Tom Van Flandern: Was the planet Mercury once a moon of Venus?

This is an essay written some years ago by the late Tom Van Flandern  which was included in his book ‘Dark Matter, Missing Planets & New Comets’. Tom, who worked for many years at the U.S. Naval Observatory, was an out of the box thinker who covered a wide range of astronomical topics, many of them well outside the mainstream. His methodology was a bit similar to my old dad’s approach to cryptic crosswords. “The clue doesn’t give you the answer, but it helps confirm you got the right answer once you’ve got it”. Leif Svalgaard says he was a crank, which in my view means he’s well worth a read. I think this article, tied in with his other solar system formation concepts, deserves to be republished for the assessment and re-appraisal of the talkshop cognoscenti and the interested visitors here.

Let us examine in detail what the consequences would be of assuming that Mercury originated as a satellite of Venus. If that were so, we might presume that Mercury formed in close orbit about Venus, perhaps by fission^a. But Mercury is four and a half times more massive than the Moon. So the interchange of energy through tidal friction between Venus and Mercury would have been enormous. Mercury’s original spin would have been halted fairly rapidly by Venus, leaving Mercury spinning once per revolution around Venus, always keeping the same face toward Venus, as for our Moon.

We can also note that both planets would have been melted by tidal heating in the early stages following escape. Assuming this occurred before Venus differentiated (before the heavy elements sank to its core), this might explain both the high density and the magnetic field of Mercury, and the absence of an appreciable magnetic field for Venus^b. Thus we have reason to believe that all three present-day planets with unusually high densities (Mercury, Venus and Earth) probably melted at one time from tidal heating. This melting is then the most probable cause of their high densities.

Tides raised by Mercury on Venus while Venus was still spinning rapidly would have caused great interior heating and out gassing, and probably a great deal of surface upheaval (mountain building). We already start to see the evolution of a planet very much like the Venus we know today: an unusually thick, dense atmosphere from outgassing to great depths below the surface; quite hot, with mountains as large as its gravity field will support; and robbed of most of its original spin. We know that 50-100 atmospheres worth of CO2 and other Venus-atmosphere constituents are trapped in carbonate rocks in the Earth’s crust, and might have been released from tidal grinding if the Earth’s Moon had been as massive as Mercury.^c

Tidal friction would take Mercury into higher and higher orbits, just as has been happening for billions of years with our own Moon. However, our Moon will not reach a distance at which it can escape from Earth. All the Earth’s spin energy will be transferred to the Moon, and the two will revolve while keeping the same side toward each other, some billions of years in the future. Mercury, however, is sufficiently massive that it takes all of Venus’s spin in the first half-billion years after formation; and Venus’ orbit is close enough to the Sun that complete escape of Mercury will occur.^d

While Mercury is still a satellite of Venus, its shape will tend to become somewhat prolate (elongated toward Venus). Although that condition will relax somewhat after escape, the planet should still retain a slightly prolate shape. This turns out to be an important factor in understanding Mercury’s present-day odd spin state (3-to-2), as we shall see.

Post-Escape Evolution
Calculations by Harrington show that escape would necessarily occur on the sunward side of Venus, and that the new orbit around the Sun which Mercury would then occupy is both stable and non intersecting with the orbit of Venus (important to avoid collisions or ejection from the solar system). The subsequent evolution of the orbit of Mercury takes it to higher orbital tilts and eccentricity (ellipticity)-indeed at times to the same values for those parameters which Mercury’s orbit has today, even though those values are higher than for any “true” planet.

Let us consider what forces might have been operative in the early solar system which could evolve Mercury’s orbit inward to its present distance from the Sun. One candidate is tides raised by Mercury on the Sun. Such tides are negligible today, for purposes of orbital evolution. However, these tides should be orders of magnitude larger than is usually assumed, as described in the chapter “sunspots and Planets”. And in the early solar system, the Sun is presumed to have been physically much larger in diameter than now, according to stellar evolution theory (its T Tauri contraction phase). There is no question that a Sun with ten times its present diameter would have produced major tidal evolution which could have separated the orbits of Mercury and Venus (assuming they already existed, and that Mercury had already escaped from Venus). Smaller solar sizes may well have accomplished the same feat, depending upon the rotation speed and extent of differential rotation of the early Sun.

Other forces which might have been operative, such as collisions or drag from leftover planetary formation material, would also operate in the direction of separating the orbits, taking Mercury closer to the Sun than Venus. It is often forgotten that some forces were likely operating to evolve the mean distances of the planets from the Sun; for if there were not, we would find leftover formation matter today in the vast number of stable orbits between the planets. In short, if the planets did not all evolve significantly in their mean distances from the Sun (changing their distances by at least a factor of two), the left-over solar system formation material would cause the solar system to more nearly resemble Saturn’s rings than discrete planets with empty space between. Even in the conventional model of solar system formation from the primeval solar nebula, the planets would have collided with masses totaling 100% of their own mass in the aggregate while they were undergoing their last doubling in mass during accretion. These aggregate collisions must have had a major effect on the mean distances of each of the planets from the Sun, which could therefore not have started at their present values even in the standard model.

We conclude that the orbit of Mercury would evolve into one with unusual eccentricity and inclination to the plane of the other planets, typical of what we see today; and that its mean distance from the Sun could have evolved to today’s values also. Venus would have lost most of its spin in the process, and its remaining slow spin could have become retrograde because of thermally-sustained high atmospheric winds being forced retrograde by tides from Mercury just prior to the satellite’s escape.

Further calculations by Harrington showed that Mercury would have had a revolution period (and therefore also spin period, since spin is locked to Venus up to escape) of somewhat over 40 days at the point of escape, which spin period it would then retain for a time in its new orbit around the Sun. Tides raised by the Sun on Mercury would gradually slow its spin. Under ordinary circumstances, this slowing would continue until it reached 88 days, Mercury’s orbital period, at which time the planet would always keep the same face toward the Sun. Instead Mercury slowed only until it reached its present 59-day, 3-spins-to-2-revolutions state. Why did its spin stabilize in that condition? Because (and I consider this the strongest argument for the correctness of the theory that Mercury was a satellite of Venus) following an escape with a 40-some-days period of spin, the next stable spin configuration for a Mercury-like body with a slightly prolate shape is the 3-to-2 spin condition. Mercury has the required prolateness (and no other planet does) to a necessary and sufficient degree. And this prolateness, which enables Mercury to lock into such an unusual spin configuration, is itself a predicted consequence of having been a satellite of a larger planet.

In summary, something gave Venus a slow backwards spin, caused it to outgas a dense atmosphere and form high mountains, and to have no natural satellites today. Something caused Mercury’s satellite-like size and appearance, giving it a prolate shape, high density, and magnetic field, and put it into a 3-to-2 spin state. Up to now, astronomers have invented separate and ad hoc explanations for each of these anomalies. But we can instead invoke a single hypothesis, from which all these anomalies are natural, deductive consequences: that Mercury is an escaped satellite of Venus!

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Footnotes

^a If Mercury fissioned from Venus due to overspin, as is suspected to be the case for the Earth’s Moon, then we would expect hemispheric asymmetry on Venus, with analogs to the Earth’s “Pacific basin” and “Atlantic ridges.” Since there are no oceans because of the heat, these might take the form of missing crust, replaced with mantle material, in one hemisphere, and perhaps a major rift or split in the other hemisphere. Or in view of Mercury’s greater mass, perhaps the entire original crust of the planet was stripped away in the fission.

^b If Mercury is an escaped satellite of Venus, perhaps most of the original iron in Venus (which ultimately produces the magnetic field) was forced up into the crust by an excessively high spin rate. Venus may have then fissioned from overspin to form Mercury, with Mercury getting most of the iron. Subsequently both bodies would have melted each other from tidal friction, which would cause the heavy elements (such as iron) to sink to the core. Such a scenario could explain why Mercury’s magnetic field is stronger than that of Venus, since Mercury would get most of the iron during the original fissioning.

As for the Earth and its Moon, a similar scenario might apply, except that the Earth’s iron was not forced to the surface, perhaps because the Earth was not as hot and molten as Venus during that phase of its formation. Whether the iron and heavy elements rise or sink depends on the rate of spin and degree of melting of the planet. For the Earth, if these elements sank to the core before the Moon fissioned, the Moon would end up without its share of heavy elements. The sinking process increases the Spin of the Earth, and raises the likelihood of a fission to form the Moon.

Interestingly, for Venus, when the heavy elements rise to the surface, its spin slows. Yet a configuration with a heavy crust and light core becomes more likely to fission at lower spin speeds because it is unstable. If it does fission, the planet that remains behind will have lost a considerable portion of its angular momentum right at the outset.

^c We might also note that Venus and the Earth have the only two surfaces in the solar system with linear mountain ranges. In this model, these were presumably built during the crust fracturing process which led to the formation of a large Moon initially in orbit around each of these two planets, and no others.

^d It is interesting to follow closely what would happen to the spin of Venus as Mercury approaches tidal escape. For Venus or any planet that rotates faster than the mean speed of its atmospheric winds, mountains and other surface irregularities impart enough momentum to the atmosphere to ensure that it moves in the same general direction as the planet spins. But when the planet’s rotation slows to below the mean wind speed, as Mercury would cause Venus to do, then the situation reverses. Mean wind speed can push on mountain ranges and other surface inegularities, and ultimately determine which direction the planet will rotate.

As Mercury tidally evolves outward, it must necessarily produce rotational drag on Venus, tending to slow the rotation of Venus. But it must raise even bigger tides on Venus’ atmosphere, causing the whole atmosphere to circulate in the retrograde direction. After billions of years, this may produce retrograde spin of the entire planet.

Today, the largest net torques on Venus’ atmosphere are due to the Earth, since its tidal forces are stronger than either Mercury’s or Jupiter’s. The Sun produces stronger tidal forces than any planet, but with no net torque. So the Earth will ensure that the atmosphere of Venus continues to receive slight retrograde impulses at every inferior conjunction of the two planets. This may explain why the spin of Venus today lies close to a resonance with Earth, despite the seemingly negligible strength of the interaction between the two planets.