The role of impact structures in localizing explosive volcanism on a contracting planet: Mercury Rebecca J. Thomas*, Dave A. Rothery, Susan J. Conway, Mahesh Anand The Open University, U.K.

European Planetary Science Congress NASA/JHUAPL/CIW, NASA/Caltech Explosive volcanism & impact structures

Impact structures appear to localize explosive volcanism. Questions arising: 1.How? 2.Can explosive volcanism can be used as a marker for ancient basin structures? European Planetary Science Congress 20153

1. Localization within impact craters European Planetary Science Congress 20154

Lessons from the Moon European Planetary Science Congress LROC Team

A feasible model for Mercury? Compared explosive volcanism in complex impact craters on Mercury (16) and the Moon (15) Where ◦ Presence of pyroclastic deposits is relatively uncontroversial ◦ Available topographic data ◦ Identifiable vents European Planetary Science Congress 20156

Findings: Energy of eruption Greater on Mercury: 1.Vent volume 2.Maximum particle range More energetic eruptions Higher volatile content powering eruption (NASA/JHUAPL/CIW) European Planetary Science Congress Mercury MoonMercury

Host crater deformation: Moon All sampled lunar sites are in craters with shallow, fractured floors. European Planetary Science Congress LROC Team

Host crater deformation: Mercury Floors are not shallow, fractured or deformed. No storage? Improbable: ◦ Multiple, closely co-located vents ◦ Large scale of deposits suggests accumulation of volatiles prior to eruption Far left: Multiple vents (-6.4° E, 48.5° S) Left: Widespread deposit (-89.3° E, 21.2° S) (NASA/JHUAPL/CIW) European Planetary Science Congress NASA/Caltech

Why no surface deformation on Mercury? Most probable cause: deeper intrusion due to Mercury’s global compression ◦ Consistent with  Energetic eruptions: chamber stable to greater overpressure  Vents at crater centre European Planetary Science Congress

Conclusions 1: How? 1. Subsurface magma storage is localized by impact crater structures on Mercury, as on the Moon …But at greater depths than the Moon due to crustal compression. 3. This gives it distinct characteristics: i.Localization of eruptions at the crater centre. ii.Larger vents and deposits because higher concentrations of volatiles are able to accumulate prior to eruption. European Planetary Science Congress More detail: Thomas et al. EPSL (in press)

2. Explosive volcanism as evidence for ancient impact basins European Planetary Science Congress

Fewer large basins than expected European Planetary Science Congress Fassett et al., JGR: Planets, 2012 Basins: Fassett et al Base image: NASA/JHUAPL/CIW

Sites of explosive volcanism + European Planetary Science Congress Compositional/spectral evidence for a circular or annular region with a distinct composition (i.e. high Mg/Si, LRM). 2. Tectonic structures forming a ring/s. 3. A circular region of thin crust and/or annulus of thick crust.

3 possible basins ≥ 500 km diameter European Planetary Science Congress Basins: Fassett et al Base images: NASA/JHUAPL/CIW

‘B54’ European Planetary Science Congress Basins: Fassett et al. 2012, base images and spectral data: NASA/JHUAPL/CIW Base image and spectral data: NASA/JHUAPL/CIW, Tectonic structures: Byrne et al. Nature Geoscience, 2014 Smooth plains: Denevi et al. JGR: Planets, 2013

Support for HMR basin European Planetary Science Congress Basins: Fassett et al. 2012, base images and spectral data: NASA/JHUAPL/CIWXRS-derived composition: Weider et al. EPSL, 2015 Spectral data: NASA/JHUAPL/CIW

Southern Basin European Planetary Science Congress Basins: Fassett et al. 2012, base images and spectral data: NASA/JHUAPL/CIW

Conclusions 2: Ancient basins European Planetary Science Congress Aligned sites of explosive volcanism = supplementary evidence for the presence of as many as 3 ≥ 500 km diameter impact basins on Mercury. Reduces the disparity in such basins versus the Moon. A greater degree of resurfacing is implied, rather than a different population of impactors.

European Planetary Science Congress Thank you for your attention.