The recent Phys.org piece Mass anomaly detected under the moon's largest crater is a review of a recently published and paywalled Geophysical Research Letter Deep Structure of the Lunar South Pole‐Aitken Basin .

Early on it says:

"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.

Later in the Phys.org article:

The dense mass—"whatever it is, wherever it came from"—is weighing the basin floor downward by more than half a mile, he said. Computer simulations of large asteroid impacts suggest that, under the right conditions, an iron-nickel core of an asteroid may be dispersed into the upper mantle (the layer between the Moon's crust and core) during an impact.

"We did the math and showed that a sufficiently dispersed core of the asteroid that made the impact could remain suspended in the Moon's mantle until the present day, rather than sinking to the Moon's core," James said.

Another possibility is that the large mass might be a concentration of dense oxides associated with the last stage of lunar magma ocean solidification.

@DavidHammen's answer to a related question elaborates on some of the assumptions in the research:

Combining the gravity and topography models required two key assumptions regarding the subsurface material in the South Pole-Aitken basin. One was that varying thicknesses of crustal material lay over mantle material, with the crustal material being less dense than the mantle material. The other assumption was that the materials were in isostatic equilibrium. This latter assumption may not be valid; it is well established that the large lunar mass concentrations outside the polar regions are in a super-isostatic state. That said, the South Polar-Aitken basin does not exhibit the signs of being in a super-isostatic state.

There is a lot of geophysics going on here that I don't understand.

But the statement in the Phys.org quote about heavy material being "sufficiently dispersed" so as not to sink to the core seems to be the opposite of "the materials were in isostatic equilibrium". Is it possible to reconcile these two? Am I comparing apples to oranges?

  • 2
    $\begingroup$ This may be better suited to a geology or physics site. $\endgroup$
    – GdD
    Jun 13, 2019 at 7:15
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    $\begingroup$ Open geologic questions on the Moon, how could it be off-topic? I find the close initiative surreal. $\endgroup$
    – peterh
    Jun 13, 2019 at 10:05
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    $\begingroup$ @GdD "may be better on" is absolutely not the same as off-topic. This question is 100% on-topic here. I have already posted a link to this and the previous linked question in What on Earth?. Since I am referencing an answer and a question in this site, and a paper about the Moon, based on two sets of lunar satellite data, I decided that the question's audience (future readers of the answers) is more likely to be here. $\endgroup$
    – uhoh
    Jun 13, 2019 at 10:06
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    $\begingroup$ I haven't voted to close @uhoh, and I'm not saying it's off topic, I'm saying that a geologist may be better placed to give you an answer. $\endgroup$
    – GdD
    Jun 13, 2019 at 10:43
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    $\begingroup$ I wish I hadn't commented now @uhoh! I think it is on topic, I was just saying that given what I know about the contributors on this site I can't think of anyone who might have an answer. That doesn't mean we won't get some great input from someone new. $\endgroup$
    – GdD
    Jun 13, 2019 at 15:29

1 Answer 1


“Super-isostatic” means that a region hasn’t (yet) relaxed to an isostatic configuration. On selenological spatial scales, things have a bit of resistance to reflow, although there’s a difference of opinion on how much resistance, the role of impact heating, etc (a starting point to read more is Kiefer et al 2012)

More to the point, dispersing a point-like mass (I.e. an asteroid, even a big one) would make it less likely that geologic processes would (slowly, really slowly) allow it to sink through the light crust and perhaps even through the denser mantle. These processes aren’t like a rock sinking; both small and large rocks sink. They’re more like putting something on a soft material like clay: a small pressure (small mass per cm2, thin object) might not sink while a larger one will. So there’s a natural sorting.

But this is outside my area, and I’m not at all sure that the colder,slower Moon still had those processes acting for this South-pole mass.

  • $\begingroup$ Thank you for the answer and the link! I'll give it a good read this weekend. $\endgroup$
    – uhoh
    Jun 14, 2019 at 4:41

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