Arising from: J. W. Head et al. Nature 434, 346–351 (2005); Head et al. reply.

Head et al.¹ interpret spectacular images from the Mars Express high-resolution stereo camera as evidence of geologically recent rock glaciers in Tharsis and of a piedmont ('hourglass') glacier at the base of a 3-km-high massif east of Hellas. They attribute growth of the low-latitude glaciers to snowfall during periods of increased spin-axis obliquity. The age of the hourglass glacier, considered to be inactive and slowly shrinking beneath a debris cover in the absence of modern snowfall, is estimated to be more than 40 Myr. Although we agree that the maximum glacier extent was climatically controlled, we find evidence in the images to support local augmentation of accumulation from snowfall through a mechanism that does not require climate change on Mars.

1. Quaternary Research Center and Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195-1310, USA

Correspondence to: Alan R. Gillespie¹ Email: alan@rad.ess.washington.edu

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