Workshop on 2001 2510.pdf

MELAS : POTENTIAL LANDING SITE FOR THE MARS 2001 MISSION. F. Costard1, N. Man- gold1, Ph. Masson1, D. Mege2 and J.P. Peulvast1. 1UMR 8616 CNRS, Laboratoire de Géologie Dynamique de la 2 Terre et des Planètes, Univ. Paris-Sud, 91405 Orsay Cedex, France, [email protected]. Département de Géotec- tonique, ESA CNRS 7072, case 129, Univ. Pierre et Marie , 75005 Paris, France.

Introduction : The French Working Group on These deposits would be volcanic ash interbedded landing site retained Melas Chasma, with layers of relatively resistant welded tuffs or (figure 1) among its highest priorities for in situ ex- mafic lava flows erupted within the chasmata [2, 3, ploration of Mars. Therefore this region is proposed 4]. Alternatively, these layers could be lacustrine by the authors as one of the most interesting site for deposits, or both volcanic and lacustrine materials - the 2001 mission. Its characteristics fit all the engi- i.e. volcanism in paleo-lake [4, 7]. neering constraints including landing ellipse and slopes [1]. Since Valles Marineris seems to be un- likely to be selected as a landing site for the Mars Sample Return missions, the Mars Surveyor mission in 2001 will be the last opportunity in the next decade to improve what we know about Valles Marineris.

Geomorphological context : The central Valles Marineris is the widest part of the equatorial trough. It is 260 km wide and 380 km long. The walls and plateau edges are dissected by large amphitheaters and displays wide regional collapses which were possibly formed along and around multiple parallel weakness zones (figure 2) initiated by tectonics [2, 3]. Removal of material may be the result of sapping processes, ice-lubricated creep, sublimation, subsur- face drainage [4] or karst collapse [5]. The removed material may have been partly incorporated into the canyon interior stratified deposits, possibly due to the drainage of a postulated lake.

Figure 2 : Block diagrams of Central Valles Marin- eris. (A) Present morphology. (B) Present structural pattern, (C) Previous structural pattern (first stage of subsidence and deposition of layered deposits). From [2, 3].

Figure 1: . Controlled photomosaic M500K-10/72CM showing the layered rocks bench. Volatile characterization of the Melas Chasma : The presence of rampart craters in the surroundings Scale : 150 x 125 km. of the troughs provides clues about the volatile distri- The nature of these stratified deposits (figure 1) is bution around Valles Marineris. Ejecta mobility can uncertain and has been discussed by many authors. be expressed by the EM ratio, of the maximum di- Several hypotheses were proposed including eolian, ameter of ejecta deposits divided by the diameter of alluvial, evaporitic, lacustrine, or volcanic origins [6]. Workshop on Mars 2001 2510.pdf

MELAS CHASMA LANDING SITE: F. Costard et al.

the parent crater. This ratio, termed the ejecta mobil- [8, 12]. Another hypothesis is the presence of ground ity (EM ratio) is assumed to correlate with the quan- water below the permafrost [13]. According to the tity of volatiles involved during emplacement [8, 9, latter author, water that fully saturates the pore space 10, 11]. The relationship between the distribution of of the rock may have produced regolith mass move- relatively high mobile ejecta around rampart craters ments, at the base of some high walls, e.g. in western and the occurrence of erosional landforms related to , though the porosity is reduced at important wall retreat or dissection (figures 3 and 4) depth by the effect of lithostatic stresses, [14]. It may strongly suggests that volatiles may have contributed have induced sapping along discontinuities in more to the widening of Central Valles Marineris troughs coherent rocks ().

Figure 3 : Distribution of rampart craters within ± 5° of the Valles Marineris. High EM ratio generally appears near the Chasma unit. From [8, 12].

Figure 4 : Variations of the EM ratio (Y axis) for 62 rampart craters according to their location on the canyon (X axis). Both regression lines exhibits a clear general rise in the EM ratio (ejecta mobility) towards the central part of the canyon. Note the enrichment in volatile materials from the margins to the Central Valles Marineris. This con- centration of volatile may have contributed to the widening of the Chasma. From [8, 12]. Workshop on Mars 2001 2510.pdf

MELAS CHASMA LANDING SITE: F. Costard et al.

In all cases, it is suggested that the EM ratio may Descent Imager that would take high-resolution pic- be closely related to the porosity of the wall rock. If tures of the region near the site before landing. this is the case, it is assumed that the upper crust in In-situ analyzes related to water processes and the central chasmata area is relatively more porous past environment with APEX are also possible be- than in peripheral areas. The regional concentration cause of the context of Melas Chasma. The topo- of volatiles may result from accumulation of under- graphic map given by MOLA [18] shows that Melas ground water from neighbouring areas. The widening Chasma is a closed depression. So if water flowed of the chasmata by sapping may have involved re- into this canyon it may have formed lacustrine de- lease of water from confined aquifer [15, 16, 17]. posits interesting for climatic and exobiological per- This interpretation might be genetically consistent spectives. These analyses would also improve the with a lacustrine origin for the layered deposits that knowledge of the geological evolution of Valles were emplaced in Central Valles Marineris. Marineris [19]. The MECA experiment would also be Melas Chasma : A safe landing site with lots of interesting in this place because this region would be scientific interests. Melas Chasma represents an a popular and scientifically interesting place to land interesting site because all instruments would be able for a manned mission. For all these reasons we think to be used in optimal way. Thanks to the new small that Melas Chasma is a first choice landing site for landing ellipse (less than 10 km), the resolution of the 2001. IR spectrometer (Mini-TES) and of the camera (Pan- References: [1] Mangold N. et al. (1999) abstract Cam) will be enough to analyze the geology and 2nd landing site workshop for 2001, Buffalo. [2] mineralogy of the geologic units of nearby scarps. Peulvast J.P. and Masson Ph. (1993) Earth, Moon The proposed sites are located on the floor of and Planets 61, 191-217. [3] Peulvast J.P. and Mas- central- Melas Chasma in the vicinity of the son Ph. (1993) Earth, Moon and Planets 61, 219-248. edge of the layered deposits. Two landing ellipses [4] Lucchitta L. et al. (1989) LPSC XX, 590-591. [5] centered on –9.8° ; 74.1°W and –10.3° ; 73.4°W are Croft S.K. (1989) LPSC IV, 88-89. [6] Nedell S.S. et presented (figure 1). Their topography appears to be al. (1987) Icarus 70, 409-441. [7] Murchie S.L. et al. flat on Viking pictures but they are overlooked (1991) LPSC XXII, 757-758. [8] Costard, F. (1989) by nearby erosional escarpments where the layered Earth, Moon and Planets 45: 265-290. [9] Barlow deposits are not concealed by talus. Should the final N.G. and Bradley T.L. (1990) Icarus, 87, 156-179. landing site be 10 km far away from the scarp, the [10] Mouginis-Mark, P.J. (1979) JGR 84, 8011-8022. resolution would be 2.8 m/pix for the PanCam and 80 [11] Kuzmin, R. et al. (1989) Solar System Res. 22, m/step for the Mini-TES. Taking a 2 km high scarp, 121-133. [12] Peulvast J.P. et al, (1999) Geomor- this last resolution will be enough to distinguish be- phology, in press. [13] Clifford C. (1993) JGR 98, tween two tens of stratigraphical units from the top to 10973-11016 [14] Mc Kinnon and Tanaka, (1989) the bottom of the scarps. If the material is volcanic, JGR 94, 17359-17370. [15] Carr, (1979) JGR 84, as it is usually argued, the result would improve the 2995-3007. [16] Higgins, (1982) Geology 10, 147- understanding of the geochemical evolution of the 152. [17] Kochel and Piper, (1986) JGR 91, E171- crustal material in the region. The geological E192. [18] D.E. et al., (1999) Science 284, study of the scarp would also be improved with the 1495-1502. [19] Weitz C.M. et al., (1999) abstract 2nd landing site workshop for 2001, Buffalo.