The fuel cell model of abiogenesis: A new approach to origin-of-life simulations

Barge, L.M., Kee, T.P., Doloboff, I.J., Hampton, J.M.P., Ismail, M., Pourkashanian, M., Zeytounian, J., Baum, M.M., Moss, J.A., Lin, C.-K., Kidd, R.D. & Kanik, I.

Astrobiology, 14, pp 254-270.

doi:10.1089/ast.2014.1140, 2014.

[JPL News]

Brief summary

Fuel cells are similar to biological cells, as well as certain geological environments, in that electrons are transferred from fuels to oxidants to produce electrical current. It has been proposed that the electrical energy in hydrothermal systems could have powered some processes that could give rise to life. We argue that fuel cells could be used to simulate some prebiotic reactions and the origin of life because of the conspicuous fuel-cell-like properties of metabolic and geo-electrochemical systems. These modular experiments could be used to investigate a variety of prebiotic catalysts and reactions, and could also be adapted to simulate seafloor systems on other planets, to see if these worlds provided enough energy to kick-start life.

Extended summary

The processes of microbial life today are analogous to those of a fuel cell, in that life maintains gradients across membranes, and harnesses these gradients to drive metabolism. Life does this using metal-containing enzyme ‘electrodes’ to transfer electrons as well as tiny molecular engines that convert proton potentials to chemical energy. The first life on Earth also likely contained some version of these fundamental components that make the biological fuel cell function (ATP-synthase, ion-selective membranes maintaining pH/electrical gradients, the electron transport chain). Certain geological environments, such as hydrothermal vents, also have fuel-cell-like properties: they can generate pH gradients and electrical current through abiotic chemical reactions, and the electrochemical energy provided in these geological settings can drive redox reactions in some ways similar to metabolism. It has been suggested that the first metabolism could have arisen in hydrothermal systems, and so we aimed to develop laboratory methods of understanding the energetic processes that bridged the gap between geological processes of the early Earth and the emergence of life on this planet.

In a paper recently published in the journal Astrobiology, we set out to examine whether origin-of-life chemistry in fuel-cell-like geochemical environments (such as hydrothermal vents) could be simulated in the laboratory as an actual fuel cell. This is a new method for combining fuel cell technology with planetary science / prebiotic chemistry simulations. We first modeled the hydrothermal system as a fuel cell: defining the surfaces of a hydrothermal chimney as the electrodes, the chimney wall as the proton exchange membrane, the hydrothermal fluid as the fuel, and the seawater as the oxidant. We grew little simulated hydrothermal chimneys in the laboratory under early Earth conditions, and created fuel cell electrodes using this chimney material as the electro-catalyst. We then tested if this material can help transfer electrons, as hydrothermal chimneys today are sometimes capable of, as are similar biological enzymes. By testing different types of materials in the simulated chimneys, these fuel cell experiments allow us to observe the particular redox / proto-metabolic reactions that might occur within a hydrothermal chimney, driven by the geological electrochemical gradients – and thus narrow down on the chemistry that might have taken place when life first arose on Earth.

One particular advantage of using fuel cell experiments to simulate geological environments is that fuel cells are modular – meaning one can easily swap out components. So, we can test different compositions of the oceans and hydrothermal fluid, as well as different minerals that might exist with hydrothermal chimneys and act as catalysts. We can investigate the effects of adding organic molecules, or even cells, and experimentally test the transition from geo-energetics to bio-energetics. Finally, there is the exciting possibility of building Mars or Europa fuel cells to simulate the water-rock interfaces on these worlds, and thus help in understanding whether life could have emerged in other planetary environments.

The rise of oxygen and the hydrogen hourglass

Zahnle, K.J., Catling, D.C. & Claire, M.W.

Chemical Geology, 362, pp 26-34.

doi:10.1016/j.chemgeo.2013.08.004, 2014.

Brief summary

The most profound change in Earth’s climate and chemistry over it’s 4.5 billion year history occurred 2.4 billion years ago when the surface oceans and atmosphere became flooded with O2 gas. The scientific community has been considering the question “What caused the rise of Oxygen” for 100 years, and the fundamental answer still remains unclear. This article was written for a commemorative journal issue celebrating the life of Dick Holland, who dedicated his scientific career to this question, as well as mentoring multiple generations (including myself) on the careful use of geochemistry to understand past environments. The paper argues for the primary importance of hydrogen escape in thinking about the evolution of the Earth, which for some reason is generally ignored by Earth scientists.

Extended summary

This paper was written in response to a paper named “What caused the rise of oxygen?” in the same special issue of Chemical Geology, dedicated to Dick Holland. We felt that the provocatively entitled paper didn’t actually answer the question, although it did create a very useful conceptual framework for one way of addressing the question. However, even this framework left out an additional angle to the story that we find important.

As a starting point, consider the starting materials. Most oxygen on Earth is bound up in silicate rocks and these bonds are nearly impossible to break in natural settings. Leaving aside the oxygen bound up in silica (and iron oxides) leaves H2O as the dominant reservoir of oxygen on Earth. This oxygen was delivered to Earth in the form of ice and hydrated minerals. The atmosphere didn’t start with any O2 gas in it, so how do you get O2 gas in the atmosphere? Despite thousands of years of alchemists trying, it turns out that, in chemistry, you can’t get something for nothing. The only way to get O2 in the atmosphere is to split apart the H2 from the O, or more precisely 2H2O -> 2H2 + O2. Left to their own devices, the H2 and the O2 would wind up reforming H2O. In thinking of the evolution of the Earth, it doesn’t really matter if this occurs in minutes or in millions of years – if the H2O reforms, it is a zero-sum game as far as O2 goes. The only way that you can permanently put O2 in the atmosphere is if you find somewhere else to permanently store the H2. So although it can seem quite complicated if you read papers on the subject, the primary aim of all “what caused the rise of O2?” papers is basically trying to find somewhere to store a vast quantity of H2.

There are really only two viable options for permanently removing H2 from Earth’s surface environment – Earth’s mantle or the rest of the Universe. Many Earth scientists, including Dick Holland, focus on the mantle as this reservoir. Somewhat confusingly, the story generally involves a concept called “mantle redox evolution” in which the volcanic gases from the mantle starts out producing more reducing gases (i.e a high H2:H2O ratio) in early times but then calms down with time and starts producing more neutral gases (i.e. a lower H2:H2O ratio). This process certainly could work, and their are many papers explaining how it might work, but they are all complicated by a simple fact. In order to make the mantle gases more oxidised with time, you have to take an already reduced mantle, and reduce some portions of it even more (i.e. sopping up all the H2), but somehow keep that very reduced portion away from the volcanic source regions. There is certainly heterogeneity in the mantle and it is admittedly a huge reservoir that could easily soak up the necessary amount of H2, but we think there is a far simpler option.

The figure shows Earth as imaged in the Lyman alpha filter by NASA’s Dynamics Explorer I satellite. Lyman alpha pictures reveal hydrogen, so all of those red pixels are hydrogen atoms in Earth’s “geosphere” (You can also see the auroral oval prominently on the night side) These hydrogen atoms have all left the gravitational influence of Earth and are swept away by the solar wind to become part of the background H in the solar system, galaxy, and universe. H is escaping the planet as you read this at 3 kilograms (6.6 US pounds) per second. There goes another 3 kg. And another. Although it seems like a lot, when you multiply this escape rate over 4.5 billion years, it’s not enough to account for all the O2 in the atmosphere. However, before there was any O2 in the atmosphere, there was a lot more H2 in the atmosphere. And the escape rate is directly proportional to the amount of H2 in the atmosphere. If you multiply the predicted amount of H2 in the early atmosphere (aided by biogenic CH4 in an interesting aside) by the escape rate, you can account almost perfectly for the amount of free O2 on Earth. We find this a simpler argument than reducing a reduced mantle to make it more oxidising.

This paper is a review, and so provides not much new data or insight. Mostly we just wanted to remind the community of our admittedly minority belief that H escape is important for the rise of O2. H escape is the accepted mechanism for explaining the oxidation state of Venus, Mars, Titan, Ganymede, Io, Europa, and Rhea in our solar system. The hope in writing this paper is that Earth scientists might gain insight from their planetary science colleagues in answering difficult questions about Earth’s evolution.

Funding the Search for Extraterrestrial Intelligence with a Lottery Bond

Haqq-Misra, J.

Journal of the British Interplanetary Society, in review.

arXiv:1311.2467, 2013.

[Astronomy Magazine] [arXiv Blog] [Business Insider] [The Raw Story] [Mashable] [OpenMinds] [Motherboard] [Taloussanomat]

Listen to the author interview! [mp3 download]

Brief summary

This paper proposes to construct a “SETI Lottery Bond” as a fixed rate perpetual bond with a lottery at maturity, where maturity occurs only upon the discovery of extraterrestrial intelligent life. Such a savings product would be managed by a financial institution and would provide a constant stream of funding for SETI facilities, such as the Allen Telescope Array, that would sustain SETI until its first discovery. The SETI Lottery Bond appeals to ethical investors who are interested in supporting SETI through investment, rather than charitable donation.

Extended summary

In this paper, I propose the establishment of a SETI Lottery Bond to provide a continued source of funding for the search for extraterrestrial intelligence (SETI). The SETI Lottery Bond is a fixed rate perpetual bond with a lottery at maturity, where maturity occurs only upon discovery and confirmation of extraterrestrial intelligent life. Investors in the SETI Lottery Bond purchase shares that yield a fixed rate of interest that continues indefinitely until SETI succeeds—at which point a random subset of shares will be awarded a prize from a lottery pool. SETI Lottery Bond shares also are transferable, so that investors can benefact their shares to kin or trade them in secondary markets. The total capital raised this way will provide a fund to be managed by a financial institution, with annual payments from this fund to support SETI research, pay investor interest, and contribute to the lottery fund. Such a plan could generate several to tens of millions of dollars for SETI research each year, which would help to revitalize and expand facilities such as the Allen Telescope Array. The SETI Lottery Bond is a savings product that only can be offered by a financial institution with authorization to engage in banking and gaming activities. I therefore suggest that one or more banks offer a lottery-linked savings product in support of SETI research, with the added benefit of promoting personal savings and intergenerational wealth building among individuals.

Galactic cosmic ray induced radiation dose on terrestrial exoplanets

Atri, D., Hariharan, B. & Grießmeier, J.-M.

Astrobiology, 13, pp 910-919.
doi:10.1089/ast.2013.1052, 2013.

[arXiv] [Astrobiology Magazine] [ABC Science]

Listen to the author interview! [mp3 download]

Brief summary

We explore the dependence of cosmic ray induced radiation dose on the strength of the planetary magnetic field and its atmospheric depth, finding that the latter is the decisive factor for the protection of a planetary biosphere. We conclude that in addition to the liquid water habitability criteria, biological radiation dose should also be considered as an important factor in constraining the habitability of a planet.

Extended summary

What are the physical conditions that make a planet habitable? The solution to this problem depends on the definition of habitability. One way to approach this problem is to study the Earth and estimate the range of physical conditions which can support an Earth-like biosphere. These include the astrophysical conditions such as the stellar spectrum and flux and also the properties of the planetary atmosphere for climate modeling. There is a tremendous interest in the search for signatures of life on planets around stellar systems, which can support liquid water on its surface (Kasting et al., 1993). However, here we focus on a different approach, where we estimate the range of physical conditions for which the radiation dose can permit a stable Earth-like biosphere. We explore various physical conditions that give rise to increased radiation dose on an exoplanet’s surface. The radiation environment of a planet consists not only of the photon and proton flux from the host star, but also the galactic cosmic ray (GCR) flux consisting of charged nuclei (mostly protons). Although the flux of GCRs is only a small fraction of the radiation flux from the host star, the average energy of individual GCR particles is higher by several orders of magnitude than photons and protons from the host star. The GCR flux depends on (1) the magnetic moment of the planet, and (2) the location of the planetary system at a particular time in the galaxy. GCR secondary particles comprise of the most penetrating ionizing radiation and its biological effects have been discussed extensively (Atri and Melott, 2013; Melott and Thomas, 2011; Dartnell, 2011).

Cosmic rays strike the planetary atmosphere and produce secondary particles, including muons, which are highly penetrating. Some of these particles reach the planetary surface and contribute to the radiation dose. Along with the magnetic field, another factor governing the radiation dose is the depth of the planetary atmosphere. The higher the depth of the planetary atmosphere, the lower the flux of secondary particles will be on the surface. If the secondary particles are energetic enough, and their flux is sufficiently high, the radiation from muons can also impact the sub-surface regions, such as in the case of Mars. If the radiation dose is too high, the chances of sustaining a long-term biosphere on the planet are very low. We explore the dependence of the GCR induced radiation dose on the strength of the planetary magnetic field and its atmospheric depth, finding that the latter is the decisive factor for the protection of a planetary biosphere. We conclude that in addition to the liquid water habitability criteria, biological radiation dose should also be considered as an important factor in constraining the habitability of a planet.