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Explaining Luna’s Farside

The Moon’s farside used to be a convenient setting for wondrous things. After all, no one had ever seen it, setting the imagination free to insert everything from paradisaical getaways (think Shangri-La in space) to secret technologies or alien civilizations. The Soviet Luna 3 image of 1959 took the bloom off that particular rose, but we also learned through this and subsequent missions that farside really does have its differences from the familiar face we see. More craters, for one thing, and fewer of the dark plains we call maria, or ‘seas.’

We can throw in measurements made by the GRAIL mission (the Gravity Recovery and Interior Laboratory) in 2012. GRAIL was a NASA Discovery-class mission that performed gravitational field mapping of the Moon as a way of examining its internal structure, a set of two probes that worked by analyzing measured changes in distance between the two craft as small as one micron. We wound up with a map of our satellite’s gravitational field that led to deeper understanding of its crust, its thermal evolution and its subsurface structure.

Here again we find variation between farside and what we see from Earth. The Moon’s crust is thicker and contains an additional layer on farside. In fact, the farside crust is on the order of 20 kilometers thicker than nearside, and GRAIL’s remote sensing data tell us that this extra crustal material is composed of silicate minerals, magmas, and rocks which are relatively high in the heavier elements. A collision between a dwarf planet and the Moon that occurred after the Moon had already formed a solid crust is now emerging as one explanation for these differences.

Image: Artist’s depiction of a collision between two planetary bodies. New research suggests the stark difference between the Moon’s heavily-cratered farside and the lower-lying open basins of the nearside were caused by a wayward dwarf planet colliding with the Moon in the early history of the solar system. Credit: NASA/JPL-Caltech.

The research presenting the theory is being led by Meng Hua Zhu of the Space Science Institute at Macau University of Science and Technology. Zhu and colleagues are using GRAIL data as fodder for computer simulations that work through impact scenarios for the early Moon, some 360 of them, to see whether they can reproduce the crustal variation we see today.

The work has produced a model that fits the bill, involving the impact of an object a bit smaller than Ceres (780 kilometers in diameter) striking the Moon’s nearside at 22,500 kilometers per hour. A second fit to the data is a smaller (720 kilometer) object hitting at 24,500 kilometers per hour. In both cases, material blown off the surface would fall back to bury the farside crust in kilometers of debris, which fits the additional crust layer found by the GRAIL mission.

Wouldn’t we see evidence on the nearside of such a titanic collision? Depending on conditions in the early Moon, the telltale signs may be hard to tease out, but the authors believe their model can explain the farside highlands and the nearside lowlands, noting such nearside features as “a large area of low-Ca pyroxene on the nearside observed by Kaguya mission that was explained to be formed via impact through melting a mixture of crust and mantle materials.” Let me quote more from the paper on this, talking about the formation of a ‘mega basin’ covering most of nearside:

…the mega basin structure may not be the same to the surface expression of a typical basin formed later. It is likely that the morphologies of mega basin were heavily modified and possibly even erased due to high internal temperature of the early Moon (Miljkovic et al., 2017), which easily allowed for isostatic relaxation processes (Freed et al., 2014). Therefore, it is highly possible that remnants of a giant impact on the early Moon may be less pronounced and significantly different from the impact basins formed later. To test whether a giant impact on the nearside is a plausible mechanism for the formation of Moon’s asymmetries, we performed a systematic numerical modeling study and quantified the outcome of such impact events.

Image: The basin-forming process for an impactor 780 kilometers in diameter (with a 200-kilometer diameter of iron core) with an impact velocity of 22,500 kph (14,000 miles per hour). In each panel, the left halves represent the materials used in the model: gabbroic anorthosite (pale green), dunite (blue), and iron (orange) represent the lunar crust, mantle, and core, respectively. The gabbroic anorthosite (pale yellow) also represents the impactor material. The right halves represent the temperature variation during the impact process. The arrows in (C) and (D) represent the local materials that were moved and formed the new crust together with deposits of material that was blasted from the impact. Credit: JGR: Planets/Zhu et al. 2019/AGU.

Simulating the impact scenarios produces variation in post-impact ejecta and crustal thickness that can reproduce the Moon’s current crust in the farside highlands. The giant impact also fits data on isotope variation in potassium, phosphorus and tungsten-182 between the surfaces of the Moon and the Earth, with the authors positing that these elements would have been added to the Moon’s constituents after its formation. The paper suggests that the impact of an 800-900 kilometer-sized body with the Moon is not unlikely given that the Moon received roughly half of the number of impacts as Mars, many in the form of planetesimals from the early system.

Image: The topographic (A), crustal thickness (B), and thorium distribution of the Moon show a dramatic difference between the nearside and farside. The star on the nearside represents the center of the proposed impact basin. The black dashed lines represent the boundary of Imbrium (Im), Orientale (Or), and Apollo (Ap) basin, respectively. Credit: JGR: Planets/Zhu et al. 2019/AGU.

My own interest in this paper is not so much the Moon itself as the fact that our abundant data on our satellite may help us understand the kind of asymmetries between hemispheres that can occur on large objects as the period of planet formation is drawing to its close. All that is part of learning about the origins of our own Solar System as well as conditions in the young systems we are now beginning to measure in circumstellar disks like the one at HD 163296 that I looked at on Friday (see HD 163296: Emerging Insights into Circumstellar Disks).

The paper is Zhu et al., “Are the Moon’s nearside‐farside asymmetries the result of a giant impact?” Journal of Geophysicl Research: Planets 20 May 2019 (abstract).

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{ 27 comments… add one }
  • Thomas R Mazanec June 3, 2019, 11:40

    I saw a reprint of an old article when I was a kid.
    It predicted just as the Farside looked, arguing that most of the maria on Nearside were from one impact (Mare Imbrium).

  • don wilkins June 3, 2019, 12:11

    Wouldn’t a collision with such a large body have lasting effects on the orbit and/or spin of the Moon?

    • Bruce D. Mayfield June 4, 2019, 23:57

      It certainly would have made significant short term effects, but tidal dynamics from Earth’s constant pull would overcome these over time.

  • ljk June 3, 2019, 12:24

    Quoting Paul Gilster in the main article:

    “The Moon’s farside used to be a convenient setting for wondrous things. After all, no one had ever seen it, setting the imagination free to insert everything from paradisaical getaways (think Shangri-La in space) to secret technologies or alien civilizations.”

    You are not kidding about the latter. Check out this 1955 video made by Walt Disney titled Man and the Moon. This incredibly well-made effort depicts what they thought the first manned lunar orbiting mission might be like ala von Braun concepts.

    Start watching at the 45:20 mark. The astronauts orbiting the Moon are on the literal dark side firing rocket flares at the surface to illuminate it for observation. At one point they light up what looks a lot like an ancient structure of unknown design and origin. The two astronauts and the narrator remain silent on what they see.

    https://www.youtube.com/watch?v=1ZImSTxbglI

    I also saw a circa 1957 World Book Encyclopedia entry on the Moon that had a pre-Luna 3 artist conception of the lunar farside. As I recall it had lots of ray craters and very few and small maria. Not sure how the artist came to these conclusions.

    • Ron S. June 3, 2019, 13:57

      My recollection is that there had been far side predictions based on impact dynamics related to tidal locking, where the near side is partially “shadowed” by Earth. Presumably the artist worked with that.

    • Enzo June 3, 2019, 16:06

      If I remember correctly (I read it when I was ~10), in “From the Earth to the Moon” by Jules Verne, while orbiting the moon, a meteorite illuminated the farside briefly. The brief glimpse shows much more earth-like conditions.

      • ljk June 4, 2019, 9:17

        I am trying to find that scene from Verne’s novel. What he does say is that while the Moon is currently uninhabited, it may have had life all the way up to intelligent and civilized beings ages ago. The three astronauts aboard the Columbiad argue over whether certain features they see are artificial or natural.

        http://www.gutenberg.org/ebooks/83

      • ljk June 4, 2019, 9:32

        Enzo, your 10 year-old self was not wrong! The illuminating meteor you reference was in the sequel, Round the Moon:

        http://www.ibiblio.org/pub/docs/books/sherwood/R-II-d.htm

        To quote:

        Suddenly, in the midst of the ether, in the profound darkness, an enormous mass had appeared. It was like a moon, but an incandescent moon whose brilliancy was all the more intolerable as it cut sharply on the frightful darkness of space. This mass, of a circular form, threw a light which filled the projectile. The faces of Barbicane, Nicholl, and Michel Ardan, bathed in its white sheets, assumed that livid wan spectral appearance which physicists produce with the factitious light of alcohol impregnated with salt.

        “Mille diables! ” cried Michel Ardan, “we are hideous. What is that ill-conditioned moon?”

        “A meteor,” replied Barbicane.

        “A meteor burning in space?”

        “Yes.”

        This globe of fire was indeed a meteor. Barbicane was not mistaken. But if these cosmic meteors, seen from the earth, present generally a light inferior to that of the moon, here in the dark ether they shone magnificently. These wandering bodies carry in themselves the principle of their own incandescence. Ambient air is not necessary for their deflagration. And indeed, if certain of these bolides pass through our atmosphere at two or three leagues from the earth, others describe their trajectory at a distance the atmosphere cannot reach. Some of these meteors, the one of the 27th of October 1844, appeared at a height of 128 leagues, the other of the 18th of August 1841, disappeared at a distance of 182 leagues. Some of these meteors are three to four kilometres wide and possess a speed up to 75 kilometres per second, [The average speed of the movement of the earth , along the ecliptic, is not but 30 kilometres per second — JV ] following a direction inverse to the movement of the earth.

        This shooting globe suddenly appearing in shadow at a distance of at least 100 leagues, ought, according to the estimate of Barbicane, to have a diameter of 2,000 metres It advanced at a speed of about two kilometres per second, that is thirty leagues a minute. It cut the projectile’s path and must reach it in some minutes. As it approached it grew to enormous proportions.

        Imagine, if possible, the situation of the travelers! It is impossible to describe it. In spite of their courage, their sang-froid, their carelessness of danger, they were mute, motionless with stiffened limbs, a prey to frightful terror. Their projectile, the course of which they could not alter, was rushing straight on this ignited mass, more intense than the open mouth of a reverberatory furnace . It seemed as though they were being precipitated toward an abyss of fire.

        Barbicane had seized the hands of his two companions, and all three looked through their half-open eyelids upon that asteroid heated to a white heat. If thought was not destroyed within them, if their brains still worked amid all this awe, they must have given themselves up for lost.

        Two minutes after the sudden appearance of the meteor (to them two centuries of anguish) the projectile seemed almost about to strike it, when the globe of fire burst like a bomb, but without making any noise in that void where sound, which is but the agitation of the layers of air, could not be generated.

        Nicholl uttered a cry, and he and his companions rushed to the scuttle. What a sight! What pen can describe it? What palette is rich enough in colours to reproduce so magnificent a spectacle?

        It was like the opening of a crater, like the scattering of an immense conflagration. Thousands of luminous fragments lit up and irradiated space with their fires. Every size, every colour, was there intermingled. There were rays of yellow and pale yellow, red, green, gray— a crown of fireworks of all colours. Of the enormous and much-dreaded globe there remained nothing but these fragments carried in all directions, now become asteroids in their turn, some flaming like a sword, some surrounded by a whitish cloud, and others leaving behind them trains of brilliant cosmical dust.

        These incandescent blocks crossed and struck each other, scattering still smaller fragments, some of which struck the projectile. Its left scuttle was even cracked by a violent shock. It seemed to be floating amid a hail of howitzer shells, the smallest of which might destroy it instantly.

        The light which saturated the ether was of an incomparable intensity, for these asteroids dispersed it in all directions. At a certain moment is was so wonderfully intense, that Michel, drawing Barbicane and Nicholl to his window, exclaimed, “The invisible moon, visible at last!”

        And through a luminous emanation, which lasted some seconds, the whole three caught a glimpse of that mysterious disc which the eye of man now perceived for the first time.

        What could they distinguish at a distance which they could not estimate? Some lengthened bands along the disc, real clouds formed in the midst of a very confined atmosphere, from which emerged not only all the mountains, but also projections of less importance; its cirques , its yawning craters, as capriciously placed as on the visible surface. Then immense spaces, no longer arid plains, but real seas, oceans, widely distributed, reflecting on their liquid surface all the dazzling magic of the fires of space; and, lastly, on the surface of the continents, large dark masses, looking like immense forests under the rapid illumination of a flash of lightning . . .

        Was it an illusion, a mistake, an optical illusion? Could they give a scientific assent to an observation so superficially obtained? Dared they pronounce upon the question of its habitability after so slight a glimpse of the invisible disc?

        Meanwhile the fulgurations [light unaccompanied by sound, Petit LaRousse —NMW] in space subsided by degrees; its accidental brilliancy died away; the asteroids dispersed in their different trajectories and were extinguished in the distance. The ether returned to its accustomed darkness; the stars, eclipsed for a moment, again twinkled in the firmament, and the disc, so hastily discerned, was again buried in impenetrable night.

  • Alex Tolley June 3, 2019, 13:04

    A multi-body pile up on the cosmic highway.

    If the Moon can have such an impactor that creates surface asymmetries, is it possible the Earth may have had something similar? Just as the Moon had thicker crust on Farside (with mountains?). is this a mechanism that could have created the major continental plate[s] early in Earth’s history, and which subsequently was reshaped and moved by plate tectonics?

    • Bruce D. Mayfield June 3, 2019, 19:01

      That’s a reasonable assumption. Mars is another case in point, with its huge northern lowlands that evidently once was an ocean basin, thought to have been formed by a gigantic impact also. Of course with the Earth constantly reworking its surface, all evidence of such would have long since been erased. How many supercontinents have come and gone in Earth’s history?

  • Ron S. June 3, 2019, 14:03

    The supposed symmetry and limited surface effect puzzles me. The Moon is now tidally locked but does rotate. Billions of years ago its rotation was faster (not yet tidally locked, or simply closer to Earth). Also much of the material would have exceeded Moon’s escape velocity or would have take days or longer to fall back. Therefore the debris trajectory would have covered a far wider surface area.

    Not only that, the shock waves would have severely disrupted the entire surface. Would those effects be large enough to leave an imprint?

  • ljk June 3, 2019, 14:22

    I was not aware that the thermal control for the rover’s biological experiment had failed. It was supposed to keep them warm for 100 days. All earlier reports said it was just supposed to last one lunar day (14 Earth days). Shades of Soviet space cover explanations?

    One Small Step for Plantkind

    By: David Grinspoon | May 30, 2019

    With a seed germinated on the lunar farside, Earth’s biosphere made its first tentative foray into the cosmos.

    https://www.skyandtelescope.com/astronomy-blogs/one-small-step-for-plantkind-biosphere-space/

  • ljk June 3, 2019, 14:26

    Andrew Jones • May 15, 2019

    Chang’e-4 may have discovered material from the Moon’s mantle

    The first science results from the unprecedented Chang’e-4 lunar far side mission are in. The mission’s Yutu-2 rover, deployed from the lander shortly after the Chang’e-4 landing on 3 January, has, with the help of the Queqiao relay satellite, returned data which suggests it has discovered material derived from the Moon’s mantle, according to research published today in Nature. The possibility of accessing mantle rocks exposed within an enormous impact basin was a major reason for attempting the challenging farside landing.

    The Visible and Near Infrared Spectrometer (VNIS) aboard Yutu-2 made the first in situ observations—detecting scattered or reflected light from surface materials—on the lunar far side. These spectra have been interpreted by the paper’s authors to represent the presence of olivine and low-calcium pyroxene, materials that may originate from the Moon’s mantle.

    Chang’e-4 set down in the Von Kármán crater within the south pole-Aitken (SPA) basin, an ancient and gigantic 2,500-kilometer-wide, 13-kilometer-deep impact basin which may have penetrated the Moon’s crust and reached the mantle below. It is visible in this GRAIL map of lunar crustal thickness as broad blue spot on the farside.

    http://www.planetary.org/blogs/guest-blogs/2019/change-4-may-have-discovered.html

  • Michael June 3, 2019, 14:51

    I am surprised it did not model the massive disruption at the antipodal point.

  • Geoffrey Hillend June 3, 2019, 18:24

    The Moon’s crust is made mostly of gabbroic anorthosite is mostly plageoclase feldspar down to forty miles which is the same composition of the silicates in Earth’s mantle which supports the giant impact hypothesis specifically pyroxene, olivine, iron and magnesium. Source The New Solar System, 4th ed. I forgot the page number.

    I don’t agree with the idea the Moon’s tungsten 182 came after the Moon’s constituents after it’s formation. As long as this article is not saying that there was no giant impact hypothesis, I agree with it since there is a small difference between the tungsten 182/tungsten 184 ratio which would not prove that there was not a giant impact hypothesis.

    I don’t agree with the idea that the tungsten 182 “would have been added to the Moon’s constituents after its formation.” From what I recall reading is that halfnium 182 is not a siderophile (iron loving elements) which has an affinity for the core and sank down into it but the halfnium 182 stayed in Earth’s mantle and decayed into stable tungsten 182, so the collision of Earth with Thea blasted Earth’s entire mantle off into space into condensed into the Moon in orbit around the Earth. There is a slightly higher ratio of tungsten on the Moon which can be explained by a higher percentage of tungsten in Thea or some dwarf planet after the formation of the Moon. Consequently, the dwarf planet collision would not invalidate the giant impact hypothesis as implied by this paper.

    Mars has more tungsten 182 than Earth which has more 184 and less tungsten 182 than Mars. Since 182 is a siderophile it sank to the core of Mars, but halfnium 182 has an affinity for the Martian crust, so the tungsten 182 in Mars crust is the result of the decay of halfnium. Earth has more 184 because there is more mantle overturn on Earth and none on Mars without plate tectonics.

  • VIY June 3, 2019, 18:59

    How is it that the far side before the impact remained a far side after the impact? This would mean no change in angular momentum which is highly unlikely even in a frontal impact, given that the moon circles Earth and will have a tangential velocity relative to a frontal impact.

    • Robin Datta June 3, 2019, 21:56

      Peeling away the mantle layers from the nearside and enfolding the farside with that material: would this have brought a denser (and heavier) core closer to the nearside surface? Maybe this could account for tidal locking holding the nearside facing the earth. And at the time of the impact, maybe the nearside need not have been “near”?

      • Bruce D. Mayfield June 4, 2019, 23:28

        A reasonable suggestion Robin. It gives the Moon an axis of greatest mass, which over time would be the most stable orientation pointing back at the mass of nearby Earth as the Moon’s spin synchronized with its orbit. At the time of impact the hit could have been anywhere around the Moon, as its rotation rate would have been much higher back then.

    • Alex Tolley June 4, 2019, 1:46

      Was it the farside during impact, or is it just the way it ended up? If the Maria on nearside are due to asteroid impacts, wouldn’t that assume that the impactors came from the direction of Earth rather than some other direction? I would have guessed that small mass distribution anomalies resulted in the Moon’s final orientation to Earth.

      • Bruce D. Mayfield June 4, 2019, 23:46

        It was just the way it ended up, I would think. Way back in our moon’s early history it was both closer to the Earth and a faster (non-synchronous) rotator. During the LHB the Moon’s spin hadn’t had enough time to have tidally locked yet. Large impacts would randomly occur anywhere, but the ones that happen to hit where the crust was thin (from the proposed great impact) would be the ones that would fill with maria forming lava flows.

  • Alex T. June 4, 2019, 4:25

    Accounting the fact, that Moon surface, hardly seeded by impact craters, no crater in the supposed impact point can mean one obviously simple thing – there was not huge impact on (close to) this specific point of Moon surface.
    Higher (Moon’s) internal temperature , as explanation of “no crater” fact, is very problematic and requires separate simulation to test required conditions for observed material distribution.

  • Peter June 4, 2019, 12:28

    Could this impactor be another object created by the giant impact that created the moon? I’m thinking about two objects orbiting Earth is similar orbits that eventually hit each other.

    • Bruce D. Mayfield June 4, 2019, 23:10

      Very plausible suggestion Peter. The timing of your idea is good too, for it most likely would have happened as the final main act of the Moon’s creation from the remnants of the great impact event on Earth.

  • Geoffrey Hillend June 5, 2019, 22:25

    A double giant impact hypothesis paper was already posted here in the past. It’s an interesting idea. I think it is less probable since on needs a double collision and the debris of both have to combine to form the Moon so they both have to hit not to far apart in time.

    Also I like the Mars sized Thea impact idea because a Mars sized body has a larger, iron core than smaller dwarf planets would and Thea’s iron core is assumed to have sank to the core of the Earth giving it a larger iron core that it would have had without the giant impact. There is a problem with a two planet collision because they both have to hit at the same angle to get the right and a fast angular momentum or rotation of the Earth.

    • Bruce D. Mayfield June 6, 2019, 10:54

      A Mars sized impactor (of Earth) with its iron core also gives Earth an extra helping of the long lived radio-isotopes like Uranium-235 (half life of 4.5 billion years) to keep Earth’s systems running for billions of years.

  • ljk June 10, 2019, 14:37

    June 10, 2019

    Mass anomaly detected under the moon’s largest crater

    by Baylor University

    A mysterious large mass of material has been discovered beneath the largest crater in our solar system—the Moon’s South Pole-Aitken basin—and may contain metal from the asteroid that crashed into the Moon and formed the crater, according to a Baylor University study.

    “Imagine taking a pile of metal five times larger than the Big Island of Hawaii and burying it underground. That’s roughly how much unexpected mass we detected,” said lead author Peter B. James,
    Ph.D., assistant professor of planetary geophysics in Baylor’s College of Arts & Sciences.

    The crater itself is oval-shaped, as wide as 2,000 kilometers—roughly the distance between Waco, Texas, and Washington, D.C.—and several miles deep. Despite its size, it cannot be seen from Earth because it is on the far side of the Moon.

    The study—”Deep Structure of the Lunar South Pole-Aitken Basin”—is published in the journal Geophysical Research Letters.

    Full article here:

    https://phys.org/news/2019-06-mass-anomaly-moon-largest-crater.html

    https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019GL082252

    Begin your 2001 Monolith jokes… now!

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