NASA Astrobiology Institute (NAI)

1. Climate, Habitability, and the Atmosphere on early Mars

   Project Investigators: Kristie Boering

   Other Project Members

   Emily Chu (Doctoral Student)

   Philip Croteau (Doctoral Student)

   Summary

   Atmospheric chemistry has profound implications for the climate and habitability of Mars throughout its history. The presence and stability of greenhouse gases and aerosols, for example, may regulate climate or force climate change. Chemical reactions in the atmosphere initiated by light (“photochemistry”) may also produce gases or aerosols that serve as a shield against ultraviolet light (as stratospheric ozone does on earth) and possibly warm or cool the surface, which, in turn, has implications for the presence and stability of water on Mars. Thus, understanding the chemical composition and physical properties of possible Martian atmospheres over time is vital to the understanding of the opportunities and challenges for early life on Mars, as well as the importance of habitat features that provide radiation protection. In this project, we are investigating in laboratory experiments how quickly photochemistry can destroy and produce various greenhouse gases and aerosols and whether or not the aerosols may serve to warm or cool the surface. We are also investigating whether or not these photochemical reactions can produce carbon-rich aerosols that might be depleted in the stable isotope carbon-13 relative to carbon-12, and thus might be mistaken for an isotopic signature produced by biological processes after the aerosols settle out of the atmosphere and become incorporated into the martian rock record or meteorites that have made it to earth.

   Astrobiology Roadmap Objectives:

   • Objective 1.1: Models of formation and evolution of habitable planets
   • Objective 1.2: Indirect and direct astronomical observations of extrasolar habitable planets
   • Objective 3.1: Sources of prebiotic materials and catalysts
   • Objective 6.1: Environmental changes and the cycling of elements by the biota, communities, and ecosystems
   • Objective 7.1: Biosignatures to be sought in Solar System materials
   • Objective 7.2: Biosignatures to be sought in nearby planetary systems

   Project Progress

   With the 30% budget cuts to the NAI teams and redistribution of the remaining funds among the BioMARS team, funding for this project ended on 9/30/2007. Over this time period, we continued experiments on (1) whether or not a photochemical haze may have formed in Mars’ early atmosphere, (2) whether or not such a haze may have warmed or cooled the surface – that is, resulted in a greenhouse or “anti-greenhouse” effect, and (3) whether such an aerosol could have settled to the surface and provided a potentially “false biosignature” in the carbon-13 isotopic composition of organic matter in the martian rock record. PhD students Philip Croteau and Emily Chu continued experiments monitoring the kinetics of formation of hydrocarbon species more complex than CH₄ and of aerosols by light scattering showing that as a simulated atmosphere increases in CO₂ relative to CH₄, particle production at first increases as CO₂ increases and then markedly decreases, similar to results found by Trainer et al., PNAS, [2007] in a very different experimental set-up but contrary to earlier photochemical model predictions by Zahnle [1986] and Kasting et al. [1983]. The implications are that organic aerosols are likely to have been more prevalent on early Earth and Mars than photochemical models have predicted and could have had a significant influence on surface temperatures (and hence the stability of liquid water at the surface), depending on their optical properties. A manuscript is in preparation for publication of these results. We also continued our collaboration with Yuk Yung and co-workers at Caltech in comparing our laboratory results irradiating CH₄ with UV light with predictions from photochemical models in order to assess the strengths and weaknesses of photochemical model reaction schemes for planetary atmospheres run at room temperature [Wong et al., 2006], relevant for understanding atmospheric photochemistry, organic hazes, and the atmospheric production rates of prebiotic chemicals on Titan and, eventually, for more oxygen-rich atmospheres on early Earth and Mars. The comparison of lab and modeling results and sensitivity studies suggest that one or several key reactions may be orders of magnitude off and/or that a reaction scheme(s) other than those in the model predominate; the comparisons also point the way to model improvements as well as which key reactions need further scrutiny in the laboratory on an individual basis. A manuscript is now in final revisions for submission to Icarus. Finally, we began a more systematic investigation of the carbon-13 isotopic composition of aerosols produced in our experiments in a collaboration with David DesMarais and Michael Kubo of NASA Ames Research Center. Work is now continuing on several aspects of this project through partial support from the NASA Planetary Atmospheres Program, which will aid in seeing the project through to completion, including both publication of the final results and the awarding of PhDs to students Croteau and Chu.

   Mission Involvement

   Cassini spacecraft and Huygens Probe

   Measurements and comparisons with planetary atmosphere photochemical models carried out under this NAI are highly relevant for understanding atmospheric data returned from these missions.

   Terrestrial Planet Finder

   Constraints on photochemistry in our terrestrial-like experimental simulations should ultimately aid modelers in predicting what sort of spectroscopic signatures may be characterisitic of terrestrial-like planets. For example, a significant amount of haze in a terrestrial-llike N2-CO2 atmosphere might indicate that a biotic or abiotic source of CH4 exists.

   Cross-Team Collaborations

   Collaborations with NASA Ames NAI: “Linking our Origins to our Future” Team Members:

   • David DesMarais and Michael Kubo: Measuring the carbon-13 isotopic composition of organic haze particles produced in laboratory experiments. Ongoing.

   • Sanford Davis: Measuring light scattering from particles, relevant for our planetary atmospheres experiments and their experiments for protoplanetary disk processes. We are sharing our experimental expertise and custom polarimeter and Davis’ and Co-I’s expertise in supercomputer calculations of the aerosol scattering matrices in order to derive the optical properties of dust and haze from experiments. Ongoing.

Publications

Adamkovics, M. & Boering, K.A.  (2004).  Laboratory studies of the photochemical formation of organic aerosols in CO2-rich environments.  Third NASA Astrobiology Science Conference.  Moffett Field, CA.