Pavonis Mons Fan-shaped Deposits — Smooth Terrain Surface Ice

Introduction and General Characteristics: The fan-shaped deposits of the Tharsis Montes generally share three major facies: 1) Ridged facies, 2) Knobby facies, and 3) Smooth facies. The ridged facies consists of narrow, parallel, concentric ridges interspersed in places with small hills [1]. The hummocky or knobby facies consists of dense concentrations of small sub-km scale circular to irregularly shaped hills [1] which are clearly highlighted by the 0.6 km wavelength in roughness maps of the area [2]. Finally, the smooth facies consists of broad convex-outward plains superposing both the ridged and knobby units. Elsewhere, we have presented evidence supporting a cold-based glacial origin for the Pavonis deposits [3]. Here, we present evidence suggesting that the smooth facies represents residual ice deposits that have persisted until the present. Previous Suggestions for Origin: Based on Viking orbiter data, several models have been proposed for the fan-shaped deposits, including massive landslides [4], glacial processes [1,5,6,7] and pyroclastic flows [1]. Scott et al [1] suggest that a combination of glacial and volcanic processes account for the emplacement of the fan-shaped deposits of both Arsia and Pavonis, interpreting the smooth facies as ash flow tuffs or lahars. The smooth facies has also been interpreted as a pyro-clastic deposit, which emanated from large graben of the northern flank deposits [7]. We have reexamined the fan-shaped deposits utilizing new data from Mars Global Surveyor (MOLA, MOC), Mars Odyssey (Themis, GRS Suite) and previous Viking data. This analysis, together with an assessment of terrestrial analogs of cold-based glaciers [8] lead us to conclude that the Pavonis fan-shaped deposits are the deposi-tional remains of cold-based glacier(s) that existed in recent Martian history. Pavonis Mons Fan-shaped Deposits: The Pavonis fan-shaped deposits (Figure 1) extend approximately 250 km northwest of the shield base along a N35°W trend [7]. These deposits cover an area of about 7.5 x 10 4 km 2 , approximately half of the area covered by the Arsia deposits. The Pavonis deposits consist of two large regions of knobby terrain separated by a region of the ridged facies and the overlying smooth facies. Smooth Facies: The smooth facies of the Pavonis fan-shaped deposits covers an area of approximately 1.2 x 10 4 km 2 (Figure 2). It is mainly concentrated in one continuous deposit north of the shield extending into the central regions of the fan-shaped deposits. The smooth deposits are characterized by broad, gentle slopes with very …