In last week’s Today’s New Reason to Believe I explained the crucial role and possible dangers that a large moon plays in a planet’s habitability.1 In a paper published April 2023 in the Astronomical Journal, two astronomers, Armen Tokadjian and Anthony Piro, explore whether large moons themselves could be habitable.2 In particular, they calculate whether a moon orbiting a planet could be tidally heated to an optimal temperature for life and, if so, how stable that optimal temperature could be. Their findings add evidence to the already-stringent requirements for habitability.
Tidal Heating of Io and Europa Tokadjian and Piro first point out that what they propose is not as far-fetched as it seems. Two of Juipter’s major moons, Io and Europa, for example, experience substantial tidal heating. Both moons are close to Jupiter; Io orbits 421,000 and Europa 670,900 kilometers from Jupiter. This proximity means that the sides of Io and Europa that are closest to Jupiter are gravitationally pulled toward the planet more strongly than their far sides. These differential gravitational forces tidally heat Io and, to a lesser degree, Europa.
Io and Europa experience additional tidal heating from the mean motion orbital resonances they experience from one another and from Jupiter’s largest moon, Ganymede. Io and Europa are in a 2:1 mean motion orbital resonance, meaning that Io makes exactly two revolutions around Jupiter for every single revolution of Europa. With Ganymede, Europa is in a 2:1 mean motion orbital resonance, while Io is in a 4:1 mean motion orbital resonance.
The tidal heating of Io has generated over 400 active volcanoes, making Io the most geologically active body in the solar system. A third of Europa’s composition is water. While the surface of Europa is frozen with a chilly surface temperature of only 90 K (–183°C or –297°F),3 its tidal heating may be sufficient to melt water deep in its interior. (It will take a satellite orbiting Europa to measure its degree of tidal heating accurately.)
Tidal Heating of Exomoons An exomoon is a moon in orbit around a planet orbiting a star other than the Sun. Several factors determine the degree of an exomoon’s tidal heating. The two researchers developed a list of the known factors and their relative importance. They determined the following outcomes.
The more massive the host planet the greater the tidal heating.
The closer the host planet the greater the tidal heating.
The denser the host planet the greater the tidal heating.
The mass, diameter, and density of the exomoon affect the degree of tidal heating.
Mean motion resonances with other large moons orbiting the planet will make substantial contributions to tidal heating.
The orbital eccentricities of the exomoon and other moons orbiting the planet will affect tidal heating.
Typically, increasing orbital eccentricity means a lesser degree of tidal heating.
Exomoons in mean motion orbital resonance with one another will inevitably generate librations (periodic variations) in their orbital eccentricities.
Taking into account all the factors contributing toward the tidal heating of a moon, the pair of researchers calculated the degree to which tidal heating would warm the surface of Io. They determined that tidal heating would warm up Io’s surface to 92 K (–181°C or –294°F). This tidal heating is greater than the heat Io receives from the Sun and is close to the 77–92 K temperature that astronomers have measured on Io’s surface.4 This measurement gave Tokadjian and Piro confidence that they had accounted for all the factors contributing toward an exomoon’s tidal heating and that their calculations were reliable. They then proceeded to determine tidal heating calculations for possibly existing exomoons.
The duo found that the greatest degree of tidal heating occurs for an Earth-like planet with two large moons in orbits of two and four days, respectively. The surface temperature of the inner moon is 481 K (208°C or 406°F). As the orbital periods of the large moons are increased, the amount of tidal heating dramatically decreases. Increasing the planet’s mass increases the minimum orbital distance of a large moon (below the minimum orbital distance, known as the Roche limit, the tidal forces exerted by the planet shatter the moon), which increases the moon’s orbital period and decreases its tidal heating. For a large moon in an 8-day orbit about a planet with the mass of Jupiter the tidal heating, independent of any stellar heating, adds up to a total of only 31°C (88°F) above absolute zero (absolute zero = –273.15°C or –459.67°F).
Tokadjian and Piro found that the lifetime of an exomoon’s tidal heating up to a temperature where liquid water becomes possible on an exomoon’s surface is brief for Earth-like planets. For rocky planets approximating the mass and size of Earth, any large moons they possess will undergo quick and substantial orbital evolution.5 The maximum time such moons will be tidally heated to permit the existence of liquid water on their surfaces is only about 1 million years. The researchers calculated that planets as massive as Neptune (17 times Earth’s mass) can retain large, tidally heated moons for as long as 5 billion years. However, for the large moons of Neptune-sized planets, tidal heating can sustain a surface temperature adequate for the possible existence of liquid water for only a few hundred million years, at most.
Detectability of Exomoons Astronomers have yet to detect and confirm the existence of an exomoon. However, they have found three possible candidates orbiting one or more of the following planets: Kepler-1623b, Kepler-1708b, and Kepler-1513b. As I noted in last week’s Today’s New Reason to Believe post, the observed dimming of Tabby’s star may be the result of fragments left over from the disintegration of an exomoon.6
The researchers note two ways that astronomers could detect and affirm the existence of exomoons either with existing astronomical instruments or with proposed instruments that are presently under consideration. One detection technique would be to observe a secondary eclipse. Astronomers can detect the eclipse of host stars by their planets passing in front of them, similar to when Venus and Mars pass between the Sun and our line of sight. It will be much more challenging, but not beyond current instrumental feasibility, for astronomers to detect the secondary eclipse of a planet’s moon passing in front of the planet’s host star.
A second technique that the two researchers propose is to detect—via transit spectroscopy—the volcanic vents of a tidally heated moon. It would be analogous to the successful detection and spectral measurements of gases emitted by Io’s volcanoes. Just as astronomers have detected neutral sodium gas being emitted from Io’s volcanoes,7 they could detect neutral sodium gas from an exomoon if that exomoon has volcanic gas emissions as great as or greater than Io’s.
Habitability of Exomoons Tokadjian and Piro’s calculations demonstrated that it is possible for an exomoon to be heated via tidal friction so that its surface is warmed to a sufficient degree where liquid water could conceivably exist. However, they also determined that it is difficult to sustain the necessary degree of tidal heating to be of any use for physical life.
To get the needed degree of tidal heating requires a rocky planet with the mass and density of Earth that possesses two or more large moons with orbital periods of only several days. If our solar system is a reasonable example, rocky planets possessing two or more large moons may be rare or nonexistent. While Mars has two moons, both are tiny. Earth’s moon is large but its formation was extraordinary, a consequence of a collision between two planets, the proto-Earth and Theia. Such a collision could produce one large moon but not two.
Rocky planets possessing large moons have a second strike against them. The time during which the tidal heating could be sufficient to sustain liquid water on one of their moon’s surfaces is short.
As the astronomers explained, the best-case scenario for a habitable exomoon would be for two or more large moons to be in close orbits about a planet as massive as Neptune. The degree of induced tidal heating would be less than it is for an Earth-sized planet but much more than for a Jupiter-sized planet. The stability of the induced tidal heating, though less enduring than for a Jupiter-sized planet, would be much longer than for an Earth-sized planet. While the degree of tidal heating would not, by itself, be great enough for the existence of liquid water, it could sufficiently supplement the heating the moon receives from the host star to reach that sufficient temperature. For that scenario to be realistic the planet must orbit its host star close to the liquid water habitable zone.
Large moons in close orbits about their planet will be tidally locked to that planet, meaning that just like Earth’s Moon their rotation rates will be the same as their revolution rates. If the moons gain a large fraction of their surface heat from their host star, then the surface heat will be too high during the day and too cold during the night for life’s survivability.
It is not enough that the surface temperature of an exomoon be suitable for liquid water. The exomoon must possess the necessary surface gravity to prevent the escape of liquid water from its surface. Our own Moon, as large as it is, falls short of the necessary surface gravity. Even a moon as large as Venus would fall short. The exomoon would need to possess the mass and density of Earth. In that event, the planet-moon system may be more accurately termed a binary planet.
Too much surface gravity could be just as big a problem as too little. An exomoon that gains a large fraction of its surface temperature from tidal heating would be far enough away from its host star that if it possesses a surface gravity any greater than Earth’s, it could retain too much ammonia and/or methane for the existence of nonexotic forms of physical life.
Habitability also requires that the exomoon possess a strong, stable, enduring magnetosphere. In three previous Today’s New Reason to Believe articles I explained why Earth possessing such a magnetosphere is nothing short of a miracle of exquisite design.8 It would be an even more outstanding miracle for a moon to possess such a magnetosphere.
As I have explained in my books, Improbable Planetand Designed to the Core, liquid water is just one of many habitability requirements. For a planet or moon to be habitable it must simultaneously reside in all 13 known planetary/lunar habitable zones. Of the known 5,374 planets9 only one simultaneously resides in more than 3 of the known planetary/lunar habitable zones—the same one that resides in all 13.
The level of design needed for a planet to be habitable is mind-bogglingly high. Research achieved by Tokadjian, Piro, and other astronomers establishes that an even higher level of design is necessary for a moon to be habitable. The fact that our planet-moon system reveals exquisitely designed features is far more than a curiosity. It’s a signature of Someone who designed Earth for human habitation.
Endnotes
Hugh Ross, “Moons: Life’s Friends or Foes?,” Today’s New Reason to Believe (blog), Reasons to Believe, May 1, 2023, https://reasons.org/explore/blogs/todays-new-reason-to-believe/moons-friends-or-foes.
Armen Tokadjian and Anthony L. Piro, “Tidal Heating of Exomoons in Resonance and Implications for Detection,” Astronomical Journal 165 (April 2023): id. 173, doi:10.3847/1538-3881/acc254.
Robert H. Tyler, Wade G. Henning, and Christopher W. Hamilton, “Tidal Heating in a Magma Ocean within Jupiter’s Moon Io,” Astrophysical Journal Supplement 218, no. 2 (June 2015): id. 22, doi:10.1088/0067-0049/21/2/22.
Takashi Sasaki, Jason W. Barnes, and David P. O’Brien, “Outcomes and Duration of Tidal Evolution in a Star-Planet-Moon System,” Astrophysical Journal 754, no. 1 (July 20, 2012): id. 51, doi:10.1088/0004-637X/754/1/51; Armen Tokadjian and Anthony L. Piro, “Impact of Tides on the Potential for Exoplanets to Host Exomoons,” Astronomical Journal 160, no. 4 (October 2020): id. 194, doi:10.3847/1538-3881/abb29e.
Miguel Martinez, Nicholas C. Stone, and Brian D. Metzger, “Orphaned Exomoons: Tidal Detachment and Evaporation Following an Exoplanet-Star Collision,” Monthly Notices of the Royal Astronomical Society 489, no. 4 (September 2019): 5119–5135, doi:10.1093/mnras/stz2464.
Apurva V. Oza et al., “Sodium and Potassium Signatures of Volcanic Satellites Orbiting Close-in Gas Giant Exoplanets,” Astrophysical Journal 885, no. 2 (November 10, 2019): id. 168, doi:10.3847/1538-4357/ab40cc.
