Simple explanation of the lunar highlands problem

[D H]

Why does the farside of the Moon have a thick crust with a landscape dominated by lunar highlands while the nearside has a much thinner crust with a landscape dominated by maria? This is what the authors of a paper recently published on the arxiv (and accepted for publication in the Astrophysical Journal Letters) call the "lunar highlands problem".

The explanation offered by the authors of the paper is extremely simple. The Moon was essentially born tidally locked, resulting the Moon's farside only received radiation from the young Sun while the nearside received radiation from both the Sun and the hot young Earth. The proximity to and temperature of the hot young Earth made that the Earth the primary energy source to the nearside. This made the farside equilibrium temperature about 250 K while for the nearside it was well over 1000 K, likely much higher. This temperature gradient would have led to lighter compounds that form crustal rock to freeze on the farside much sooner than on the nearside. *If* mixing was slow enough (but not too slow), this would in turn have led to the marked mismatch we now see between the near and far sides of the Moon.


A. Roy, J.T. Wright, and S. Sigurdsson, "Earthshine on a Young Moon: Explaining the Lunar Farside Highlands", arXiv:1406.2020v1 (2014), accepted for publication in ApJL
http://arxiv.org/abs/1406.2020v1

 

[Ophiolite]

Interesting idea: I am concerned about the proposition that the moon was tidally locked almost from its formation.

One of the pieces of data the authors use to reach this conclusion is sourced from Garrick-Bethell, I., Wisdom, J., & Zuber, M. T. (2006, Science, 313, 652). However, this paper, using these data surmises that "...the Moon may have once been in a 3:2 resonance of orbit period to spin period, similar to Mercury’s present state. The possibility of past high-eccentricity orbits suggests a rich dynamical history...".

I await the formal reaction to the proposition with interest.