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Untangling the Effects of the ‘Big Whack’

Seasonal change on our planet is relatively moderate because the Earth has a small axial tilt. Just how that situation arose makes for interesting speculation, and a series of scientific papers that have been augmented by a new analysis in Nature from Matija Ćuk (SETI Institute) and Sarah Stewart (UC-Davis). Working with colleagues at Harvard and the University of Maryland, the scientists have created computer simulations showing that the early Earth experienced a day as short as two hours, and had a highly tilted spin axis.

How we get from there to here is the question, and it’s one that Ćuk and company answer by examining the collision that spawned Earth’s Moon. The impact theory sees the Moon forming from the debris of the collision between an infant Earth and a Mars-sized protoplanet.

It was Ćuk and Stewart who suggested some four years ago that following the ‘Big Whack,’ the Earth’s rotation period was closer to two hours than the five that earlier work had suggested. The Moon would have formed much closer to the Earth than it is today, with the Earth losing much of its spin and a good deal of its tilt as the Moon’s orbit widened.


Image: Artist’s depiction of a collision between two planetary bodies. Such an impact between the Earth and a Mars-sized object likely formed the Moon. Credit: NASA/JPL-Caltech.

All this fits with what we see today, with the Moon continuing to move gradually away from the Earth as our planet’s spin continues to slow. The slowdown is produced by the tides the Moon raises on our surface, which dissipate energy continually as they interact with the oceans.

If we assume a fast spinning early Earth, the ejection of material following the impact can produce a Moon similar in composition to Earth’s mantle, which is what we see in lunar rocks. Now we get into the realm of orbital interactions that determine the system’s evolution. For today’s Moon has a tilt of its own, about five degrees off from Earth’s orbital plane. This is true despite the likelihood that internal friction due to tidal effects by the Earth should have had a profound effect on the Moon’s tilt, decreasing it over a billion-year time frame.

The tilt of the Moon’s orbit, in other words, must have once been much greater than it is today. Ćuk, Stewart and colleagues Douglas Hamilton (University of Maryland) and Simon Lock (Harvard) believe we can arrive at today’s situation if we begin with an Earth that, not long after the impact, was spinning essentially on its side, with the Moon orbiting the equator. With an axial tilt of over 70 degrees, this situation will not last, with solar gravitational forces creating an eccentricity in the Moon’s orbit that produces strong tidal flexing within it.

These internal lunar tidal effects, in the view of the scientists, would have produced a counter-force against the tidal effects from the Earth that would have been pushing the Moon into a wider orbit. During this period, Earth would have continued to lose its spin, but rather than going into a wider orbit, the Moon’s orbit would have become increasingly tilted.

It would only be as the Earth’s rotation continued to slow that the Moon could break out of this deadlock and continue moving away from our planet. Its subsequent torque on the Earth’s spin axis is what Ćuk and team believe began to move the Earth’s axial tilt into more moderate territory. And tidal flexing inside the Moon would have helped to shrink its orbital inclination, so that today it is within five degrees of the orbital plane of the planets.

The paper summarizes the situation, with interesting exoplanet implications. Note the reference in the passage below to the ‘Cassini state,’ which is a system that obeys the laws of the Moon’s motion with respect to the Earth that were originally stated by the Italian astronomer Giovanni Domenico Cassini (for whom our Saturn orbiter is named):

Our high-obliquity model is at present the only model we are aware of to explain the origin of large past lunar inclination, which was subsequently reduced by strong obliquity tides at the Cassini state transition… [O]ur results support high-AM [angular momentum] giant-impact scenarios for lunar origin. An initially high-obliquity Earth is consistent with the expectation of random spin-axis orientations for terrestrial planets after giant impacts, and the dynamics discussed here naturally reduces Earth’s obliquity to values that are low to moderate.

Which is interesting indeed, because here we have a mechanism that can gradually bring a high obliquity exoplanet to a much lower axial tilt, thereby making moderate seasons possible. Is a large moon critical for planetary habitability? Ćuk comments on the same point:

“This work shows that there are multiple ways a planet could get a small axial tilt, making moderate seasons possible. We thought Earth was this way because of the direction of the giant impact 4.5 billion years ago, but it looks like Earth achieved this state later through a complex interaction with the Moon and the Sun. I wonder how many habitable Earth-like extrasolar planets also have a large Moon.”

Centauri Dreams’ take: Interacting gravitational influences within the Solar System have a great deal to do with habitability, as the work of the Serbian astronomer Milutin Milanković has demonstrated. Milanković (working while a prisoner of war during World War I) found rhythmical climate cycles that deep sea core analysis confirmed in the 1970s, all related to gravitational nudges involving the motions of Jupiter, Saturn and the Moon.

David Grinspoon writes about Milanković in his fine new book Earth in Human Hands (2016), pointing out that while our Moon has kept Earth’s rotational axis stable at a 23.5 degrees tilt from the Sun, Mars (currently at about 25 degrees of tilt) varies from 15 to 35 degrees and sometimes more over a period of 120,000 years. Rhythmical climate torques also show up on Titan. Clearly such interactions will have to be taken into account as we examine young systems around other stars and their likely evolution.

The paper is Ćuk, “Tidal evolution of the Moon from a high-obliquity, high-angular-momentum Earth,” published online by Nature 31 October 2016 (abstract).


Comments on this entry are closed.

  • Joe November 1, 2016, 12:04

    This article is a good argument for the Rare Earth theory. Note the moon had to be just the right size when it collided with Earth and it had to hit at just the right angle to go into a stable orbit. If the moon had been larger, the collision might have destroyed the Earth. If it had been smaller it’s effect would have been less. But because the moon is the size it is, we got moderate seasons and tides. Tides of course were a factor in the movement of early life from water to land. Overall, the odds of such a lucky collision happening in other star systems is pretty low.

    • Torbjörn Larsson November 2, 2016, 18:33

      Nothing is a good argument for an idea based on bayesian factor addition. You can chose any outcome you like, so it isn’t testable – it isn’t a theory.

      The tidal zone may have been useful for speeding up migration to land for some species, but we are talking of a difference in perhaps a few thousands of year of evolution vs 4 billion. Not significant.

      • Darrell Ernst November 3, 2016, 20:53

        I agree. We don’t have enough data to warrant assigning any significant probability to any Rare Earth hypotheses. It is trivially true that every previous moment in the history of the Earth has had an impact on its future to one degree or another and that that history has shaped the emergence and evolution of life on Earth. But drawing the conclusion that a given exo planet’s history must be sufficiently similar to Earth’s in order for complex life to have arisen on it seems completely unwarranted.

  • RobFlores November 1, 2016, 16:41

    We need to step back a bit from this “these type of young proto planets collisions are rare” view
    I think they would not be that rare, because of the disturbances of migrating larger planets and other multi body instabilities.
    What is probably rare is to have two rather massive rocky worlds collide and have ONE survive.

    Is it more likely that such a collision would produce an
    Asteroid belt where at the orbital expanse where the collision took place?

  • andy November 1, 2016, 16:42

    On the subject of close encounters between celestial bodies, there’s a new paper in A&A forthcoming section about the Gaia results for the approach of Gliese 710 to our solar system. Minimum distance is decreased to 13366 AU with a 90% uncertainty of 6250 AU.

    Berski & Dybczyński (2016) “Gliese 710 will pass the Sun even closer. Close approach parameters recalculated based on the First Gaia data release.” (pdf link)

    • ljk November 3, 2016, 9:11

      Now we have to wonder what else will be coming our way closer than first thought? And we have the Andromeda galaxy collision in a few billion years, which will really play havoc with the stars even though astronomers claim such mergers rarely have individual stars collide. Remember, these are the same folks who once said that binary stars could not keep planets in stable orbits around them.

      • andy November 4, 2016, 16:18

        Nice use of the poisoning the well fallacy. I’d actually like a reference for that claim about the stability of binary star planets (I’m aware the claim has been made in science fiction, e.g. Solaris). After all, triple star systems had been observed for quite some time, which isn’t an entirely different class of problem.

        The issue of stars hitting each other in galactic collisions has more to do with the extremely small volume of a galaxy taken up by the stars themselves rather than the space between them, it is a rather different kind of problem to stability of planets within binaries.

    • ljk November 3, 2016, 9:24

      Anderson’s book Vistas of Many Worlds will no doubt have to be updated:


  • Michael November 1, 2016, 18:25

    Two hours rotation, it must have looked like a glowing rotating liquid smarty. The earthquakes must have been huge as the planet cooled and the crust solidified.

  • James Stilwell November 1, 2016, 19:42


  • Torbjörn Larsson November 2, 2016, 18:19

    That Mars (and Venus and Mercury) has the same axis orientation shows that large moons are not a factor for habitability. (In fact, our Moon is a tad too large, so if not the Sun would have sterilized Earth by then the future mutual tidal lock results in an inhabitable planet of erratic axis tipping at ~ 6 Ga.)

    • Mark Zambelli November 4, 2016, 13:16

      It’s unclear if Venus ever flipped or not (the 177° tilt explains its retrograde rotation but that may not have actually happened through turning upside down). If some other mechanism caused the retrograde behaviour (ie liquid mantle/tidal interactions) then we should be thinking 3° tilt instead (just like Jupiter) and that leads to assuming either axial tilt prefers lining up to orbital planes, or, we have a coincidence to see them that way today.

      Mars’ tilt varies a lot over time and I’m going to play devil’s advocate here and say “so what?”… Maybe it’s a lot better for early life to be plunged between extremes as it strengthens it’s evolutionary ‘card-hand’ giving it a lot more advantages over any life evolving in a much more stable environment. Change may envigour (not too extreme though else you could damage the chances so badly there would be little or no progress)

      • ljk November 4, 2016, 17:10

        Or does a harsh life also create harsh creatures, who are focused on survival and conquest rather than cooperation and reflection? Do such beings ever really evolve beyond that attitude?

        Humanity did not start becoming civilized until life for certain groups went beyond mere base survival. One might even question if we are truly civilized as a whole even now.

        • Mark Zambelli November 7, 2016, 13:19

          Harsh creatures with an agressive outlook?…quite possibly, yes. I have always been of the mind that creatures of that ilk, ie those that cannot move beyond a ‘war-like’ phase etc, are destined to doom themselves.

  • ljk November 3, 2016, 9:08

    Speaking of objects from space impacting other worlds, look what the Curiosity rover found driving about on Mars:


  • ljk November 3, 2016, 12:46

    Quoting from the main article:

    “David Grinspoon writes about Milanković in his fine new book Earth in Human Hands (2016), pointing out that while our Moon has kept Earth’s rotational axis stable at a 23.5 degrees tilt from the Sun, Mars (currently at about 25 degrees of tilt) varies from 15 to 35 degrees and sometimes more over a period of 120,000 years. Rhythmical climate torques also show up on Titan.”

    Question: Is Saturn not close and massive enough to stabilize Titan, or is the cause of its axial wobblings? And do any of the other neighboring moons also affect Titan’s stability?

    • Mark Zambelli November 4, 2016, 13:20

      I wonder if Titan’s globally disconnected ice layer plays a role in preventing such stability?

  • ljk November 4, 2016, 9:42

    The solar system may have ejected Moon and Mars-sized worlds into interstellar space

    By crunching numbers, astronomers discover that many worlds may be gone forever from our corner of the cosmos.

    By Charles Q Choi | Published: Tuesday, November 01, 2016

    Full article here:


    To quote:

    Starless planets may not be rarities. “A startling result made a few years ago was that there are as many as one or two free-floating Jupiter-size planets per star,” says astrophysicist Thomas Barclay at NASA Ames Research Center in California.

    So far the known orphan worlds have all been ones large enough for current observatories to discover in the vast spaces between the stars — gas giants like Jupiter and Saturn, instead of rocky planets like Earth and Mars. Still, the upcoming Wide Field Infrared Survey Telescope (WFIRST), an infrared space observatory that NASA plans to launch in the 2020s, “could find Earth-mass and potentially Mars-mass material,” Barclay said.