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Assessing Exomoon Habitability

Yesterday’s post on exomoons and their possibilities as abodes for life leads naturally to new work from René Heller (Leibniz Institute for Astrophysics, Potsdam) and Rory Barnes (University of Washington). We’re finding planets much larger and more massive than Earth in the habitable zone, as the recent findings of the Planet Hunters project attest. What can we say about the habitability of any large moons these planets may have? In their paper, Heller and Barnes look at the issues that separate exomoon habitability from habitability on an exoplanet itself.


If Earth-sized satellites of giant planets exist, they may have certain advantages over terrestrial planets in the same orbit, depending on the host star. We know that M-class dwarfs are by far the most common kind of star in the galaxy, and that habitable zone planets around one of these will probably be tidally locked, with one hemisphere permanently facing the star and the other in permanent darkness. Extreme weather conditions would result, creating severe limitations on the size of any habitable regions. But an Earth-mass exomoon around a gas giant will be locked not to the star but to the planet, a configuration that could stabilize the climate and prevent the dark side atmosphere from freezing out and the bright side atmosphere from evaporating.

Image: Artist’s conception of two extrasolar moons orbiting a giant gaseous planet. (Credits: R. Heller, AIP)

The stability of the atmosphere remains an issue, though, dependent on the size of the moon, the type of atmosphere and the intensity of incoming radiation. The researchers believe a nitrogen-dominated atmosphere would be stripped away by ionizing extreme ultraviolet radiation (EUV) in some habitable zone situations. This is interesting so let me quote the paper:

If Titan would be moved from its roughly 10AU orbit around the Sun to a distance of 1AU (AU being an astronomical unit, i.e. the average distance between the Sun and the Earth), then it would receive about 100 times more EUV radiation, leading to a rapid loss of its atmosphere due to the moon’s smaller mass, compared to the Earth. For an Earth-mass moon at 1AU from the Sun, EUV radiation would need to be less than seven times the Sun’s present-day EUV emission to allow for a long-term stability of a nitrogen atmosphere. CO2 provides substantial cooling of an atmosphere by infrared radiation, thereby counteracting thermal expansion and protecting an atmosphere’s nitrogen inventory.

For that matter, how massive does a moon need to be to create the magnetic shield that can sustain a long-lived atmosphere, and to drive tectonic activity and support a carbon-silicate cycle? The authors peg the figure at somewhere above 0.25 Earth masses, a figure that can be adjusted depending on the moon’s composition and structure. As I mentioned yesterday, the ongoing Hunt for Exomoons with Kepler (HEK) project pegs 0.2 Earth masses as the minimum size for an exomoon detection using current technology.

How large exomoons form is a question we’ve been kicking around here in the comments. If a gas giant is assumed to have formed well beyond the snow line and then migrated to its position in the habitable zone, would it bring with it a massive enough moon to sustain habitability? One recent paper (Canup and Ward, 2006) has shown that moons formed in the circum-planetary disk of a gas giant will have masses in the range of 10-4 times that of the planet’s mass. The paper runs through formation and capture scenarios and notes the work of Takanori Sasaki (Tokyo Institute of Technology), who suggests the formation of Earth-mass moons is indeed possible. We’re left with evolving ideas in what is clearly an active area of research.

Tidal heating is also a key factor in exomoon habitability, capable of causing intense magmatism and resurfacing on the moon’s surface in close orbits and enough volcanic activity to render the moon uninhabitable. But Heller and Barnes see scenarios where a moon becomes habitable only because of tidal heating, such as when the host planet orbits outside the outer edge of the habitable zone. For that matter, tidal heating can drive plate tectonics. Here the authors point to Europa, where tidal effect provides the heat to sustain a subsurface ocean of liquid water.

A theoretical model emerges, estimating the minimum distance a moon can be from its host planet and still allow habitability. The authors call the inner part of the circum-planetary habitable zone the ‘habitable edge.’ Moons inside the edge can run into runaway greenhouse effects because of tidal heating, while those outside the habitable edge are habitable by definition:

Similar to the circumstellar habitable zone of extrasolar planets… we conclude that more massive exomoons may have somewhat wider habitable zones around their host planets – of which the inner boundary is defined by the habitable edge and the outer boundary by Hill stability – than do less massive satellites. In future investigations it will be necessary to include simulations of the moons’ putative atmospheres and their responses to irradiation and tidal heating. Thus, our irradiation plus tidal heating model should be coupled to an energy balance or global climate model to allow for more realistic descriptions of exomoon habitability.

The minimum distance model for a planet to orbit and still be habitable should come in handy if we do find some candidate moons through the HEK project. “This concept will allow future astronomers to evaluate the habitability of extrasolar moons. There is a habitable zone for exomoons, it’s just a little different than the habitable zone for exoplanets,” Barnes said. But getting spectroscopic biosignatures in the atmospheres of any detected exomoons awaits next-generation space telescopes, so for now we’re left to use orbital configurations and studies of exomoon composition to assess potential habitability. One upcoming positive is the 2022 launch of ESA’s Jupiter Icy Moons Explorer mission, which should offer insights into the massive Galilean moons from tidal effects to surface chemistry and useful data on their interiors.

The paper is Heller and Barnes, “Exomoon habitability constrained by illumination and tidal heating,” accepted by Astrobiology (preprint). A news release from the Leibniz Institute for Astrophysics is available.


Comments on this entry are closed.

  • Eniac January 20, 2013, 20:07


    Well…..how much does it help? Give me something simple like a water column on Callisto vs. a water column on Earth. If I am a thousand feet down on Callisto what is the pressure compared to Earth?

    Callisto’s gravity is about 1/8 of Earth’s, therefore the pressure under any given height water column will be one eighth that on Earth. Conversely, for the same amount of pressure you need eight times the water column height on Callisto.

  • FrankH January 20, 2013, 20:09

    @Rob Henry – I’m quite aware of hydrodynamic escape from an atmosphere. If anything, it could make the losses GREATER than just Jean’s escape would lead you to believe.

    I don’t understand your issue with the Atmospheric Retention plot (http://astro.unl.edu/naap/atmosphere/animations/gasRetentionPlot.html).

    They use Jean’s escape and the rule that if the escape velocity of a planet is about 6x the average velocity of a gas in is atmosphere, the gas will be retained for billions of years.
    The 10x Avg is where the gas will absolutely be retained. You could argue that they should use 6x (they do – it’s where the color fades).

    Yes, their calculations make some assumptions, but the graphs are not wrong. It’s, in fact, quite accurate and a lot faster than calculating the numbers.

    If your calculations show that Jean’s escape is incorrect or otherwise wildly inaccurate, please post them here or link to articles that claim that it is wrong.

  • Rob Henry January 21, 2013, 16:05

    FrankH, from your last post it seems that you also realise 6x rms is figure that we should be using for an unbiased gauge of potential for atmospheric retention of exoworlds. Equally clear is that you are not, or our figures would be identical.

  • FrankH January 22, 2013, 16:13

    @Rob Henry- You’re the one who seems to have an issue with the web app results. Jeans escape is pretty much the floor for atmospheric escape. Add hydrodynamic escape and atmospheric heating and sputtering (for a planet with a weak magnetic field) from a stellar wind and the 6x figure looks optimistic.
    Your example of a world with an escape velocity of 3.1km/sec and a temperature of 300K would be right at 6x or slightly below. Great for retaining some gases using a “spherical cow” assumption, but in a realistic evolving system it’s unlikely.

  • Rob Henry January 22, 2013, 20:50

    Frank, I think your last comment gets us to the crux. I do not have any issue with web based applications, and certainly not with that one, only their misuse.

    That is a great site for world designing, and informing people of the current best ballpark guess of atmospheric retention. This is very poor if we want to examine the theoretical floor size for atmospheric retention of Jovian moons in their most likely HZ. This is why we must work straight from theory.

    Speaking of this – I noticed that you have never addressed the very important issue of the relationship of exobase temperature to surface temperature. On Venus the exobase is about half the surface temperature, on Earth, about three times it. Perhaps this time you can finally see the problem, and that a site that is great for worldbuilding is not so good here.

  • ljk February 16, 2013, 19:40


    Comet dust seeding life to Jupiter moons?

    by Staff Writers

    Boulder, Colo. (UPI) Feb 15, 2013

    Comet dust may have seeded Jupiter’s moons, including Europa and its liquid ocean beneath an icy crust, with the raw ingredients for life, U.S. researchers say.

    Asteroids and comets rich in the carbon-containing compounds that are key to life on Earth have been captured by Jupiter’s gravity, becoming orbiting moons that frequently collided as they settled into new orbits billions of years ago and created a fine dust of those compounds, they say.

    The question is, where has all that dust gone?

    Computer models suggest Jupiter should have captured about 70 million gigatons of rocky material but less than half that amount remains as irregular moons orbiting the planet.

    William Bottke of the Southwest Research Institute in Boulder, Colo., said the ground-up material would have fallen toward Jupiter, dragged by gravity and blown by the solar wind and almost half of it would have hit Jupiter’s largest moons, including Callisto, Ganymede and Europa.

    Images from NASA’s Galileo spacecraft have shown dark material on Ganymede and Callisto.

    “Callisto literally looks like it’s buried in dark debris,” Bottke told NewScientist.com, noting the surface of Ganymede looks similar.

    In comparison, Europa’s surface appears relatively clean but cracks in the moon’s icy crust suggest material is being cycled from the surface to deeper inside.

    Carbon-rich debris settling on Europa may have been incorporated into the ice and made it into the ocean, Bottke said.

    “Would it be important in Europa’s ocean? It’s hard to say,” he said. “But it is kind of interesting to think about.”

  • GaryChurch February 25, 2013, 14:00

    “Callisto’s gravity is about 1/8 of Earth’s, ”

    Thanks Eniac; not as wonderful as I thought but still pretty good. I really would like to find out what the water pressure is on these moons under miles of ice. Hardsuits can go down to 2000 feet so that would be 16,000 feet on Callisto- but if the ice increases the pressure the same as water then it is a very different story.