Habitable Moons – Two Decades Later

Habitable moons have been the spectacular settings for a number of science fiction stories over the decades. For scifi movie enthusiasts of my age, the forest moon of Endor, which appeared in the 1983 film “Star Wars: The Return of the Jedi”, is a notable example. But starting in late 1995 with the discovery of giant extrasolar planets orbiting in and near the habitable zone of Sun-like stars, the possibility that giant planets with Earth-size moons could be potentially habitable began to spark the interest of some in the scientific community as well. And not “habitable” like Jupiter’s moon Europa potentially is with a tidally-heated ocean that could provide an abode for life protected from the vacuum of space by kilometers of ice, but “habitable” like the Earth with conditions that allow for the presence of liquid water on the surface for billions of years with the possibility of life and maybe a technological civilization evolving.

The cover of the December 1998 issue of Sky & Telescope. (S&T)

On January 17, 1997, Nature published the paper “Habitable Moons Around Extrasolar Giant Planets” by Darren Williams, James Kasting (considered by many to be the father of our modern understanding of Earth-like habitability) and Richard Wade (Pennsylvania State) – the first serious review of habitable moons to appear in a peer-reviewed science journal. At this time, I was on the editorial board of SETIQuest Magazine and a regular contributor of feature length articles. Having read the Nature paper and being in contact with (then graduate student) Darren Williams, I took the opportunity to write a fully-referenced review on the topic of habitable moons in the spring of 1997 (see “Habitable Moons: A New Frontier for Exobiology”). I even went so far as to take a stab at making a VERY rough estimate at the number of habitable moons in our galaxy (an estimate in desperate need of updating after over two decades of exoplanet search results). Since my research showed it was likely that habitable moons would tend to come in groups of two or more, I further speculated about the possibilities of life originating on one of these moons being transplanted to a neighbor via lithospermia. And since I did not have to contend with scientific peer-review for this article, I even speculated about the effects multiple habitable moons would have on a spacefaring civilization in such a system with so many easy-to-reach targets for exploration and exploitation.

The following year, I was approached by the staff of Sky & Telescope to write a popular-level article on habitable moons for a special magazine issue about the search for extraterrestrial life. That article, entitled “Habitable Moons”, (along with another piece on SETI I cowrote with associate editor, Alan MacRobert) was published 20 years ago in the December 1998 issue of Sky & Telescope. Even after two decades, this article still nicely addresses the central scientific issues about the potential habitability of moons and is shared here as it was originally published.

Habitable Moons – December 1998

One of astronomy’s most exciting discoveries in recent years has been the detection of worlds orbiting stars other than our own. More interesting still is that some of these extrasolar giant planets (EGPs) orbit within the “habitable zones” of their suns – that is within the range of orbital distances where liquid water should exist and, in theory, life could thrive. According to conventional wisdom, however, it is highly unlikely that life, much less intelligent life, could arise of these giant, gaseous planets. But what about their moons? Could they support habitable, Earth-like conditions where a technological civilization might evolve?

Below its icy crust, Jupiter’s moon Europa has a tidally warmed ocean possibly capable of harboring life. Ganymede, Callisto, and perhaps even Triton and Pluto may have internal oceans too. However, such airless worlds are not considered ‘habitable’ places where a technological civilization might evolve. (NASA/JPL/DLR)

In our own solar system, the larger a gas giant the greater the total mass of its satellites. So perhaps EGPs more massive than Jupiter have moons as large as Mars. Brown dwarfs, which are even more massive and have also been found closely orbiting a few nearby Sun-like stars, might have moons larger than the Earth itself. Worlds this big could be quite livable. (Since brown dwarfs are failed stars, their companions should probably be labeled planets. However, the classification of some of the recently discovered extrasolar companions is still controversial, so for convenience, I will refer to the companions of both EGPs and brown dwarfs as moons.)

One factor determining a moon’s habitability is the stability of its orbit., which can be disrupted by the close proximity of its sun. Studies of multibody systems suggest that a moon with a period of revolution of less than about 45 to 60 days will remain bound to an EGP or brown dwarf that orbits 1 astronomical unit from a Sun-like star. The major moons of our solar system’s gas giants all have orbital periods between 1.7 and 16 days regardless of the mass of the planet. This suggests that the total angular momentum of a gas planet’s system of moons is also roughly proportional to the planet’s mass. If a similar scaling law applies to the moons of EGPs and brown dwarfs, a 16-day orbit is still significantly shorter that the upper limit for stability. At the other end of the range a 1.7-day orbit puts the moon well outside of the Roche limit, where a moon would be sheared apart by tidal forces. Assuming there are no impediments to moon formation, EGPs and brown dwarfs in a star’s habitable zone can thus have large moons in very stable orbits.

Keeping an Atmosphere

For a moon to be ideally habitable, it must have an appreciable atmosphere. Obviously such a body must be larger than the Earth’s own airless Moon, which has a mass 0.012 times that of the Earth, but how much larger? Darren Williams, James Kasting, and Richard Wade (Pennsylvania State University) have examined this problem in detail and found that a number of processes allow a world’s atmosphere to escape.

The first is well known. Some gas atoms in the upper atmosphere attain speeds faster that the escape velocity as a result of thermally driven collisions and are soon lost. For the atmosphere to be retained for long periods, this process must be kept very slow. Either the temperature at the top of the atmosphere must be low, or the world must be massive enough to have a high escape velocity. For a body with a Mars-like density and an Earth-like atmospheric temperature structure, calculations show that it must have a mass of at least 0.07 Earth to retain most of its atmosphere for 4.6 billion years (the current age of the Earth). The escape of some atmosphere is not necessarily fatal. On Earth carbon dioxide can be replenished from the vast stores of this gas locked up in carbonate deposits. However, the loss of other biologically important gases, like nitrogen, is irreversible. A major loss process for nitrogen is called dissociative recombination. This process starts when a positively charged nitrogen molecule at the top of the atmosphere combines with an electron to produce a pair of free nitrogen atoms. The energy released by this reaction gives the now neutral atoms enough of a kick to permanently escape. Estimates based on Mars’ nitrogen loss rate indicate that dissociative recombination becomes negligible for a world with a minimum mass of 0.12 Earth at a distance of 1 a.u. from a Sun-like star.

A potentially greater threat to a moon’s atmosphere is sputtering. This process occurs when an energetic charged particle collides with a gas molecule and the rebound kicks the molecule into space. The magnetospheres of the gas giants in our solar system, and presumably of EGPs and brown dwarfs, contain radiation belts potent enough to completely erode the atmosphere of an orbiting Earth-like world in only a few hundred million years.

Surprisingly, Jupiter’s moon Ganymede has a magnetic field produced by dynamo action in its core. A magnetic field would be needed to shield any habitable moon’s atmosphere from erosion by the high-energy particles expected in the magnetosphere of a giant planet or brown dwarf. (NASA/JPL)

One way to blunt this form of atmospheric loss is through the shielding effects of a strong magnetic field. Measurements by NASA’s Galileo spacecraft at Jupiter hint that large moons might have magnetospheres of their own with the required strength. Galileo unambiguously detected a strong Earth-like magnetic field around Ganymede, which has a mass of only 0.025 Earth. The situation with Io, with a mass of 0.015 Earth, is a bit more ambiguous but still promising. Researchers once believed that small bodies like Jupiter’s Galilean moon could not possess such strong fields. But these moons orbit deep inside of Jupiter’s own powerful magnetosphere. According to models developed by Gaeme Sarson (University of Exeter) and his colleagues, a strong ambient field is though to help initiate the circulation needed to produce a vigorous dynamo effect in the core of even a slightly active moon, leading to a strong field for the moon itself. Taken together, these observations and models hint that planet-size moons can maintain protective magnetic fields that they would not otherwise have in isolation.

Maintaining Geologic Activity

For a moon to have an active dynamo producing a magnetic field it must have a source of internal heat. But even more internal heat is required to drive another process that may be necessary: the geologic activity needed for the carbonate-silicate cycle, which controls global atmospheric temperatures. Without this important cycle to maintain atmospheric carbon-dioxide levels, an otherwise habitable moon will experience a perpetual ice age much as Mars does today.

The most important current source of internal heat for our solar system’s terrestrial planets comes from the decay of radioactive isotopes. This heat production decreases with time, however, and small bodies cool faster than large ones. As a result, large planets like Earth can support the carbonate cycle longer than small planets like Mars. While the amount of internal heat needed is still a subject of debate, it is estimated that a world’s mass must be at least a quarter that of the Earth to maintain this cycle for 4.6 billion years if radiogenic sources are the only heat producers.

But large moons orbiting an EGP or brown dwarfs have an additional source of energy that out terrestrial planets lack: tidal heating. A spectacular example of this phenomenon is Jupiter’s moon Io. The constant flexing of Io’s surface as the moon circles Jupiter in its slightly eccentric (elliptical) orbit generates enough heat to make it the most volcanically active body in the solar system.

Small planets and moons usually cool quickly and become geologically inactive. But activity is needed to drive the carbonate-silicate cycle that regulates greenhouse gases in an atmosphere. Io, shown here, is violently volcanic despite its Moon-like size due to tidal heating. The same process might help make other small moons livable. (NASA/JPL/University of Arizona)

Europa may be experiencing a lesser degree of tidal heating, which is still enough to maintain the ocean of liquid water that apparently exists beneath its icy shell. Ganymede has a magnetic field and evidence of past geologic activity that both suggest this largest moon of Jupiter experienced episodes of enhance tidal heating as its orbit evolved through distorting resonances with its siblings over the past few billion years. The magnitude of tidal heating depends on a number of factors such as the mass of the moon and its primary, the moon’s internal structure, the size and eccentricity of its orbit, and the orbits of its near neighbors. Large moons around EGPs probably experience some amount of tidal heating that help maintain habitable conditions far longer than even much larger terrestrial planets could in isolation.

Length of Day

Another potential problem for habitable moons is the length of their day. Computer models show that any large moon orbiting an EGP or brown dwarf becomes locked into synchronous rotation (with one side of the moon always facing the planet) within a few hundred million years. This, of course, has happened with our own Moon and many of the large moons in the solar system. Assuming that large moons typically have orbital periods of 1.7 to 16 days, any potentially habitable moon would have a “day” several times longer than Earth’s. Simple calculations by Stephen Dole of the Rand Corporation in the 1960s showed that the surface of a body with an Earth-like atmosphere would become uninhabitable when the period of rotation exceeds 4 days, due to the large swings in surface temperature.

In reality, the situation for slowly rotating moons would probably not be so bleak. Monoj Joshi and Robert Haberle (NASA/Ames Research Center) and their colleagues have been investigating the effect of synchronous rotation on the habitability of planets closely orbiting red-dwarf stars. Such a planet would become tidally locked so that one hemisphere always faces its sun while the other experiences perpetual night.

Joshi and Haberle’s latest computer models have shown that an atmosphere with a carbon-dioxide pressure of only 1 to 1.5 bars (a bar is the atmospheric pressure of the Earth) not only maintains habitable conditions on a synchronously rotating planet but also allows liquid water to exist even on the planet’s perpetually dark side. A thick carbon-dioxide atmosphere (one rich in this infrared-trapping “greenhouse gas”) retains heat better than a thin Earth-like (nitrogen-oxygen) atmosphere, and it allows this heat to be transferred more efficiently to the dark side via large-scale circulation patterns.

The situation for a slowly rotating moon should be less extreme than for a synchronously rotating planet. While simulations with such a moon have yet to be performed, even modest additions of carbon dioxide to a moon’s atmosphere (which would probably exist as a natural consequence of the carbonate-silicate cycle) will likely allow it to remain clement despite having a day as long as two weeks. Clouds and large bodies of water, which have not been taken into account in models to date, should further moderate temperature extremes.

Problem of Eccentricity

A number of recently discovered EGPs and brown dwarfs have mean orbital distances that lie inside the habitable zones of their suns. The companions of the Sun-like stars 47 Ursae Majoris and HD 29587 in Perseus, while near the outer limits of their system’s habitable zones could possibly support water-bearing moons. Unfortunately many of the other known EGPs and brown dwarfs have eccentric orbits that would complicate habitability due to large swings in the amount of sunlight reaching them. The mean insolation of the EGP orbiting 16 Cygni B, for example, is about half that of the Earth, but this level ranges from 20 percent to as much as 260 percent of the sunlight on Earth because of the planet’s eccentric orbit.

As in the case of planets with long days, the presence of a dense carbon dioxide atmosphere could lessen these extremes. Since the companion to 16 Cygni B lies on average in the outer portion of its system’s habitable zone, any large moon it possess could have the required dense atmosphere as a result of the carbonate-silicate cycle. Other candidates in this category include the brown dwarfs orbiting HD 110833, BD -4° 782, HD 18445, and HD 217580.

The Future

The detection of moons suitable for life will probably be more difficult than the detection of habitable terrestrial planets. None of the telescopic systems that have been proposed to detect Earth-size planets around nearby stars will have the resolution required to separate the image of a moon from its primary. Photometric searches for planetary transits, such as the proposed Kepler mission, might have better luck. But the ever-changing position of these moons in relation to their primaries will require the observation of many transits to isolate a moon’s photometric signature. Given the difficulties, the unambiguous detection of a moon, habitable or otherwise, is probably decades away.

Moons of extrasolar planets won’t be spotted any time soon. Even NASA’s proposed Terrestrial Planet Finder, which will look for Earth-like planets orbiting nearby stars, won’t have the resolution needed to see satellite bodies. (NASA)

Much theoretical work remains to be done to determine the possible abundance of these bodies. A detailed understanding of the origins of brown dwarfs and EGPs is required, not to mention the origin of moon systems themselves. The distribution of volatiles like water in a moon system, and how this is affected by the thermal history of its primary, will also have to be better understood.

Still, there may be millions if not hundreds of millions of habitable moons in our galaxy. Given that large moons generally occur in groups among the gas giants in out solar system, habitable moons could also occur in groups of two or more per planet. It’s anyone’s guess what the implications may be for the abundance of life and the possible development of extraterrestrial intelligence.

Postscript – December 2018

Research over the last two decades continues to support the notion that some moons of extrasolar planets (or “exomoons”) could be potentially habitable in an Earth-like sense under a reasonable range of conditions (see “Habitable Moons: Background and Prospects”). The ongoing study of the planets and moons in our solar system, as well as the processes which shape their environments, along with the flood of extrasolar planet discoveries have helped scientists address the issue of habitable exomoons with increasing fidelity. While there have been some potential exomoon detections in the photometric data gathered by NASA’s recently completed Kepler mission (see “The Case for a Moon of Kepler 90g”), the announcement in October 2018 of a Neptune-size exomoon candidate orbiting the giant planet Kepler 1625b seems to be on an increasingly firmer footing as new observations are made (matching my prediction in 1998 that the first detection of exomoons was “probably decades away”). As our instruments and techniques continue to improve, it will only be a matter of time before a potentially habitable exomoon candidate is identified and our predictions are finally tested by observations.

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