Sixth International Conference on Mars (2003) 3091.pdf TONGUE-SHAPED LOBES ON MARS: MORPHOLOGY, NOMENCLATURE, AND RELATION TO ROCK GLACIER DEPOSITS. D. R. Marchant1 and J. W. Head2, 1Department of Earth Sciences, Boston University, Boston, MA 02215 [email protected], 2Department of Geological Sciences, Brown University, Providence, RI 02912. Introduction: Recent work based upon Mars Global Different processes and environments could produce Surveyor (MGS) data [1,2], in conjunction with previous similar or gradational landforms and thus the landforms analyses of Viking data [3], suggests that rock glaciers, simi- themselves might not be unequivocal indicators of a specific lar in form to those found in polar climates on Earth, have origin. Thus, we follow Whalley and Azizi [1] and suggest been an active erosional feature in the recent geologic history that non-genetic descriptive terms be used for the martian of Mars. A wide range of literature exists describing rock features. glacier characteristics, form, and distribution, but diversity of Martian Features: We have developed a descriptive, opinion exists on rock glacier nomenclature and genesis. non-genetic nomenclature for the topography, features and In this contribution we outline the two-fold genetic clas- deposits associated with these structures. sification of Benn and Evans [4] for terrestrial rock glaciers, We call on established anatomical morphology for no- and then propose a non-genetic descriptive set of terms to be menclature. The tongue-shaped lobe can be divided into the applied to martian features. tip, or apex, the blade (the flat surface just behind the tip), Terrestrial Classification: Benn and Evans [4] applied the body and its rear part (the dorsum), and the tongue root. a simple two-fold genetic classification system to terrestrial Ridges along the edges are called marginal ridges and those rock glaciers, dividing them into periglacial (Fig. 1) and concave outward ones at the tongue apex are called apical glacial (Fig. 2) groups. ridges. Within the tongue body and in the dorsum area occur Periglacial Rock Glaciers. These are purely periglacial a series of chevron-shaped ridges with the apex of the chev- features that form by deformation of the lower parts of talus rons pointing toward the tongue apex. The central part of the slopes into bulging lobes, and do not involve the presence of tongue is depressed. The tongue root zones occur in the up- glacial ice (Fig. 1). This will yield either isolated or overlap- per parts of crater walls in subdued alcoves. The tongue ping protalus lobes (‘L’ in Fig. 3), and protalus ramparts, as width narrows by at least a factor of two from the root zone seen in the middle left and far right of Fig. 3. toward the tongue body. Chevron ridges occur primarily Glacial Rock Glaciers. These originate from the pro- within this transition zone. Surrounding the tongue-shaped gressive burial of a core of glacier ice by a boulder-rich de- lobes are linear to elongate smoothed and subdued ridges and bris mantle, and its subsequent downslope movement and mounds, some with scalloped margins. The scalloped mar- deformation (Fig. 2), and are considered transitional to de- gins occur on the inside and the broad structure often mimics bris-covered glaciers. These are often found in cirque basins the tongue shape. In Fig. 4, subdued channels emerge from (Fig. 3), where significant amounts of debris are deposited on the apices of these broad features. We interpret these broad glacial surfaces by slope processes and debris is concentrated features to be remnants of earlier larger and more extensive by melt-out or sublimation processes. lobes. The contrasting morphology of these types of deposits is Terrestrial Analogs: The Antarctic Dry Valleys offfer a illustrated in Fig. 3. On the left, overlapping protalus lobes Mars-like environment in which many of the features shown form at the base of a rock glacier forming at the lower art of in Fig. 2 can be seen. In Fig. 6, near Pearse Valley, protalus the valley wall, while an extensive, single protalus lobe is lobes are being produced. Water draining from the rock seen at the base of the valley wall in the center right. In con- outcrops in the background is soaking into the ground and trast, in left center, a valley glacier is seen emerging from an producing gelifluction lobes in the permafrost. Continued ice cap on the summit of the plateau. Earlier advance of this drainage is dissecting the lobes and forming deltas and fans valley glacier overrode the periglacial rock glaciers, extended between the lobes and the lake on the valley floor. onto the valley floor, and formed a moraine. Subsequent In Fig. 7, near Beacon Valley, a cirque hosts the accumu- retreat to its present position was accompanied by melting lation zone of a rock glacier extending for ~2 km down a and drainage, locally breaching the moraine. Because of its steep-sided valley. On the opposite side, snow and ice are origin from the ice cap at the summit of the plateau, the val- accumulating in cirques to form glaciers that extend down- ley glacier is not covered with debris and has a dominantly slope, producing distinctive tongue-shaped moraines at their ice-free surface. margins. In the center, a major cirque disrupts the linear continuity In Fig. 8, a tongue-shaped ice-cored rock glacier extends of the valley wall and provides a local environment for the down a narrow valley into the surrounding valley floor. Note accumulation of snow. Continued accumulation builds a that the ridges on the rock glacier continue throughout the glacier which becomes covered with debris because of the length of the feature, indicating the presence of a surface till steep and active slopes of the cirque walls (Fig. 2). The ice- and its deformation into nested ridges. Compare this rock cored glacial rock glacier flows downslope out of the cirque glacier morphology to the moraine in the glaciers in Fig. 7, and down the valley wall onto the valley floor where it and to the lobe-like morphology of the protalus lobes of broadens to form a spatulate or Piedmont-type glacial rock gelifluction origin in Fig. 6. The transverse profile of the glacier. In the middle-right, a smaller cirque also forms an rock glacier is also flat to convex, as opposed to the concave environment of snow and ice accumulation and debris cover nature of the lobate moraine of the glacial deposits in Fig. 8 from the cirque walls. Subsequent melting of glacial ice once the ice has retreated. within the two glacial rock glaciers has caused a linear de- Conclusions: On the basis of the characteristics of the pression and stream to form in the tongue-shaped rock gla- tongue-shaped lobes on Mars, their associated features, and cier. Melting has caused nicks, a small lake, and streams in comparison to features in the Antarctic Dry Valleys of the spatulate rock glacier. known origin, we can reach the following conclusions: Sixth International Conference on Mars (2003) 3091.pdf 1) A variety of features on Mars are very similar to fea- age of ice in the original deposit was very high, and tures on Earth that form in glacial and periglacial sublimation and melting led to retreat, and subsi- environments. dence and downwasting of any debris cover, leaving 2) Confusing nomenclature, genetic classifications, marginal morainal ridges as a main feature. possible form convergence or equifinality, and un- 7) The presence of fainter, broad lobe-like features certain origins of many of the terrestrial examples with scalloped margins of similar orientation sug- all make direct application of general Earth morpho- gest the former wider extent of such activity. logical comparisons to Mars difficult. 8) The lack of cross-sectional convexity in these 3) We therefore have developed a descriptive and non- tongue-shaped lobes and related deposits suggests genetic nomenclature for these features. that the ice involved in their formation is now 4) Careful comparison of the Mars features to well- largely gone. This suggests that conditions in the studied Earth analogs in the Mars-like environment past favored the formation of active glaciers and of the Antarctic Dry Valleys can lead to insights into glacial landforms, and that the present time is more the origin of these features on Mars. equivalent to an interglacial period. 5) The morphology and characteristics of the Mars fea- 9) The presence of these features on pole-facing inte- tures examined in this study have been carefully rior crater walls suggests that this micro- compared to three types of features in the Antarctic environment is very favorable for the accumulation Dry Valleys: 1) gelifluciton lobes, 2) rock glaciers, of snow and the initiation of local glaciation. and 3) alpine glaciers. 10) This type of glaciation appears to be a significant 6) The tongue-like lobate, concave nature of these fea- process in the modification of crater walls and tures is very similar to alpine glaciers and debris- floors. covered glacier deposits. In these cases, the percent- References: [1] Whalley, W. B. and Azizi F. (2003) JGR, 104 [5] Johnson, P.G. (1984), Annals of the Association of American (E4),8032, doi: 10.1029/2002JE001864. [2] Head J. W. and Mar- Geographers, 74, 408-419. [6] Martin and Whalley (1987), Progress chant, D.R. (2003) Geology, (accepted; in press). [3] Lucchitta, B. K. in Physical Geography, 11, 260-282. [7] Humlum, O. (1982), Norsk (1981) Icarus, 45 (2), 264-303. [4] Benn, D.I. and Evans, D.J.A. Geografisk Tidsskrift, 82, 59-66. [8] Potter et al. (1998), Geograf- (1998) Glaciers and Glaciation, Arnold Publishers, London. iska Annaler, 80, 251-265.