Life Not-As-We-Know-It: Methane Biochmeistry

The recent confirmation of the existence of a molecule called Acrylonitrile (also known as Vinyl Cyanide) in the atmosphere of Titan has me thinking about the possibilities of alternative biochemistries. This confirmation (following up on previous data from several years earlier) is interesting because Acrylonitrile has been shown in a computer model done in 2015 to self-assemble into sheets and vesicles when dissolved in liquid methane similar to how lipids form membranes in liquid water. These structures are dubbed “Azotosomes”, similar to the term Liposome for the lipid bi-layer structure that makes up the cell membranes of terrestrial life. While definitely not a sure sign of alien life, it does raise some very important questions: Can life, or something similar, arise in conditions vastly different than that of Earth? Could there be aliens swimming in the methane seas of Titan, perhaps?

 

F3.large

The Azotosome, composed of alternating arrangements of acrylonitrile molecules. From Stevenson et. al.

 

Before trying to tackle this question, let’s review some facts about Titan.

Titan: Nature’s Petrochemical Lab

Titan is the largest moon of Saturn. It is unique among any of the moons in the Solar System in that it has a complex, substantial atmosphere which is chemically active and dynamic, with clouds, haze, and even rain. Titan’s atmosphere is composed almost entirely of Nitrogen, with a few percent methane and ethane and trace amounts of other hydrocarbons. The thick orange haze is a result of ultraviolet light from the sun breaking down methane and other molecules and reforming them into complex, long-chain hydrocarbons. In effect, the whole moon is shrouded in a world-wrapping blanket of natural smog.

4822568056_d8c05e861d_o

Crescent-phase Titan, with a dramatic backlit atmosphere. Image taken by NASA’s Cassini spacecraft.

Titan is also absolutely frigid. With an average surface temperature of -180 C°, it is far too cold to support any kind of life we know. Titan’s crust and surface are made up mainly of water ice instead of rock, and at such cryogenic temperatures, it is materially similar to granite in terms of hardness.

Titan is also the only world in the solar system other than Earth that has stable bodies of liquid on its surface. As it is far too cold for water to ever be a liquid, Titan’s seas, lakes, and rivers are instead filled with liquid methane and ethane. Vast seas larger than Earth’s Great Lakes in North America dominate the northern polar region. Titan’s lakes are fed by a methane cycle analogous to the water cycle on Earth; methane evaporates from the surface, condenses into clouds, falls back down as methane rain, and carves out channels and rivers as it flows back to the lakes and seas to evaporate again.

PIA17655_1355x676

Titan’s northern polar region plays host to massive seas of liquid methane, as well as smaller lakes which dot the moon, as seen in this radar-image map produced by the Cassini spacecraft. Here, the liquid surfaces show up as dark, non-reflective regions since they absorb almost all of the radar energy.

In addition to the methane rain, the complex photochemistry in Titan’s haze layers produces a chemical precipitate called tholin, which is a tar-like mix of various hydrocarbons. This tholin probably coats the ground like a thick mud in some places. Additionally, the equatorial regions are home to vast and gigantic dune fields. Instead of silicate sand, these dunes are made up of hydrocarbon grit mixed with grains of water ice eroded from the hillsides.

As alien as it is, superficially Titan is quite Earth-like. In fact, the chemistry happening in the atmosphere and on the surface is in some ways very similar to what was thought to have happened on early pre-biotic Earth. So could the precursors to life be present on Titan? Perhaps. But if anything is actually alive here, it would have to be very, very different than anything we know. First, with no liquid water, any Titanian life would need something else to act as a solvent for its biomolecules. Water is such a good solvent mainly because it is a polar molecule; the oxygen atom is highly electronegative compared to the two hydrogen atoms bonded to it, and this gives it a slight charge imbalance. Methane, on the other hand, is non-polar, having a more balanced charge distribution. This makes it a poor solvent and as such, the list of compounds soluble in methane is far shorter than for water.

Exotic Life

So we have a molecule that can spontaneously form membrane-like structures that would work well for partitioning their interiors off from the surroundings, one of the first basic requirements for a living cell. And it should work in cryogenic liquid methane. Do we know of any molecules or compounds that could carry out pre-biological or even fully-blown biological reactions inside of it? After all, there’s little point in speculating about alternative biochemistries if there is no actual biochemistry to be had.

Noted astrobiologist Chris McKay suggests that one possible chemical pathway life on Titan could exploit is to react naturally-occurring acetylene with Hydrogen, which could serve as an energy source to power exotic metabolisms (unfortunately the paper is behind a paywall, but the abstract is available for free). This reaction would also release methane as a waste product, helping replenish the chemical as it is destroyed by solar UV radiation in the upper atmosphere. If such a process was happening on or near the surface of Titan, we’d expect to see a sudden drop in the concentration of Hydrogen near the surface, as well as far less acetylene than would be expected.

Curiously enough, in 2010, Johns Hopskin University research Darrell Strobel found just such evidence. While analyzing the concentration of Hydrogen in different layers of Titan’s atmosphere, Strobel discovered that Hydrogen was very abundant in the upper atmosphere, but rather depleted further down, driving a downward flow towards the surface. Most interestingly, however, was the fact that the Hydrogen seemed to just disappear near the surface. Another paper published that same year observed far less acetylene than expected near the surface as well.

Could these be a potential smoking gun for aliens? Well, maybe. Strobel himself agreed that it was consistent with McKay’s predictions. However, there could be any number of non-biological explanations for the missing Hydrogen and acetylene, as well as non-biological mechanisms for methane replacement. It could even just be human error. Ultimately, another spacecraft will have to be sent to Titan to follow up on these results. But frustratingly, with Cassini fated to dive into Saturn’s atmosphere this Septemeber, ending its 13-year long mission, no spacecraft will available to study Titan up close in the foreseeable future.

But a few days ago I came across an abstract from a paper published in 2015 about the potential for a group of molecules called polyethers to function as the backbone of a genetic biopolymer in liquid hydrocarbons. Unfortunately, the paper is behind a paywall, but I was able to request a copy from my university library. This is incredibly important since the only naturally occurring genetic biopolymer we know of needs to be dissolved in liquid water to function; DNA. There is one downside, though: the methane seas of Titan are far too cold for polyethers to remain soluble. If there is life on Titan, it’s DNA-equivalent molecule won’t be using polyethers. Perhaps something else on Titan could do the job, but right now, it’s not clear what that could be. It’s possible that Titan might just be too cold for even exotic hydrocarbon life. Does that rule out Titan as a home for life? Not definitively, I think. Even if the surface is too cold even for exotic hydrocarbon biochemistry, Titan is thought to have a subsurface water-ammonia ocean deep beneath its icy crust. That life, however, would be much more difficult to find.

Polyether-based genetic molecules would be well-suited to warmer worlds with a similar environment to Titan, though. “Warm” is a relative term here, of course; 85 to 231 K, or -188 C° to -42 C°. And rather than being dissolved in liquid methane, these polyethers would be most soluble in liquid propane. And while methane and propane are both non-polar molecules and therefore are rather poor solvents, in general, the warmer a substance is the better a solvent it becomes. Thus, a “warm” propane ocean is a better solvent than lakes of cryogenic liquid methane.

WolframAlpha--methane_phase_diagram__Result____2017_08_01_17_27

Phase diagrams of methane and propane, generated in WolframAlpha.

 

The area of the phase diagram over which a substance is liquid plays a huge role in how well that substance can form stable bodies of liquid on the surface of a world. Compared to propane, methane is a liquid over a far smaller range of temperatures and pressures. On Titan, this causes the lakes and seas to be confined to the colder polar regions. Water’s liquid range is also rather broad, which is another reason why it is such a good solvent for biological processes. On a world even marginally warmer than Titan, methane would likely be unable to condense into lakes and seas (although the upper atmosphere could get cold enough for clouds to occasionally form), but propane would still be liquid, and a propane cycle could drive weather patterns similar to Earth and Titan. Azotosomes could still possibly form, and now they will also have interesting molecules to stuff inside themselves. Provided Hydrogen and acetylene are also present, the same hydrocarbon-based metabolism outlined by McKay could also play out inside them. Now all that’s needed is an actual self-replicating, information-carrying molecule that can undergo Darwinian evolution.

So we’ve got a potential backbone, but we still need a nitrogenous-base analog; something to actually encode genetic information with.  None are currently known, but that doesn’t mean they don’t exist. There are hundreds of billions of planets in our galaxy, each one is an independent chemical laboratory carrying out world-spanning experiments in parallel. If there are viable alternative biochemistries which don’t need water, they exist somewhere.

Until next time, keep looking up.

References and Additional Reading:

James, S., Lunine, J., & Clancy, P. (2015). Membrane alternatives in worlds without oxygen: Creation of an azotosome. Science Advances, 1. Retrieved from http://advances.sciencemag.org/content/advances/1/1/e1400067.full.pdf

Strobel, D. (2010). Molecular hydrogen in Titan’s atmosphere: Implications of the measured tropospheric and thermospheric mole fractions. Icarus. Retrieved from https://web.archive.org/web/20120824195338/http://astrobiology.jhu.edu/wp-content/uploads/2010/06/Icarus-2010-Strobel.pdf

McLendon, C., Opalko, F. J., Illangkoon, H. I., & Benner, S. A. (2015). Solubility of Polyethers in Hydrocarbons at Low Temperatures. A Model for Potential Genetic Backbones on Warm Titans. Astrobiology, 15(3), 200-206. doi: 10.1089/ast.2014.1212

Cook, J., & Weselby, C. (2010, June 03). What is Consuming Hydrogen and Acetylene on Titan? Retrieved from https://web.archive.org/web/20110629185640/http://www.jpl.nasa.gov/news/news.cfm?release=2010-190

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Worlds Around Brown Dwarfs: When is a Planet a Planet?

I’ve always found brown dwarfs to be fascinating objects, and as such I’ve recently been re-reading papers about their formation and unusual chemistry. Brown dwarfs are strange, transitionary objects; they’re sort of in between being a planet and a star. They start out like a normal star would, forming from a collapsing cloud of interstellar dust and Hydrogen. The only difference is that the object in the center of this collapsing cloud is not big enough to achieve the crazy-high temperatures and pressures in its core to start fusing Hydrogen into Helium as stars do; instead, it merely glows from the heat of its birth and the energy released through gravitational contraction. As a result, brown dwarfs are very dim. Most of the light that they emit is in the invisible infrared part of the spectrum, and many would appear to glow magenta to the human eye.

 

brown_dwarf

My interpretation of a world transiting its brown dwarf host.

 

Early in their history, some of the largest brown dwarfs may be capable of fusing Hydrogen’s bigger brother isotope Deuterium, or even Lithium, but this process is short-lived. Most, if not all of a brown dwarf’s energy comes from the release of heat from its highly compressed interior. They are, in essence, failed stars.

Incidentally, you may have heard it said that Jupiter is a failed star; that had it been just a little more massive, it would have ignited into a second Sun. That’s not quite true, as you’d actually need to toss about 80 more Jupiters into Jupiter before it would become massive enough to approach the smallest known true stars. 80 Jupiter masses is about how small most astronomers think a star can get and still be a star. Any lighter and you can’t get high enough pressures and temperatures to fuse Hydrogen. Brown dwarfs occupy the mass range between about 13 and 80 Jupiter masses (although the lower bound of 13 Jupiter-masses is rather fuzzy and is more of a rule-of-thumb).

There’s a lot of really cool things (pun intended) about brown dwarfs, many of which deserve their own posts (and probably will get them eventually). But something that’s been on my mind recently is just what do you call an object that orbits one? Brown dwarfs aren’t technically stars, but they’re not planets either. So would an object in orbit around a brown dwarf be a planet, or a moon? This post will be a weird mix of both astronomy, linguistics, and categorization, so I apologize in advance.

There are examples of brown dwarf companions that have been discovered over the last few years. Most of these are very large objects with several times the mass of Jupiter. In terms of the system’s mass ratio, these are more similar to a binary star system than to a planetary system. But there is no reason why a brown dwarf couldn’t form smaller, terrestrial planets. In fact, given their low mass, brown dwarfs are expected to preferentially form terrestrial worlds over larger gas giants (and because of the nature of expected chemistry in a brown dwarf’s protoplanetary disk, those planets are likely to be very different than anything we know… but that’s a story for another post). However, there is right now at least one example of a low-mass companion orbiting a brown dwarf: OGLE-2013-BLG-0723LB/Bb. This world is about 0.7 Earth-masses, a little smaller than Venus, which would make it a terrestrial planet assuming it was small and rocky. But is it a planet, or is it a moon? The situation is further complicated by the fact that the brown dwarf itself orbits a low-mass star. Is this then a binary system of star and brown dwarf with a planet, or a single system of one star containing a brown dwarf with a large moon?

This might seem like a simple matter of semantics, but classification and taxonomical schema are important in trying to understand things in nature. Unfortunately, nature is very rarely, if ever, neatly organized into discrete categories that can fit nicely into little boxes. There’s a lot of fuzziness, nuance, and overlapping categories. Everyone remembers when the International Astronomical Union (IAU) reclassified Pluto to a “dwarf Planet”. While I agree with creating a new bin for objects like Pluto (which are more similar to each other than they are to any other object in the Solar System), doing so opened up a huge can of worms, and underscored just how vague our ideas of “planet-ness” are.

The IAU decided that in order to be an official planet, an object had to:

  1. Orbit the Sun (or another star; in which case it was an exoplanet)
  2. Be sufficiently massive enough to be rounded out into a spheroid by its own gravity
  3. Be gravitationally dominant over all other objects in its orbit (to be able to “clear the neighborhood”)

There are plenty of reasons why these criteria alone are insufficient to define a planet, but it presents a thorny issue when it comes to objects orbiting brown dwarfs. Again, they’re not stars, so anything orbiting them fails to meet the IAU’s definition of planet by virtue of not orbiting a star. But the brown dwarf is not a planet either, so their companions can’t quite be called moons. Yet the objects we find orbiting brown dwarfs are very obviously planet-like in terms of size and mass, especially when they’re several Jupiter-masses and therefore gas giant worlds. And certainly no one would ever mistake a gas giant for a moon. Clearly, we need a better system for classifying these objects. Or at the very least a better definition of planet. Right now, it’s all quite a mess, and frankly a little hand-wavy.

I’m not sure exactly how we should draw the line between planet and not-planet, or if such a line can really be meaningfully drawn (I truthfully don’t think it can). There are far too many outlier cases, like pulsar planets that orbit stellar remnants, planets orbiting black holes, binary planets, dwarf planets, and even rogue planets that don’t even orbit anything but instead roam the galaxy unbound to any object. But my thought is that if an object orbiting a brown dwarf meets the other two requirements in the IAU’s definition, then it’s a planet. After all, the IAU’s definition hasn’t stopped astronomers from calling any of the above “planets”. This might spawn a separate post about the need to overhaul our definition of planet entirely, as I feel that’s a discussion the astronomical community is going to have to have at some point.

Until next time, keep looking up.

References and Additional Reading:

Pascucci, I., Herczeg, G., Carr, J. S., & Bruderer, S. (2013). The Atomic and Molecular Content of Disks Around Very Low-mass Stars and Brown Dwarfs. The Astrophysical Journal, 779(2). Retrieved from http://iopscience.iop.org/article/10.1088/0004-637X/779/2/178/pdf

Bolmont, E., Raymond, S. N., & Leconte, J. (2011). Tidal Evolution of Planets Around Brown Dwarfs. Retrieved from https://arxiv.org/abs/1109.2906.

Udalski, A., & Jung, Y. K. (2015). A Venus-Mass Planet Orbiting a Brown Dwarf: A Missing Link Between Planets and Moons. The Astrophysical Journal. Retrieved from http://iopscience.iop.org/article/10.1088/0004-637X/812/1/47/pdf

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Tapered Rotating Habitats for Settling Low-Gravity Worlds

This has been an idea that has been bouncing around in my head for a while now. My original idea was to turn it into a paper, and I still might one day if I flesh it out more.

The long-term effects of living in a microgravity environment have long been known to be severely detrimental to the human body. One simple solution to this problem in free space is to build space stations which rotate at several rotations per minute (RPM) in order to provide a kind of artificial gravity through centripetal force. In order to reduce the negative effects brought about by Coriolis forces, such as nausea and disorientation, such space stations are typically quite large, often reaching several kilometers in diameter. Utilizing the constant and unobstructed sunlight available in free space for effectively unlimited power, such habitats are typically estimated to be able to house populations of several thousand to several million people, depending on size. However, when it comes to settling on a planetary surface, one is stuck with whatever natural gravity the object has, which for every solid world in the Solar System, is typically much less than Earth’s. Currently, it is not known how the human body adapts to living in fractional gravities, and the only data available is in the case of “zero gravity” and Earth gravity; opposite ends of the spectrum (the time Spent on the Moon, a few days total for each Apollo mission, was not enough time to establish the long-term effects of Lunar gravity on the human body). Assuming that physiological responses to gravity scales linearly with gravitational force, it can be surmised that the kinds of long-term degradation observed in extended periods of freefall would likely happen in reduced gravity, albeit to a degree relative to the gravity experienced. If this is the case, safe and long-term habitation of low-gravity worlds can still be made possible by constructing large rotating habitats buried in the surfaces of these worlds, with walls sloped at an angle so that the local force of gravity and the artificial centripetal gravity balance out to create a net resultant force.

When it comes to settling the Solar System, most people envision sprawling arrangements of habitats and other buildings scattered across a planetary surface, typically on the Moon or Mars. In science fiction, these are often seen as the precursors to urban vistas similar to what we see on Earth. However, this vision ignores two critical factors about living on most planetary surfaces; the lack of protection from radiation (in the form of cosmic radiation and solar flares), and the strength of gravity. In most science fiction settings, both of these are typically handwaved away through some kind of unexplained technology, typically some kind of artificial gravity generator in the case of gravity (if it’s mentioned at all), and radiation is typically ignored entirely. In reality, the reduced-gravity environment of many worlds throughout the Solar System may present a challenge when it comes to human settlement. While radiation can be mitigated by simply surrounding habitats with regolith-filled sandbags or burying them in trenches, the natural gravity of a planet or moon cannot be changed.

The only known method for generating artificial gravity is through rotation. This concept has been included in various designs for fictional spacecraft, such as the Discovery from 2001: A Space Odyssey, ISV Venture Star from Avatar, and the Endurance from Interstellar, as well as a few designs for future spacecraft such as NASA’s Discovery II and Nautilus X spacecraft. It has also been used in the design of massive, permanent orbital settlements meant to house many thousands or hundreds of thousands of humans in free space, popularized by physicist Gerald O’Neill in 1977 in his book The High Frontier, and expanded upon by many authors and researchers since. Indeed, I plan on writing about this topic extensively in the future. The idea of such a rotating space habitat can also be applied to the surface of a low-gravity world, after carefully considering some subtle alterations to account for the host world’s weak but non-zero natural gravity.

The ideal design for this, I think, would be a tapered cylinder. The diagram below shows what I mean:

diagram2

Diagram of a tapered cylindrical habitat

The habitat consists of an outer shell to house the rotating section, similar to the frame of a clothes drier in which the spinning drum is housed. A modest layer of regolith on top of the shell provides radiation shielding from cosmic rays and solar particles. A central, non-rotating axial shaft serves as both an axis of rotation and also an access elevator. Driven into the deeper bedrock are the radiators for the habitat’s life support and power systems, which conduct waste heat into the surrounding rock.

The habitat itself is shaped a bit like a bowl with straight sides. The side walls are angled at a slope which, when the habitat is under spin, creates a sensation of normal, “flat” gravity near the equator. One glaring issue with this design is that as the diameter becomes smaller towards the bottom, the centripetal force becomes stronger and stronger while the natural force due to gravity remains constant, resulting in a sensation of walking uphill.

diagram1

Force diagram of the rotating tapered habitat.

The following tables list values of θ required to generate a given amount of artificial gravity for selected Solar System objects with low but non-negligable gravity:

Lunar Carousel

Resultant Gravity Wall Angle θ (To Verticle)
1 G (9.81 m/s2)

9.5º

0.75 G (7.35 m/s2) 12.7º
0.5 G (4.90 m/s2) 19.7º
0.38 G (3.72 m/s2) 25.8º

Ceres Carousel

Resultant Gravity Wall Angle θ (To Verticle)
1 G (9.81 m/s2)

1.6º

0.75 G (7.35 m/s2) 2.2º
0.5 G (4.90 m/s2) 3.3º
0.38 G (3.72 m/s2) 4.3º

Europa Carousel

Resultant Gravity Wall Angle θ (To Verticle)
1 G (9.81 m/s2)

7.7º

0.75 G (7.35 m/s2) 10.3º
0.5 G (4.90 m/s2) 15.5º
0.38 G (3.72 m/s2) 20.7º

Ganymede Carousel

Resultant Gravity Wall Angle θ (To Verticle)
1 G (9.81 m/s2)

8.4º

0.75 G (7.35 m/s2) 11.2º
0.5 G (4.90 m/s2) 17º
0.38 G (3.72 m/s2) 22.6º

Callisto Carousel

Resultant Gravity Wall Angle θ (To Verticle)
1 G (9.81 m/s2)

7.2º

0.75 G (7.35 m/s2) 9.6º
0.5 G (4.90 m/s2) 15.5º
0.38 G (3.72 m/s2) 20.7º

This is one possible solution for how to deal with living on low-gravity worlds, but it probably isn’t the only one. Personally, I think that if we’re going to try to settle a broad array of planet types, we should adapt ourselves to their environments rather than attempting to adapt the environment to us, through a combination of cybernetic augmentation and genetic engineering. Of course, these are not strict binary paths. A mixture of both is a likely and desirable outcome.

Until next time, keep looking up.

References and Additional Reading

O’Neill, Gerard K. “The Colonization of Space.”Phys. Today Physics Today 27.9 (1974): 32. Web. http://www.nss.org/settlement/physicstoday.htm

 Miller, J., L. Taylor, C. Zeitlin, L. Heilbronn, S. Guetersloh, M. Digiuseppe, Y. Iwata, and T. Murakami. “Lunar Soil as Shielding against Space Radiation.”Radiation Measurements 44.2 (2009): 163-67. Web. http://www.lpi.usra.edu/meetings/nlsc2008/pdf/2028.pdf

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The Paleocene-Eocene Thermal Maximum: An Introduction to Rapid Climate Change

This is the first in a series of posts that will be discussing anthropogenic climate change. But before I begin talking about our current bout of global warming, I think it would be good to start off by looking at past episodes of natural rapid warming in order to better understand what happens when the planet undergoes a sudden transition into a greenhouse state. The most well-known of these events occurred around 55.5 million years ago in the Paleogene period, between the Paleocene and Eocene epochs, called the Paleocene-Eocene Thermal Maximum (or PETM).

There were actually a number of smaller temperature spikes during the Eocene, but none were as dramatic as the PETM.

And it’s quite a strange event at that. The PETM is marked by an absolutely massive drop in the ratio of carbon-13 to carbon-12 isotopes in marine sediments (isotopes, for those unfamiliar, are variations of elements whose nuclei have a different number of neutrons than the “primary” atom; your typical garden-variety carbon has 12 protons and 12 neutrons. Carbon-13 has 12 protons and 13 neutrons, hence the “13”). This is important for a number of reasons. First, most organisms, especially plants, tend to prefer the lighter carbon-12 over the heavier carbon-13. Because the carbon that was released into the atmosphere during the PETM is so depleted in 13C, that implies it was biogenic, since life is very good at discriminating elements by isotope. Additionally, the oceans would have absorbed a large amount of this CO2 , reducing the pH (resulting in ocean acidification as the dissolved CO2 forms carbonic acid) which then also increases the amount of carbonate ions (since a more acidic ocean is better able to dissolve carbonates from rocks and shells).

Even today, it’s not clear what the actual source of all this carbon was. Some of the most popular ideas include pockets of methane trapped in deep sea ice called methane clathrates. While these clathrates are normally stable, when deep-ocean temperatures rise above a critical value, they can very rapidly destabilize and release a huge amount of methane in a short amount of time. Methane is a potent but short-lived greenhouse gas, and is 34 times as powerful as CO2 over a 100-year period. But methane in the upper atmosphere is short-lived due to reactions with free hydroxyl ions which break it down into CO2 and water vapor. Biogenic methane trapped in clathrates would also have a reduced 13C/12C ratio. Other ideas include widespread peat and coal fires, but a lack of soots and other combustion byproducts in sediment samples tends to rule this out. A lack of glaciation in Antarctica could have allowed for the release of large amounts of CO2 from thawing permafrost deposits as they warmed each summer. Whatever was the trigger, the end result was that a tremendous amount of CO2 was injected into the atmosphere and kicked off an unprecedented spurt of planetary warming.

The Paleogene itself emerged from a world which had just recovered from the aftermath of a catastrophic asteroid impact. This infamous extinction event marks the boundary between the Cretaceous and Paleogene periods, and is most popularly known for the death of nearly every species of non-avian dinosaurs. The general climate trend of this time was one of cooling (although the climate was still much warmer than that of today’s). The prevailing warmer and wetter conditions of the preceding Mesozoic era gave way to a cooler and drier phase which continues into the present day. Pangaea had long ago fractured and split apart, and the continents were already taking on familiar shapes and orientations.

050marect

The arrangement of the continents, ~50 million years ago. Note the lack of glaciation on most of Antarctica and Greenland. Source.

Despite this cooling trend, it should be noted that the PETM was happening in the context of an already-warmer climate (estimates place the mean global temperature during the Paleogene at around 18° C; 4° higher than the modern average). During much of the Paleogene, there wasn’t very much ice coverage at either pole. Indeed, evidence suggests that for much of the period the poles may have been entirely ice-free. In fact, the Paleogene opened on a warming trend that peaked in the mid-Eocene during the “Eocene Optimum”, where average temperatures were near where they were at the height of the PETM, before the cooling trend that marked the rest of the Paleocene took dominance. The difference between the PETM and the Eocene Optimum was that it took nearly 5 million years for temperatures to ramp up to their max during the Optimum (and then about another 5 million years to fall back down), while the PETM was a brief, intense spike that lasted at most 200,000 years. A geological eyeblink.

65_myr_climate_change

This graph plots polar ocean temperature as a function of time, using Oxygen-18 ratios in marine fossils as a proxy for temperature. Source and explanation.

What Causes Changes in Global Temperature Over Geologic Time?

Before continuing on, let’s take a step back and talk about how and why climate changes generally. Earth’s climate, or indeed, the climate of any planet, is a vastly complex system. Therefore, it’s difficult to ascribe changes in climate to a single variable. In fact, most changes in Earth’s climate have been a result of several factors all working together, forming feedback loops that either re-enforce each-other, or sometimes work against each-other. For example, while one of the major leading causes of modern anthropogenic climate change is the release of the greenhouse gas Carbon Dioxide, other factors such as deforestation, agriculture, over-hunting/fishing, and urbanization all affect the climate. Deforestation, in particular, brightens the surface of the Earth by removing dark, light-absorbing vegetation and exposing the brighter surface beneath. This increases the albedo of the planet, reflecting more sunlight back to space and cooling the surface. However, forests help lock away a significant portion of carbon from the atmosphere, playing a vital role in the carbon cycle. While their removal results in a slight cooling effect, it also destroys an important carbon sink and thus also contributes to warming the planet as we increase the amount of CO2 in the atmosphere.

These are only some of the things that can influence climate. It’s by no means an exhaustive list, and I am not a climatologist. Real climate is very messy, chaotic, and sometimes hard to explain. A whole blog could be dedicated to the subject and still not manage to encompass them all to the detail they deserve. What I present here is just a brief, simple overview.

There are several factors, both internal and external to the Earth, that can influence climate. Some of the more obvious ones are giant volcanic eruptions, which can inject huge amounts of sulfur dioxide and volcanic ash into the stratosphere. The SO2 forms aerosols which, in combination with the volcanic ash, block out sunlight and cool the planet. Asteroid impacts, such as the aforementioned end-Cretaceous impact that famously killed most non-avian dinosaurs 66 million years ago, also act similarly to volcanic eruptions as they can inject vast quantities of rock and soot into the atmosphere. The resultant fireball upon impact can also ignite forest fires many thousands of kilometers away from the point of impact, releasing huge quantities of CO2. But for all their violence, volcanoes and asteroids typically only result in rather short-term changes in climate.

Longer-term climate changes often result from roughly cyclical variations in the orientation of the Earth relative to the Sun, the shape of the Earth’s orbit, and even the intensity of the Sun itself. The Earth’s orbit is neither circular nor stable. It’s an ellipse, and it wobbles around, or precesses, with a regular period. This is known as apsidal precession, and when traced out over a full period, results in pretty spirograph patterns. For Earth, the average period of apsidal precession is around 112,000 years. However, the Earth is also tilted on its axis. Currently, Earth’s axis is tilted about 23.44° relative to the plane of its orbit; but this too is subject to precession. Like a gargantuan spinning top, the Earth’s axis takes around 26,000 years to complete one full rotation. That is, if you tracked the movement of the Earth’s axis through time, it would trace out a cone. But that’s not all. The axial tilt itself fluctuates between 22.1° and 24.5° on a 41,000-year cycle. And all of these cycles of precession and wobbling stack on top of each-other, making for a very complicated overall motion. These changes are what are collectively referred to as Milankovitch cycles, and generally speaking, most long-term climate variations map well to them.

Other variables include variation in the intensity of sunlight hitting the top of the atmosphere. Over very long periods of time, changes in average solar luminosity can also cause changes in climate. (This is unrelated to sunspot activity, such as the famous “Maunder Minimum”, a period between 1645 and 1715 where very few sunspots were observed. This happened to coincide with the “Little Ice Age” around 1650 when Europe and North America experienced colder-than-average temperatures. While the relationship between sunspot cycles and climate is poorly understood, it is somewhat contentious in the scientific community as to whether a causal relationship exists between the Maunder minimum and the Little Ice Age.) Even the arrangement of the continents can affect climate, since as continents move they can change ocean circulation patterns which deliver warm and cold waters to shorelines. The surfaces of continents are also generally brighter than the dark oceans, and so more continents near the equator tends to cool the planet by reflecting more sunlight back into space, whereas more open equatorial ocean would warm it.

All of these factor into climate change in some way or another. But the major driver for both the PETM and our modern warming has to do with atmospheric chemistry, specifically the balance of greenhouse gasses and what happens when you suddenly add a boatload of it into the atmosphere in a geological instant.

How does the PETM Relate to Anthropogenic Climate Change?

We’ve seen that the massive injection of CO2 can cause an equally rapid change in climate and global temperatures in both the paleoclimate data from the PETM, and unfolding in real time as we continue to pump gigatons of CO2 into the atmosphere every year. The PETM demonstrates clearly enough the relationship between greenhouses gasses and temperature shifts, as a spike in one lead to a spike in the other. And as the closest analog to our current situation, it may hold key insights into what we might expect to see if we don’t act on climate change right now.

To help put things into perspective, the average carbon loading rate during the PETM (which is thought to have occurred in multiple pulses, with the main pulse lasting around 2,000 years) was around 1 Pg (a petagram, or 1015 grams) per year during the main 2,000-year pulse. In 2014, a revised estimate for the amount of COreleased annually due to burning fossil fuels and other human activities was measured to be around 40 Pg/year, a staggering 40 times higher than the average flux during the PETMAnd while that rate hasn’t been constant over the last 150 years or so (it’s been steadily increasing since the start of the industrial revolution), when compared to the 2,000 years it took for CO2to accumulate during the PETM, it’s enough to make one pause. For comparison, volcanoes “only” emit around  200 million tonnes, or 0.2 Pg, of CO2 annually.

As an aside, anyone that claims that volcanoes put out more CO2 than human activity is either misinformed or outright lying. Besides that fact that the ash and soot thrown up during eruptions create a cooling effect much stronger than any greenhouse warming from volcanic CO2.

land_ocean

Global average temperatures have already risen by a full degree Celsius in the last 100 years, with much of that warming happening in the last 30 years or so. Nearly every month in 2016 has been the hottest month on record. The average CO2 concentration in the atmosphere is officially over 400 ppm, over twice the pre-industrial level, and sea levels continue to rise steadily at a rate of 2.9 mm/year. If the PETM is anything to go by, if we as a global civilization continue to burn fossil fuels and release ever-increasing amounts of CO2 into the atmosphere, we might create the same kind of intense warm period in only a matter of centuries. The PETM shows just how sensitive the climate system is to changes in CO2 concentration. And if much of the warming was in fact due to a release of methane from deep-sea clathrates and polar permafrost deposits, a rapid warming event due to high CO2 levels like the kind we’re continuing to produce could very well trigger such a runaway destabilization in the future. This is especially disturbing since there are signs that this process may have already begun.

A change in global temperature of just 2° by the end of the century would be significant enough. We would be extremely lucky if we could cap warming at that level. But, despite international efforts such as the Paris Agreement, I don’t think that many of the industrialized nations, especially the United States given the recent President-elect’s statements on climate change policy, will do much beyond token efforts at seriously decarbonizing their energy infrastructures. If worst comes to worst, can our civilization adapt to a world that is 8° warmer? Even though such a change would likely play out over centuries, it would be a change that is, as far as we know, unprecedented in the geologic record, happening many times faster than the PETM. What kinds of challenges would that present? It would certainly have impacts on agriculture, trade, water and food security, weather, and countless other facets of life.

This problem isn’t going to go away. The longer we continue to drag our feet, the worse it will be for all of us. We’re already feeling the early signs of climate change. This will be a centuries-long process, but that is not an excuse to hesitate to act. We know what the climate is capable of during natural warming events. What we are doing is far quicker and has the potential to be far more devastating. That, in part, is why we study the past. History, both natural and cultural, is full of lessons that can help steer us towards a better future. We just have to listen.

References and Additional Reading:

McInerney, F. A., & Wing, S. L. (2011). The Paleocene-Eocene Thermal Maximum: A Perturbation of Carbon Cycle, Climate, and Biosphere with Implications for the Future. Annual Review of Earth and Planetary Sciences. Retrieved from http://www.whoi.edu/fileserver.do?id=136084&pt=2&p=148709

Archer, D., & Buffett, B. (2005, March 3). Time-dependent response of the global ocean clathrate reservoir to climatic and anthropogenic forcing. Geochemistry Geophysics Geosystems. Retrieved from http://geosci.uchicago.edu/~archer/reprints/archer.2005.clathrates.pdf

Le Quéré, C., Peters, G. P., & Andres, R. J. (2014). Global carbon budget 2013. Earth System Science Data. Retrieved from http://www.earth-syst-sci-data.net/6/235/2014/essd-6-235-2014.pdf

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On Coffee and Nuclear Power

In which yet another blog is added to the exponentially growing mass of thoughts and opinions that is the internet.

Despite the name, I don’t plan on writing very much at all about coffee (even though coffee is what is at fault for the existence of this blog). Although there will probably be at least one or more rants about nuclear power and why people seem to be so gosh-darned resistant to it and how to get them to change their minds (you can take a guess as to where I stand on that issue). I do plan on writing about topics such as energy policy, climate change, and sustainable living. Additionally, the other major focus of this blog will be topics related to space exploration and settlement, such as in-situ resource utilization, spacecraft propulsion methods, designs for space habitats, and planetary science, all of which have a surprising amount of overlap with the first set of topics.

In fact, it’s my goal to explore the boundaries where these topics overlap, in order to show how exploring and settling space can help improve life for people living on Earth while also making humanity as a whole a space-faring, multi-planetary civilization.

For my first set of posts, I’m planning on writing about an event in the geologically recent past called the Paleocene-Eocene Thermal Maximum, and how it can be used as a proxy to better understand and predict the current episode of global warming being caused by the carbon dioxide emissions of our fossil fuel powered global civilization. Using the paleoclimate data as a guide, can human civilization adapt and survive if climate change continues unabated? What factors are similar between now and the PETM, and what’s different? How good of a model is the PETM to current anthropogenic warming?

I might not get to all of those questions in one post, and I’m leaning towards making it a series of posts instead. I’ll try to keep things relatively upbeat, given the subject matter. But that’s what you can look forward to. I also have some plans for some posts involving Martian water ice extraction and optimal designs for rotating space habitats, so there’s definitely enough material bouncing around in my head to provide a varied and (hopefully) interesting stream of content for a while.

Until then, keep looking up.

 

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