The Western Libya Montes Valley System on Mars: Evidence for Episodic and Multi-Genetic Erosion Events

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THE WESTERN LIBYA MONTES VALLEY SYSTEM ON MARS: EVIDENCE FOR EPISODIC AND MULTI-GENETIC EROSION EVENTS. R. Jaumann1,2, A. Nass1,3, D. Tirsch1, and D. Reiss4; 1DLR, Inst. of Planet. Expl. Rutherfordstrasse 2, 12489 Berlin, Germany ([email protected]); 2Dept. of Earth Sciences, Inst. of Geosciences, Remote Sensing of the Earth and Planets, Freie Universitaet Berlin, Germany; 3Dept. of Geographie, Geomatics Section, University of Potsdam, Germany; 4Inst. of Planetology, Westfälische Wilhelms-Universität, Münster.

Abstract: Martian valley networks have been cited as power and thus large amounts of water in order to in- the best evidence that Mars maintained flow of liquid cise valleys even into loose sediment and to move water across its surface. Although internal structures gravel and blocks. Moreover, on Mars we have to ex- associated with a fluvial origin within valleys such as pect compact basaltic rock as the common surface ma- inner channels, terraces, slip-off and undercut slopes terial. Thus, it might not be likely that spring-driven are extremely rare on Mars [1] such features can be erosion processes like groundwater exfiltrating along identified in high-resolution imagery [e.g. 2,3]. How- the base of a headwall and subsequent collapse of the ever, besides internal features the source regions are an head front are the dominant erosion process to incise important indicator for the flow processes in Martian the Martian valley networks [22]. Little is known valleys because they define the drainage area and thus about paleo-discharge values but the few cases where constrain the amount of available water for eroding the discharges of exposed and preserved interior channels valley network. Furthermore, the morphology of the have been estimated show that the amount of water source regions and their topographic characteristics within the valley systems coincide with that of dimen- provide information about the origin of the water. On sionally comparable terrestrial rivers valleys [23,24,3]. Mars, valley networks are thought to have been This indicates that the erosion of the valley networks formed by retreating erosion where the water is sup- was the result of water running on the surface at least plied from the sub-surface. However, the mechanisms periodically in terms of high magnitude flash floods that are responsible for the release of ground water are [3]. However, the water release mechanism is still con- poorly understood. The three-dimensional highly re- troversial. As precipitation is unlikely due to climate solved data of the High Resolution Stereo Camera conditions [13,14] and seepage is equivocal [22] other (HRSC) on the Mars Express Mission [4] allow the possibilities might be short anomalous wet periods detailed examination of valley network source regions. immediately following large impacts [25] or basal melting of large snow deposits [26]. In this paper, we Introduction: Martian valley networks superficially explore a region in the western Libya Montes where a resemble terrestrial drainage systems and thus have well-developed valley system emanates from the edge been taken as evidence that Mars maintained flow of of a lava front indicating magma with ground ice or liquid water across the surface [5,6]. Almost all of snow pack interaction as possible water release mecha- these valleys are concentrated in the ancient cratered nism. Moreover, at least two lava streams of different Martian highlands and are considered to be very old. ages show the same valley source features that might The valley networks have been cited as the best evi- be due to multiple water release events. In addition, an dence of warm and wet conditions early in the history interior channel allows the estimation of palaeo- of Mars [e.g. 7,8,9]. An early warm and wet Mars with discharges and waterfall-like features at the valley sustained precipitation is, however, being questioned heads indicate specific water release processes. because of the reduction of an early atmosphere due to losses of molecules by impact erosion [10], solar wind Western Libya Montes: The Libya Montes at about [11], significant lost of CO2 molecules due to thermal 80°E to 100°E and 2°S to 6°N border the southern escape [12], the theoretical difficulties to sufficiently margin of the Isidis Planitia rim. warm Mars with a CO2-H2O greenhouse [13,14] dur- The Libya Montes are one of the oldest and most ex- ing the lower sun output in the planet’s early history tremely eroded surface units (Nplh) on Mars [27,28]. [15], and the lack of large weathering deposits on The contact between the northern plain materials of Noachian surfaces [16,17]. Nevertheless, the morphol- Isidis Planitia and the Libya Montes highlands appear ogy of many valley networks indicates that fluvial ero- at about -3.5 km altitude, between 81°E to 95°E at sion was responsible for the valley formation [e.g. about 4°N. The Libya Montes consist of massifs and 5,6,18]. Although the basic erosional process is still hummocky terrain interbedded by local basins and controversial, geologic interpretations focus on ground dissected and flooded areas. Three large valley sys- water sapping [6,18] or surface runoff [19,9,20] or a tems build the main drainage of many smaller valleys combination of both [21]. The morphology and dimen- throughout the Libya Montes. Hence these valleys are sions of valley networks require high-energy erosional

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unnamed we follow the nomenclature of [29], who mid-Amazonian and also bear evidence for changing called them Western, Middle and Eastern Valley. water release mechanisms with time. Noachian massif materials (unit Nm) form rugged mountainous features and have been categorized as the Source Region: Whereas the source region of the east- type area of “mountainous terrain” [30]. Individual ern part of the valley is dominated by a dendritic dis- massifs occur as isolated peaks or as arrays of peaks sected drainage area within very old geologic units resembling the remnants of impact crater rims as well (Fig. 2), the style of the western valley catchments is as possible remnants of the Isidis basin concentric different. There the source region of the valley is cov- structure. These oldest unit of the Libya Montes have ered by a series of lava flows. Even the upstream part mostly been formed in context with the Isidis event of the western valley is covered by lava flows that and pre-date the earliest recorded fluvial events. How- overflow the original interior channel. Within this part ever all massifs bear evidence of extensive erosion on of the valley a younger interior channel cuts the lava their flanks as indicated by small valley-and-spur relief flow (Fig.3) indicating multiple flow events. Tributar- [29] postdating their formation. The mountainous ter- ies are rare, short, and not dendritically arranged in rain are surrounded by rugged areas (unit NHf) inter- this area. The valley system emanates directly from a preted as either pediments or bajadas cut and deposited lava plain and cascades down over from the adjacent massifs [29]. Short drainage networks are common in these fluted and dissected materials that postdate the massif materials unit but are still Noachian in age. The massif material unit and their adjacent deposits are dissected by extended relatively integrated broad valley systems. These valleys are characterized by floodplain-like surfaces dissected by channels. These dissected plain (unit Hd) stratigraphi- cally occur in the late Noachian and extend into the late Hesperian [29]. Topographic lows in crater floors, between the massifs and depressions in the larger valley systems may have acted as temporary catchment basins and are filled with sedimentary intermontane plains and highland basin materials (unit Hi) [29] of similar ages as unit Hd. The dissected plain unit builds the main drainage area of the Libya Montes as the Western, Middle, and Eastern Valley. All main valley Fig. 1: Western Libya Montes valley system (left: systems terminate in Isidis Planitia with clear evidence HRSC nadir image; right: MOLA, blue to white indi- of depositional fans of relatively rocky material within cates – 5000m to 3000m). the Hesperian terminal plains material (unit Ht) that marks the transitional zone of the basin margin [29]. The Syrtis Major shield volcanoes Nili Patera (8.9°E, 67°N) and Meroe Patera (6.9°N, 68.6°E) are located west to the Libya Montes. Distal lava flows (unit Hsm, [28]) of the Syrtis Major Planum volcano system in- vaded the western part of the Libya Montes reaching the source regions of the Western Valley system. The Western Libya Montes Valley cuts the highlands between 1.4°N to 3.5°N and 81.6°E to 82.5°E and originates in a region that is affected by the Syrtis Ma- jor lavas. The valley consists of a western and eastern branch and drains down to Isidis Planitia over a distance of 400 km (Fig. 2). Most of its distance the western branch of the valley exhibits an interior channel that allows to constraint palaeo-discharge and erosion budgets [3]. In this paper, we focus on the Western Fig. 2: Dendritic pattern of the eastern branch of the Valley system because it documents the erosion his- western Libya Montes valley system (THEMIS night- tory of the Libya Montes from the mid-Noachian to the time IR image).

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old highland materials for a distance of about 20 km ing to discharge analyses discussed in [3], the amount and a height difference of 700 m (Fig. 3). The stag- of water released within this Amazonian aged interior gered cascades have slopes of 6°. This is high com- channel is estimated to be about 400 m3/s. This is an pared to the overall valley slope of 0.1° but small com- order of magnitude less than the discharges measured pared with slopes of waterfalls on Earth. However if for the early Hesperian period in the same region [3]. we consider slope reduction by erosional undercut and collapse, this remaining slope will coincide with for- mer cataracts. Although we cannot exclude an early period of precipitation, as indicated by the dendritic pattern of the eastern branch, most of the western valley has been formed by retreating erosion caused by subsurface water release.

Fig. 3: Source region of the western Libya Montes valley system. The valley emanates abrupt at the end of a lava flow, cascades down the boundary of old highland materials and incises into younger lava flow (HRSC stereo image). Fig. 4: Ages of different surface units in the western Libya Montes valley source region as derived from The close correlation of valley erosional structures and crater frequency measurements (see Fig. 4a, 4b, 4c for lava emplacement indicate a volcanically triggered agedating plots of areas marked by asterisk). water release mechanism either by hydrothermally driven expulsion of groundwater or more likely by melting and mobilizing ground ice due to heat induced by lava.

Stratigraphic Sequences: The analysis of stratigraphic sequences in the western branch of the valley system in terms of crater frequency measurements [31,32] indicates a series of subsequent volcano-fluvial events (Fig. 4, 5). In the Noachian < 3.7 Ga, Isidis rim material (Nm, Nplh) was initially fluvially eroded and erosional debris (Hd1) was deposited [3]. In the Hes- perian (3.5 to 3.4 Ga), volcanic activity (Hs1) triggered water release followed by deposition of fluvial sediments (Hd2) 3.35 Ga ago [3]. In the Amazonian volcanic activity occurred during the period 1.4 to 1.1 Ga ago with lava flows (As2 and As3) and related fluvial activity as indicated by the Fig. 4a: Crater frequency measurement for area * interior channel in Fig. 3, which cut the lavas of unit (absolute age determination after [31]). As3. The interior channel dissecting lava flow As3 might have been the last fluvial event in this region. Accord-

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Summary: The western Libya Montes valley was epi- sodically active for about 2 billion years with major episodes in the Noachian (> 3.7 Ga), the Hesperian (3.5 to 3.4 Ga), and the Amazonian (1.4 to 1.1 Ga). Whereas precipitation may have dominated the fluvial activity during the Noachian as indicated by the dendritic drainage pattern in the older eastern branch, the Hesperian and Amazonian fluvial activities might have been triggered by volcanic processes such as ground ice melting or hydrothermal water release. The Hespe- rian discharge rate of 4800 m3/s indicates fast erosion of the valley that might have been cut down within a million years [3]. This suggest rather episodic than sustained flows, which is consistent with the observa- Fig. 4b: Crater frequency measurement for area ** tion of volcanically related water release processes. (absolute age determination after [31]).

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