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The Geographic Distribution of Boulder Halo Craters at Mid-to-High Latitudes on MarsExtensive evidence exists for ground ice at mid-to-high latitudes on Mars, including results from neutron spectroscopy [1-3], thermal properties [4-5], geomorphology [e.g., 6-9], and the in situ observations of Mars Phoenix [10]. This ground ice has been hypothesized to be emplaced diffusively and fill pores [11], or to have accumulated by ice and dust deposition that draped or mantled the terrain [7, 12]. These two processes are not mutually exclusive; both potentially have occurred on Mars [5]. One of the landforms found in areas where ground ice is common on Mars are boulder halo craters [e.g., 13-15] (Figure 1), which are topographically muted impact craters that are filled by ice-rich regolith. They are outlined by boulders that trace a circular outline of the original crater rim. Boulder halos generally have distinctly higher boulder densities than the surrounding background plains and have few boulders in their interiors. The mechanism of boulder halo crater formation is somewhat uncertain. Our working model is that an impact event occurs with sufficient size to excavate to a depth greater than the boulder-poor, ice-rich soils. Excavated boulders are deposited around the crater's rim and in its proximal ejecta. Quite rapidly [14], the crater becomes infilled by icy soil. Rather than being buried, boulders in the halo remain at the surface, perhaps be-cause they 'float' relative to finer-grained materials [14, 16]. Regardless of the details of this process, the life-time of boulders at the surface is much greater than the timescale needed to remove most of the craters' topography. Physical weathering of rocks must be greatly out-paced by crater infilling (the opposite of what is typical, e.g., on the Moon [17]). The rapidity of this infilling is easiest to understand if icy mantling material is deposited and accumulates, rather than simply being added by pore filling of soils. If this model is correct, boulder halos only form when they excavate rock-producing materials from beneath the upper surface. Thus, the distribution and size of craters that result in boulders halos may provide in-sight into the thickness of the ice-rich surface layer in different locations. Note that this thickness is necessarily that of the ice-rich layer at the time of impact, not at present. This study is an initial survey of boulder halo crater locations in the 50deg to 80degN and 50deg to 80degS latitude bands on Mars.
Document ID
20170002461
Acquisition Source
Marshall Space Flight Center
Document Type
Conference Paper
Authors
Rader, L. X. (Mount Holyoke Coll. South Hadley, MA, United States)
Fassett, C. I. (NASA Marshall Space Flight Center Huntsville, AL United States)
Levy, J. S. (Texas Univ. Austin, TX, United States)
King, I. R. (Harvey Mudd Coll. Claremont, CA, United States)
Chaffey, P. M. (Mount Holyoke Coll. South Hadley, MA, United States)
Wagoner, C. M. (Mount Holyoke Coll. South Hadley, MA, United States)
Hanlon, A. E. (Mount Holyoke Coll. South Hadley, MA, United States)
Watters, J. L. (Texas Univ. Austin, TX, United States)
Kreslavsky, M. A. (California Univ. Santa Cruz, CA, United States)
Holt, J. W. (Texas Univ. Austin, TX, United States)
Dyar, M. D. (Mount Holyoke Coll. South Hadley, MA, United States)
Date Acquired
March 23, 2017
Publication Date
March 20, 2017
Subject Category
Lunar And Planetary Science And Exploration
Report/Patent Number
MSFC-E-DAA-TN38531
Meeting Information
Meeting: Lunar and Planetary Science Conference
Location: The Woodlands, TX
Country: United States
Start Date: March 20, 2017
End Date: March 24, 2017
Sponsors: Lunar and Planetary Inst., Universities Space Research Association
Distribution Limits
Public
Copyright
Public Use Permitted.
Keywords
Ice
geomorphology
Mars

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