Thread by @PeterBJames

Our paper on the deep structure of the Moon's South Pole-Aitken basin has been published (co-authors include @ThePlanetaryGuy, @maria_zuber, and three more non-tweeting authors). There are several "massive" results packed into this tiny paper. [1/n]

agupubs.onlinelibrary.wiley.com/doi/full/10.10…

The SPA basin is huge, with an inner rim approximately 2000 kilometers long... imagine a hole in the ground stretching from Washington DC to Waco, TX. In terms of topography (this map courtesy of @corrinerojas) it is one of the defining features of the Moon's far-side. [2/n]

It's the largest preserved basin (i.e., super-sized crater) in the Solar System. While bigger impacts almost certainly occurred—such as the impacts that may have formed Mars's northern lowlands or the Moon's Procellarum terrain—most traces of those basins have been lost. [3/n]

...This makes the SPA basin one of the best natural laboratories for studying catastrophic impact events, an ancient process that shaped all of the rocky planets and moons we see today. [4/n]

The gravity field (i.e. the subtle changes in the strength of gravity) above the SPA basin is determined by the distribution of mass underground, so this is one of our best tools for understanding the basin's formation and evolution. [5/n]

For example, if you assume that all gravity anomalies are produced by (1) topography, (2) density variations in the crust, and (3) Moho undulations, you can use theory developed by @mwczrk to map the thickness of the Moon's crust. [6/n]

Moho undulations generate gravity anomalies, but unexpected density anomalies in the crust or mantle can generate identical anomalies. Unfortunately, this means that the interpretation of geophysical data is often ambiguous or "non-unique". [7/n]

In this paper we alleviated the issue of non-uniqueness by assuming that residual stresses should be minimized. This isn't the case everywhere on the Moon (e.g., mascons and ejecta top-loads), but it's reasonable for SPA. [8/n]

This leads to the first massive result: we've found a huge mass excess in the Moon's mantle under the southern portion of the SPA basin. That section of the mantle has at least *2 quadrillion metric tons* more mass than normal. [9/n]

...That's a lot of mass. Imagine taking a pile of metal ~5x larger than the Big Island of Hawai'i and burying it underground – that's roughly how much unexpected mass we detected. [10/n]

...That particular analogy is relevant to one of the compelling explanations for this observation: models by Jordan Kendall & the Purdue group predict that the iron-nickel core of the basin-forming impactor would have been dispersed into the upper mantle. [11/n]

We did the math to show that a sufficiently dispersed impactor core could remain suspended in the Moon's mantle until the present day, rather than sinking to the Moon's core. [12/n]

Another possibility for this large mass anomaly is a concentration of dense oxides associated with the last stage of magma ocean cooling, as predicted by @ltelkins and others. (Illustration: Jennifer Rapp / @LPItoday) [13/n]

...If 20% of these oxides were stranded in the upper mantle under SPA, that would be sufficient to explain the gravity data. We didn't propose a mechanism for accomplishing this though, so the oxide explanation is still speculative. [14/n]

The anomaly is not particularly correlated with surface geology. It overlaps with the "SPACA" terrain identified by @_DanOnTheMoon_, but it's offset somewhat. The lack of a strong correlation w/ surface features supports the conclusion that this is a deep mass anomaly. [15/n]

The second major result in this paper: the Moon's biggest crater just got ~60 km bigger.

(To be clear, the Moon hasn't changed, but we now have new topography data showing that we previously underestimated SPA's size). [16/n]

The previous estimate for SPA's inner rim dimensions was made by fitting an ellipse to Clementine topography, which had a gap at the south pole. When we perform the same analysis with LOLA topography & crustal thickness, we see that the southern rim is closer to the pole. [17/n]

The "South Pole-Aitken" basin is living up to its name, in the sense that the new inner rim ellipse more nearly coincides with the actual south pole. [18/n]

NASA is planning to send astronauts to the South Pole, and now we know that several of the massifs in that region were formed by this huge impact! [19/n]

These new best-fit ellipses show that the major axis of the basin is oriented approximately N17ºW, which is consistent with the analysis of Garrick-Bethell & Zuber (2009) but significantly different from the assumptions of other papers such as Schultz & Crawford (2011). [20/n]

Schultz & Crawford postulated that the antipode of the Procellarum KREEP terrain coincides with the point of first contact by the SPA impactor. This was a novel explanation for the origin of the KREEP basalts (note: a caveat is on the way). [21/n]

However, that paper was problematic; for example, the plotted antipodes in this figure are inconsistent by hundreds of kms. This inconsistency was a point of discussion at GRAIL team meetings, and I can't remember if I or @planetpatman was first to call attention to this. [22/n]

Furthermore, the basin orientation assumed by Schultz & Crawford is >30º off from the orientation we have determined. This theory for the origin of KREEP basalts was reported by WIRED magazine and others, but it needs a new analysis if it is to be taken seriously. [23/n]

This is a good time to note that Pete Schultz is the person who inspired me to pursue planetary science when I was a freshman at Brown University. Scientific disagreements such as this are in no way an indication of personal disrespect. [24/n]

The weight of the mantle mass anomaly is literally pulling the basin floor downward by about a kilometer, creating a central topographic depression (on the left, marked by the dashed circle). This coincides with a free-air gravity low (right). [25/n]

Previous models of the crustal thickness predicted that SPA's central depression resulted from thinner crust, but our new inversion with a mantle mass anomaly suggests that the crust in the central depression is just as thick as it is in other parts of the basin floor. [26/n]

It's surprising that the crust inside the SPA basin is as thick as it is, because computer models indicate that an impact of that size should have completely excavated the crust along with a good chunk of the upper mantle. So where did all of that crust come from? [27/n]

There are a few ideas: Melt sheet differentiation could have created a >10 km norite crust, and weak crustal material could have flowed inward after the impact (as modeled by the likes of @brcjohns and @SarahTStewart); ejecta deposits may have helped as well (@noahpetro). [28/n]

In any case, the new crustal thickness solution implies that the crust inside SPA is a few kilometers thicker than previously estimated (~16 km). And, if we geophysicists are underestimating the density of the SPA floor, the crust may be even thicker than that. [29/n]

That's the quick summary. I want to give a special thanks to @Overleaf for making this the most pain-free LaTeX editing experience I've had to date! [30/30]