Geothermal heating enhances atmospheric asymmetries on synchronously rotating planets

Haqq-Misra, J. & Kopparapu, R.K.

Monthly Notices of the Royal Astronomical Society, 446, pp 428-438.

doi:10.1093/mnras/stu2052, 2014.

[arXiv] [Sciworthy]

Brief summary

Earth-like planets that orbit small, red stars (M-dwarfs) might be able to sustain liquid water on the surface and provide habitable conditions where life could develop. However, such planets orbit their parent star so closely that they are prone to fall into synchronous rotation so that the “sub-stellar point” of the planet always faces the star and is constantly heated, while the opposing “anti-stellar point” never receives starlight and is constantly cooled. These atmospheres can be at risk of collapsing into huge ice caps on the anti-stellar side, but the large-scale motions of the atmosphere can provide enough energy transport from the warm side to the cold side to keep the climate stable.

In this study we use a general circulation climate model (GCM) to find that geothermal heating from tidal interactions can act to amplify warming on the night side of a synchronously rotating planet due to changes in energy transport by large-scale atmospheric dynamics. We show that the patterns of circulation on these planets (which on Earth is known as the Hadley circulation) changes direction depending on the eastward or westward position from the sub-stellar point. We also demonstrate the presence of a cross-polar circulation that transports energy and mass from the sub-stellar to anti-stellar point across the northern and southern poles and also contributes to climate stability. Understanding the impact of physical processes on the dynamics of the atmosphere is critical to assessing the habitability of terrestrial planets orbiting low-mass stars.

Damping of glacial-interglacial cycles from anthropogenic forcing

Haqq-Misra, J.

Journal of Advances in Modeling Earth Systems, 6, 294-299.

doi:10.1002/2014MS000326, 2014.

[arXiv] [Sciworthy]

Brief summary

Geologic records over the past million years indicate a 100,000 year cycle in the extent of Earth’s surface covered by ice. These ice age cycles are a result of variations in Earth’s orbital geometry, but it is unclear how these variations will continue in the presence of significant human emissions. Here I develop a simple climate model to demonstrate the potential for human-induced climate change to damp out these variations in ice coverage, which suggests that human actions today could have long-lasting impacts into the future.

Extended summary

Long-term patterns in Earth’s climate show glacial cycles that correspond to variations in Earth’s orbital geometry and affect the overall amount of sunlight that the planet receives. Known as “Milankovitch cycles”, these variations are observed in geologic reconstructions of temperature and isotopes to show periodic changes every 23,000, 41,000 and 100,000 years. The first two of these correspond directly to changes in Earth’s tilt (i.e. obliquity) and wobble (i.e. precession), but the longer 100,000 year variations in orbit seem too weak in magnitude to drive the strong climate signals we observe.

One solution to this problem is that the climate system itself amplifies these small changes to create more noticeable periodic signals. These amplification mechanisms could be the large thermal intertia of the oceans, the vast energy required to move giant ice sheets, or long-term cycles in greenhouse gases such as carbon dioxide and methane. Any combination of mechanisms such as these could magnify small changes in sunlight from Milankovitch cycles and create the dominant 100,000 year cycle in ice coverage seen in the geologic record.

Studying this problem has proven to be challenging because of the long time scales involved. Most contemporary climate models are focused on patterns of climate on Earth today and in the near future of a few hundred years from now, but few modelers have focused their attention on the more distant future of climate. In this paper I develop a simple climate model that uses stochastic (i.e. randomly generated) forcing to achieve a state of resonance that displays a 100,000 year cycle in ice coverage. The model is an idealization of the more complicated Earth system and provides a tool for exploring the behavior of climate over these long time scales.

Calculations with this model show that the influence of human emissions into the atmosphere can affect the presence of the ice age cycles, either by damping the magnitude of changes or by ceasing the cycles altogether. The simplified calculations here cannot predict exactly when this should occur, but this study points toward the existence of a threshold beyond which ice age cycles may cease as a result of human emissions.

The future of Earth’s climate is becoming increasingly marked by the presence of human activity. Depending on the course of events over the next few hundred years, we may find that the damping or cessation of ice age cycles is yet another indicator of the dawning of the age of the anthropocene.

Habitable zones around main-sequence stars: Dependence on planetary mass

Kopparapu, R.K., Ramirez, R.M., SchottelKotte, J., Kasting, J.F., Domagal-Goldman, S. & Eymet, V.

Astrophysical Journal Letters, 787, L29.

doi:10.1088/2041-8205/787/2/L29, 2014.


Listen to the author interview! [mp3 download]

Brief summary

Estimates of habitable zones around other stars assume an Earth-mass planet with Earth-like atmospheres. But recent exoplanet discoveries are unveiling a wide range of rocky planets: mostly larger in mass/size than our Earth because our detection methods are sensitive to larger planets. So the question is: How different are the habitable zones for rocky planets that are not of Earth-size? Here we discuss this question, concluding that habitable zones are wider for larger mass planets than smaller ones.

Extended summary

Identifying habitable (and possibly inhabited) planets around other stars is one of the greatest long-term goals of current exoplanet surveys. If one is to find Earth-like exoplanets in the habitable zones (HZs), one needs to know at what distance they should be found from their parent star. Most of the HZ limits that we see in the exoplanet literature were based on a 1-D climate model initially developed by Jim Kasting at Penn State. Jim and his collaborators published a seminal paper in 1993 about Habitable Zones around main-sequence stars (stars that fuse hydrogen into helium in their core, like our sun). According to that paper, the inner edge of the HZ is at 0.95 AU, and the outer edge is at 1.67 AU. (AU stands for “astronomical unit”, and 1 AU equals the mean Earth-sun distance.)

For two decades, the Kasting et al. (1993) results were the prime source for HZ estimates. In 2013, I (along with other collaborators including Jim) published revised HZ estimates, updating Jim’s old climate model. At the time Jim published his paper, no exoplanet was discovered around any Main-sequence star. People were not sure if Earth-size/mass planets even existed. Two decades later several observational surveys detected several super-Earth sized planets, indicating that small planets do exist and, in fact, are probably more common than larger planets. With this in mind, we set to re-calculate HZs. We updated several things in the climate model, but the most important one was how strong water and carbon dioxide absorb infra-red (IR) radiation. As you may know, infra-red radiation is extremely important for the greenhouse budget of a planet. The stronger the absorption of IR, the greater the greenhouse effect on a planet.

We considered an Earth-mass planet around different kinds of stars, and derived HZ limits. To our surprise, we found that the inner edge of the HZ (i.e, how close to a star one can push a planet before all the water on the surface is evaporated) is at 0.99 AU! (and the outer edge was almost the same as Jim’s old result: 1.70 AU). Remember that, by definition, Earth is at 1 AU. So this implies that we are just a step away from being un-inhabitable. There is a caveat: Our 1-D model can not model clouds, and water clouds can cool a planet by increasing the planet’s albedo (how much sunlight is reflected to space). A higher albedo means more sunlight is reflected, potentially cooling the surface of a planet. So, our 0.99 AU inner edge is a pessimistic limit. And that is ok. We want to be conservative in our HZ estimates, so that we don’t over count the number of potentially habitable planets in our Galaxy.

After we published this paper, several researchers in the exoplanet community asked us what are the HZ limits for super-Earth mass planets? or sub-Earth mass planets? That is the paper I will explain briefly here. To perform this calculation, we assumed that the background Nitrogen pressure scales with the size (or mass) of a planet. Meaning larger mass planets have more N2 (they acquire more volatiles during the formation). For the inner edge of the HZ, We found that the amount of N2 in the atmosphere just delays when the planet loses the surface water into the atmosphere, but does not affect the domination of water vapor to the greenhouse effect. In essence, more N2 acts as a light-scattering gas, increasing the albedo, to an extent. But as the planet warms more and more (as we push it closer to the star to find the inner edge), water-vapor completely dominates the atmosphere, and any scattering of light by N2 becomes negligible.

What actually matters is the gravity of the planet. A lower mass planet has less gravity. So compared to a larger mass planet, it has more water-vapor at a given height (it is puffy). When there is more water-vapor in the atmosphere, it absorbs more IR radiation, increasing the greenhouse effect. Hence, one can not push a lower mass planet as close to a star as a higher-mass planet (because as the Star’s radiation increases, more heat is absorbed and the low mass planet reaches it’s inner edge sooner than a high-mass planet). The outer edge of the HZ does not change much at all due to competing effects of albedo and the greenhouse effect of carbon-dioxide atmosphere. That is why the width of the HZ for a low mass planet is small compared to a high-mass planet. And that is the essence of our paper.

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.