper discusses, on reflected light and direct imaging of the planet, not on transmission spectroscopy that will potentially allow examination of atmospheric layers, not just those where haze will interfere with.

  • Michael C. Fidler March 10, 2018, 22:49

    Early Earth had a hazy, methane-filled atmosphere.

    “More than 2.4 billion years ago, Earth’s atmosphere was inhospitable, filled with toxic gases that drove wildly fluctuating surface temperatures. Understanding how today’s world of mild climates and breathable air took shape is a fundamental question in Earth science.

    New research from the University of Maryland, the University of St. Andrews, NASA’s Jet Propulsion Laboratory, the University of Leeds and the Blue Marble Space Institute of Science suggests that long ago, Earth’s atmosphere spent about a million years filled with a methane-rich haze. This haze drove a large amount of hydrogen out of the atmosphere, clearing the way for massive amounts of oxygen to fill the air. This transformation resulted in an atmosphere much like the one that sustains life on Earth today.”

    Read more at: https://phys.org/news/2017-03-early-earth-hazy-methane-filled-atmosphere.html

    They should be using UV to get a better simulation, what frequencies would a plasma generate?

  • Geoffrey Hillend March 11, 2018, 0:58

    Quote by Alex Toley: “The impact of haze is, as the paper discusses, on reflected light and direct imaging of the planet, not on transmission spectroscopy that will potentially allow examination of atmospheric layers, not just those where haze will interfere with.”

    Direct imaging will result a combination of all the layers, but through a thinner part of the atmosphere so It will give only a single group spectra. Sometimes one is getting only a thin portion of the atmosphere where the starlight is shining through with transit spectroscopy. Light polarization techniques should work well with transit spectroscopy since one only needs the polarized light. One is also getting a smaller part of the atmosphere as well with transit spectroscopy and the thicker layers are of course the closest to the planet with where there are more clouds. Both techniques should show the same gases though and it’s better to have a direct image if possible.

    Direct imaging works the same way as the spectroscopy of the planets in our solar system. For example, looking at Mars with your own telescope with a diffraction grating or spectrometer attached to one end of it. Light passes through the atmosphere of Mars, it reflected off the surface and back out into space to the telescope. Some of the light is absorbed by the planets atmosphere. The absorption works through the process of light scattering and light polarization. With transit spectroscopy, the light does not have to hit the planets surface. It only has to pass through a portion of the atmosphere to be absorbed and re emitted by a gas of the appropriate wavelength. A particular atom or molecule will only absorb light or EMR at a very specific frequency and produce spectral lines or bands which show the distinct energy levels of that atom. We should get the same spectra with direct imaging as with transit spectroscopy since the light is absorbed by gases when it passes through at atmosphere which might include reflection. Transit spectroscopy is the the star plus light of the planet minus the light of the star which yields the planet light or spectra.

    If the haze is just water vapor, some visible light should still pass through it to yield spectra in the visible spectrum if the haze is not too thick.

    • Alex Tolley March 11, 2018, 12:20

      Geoffrey to understand this better, do you have a good online reference regarding different spectrographic techniques for planetary atmosphere characterization?

  • Geoffrey Hillend March 11, 2018, 18:06

    I am sorry, I don’t know of any links. I am drawing from my own memory from knowledge I got from reading books on meteorology, spectroscopy, astrophysics, youtube, etc and scattered information from different web sites. It’s pretty much common sense though. For example. If you are standing on the ground and can see the horizon you are looking through the thickest part of the atmosphere which is the troposphere. If you look straight up, the air thins out pretty fast so the air pressure on mount Everest is only around one quarter of sea level pressure or around 250mb vs the average surface pressure of 1000mb or one bar. My point here is when you see the Sun set you get much more red light or long wave visible light than blue since the shorter wave blue light gets blocked or absorbed since it has to go the the thickest part of the air. You can imagine an eclipse of an exoplanet of it’s star and you can see the light goes through the edges; the occultation happens so the top thin layer gets hit first and the the rest but I think that they use the whole eclipse so light from the star passes around both sides of the planet’s atmosphere which is only a small portion of the whole that is restricted during an eclipse. With direct imaging, the light gets reflected off the entire surface so you get more surface area of light being reflected back from the whole half of the planet depending on the phase, full, gibbous or half illuminated as the planet moves around the star on its orbit. With direct imaging, the starlight is absorbed by a planets atmosphere twice. The starlight first gets absorbed as it comes through the air to hit the surface and then it gets absorbed again after being reflected back out into space. We can get the whole combined spectra of the planets atmosphere with the both techniques so they support each other.

    Light polarization is when the waves take only a specific angle or preferential alignment; For example you can have linear, circular and elliptical polarization. Starlight is not polarized or does not have a preferential alignment since all the different angles or types are mixed so we can differentiate easily faint light with light polarization which would stand out from ordinary starlight. I am not an expert in spectroscopy but I know that light polarization needs scattered light like our blue sky which is light polarized nitrogen scattering. One would get less light polarization with a completely cloud covered world but would still have some polarization from the thinner top of the atmosphere. Clouds might add some infrared polarization.

  • Geoffrey Hillend March 12, 2018, 16:49

    With our present technology and the James Web Space telescope we won’t be able to resolve the phases of an exoplanet in orbit but only see a dot or pin point of light which is considered direct imaging which will be enough to get spectra.

  • Daniel Högberg March 13, 2018, 15:47

    Maybee the wrong place for this question but here goes: Is it possible for a super earth, say 3 times the size of earth to have a surface gravity that we humans could endure? Could it even be as low as one G, or this impossible due to planet size /mass? Thanks!

    • Michael March 14, 2018, 3:59

      This depends a lot on the materials accreted during the planets formation. If mostly ice/water at three times the volume of earth I would think so possibly be even less than 1 g, if rock then no way.

  • Michael C. Fidler March 15, 2018, 10:26

    The Role of Atmospheric Compositions.


    A Spectral Tour of Planetary Atmospheres:
    Venus, Earth, Mars, Giant Planets, Titan.


    Spectroscopy of exoplanets.


    Transmission spectra of exoplanet atmospheres.


    Characterizing Transiting Planet Atmospheres through 2025.


  • Michael C. Fidler March 15, 2018, 10:32

    The Transiting Exoplanet Community Early Release Science Program for JWST.

    The James Webb Space Telescope (JWST) presents the opportunity to transform our understanding of planets and the origins of life by revealing the atmospheric compositions, structures, and dynamics
    of transiting exoplanets in unprecedented detail. However, the high-precision, time-series observations required for such investigations have unique technical challenges, and prior experience with Hubble,
    Spitzer, and other facilities indicates that there will be a steep learning curve when JWST becomes operational. In this paper we describe the science objectives and detailed plans of the Transiting Exoplanet Community Early Release Science (ERS) Program, which is a recently approved program for JWST observations early in Cycle 1. We also describe the simulations used to establish the program.
    The goal of this project, for which the obtained data will have no exclusive access period, is to accelerate the acquisition and diffusion of technical expertise for transiting exoplanet observations with JWST, while also providing a compelling set of representative datasets that will enable immediate scientific breakthroughs. The Transiting Exoplanet Community ERS Program will exercise the time-series modes of all four JWST instruments that have been identified as the consensus highest
    priorities, observe the full suite of transiting planet characterization geometries (transits, eclipses, and phase curves), and target planets with host stars that span an illustrative range of brightnesses. The
    observations in this program were defined through an inclusive and transparent process that had participation from JWST instrument experts and international leaders in transiting exoplanet studies.
    The targets have been vetted with previous measurements, will be observable early in the mission, and have exceptional scientific merit. Community engagement in the project will be centered on a twophase
    Data Challenge that culminates with the delivery of planetary spectra, time-series instrument performance reports, and open-source data analysis toolkits in time to inform the agenda for Cycle 2 of the JWST mission.


  • Michael C. Fidler March 15, 2018, 10:37

    Surface and Temporal Biosignatures.

    “Recent discoveries of potentially habitable exoplanets have ignited the prospect of spectroscopic investigations of exoplanet surfaces and atmospheres for signs of life. This chapter provides an overview of potential surface and temporal exoplanet biosignatures, reviewing Earth analogues and proposed applications based on observations and models.
    The vegetation red-edge (VRE) remains the most well-studied surface biosignature. Extensions of the VRE, spectral ‘edges’ produced in part by photosynthetic or nonphotosynthetic pigments, may likewise present potential evidence of life. Polarization
    signatures have the capacity to discriminate between biotic and abiotic ‘edge’ features in the face of false positives from band-gap generating material.
    Temporal biosignatures — modulations in measurable quantities such as gas abundances (e.g., CO2), surface features, or emission of light (e.g., fluorescence, bioluminescence) that can be directly linked to the
    actions of a biosphere — are in general less well studied than surface or gaseous biosignatures. However, remote observations of Earth’s biosphere nonetheless provide proofs of concept for these techniques and are reviewed here. Surface and temporal biosignatures provide complementary information to gaseous biosignatures, and while
    likely more challenging to observe, would contribute information inaccessible from study of the time-averaged atmospheric composition alone.”


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