An Assemblage of Lava Flow Features on Mercury
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JOURNAL OF GEOPHYSICAL RESEARCH: PLANETS, VOL. 118, 1303–1322, doi:10.1002/jgre.20052, 2013 An assemblage of lava flow features on Mercury Paul K. Byrne,1 Christian Klimczak,1 David A. Williams,2 Debra M. Hurwitz,3,6 Sean C. Solomon,1,4 James W. Head,3 Frank Preusker,5 and Jürgen Oberst5 Received 11 July 2012; revised 26 November 2012; accepted 15 January 2013; published 17 June 2013. [1] In contrast to other terrestrial planets, Mercury does not possess a great variety of volcanic features, its history of volcanism instead largely manifest by expansive smooth plains. However, a set of landforms at high northern latitudes on Mercury resembles surface flow features documented on Earth, the Moon, Mars, and Venus. The most striking of such landforms are broad channels that host streamlined islands and that cut through the surrounding intercrater plains. Together with narrower, more sinuous channels, coalesced depressions, evidence for local flooding of intercrater plains by lavas, and a first-order analysis of lava flow rates, the broad channels define an assemblage of flow features formed by the overland flow of, and erosion by, voluminous, high-temperature, low-viscosity lavas. This interpretation is consistent with compositional data suggesting that substantial portions of Mercury’s crust are composed of magnesian, iron-poor lithologies. Moreover, the proximity of this partially flooded assemblage to extensive volcanic plains raises the possibility that the formation of these flow features may preface total inundation of an area by lavas emplaced in a flood mode and that they escaped complete burial only due to a waning magmatic supply. Finally, that these broad channels on Mercury are volcanic in nature yet resemble outflow channels on Mars, which are commonly attributed to catastrophic water floods, implies that aqueous activity is not a prerequisite for the formation of such distinctive landforms on any planetary body. Citation: Byrne, P. K., C. Klimczak, D. A. Williams, D. M. Hurwitz, S. C. Solomon, J. W. Head, F. Preusker, and J. Oberst (2013), An assemblage of lava flow features on Mercury, J. Geophys. Res. Planets, 118, 1303–1322, doi:10.1002/jgre.20052. 1. Introduction relations between Mercury’s smooth plains and surrounding terrain, together with estimates of plains volumes far in ex- [2] By returning images of extensive smooth plains deposits cess of those expected from impact processes alone, as strong within and surrounding the Caloris basin and other large fl evidence for an origin by volcanism [Murray et al., 1975; impact basins on Mercury, the Mariner 10 ybys raised the Strom et al., 1975; Dzurisin, 1978; Kiefer and Murray,1987; prospect that extensive surface volcanism had occurred on Robinson and Lucey, 1997]. the innermost planet. Some workers considered these deposits [3] Strong support for widespread surface volcanism on to be of impact origin, regarding them as morphologically Mercury was provided by the MErcury Surface, Space similar to lunar highland plains known to have been fl ENvironment, GEochemistry, and Ranging (MESSENGER) emplaced as uidized impact ejecta deposits [Wilhelms, spacecraft from observations made during its three Mercury 1976; Oberbeck et al., 1977]. Others viewed embayment flybys [Solomon et al., 2011]. Near-global imaging of the planet revealed extensive smooth plains deposits that showed evidence for sequential embayment of impact basins and ejecta and that were spectrally homogenous but differed 1Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C., USA. in color from surrounding terrain, featured lobate margins 2School of Earth and Space Exploration, Arizona State University, morphologically similar to lava flow fronts, partially buried Tempe, Arizona, USA. numerous impact craters, and ranged in thicknesses from 3Department of Geological Sciences, Brown University, Providence, hundreds of meters to several kilometers [Head et al., Rhode Island, USA. 4 2008, 2009a; Denevi et al., 2009; Prockter et al., 2010]. Lamont-Doherty Earth Observatory, Columbia University, Palisades, fl New York, USA. From yby data, an especially large region of smooth plains 5Institute of Planetary Research, German Aerospace Center, Berlin, was identified at high northern latitudes [Denevi et al., 2009; Germany. Fassett et al., 2011] (Figure 1, label 1); early results from 6 Lunar and Planetary Institute, Houston, Texas, USA. orbit indicated that, although not obviously related to one Corresponding author: P. K. Byrne, Department of Terrestrial or more large impact basins, these plains cover some 6% of Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road Mercury’s surface and appear to have been emplaced by N.W., Washington, DC 20015–1305, USA. ([email protected]) low-viscosity, high-effusion lavas in a flood-mode style ©2013. American Geophysical Union. All Rights Reserved. similar to large basaltic igneous provinces on Earth [Head 2169-9097/13/10.1002/jgre.20052 et al., 2011]. 1303 BYRNE ET AL.: LAVA FLOW FEATURES ON MERCURY approach of Williams et al. [2001a] to calculate a range of lava flow rates through the channel and thus fill times for the downstream flooded basin, providing first-order quantitative data for lava flows on Mercury. We also review an apparent topographic anomaly within two broad channels that suggests that long-wavelength modification of the regional topography has occurred since channel formation. Finally, we synthesize our observations and relate the development of the flow feature assemblage to the volcanic history of Mercury in general. 2. Methods [7] The basis for our photogeological mapping was a Mercury Dual Imaging System (MDIS) [Hawkins et al., 2007] wide-angle camera (WAC) global image mosaic [cf. Becker et al., 2012], with a mean resolution of 250 m/px. This base map was supplemented by MDIS narrow-angle camera (NAC) high-resolution targeted images (at ~25 m/px resolution) for areas of specific interest. All data were projected and analyzed within an ESRIW ArcMap™ 10.0 geographical information system (GIS) (Figure 2). [8] Our mapping approach involved delineating the Figure 1. An orthographic sketch centered at 45 N, 90 E, fl showing the locations and spatial extent of (label 1) the northern margins of landforms we attributed to lava ow and yielded smooth plains [Head et al., 2011], outlined in black, and an interconnected network of outlines that encloses the (label 2) the contiguous smooth plains deposits described smooth plains deposits within the northern assemblage in this study, outlined in red. (Figure 3). These outlines served as the basis for such morphometric determinations as channel length, flooded basin diameter, and coalesced depression diameter. A [4] A major challenge associated with understanding the gridded digital terrain model (DTM), derived from Mercury development of large volcanic provinces, such as Mercury’s Laser Altimeter (MLA) topographic profiles [Zuber et al., northern smooth plains, is the burial of source vents and 2012], provided elevation data where coverage permitted flow-related features by successive lavas, rendering the deter- and from which we identified regional topographic trends mination of flow directions, unit thicknesses, and detailed and downslope directions. emplacement histories difficult. Such burial adds to the [9] To calculate lava flow rates through a single broad paucity of primary volcanic landforms on Mercury [Strom channel, Angkor Vallis, we employed a set of equations et al., 1975]. Although smooth plains are abundant, the for confined (channel- or tube-fed) flows, taken from surface of the innermost planet lacks the large shield Williams et al. [2001a], to calculate iteratively the Reynolds constructs and abundant erosional rilles commonly observed number (Re), friction coefficient (l), and flow velocity (u) on other volcanic worlds [e.g. Hulme, 1973; Whitford-Stark for two candidate lava viscosities. By then taking an and Head, 1977; Greeley and Spudis, 1981; Head et al., estimate for the volume of volcanic fill within a basin at the 1992; Schenk et al., 2004], and evidence for intrusive activity assumed downstream end of Angkor Vallis, we determined is also scarce [Head et al., 2009b; Klimczak et al., 2010]. corresponding basin fill times (see section 3.2). [5] Mercury is not entirely without volcanic flow features, however, as evinced by the recent discovery of an assemblage 3. Results of channels, coalesced depressions, and related landforms that escaped burial and are proximal to the southeastern margin of 3.1. Photogeological Mapping the northern smooth plains deposits [Byrne et al., 2011; Head [10] Our study area encompassed the northern flow feature et al., 2011] (Figure 1, label 2). Together with other candidate assemblage, which lies almost entirely between the 50N lava flow features elsewhere on Mercury [Hurwitz et al., and 70N parallels and the 90E and 120E meridians 2013], these landforms can provide important information (Figure 2). The flow features themselves are situated within about the behavior, duration, and volume of surface volcanism a region of rough, intercrater plains [e.g., Trask and Guest, on Mercury that, in turn, serves as a basis for understanding 1975; Denevi et al., 2009]. The northern plains documented the development of smooth plains deposits across the planet. by Head et al. [2011] lie to the northwest, and the Caloris [6] In this paper, we document in detail this northern flow- exterior plains [e.g., Murchie et al., 2008] are to the southeast. feature assemblage,