Planet Hunters Talk

Habitable zone(s)

  • zuloo37 by zuloo37

    In addition to the common habitable zone we consider, the one from 273-373 K (0-100 degC) for liquid water, there are also two other types of habitable zones that can support some type of life using a solvent different than water: ammonia or methane. Of course, while temperature is the main factor, atmospheric pressure is the next most important.

    At one atmosphere of pressure (since we did all the measurements here on Earth), the liquid range for ammonia (NH3) is 195.5-239.8 K (-77.7 to -33.3 degC), and the liquid range for methane (CH4) is 91-109 K (-182 to -164 degC). However, there is evidence that Titan, a moon of Saturn, has liquid methane on its surface because its surface temperature is around 94 K and the atmospheric pressure is about 1.5 x what it is on Earth. In fact, there's a whole "methane cycle" analogous to the water cycle on Earth, where methane lakes evaporate into clouds of methane, which rain liquid methane back down into the lakes.
    Unfortunately, our system doesn't have any planets that can have liquid ammonia on their surface, though Mars might be able to if it had a more significant nitrogen atmosphere.

    I recommend viewing transits in several IR wavelengths so that if light shines through the planet's atmosphere at all, we could potentially see the concentrations of different gases using the absorption maxima of those gases and determine whether the planet has atmospheric composition necessary to support life.

    Water was a deciding factor in Earth's evolution, but Earth did go through a phase where it had no water nor oxygen (except in silicates/phosphates.. No oxygen gas). The first steps in abiogenesis require PAHs like graphene and graphite. Carbon-14 impurities in these beta decay into nitrogen, and if two carbon-14 atoms are adjacent in a graphene lattice, they decay into nitrogen gas. This process is the main source of the 80% nitrogen atmosphere on Earth and the 95% nitrogen atmosphere on Titan. Then, for the next step, in some solvent (methane, ammonia, or water), methane, ammonia, and water have to react together.
    The compounds formed include hydrogen gas, methylamine, methanimine, hydrogen cyanide, methanol, formaldehyde, carbon monoxide, hydroxylamine, azanone, and the most important but least researched, apparently: methanolamine / aminomethanol, the simplest amino alcohol. Amino alcohols can polymerize in methane or ammonia to form protein-like structures that are capable of catalyzing a wide range of chemical conversions. Water tends to break them down into their monomers, and oxygen converts them to formamide and water. But in an oxygen-free environment, like that on Titan, amino alcohols are essential for metabolism.

    UV light from the Sun acts on methane and nitrogen in the atmosphere to form hydrogen cyanide, acetylene, cyanogen, diacetylene, cyanoacetylene, etc, releasing hydrogen in the process. Primitive life living in the methane lakes takes in acetylene (HCCH) and hydrogen to metabolize it to ethylene (H2CCH2). Ethylene is essential because it reacts with methane to form propane, and with ethane to form butane, forming all heavier hydrocarbons which form the cellular membranes of life there. Hydrogen is added to ethylene to make ethane, and hydrogen can be added to ethane to make methane. The ammonia and water on Titan is below the surface, but probably still available as they come up through the bottom of the lakes, where they react with methane, ethane, propane, etc to form amino alcohols which the life can use. In order to float or sink, the life probably uses relative ion concentrations inside and outside the membrane, with sodium channels of some sort made from polymers of amino alcohols, possibly. Ammonia based life is quite similar. Amino alcohols came first, and they still formed by the reaction of methane, ammonia, and water.

    The difference in what we now think of as water-based life is that we no longer use amino alcohols, because all life on Earth that did use amino alcohols either adapted to the increasing concentrations of oxygen or went extinct during the great oxidation event. The particular pathway for amino alcohols to be converted into the corresponding amino alcohol is simple: add formic acid (HCOOH) to an amino alcohol like aminomethanol (H2NCH2OH), and a simple condensation reaction forms the amino acid glycine (H2NCH2COOH), the simplest amino acid. In the past, there was a variety of complex multicellular life on Earth using amino alcohols, mostly things that we would call giant insects. It's not a coincidence that the only insects that are left make formic acid to convert amino alcohols into amino acids, since oxygen breaks amino alcohols down into formamide otherwise, and life that tried to keep using amino alcohols found that their proteins disintegrated in the oxygen environment. Eventually the mitochondria, which adapted to use oxygen for energy, took over and can be found in almost every animal and plant cell on Earth.

    What I'm trying to say is that planets with liquid ammonia or methane can probably also support complex life, and don't necessarily need oxygen. However, planets with an excess of water inevitably have some of that water converted into oxygen by life, and that oxygen is converted to ozone by lightning, and that ozone blocks a lot of the UV light from the Sun, making genetic mutations less likely and letting live evolve. However, other planets with liquid solvents have their own mechanisms to block UV light, like the complex organic molecules in the atmosphere of Titan, or hydrazines, diazenes, and other azanes/azenes in the atmosphere of a planet with liquid ammonia on its surface.

    I should probably also mention that while hydrogen cyanide is toxic to oxygen-metabolizing life because the cyanide ion binds irreversibly to hemoglobin and cytochrome c oxidase, HCN is an essential precursor to nucleobases like adenine (5 HCN --> Adenine in an exothermic reaction, especially in the presence of ammonia). Seasonal cycles on Titan have HCN clouds followed by methane lakes in the same place, and I'd like to say it's glaringly obvious that the HCN condensed into adenine and other nucleobases and life used those resources and produced the methane lakes. There are at least eight nucleobases that can be formed by oligomeric condensation of HCN with ammonia and/or water. Phosphates probably aren't common in an oxygen-free environment, but phosphorus nitride might be. Reaction of alcohols and amines (and amino alcohols) with PN makes polymers that twist into a helical shape. Attach those nucleobases and you have a genetic storage molecule. There may even be a simpler version of the ribosome using poly phosphonitrile nucleic acids, catalyzing the genetic code directed synthesis of poly amino alcohols. There's good evolutionary evidence that such a ribosome would read nucleobases in groups of two instead of groups of three, since the first two letters are most important for translation. Not that I know exactly how life evolved from phosphonitrile-amino alcohol ribosomes to phosphate-amino acid ribosomes, but I know all life is related. The oxidation event probably broke down poly phosphonitriles into phosphates. Something interesting is that while the primitive life reads bases two at a time, there are still 64 combinations possible since there are eight different nucleobases. The base pairing between these has more options that the typical RNA/DNA base pairs, though.

    But I digress. There are three habitable zones: One for liquid water, which we've been using, one for liquid ammonia, farther out, and one for liquid methane, even farther out. But atmospheric pressure can potentially change the range of all three of these types of habitable zones. At higher atmospheric pressures, there's a larger liquid range for water. At lower atmospheric pressures, there's a smaller liquid range for water, and it disappears completely at or below 0.006 atm. Above 100 atm, ammonia has no liquid state, and below 1/10 atm, ammonia has no liquid state. Below 1/10 atm, methane has no liquid state, and above around 2000 atmospheres, methane has no liquid state.

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