The moderators for the instrumentation session were Phil Neches of Teradata Corporation and Nilton Renno of the University of Michigan. Neches began the session by declaring himself more of a technologist than a scientist. He said that instrumentation is where science, technology, and commerce come together. Neches then introduced the first presenter.
Morgan Cable of the Jet Propulsion Laboratory began her talk by describing three types of plumes in our solar system. Volcanic plumes, such as Io’s Loki, are usually rich in sulfur but deficient in water. Cometary plumes are rich in CO2 and H2O and are induced by sunlight. The most interesting type of plume when it comes to life, however, is the type emitted by ocean worlds. Enceladus is one such confirmed example, and Europa also has a potential plume detection.
Enceladus has roughly 100 distinct jets emanating from the tiger stripes on its southern, polar region (see Figure 5.1). These form into a single plume high above the surface. The plume is modulated by diurnal tidal flexing, but appears to be steady at least since the time of Voyager and probably earlier, since it feeds Saturn’s E-Ring. This plume contains both gas and solid particles. The Cassini Ion and Neutral Mass Spectrometer (INMS) has determined the plume to be rich in H2O, CO2, CH4, NH3, and heavier hydrocarbons all the way up to its mass limit of 100 atomic mass units. Particles observed by the Cassini Cosmic Dust Analyzer (CDA) include water ice, salts, silica, and organics. The size and oxidation state of the silica nanograins suggests hydrothermal activity on Enceladus.
Regarding plumes in general, she said that if one wants to look for biosignatures in the plume, one must measure not only its composition, but also the relative abundances. The source of the plume must also be determined, whether it be from the subsurface ocean or from the surface itself. The plume grains are presumably where biomarkers would be concentrated (potentially even cells). Their size distribution and formation mechanism (e.g., spray aerosols) are also important. The plume’s overall structure and dynamics must also be known. Cable said that all of this must be placed in its environmental context.
Instrumentation Classes from the Past
Cable then began to go through several instrument classes that have been used to study plumes or could be used more in the future. She started off with the mass spectrometer, such as the Cassini INMS or the Europa Mass Spectrometer for Planetary Exploration/Europa (MASPEX), which are able to target gas and, occasionally, ice grains. Mass spectrometers have an extensive flight heritage. Recent advancements have been made in mass resolution and sensitivity. The Cassini INMS could not distinguish between CO and N2 (both 28 atomic mass units), but MASPEX can, since it allows for resolving ambiguities with respect to the atomic mass units of a molecule and also enables isotopic investigations of other molecules, some of which are related to life. Deconvolving complex mixtures of materials can be difficult though. This can be mitigated by trapping material and then slowly releasing them by increasing the temperature. However, an Earth-based laboratory would be best.
Cable then explained that dust detectors, such as those on the Cassini, Stardust, and Europa Clipper missions, are designed for grains containing salts, ions, and organics. The reflectron design has an extensive flight heritage and is made specifically for plumes (i.e., low-density particles sampled at high velocity). They can make rapid measurements, allowing for the measurement of plume structure during a fly-through. A disadvantage to this is that the ionization of incoming grains is dependent on both the mass of the particles and the velocity of the collision. Again though, bringing material back to a laboratory on Earth would be best. However, this is not often possible. Currently, no technology exists to trap and preserve plume particles for long durations other than aerogel, which nonetheless has a lot of organic contamination and is not conducive to life investigations.
Near-infrared spectroscopy, like on the Cassini and Europa Clipper missions, can also target both gaseous and solid particles in plumes, although sensitivity is limited for trace species. Imaging spectroscopy can also provide information on the plume’s structure and grain size. However, spectroscopy can only identify certain functional groups, like an N-H or C-O stretch, but cannot unambiguously identify them as a constituent of glycine, for example. Near-infrared spectroscopy can also look for particles being deposited on the surface by looking at the body’s surface albedo.
Ultraviolet (UV) spectroscopy also has extensive flight heritage; it has been used on both Cassini and Juno and is planned for the Europa Clipper mission. It can identify plumes at a distance and look for hydrogen and oxygen auroras and simple organics. Similar to near-infrared spectroscopy, the UV technique can also look at the body’s surface albedo to look for particles being deposited. Again though, sensitivity for trace species is limited.
Microwave (submillimeter) radiometers, such as the Microwave Instrument for the Rosetta Orbiter (MIRO), also have a long history. They can observe three-dimensional (3D) plume structure and dynamics, but can only
observe gas-phase polar molecules. A microwave radiometer is able to measure the temperature of water, but like the spectrographs, it has a limited sensitivity to trace species.
Instrument Classes of the Future
Going back to the mass spectrometer, Cable said that adding a gas or liquid chromatograph could be a very powerful tool. It could allow for the detection of amino acids (including their chirality), proteins, lipids, and other biosignatures. However, these instruments are tailored for certain species, and one has to be aware of potential confounding species as well. The need for greater sensitivity and collection of a sufficient sample could also prove problematic. Another challenge is the complexity of the instrumentation needed to capture and examine particles.
Cable mentioned laser desorption ionization mass spectrometry as another technique—one that she believes will be used on Mars. It uses a soft ionization method that is great for large molecules like DNA, RNA, and proteins. Again though, a very complex instrument is needed to capture and concentrate these plume particles when the instrument is flying at several kilometers per second.
Raman spectrometry would target functional groups, such as amines, carboxylic acids, and ketones. Coupled with microscopic imaging, this could be used to confirm whether a particle is indeed a cell. It is not capable, however, of unambiguously identifying complex organic molecules and again requires complex instrumentation.
Immunoassay-based microfluidic chips, like the LifeMarker Chip, can also identify complex biomolecules like DNA, RNA, and proteins. Disadvantages of the LifeMarker are that it can only target Earth-like molecules and again requires complex instrumentation.
A microchip capillary electrophoresis with laser-induced fluorescence can detect a wide range of organic molecules: chiral amino acids, lipids, amines, thiols, fatty acids, DNA, RNA, and proteins. As with the previous methods discussed above it requires a complex instrument to capture and concentrate these particles.
The last instrument class Cable described was microscopic imaging. This technique would be able to see individual cells and their movements, which would be a “smoking gun” for life. Microscopic imaging, again, requires complex instrumentation to capture and concentrate particles. Another problem may be that whole cells might not be common in plume grains.
Planning Future Missions
Cable then went through a list of things to consider when designing a plume fly-through mission. The altitude of a fly-through is important, as this determines the plume density and particle size distribution the spacecraft will encounter. For example, grain sizes of 10 microns in the plume of Enceladus can be reached safely at a typical flyby altitude, whereas a grazing swing past Europa necessary to see 10-micron-sized grains could be dangerous, especially considering planetary protection issues. The issue of flythrough speeds has two opposing arguments. High velocities are needed to ionize the species when hitting the collection device, but higher velocities also increase fragmentation. One solution is choosing an intermediate speed, while another option is doing both slow and fast flybys. The capture medium is another issue. Metal plates are simple and can help ionize the particles, but they can cause fragmentation. Aerogel preserves the particles, but it is difficult to then extract the particles from the aerogel afterwards. Another consideration is the number of fly-throughs that are needed. Repeated fly-throughs can either repeat measurements to see variations over time or can be used to build up larger concentrations for later analysis. The choice of which species to target, in terms of molecules or whole cells, gaseous or grains, and whether they are susceptible to fragmentation, is another important consideration. Finally, Cable finished by saying that everything needs to be understood in context and that any in situ life detection would require a sample return mission to confirm it.
The first question from the audience challenged Cable’s concluding thought and asked why a sample return mission would be needed rather than a lander. Cable accepted that a lander could absolutely be included. The
audience member followed up by asking whether a lander is needed before sample return or whether it could be skipped in favor of doing sample return sooner. Cable admitted that that was the process for Mars, but made the distinction that getting to Mars is much quicker than getting to the outer solar system. She then questioned how long we would be willing to wait to follow the progression from fly-through to lander to sample return. The participant said that she is suspicious of the length of time sample return missions require. The martian sample return mission does not plan to do any sample curation prior to return. A further problem with the outer solar system, she said, is that the icy and volatile materials would need to be stabilized. Therefore, she preferred a lander. Another participant then said that Cassini has already justified a sample return and suggested that an in situ and a sample return mission should be planned together. Cable agreed. Referring to the capture and cryogenic stabilization of a sample return, another audience member said that even a non-pristine sample would be interesting. He said that most things don’t decompose at 25°C over 10 years anyway.
Some of the instruments Cable mentioned were going to be on the new Europa mission, one workshop participant said. The audience member then asked what type of mission (and timeframe) she is looking for to address these questions for Enceladus. Cable said “5 years ago” because Cassini will be lost soon, which means that no new in situ information about the Saturnian system will be available. While admitting that everybody has their favorite planet or moon, she said that Enceladus is just spewing free samples from its south pole. She said that Cassini has done wonderful things, but it was never designed to be a seafaring or life-detection mission. Its mass spectrometer cut-off at 100 atomic mass units is too low to include more than a couple of tiny amino acids. She wants to extend the mass range and get new instruments with their advancements in sensitivity and maybe even a sample return mission.
Another member of the audience then asked whether the onboard computation capacities and downlink rates could support these new mission instruments. Cable said that the mass spectrometry technique creates a lot of data. However, it also has the processing capacity to quickly look at the data and select the most informative mass spectra to transmit back to Earth.
A member of the audience then requested that Cable elaborate on the importance of measuring not only the abundances of molecules, but also their ratios. He said that the biosignatures would presumably be coming from the bottom of a deep ocean and asked if there were cause for concern that the chemistry in the oceans or plume could alter the signals. Cable answered by saying that these measurements need to be understood in context. Issues like ions dissolved in the ocean, the transport time for molecules to get to the surface, and how they’re turning into aerosols and particles in the air must be understood. She said that, barring some tentacle waving hello, there will not be a simple yes or no at first.
Lastly, a participant at the workshop then commented that, with respect to the detection of organic molecules, instrumentation may have different levels of sensitivity to different types of molecules, such as heavy versus light. She then said that instruments are getting down into the parts per trillion range. However, organic contamination must be kept equally as low, which is incredibly challenging to do. This is why, Cable concluded, she wanted to ultimately have a sample return mission where these sensitivity questions could be thoroughly addressed.
Shawn Domagal-Goldman of NASA Goddard Space Flight Center began his talk by emphasizing the need for collaboration and complementarity in the search for biosignatures. He said that the properties of the exoplanet population discovered to date have caused us to reconsider how planetary systems form and evolve.
The next major step, he said, is to characterize their chemical compositions. It has already been done a bit with the Hubble and Spitzer space telescopes, but it will ramp up with the launch of the James Webb Space Telescope (JWST) and then later with the Wide Field Infrared Survey Telescope (WFIRST). The most successful planet discovery techniques to date, the radial velocity and transit techniques, are biased towards planets close to their host stars. Conversely, WFIRST’s microlensing mission will be biased towards the outer planets, completing the census of exoplanets. Its coronagraph could also characterize gas giant exoplanets, and a starshade—an option that has been studied but not approved—could enable the search for potential biosignatures. Extremely large ground-based telescopes are now being developed that should be able to do not only transit spectroscopy (which would be biased
towards the stratosphere), but also direct imaging of planets—perhaps even rocky ones—in the habitable zones of M dwarfs. However, Domagal-Goldman is concerned about the habitability of planets orbiting M dwarfs due to the possibility that they would have lost their atmospheres due to high-energy radiation from their host stars.
Potential and Desired Telescope Specifications
Therefore, Domagal-Goldman said, we should think about how to complement JWST and the extremely large, ground-based telescopes and look beyond M dwarfs. One question is what the wavelength range should be. Ideally, a telescope will need to look at wavelengths shorter than both those JWST is able to observe and those for which ground-based adaptive optics are optimized—that is, the UV and visible. The near-infrared is nonetheless also an interesting region. The wavelength range can help mitigate false positives and increase user knowledge of the environmental context (see Figure 5.2). For exoplanets, this means identifying as many gases and their abundances as possible, which means a wide wavelength range. Domagal-Goldman likes the idea of using the flux or kinetics of biosignatures as a discriminant, a conclusion independently reached in the earlier presentation by Tori Hoehler. Abiotic production of many molecular species proceeds at a rate that is orders of magnitude lower than would be expected through production by life. This contrasts with the concentrations of gases, for which abiotic processes can lead to higher values of oxygen and ozone than biotic ones.
The challenge to identifying or constraining fluxes is that it requires a great deal of environmental context. The teams for LUVOIR and HabEx, he said, want the lower wavelength cutoff to be set at about 100 nm to characterize the far UV starlight that produces a lot of photochemistry as an important constraint on the abiotic sources of oxygen and ozone. However, the wavelength cutoff for directly imaging planets would be 300 to 400 nanometers. Although WFIRST is only planning a maximum wavelength of 1 micron, LUVOIR and HabEx are considering going out to 2 to 3 microns. This would yield the detection of O2, O3, H2O, CH4, and high levels of CO and CO2. This set of gases would allow for identification of high flux rates of O2 and CH4 to modern Earth’s atmosphere, and discrimination of Earth’s O2 as biogenic in origin. Obtaining this wide wavelength range is difficult. Maintaining UV capability requires clean mirrors, but when cooling the telescope below 260 K in order to observe in
the infrared, precipitates will appear on the mirror’s surface. Keeping the telescope above 260 K, however, will degrade observations beyond about 1.8 microns. On the other hand, the extension out to at least 1.8 microns will maintain the ability to detect the suite of gases required to constrain oxygen fluxes.
Another technical challenge, according to Domagal-Goldman, is the starlight suppression. Using a starshade flying in formation significantly lessens the burdens placed on the telescope itself. Another advantage of a starshade over a coronagraph is that there is no outer working angle, meaning that outer planets potentially as far as Kuiper belt distances will remain visible. However, a major disadvantage is that, as the observations move to longer wavelengths or are made with larger telescopes, the starshades become quite large. To observe at 2 microns on a mission like HabEx or to observe at any wavelengths with a large telescope like LUVOIR, a starshade with a diameter of about 100 m is required. Packing a starshade of that size would be challenging. The biggest challenge, he said, may be the edge tolerance of the starshade petals. A coronagraph, on the other hand, suppresses the starlight within the telescope. However, waveform distortions from the optics must be corrected. In a segmented mirror, this can be done by making very stiff segments and ensuring they do not move with respect to each other with active control systems. The mechanisms for doing this have all flown before, and the control systems required are already in operation on ground-based systems such as Keck. Thus, in theory, all the components are in place for this to work. In practice, however, this has not yet been demonstrated at the systems level at the precision required to suppress starlight sufficiently to detect and characterize potential Earth-like worlds.
Both LUVOIR and HabEx are proposed to observe potentially habitable planets and search for potential biosignatures. However, HabEx will be optimized for planets while enabling a broader range of general astrophysical observations. LUVOIR, on the other hand, will be a general observatory for a variety of astrophysical goals, including exoplanets. The two missions also have different levels of ambition. HabEx aims to search for planets around enough stars to have a very good chance at characterizing at least one rocky planet in the habitable zone of another star. LUVOIR, on the other hand, will attempt to characterize dozens of such worlds. LUVOIR will also be able to constrain the abundance of any property on those worlds, including a biosignature or combination of biosignatures, to a level of ~10 percent. Due to the uncertainty of future budgets and scientific and technological discoveries, Domagal-Goldman wants several options to be prepared for different future realities.
Each mission has two different potential architectures depending primarily on the telescope’s aperture size. For reference, the Hubble Space Telescope and WFIRST each have a diameter of 2.4 m. The HabEx team is considering using either one 4-m, monolithic mirror or a 6.5-m segmented mirror (either hexagonal or pie-shaped), the same size as JWST. The LUVOIR design team is deciding between a 9-m and a 16-m architecture, both segmented. This is the largest telescope a launch vehicle could reasonably fit. He then simulated an observation of Europa with a ~10-m LUVOIR, which would be able to clearly see the structure of the claimed plume. Domagal-Goldman then simulated the number of potentially Earth-like planets observable as a function of aperture size. For a telescope with a 4-m, 8-m, or 16-m aperture, approximately 6, 25, or 100 potentially Earth-like planets would be observable, respectively. The more candidates observable, the more precise the constraints are on the fraction of rocky planets in the habitable zone. However, these telescopes won’t just find potential Earths. They will find everything more detectable than Earth too. Even for the 4-m mission, dozens of other (likely) uninhabitable worlds will also be discovered, such as a warm Titan. A 12-m mission would likely find Jupiter analogs and warm Jupiters. Larger apertures allow for a higher cadence of observations, so more of the temporal domain is observable. This opens up techniques such as longitudinal mapping of planetary surfaces and maybe even latitudinal mapping using seasonal or orbital variations.
Instruments are being considered for both telescopes. Domagal-Goldman said that both are likely to have a starlight suppression technique, probably a coronagraph. LUVOIR is planning an instrument called the Optical-IR Band Spectroscopy Coronagraph for Understanding Rocky Atmospheres (OBSCURA). The goal for OBSCURA is to get a contrast ratio of <1010 with low resolution spectroscopy (R > 150) from 0.2 to 0.4 or up to 1.8 to 2.4 μm if the stretch goal is met.
Another LUVOIR instrument is the UV Multi-Object Spectrograph (LUMOS). This would extend from the far- to the near-UV and have a high resolution of about R ≈ 100,000. When used in multi-object mode, its resolu-
tion would be “medium resolution.” It would also have near-UV imaging capabilities. It will be a major upgrade of Hubble’s Space Telescope Imaging Spectrograph (STIS). LUMOS would provide contextual information on the host stars of potentially habitable worlds. The High Definition Imager (HDI) would be similar to the Hubble Wide Field Camera 3 (WFC3) and would observe in the optical to near-infrared with a field-of-view of 4 to 6 arcminutes. It could possibly allow for high-precision astrometry to measure planet masses. Domagal-Goldman explained that HabEx is considering a “workhorse” UVOIR camera and a UV spectrograph. The UVOIR camera would deliver similar kinds of science to LUVOIR’s HDI instrument, and the UV spectrograph would deliver similar science to LUVOIR’s LUMOS instrument. The HabEx team will select one of these two instruments and leave an “open bay” for a second astrophysics instrument, which could be used for another instrument. This could be the other instrument that was considered, a foreign contribution, or something else.
The fourth and final proposed LUVOIR instrument is a high-resolution spectrograph (up to R ≈ 100,000) with high photometric precision for transits. Potentially, it could also precisely measure radial velocities in order to obtain planet masses. It can also be combined with the coronagraph via a fiber feed to the spectrograph to deliver ultra-high resolution spectra of exoplanets, which can help identify the presence or absence of specific molecules by the pattern of their individual absorption lines. Domagal-Goldman suggested this could be a powerful way to identify individual molecules even at low abundances. This would help improve the context for any potential biosignatures, thereby improving the confidence that they were sourced from biology.
Each telescope has technological challenges commensurate with their levels of ambition. The biggest challenge, according to Domagal-Goldman, is the starlight suppression. If the telescope uses a coronagraph, it must be highly stable and compatible with the entire telescope, including segmented mirrors if they are used. If it uses a starshade, the problems of deploying the starshade, flying in formation, and manufacturing the petals’ edges need to be fixed. Other challenges include needing a heavy-lift launch vehicle and ensuring the compatibility of UV observations with a coronagraph.
A member of the audience said that his exoplanet friends told him that clouds are a big problem in measuring spectra of hot Jupiters. He then asked about how clouds could confound the detection of biosignatures on terrestrial planets. Domagal-Goldman told the audience member that his friends (with all due respect) were wrong to view clouds only as a problem because the formation of the clouds is an important planetary process in itself. He admitted that they do block and refract photons from lower in the atmosphere, but that it is a bigger problem for transit spectroscopy than it is for direct imaging. He said that simulations show that some photons from the planet’s surface do get through the clouds though. Domagal-Goldman finished by wishing that people would think of clouds as conveyors of information.
Jennifer Eigenbrode of NASA Goddard Space Flight Center started by thanking the members of the Mars Science Laboratory (Curiosity) team and the Sample Analysis at Mars (SAM) team for all their work. She then recounted a story about when Curiosity rover landed. During the landing event, she informally polled the team and found out that the majority of people in the room doubted that they would find organics on Mars, especially not in the top 5 cm of the martian surface. Eigenbrode said that she is now convinced that organics are widely distributed over the martian surface and throughout the rock record.
SAM Analysis Techniques
Eigenbrode described the two in situ analysis techniques on SAM. First is the detection of bulk gas composition via evolved gas analysis (EGA), which can reveal the presence of refractory organic matter. Second is the detection of molecules with gas chromatography–mass spectrometry (GCMS). The SAM measurements of organic volatiles begin by heating up a crushed sample of rocks or soil to about 860°C. As gas comes off, a small portion of it gets
“sniffed” into the EGA. This gives an indication of the evolution of the bulk gas all mixed together. The rest of the gas is trapped, and certain analytes are released into the GCMS. The GCMS can identify specific molecules.
Two processes occur, Eigenbrode explained, when heating a sample of pure organic material using helium: thermal desorption and pyrolysis. Thermal desorption is the process where smaller, non-bonded molecules are volatized. When the heated material is purely organic, this process can occur up to about 400°C. Pyrolysis, which proceeds mostly at higher temperatures, is the actual breaking of bonds. This produces two peaks, one for thermal desorption at lower temperatures and one for pyrolysis at higher temperatures (see Figure 5.3). On Mars, the thermal desorption peak is split into two (again, see Figure 5.3). A high abundance of O2 is evolved at approximately 200°C to 300°C, with thermal desorption being dominant at lower and higher temperatures. A typical source of the O2 is the breakdown of oxychlorine phases, such as perchlorate. Pyrolysis remains the strongest process at the highest temperatures. It can break macromolecules apart. In natural material that we know of, organic matter is 75 to 90 percent macromolecular. Macromolecules with more functional groups are more easily broken. In the SAM data, there is a well-known, well-characterized background signal that mostly appears during the thermal desorption phase.
Rocknest and Mojave 2 Sample Results
The first site that Curiosity visited was Rocknest, Eigenbrode recalled, an Eolian Drift on the Gale Crater floor often called “martian soil.” Using evolved gas analysis (EGA) shows a background signal clearly visible, but there is also a bump at 825°C from the release of refractory organic material. This bump shows C1, C2, C3, and possibly C4 signals. Eigenbrode said that this is probably a reduced carbon source bound up in a mineral, which had to break down in order to release the carbon. If it was just a refractory organic material in a macromolecule, the bump would have been more smeared out. This sample’s bulk chemistry as measured by the Alpha Particle X-ray Spectrometer (APXS) is similar to that of Meridiani and Gusev. These locations are mostly basaltic and are thought to be a global signature, implying that the reduced carbon phase might also be global.
Eigenbrode then moved on to stratigraphy, focusing on two samples of mudstones. Mudstones are fine-grained, which is difficult for water to get through. This implies a better chance that organic materials may be preserved inside them. One sample is from Yellowknife Bay, Cumberland, and the other one is at Pahrump Hill/Marias Pass called Mojave2, but both are considered lake lacustrine deposits. However, they were deposited at different times and have different compositions. Thermal desorption of the Cumberland sample showed chlorinated C1 to C4 hydrocarbons and benzene.1 EGA found a set of high temperature (>500ºC), correlated peaks for single-ring,
1 C. Freissinet, D.P. Glavin, P.R. Mahaffy, K.E. Miller, J.L. Eigenbrode, R.E. Summons, A.E. Brunner, et al., 2015, Organic molecules in the Sheepbed Mudstone, Gale Crater, Mars, Journal of Geophysical Research: Planets 120:495.
aromatic hydrocarbons with masses of up to about 100 atomic mass units, which she said may be macromolecular material. A similar thing is seen for C1 to C4 alkyl hydrocarbons, but they are relatively weak signals. Eigenbrode said that this suggests that a macromolecular species (or something else very refractory) is undergoing pyrolysis. If you add in ionizing radiation and metal catalysts, which are known to be present because the material is basaltic, a Fenton-like reaction is possible. When macromolecular material is broken down, smaller, oxygenated molecules are made. These can then be chlorinated to produce C1 to C4 chlorinated molecules and chlorobenzene, which are equivalent to the types of molecules detected during thermal desorption.
The Mojave2 sample is at the bottom of the Lower Mound outcrop at Gale Crater. EGA shows large peaks from C1 to C4 and potentially even C5 alkyl hydrocarbons at high temperature. The possibility of nitrogen and oxygen atoms, however, makes this difficult to interpret. The single-ring, aromatic hydrocarbons, such as chlorobenzene and toluene, also display peaks. Organic sulfur volatiles are also detected, such as thiophene, methanthiol, and dimethylsulfide, which are confirmed by GCMS. These are not seen in the Cumberland blank sample. Eigenbrode performed this same analysis on the Tissint martian meteorite, which fell in Morocco in 2011, and found consistent results.
Eigenbrode’s conclusion is that refractory organic matter is present on Mars. The source of this material, she said, is still unknown. It could have an abiotic igneous or hydrothermal origin, which has been suggested for the Tissint meteorite. Another potential source of organic matter on Mars is meteorites. After erosion, the organic material could then be concentrated in lake beds. A biological origin is also possible if the material was heavily processed, since heavier molecules were not seen. Meteorite impacts or irradiation might have broken down this material into smaller molecules. The implication of her conclusion is that organic matter might be widespread on the martian surface and in the rock record. The fact that this was discovered in a lake bed supports the hypothesis that the lake in Gale Crater was potentially habitable ~3.6 Gyr ago. Looking forward, Eigenbrode said that these organics could help support habitability on Mars now and in the future.
An audience member asked about the presence of nitrogen compounds on Mars, or lack thereof, and what this might imply for life. Eigenbrode said that there are nitrates on Mars.2 However, they were not able to distinguish whether or not the organic material contains nitrogen, especially for the expected C1 to C5 hydrocarbons.
According to Dr. Ben Clark’s earlier talk, the Viking pyrolysis stopped at 500°C and missed these organic signals. A conference participant then asked why the SAM limit seemed to be 825°C, implying that more interesting data could be revealed at higher temperatures. Eigenbrode answered that they can go to higher temperatures; they hit 860°C regularly. However, it takes a lot of power to go higher. With limited power resources and insufficient initial results to demonstrate the need to go hotter, they usually choose not to. The audience member then asked what the results might be if they went to consistently higher temperatures. Eigenbrode said the amount of organics might be increased if they were trapped in minerals that only break down at hotter temperatures. She then said that this might be an important process on Mars.
In the energy crisis of the 1970s, one audience member noted, the United States put a lot of money into coal structures, which would contain thiophene. He then asked how much coal could diagenize before its biological origin would no longer be apparent. Thiophene, he also noted, is found in some meteorites. The audience member finds thiophene interesting, since it is a diagenesis product of sulfur-containing biology. Eigenbrode responded that thiophene is usually formed by a C4 structure. In the presence of sulfur materials and at different pH or temperatures, you can sulfurize diene into thiophene. Thiophene is found in abiotic material as well as biological materials. She therefore thinks thiophene is not a good biosignature. However, Eigenbrode said, they also found methylthiophene, a C5 molecule, and they know that they didn’t bring any C5 molecules with them on SAM.
Another member of the audience then asked what her message is to Gil Levin, the principal investigator of the Viking labeled-release experiment, who in 1997 claimed (and continues to claim) that the 1976 Viking Lander
2 J.C. Stern, B. Sutter, C. Freissinet, R. Navarro-González, C.P. McKay, P.D. Archer, Jr., A. Buch, et al. 2015, Evidence for indigenous nitrogen in sedimentary and aeolian deposits from the Curiosity rover investigations at Gale Crater, Mars, Proceedings of the National Academy of Sciences of the U.S.A. 112:4245.
positively detected microbial life on Mars. Eigenbrode answered by saying that GCMS was the right approach for Viking and that the labeled release experiment was a genius idea. An audience member then asked how she would interpret the labeled-release experiment. Eigenbrode said that she doesn’t think that her results have any impact on it. However, degraded products, such as smaller carbon compounds like the chlorohydrocarbons in the Cumberland sample, could have easily oxidized under different types of conditions, such as the labeled-release experiment.
A workshop participant then asked what the concentration of organic material is in concretion-rich martian soil and in the mudstones. Eigenbrode said that the answer is still a work in progress, but that it is in the parts per million range.
One participant at the workshop then asked what kind of instrument Eigenbrode would like to send to Mars next in order to study the organic material. She replied that she wants to learn how the organic molecules are preserved, what mineral associations they are contained in, and how they got in them. One way to answer this question, she said, is to take the refractory organic material, break it up, and get more mineralogical information out of it. Another way is to look at the material with spectroscopy. It could even require sample return. She is concerned, however, that the organic material might be heterogeneously distributed, so that follow-up missions might not see organics depending on the technique and the sample location. Eigenbrode wants more details on the structure and the context of the organics.
A member of the audience then asked how they know that the peak at 825°C is not contamination from Earth and asked if that experiment has been done on soil or minerals on Earth to see if similar peaks occur. Eigenbrode said that the Rocknest sample was analyzed four times, two of which showed the peak. Prior to those tests, they did a blank that used just the instruments. The peak also was not seen in other samples. This shows heterogeneity on the martian surface. They tried looking at this material with a GCMS, but because the temperature is so hot, the particles were hard to trap, especially with a sample containing a lot of chlorine and sulfate, which alter the trapping conditions. Therefore, the level of trapped material may not have been sufficient for detection. They did see some chloromethane, but are not sure what the origin was or whether it could be from the instrument. As far as Earth-based tests, she has tried, but has been unable to reproduce the signal. They have maybe found CO, CO2, and CH4, but nothing with more carbon atoms. Every other sample they have tried has exhibited traditional behavior with only two peaks: thermal desorption and pyrolysis.
Lastly, a participant at the workshop then asked how to get around the problem of perchlorate chemistry when heating the sample. Eigenbrode said that, in order to avoid the oxidation of organics by the oxygen produced by perchlorate when heating, you have to stick the oxygen somewhere, such as providing other materials for the oxygen to latch onto, like strongly alkaline-like materials or reductants to buffer the oxidation. She thinks that the tetramethylammonium hydroxide (TMAH) experiment on SAM could diminish perchlorate’s effects, but they haven’t yet run those experiments with martian samples. Lastly, Eigenbrode offered another strategy, which would be to do a two-step process: (1) heat up the sample to lower temperatures to drive off the perchlorate oxygen, and (2) continue to higher temperatures to examine the pyrolysis aspect of the experiment.