[HN Gopher] Differences between Moon's near and far sides linked...
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Differences between Moon's near and far sides linked to colossal
ancient impact
Author : wglb
Score : 70 points
Date : 2022-04-11 14:31 UTC (1 days ago)
(HTM) web link (www.brown.edu)
(TXT) w3m dump (www.brown.edu)
| prewett wrote:
| The article doesn't seem to answer the question of why the heat
| plume distributed stuff on the nearside rather than evenly, or on
| the top side. If the south pole was hit, it seems like the heat
| should flow evenly on both sides, and if anything case the
| deposition to happen towards the north pole, rather than the near
| side.
| andrewflnr wrote:
| Reading the linked paper and Wikipedia, it looks like the
| center of the KREEP concentration is roughly antipodal to the
| impact crater, which itself is not exactly on the south pole.
| So at least the symmetry side of things is in order. But yeah,
| this PR article is exceptionally bad at taking a long time to
| say almost nothing of interest.
|
| [0] https://www.science.org/doi/10.1126/sciadv.abm8475 [1]
| https://en.wikipedia.org/wiki/South_Pole%E2%80%93Aitken_basi...
| waynecochran wrote:
| Yes -- that is what I was wondering. The following statement is
| given with no explanation -- was Earth's gravity involved?
| As the heat plume spread beneath the Moon's crust, that
| material was eventually delivered en masse to the nearside.
| akomtu wrote:
| A wild speculation: perhaps Moon is the huge asteroid that hit
| and liquefied the planet that was there before Earth, so some of
| its material sticked to that asteroid and formed Moon, while the
| rest formed Earth. In this case, the two sides of Moon show the
| direction of the impact.
| Sharlin wrote:
| I mean, the prevailing theory _is_ that the Moon formed in the
| aftermath of the Theia-proto-Earth collision. Indeed the
| existence of the Moon is why the Theia hypothesis was proposed
| in the first place. But the bulk of Theia's mass (which would
| have been ~10% of Earth's mass) is thought to have mixed with
| Earth's and sunk into the core /mantle, while most of the Moon
| is made of same material as the modern Earth's crust.
|
| However, the differentiation of the Moon's near and far sides
| must have happened later, presumably during what's called the
| late heavy bombardment ("late" is relative here; we're still
| talking ~4 Ga ago). Although I believe some recent research has
| cast doubt on whether the LHB actually happened at all.
| JKCalhoun wrote:
| I recently took an old-school "voxel" algorithm and munged some
| NASA moon terrain data to try to create a game.
|
| I really have enjoyed learning about the Moon's geography just
| from flying around the terrain, picking locations for "lunar
| bases" and such. Fascinating cracks and valleys, grooves, a
| straight mountain range -- in general things I was not expecting
| on a planetoid without water/erosion.
|
| It really is stark how different the far side of the Moon is from
| the near side. Insanely pock-marked with so many craters.... I
| assumed there was an Earth component to that in the same way the
| Earth has locked the Moon into an orbit/rotation that is always
| facing us.
|
| I'm become very keen to learn more about the Moon now.
| Teever wrote:
| Can you talk more about the moon data munging?
|
| I just started some Blender tutorials and the first thing I
| want to try once I get some tutorials under my belt import dats
| from the Moon/Mars.
| JKCalhoun wrote:
| Sure.
|
| For the Moon the (images) data seems to be bucketed with
| regard to pixels-per-degree (PPD), the more coarse data set
| being 64ppd data (finer resolution for the Moon goes at least
| as high as 512ppd that I have found). Sadly, my workflow
| (image editors, and Blender as you mentioned) are already
| straining with the 64ppd data-set so I did not try to go
| higher resolution. It means though that my Moon is wildly
| small -- the lunar bases that seem to be a paltry 7x7 or 9x9
| voxels are in fact the size of a small town.
|
| I began with NASA's "CGI Moon Kit" [1] which, as I say, is
| 64ppd. There is both a "color data" image and an "elevation
| data" image (where the greyscale value of the image
| represents elevation). Polar data was garbage (and would look
| wildly distorted in my tile-based voxel algorithm so I
| confess that I tossed it -- cropped everything outside
| 60-degrees latitude basically - I just limit fuel in the game
| to keep pesky kids from trying to get too deep into the polar
| regions).
|
| Even at 64ppd and poles cropped, I still ended up with two
| images that were 23040 x 11264 pixels in size. This turned
| out to be a challenge for most of the image editing apps I
| have. Blender handled it (but barely).
|
| With both color + elevation images I followed an online
| tutorial [2] meant for cartographers wanting to use Blender
| for relief shading. NASA's moon map had no shadows (probably
| a good thing). I was not going to try to real-time
| shade/light the voxel terrain so I settled on "burning in"
| the shadows with Blender. While my MacBook ran some 8 hours
| or so (over night) to generate the relief-shaded lunar map,
| the result was quite nice. As a result, it is near sunset
| everywhere on my Moon, ha ha.
|
| With a relief-shaded image representing the color and a same-
| sized image representing the elevation, I wrote a custom app
| to read both files into memory and then generate the correct
| number of 512 x 512 tiles that contain both the elevation
| data and RGB data in a single raw file. Now I have 45 x 22
| raw tiles covering the entire 64ppd Moon. The voxel algorithm
| uses simple math to figure out which tile a given voxel is to
| be found, reads the elevation/color data.
|
| In hindsight, I could have output PNG files or something with
| alpha channel (where alpha would be interpreted as
| elevation). As it turned out, the Moon color was so close to
| greyscale that I ended up treating it as such and used only
| the green channel when creating my Moon tiles (so the tile
| data has only a single 8-bit value for elevation and a single
| 8-bit value for "color" per voxel -- cut the raw tile size in
| half).
|
| Still wrapping up the game [3]. I'll put the sources up on
| Github end of the month, I think.
|
| [1] https://svs.gsfc.nasa.gov/4720
|
| [2] https://somethingaboutmaps.wordpress.com/blender-relief-
| tuto...
|
| [2] https://mooncraft2000.com
| 127 wrote:
| I did some cropping of the LOLA dataset to a .raw that
| Unity3D could understand as terrain input:
| https://github.com/amb/SplitTIFF ... a couple of years ago
|
| Happy to hear any better alternatives for the task.
| JKCalhoun wrote:
| That is very cool. I starred your repo, if I try to up
| the PPD I'll have a closer look.
| bdefore wrote:
| Fascinating. Are there upcoming plans from any space agency
| to improve that 64PPD with upcoming or existing satellites
| or Earth-based capture of moon topography?
| JKCalhoun wrote:
| There are 512ppd data sets [1] already. As I say, I found
| them unwieldy with my workflow. A new workflow would be
| required to process these into "game tiles".
|
| [1] https://astrogeology.usgs.gov/search/map/Moon/LRO/LOL
| A/Lunar...
| bdefore wrote:
| Whoops. I missed that you mentioned higher PPD is
| available. If your workflow was improved to accommodate
| it, I presume it would produce more, for lack of a better
| phrase, life-like terrain? Would a much higher PPD be
| required to reach a threshold to improve the experience.
| And are there plans coming to get finer detail than
| 512PPD?
| JKCalhoun wrote:
| It occurred to me that, beyond 512ppd, you could
| "synthesize" higher-res detail. I don't know how much of
| a cop-out that is though.
|
| There may be >512ppd data though. IANAS (I am not a
| selenologist).
| gehwartzen wrote:
| Becouse of the orbital lock it's interesting to think of it
| like a shield for earth. I imagine a lot of those craters were
| by asteroids that would have hit earth, esp the perfectly round
| ones in the center. The oblong ones or ones towards the edges
| might have come at an angle that wouldn't have hit earth. Seems
| like a fascinating way to study earths impact history if we end
| up exploring the dark side of the moon in more detail.
| bdefore wrote:
| I believe consensus is that the moon will eventually drift
| away from the Earth. Do such impacts, predominantly on the
| far side of the moon, apply enough force to change that
| prediction? Are such projections of whether the moon leaves
| Earth orbit (or instead collides with Earth) accounting for
| this?
| throwanem wrote:
| The moon _is_ drifting away from Earth at a rate of about
| four centimeters per year [1]. Inbound impacts on the far
| side won 't significantly affect this because of the way
| orbital mechanics works. To lower an orbital altitude, you
| need thrust opposing the orbital vector. (This is why
| spacecraft orient the way they do, with engines pointing in
| the direction of travel, for retrofire.) Thrusting toward
| or away from the orbital center of gravity (in this case
| Earth) can affect the circularity of the orbit, but that's
| all. In the case of the moon intercepting an impactor
| that'd otherwise strike Earth, the thrust vector is
| necessarily toward Earth, so the effect would be to render
| the moon's orbit infinitesimally more elliptical, with the
| increase along the major axis balanced by an equally
| minimal decrease along the minor axis. (To the extent the
| force vector of the impact is displaced from the axis
| through the moon's own center of gravity, that will affect
| the spin of the moon, but not its orbit.)
|
| According to the linked article, it will take about fifteen
| billion years for the moon to escape Earth orbit entirely.
| I haven't checked, but I believe that's considerably longer
| than it is expected to take for the Sun to expand as a red
| giant and render Earth uninhabitable. Certainly more
| wondrous than either event would be for anything then
| living to remember having once had distant ancestors who
| called themselves "human"...
|
| [1] https://www.google.com/amp/s/public.nrao.edu/ask/what-
| happen...
| bdefore wrote:
| It's compelling to me that the Earth and its moon are so
| tightly linked that their relationship is projected to
| outlast a life-providing sun. I believe this is moreso
| than any other moon-planet pairing in the solar system?
| Given what we know of incoming asteroids and comets, the
| probability of their impacts hitting with enough force at
| a trajectory you describe with enough significance to
| change the moon's orbit is expected to be low . And we're
| relatively confident of that?
| throwanem wrote:
| I don't have numbers or references here, but my intuition
| from a lifetime of space nerding is that, if the moon
| were struck with enough retrograde force to materially
| lower its orbit, in the _best_ case that impact would
| liberate debris of mass and vector such that our first
| concern would be planetwide disaster response. (In the
| worst case, it might shatter the moon entirely and
| distribute a significant fraction of its total mass
| across Earth 's surface; in such a scenario, the survival
| of eukaryotic life anywhere on Earth qualifies as a
| million-to-one-shot win.)
|
| But the odds of such a collision I suspect are
| significantly lower even than of a similarly sized
| impactor striking Earth. Granted that the moon helps a
| great deal in sweeping space near Earth of objects that
| might otherwise enter our atmosphere, but something with
| the kind of kinetic energy it would take to produce these
| outcomes isn't going to spend long enough within the
| gravitational influence of the Earth-Moon system to be
| significantly affected before impact. It'd be like trying
| to swerve an ICBM warhead by blowing really hard on it,
| and I choose that simile with care - if this ever _does_
| happen, I think it might constitute a clear indication
| that someone somewhere in the universe really hates us!
| JKCalhoun wrote:
| I also am fascinated by the idea of Nemesis [1] -- an
| infrequent event that causes mass-impact throughout our solar
| system.
|
| [1] https://en.wikipedia.org/wiki/Nemesis_(hypothetical_star)
| bdefore wrote:
| Do we have an idea of what percentage of impacts were
| prevented this way? What likelihood is there that the moon
| saved humanity (more than once?) since the time that we
| descended from the trees?
| gehwartzen wrote:
| I imagine once you figure out which craters correspond to
| ones that came in at an angle which would have hit earth
| and compare that to all the impacts you have had on earth
| one could figure it out.
|
| The thing is that on the moon the craters remain visible
| for a much longer period of time while on earth they get
| covered up by plate shifts, erosion, ocean impacts, etc. So
| instead it might be a way to calculate how many asteroids
| likely have hit earth in general throughout history whether
| we have found the craters on earth or not. Then you could
| calculate based on the size of the craters on the moon
| which ones would have been earth catastrophic events
| (taking into account any diminishing impact energy due to
| our atmosphere). Which would tell us how many the moon
| saved the earth from. In terms of how many since human
| ancestors were evolving we would have to then date earth
| catastrophic craters on the moon. I'm not sure if this can
| be done through photographs or if we would have to get
| surface samples.
|
| Then comparing that number to the fraction of earth
| covered/protected by the moon to the total area on earth we
| would have a decent idea of how many of these events
| happened one earth even if we haven't discovered the
| evidence for them on earth.
| getpolarized wrote:
| There's a MASSIVE impact crater on the far side of the moon which
| we never see.
|
| Imagine if it was inverted. It would look like a MASSIVE eyeball
| staring at us during our evolution.
|
| Imagine how that would have impacted religion!
| [deleted]
| mageofpanthera wrote:
| wishawa wrote:
| This comment sounds AI-generated. If you're a real person can
| you explain the point you were trying to deliver and why you
| chose to write this way?
| mageofpanthera wrote:
| Machines do not have narrative-interpretation bugs associated
| with Superiority Complex, humans do, Arthur.
| klyrs wrote:
| I've met some old-school hippies who actually talk like that,
| and appear to completely believe the pseudoscience.
| xen2xen1 wrote:
| You mean self importance?
| mageofpanthera wrote:
| Oh, I remember that word now: "pseudoscience"; it used to
| be regurgitated everytime someone stepped into a pool of
| reality-altering facts. They never manage to pin down the
| 'pseudo' or the 'science', or contribute (like old-school
| hippies) anything to the collective.
| klyrs wrote:
| Yeah, but sell a finger for the price of a lid, and your
| hippie suddenly believes in concrete facts, scientific
| measurements and capitalism...
| bdefore wrote:
| Is that what happened here? My head went in a few orbits over
| this comment.
| [deleted]
| h2odragon wrote:
| > The nearside is home to a compositional anomaly known as the
| Procellarum KREEP terrane (PKT) -- a concentration of potassium
| (K), rare earth elements (REE), phosphorus (P), along with heat-
| producing elements like thorium. KREEP seems to be concentrated
| in and around Oceanus Procellarum, the largest of the nearside
| volcanic plains, but is sparse elsewhere on the Moon.
|
| Bait. Big pile of things we want, placed where we can see them
| but not easily reach...
| sfblah wrote:
| I thought Kazakhstan produced the best potassium.
| JKCalhoun wrote:
| Indeed, the Moon is a harsh mistress.
| rhplus wrote:
| _Why the two sides are so different is one of the Moon's most
| enduring mysteries._
|
| How much can be explained by the earth shielding the moon from
| impacts to its near-side?
| spaetzleesser wrote:
| That's what I was thinking. Or that the earth gravity would
| cause tidal forces.
| vikingerik wrote:
| Not much. The Earth subtends a width of only 2deg in the Moon's
| sky - it covers only about 1/10000 of the possible approach
| vectors to the Moon.
|
| Think about a lunar eclipse for comparison: the Earth
| intercepts the Sun-Moon vector for a couple hours every few
| months, roughly 1/10000 of the time.
|
| The Moon is a smaller target (1/4 the diameter, 1/16 the cross
| section, also 1/81 the mass and gravity), so it gets hit less
| because of that, but not because of Earth shielding it.
| cperciva wrote:
| I don't think that calculation is quite right. An asteroid on
| a collision course with the moon doesn't need to _hit_ the
| Earth in order to avoid hitting the moon -- simply coming
| close to the Earth will alter its course.
| uncertainquark wrote:
| This is yet another (most welcome) hypothesis in explaining the
| sheer differences in our Moon's nearside and farside. Just last
| month I wrote a feature article explaining the other leading
| theories for what caused these nearside-farside differences if
| anyone's interested: https://blog.jatan.space/p/the-two-faced-
| moon
|
| The Moon is so much more interesting than most people think.
| frankohn wrote:
| I was wondering if a possible explanation is that, due to the
| gravity attraction of the earth, the hot, heavy, inner part of
| the moon, which is richer in heavy elements, I guess, like iron
| is closer to the near-side surface. Since in the far-side the
| hot, heavy part is too far from the surface the eruption
| activity is small or non-existing.
|
| Because the moon is tidally locked to the earth the pull always
| act in the same direction.
|
| I was imagining the moon as a drop of liquid in the vacuum
| space with a denser core at the center. The heavier core would
| be at the center because of the moon's own gravity. In the
| presence of the earth's gravity the denser part of the "liquid
| drop" will be centered more closely to one side and so the
| volcanic activity would be strongly biased.
| OscarCunningham wrote:
| For the same reason there are two tides, you would expect the
| drop to be pulled towards Earth, but also thrown out away
| from Earth.
|
| https://en.wikipedia.org/wiki/Tidal_force
| frankohn wrote:
| No, because the moon is tidal-locked to the earth so the
| pull is always in the same direction. Tidal-locked means
| the moon always show the same face to the earth.
| CorrectHorseBat wrote:
| Won't there be centrifugal pull (push?) on the other
| side? Just like there is tide on the moon side and on the
| other side
| false-mirror wrote:
| You're referring to the fictitious centrifugal force [0]
| which is actually a minor change in inertia--very much
| out shined by the centripetal force of gravity. For
| example, on earth if you weigh 200 lbs on the North Pole,
| you'd weigh about 199lbs on the equator. On the moon,
| this tiny effect would be wiped out by gravity.
|
| [0]:https://en.wikipedia.org/wiki/Centrifugal_force
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