≡ Menu

Lunar Recession: Implications for the Early Earth

It was in 1775 that Pierre-Simon Laplace developed his theories of tidal dynamics, formulating in the following year a set of equations to explain the phenomenon at a greater level of detail than ever before. Looking at the Moon on a frosty winter night, it’s pleasing to realize that there is a mountainous region at the end of Montes Jura in Mare Imbrium that is called Promontorium Laplace. Surely the French astronomer and mathematician would have been pleased.

One result of Laplace’s calculations was his pointing out that the Moon’s equatorial bulge was far too large to be accounted for by its current rate of rotation. Here we’re dealing with conditions of formation of an object thought to have been the result of a collision between the Earth and a Mars-sized planet early in our system’s evolution. I seldom write about the Moon in these pages, but today’s story on its development catches my eye because it relates to the early history of our own world and the Solar System itself. For Chuan Qin (now at Harvard University) and colleagues have modeled how quickly the hot young Moon receded from the Earth.

The current rate of the Moon’s recession from the Earth is about 4 centimeters per year. But what was the recession rate in the earliest periods of its formation?

Image credit: University of Colorado at Boulder.

The tidal bulge at the equator evidently has much to tell us. A hot, fast-rotating early Moon would have possessed a much larger equatorial bulge than today’s. As the Moon moved farther from the Earth and its rotation slowed, the bulge would have shrunk until, cooled and hardened, a permanent ‘fossil’ bulge remained in its crust. Working with a model adjusting the relative timing of lithosphere thickening and lunar orbit recession, Qin and team have found that the pace of the lunar recession was slow, lasting for several hundred million years in an era roughly four billion years ago.

If this dynamic modeling is correct, it can tell us something about the early Earth, says Shijie Zhong (University of Colorado at Boulder), a co-author on the paper:

“The moon’s fossil bulge may contain secrets of Earth’s early evolution that were not recorded anywhere else. Our model captures two time-dependent processes and this is the first time that anyone has been able to put timescale constraints on early lunar recession.”

The new model has implications for the hydrosphere, the combined mass of water on the early Earth. The researchers argue that the Moon’s equatorial bulge is evidence that Earth’s energy dissipation in response to tidal forces would have been greatly reduced in this period. That’s assuming that a hydrosphere even existed in the Hadean, a geologic eon that began with the planet’s formation some 4.6 billion years ago and ended roughly 4 billion years ago. From the paper:

Viable solutions indicate that lunar bulge formation was a geologically slow process lasting several hundred million years, that the process was complete about 4 Ga when the Moon-Earth distance was less than ~32 Earth radii, and that the Earth in Hadean was significantly less dissipative to lunar tides than during the last 4 Gyr, possibly implying a frozen hydrosphere due to the fainter young Sun.

The paper makes the case that Earth’s hydrosphere may have been frozen during the time of the Moon’s formation, making for little tidal dissipation. One possibility emerging from that is a faint young Sun radiating about 30 percent less energy than today. A ‘snowball Earth’ in the Hadean could have been the result, but we have no direct evidence in the geological record for this. Qin and team intend to continue work on their model as they dig deeper into the Moon’s evolution in a period ending with the Late Heavy Bombardment some 3.8 billion years ago.

The paper is Qin et al., “Formation of the Lunar Fossil Bulges and Its Implication for the Early Earth and Moon,” Geophysical Research Letters 2 February 2018 (abstract).

tzf_img_post

{ 15 comments… add one }
  • John walker February 8, 2018, 16:26

    Hadean zircon and greenstone belts like the Isua and Nuvvuagittuq have shown that the Earth had a substantial liquid hydrosphere during the period in question. A 2016 paper from Carl Wunsch at MIT suggested that a global ice shield at least of the strength speculated in the proterozoic Snowball Earth phase would not have had a devastating effect on tidal dissipation. https://www.google.de/url?q=http://ocean.mit.edu/~cwunsch/papersonline/tides_ice_cover_icarus2016.pdf
    With no access to the paper from Qin et al. I wonder about the degree of freezing that is hypothesized therein.

  • Tom Mazanec February 8, 2018, 17:05

    wasn’t the Hadean named after Hades ie Hell? Wasn’t the surface supposed to be molten?

  • Geoffrey Hillend February 8, 2018, 17:36

    Interesting. more support for the large impact hypothesis.

  • Alex Tolley February 8, 2018, 17:58

    The paper’s math is way over my head, but the constraints on the Hadean Earth’s surface have consequences.

    If, as the authors’ suggest, the Hadean Earth was in a snowball state needed to reduce tidal dissipation, then life’s genesis must have been extremely fast at the end of this period (4 gya) to result in fossils 3.5 gya and possibly even as early as 3.8 gya. This suggests to me that life would have started in the ocean depths at vents that were erupting below the surface ice.

    A snowball Earth in the Hadean seems to obviate any need for a warm surface maintained by CO2, CH4 and possibly H2 for the necessary greenhouse effect with a faint sun.

    Another possibility is that the oceans were largely absent during the Hadean, perhaps being created by cometary impacts. But if that is the case, then the genesis would be similarly constrained in time to models of life evolving in “warm ponds” on the surface. Which may, in turn, lend credence to the panspermia theory, perhaps originating on Mars or outside the solar system.

    Then again, maybe the authors’ models are wrong about the lunar bulge despite the claimed robustness of the variables to the result.

  • Michael February 9, 2018, 2:17

    I don’t believe the continents had formed yet so tidal dissipation would have been weak as there was no significant land masses to interact with. Any land masses that would have formed would have been weakly held up as well due to the higher ground temperature.

  • Geoffrey Hillend February 9, 2018, 2:20

    I wouldn’t jump to conclusions. Life that began without any light has yet to be proven. The thermophiles in our sea migrated there from the surface. Sunlight might have been needed for it too start but even if it didn’t sunlight is still needed for photosynthesis. This theory hypothesizes that UV radiation began life: http://www.sciencemag.org/news/2017/07/how-sunlight-might-have-jump-started-life-earth

  • djlactin February 9, 2018, 4:09

    The speculation on temperatures of Hadean Earth (and early Mars) assume implicitly that they were the same distance from the Sun as now. Maybe not so: The Grand Track model and the Nice model of solar system evolution have Jupiter, Saturn, Uranus and Neptune migrating in and out. I think we can assume that the inner planets were also affected. To my point: maybe Earth and Mars have also migrated outward. At 4 Gya Earth could have been sufficiently closer that solar flux was comparable to today’s.

    • ljk February 9, 2018, 10:08

      How can we find out if the planets of the Sol system were in different orbits 4 billion years ago?

  • Paul Higginbotham February 9, 2018, 10:39

    I have the feeling, that the lunar spin, could have reversed the present 4cm per year retreat, by spin of moon reduction, while close during hadean times, by spinning tidal reduction of lunar surface, thus masking earth hydrological tidal retardation. Thus the reduction of lunar angular momentum, reversed the tidal retardation of lunar distance, but not earth spin reduction, which also carried on, much stronger then, thus spinning faster during the early hadean, and reducing day length more rapidly.

  • andy February 9, 2018, 16:28

    Talking of cold planets in the early solar system, there are some recent results on Martian clays that suggest a cold climate for ancient Mars, with only sporadic warm periods (possibly triggered by impacts).

  • Charlie February 12, 2018, 2:19

    One interesting aspect of this entire conversation is the fact that I believe most people are unaware that the orbital mechanics of the Moon-Earth system is gravitationally coupled so strongly that there has been since the formation of the system a tremendous amount of angular momentum transfer between the two bodies.
    A very brilliant nineteenth century mathematician by the name of G. W. Hill was among the first to show mathematically that the moon will undergo a constant regression in distance from the earth, but eventually it will have a maximal limit which due to the physics will put a On how far out the moon can ultimately recede to.

    One of the interesting results of this is that ultimately that total eclipses, such as we saw last August in 2017, will cease to occur in about 600 million years and there will be only annular eclipses. After that, on earth. The mechanics of these angular momentum transfers are extremely interested in the way that they effect the spin of the planet and also to the seasons that life depends on here on earth.

  • Geoffrey Hillend February 12, 2018, 17:14

    Charlie, when you say most people you must mean the layman because the gravitational coupling of the Earth Moon system is planetology 101 in most universities. It was explained in my junior college Moons and Planets book by William K. Hartmann 1983 version. Also in The Realm of the Terrestrial Planets, Znenek Kopal. You are right about the angular momentum which was supplied by the the collision of Theia in the Giant Impact hypothesis. The spin couldn’t reverse itself since it has to start with more angular momentum or faster spin. Also there is also fossil evidence of that there were shorter days, 21 hours in the Cambrian 500 million years ago and more days in the year like 400; the evidence comes from the shells of scallops with 400 lines for days, much longer shoreline tidal movement and etc.

    The friction of the sea on the Earth slows down its rotation and causes the rotational momentum of the Earth to be transferred to the orbital momentum of the Moon. The tides of the Moon still slow down the rotation of the Earth even without a liquid sea but the slowing will still be less without a liquid sea or with a frozen sea, but the Moon bulge hypothesis is valid.

  • Geoffrey Hillend February 16, 2018, 17:09

    Pardon me for the mistake. The other book I read with the gravitation coupling of the Earth-Moon system was called “Exploration of the Universe by George O. Abell. 1982 version, which was the astrophysics textbook for the junior college I attended.

Leave a Comment