Superposed glaciers on Mars: what, where, when, and why?

Post contributed by Adam J. Hepburn, Department of Geography and Earth Sciences, Aberystwyth University, UK.

Mars hosts abundant glacier-like landforms throughout its mid-latitudes, the presence of which necessitates major shifts in climate relative to present conditions. These ice-rich viscous flow features (VFFs) are typically found in coalescing, size-hierarchical systems whereby lower-order glacier-like forms (GLFs; ~5 km long) flow from alcoves and merge with higher-order lineated valley fill (LVF; 100s of km long). Several larger VFFs have been dated previously, indicating Mars underwent glaciation in the past several hundred million years, during the late Amazonian epoch.  However, several authors have noted examples of GLFs flowing onto, rather than into, LVFs (Image 1), and hypothesised that these may correspond to a more recent phase of glacial activity. We used crater dating to ascertain that—in addition to the earlier phase of widespread regional glaciation—these superposed GLFs (SGLFs) were formed following at least two major cycles of more recent alpine glaciation, the latter of which ended ~2 million years ago.

Image 1: Superposed glacier-like form (SGLF) flowing onto the underlying viscous flow feature (underlying VFF), in the Protonilus Mensae region of Mars. (A–B) North-up orientated HiRISE image (ESP_018857_2225) image of an SGLF (light blue) emerging from an alcove and flowing onto lineated valley fill (dark blue). Approximate location of image centre is 42.23◦ N, 50.53◦ E. Reproduced from Hepburn et al, 2020.

Reconstructing Mars’s glacial chronology informs our understanding of its physical environment and past climate. Ice-rich viscous flow features (VFFs) are distributed within the 30–50° latitude zones of each hemisphere and are regarded not as active glaciers, but as the recessed relics of more extensive past glaciation(s). Buried beneath sediment which protects them from sublimation that would otherwise occur today, the presence of VFFs today necessitates major changes in Mars’s past climate, driven by major shifts in the spin and tilt of the planet. It is possible to estimate the timing of these climatic changes, or glaciations, by comparing the number and size of craters (their size-frequency) on VFFs to isochrons of predicted crater size-frequency for given surface ages. Typically, VFFs appear to be 100s of millions of years old and indicate that periods of glacial accumulation, possibly including those suitable for life, occurred relatively recently in Mars’s geological history. However, only a handful of large VFFs have been dated, and uncertainties associated with crater dating on icy surfaces—including the properties of the surfaces themselves—make comparison between ages, and the establishment of a martian glacial chronology, difficult.

On Earth, smaller glaciers typically respond quickest to changes in climate, and smaller GLFs are expected to be the most recently active glaciers on Mars. Several superposed glacier-like forms (SGLFs) have been noted throughout the Protonilus and Deuteronilus Mensae regions of Mars’s mid-latitudes, thought to correspond to this more recent glacial activity. Rather than smoothly coalescing, SGLFs debouch from alcoves onto underlying VFFs and terminate abruptly at a sharply defined terminal boundary (Image 2). Their apparent superposition on underlying VFFs indicates that SGLFs are younger than their underlying VFFs, but the formation age differences between these forms had not been quantified.

Image 2: Superposed glacier like forms (SGLFs) and their underlying viscous flow features (underlying VFFs), with model ages (in millions of years) derived from crater counting. All images are Context Camera mosaics in north-up projection. (A) SGLF (42.23◦ N, 50.53◦ E) flowing onto underlying VFF, Protonilus Mensae. (B) SGLF (40.375◦ S, 103.05◦ E) flowing onto underlying VFF, Eastern Hellas. (c) SGLF (44.02◦ N, 47.30◦ E) flowing onto underlying VFF, Deuteronilus Mensae. (d) SGLFs (41.73◦ S, 108.66◦ E) flowing onto underlying VFF, Eastern Hellas. The different types of VFFs, all being remnants of former-glaciation, are organised in a hierarchy of coalescing flows. SGLF superposition is evidenced by the frontal contact of a raised lobe or moraine-like ridge (marked by white arrows) and surface lineations mismatched from those on the underlying VFF unit. Reproduced from Hepburn et al, 2020.

We identified and examined 320 SGLFs throughout the mid-latitudes of Mars (Image 3), and used crater dating to confirm the apparent relative age difference between SGLFs and underlying VFFs in terms of the crater size-frequency distribution and the absolute ages of each landform (Image 2).  Underlying-VFF ages appear to be spread evenly over the last 300 million years and are consistent with previous work. However, the SGLF ages are bimodal, clustering at ∼2–20 and ∼45–65 million years ago, indicating two glacial growth–recession episodes in the recent martian mid-latitude glaciation record. Dating SGLFs provide a first glimpse into the last stages of mid-latitude glaciation and highlight a complexity that is probably common in Mars’s glacial evolution—similar to Earth’s.  

Image 3: Locations of superposed glacier-like forms (SGLFs) on Mars. Plot symbols distinguish SGLFs formed during the two clusters of glacial recession phases. The latitudinal range of SGLFs displays strong hemispheric symmetry typical of viscous flow features produced by mid-latitude glaciation. The northern SGLFs population is concentrated in the crustal dichotomy boundary of Mars, and the southern SGLF population near large impact basins. Locations of all glacier-like forms are shown for comparison. Background colours show Mars Orbiter Laser Altimeter gridded topography in cylindrical projection. Reproduced from Hepburn et al, 2020.

Further reading:

Hepburn, A. J., Ng, F., Livingstone, S. J., Holt, T., & Hubbard, B. (2020). Polyphase mid-latitude glaciation on Mars: Chronology of the formation of superposed glacier-like forms from crater-count dating. Journal of Geophysical Research: Planets, 125, e2019JE006102. https://doi.org/10. 1029/2019JE006102

Dickson, J. L., Head, J., & Marchant, D. R. (2008). Late Amazonian glaciation at the dichotomy boundary on Mars: Evidence for glacial thickness maxima and multiple glacial phases. Geology, 36(5), 411–414. https://doi.org/10.1130/G24382A.1

Berman, D. C., Crown, D. A., & Joseph, E. C. S. (2015). Formation and mantling ages of lobate debris aprons on Mars: Insights from categorized crater counts. Planetary and Space Science, 111, 83–99. https://doi.org/10.1016/j.pss.2015.03.013

Milliken, R. E, Mustard, J. F., & Goldsby, D. L. (2003). Viscous flow features on the surface of Mars: Observations from high-resolution Mars Orbiter Camera (MOC) images. Journal of Geophysical Research, 108(E6), 5057. https://doi.org/10.1029/2002JE002005

Head, J. W., Marchant, D. R., Agnew, M. C., Fassett, C. I., & Kreslavsky, M. A. (2006). Extensive valley glacier deposits in the northern mid-latitudes of Mars: Evidence for Late Amazonian obliquity-driven climate change. Earth and Planetary Science Letters, 241(3-4), 663–671. https://doi.org/10.1016/j.epsl.2005.11.016

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