Mathematics – Logic
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..662e&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
Introduction: The Noachian highland Libya Montes, located at the southern edge of Isidis Planitia, represents one of the oldest regions [1] that have been most heavily modified by fluvial processes. In this ancient region, long and broad "longitudinal valleys" are pervasive. They are indication for intensive, longterm and multiple fluvial processes [2-4]. In addition, widespread "dendritic valley networks", particularly their extended branching, are evidence for atmospheric precipitation [5]. Within the Libya Montes, three distinct valley systems are defined by [2]: a western, a middle and an eastern system. Here, we present our morphologic and stratigraphic investigations of the eastern valley system located between 75°E and 90°E and 5°N and 5°S (Figure 1). Here we focus on investigating morphologic features in order to produce a morphologic map. A morphologic map was constructed on the basis of visual mapping. In a second step, we performed crater counts for our geomorphologic units in order to determine their stratigraphy. Data: Several datasets from current missions, including the High Resolution Stereo Camera (HRSC), the Context Camera (CTX), the High Resolution Imaging Science Experiment (HiRISE), the Mars Orbiter Camera (MOC), the Mars Orbiter Laser Altimeter (MOLA) and the Thermal Emission Imaging System (THEMIS) were used to create our map. These images were also used for the regional and local analysis of morphologic features and their stratigraphic relationships. Morphology: Our morphologic mapping builds on the classification of [2], but has been altered based on newer, higher resolution data. Therefore, existing units were modified and a couple of new units were added. Our detailed morphologic map is shown in Figure 2. The Noachian highlands (Nm) are the oldest surfaces in the study area and are shown in dark-brown in the morphologic map (Figure 2). Adjacent to the basement material, unit NHf displays steep and heavily degraded slopes. Widespread within the NHf are small "dendritic valley networks" (Figure 3) which are shown in dark green and labeled with NHf_d. The "longitudinal valleys" are long stretched and broad in geometry (Figure 3). They are shown as unit NHf_l. Downstream and within their middle reaches, the "longitudinal valleys" become incorporated into broad plains. The "longitudinal valleys" are interrupted several times by "intermontane plains" Hi_ip (light blue) and "highland basins" Hi_hb (dark blue). Stratigraphy: In total, we performed 141 crater counts on 62 homogeneous surface units. Our model ages [6-7] determined by crater counts vary between ~4.1 and ~3.3 Ga. This corresponds to the period from the middle Noachian to the upper Hesperian [6]. The oldest model ages were measured in the "Noachian massifs" (Nm) within the range from ~4.1 to ~3.8 Ga. These remnants of the ancient highlands show an average age of ~4.0 Ga. The "dendritic valley networks" show the same average age of ~4.0 Ga, which corresponds to a formation within the middle Noachian. Our crater counts reveal that the formation of the dendritic drainage patterns occurred within ~300 My, between ~4.1 and ~3.8 Ga. The sloped surfaces next to the "longitudinal valleys" exhibit model ages of approximately ~3.8 to ~3.5 Ga. The main valley of eastern Libya Montes shows a model age of ~3.5 Ga (Figure 4). Hence, the surfaces are upper Noachian and lower to upper Hesperian in age. The average age of approximately ~3.7 Ga corresponds to the Noachian-Hesperian transition. The difference in age between the older middle regions of the "longitudinal valleys" and their younger downstream regions, amounts to nearly ~200 My. Our results are consistent with the ages of the western valley system, which has been dated to be ~3.7 - ~3.3 Ga old [4] The age determinations for units Hi_ip and Hi_hb showed a formation period of about ~500 My within the range from ~3.8 to ~3.3 Ga. The average age amounts to ~3.6 Ga. We find that in this part of Libya Montes, fluvial activity might have come to an end between 3.6 and 3.5 Ga ago. Conclusions: Based on our study, we conclude that the Libya Montes highlands are one of the oldest regions on Mars and have been heavily modified by fluvial processes. Based on our age determinations we conclude that the "dendritic valley networks" (average age ~4.0 Ga) have formed earlier than the "longitudinal valley" systems (average age ~3.7). Due to the distinct morphology of the "dendritic valleys" and their emergence at local summits, we propose that an initial valley formation in the Noachian Epoch was controlled by precipitation and surface runoff. We propose that fluvial activity was high during the Noachian, with a maximum at the Noachian- Hesperian transition. References: [1] Scott & Tanaka (1986) U.S. Geol. Surv. Misc. Invest. Ser., Map I-1802-A. [2] Crumpler & Tanaka (2003) JGR, 108, ROV 21-1. [3] Jaumann et al. (2005) GRL, Vol. 32, L16203. [4] Jaumann et al. (2007) LPICo. 1353. [5] Mangold et al. (2005) Science, Vol. 305, 78-81. [6] Hartmann & Neukum (2001) Space Sci. Rev. 96, 165-194. [7] Ivanov (2001) Space Sci. Rev. 96, 87-104.
Erkeling G.
Hiesinger Harald
Jaumann Ralf
Reiss David
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