Volatile-rich impact ejecta on Mercury

Post contributed by Dr Jack Wright, School of Physical Sciences, The Open University, UK.

The Caloris basin is the largest (~1,500 km across), well-preserved impact structure on Mercury (Image 1a; Fassett et al., 2009). Hummocky plains around Caloris host numerous, steep-looking, conical knobs (Image 1b). The obvious explanation for the hummocky plains is that they formed from material ejected by the Caloris impact ~3.8 billion years ago. It follows that the knobs probably formed from discrete ejecta blocks. What isn’t obvious is why many of these blocks, which hypothetically could have formed with a variety of shapes, exist as steep cones in the present day. If these knobs really did form as Caloris ejecta, then they offer a rare opportunity to study materials ejected from Mercury’s interior with remote sensing techniques.

Image 1: Mercury and the circum-Caloris knobs. (a) Enhanced colour limb view of Mercury from the MESSENGER spacecraft. The Caloris basin’s interior is made of volcanic plains that appear orange in this data product. The arrow indicates the location of (b). (b) Examples of circum-Caloris knobs just outside the Caloris rim. Mosaic of MESSENGER MDIS WAC frames EW0220807059G, EW0220807071G, and EW0220763870G. ~86 m/pixel.

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