Mathematics – Logic
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..663p&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Mathematics
Logic
Scientific paper
ABSTRACT Introduction Isidis Planitia is one of the many areas on Mars containing thumbprint terrain (TPT), a term coined to reflect the resemblance in Viking images to fingerprints. Other instances occur in Argyre, Hellas, Arcadia Planitia, and Utopia Planitia. The terrain is found where Greeley and Guest (1987) have defined the Hesperian Ridged Plains (Hvr) unit. However, landforms comprising the TPT in Isidis are markedly different in morphology from those in the northern plains. The purpose of this study is to conduct a systematic examination of the TPT in Isidis Planitia using high-resolution imagery, and to propose a hypothesis for its genesis. Northern Plains TPT Morphology: TPT landforms include branching troughs and medial ridges forming whorled lobes and mounds, most with basal scarps or terraces. TPT has been described as consisting of parallel, en echelon, or nested sets of regularly spaced curvilinear ridges or aligned hills [1]. The ridges were estimated to be 0.5-2.5 km wide and 1-40 km long, with a characteristic spacing of 2-6 km. Whorled lobes of TPT are 75-150 km wide, with heights ranging from 10-200 m. Previous work identified 22 areas of TPT, covering 3000- 420,000 km2 in the Northern Plains at elevations between 0 and -2 km. In Utopia, TPT includes branching troughs and medial ridges [5], and TPT is closely associated with troughs in at least 9 other Northern Plains areas [1]. Northern Plains TPT Origin: MOLA topography supports the hypothesis that TPT and associated trough systems in Utopia and Arcadia Planitae are glacial features [1,2,3]. Possible mechanisms include formation of ridges as moraines and troughs as eskers formed by wet-based continental glaciers. The absence of drumlin fields suggests that the glaciers responsible for forming the topography may have been cold-based and thus did not deform the substrate so as to form drumlins [4]. A puzzling characteristic of Mars alleged glacial landscapes is that they are morphologically pristine, though they must be at least hundreds of millions of years old [1]. Isidis TPT The physical appearance of the Isidis features differs from that of the Northern Plains TPT. Isisdis TPT includes wrinkle ridges and curvilinear ridges. Wrinkle ridges are oriented radially and concentric to the basin structure, form cells of 180 km in diameter, and occur throughout the basin over a range of elevations. They are on the order of 75-150 m high and less than 70 km wide [6]. In this study, we focus on the curvilinear ridges and associated features, using THEMIS daytime IR data. Curvilinear ridges are 10-50 m high, and <5 km wide, with a large number 1km wide. Ridges consist of connected cones with central depressions (30-50 % of basal diameter). Cones are often connected to each other midway through their height but sometimes share portions of their rims as well. Basal diameters of the cones vary from 600-1000 m (nearly twice the size of cones seen in northern plains TPT) [6]. Spatial Patterns Closely spaced ridges are sub-parallel to parallel. Mapping of TPT features in Isidis Planitia shows four domains of distinct morphology (Figure 1). Domain 1 consists of chain-like ridges of cones concentrated to the southern and western regions of the basin. Domain 2 consists of isolated cones localized in the basin center. Domain 3 is the Syrtis Major Isidis Planitia transition zone and consists of clusters of knobs, mesas, and large single scarps [7]. Domain 4 (west of the transition zone and along the outer regions of the basin) consists of smooth terrain lacking a significant number of cones, or knobs and mesas. The most detailed mapping in this study has been completed for Domains 1 and 2. In Domain 1, an apparent pattern emerges along a boundary trending E-W at 12 N. Cone-chains located north of this boundary show a preferential N-S alignment, convex toward the east. Cone-chains located south of the boundary show a preferential E-W alignment, convex toward the south. Cone-chains occupy the region previously mapped as Hvr [8]. Origin of the Isidis TPT features Glacial formation mechanisms are generally accepted for formation of TPT in Argyre, Hellas, Arcadia, and Utopia. The TPT of Isidis Planitia is markedly different in morphology, and so deserves a fresh examination. We look to terrestrial analogues of rootless cones. The underlying mechanism is the interaction between magma (or lava) with a volatile (possibly water) rich substrate. Top-heating A top-heating model would involve lava flows and Vastitas Borealis Formation (VBF) materials known to cover the floor of Isidis [7]. In this model, Syrtis Major eruptions would produce tube-fed lava flows overlying volatile-rich materials derived from the Northern Plains, heating the wet substrate from above; interaction between the hot lava tubes and the substrate would produce chains of rootless cones. This model is expected to play a role in the formation of the TPT features, though it is doubtful if it can act solely to form the features, because the model is topography dependent and cannot readily account for both large- and smallscale patterns. Bottom-Heating The spatial patterns seen in planview of TPT in Isidis Planitia show a strong similarity to surface and seismic expressions of sills the Karoo basin in South Africa and the North Rockall Trough in the NE Atlantic on Earth. This leads to the idea that a sill or cone sheet complex beneath the Isidis basin, possibly linked to Syrtis Major, could drive bottom heating of a volatile rich substrate, leading to the formation of aligned rootless cones comprising the thumbprint terrain of Isidis Planitia. Though similar to the top-heating model, a system of intrusive structures heating from below would be independent of the current local topography of the basin, and is thus favorable. Such a mechanism could further explain the small-scale deviations in spatial alignment from the regional trend. The branch-like evolution of a sill complex can result in the non-uniform development of sills [11], such that daughter bodies of a parent sill can vary in size and vertical and horizontal distribution, and thus result in different abutment relationships, causing varying surface manifestations of the hybrid sill tips. A combination of both models is also a likely candidate for the genesis of TPT in Isidis. We are continuing to investigate the details of the two models by analyzing additional datasets and terrestrial analogues. References [1] Kargel et al. (1995) JGR-E, 100, 5351-5368. [2] Pomerantz, W.J and Head III, J.W (2003) LPSC XXXXIII, Abstract 1277. [3] Chapman M. (1994) Icarus, 109(2), 393-406. [4] Head III, J.W and Marchant (2003) LPSC XXXXIII, Abstract 1247. [5] Scott and Underwood (1991) Proceedings of Lunar Planet. Sci, 21, 627-634. [6] Hiesinger, H. and Head III, J.W (2003) 6th Intl Conf. on Mars, Abstract 3061. [7] Ivanov, M.A and Head III, J.W (2003) JGR-E, 108, E6. [8] Greeley, R. and Guest, J.E (1987), US Geol. Surv. Misc. Invest. Ser., Map I-1802-B. [9] Cartwright, J. and Hansen, D.M (2006) Geology 34(11), 929-932. [10] Hansen, D.M and Cartwright, J. (2006) Journal Geol. Soc. London 163 (3), 509-523. [11] Thomson, K., and Hutton, D. (2004) Bull Volcanology, 66, 364-375.
Ghent Rebecca R.
Pithawala Taronish M.
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