The Circumstellar Habitable Zone (CHZ), Its Limitations, and the Role of Tidal Force
In astrobiology, the concept of the Circumstellar Habitable Zone (or CHZ) (sometimes colloquially referred to as the “Goldilocks Zone”) is linked to the assumption that liquid water is considered likely to be essential to life beyond Earth. Particularly, the CHZ generally refers to the region (a 3D, shell-shaped region) around a particular star that is considered to be in the right range of distance from the star such that a planet (with an atmosphere – required for surface liquid water) located in that region could potentially have surface liquid water. So, the concept generally goes, such a planet must be neither too close to the star (and thus too hot for surface liquid water), nor too far from the star (and thus too cold for surface liquid water). Generally, of course, if water is too hot, it becomes a gas (it boils); if it is too cold, it becomes a solid – ice (it freezes).
The CHZ concept has been useful in assessing exoplanets for speculated potential habitability; however, over time it is becoming increasingly recognized that the utility, comprehensiveness and accuracy of the CHZ concept has substantial limitations. There are a number of reasons for this, which include:
(1) It has recently become increasingly clear that a number of moons in the Solar System that are outside the generally recognized CHZ of the Sun, and which do not have substantial enough atmospheres to support surface liquid water, nonetheless probably have large regions of subsurface liquid water. These include Europa, moon of Jupiter, and Enceladus, moon of Saturn. It is considered likely that tidal forces largely provide the heat needed to keep this subsurface water liquid, and that surface ice provides the required pressure (rather than an atmosphere, as required for surface liquid water).
(2) Increasingly, moons of the Solar System as well as exomoons (moons in other solar systems) seem to be promising candidates for harboring life beyond Earth, in addition to exoplanets. The likely presence of large regions of subsurface liquid water on (in) moons in the Solar System is one reason for this. Another is the increasing evidence for energetic activity on such moons – such as likely geysers on Europa and volcanic activity on Io, another moon of Jupiter, which can be friendly to life. Still another is recent discoveries of extended habitats of microbial and aquatic life around hydrothermal vents in the deep seas of Earth. This life does not require interaction with an atmosphere (being deep underwater) and appears to be ultimately dependent not on energy from the Sun (even indirectly), but instead on energy derived from chemicals dissolved in vent fluids.
(3) Increasingly, tidal forces seem likely sufficient to allow for subsurface liquid water to exist on celestial bodies outside of a traditionally defined CHZ, and particularly on moons (including exomoons), where the large force of gravity from the massive host planet allows for strong tidal forces. Tidal forces are also believed to potentially provide for substantial sources of energy on moons, such as hydrothermal vents, plate tectonics, geysers or volcanoes.
(4) Tidal locking, once generally thought to greatly reduce, or eliminate, the probably of habitability of moons, is increasingly considered to be not necessarily prohibitive to, and in some cases to even potentially be helpful to, habitability of moons. Particularly, it is thought that tidal locking can cause increased tidal forces and consequent potentially helpful tidal heating of the moon, without, in some cases, necessarily causing prohibitive temperature extremes.
(5) Although perhaps less significant than the foregoing reasons, the (very) speculative possibility of life with a non-water liquid solvent (such as methane or ammonia) also inspires a search for habitable bodies outside a CHZ; these compounds are liquid at much lower temperatures than water, so that celestial bodies can maintain them as liquid much further out from a star than the CHZ (for example, surface liquid hydrocarbon lakes are believed to exist on Titan, moon of Jupiter).
Given the increasingly recognized limitations on the CHZ concept, new constructs have been proposed, including the concept of a circumplanetary habitable zone, or a habitable edge around a planet (see, for example, http://www.astrobio.net/news-exclusive/the-habitable-edge-of-exomoons/).
Since tidal forces are increasingly considered potentially key to the potential habitability of moons outside traditionally defined CHZs, a description of tidal forces and tidal locking follows. Since the force of gravity decreases with distance, the “pull” is not the same on all portions of a body. For example, the “pull” on the Earth’s moon, from the gravity of the Earth, is stronger on the face of the moon closest to the Earth (the near side), and lesser on the face of the moon farthest from the Earth (the far side). This differential across the body results in a stretching or bulging effect, which, in turns, disturbs the body itself, causing friction and heat. Interestingly, this also works in reverse, so to speak; gravity from the moon causes a tidal force on the Earth, which is related to the tides of the Earth’s oceans. Furthermore, a strong enough tidal force, such as may be the case with a moon orbiting closely to its host planet, can cause a moon to rotate such that the same side of the moon always faces the host planet – this is called tidal locking. Tidal locking can result in temperature extremes on the near and far sides of the moon, but also provides potentially life-friendly energy.
In the above image, the green region represents the CHZ around a star, while the red region is inside the CHZ (and generally too hot), and the blue region is outside the CHZ (and generally too cold). In general, the hotter the star, the further out the CHZ. Image Credit: Astronomy Education at the University of Nebraska-Lincoln Web Site (http://astro.unl.edu).In the above image, the blue circle can be a planet, such as the Earth, and the satellite can be a moon, such as the moon. As indicated by the force arrows, the tidal force caused by the gravity of the moon causes a stretching, or bulging, effect on the Earth (which causes the ocean tides). Conversely, the tidal force caused by the gravity of a planet on a moon causes a “stretching” effect on the moon, and with it, friction and tidal heating. Image available at https://commons.wikimedia.org/wiki/File:Field_tidal.svg, author: user – User:Krishnavedala.
