Evidence of past expansion of the Moon.

[Florian]

An interesting article just published in Science:

Jeffrey C. Andrews-Hanna et al "Ancient Igneous Intrusions and Early Expansion of the Moon Revealed by GRAIL Gravity Gradiometry"

"The earliest history of the Moon is poorly preserved in the surface geologic record because of the high flux of impactors, but aspects of that history may be preserved in subsurface structures. Application of gravity gradiometry to observations by the Gravity Recovery and Interior Laboratory mission results in the identification of a population of linear gravity anomalies with lengths of hundreds of kilometers. Inversion of the gravity anomalies indicates elongated positive density anomalies interpreted to be ancient vertical tabular intrusions or dikes, formed by magmatism in combination with extension of the lithosphere. Crosscutting relationships support a pre-Nectarian to Nectarian age, preceding the end of the heavy bombardment of the Moon. The distribution, orientation, and dimensions of the intrusions indicate a globally isotropic extensional stress state arising from an increase in the Moon’s radius by 0.6-4.9 km early in lunar history, consistent with predictions of thermal models."

This diking activity around 3.8 is remarkable and could be a hint for an earlier activity of the Moon. The estimated radius increase is modest, but is the lower limit because lithospheric extension is not systematically accompanied by intrusion.

[sathearn]

Here's an excellent discussion of this new evidence, elucidating for a more general audience the inference that linear dike-like structures, distributed isotropically around the lunar surface, may indicate some post-formation expansion early in the Moon's history.

http://www.planetary.org/blogs/emily-la ... sults.html

Hat tip to dwkjo in the "Expanding Earth My Ass" comments section for pointing out this source.