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.

High-resolution MESSENGER images show material that has cascaded down the sides of knobs and partially obscured impact craters that are demonstrably younger than Caloris and the original emplacement of the blocks (Image 2a; Wright et al., 2020). On many knobs there are hollows (Image 2b), which are geologically young landforms that form by loss of some material that is volatile under Mercury’s surface conditions. This suggests that many knobs contain these volatiles. Material from one knob has encroached on a fault scarp that cuts the Caloris rim (Image 2c,d). These observations show that the knobs have been substantially modified by mass-wasting since their original emplacement over the course of hundreds of millions of years.

Image 2: Knob relationships with other landforms. (a) Knob material burying a crater (yellow arrow) on volcanic plains that post-date Caloris. MESSENGER MDIS frame EN0258398544M, 43 m/pixel. (b) Knob with extensive hollows, indicating a volatile component within the knob. MESSENGER MDIS NAC frame EN1045703448M, 29 m/pixel. (c) Thrust scarp cutting the Caloris rim (yellow arrows). White box shows the location of (d). (d) Knob material on a thrust scarp (yellow arrows) that cuts the Caloris rim and the volcanic plains in and around the basin. This demonstrates that mass-wasting activity was long-lived after the Caloris ejecta blocks were emplaced. MESSENGER MDIS NAC frame EN0220720885M, 19 m/pixel. MESSENGER MDIS global monochrome mosaic, 166 m/pixel.

Elsewhere on Mercury, hollow formation has triggered mass-wasting (Blewett et al., 2013), so volatile-loss from the knobs could explain why knob materials have been able to bury young impact craters. Mass-wasting is a geomorphic process that generates cohesionless material that builds slopes up to the angle of repose (~32°). Topographic profiles of the knobs, generated with laser altimetry and shadow measurement techniques, show that their slopes are consistent with formation by mass-wasting. So it seems possible that Caloris excavated volatile-bearing ejecta blocks that have since degraded into cones. This landform progression is similar to molard formation in permafrost environments on Earth (Image 3; Morino et al., 2019).

Image 3: Molards on Earth. (a) Ice-cemented rockfall block from Móafellshyrna. Photographed 2012. (b) A molard that formed from the block in (a) after three years of thawing. Photographed 2015. The white bars in (a) and (b) indicate vertical scales of 2 m. Image credited to Morino et al. (2019).

If this model of knob formation is correct (Image 4), then it implies that the volatiles that form hollows and knobs can be found in Mercury’s deep interior, from which Caloris ejecta blocks were derived. This result requires that Mercury’s silicates were not depleted in volatiles during the planet’s formation. This poses a challenge to many formation models for Mercury, which commonly invoke some cataclysmic event in Mercury’s early history, such as a giant impact, to explain the planet’s disproportionately large iron core. Heating from such energetic events would preferentially strip the planet of its volatiles.

Image 4: Hypothesis for volatile-rich circum-Caloris knob formation and evolution on Mercury. Knobs were originally emplaced as volatile-bearing Caloris ejecta blocks with arbitrary shapes. Loss to space of a structurally integral volatile component caused the inert material to disaggregate and collect at the foot of the block building slopes to the angle of repose. This exposed fresh material and caused the scarp to retreat. Once the volatile component was exhausted or entombed beneath a sufficient lag deposit, scarp retreat ceased and impact gardening took over, leading to a rounded knob summit.

Further reading

Blewett et al. (2013) Mercury’s hollows: Constraints on formation and composition from analysis of geological setting and spectral reflectance. J. Geophys. Res. Planets, 118, 1013–1032. https://doi.org/10.1029/2012JE004174

Fassett et al. (2009) Caloris impact basin: Exterior geomorphology, stratigraphy, morphometry, radial sculpture, and smooth plains deposits. Earth Planet. Sci. Lett., 285, 297–308. https://doi.org/10.1016/j.epsl.2009.05.022

Morino et al. (2019) Molards as an indicator of permafrost degradation and landslide processes. Earth Planet. Sci. Lett., 516, 136–147. https://doi.org/10.1016/j.epsl.2019.03.040

Wright et al. (2020) Modification of Caloris ejecta blocks by long-lived mass-wasting: a volatile driven process? Earth. Planet. Sci. Lett., 549. https://doi.org/10.1016/j.epsl.2020.116519

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