Tidal energy of the Moon in the form of gravitational attraction for the Earth would actually cause the solid crust of the Earth to stress with each revolution of the Moon. (28)
A secondary (tidal) heating event in the Earth-Moon system is proposed for both the lunar mare and Earth’s continental flood basalts. We call this tidal heating because it shares a similarity with the ocean tides in that the gravitational pull of another body causes the deformation of the Earth’s shape. Because the oceans are very viscous, gravitation causes their shape to change readily while the molding of the shape of the planet Earth itself is less visible. Essentially, it is a gravitational massaging that causes subtle deformation and melting pockets on the two bodies involved. As the gravitational energy was manifested in the physical stresses of these planetary bodies, heat was generated as a form of orbital energy dissipation.
During the Apollo Missions it was assumed that planetary bodies contain only the heat of formation (accretion) and isotopic decay, and this initial heat was thought to gradually be lost over time. Earth’s moon and Mars were both thought to be smaller planetary bodies which had lost all of this primordial heat and thus were “dead” bodies. The Earth was thought unique as the only “active” planet in the solar system with its plate tectonics, volcanic resurfacing, and biological life. Earth was assumed to still have its heat from accretion only because of its relatively large size. In general, secondary or tidal heating was not considered a significant contributor to overall planetary heating. Recent findings throughout the solar system make us aware that tidal heating caused by the gravitational pull of two bodies is very much a factor.
What Does Io Tell Us?
The Voyager flyby in 1979 of Io, Jupiter’s innermost moon, introduced a revised concept of tidal heating. (5,6) We’ve learned that gravitational stresses produced by Jupiter cause the heating of Io and Io is now recognized as the “most” active planetary body. The pattern of tidal heating on Io, interestingly enough, is found to be similar to the distribution of the mare on Earth’s moon.
Io’s episodic heating cycle results in massive heat loss in the eruptive phase, as heat is dumped onto the surface through basaltic outpourings. This heat loss cools Io, making it less conducive to Jupiter’s tidal pull, thus reducing the effect of tidal heating. The heating cycle then begins again; the process eventually results in the resurfacing of Io with layer upon layer of flood basalts.
Thus, Voyager “proved” tidal heating is an effective energy-transfer system. Other bodies have since been found to be impacted by tidal heating as well, most recently Saturn’s moons Enceladus and Dione, which have both shown evidence of geysers. (7)
Tidal Heating in the Earth-Moon System
The nature of Io’s heating reveals how tidal heating would be expected to play out in the Earth-Moon system during a close approach of the Moon to the Earth. Tidal heating in the Earth-Moon system would impact both bodies. Geologic evidence of a tidal heating event on the Earth and Moon, expressed by the outpouring of basalts, could confirm a close approach of the Moon to Earth and allow for the place of this event in the geologic timescale. A mechanism for “reheating” in the Earth-Moon system could provide the energy source that drives the Earth’s current dynamic systems and thus offers an explanation for why the Earth has remained so geologically active. Although the Moon is not appreciably heated by tidal heating at its current distance from the Earth, the potential impact of tidal heating at the previously close distance should be investigated.
Volcanism in the Solar System Today
Tidal heat is manifested in volcanic activity and surface melting, generally resulting in resurfacing of planetary bodies affected by tidal stresses. Tidal heating in some form may well be active in Mercury, Venus, Earth, Mars, Galilean satellites, and Saturn moons. (2) Io is now considered the “most geologically active body in the solar system”; New Horizons photographed a 200-mile-high plume from Io’s volcano Tvashtar, glowing lava and gas pockets. (8)Volcanic activity on Mars appears to have continued into the present (9) and the discovery of slope wash and alluvian deposition, possibly deposited within last 5 years, indicated an even more active Mars. Venus has been found to have large volcanoes and Triton has been found to have cryptovolcanoes. Overall, flood basalts in the solar system are much more common than once thought and have been found in many cases to be much more recent than had previously been believed.