Role of Volatiles in the Emplacement of Ejecta Deposits around Martian Impact Craters

Introduction: Impact craters are a dominant geological landform on Mars, the most Earth-like planet in the Solar System. The martian impact cratering record is more diverse than for Earth and the other terrestrial planets [e.g., 1]. Of particular interest is the presence of multiple layers of lobate or fluidized proximal ejecta deposits surrounding martian impact craters. These are collectively termed " layered ejecta structures " [2]. Fluidized ejecta deposits have been attributed to either (1) interaction of ejecta with volatiles derived from the subsurface [e.g., 3], or (2) ejecta interaction with the atmosphere [e.g., 4]. Earth possesses both an atmosphere and volatile-rich (i.e., sedimen-tary) rocks so that similar ejecta emplacement styles may be expected. Observations of proximal ejecta deposits around martian impact craters: The characteristics of lay-ered ejecta deposits surrounding martian impact craters have been documented by many workers, most recently by Barlow [5]. In summary [after 5], ~89 % of the catalogued martian impact craters >5 km in diameter that retain ejecta deposits display lobate or fluid-ized ejecta deposits. Three main types of layered ejecta structures have been recognized, comprising single (SLE), double (DLE), or multiple (MLE) layers of ejecta, with the SLE morphology being the most predominant type. All layered ejecta blankets are characterized by a ridge or rampart at their outer edge [3]. It is clear that the features of fluidized ejecta deposits around martian impact craters do not reflect just simple ballistic emplacement, as with the majority of lunar and mercurian craters. The ejecta emplacement process also involves a component of radial flow [3]. Observations of proximal ejecta deposits around terrestrial impact craters: Recent studies of several well preserved terrestrial impact structures have provided important new information about the emplacement mechanisms of proximal impact ejecta deposits. This work draws on studies at Meteor Crater and Lonar Crater (both <2 km in diameter and SLE structures), the Ries and Haughton structures (both ~24 km in diameter and DLE structures), and Chicxu-lub (a ~180 km diameter DLE or MLE structure). For SLE structures: • Possible evidence of fluidized ejecta deposits have been noted around Meteor Crater [6]; however, the necessary detailed field studies have not been carried out to date. • Recent workers at the Lonar Crater have documented a component of ground-hugging surface flow following ballistic deposition of the ejecta blanket [7];