Massive degraded impact crater on Mercury - chemical evidence? 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):



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?
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