Volatiles at Mercury’s surface

1. Volatiles at Mercury’s surface David Rothery, Rebecca ThomasDepartment of Physical Sciences The Open University D.A.Rothery@open.ac.uk Laura KerberLab MétéorologieDynamique du CNRS, Université Paris 6HEWG, Key Largo, 15 May 2013

2. Ice in permanently-shadowed craters • S in the regolith (2-5% from XRS, poor spatial resolution) • Explosive volcanism • Hollows – cold-trapped cometary – need volatiles in magma to drive explosions – an ongoing process? Volatiles:

4. Head et al., (2009), EPSL, 285, 227-242

5. ‘Kidney-shaped vent’, RS03

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7. Hollows on Mercury Rebecca Thomas, Dave Rothery, Susan Conway & Mahesh Anand Open University, UK Hollows on the peak and floor of Eminescu crater (JHU/APL & ACT)

8. What are hollows? BCFDs viewed by Mariner 10 and MESSENGER flybys = hollows. Hollows over much of the floor of Hopper crater (left) and in a scattered cluster in another crater (right) (JHU/APL & ACT)

9. Analogue: ‘Swiss cheese’ terrain Form by sublimation of CO2 ice on Mars’ south polar cap. Similar morphology, though not a perfect analogue. b. a. Formed by sublimation? a. Polar ‘Swiss cheese’ terrain on Mars, b. enhanced-colour image of hollows in Raditladi basin, Mercury.

10. What didn’t we know? Global distribution of hollowing Composition of the material being lost. Source of the material being lost. Mechanism for removal.

11. Survey conducted 20°wide strips, 22% of the planet’s surface (JHU/APL & ACT)

12. Clusters of hollows identified 104 clusters, covering c.31,500km2 (JHU/APL & ACT)

13. How do hollows form? Loss of an at least moderately-volatile substance. Removed without melting. Candidate mechanisms: Sublimation Space weathering e.g. photon-stimulated desorption (PSD), solar wind sputtering (when s-w pushes magnetopause close to the surface ) Micrometeorite bombardment (estimated to be too slow to form hollows, Blewettet al. 2011)

14. Controlled by insolation? Relevant to sublimation and PSD – photon and heat-dependent. Hollows form on slopes with a preferred aspect in 20% of cases. Strong preference for sun-facing slopes: weak correlation with insolation.

15. Controlled by insolation? On a regional scale: 11.5x more hollowed area in the ‘hot pole’-crossing strip than the ‘cold pole’-crossing strip. Very few hollows in polar regions – though this may be due to observational bias/substrate.

16. Solar wind sputtering? Expected to be especially intense at polar and possibly equatorial regions. Largest total hollowed area is equatorial. But not consistent over all strips, and little hollowing in polar regions. Contribution from solar wind sputtering not ruled out but not positively supported.

17. Sublimation and possibly PSD are best-supported due to correlation of hollows with insolation. Some loss by sputtering is not ruled out. Weakness of the correlation to insolation suggests threshold is low or another factor is more important, i.e. presence of hollow-forming material at the surface. Hollow-formation mechanisms: Conclusion

18. Identifying the material being lost 86% of hollow clusters observed formed in regional or localised low reflectance material. Bright deposits appear to be the products of formation: on hollow floors and in diffuse halos around them. LRM and hollowing in Chekhov crater (JHU/APL & ACT)

19. Identifying the material being lost Hollows form in the low reflectance substrate. Hollows do not form on smooth, high-reflectance volcanic plains: none in northern plains, Caloris plains. Suggests the hollow-forming volatile is a component of low reflectance material and not of plains lavas. Unnamed crater at the rim of Rembrandt basin. (JHU/APL & ACT)

20. How does this unstable material reach the surface? 1. Exhumation • Hollows occur within impact craters in 92% of cases. • Suggests craters facilitate the transfer of the hollow-forming material to the surface. • In most cases, the theory (Blewett et al. 2012) that they are exhumed during crater formation appears to be valid. Atget crater in the Caloris plains (above) and Renoir basin (below). (JHU/APL & ACT)

21. How does this unstable material reach the surface? 2.Later exposure • Old, degraded craters: hollowing localized in superposed simple craters and thrusts crossing the crater. • Suggests hollow-forming volatiles remained in the near-surface after hollowing ceased at the surface. • Small impacts and fault-bend folding expose these to surface conditions long after crater formation. Unnamed degraded crater at-57.4°E,58.3°N. (JHU/APL & ACT)

22. How does this unstable material reach the surface? 3. Migration through crater fills • Hollows form on crater fills where they bury peak ring. • Rest of peak ring also has hollows, rest of floor does not. • Pitted texture • Suggests migration through floor material, undermining it. • Possibly also through pyroclastic deposits. Hollows and pitting in Sousa crater. (JHU/APL & ACT)

23. How does this unstable material reach the surface? 4. Migration up crater-related faults? • Suggested by association with pyroclastic volcanism: • 78% of observed pits have surrounding hollows. • All of these also have spectrally-red deposits. • Pyroclastic pits form above crater-related faults e.g. terraces, peak structures. • Both volatile-bearing magma and hollow-forming material migrate up crater-related faults? Unnamed crater at -3.6°E, 25.6°N. (JHU/APL & ACT)

24. How does this unstable material reach the surface? 4. Migration up crater-related faults? cont. • Even in the absence of pyroclastic volcanism, hollows often form preferentially on peak structures or terraces. • But these faulted areas are also the areas exhumed from greatest depth. • Exhumation and migration equally-well explain their localisation. • Post-cratering migration from depth is not proved. Unnamed crater at -3.6°E, 25.6°N. (JHU/APL & ACT)

25. Theory of hollows in a nutshell • Hollows form by the loss of volatile material which is a component of low reflectance material. • This may occur by sublimation. • This material is exhumed from depth by impact craters and may also migrate to the surface on a local or deeper scale. • Suggests there is a globally-extensive subsurface volatile-bearing layer which is spectrally dark. • What is it? • Cumulates? (Denevi et al., 2009) • Primary crust? (Rothery et al. 2010) • Certainly different from plains lavas.

26. Looking forward: Composition • If we can determine the composition of the dark material and the bright products, we can: • constrain the formation process and • begin to understand the processes which lead to the accumulation of this deposit. • The higher-resolution spectral data from BepiColombo will hopefully provide this information. MPO, an artist’s impression (ESA)

27. Theory of hollows in a nutshell • Hollows form by the loss of volatile material which is a component of low reflectance material. • This may occur by sublimation. • This material is exhumed from depth by impact craters and may also migrate to the surface on a local or deeper scale. • Suggests there is a globally-extensive subsurface volatile-bearing layer which is spectrally dark. • What is it? • Cumulates? (Denevi et al., 2009) • Primary crust? (Rothery et al. 2010) • Certainly different from plains lavas.