Mars Polar Science 2000 4110.pdf

HIGH-LATITUDE MARTIAN IMPACT PALEOLAKES: THE POSSIBLE CONTRIBUTION OF SNOWFALL AND ANCIENT GLACIERS IN THE LACUSTRINE ACTIVITY ASSOCIATED TO ARGYRE AND HELLAS . N. A. Cabrol and E. A. Grin. SETI Institute. NASA Ames Research Center.

Introduction: Hellas and Argyre show impact cra- like depressions, and flows down, as shown by ter paleolakes, which morphologies could have sinuosities, junctions, and erosional forms associ- been associated to glacial and sub-glacial proc- ated with the flow in crater. The combina- esses, implying the existence of snowfall and an- tion of observations could be consistent with a the cient glaciers. Some of them show as well a hy- remnants of an ancient glacier-like feature. To drothermal contribution related to the presence of form a glacier, precipitation of snow is required. volcanic centers. We will see that recent martian climate modeling by Haberle et al., (2000) shows that such precipi- Argyre: The Argyre group counts 8 mem- tation were likely to happen repetitively in Argyre bers, that were supplied by the Doanus Vallis through time. system. Some of them display spectacular fluvio- lacustrine structures (e.g., deltas, sedimentary ter- In addition to the recent lakes associated races), and evidence of several episodes of flow or with the filling of craters by potential basal melt in one sustained long episode. The material deposited glacier, more recently Parker et al., (2000), basing both on the channel lakebed floor shows a small their observation on MOLA, MOC and TES data population of superimposed impact craters (1 to 5 have shown the potential for the complete filling of over areas ranging from 10,000 to nearly 15,000 the Argyre basin by a large body of water that km2). Many craters are subdued or partially filled would have overtopped the northern rim of the by this unit. The best estimate would give a Mid- basin during the . Therefore, Argyre it- dle age to this unit (Cabrol and Grin, self should account for another large impact basin 2000)), placing it in climate conditions close to paleolake. what is experiencing today. Although the crater sample is small and the area reduced, the worst case estimate still puts this geological unit at Hellas: The Hellas region is covered by the fringe of the Upper and Lower extended lava flows from Hadriaca Patera that Amazonian. resurfaced most of the preexisting units surround- ing the volcano on the basin rim (Potter 1976). One of the most striking systems of the The Hellas basin itself seems to have been the lo- Argyre group is centered 57°S and 18°W. A valley cus of substantial lacustrine activity late in its network enters the Darwin Crater from the East history. Successive lakes may have been associ- rim. Bright deposits are visible on the top of local ated with the activity of Dao, Harmakhis and Reull crests near the east rim of the crater. This Viking Valles, as shown by clear terrace morphologies image was taken during the extended mission by and smooth deposits that could indicate the pres- the Orbiter 2 on February 1978 during orbit 533 ence of a shoeline. The latest lava flow episodes with Ls≈42°. It was Spring in the northern hemi- were proposed to be at the margin of the Hesperian sphere, and fall in the southern hemisphere. The and Amazonian periods ( and Guest, polar CO2 ice cap was growing. The latitude 1987). The three channels cut these deposits and of the edge of the south cap at Ls=42° would have therefore could be more recent. They could be also been close to 56°S. latitude. So its possible contemporary to these lava flows and triggered by that the bright deposit is CO2 ice beginning to de- the volcanic activity. The very few number of su- posit on the regional crests. The Viking image perimposed craters on the channel material could shows also material in the valleys that enters Dar- support the hypothesis that the channel activity win downslope. This material is different from lasted long enough after the cessation of the vol- debris slope material. It finds its source on cirque- canic activity. The nature of the material that en- Mars Polar Science 2000 4110.pdf

HIGH-LATITUDE MARTIAN IMPACT PALEOLAKES: N. A. Cabrol and E. A. Grin

tered the Hellas basin from these channels resem- However, we could not find one truly superim- bles the lineated material of the , posed ≥2 km-diameter on the 5000 which is assumed to be ice/water rich. km2 deposits left by the valley networks, but only four smaller impacts (≤1.5 km-diameter with large lobate ejecta). This could indicate a recent setting, Three smaller impact basins experienced and/or a recently exhumed deposit. The source ponding probably during the same period. In some region of the valley networks is located on the places, lava flows interact with the channels that northwest and west rims of . One system is enter the three crater basins. We assume then that centrifugal and heads north, while the other is en- ponding might have occurred in these basins at the tering Secchi. The source area is very limited but same time as the latest volcanic activity. The mor- the valley networks are well-developed and pre- phological evidence does not point toward long- served, favoring the hypothesis of a relatively re- sustained lacustrine activity at geological scale, cent activity. Plausible source of water supply in- but rather short ponding associated with possible clude emergence of groundwater and melting of limited hydrothermal activity. One crater basin is snow packs. located 500 km of Hadriaca Patera, on the north- western flank of the volcano. The channel that supplied this crater can be traced 400 km up- stream and disappears. The morphological evi- dence left is not conclusive enough to tell if this upstream point is the source of the channel, or only one segment overtopped by a lava flow. In the crater itself, the morphology of the deposits does not resemble that of a lava flow, and some terrac- ing and deposits can be observed. For these rea- sons, we would favor temporary ponding of water instead of filling from fluid lava.

On the east flank of Hadriaca Patera, two communicating smaller craters located between 39°S and 40°S latitude and 260°W longitude form an open system. The channel supplying them starts in the mountain units at the junction of Reull Val- lis and . The southern crater ba- sin is breached and material is deposited into . The morphology of the deposits suggests a highly-loaded, mud-like flow. The stratigraphic age of the channel supplying these two basins is comparable to the latest lava flows, and support the potential interaction between volcanic activity and ground ice, as proposed by the previous work of Squyres et al., (1987) about this region.

Secchi- At 58° S and 260°W, the 200-km diameter Secchi crater shows clear evidence of flowing and deposition in its north and west parts. In its case, the timing of such events is difficult to establish. The Secchi basin is a Noachian basin.