Lunar and Planetary Science XXXIII (2002) 1428.pdf

CHASMA BOREALE AND RELATED NORTH POLAR DEPOSITS: NEW INSIGHTS INTO MECHANISMS OF FORMATION Kathryn E. Fishbaugh1 and James W. Head III2, 1Brown University, Dept. Geol. Sci., Box 1846, Providence, RI 02912, [email protected] , [email protected]

Introduction: Boreale, a large reentrant in the the cap margin. Outflow of meltwater, on the other hand, North Polar cap of (Fig. 1), is distinct in scale, detailed occurs at the base. Therefore, it forms a tunnel [10] that topography, and orientation from the spiraling troughs that begins at the base of the polar cap and may later grow into a characterize the majority of the polar cap. reentrant. One purpose of this study is to characterize the structure (5) There are lobate deposits at the mouth of the chasma and morphology of using new, detailed to- (Figs. 1 and 2a). We have not found any examples of com- pographic data from MOLA [1] (Fig. 2) and high-resolution parable steep-sided lobate or fan-shaped eolian deposits on MOC and Viking images. The second purpose is to review or elsewhere on Mars. If katabatic winds slowly proposed formation mechanisms (glacial flow and ablation carving the chasma were to carry enough sediment to form [2]; outflow by meltwater [3,4]; and katabatic winds and such lobate deposits, then one would expect more gradually- sublimation [5]) and to assess their viability in light of the sloping and more dispersed sediments since katabatic winds new data. increase in speed and strength as they flow from the top of Characteristics and Implications for Formation: To the ice [7,8]. Regardless, we would expect winds to form form the chasma by Fisher's accublation model [2], a topog- dunes of these lobate deposits if they were eolian in origin, raphic depression would already need to exist. If the chasma just as has been formed elsewhere in the North Polar region formed simply by ice flow to create a large lobe, required and has been found within the chasma itself. Rather, the flow rates [2,6] would be much too high. Thus we are left origin of these deposits that we favor is one of deposition with two mechanisms: katabatic wind erosion and out- from meltwater that, when it reached the ice cap terminus flow of meltwater. We list the main characteristics of the and dispersed laterally, unloaded its sediment in a fan. This chasma and related deposits and assess them in terms of behavior has been observed in association with terrestrial these two theories of formation. jökulhlaups [11,12]. (1) Chasma Boreale is by far the largest reentrant in the (6) Streamlined deposits are associated with the smaller polar cap. Meltwater outflow acts from the base of the cap chasmata (Fig. 2) and with the mesa beyond the mouth of and can erode large amounts of ice through the heat of the Chasma Boreale (Fig. 1). While eolian processes may meltwater itself and through heat generated by friction and streamline deposits, wind direction maps produced by Tsoar turbulence. Katabatic winds act from the surface down- et al. [13] do not show directions consistent with the par- wards. There is no evidence (on Earth [7,8] or Mars) that ticular shape of these deposits. Winds have modified these katabatic winds, even aided by sublimation, are strong deposits, however, as there are dunes on top of them, and enough to erode such a large reentrant. A large depression is they may have even been deposited by wind. Outflowing needed to strongly enhance the winds, and no other trough meltwater could mold these deposits into streamlined shapes. has enlarged to such a scale. In addition, the initiation of (7) Examination of all available MOC and high- chasma formation and the evolution of its morphology are resolution Viking images of Chasma Boreale shows that not addressed by katabatic wind models [5]. dunes of all forms are ubiquitous on the floor of Chasma (2) The chasma is oriented downslope from the cap cen- Boreale. No obvious fluvial features were found. As noted ter, and the floor (on average) has a negative slope (Fig. 1). by Benito et al. [4], it is likely that eolian depositional fea- Meltwater collected beneath the cap would (on average) ul- tures would mask any existing fluvial features. Linear ridges timately flow downslope from the point of melting. Of and troughs seen in MOC images of the smaller chasmata course, this orientation is also consistent with a glacial flow are similar to the seen in fretted origin and with katabatic winds. The orientation also fol- channels [e.g., 14, 15, 16] which was interpreted to be post- lows that of water or wind directed by the Coriolis effect. channel formation, late-stage, slope-related modification of However, one of the smaller chasmata to the west does not ice-rich debris which has since undergone sublimation. follow this direction (Fig. 2). (8) The sinuous channel extending to the west from the (3) Chasma Boreale begins at a deep, enclosed depres- westernmost, small chasma (Fig. 2) seems to require outflow sion (Fig. 1), and the smaller chasmata show evidence of of water. Eolian processes are highly unlikely to form such collapse at their head (Fig. 2a). Katabatic winds, even aided sinuosity or such narrow and deep a valley. by sublimation, are unlikely to induce such large-scale col- Summary: We use new, high-resolution MOLA data lapse. On the other hand, outflow of glacial meltwater on and MOC and Viking images of the chasma region to assess Earth is known to be associated with collapse at the point of hypotheses of origin of Chasma Boreale. We find that these melting [9]. In addition, this process acts from the base up- new data support initial formation by several stages of melt- wards, rather than from the surface downwards as is the case water outflow with later modification by katabatic winds and for katabatic winds, so that a depression reaching to the base sublimation. MOLA data also reveal evidence for similar would be more likely. events of smaller magnitude elsewhere on the margins of the (4) The floors of both Chasma Boreale and the smaller polar cap. Still unknown is what triggered the melting, a chasmata have elevations close to that of the surrounding quantitative measurement of flow parameters, when the plains (Figs. 1, 2c). This is not observed in any polar melting took place, and how melting relates to the retreat of troughs except those on the extreme periphery of the cap and the polar cap [17]. Recent work suggests that basal melting is not expected with katabatic wind erosion which should of the polar caps is plausible under intermediate obliquity produce a gradually sloping floor from the point of origin to Lunar and Planetary Science XXXIII (2002) 1428.pdf

FORMATION OF CHASMA BOREALE: K. E. Fishbaugh and J. W. Head III conditions [18]. We are now correlating these events with [11] Maizels, J. (1989). J. Sed. Petrol. 59, 204-223. [12] Brennand, others in the polar and circumpolar region [19]. T. and J. Shaw (1996). Sedimentary Geology 102, 221-262. [13] References: [1] et al. (1999). Science 284, 1495-1503. Tsoar et al. (1979). J. G. R . 84, 81676-8181. [14] Lucchitta, B. [2] Fisher, D. (2000). Icarus 144, 289-294. [3] Clifford, S. (1987). (1984). J. G. R. 89, B409-B418. [15] McGill, G. (2000). J. G. R. J. G. R. 92, 9,135-9,152. [4] Benito, G. et al. (1997) Icarus 129, 105, 6945-6959. [16] Carr, M. (2001). J. G. R., in press. [17] Fish- 528-538. [5] Howard, A. (2000). Icarus 144, 267-288. [7] Ball, F. baugh, K., and J. Head (2000). J. G. R. 105, 22,455-22,486. [18] (1957). Tellus 10, 201-208. [8] Kodama, et al. (1985). Ann. Glaciol. Kreslavsky, M. and J. Head (2002). LPSC 33. [19] Fishbaugh, K., 6, 69-62. [9] Björnsson, H. (1992). Ann. Glaciol. 16, 95-106. [10] and J. Head (2002). J. G. R. 105, in press. Blown, I,. and M. Church (1985). Can. Geotech. J. 22, 551-563.

B

Figure 1. Topographic profiles across the summit of the north polar cap (Profile A-A’) and across the portion of the cap containing Chasma Boreale (B-B’). Top, location of profiles. Bottom, annotated profiles. Note 3 km difference between pole summit and Chasma Boreale floor. Also note the scale differences between the troughs and Chasma Bo- reale, and the similar elevations of the headward Chasma Boreale depressions and the North Polar Basin floor. Pro- files from gridded MOLA topography at a resolution of 200 pix/deg.

C

Figure 2. Structure and relationship of small chasmata and a sinuous channel in the layered terrain at the margin of the north polar cap near Chasma Boreale. (a) MOLA topog- raphic gradient map showing the area just to the east of A Chasma Boreale and an annotated sketch overlain. North (poleward) is to the bottom. Arrows indicate interpreted flow direction. (b) Location map for the profiles shown in c). (c) MOLA altimetric profiles show details of relation- ships of these features.