Icarus 277 (2016) 433–441 Contents lists available at ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus Geomorphology and volcanology of Maat Mons, Venus ∗ Peter J. Mouginis-Mark Hawaii Institute Geophysics and Planetology, University of Hawaii, Honolulu, HI 96822, USA a r t i c l e i n f o a b s t r a c t Article history: Full-resolution (FMIDR) Magellan radar backscatter images have been used to characterize the geology Received 23 December 2015 and volcanology of the volcano Maat Mons on Venus. This volcano has often been identified by remote Revised 8 April 2016 sensing techniques as one of the volcanoes on the planet that could have been recently active, and is the Accepted 13 May 2016 highest volcano on Venus with a relief of ∼9 km. The summit of Maat Mons is characterized by a caldera Available online 4 June 2016 complex ∼26 ×30 km in diameter with at least six remnant pit craters ∼10 km in diameter preserved in Keywords: the walls of the caldera, suggesting that multiple small volume ( < 16 km 3) collapse events formed the Venus surface caldera. Four lava flow types, described as “digitate flows”, “sheet flows”, “fan flows” and “filamentary Volcanism flows”, can be identified on the flanks. Three rift zones can be identified from the distribution of 217 pit Geological processes craters > 1 km in diameter on the flanks. These pits appear to have formed by collapse with no effusive activity associated with their formation. No evidence for explosive volcanism can be identified, despite the (relatively) low atmospheric pressure ( ∼55 bar) near the summit. There is also a lack of evidence for lava channels, deformation features within the caldera, and thrust faults on the flanks, indicating that the physical volcanology of Maat Mons is simpler than that of typical martian and terrestrial shield volcanoes. Preservation of fine-scale (3–4 pixels) structures within the pit craters and summit pits is consistent with geologically very recent activity, but no evidence for current activity can be identified. © 2016 Elsevier Inc. All rights reserved. 1. Introduction Detailed geomorphic mapping of the summit area and flanks of the volcano extending up to ∼100–120 km from the summit Over the past 30 years, several investigations have hinted that ( Fig. 2 ) has been conducted here to better characterize the styles Venus is volcanically active today, but none have been defini- of volcanism at Maat Mons. Despite the importance of Maat Mons tive. Episodic injection of sulfur dioxide into the atmosphere for investigating recent volcanism on Venus, the available data > ( Esposito et al., 1988 ), high radar emissivity at elevations 2.5 km sets for such analysis are scarce, even by comparison with else- above the 6051 km mean planetary radius ( Robinson and Wood, where on the planet. Only left-looking Magellan synthetic aperture 1993 ), visible and infrared emissivity measurements of the surface radar (SAR) data are available for the entire volcano (right-look ( Smrekar et al., 2010 ), and enhanced microwave thermal emission data are missing for the summit), and no stereo-derived topogra- ( Bondarenko et al., 2010 ) have all been proposed as indicators of phy ( Lerberl et al., 1992; Gleason et al., 2010 ) is available. FMIDR recent eruptions. Maat Mons (194 °E, 1 °N) ( Fig. 1 ) is possibly the Magellan radar backscatter images were used for this study; at best candidate for a recently active volcano on Venus, by virtue the latitude of Maat Mons, these data have a spatial resolution of of the spatial variability of radar emissivity values at the summit 108 m (cross track) and 110 m (along track) prior to projecting the ( Klose et al., 1992; Robinson and Wood, 1993; Campbell, 1994 ), data. Thus the intrinsic preserved resolution of the radar images > near-infrared spectra ( Shalygin et al., 2012 ), and high ( 8 km) to- is probably no better than ∼150 m x 150 m. Topographic data come pographic relief which suggests that the volcano is still being from the Magellan nadir-looking radar altimeter that has mapped constructed. In addition, Magellan gravity data show that the Atla the surface at a horizontal resolution of 10–30 km ( Ford and Pet- ∼ Regio region ( 10°S to 25°N, 180° to 215°E), where Maat Mons is tengill, 1992 ), so this has resulted in poor knowledge of the sum- located, is one of the areas on Venus that could be situated over mit caldera geometry and the detailed shape of the upper slopes an active hot spot and thus is consistent with the hypothesis that of the volcano ( Fig. 3 ). Furthermore, Maat Mons is located on the Maat Mons could be active today ( Smrekar, 1994; Shalygin et al., Equator at 194 °E, so that the volcano is never visible from Earth- 2012 ). based radar ( Campbell and Campbell, 1992 ) thereby precluding any multi-incidence angle radar studies of the texture of lava flows. ∗ Tel: + 18089566490. Magellan SAR data have an incidence angle of ∼45 ° over Maat E-mail address: [email protected] , [email protected] Mons. http://dx.doi.org/10.1016/j.icarus.2016.05.022 0019-1035/© 2016 Elsevier Inc. All rights reserved. 434 P.J. Mouginis-Mark / Icarus 277 (2016) 433–441 Fig. 1. Location map for Maat Mons. Study area of Maat Mons ( Fig. 2 ) and the NW lava flow field ( Fig. 8 ) are identified. “V1” and “V2” denote the Vega 1 and Vega 2 landing sites. Mosaic covers an area from 30 °N to 45 °S and 58 ° to 215 °E. Part of JPL image PIA00256. Fig. 2. Magellan SAR mosaic of Maat Mons. Superposed contours from the Magellan radar altimeter are at 500 m intervals. White boxes show the locations of subsequent figures. Geographic area extends from 0.3 °S–2.2 °N, 193.4 °E–196.2 °E. Topographic data from Ford and Pettengill (1992) . Magellan image mg_0 024/f0 0n194. See Fig. 1 for location. 2. New mapping of the summit craters. Collectively, the mapping permits insights into several characteristics of the volcano, including: This study of Maat Mons has included an analysis of the distri- bution of pit craters on the flanks, the spatial distribution of lava (a) An investigation of the role of elevation on the degassing flow fields (radar-bright and radar-dark flows), and the morphology of magmas on Venus. In particular, a search for evidence P.J. Mouginis-Mark / Icarus 277 (2016) 433–441 435 Fig. 4. Summit caldera of Maat Mons. There are numerous collapse features (iden- tified in the insert at top right) within the broad collapse feature that forms the Fig. 3. Pair of oblique views of Maat Mons derived from Magellan radar images ∼26 ×30 km diameter caldera. See Fig. 2 for location. Radar look-direction is to- and altimetry. The summit elevation of the volcano is 8860 m, and the lowland to wards the right. Magellan images mg_0 024/f0 0n194/ff21 and ff22. the NW is at ∼−100 m, so there is ∼9 km difference in elevation. These two views are from slightly different angles viewed from the north-east, but the flank profiles ( Senske et al., 1992; Stofan et al., 2001 ). Magellan radar backscat- and geometry of the summit appear to be quite different due to the low spatial resolution of the Magellan topographic data. Numbers identify the same features in ter and altimetry indicate a narrow rim to the west that served to each image. Top view is JPL image PIA00254 (vertical exaggeration is 22.5 ×) and contain the flows on the caldera floor, and this is consistent with lower view is JPL image PIA00106 (vertical exaggeration is 10 ×). the impression that flows from within the caldera have spilled out to the east and the north sides of the caldera. On Earth, it is recognized that calderas are about the same size of explosive eruption at the lower-pressure high elevations, as the magma chambers beneath them ( Walker, 1988; Parfitt and which would set constraints on the volatile content of the Wilson, 2008 ), and many large calderas may have evolved incre- magmas. mentally in response to a succession of moderate-sized eruptions (b) Consideration of the probable size and spatial migration of ( Walker, 1984 ). Collapse features within a caldera are inferred to the magma chamber within the edifice of Maat Mons, based have formed by collapse due to evacuation of a shallow magma upon the distribution of collapse craters within the sum- chamber ( Walker, 1988 ), so that the spatial distribution of the six mit caldera. If there is evidence for different collapse crater smaller pits around the perimeter of the Maat Mons caldera there- sizes, then this might provide information on the size of the fore suggests that the magma chamber within the edifice could magma chamber over time. have migrated with time, comparable to the collapse features iden- (c) An interpretation of the distribution of pit craters and frac- tified within the calderas of Olympus and Ascraeus Montes on tures on the flanks. The spacing and orientation of pit Mars ( Mouginis-Mark, 1981; Mouginis-Mark and Rowland, 2001 ). craters may be used as potential indicators of rift zones Head and Wilson (1992) predicted that large volcanic edifices on within the volcano, placing constraints on the internal Venus should have both a deep and shallow magma reservoir. The “plumbing system”. Specifically, the widths, lengths, and size and distribution of intra-caldera pits may therefore allow in- depths of dikes are important for the identification of the in- ferences to be made on the geometry of the shallow magma cham- ternal structure of the volcano ( Head and Wilson, 1992 ). The ber.