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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

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characteristics and formation of an unusual martian impact crater ejecta deposit

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.)

summary
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.
observations
Observations

140 LARLE craters 1-km-diameter in 75 latitude zone

  • Called Low-Aspect-Ratio Layered Ejecta (LARLE) craters due to very low aspect ratio (A = thickness/length)

LARLE layer displays dune-like features but few to no ejecta blocks and terminates in a sinuous, almost-flame-like pattern.

Largest LARLE crater is 12.2 km in diameter. Median diameter = 2.7 km .

observations1
Observations

Consistently found in fine-grained (and usually ice-rich) mantle deposits

Override pre-existing terrain rather than being deflected

LARLE layer can extend up to 21 crater radii from crater rim.

Many characteristics in common with pedestal craters.

formation
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.

conclusions
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