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The planet Mercury has a myriad of impact features - this has been known for some time. However, observe in the following images, a relatively large 'High-Mg region' detected on the planet (below):

enter image description here

Image source: Messenger website

From the group's press conference website, they state that:

The unusual composition of the high-Mg region and the relatively thin crust in this region suggest that it may be the remains of an ancient and highly degraded impact basin.

How is the 'unusual composition' of this region suggestive of being the remains of an ancient, highly degraded, impact basin?

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  • $\begingroup$ The composition is "unusual" in that it is not an expected composition for Mercury and the source of the "unusually" high amount of Mg is likely from outside Mercury, namely asteroids/comets. Its the same principle with water on Earth. We don't expect Earth to naturally have as much water as it does, so we assume much, if not most, was brought in by comets impacting the Earth. $\endgroup$ – zephyr Nov 7 '16 at 19:02
  • $\begingroup$ @zephyr okay, thanks for the explanation - I literally can't tell, but are the quotation marks supposed to be sarcastic? $\endgroup$ – user14785 Nov 8 '16 at 2:57
  • $\begingroup$ No, I was merely quoting the term from the press conference. I can try to expand my comment into a full answer if you'd like but I'm not sure how much more there is to say. $\endgroup$ – zephyr Nov 8 '16 at 3:01
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There is still some research to do to definitively answer this question. Having said that though, according to research published in the paper "Evidence for geochemical terranes on Mercury: Global mapping of major elements with MESSENGER’s X-Ray Spectrometer (Weider et al. 2015), the high Mg region (HMR) could be due to the consequences of an impact event - the authors from the article suggest that:

therefore, that the HMR may also be an impact related feature, and that its high Mg/Si ratio is evidence of mantle excavation during a large impact event early in Mercury’s history (>4.1 Ga).

and later, partly resurfaced by subsequent volcanism and smaller impact events.

The authors elaborate further that the mechanism behind it could be

If an ancient large impact at the HMR site penetrated into a vertically differentiated mantle (with additional lateral variations possible) and reached a deep, otherwise unsampled layer (i.e., the lherzolitic source proposed by Charlier et al. 2013*), the HMR composition would be the signature of high-degree partial melting of that mantle source.

Additional resource cited:

Charlier et al. 2013, Phase equilibria of ultramafic compositions on Mercury and the origin of the compositional dichotomy, Earth and Planetary Science Letters

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