Tri-axial Mars - the Mars Kim Stanley Robinson forgot

A synchronous moon as a possible cause of Mars’ initial triaxiality

https://arxiv.org/pdf/2408.14725

This came out of the backlog. Some editing from that.

28 August 2024 - A synchronous moon as a possible cause of Mars’ initial triaxiality

Oh, that's interesting. Mars presents a lot of questions because it is the closest Earth-a-like we can study in any detail.

On the other hand, many people forget how different Mars is to Earth (@twitter.com@elonmusk - are you listening? Of course not - you talk, not listen.) Yes (FTFA), "It turns out that a moon of less than a third of the lunar mass was capable of producing a sufficient initial triaxiality." may be true, but it glosses over that Mars is now (and probably always was) one tenth of Earth's mass. Is that comparison with the Moon in absolute mass, or relative mass? In either case it is ridiculously larger than Phobos or Deimos, or their combination.

Where did this Moon go? And why?

I saw an interesting SETI "lunchtime lecture" on the Martian "hemispheric dichotomy" (N. Polar Basin vs Southern Highlands) a number of years ago. Accepting the "giant impact" hypothesis for that structure (itself a natural expectation of "hierarchical growth" [should that be "oligarchic growth"? From Wiki, The next stage is called oligarchic accretion. It is characterized by the dominance of several hundred of the largest bodies - oligarchs - which continue to slowly accrete planetesimals. No body other than the oligarchs can grow. ] - little things accrete to make bigger things - models of planetary growth), then the possibility that after the last "giant impact" the body is significantly non-spherical becomes ... well, plausible, but not guaranteed. Late-stage impacts are going to deliver a lot of energy so that the planet is effectively a droplet of a low-viscosity fluid. And you've got to have a large enough body ("Moon-size", or larger ; the Moon is about 1.25% of the mass of the Earth), close enough to affect the shape of the (slowly) cooling mass.

Time to RTFP!

"Motivation :" Mars’ triaxiality makes itself most evident through the equatorial ellipticity produced by the Tharsis Rise and by a less prominent elevation located almost diametrically opposite to Tharsis and constituted by Syrtis Major Planum and an adjacent part of Terra Sabaea Yeah, well we all know Tharsis - volcanoes, possibly still recently active. Maybe a mark of "single plate tectonics and where the heat gets out. Tharsis, volcanic peaks excluded, is about 7km above the mean elevation of the planet (or is it to a reference elevation, not a "mean" - a bit of Martian cartography I'll have to check up on) while the elevation he gives for Terra Sabaea is only 2.1~2.3 km. The author then goes on to consider the ellipticity of Mars without the Tharsis contribution (which the mappers, Zuber and Smith (1997), had also considered). Even [without Tharsis] Mars retained much of its triaxiality. - Which I'll take as read. They then propose the initiation of a "seed" triaxial component from their putative moon, later amplified by tectonic processes dumping heat and magma onto the Tharsis high point. Unfortunately, this gets rather iffy already. Mars is reported to undergo a lot more "polar wander" than Earth (justifying the horrible SF consequences of losing the Moon, and all sorts of other doom) and that the current near-polar position of the North Polar Basin and the (sub-equatorial) Tharsis bulge are near-coincidence. I don't think you can have both at the same time. I agree with this next quote - but am not blind to the problems of moons turning up then going away : The seed asymmetry of the equator was considerable if the synchronous moon existed already at the magma-ocean epoch, and was weaker if the moon showed up at the solidification stage.

Whence had it come, whither gone?

The author's title, not mine. But yes, it's a big question.

Had the impact happened during the magma-ocean stage, it would hardly have influenced the subsequent development of Mars’ global structure.

I couldn't put it more succinctly myself. See my above "droplet of a low-viscosity fluid" comment.

On the other hand, had it [a large impact] happened during the formation of crust, it may have, speculatively, left some signature - whence the question arises whether that impact could be the one responsible for the north-south hemispherical dichotomy, a theme beyond the scope of our study.

I don't think the author has seen Marinova's SETI lecture on her work, or the associated papers. Her modelling of a Polar-basin forming impact has the redistribution of 10~20 km thickness of crustal thickness from the (putative) impact site to the rest (other 2/3) of Mars' surface - which would literally outweigh this proposed minor lunar re-shaping. There's the non-trivial point too that the crust and upper mantle would have isostatically adjusted towards following the (gravitational) spheroid or (rotational ellipsoid. Rocks are not solid, even on a cold, dead planet like Mars - they creep under forces.

He doesn't really address the "whence" question - he lists some features of protoplanetary discs, and says they might be factors, while ignoring the blunt fact that most people in the field accept the really large satellites in the Solar system (Luna, Charon) are the products of "giant impacts", and this "Nerio" (some Roman mythological associate of Mars/ Ares) would fall into that category too.

What does he say about "whither"? Well, he blames it on the LHB (Late Heavy Bombardment), with a proviso that it would have to have been early in the LHB, so that later LHB impacts would overprint the expected equator-biased impacts from bits of the moon falling to Mars.

Colour me unconvinced on that front. It's plausible, but far from convincing. The whole "LHB" concept is itself rather dependent on a relatively small number of radiometric dates from a relatively small area of the Moon, all rather close to the Imbrium Basin. There are geological challenges from terrestrial observations too. It's an idea seriously needing better support (e.g. from sample-return missions from the Lunar far-side).

The remaining 27 pages of the paper are mathematical arguments which are over my head. The author obviously thinks they show that his sequence of events is mathematically plausible, and I'm willing to accept that (besides, it's plain from the reference list, that this is his field, and he's worked with many others in this area, and presummably they accept this work when they reviewed the paper. "plausible" ≠ "true".

My summary : plausible, but I don't think it's likely. Worth a read ; not worth studying the maths (which I assume is correct).

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