Proposal Abstract
A major question of the history of our solar system concerns the faint young sun paradox and whether the sun’s lower luminosity in the Hadean and Achaean eons was compensated by higher contents of gases with stronger greenhouse potential. Methane is such a gas, and is probably the one that is most easily envisaged in high concentrations in Earth’s early atmosphere. It is, however, not at all clear whether it would have been degassed from Earth’s mantle, formed abiotically through weathering reactions in the presence of water, or even biogenically by early microorganisms in the subsurface. The importance of methane is, however, not restricted to our own planet. Methane has been detected in the atmosphere of Mars and the compound is an important component of the outer gaseous planets and their satellites. Methane-induced chemistry is known to take place in extrasolar planets and even in the interstellar medium.
We propose to create a team of about 13 specialists in the complementary fields of astronomy, chemistry, microbiology, geology, planetary science, and physics to address a multitude of questions related to the formation and destruction of methane. Questions that will be addressed are, for instance, the formation of methane in the interstellar medium and its role in star-forming regions and protoplanetary disks, the origin and cycles of methane on Titan and other icy moons, and the implications of the existence of methane in the atmosphere of Mars and exoplanets. We will also look into the mechanisms for the formation of complex biomolecules in methane-dominated atmospheres and aquatic environments, such as the ocean floors and Earth and some of the satellites of the outer gaseous planets. We will try to elucidate to what extent microorganisms can survive on methane solely and which terminal electron acceptors that would be available for them under different environmental conditions. The proposed ISSI team will thoroughly review the role of methane in planetary systems and we expect that it will create novel cross-discipline research projects for the future.
Scientific rationale
The study of the formation and destruction processes of methane is important for several different reasons. Methane has a greenhouse gas potential about four times stronger than carbon dioxide on a molar basis. Vast amounts of methane are stored on Earth as solid methane clathrates in the continental slopes and permafrost regions of the Arctic. Such clathrates are destabilized by increasing temperatures and/or decreasing pressure, thus releasing molecular methane gas to the atmosphere. The atmospheric residence time of methane on Earth is about 14 years. Methane-rich plumes in deep ocean waters on Earth are known to be linked to hydrothermal circulation in the ocean floor. Fischer-Tropsch type reactions convert CO2 to CH4 by reaction with H2 produced during serpentinization of olivine in ocean floor basement rocks. During the early ages of Earth, methane could have been a major constituent of the atmosphere and thus have played a pivotal role in the climate of the primeval planet and the origin of life. Methane, together with ammonia, is known to form hydrogen cyanide (HCN) in a variety of environments. HCN is central to most reaction pathways leading to abiotic formation of simple organic compounds containing nitrogen, such as amino acids and nucleotide bases (purines and pyrimidines). It is, therefore, considered to have been a key compound for prebiotic chemistry and the origin of life.
The importance of methane is not restricted to our own planet. Very recently, methane has been detected in the atmosphere of Mars, which raised the important question how this compound is formed and released on a planet with a highly oxidizing surface. Furthermore, methane is an important component of the outer gaseous planets. Saturn’s satellite Titan even has a cycle of solid liquid and gaseous methane, similar to the water cycle on Earth. Ionization of methane by magnetospheric electrons, cosmic rays and solar wind particles (when Titan emerges from Saturn’s magnetosphere) and photolysis by solar UV rays trigger a complex chemistry which can even lead to primitive biomolecule precursors and polymeric tholines (which, upon hydrolysis, have shown to yield amino acids and nucleobases). Furthermore, methane has been detected by the Cassini spacecraft in the plumes emerging from the southern polar region of Enceladus. The question if this compound has been formed in situ (e.g. by Fischer-Tropsch-type synthesis on glaciated moons) or delivered earlier onto the satellite and just released by the geyser is still under debate. Cooperative efforts by geologists, planetary scientists and physicists are necessary to solve such issues.
Methane-induced chemistry in planetary atmospheres is also taking place in extrasolar planets. The compound has been detected in transiting hot Jupiter-type exoplanets and, very recently, methane, together with water and traces of CO2 was identified as components in the atmosphere of HD 209458b. Furthermore, non-local thermal equilibrium conditions for methane have been observed in an extrasolar planet, which further enhances the complexity of methane-dominated atmospheres. An evolved chemistry that started by dissociation and ionization of methane is probably going on in the ionospheres of many exoplanets, and a new generation of powerful telescopes will be able to throw more light on the processes happening in these environments through possible identification of other substances formed from methane there.
Methane is also present in the interstellar medium, where, due to the low temperatures in this environment, it can freeze out on carbonaceous and silicate grains together with other compounds to form ices. In star-forming regions the temperature augmentation through the collapse causes these ices to evaporate, sharply increasing the molecular abundances, which enables formation of complex species and also influences the formation of planetary systems. Molecules formed in protoplanetary disks can finally be captured by the gravitational field of the planets becoming part of their atmospheres or getting frozen in as component of planetary ices.
Gas-phase methane observations in interstellar objects have been severely hampered by its lack of dipole moment, which prohibits mm and submm detections. However, the new telescope array ALMA with its superior resolution and signal-to-noise ratio provides an unprecedented opportunity to search for deuterated isotopomeres like CH3D, which possess a weak permanent dipole moment.
Research on the chemical and physical processes of methane in planetary atmospheres encompasses a number of different subjects ranging from observational astronomy to geochemistry. In order to fully understand the methane chemistry in these environments, research efforts in a multitude of fields are necessary. The proposed ISSI team plans to thoroughly review this subject from a multidisciplinary angle and create a forum for development of novel cross-discipline research projects concerning the role of methane in planetary systems.
Goals
As mentioned above, the formation of methane and the subsequent processes induced that compound in space and planetary atmospheres is a truly interdisciplinary subject, which unfortunately brings about that scientists engaged in the area are spread about very different subjects. Therefore we aim to bring together a team of about 13 scientists to develop collaborative research strategies on the following subjects trying to answer the following questions:
- Methane formation in the interstellar medium
- Which feasible mechanisms exist for methane production on grain surfaces and in the gas phase?
- Which interstellar environments are carbon-rich and foster methane
formation?
- Which future observational projects of methane isotopomers can be devised to elucidate the formation of methane in dark clouds?
- Methane in star-forming regions and protoplanetary disks
- What can high-resolution observations of methane in protoplanetary disks tell us about the role of methane in planet formation?
- How does methane influence the formation of planetary systems?
- How can the rich chemistry initiated by methane in star-forming regions be elucidated?
- The methane cycle on Titan
- What is the origin of methane on icy moons, especially Titan? What can isotope ratios tell us about this?
- What is the chemistry associated with methane conversion to complex organics? What are the final products?
- What are the production and loss processes of methane in Titan’s atmosphere and what is their relative importance?
- Which conclusions can we draw from Titan’s methane chemistry on the one of early Earth and possible terrestrial atmospheres on exoplanets?
- What is the role of methane in Titan’s weather and climate?
- Formation of complex (bio)molecules in methane-dominated atmospheres and aquatic environments
- Is methane an indicator of the occurrence of more complex organic molecules?
- Is methane in equilibrium with other simple organic compounds?
- Does methane participate in abiotic reactions with nitrogen compounds to form N-heterocycles and amino acids?
- Methanogenesis on the ocean floors
- What is the history of atmospheric methane abundance on Earth?
- How was methane produced during different ages of our planet?
- Does active transport of methane always occur along faults to point sources on the ocean floor?
- Can methane be formed on the bottom of subglacial oceans on icy moons?
- Methane on Mars- a sign of conditions supporting life
- Which research projects on methane should we plan for projected missions to other planets and their satellites?
- Does the formation of methane on Mars depend on degassing, weathering reactions or biogenic production?
- Is there a pervasive flux of methane or are sources local?
- Do the seasonal variations of methane in the Mars atmosphere depend on gas hydrate formation and dissolution or other mechanisms?
- Methane on exoplanets
- How abundant is methane in different exoplanet atmospheres?
- What can recent observations tell us about the methane abundance and the temperature profile in atmospheres of exoplanets?
- How can we use future telescopes best to elucidate atmospheric chemistry of exoplanets?
- Methane as a substrate for heterotrophic organisms
- Can heterotrophic organisms use methane as a substrate solely?
- What are the potential electron acceptors outside Earth?
Timeliness of the project
The recent successes of the Cassini spacecraft collecting a cornucopia of data on the ionosphere of Titan and the identification of hydrocarbons in plumes from geyser eruption of Enceladus, the identification of methane outflows from Mars as well as the identification of methane in exoplanetary atmospheres have undoubtedly moved methane-induced processes on planets in the focus of interests. The multitude of recent papers in high profile journals like Nature and Science warrant that. This offers a unique chance to combine research efforts from different fields to make a crucial step forward in the understanding of the role of this substance in oceans and atmospheres of planets and their satellites.
Expected output
One of the tasks of the team would be review extensively the state of knowledge in the field of methane-induced processes in space and on planets: These efforts should result in a series of review articles in a special issue of a high-quality scientific journal. Moreover, new cooperative research projects designed by the team will result in further publications.
