Origin and Evolution of the Vastitas Borealis Formation in the Vicinity of Chryse and Acidalia Planitiae, Mars

46th Lunar and Planetary Science Conference (2015) 1457.pdf

ORIGIN AND EVOLUTION OF THE VASTITAS BOREALIS FORMATION IN THE VICINITY OF CHRYSE AND ACIDALIA PLANITIAE, MARS. M. R. Salvatore1 and P. R. Christensen1, 1School of Earth and Space Exploration, Arizona State University, 201 E. Orange Mall, Tempe, AZ 85287-6305, [email protected].

Introduction: The martian surface is characterized Methods: Morphological and stratigraphic properties by an ancient topographic dichotomy, with heavily cra- were investigated using the High Resolution Imaging tered highlands to the south and the less cratered lowlands Science Experiment (HiRISE [11]), Context Camera to the north [1]. The Vastitas Borealis Formation (VBF) (CTX [12]), and Thermal Emission Imaging System is the uppermost geologic unit over much of the northern (THEMIS [13]) visible camera. Visible/near-infrared lowlands, is separated into an interior and annular mar- (VNIR) spectral data were derived from the Compact ginal unit [2], and is thought to represent the final stages Reconnaissance Imaging Spectrometer for Mars (CRISM of water-related sedimentary activity in the northern [14]) instrument, while thermal infrared (TIR) spectral plains of Mars [1-5]. Many studies have identified unique data were acquired using the THEMIS infrared camera morphological features within the VBF [2, 5, and refer- and the Thermal Emission Spectrometer (TES [15]) in- ences therein], including subdued topography [e.g., 3], strument. THEMIS nighttime infrared data were used to mud volcanism [e.g., 6], graben networks [e.g., 7], thin derive the thermal inertia of different surfaces [16], which and lobate flow features [e.g., 8], and sedimentary layer- is indicative of the grain size and/or induration of the ing in the shallow subsurface [9]. However, only recently surface materials. Mars Orbiter Laser Altimeter (MOLA has a single hypothesis been proposed to explain all of the [17]) data were used for topographic analyses. newly available datasets [5, 10]. In this study, we evalu- Results and Discussion: We assess the five predic- ate the hypothesis that the VBF was derived from the tions listed earlier as a means of testing the hypothesis burial, compaction, and expulsion of water-laden sedi- that the VBF formed through the burial, compaction, and ments (“mud”) following the discharge of the circum- expulsion of mud from the shallow subsurface [5]: Chryse outflow channels. This mud expulsion hypothesis (1) Thickening sedimentary layer. The properties of helps to unite the range of morphological, thermophysical, and spectral properties of the VBF and the plains of Chryse and Acidalia Planitiae under a single mechanism. Hypothesis and Predictions: Based on the previous work and recent analyses throughout this region, we pro- pose the following timeline of geologic events (Fig. 1) [5]: (a) the pre-outflow channel landscape of the northern plains; (b) deposition and burial of fine-grained, water- laden sediments derived from the circum-Chryse outflow channels; (c) compaction of these sedimentary materials by subsequent coarser outflow channel sediments; (d) expulsion of the fine-grained, water-laden sediments onto the surface due to overpressurization, resulting in deposi- tional constructs, fracture formation, and diagnostic flow morphologies; and (e) deflation of the surface in response to the removal of these sedimentary materials. Predictions. Using this hypothesis as a geologic framework, we can make several predictions that can be tested using the available orbital datasets [5]. If the VBF formed as a result of the burial, compaction, and expulsion of water-laden sediments, then there should be evi-

dence of (1) a thickening sedimentary layer in the subsur- Figure 1. Hypothesized evolution of the VBF in Chryse and face with distance from the outflow channels; (2) similar Acidalia Planitiae. (a) pre-outflow channel terrain, followed by subsurface properties in the VBF marginal unit and the the deposition of fine (b) to progressively coarser (c) outflow surrounding plains of Chryse Planitia; (3) surface defla- channel sediments. (d) Compaction and expulsion of fine- tion as a result of the expulsion of water-laden sediments; grained, water-laden sediments onto the surface. The formation (4) similar thermophysical properties associated with of mud volcanoes and lobate flow features (1), as well as polyg- materials of the same predicted origin; and (5) regional onal fractures (2) are a result of this (e) Surface deflation in spectral signatures that are consistent with the proposed response to subsurface sediment removal. From [5]. formation mechanism. 46th Lunar and Planetary Science Conference (2015) 1457.pdf

crater ejecta deposits can be used as a proxy to character- interior unit and the surrounding plains of Chryse Planitia ize the properties of the subsurface. Within the VBF mar- also share a similar origin that is unique from the VBF ginal unit and much of the surrounding plains of Chryse marginal unit and mud volcanoes. Planitia, craters of comparable size exhibit widely differ- (5) Regional spectral signatures. The composition of ent ejecta properties, indicative of a highly variable sub- the VBF interior unit in Acidalia Planitia was previously surface. This observation suggests that the subsurface derived using both VNIR [19] and TIR [20] orbital data properties in this region are heterogeneous over short [21]. Both datasets suggest an overall basaltic composi- spatial scales and at shallow depths. Further north into the tion for this material, and both agree that the identified VBF interior unit, crater ejecta are largely homogeneous pyroxene compositions are dominated by low-calcium regardless of crater diameter. These similar properties pyroxenes. Such compositions are more indicative of indicate that the subsurface becomes more homogeneous Noachian-aged basalts than Hesperian-aged basalts [22], to a greater depth with increasing latitude. which is consistent with the plains of Chryse Planitia and (2) Similarity between Chryse plains and VBF mar- the VBF interior unit being derived from materials ginal unit. Sedimentary layers associated with outflow sourced from the late-stage outflow channel activity. channel deposits have been observed outside of the VBF These late-stage outflow channel sediments are derived in the plains of Chryse Planitia [9]. Similarly, crater ejec- from the deepest (oldest) areas within the channels, rela- ta morphologies and thermophysical properties that are tive to the younger materials derived from higher in the common in the VBF marginal and interior unit are also stratigraphic column. observed in the surrounding plains outside of the VBF. Conclusions: The mud expulsion hypothesis for the These observations suggest that the previously mapped origin and evolution of the VBF satisfies all previous and extent of the VBF may simply be a surficial expression of currently observed morphological, thermophysical, and a more widespread subsurface sedimentary unit. The spectral properties of Chryse and Acidalia Planitiae [5, more diagnostic surface properties associated with the 10], and was recently proposed for Utopia Planitia [23]. VBF may only have formed where the subsurface sedi- This formation mechanism indicates that the northern mentary accumulations are thickest and where subsequent plains underwent immense and rapid sedimentary modifi- surface deformation could occur. cation both during and after outflow channel activity. (3) Surface deflation. Evidence for surface deflation is While the presence of a northern ocean on Mars remains observed throughout the VBF [5]. High-standing buttes uncertain, it is clear that oceanic amounts of liquid water and mesas have been identified with perched collar-like were likely required to produce the observed properties morphologies and no observed contact between their ba- associated with the VBF. ses and the surrounding plains. On average, these collars References: [1] Tanaka K. et al. (2014), USGS Map, are 123 ± 36 m above the surrounding plains. These col- 3292. [2] Tanaka K. et al. (2005), USGS Map, 2888. [3] lars are thought to indicate the original height of the sur- Kreslavsky M. & Head J. (2002), JGR, 107, rounding terrain that has since deflated due to the expul- 10.1029/2001JE001831. [4] Lucchitta B. et al. (1986), JGR, 91, sion of material from the subsurface. Applied evenly over E166-E174. [5] Salvatore M. & Christensen P. (2014), JGR, the observed extent of these buttes, a deflation of 123 m 10.1002/2014JE004682. [6] Oehler D. & Allen C. (2010), Ica- represents approximately 3 x 105 km3 of expelled material rus, 208, 636-657. [7] Buczkowski D. et al. (2012), JGR, 117, from the subsurface. Assuming fluid-laden sediments 10.1029/2011JE003934. [8] Tanaka K. (1997), JGR, 102, with a porosity of 35%, consistent with inferences from 4131-4149. [9] Salvatore M. & Christensen P. (2014), Geology, [18] based on the modeled dielectric constant of the sub- 42, 423-426. [10] Ivanov M. et al. (2014), Icarus, 228, 121- surface, the estimated volume of water expelled from the 140. [11] McEwen A. et al. (2007), JGR, 112, subsurface would be 1.05 x 105 km3, which is slightly less 10.1029/2005JE002605. [12] Malin M. et al. (2007), JGR, 112, than the volume of Lake Superior. 10.1029/2006JE002808. [13] Christensen P. et al. (2004), (4) Similar thermophysical properties. Several differ- Space Sci. Rev., 110, 85-130. [14] Murchie S. et al. (2007), ent units throughout Chryse and Acidalia Planitiae exhibit JGR, 112, 10.1029/2006JE002682. [15] Christensen P. et al. comparable surface properties and thermal inertias. For (2001), JGR, 106, 23823-23871. [16] Fergason R. et al. (2006), example, the marginal unit of the VBF and the previously JGR, 111, 10.1029/2006JE002735. [17] Smith D. et al. (2001), identified mud volcanoes exhibit comparable thermal JGR, 106, 23689-23722. [18] Mouginot J. et al. (2012), GRL, inertias (140 J m-2 K-1 s-0.5 and 135 J m-2 K-1 s-0.5, respec- 39, 10.1029/2011GL050286. [19] Mustard J. et al. (2005), tively). Similarly, the plains of Chryse Planitia outside of Science, 307, 1594-1597. [20] Rogers D. & Christensen P. the VBF and the VBF interior unit exhibit identical ther- (2007), JGR, 112, 10.1029/2006JE002727. [21] Salvatore M. et mal inertias (165 J m-2 K-1 s-0.5). These properties suggest al. (2014), EPSL, 404, 261-272. [22] Bibring J.-P. et al. (2006), that the VBF marginal unit and the observed mud volca- Science, 312, 400-404. [23] Ivanov M. et al. (2014), Icarus, noes are composed of the same materials, while the VBF 248, 383-391.