1. CHARACTERISTICS AND FORMATION OF AN UNUSUAL MARTIAN IMPACT CRATER EJECTA DEPOSIT Nadine G. Barlow Dept. Physics and Astronomy Northern Arizona University Flagstaff, AZ 86011-6010 Nadine.Barlow@nau.edu Office: 928-523-5452 Cell: 928-600-9253 Co-authors: Joseph M. Boyce (U. HI) and Lionel Wilson (Lancaster U.)

2. Summary We have identified an unusual type of ejecta deposit around generally small Martian impact craters at high latitudes. This deposit is much more extensive than a normal ejecta blanket and requires a different emplacement mechanism. We propose that a dust-laden gravity-driven density current (“base surge”) is responsible for emplacement of this extensive outer layer.

3. Observations 140 LARLE craters  1-km-diameter in  75  latitude zone LARLE layer displays dune-like features but few to no ejecta blocks and terminates in a sinuous, almost-flame- like pattern. Called Low-Aspect-Ratio Layered Ejecta (LARLE) craters due to very low aspect ratio (A = thickness/length) Largest LARLE crater is 12.2 km in diameter. Median diameter = 2.7 km.

4. Observations Consistently found in fine- grained (and usually ice-rich) mantle deposits LARLE layer can extend up to 21 crater radii from crater rim. Override pre-existing terrain rather than being deflected Many characteristics in common with pedestal craters.

5. Formation LARLE deposits are sufficiently different from normal Martian layered ejecta blankets that they require a different emplacement mechanism. We propose a base surge, similar to the dust-filled, gravity-driven debris flows seen around nuclear explosions and explosive volcanic eruptions. Deposit is prevented from removal by the wind by cementation of upper layer by salts transported upward by water in the LARLE deposit. Base surges, like this one at Sedan nuclear event, are radially expanding, dilute, turbulent clouds of suspended fine-particles driven by the density contrast between the clouds and the ambient atmosphere.

6. Conclusions We have identified 140 Martian impact craters with an extremely extensive ejecta deposit (“LARLE craters”) We propose that the LARLE deposits are emplaced by a turbulent cloud of fine-grained particles (base surge), which is different from the formation of normal Martian layered ejecta deposits. Salts deposited on surface of LARLE layer prevent this deposit from being quickly removed by the Martian winds. Pedestal craters are eroded versions of LARLE craters. For more information: Dr. Nadine G. Barlow Cell: 928-600-9253 Slides posted at www.physics.nau.edu/~barlow