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DISCOVERY OF A LARGE RAYED CRATER ON MARS: IMPLICATIONS FOR RECENT VOLCANIC AND FLUVIAL ACTIVITY AND THE ORIGIN OF MARTIAN

Introduction: Recent impact craters on the Moon, Mercury, and icy Galilean satellites have produced bright rays extending hundreds or thousands of kilometers from the crater rims. A large rayed crater has never previously been seen on Mars, and it has been assumed that the active aeolian environment would quickly remove the ephemeral ray material [1]. Here we report the discovery of a 10-km diameter crater in the young volcanic plains of Cerberus (SE Elysium Planitia) with rays (apparent in THEMIS IR mosaics) extending more than 800 km. The rays are associated with ~10 5 to 10 7 secondary craters ranging from 15 to 100 m in diameter. This quantity of secondary craters, if typical, suggests that most small craters seen in NA-MOC images are secon-daries. About 75% of the craters superimposed over Athabasca Valles [2] originated from this single impact event. Attempts to date very young surfaces from the density of small craters are limited by the fact that secondary cratering is highly clustered in space and time, increasing the uncertainties by a factor of 30 or more. The 10-km Cerberus crater may be the youngest crater on Mars of this size class, perhaps < 10 6 yrs old [3]. The presence of flow ejecta suggests that ground ice was present at depths of a few hundred meters even in very recent times at this equatorial location. Mars meteorite EET79001 (basaltic glass) has an ejection age of less than 1 Ma [4] and could have originated from this crater. Observations by MOC: Small (15-100 m) bright-rayed craters on Mars [5] have been extremely puzzling. They appear very fresh and well-preserved, with bright ejecta and fine rays extending up to distances of ~10 crater diameters. They are strongly clustered both locally and globally (most are in the Cerberus region). The crater rims are generally circular, but those imaged at the highest resolution appear more angular and unusually shallow (for primary craters). Some of them consist of very tight clusters of craters. They range in size from the limits of MOC resolution (~10 m diameter) up to ~100 m; a few are slightly larger. It is very difficult to map out the distribution of these craters from the sparse NA-MOC coverage. Observations by THEMIS: THEMIS IR images (day and night) have revealed exquisite detail in well-preserved impact craters [6]. There are strong variations in thermal inertia (TI) and albedo, apparent from early …