Balme, Matthew

Conway, Susan

Pegg, David

Rothery, David

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsExplosive Vents on Mercury: Commonplace MultipleEruptions and Their ImplicationsConference or Workshop ItemHow to cite:Pegg, David; Rothery, David; Balme, Matthew and Conway, Susan (2019). Explosive Vents on Mercury: CommonplaceMultiple Eruptions and Their Implications. In: 50th Lunar and Planetary Science Conference 2019, 18-22 Mar 2019,Houston, TX.For guidance on citations see FAQs.c© [not recorded]Version: Version of RecordLink(s) to article on publisher’s website:https://www.hou.usra.edu/meetings/lpsc2019/Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukEXPLOSIVE VENTS ON MERCURY: COMMONPLACE MULTIPLE ERUPTIONS AND THEIR IMPLICATIONS.  D. L. Pegg1, D. A. Rothery1, M. R. Balme1, and S. J. Conway2.  1The Open University, Milton Keynes, MK7 6AA, UK david.pegg@open.ac.uk. 2LPG Nantes – UMR CNRS 6112, Université de Nantes, France.   Introduction:  The MESSENGER mission imaged high albedo red spots (now referred to as faculae) on the surface of Mercury [1-3]. Faculae show little relief apart from central rimless, non-circular depressions. They are interpreted to be explosive volcanic vents surrounded by explosive volcanic deposits [1,2, 4]. Their global distribution, setting and overall shape have been studied [4-8], but little work has been under-taken on their internal structure: only the vent within Agwo Facula has been described in detail [9], where several vents inside a common rim were identified, making it a compound volcanic vent [10].  We present here a larger study of all compound vent sites on Mercury, in which their internal structures are described and analyzed. Methods: We used the high resolution MDIS im-ages [11] of faculae within the database of [6] to study vents contained within them. Where a vent structure was clearly resolved and had two or more immediately adjacent or overlapping vents, we classified it as com-pound(eg. Fig 1) (unless the vents were clearly separat-ed within the same structure, such as on different parts of a peak-ring)).  We identified compound vents based on internal structures, predominantly septae (narrow ridges be-tween vents). Distinct volcanic craters (non-circular with defined, but not raised, rims) were also identified. Fig 1: Example of a compound vent system on Mercu-ry.  Sinusoidal projection centered on the vent. Fig 2: Global distribution of compound volcanic vents identified on Mercury, using the Mercury global color basemap [12]. 1273.pdf50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132)We used cross-cutting relationships to infer relative timing where possible. In a few cases, where image resolution was sufficient, we used the relative internal roughness of each vent to aid inference of relative tim-ing: smoother surfaces being assumed to be older (blanketed by a thin layer of younger volcanic ejecta, or muted by thicker regolith).  Results: There are 64 compound vents within the 184 faculae on Mercury (Fig 2), so at least a third of all faculae have compound vents at their source. This is likely to be an underestimate, due to illumination con-straints and the limited availability of high resolution images. We noted a wide variety of individual vent sizes, and that vents within a single compound vent system can have different depths. There is no clear pattern to the global distribution of compound vents. Globally the proposed link be-tween faculae and tectonic structures cannot be seen [13], but the morphologies of individual vents are often controlled by structural features. Internal features: Within most vent sites there is lit-tle evidence of the constructive edifices previously identified at some pits [8], suggesting that the dominant style of volcanism is explosive excavation. This in-cludes newly-recognized features from our study: small-scale pits on the edges of the larger vents within a compound structure (e.g., Fig 3). These pits do not have the spectral characteristics of hollows seen on Mercury [2,4,5,14] and in some cases appear to follow existing structures within the pit. Discussion: Previous work has shown that explosive vents on Mercury formed over a long period (and pos-sibly as recently as the Kuiperian) [6,8]. It is not possi-ble to make estimates of the length of time individual vent sites were active, both due to their small size and their rough appearance, which makes impact crater statistics unreliable. The large proportion of faculae with compound vents suggests that at least a third of the vents were not made simply by single eruptive events. The different scales of vents observed, includ-ing the much smaller end-stage textures and overlap-ping vents, suggests that many vents on Mercury were active for prolonged periods of time. This prolonged activity suggests that the dyke systems that presumably fed these vents must have been recharged with vola-tiles. Possible mechanisms for this include magmatic recharge tapping a volatile rich mantle [15], or assimi-lation from volatile rich crust [16].  Given the small size of many of these features it seems likely that some have not yet been located due to the low resolution of images available. This is also true for single vents. Conclusion: A large proportion of explosive vol-canic vents on Mercury are compound structures indic-ative of multiple eruption events at that site. This sug-gests that magma sources were long-lived and recur-rent. Volatiles were replenished over time, either through magma recharge or assimilation. This has wide implications for the internal plumbing of these sites, suggesting that, once established, they can be long-lived. Multiple eruption centers associated with each facula must be accounted for in models of eruption velocities, which often assume a single central vent. Further study by BepiColombo will provide more in-formation on these structures and their implications for Mercury’s volatile rich volcanism. Acknowledgements: STFC for funding my stu-dentship (grant number ST/R504993/1).  References:  [1]Head, J. W., et al., (2008). Science, 321(5885), 69-72. [2] Head, J. W., et al. (2009) EPSL 285.3-4: 227-242. [3]Blewett, D.T, et al. (2009) EPSL, 285, 272-282. [4] Kerber, L., et al., (2009). EPSL, 285(3-4), 263-271. [5] Goudge, T. A., et al., (2014). JGRP, 119(3), 635-658. [6] Thomas, R. J., et al. (2014). GRL, 41(17), 6084-6092.[7] Thomas, R. J., et al. (2014). JGRP, 119, 2239-2254. [8] Jozwiak, L. M., et al. (2018). Icarus, 302, 191-212. [9]Rothery, D. A., et al., (2014). EPSL, 385, 59-67. [10]Davidson et al., Ency-clopedia of Volcanoes, Academic Press, San Diego (2000), pp. 663-681. [11] Hawkins, S. E, et al. (2007) Space Science Reviews 131.1-4 247-338. [12] Denevi, B. W., et al, (2018). SSR, 214, 2.[13] Klimczak, C, et al., (2018) Icarus 315 (2018): 115-123.[14] Kerber, L. et al, (2011) PSS, 59, 1895-1909. [15] Wright, et al., MERCURY 2018, 2047. [16] Nittler, R.D. et al. (2011) Science, 333, 1847-1850.    Fig 3: Vent floor within Nathair Facula highlighting small pits around the texturally roughest (youngest?) part of vent.  1273.pdf50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132)

Explosive Vents on Mercury: Commonplace Multiple Eruptions and Their Implications

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