| Summary: | Glacial landforms including lobate debris aprons are a globally distributed water ice reservoir on Mars preserving ice from past periods when high orbital obliquity permitted non-polar ice accumulation. Numerous studies have noted morphological similarities between lobate debris aprons and terrestrial debris-covered glaciers, an interpretation supported by radar observations. On both Earth and Mars, these landforms consist of a core of flowing ice covered by a rocky lag. Terrestrial debris-covered glaciers advance in response to climate forcing, driven by obliquity-paced changes to ice mass balance. However, on Mars, it is not known whether glacial landforms that were emplaced over the past 300-800 Ma formed during a single, long deposition event or during multiple glaciations. Here we show that boulders atop 45 lobate debris aprons exhibit no evidence of sequential comminution, but are clustered into bands that become more numerous with increasing latitude, debris apron length, and pole-facing flow orientation. Boulder bands are prominent at glacier headwalls, consistent with debris accumulation during the current martian interglacial. Terrestrial debris-covered glacier boulder bands occur near flow discontinuities caused by obliquity-driven hiatuses in ice accumulation that form internal debris layers. By analogy, we suggest that martian lobate debris aprons experienced multiple cycles of ice deposition, followed by destabilization of ice in the accumulation zone leading to boulder-dominated lenses, and subsequent ice deposition and continued flow. Correlation between latitude and boulder clustering suggests that ice mass balance works across global scales on Mars. Individual lobate debris aprons may preserve ice spanning multiple glacial/interglacial cycles.
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