Paleoglaciology on Mars
The recent (Feb. 2021) arrival of the Perseverance rover at Jezero crater on Mars provides a new incentive to look at paleoglaciation on the Red Planet. Given that Mars has polar ice and features suggestive of glacial action that can be observed from orbit, it can be assumed that some parts of the planet which are now ice-free have experienced glacial environments in the past. It is of interest to understand how and why Mars reached its present cold, desiccated condition. Using paleoglaciology to infer past changes in climate can potentially help to interpret the climate evolution of Mars. This objective aligns with the objective of the present analysis of ice-disrupted bedrock on the Avalon Peninsula.
Moving warm-based glaciers can shape hills and valleys by mechanical erosion and can leave deposits of glacially-transported rock fragments that are clearly indicative of past glacial action. It is less clear how this geomorphic evidence of past glaciation can be linked to climate evolution on Earth or on Mars, other than to demonstrate that climate must have transitioned from glacial conditions to non-glacial conditions at some point in the past. Glaciofluvial erosion can likewise provide general evidence of past glacial activity. While evidence of catastrophic flooding might suggest rapid ice melting, hence rapid climate warming, sudden melt-pond drainage can also cause catastrophic flooding. Furthermore, it can be ambiguous as to whether an eroded channel or fan was formed subglacially or subaerially.
Cold-based glaciers do not leave behind the readily-recognized large-scale geomorphic markers that characterize past warm-based glacial activity. Although less recognizable from a distance, the fracturing and displacement (brecciation) of bedrock by cold glacial ice nevertheless provides an informative means to infer past glacial activity and related changes in climate. Asymmetric erosion of hills, specifically lee-side brecciation indicative of negative relative ice pressure, provides evidence of past ice flow and ice flow direction. Observations on the Avalon Peninsula clearly indicate that lee-side brecciation, usually interpreted as plucking by warm-based glacial ice, can also result from the action of cold glacial ice deforming via creep. Reduced local ice pressure is the primary driver of lee-side brecciation in all circumstances. This rule applies on Mars as well as on Earth.
Fracturing of bedrock in a cold subglacial environment can be caused by thermal action, by direct mechanical action (a result of ice-induced stress) or a combination of both. Bedrock brecciation by ice on low-sloping or horizontal bedrock surfaces (bedrock frost heave or ice-induced fissures) is indicative of a thermally-driven process whereby intruding subglacial groundwater freezes and converts heat flow energy into work. This process requires a temperature gradient and can occur subaerially or subglacially. Subaerial bedrock frost heave and related bedrock-brecciation processes rely on fluctuating temperatures and associated heat flows derived from solar heating or changes in the weather. Subglacial bedrock frost heave and related processes are powered by temperature gradients in bedrock and in overlying glacial ice.
Instances of thermally-driven brecciatrion of bedrock seen on the Avalon Peninsula can be shown, in most cases, to have occurred in a cold subglacial environment with pressurized subglacial groundwater providing an essential input to the process. Many instances of mechanical brecciation of bedrock (including lee-side brecciation) seen on the Avalon can also be shown to have occurred in a cold subglacial environment, sometimes incorporating a component of groundwater-supported thermal brecciation. Any area on Mars that is presently glaciated would experience cold-based glaciation because of extremely cold weather and low heat flux from the interior of the planet. This assumption could reasonably be extended back in time until the Martian climate switched from permitting liquid water to flow on the surface to its present desiccated and frozen condition. Analogies can thus be drawn between the deglaciation of parts of the Avalon under cold glacial conditions and the deglaciation of areas on Mars. A key difference is that Martian deglaciation probably occurred by sublimation, whereas Avalon deglaciation occurred by fast melting and creep.
If Mars never experienced warm-based glaciation anywhere, then the analogy between Martian cold deglaciation and Avalon cold deglaciation partially breaks down. In particular, without an extended interval of thick warm-based glacial ice cover, permeable bedrock might never develop a deep reservoir of pressurized groundwater. Thermally driven subglacial bedrock brecciation observed on the Avalon required a supply of subglacial groundwater rebounding to the surface as the weight of glacial ice cover diminished. At a site like Jezero crater, a cold glacier overlying pressurized groundwater could have resulted from the freezing of a lake from top to bottom. It is also possible that the lake was always frozen over, with water draining into and out of the crater underneath thick ice. As climate cooled, a subglacial lake could freeze to the bottom. The possibility of thermal-origin subglacial bedrock brecciation at Jezero crater, analogous to that observed on the Avalon, can thus not be entirely excluded.
If Jezero crater was once occupied by a cold glacier, then evidence obtained from observations on the Avalon Peninsula can be extrapolated to infer that mechanical brecciation of bedrock by ice was a near certainty. No groundwater is needed to support the fracturing of rock by cold glacial ice. Ice is unique among minerals in its ability to undergo relatively rapid plastic deformation at cold temperatures. All forms of bedrock will, in contrast, be brittle when in contact with cold ice. As cold glacial ice undergoes creep, rock in contact with with the ice will experience shear stress, which in many contact geometries will translate to a component of tensile stress. If bedrock contains joints or planes of crystallographic weakness, then the rock will fail when exposed to sufficient deviatoric stress imparted by cold glacial ice. This principle applies to large boulders as well as to bedrock. Observations from the Avalon Peninsula show that loose boulders and small bedrock protrusions have been fragmented while embedded in deforming cold ice.
A cursory look at recent photos recorded by Perseverance at Jezero crater shows an abundance of boulders, fragmented bedrock and sharply defined cliff edges. All these features can be indicative of past cold glaciation. Many inland cliffs on the Avalon Peninsula have been cut from glacially-sculpted bedrock landforms by thermal and mechanical action beneath late-stage cold-based glaciers. Based on the presently available preliminary observations, the Martian features could be glacial or they might instead reflect subaerial brecciation by ice (the Martian equivalent of freeze-thaw weathering), fluvial erosion (catastrophic flooding), glaciofluvial erosion, erosion by wind-driven sand or even the remnants of ancient impacts or volcanic explosions. On the Avalon, it is often possible to attribute specific instances of bedrock brecciation unambiguously to cold subglacial action. For example, a joint block that has been ejected from a still-visible cavity, and transported a short distance from its point of origin, establishes a cold subglacial origin. Bedrock frost heave requires subfreezing conditions and subsequent transport strongly implies glacial ice movement.
Perhaps the most interesting paleoglacial observation that might be made at Jezero crater would be to find indications of thermal-origin subglacial bedrock brecciation. The bedrock at the Perseverance landing site is presumed to be sedimentary. On the Avalon Peninsula, tilted ancient sedimentary beds provide an optimal environment for finding instances of subglacial bedrock frost heave. Groundwater confined to a gently-dipping permeable contact between relatively impermeable beds rebounded from depth during deglaciation and disrupted frozen bedrock just below the ice-bedrock interface. Similar observations at Jezero could be tied to a sequence of climate events, beginning with groundwater saturating the bedrock. This would be followed by development of a cold glacier, freezing of the bedrock and penetration of a freezing front downward into the rock, but not too deeply. Then, the overlying glacier would need to depart via sublimation, creating a pressure imbalance whereby groundwater moved upward from depth, freezing as it approached the glacier base.
Clearly, it is unlikely that all of the above conditions were realized at the location where Perseverance is operating. Set off against this low probability of occurrence would be the relative ease of finding frost-heaved joint blocks in wide-field ground-level photography, should they exist. On the Avalon, frost-heaved joint blocks are difficult to recognize in aerial photography looking straight down, even from altitudes as low as 15 meters. However, frost-heaved joint blocks can readily be recognized in ground-level photographs taken from hundreds of meters away. Frost-heaved joint blocks stand out from other boulders when they are tall relative to their width. Most boulders lie on the ground at an orientation that approximately minimizes the height of their center of mass. Seeing a tall, thin boulder standing on edge is a clue that it might be supported by being anchored in underlying bedrock. Two examples are shown in the following screen grabs from a recent panorama released by NASA from the Perseverance rover at Jezero crater.
Moving warm-based glaciers can shape hills and valleys by mechanical erosion and can leave deposits of glacially-transported rock fragments that are clearly indicative of past glacial action. It is less clear how this geomorphic evidence of past glaciation can be linked to climate evolution on Earth or on Mars, other than to demonstrate that climate must have transitioned from glacial conditions to non-glacial conditions at some point in the past. Glaciofluvial erosion can likewise provide general evidence of past glacial activity. While evidence of catastrophic flooding might suggest rapid ice melting, hence rapid climate warming, sudden melt-pond drainage can also cause catastrophic flooding. Furthermore, it can be ambiguous as to whether an eroded channel or fan was formed subglacially or subaerially.
Cold-based glaciers do not leave behind the readily-recognized large-scale geomorphic markers that characterize past warm-based glacial activity. Although less recognizable from a distance, the fracturing and displacement (brecciation) of bedrock by cold glacial ice nevertheless provides an informative means to infer past glacial activity and related changes in climate. Asymmetric erosion of hills, specifically lee-side brecciation indicative of negative relative ice pressure, provides evidence of past ice flow and ice flow direction. Observations on the Avalon Peninsula clearly indicate that lee-side brecciation, usually interpreted as plucking by warm-based glacial ice, can also result from the action of cold glacial ice deforming via creep. Reduced local ice pressure is the primary driver of lee-side brecciation in all circumstances. This rule applies on Mars as well as on Earth.
Fracturing of bedrock in a cold subglacial environment can be caused by thermal action, by direct mechanical action (a result of ice-induced stress) or a combination of both. Bedrock brecciation by ice on low-sloping or horizontal bedrock surfaces (bedrock frost heave or ice-induced fissures) is indicative of a thermally-driven process whereby intruding subglacial groundwater freezes and converts heat flow energy into work. This process requires a temperature gradient and can occur subaerially or subglacially. Subaerial bedrock frost heave and related bedrock-brecciation processes rely on fluctuating temperatures and associated heat flows derived from solar heating or changes in the weather. Subglacial bedrock frost heave and related processes are powered by temperature gradients in bedrock and in overlying glacial ice.
Instances of thermally-driven brecciatrion of bedrock seen on the Avalon Peninsula can be shown, in most cases, to have occurred in a cold subglacial environment with pressurized subglacial groundwater providing an essential input to the process. Many instances of mechanical brecciation of bedrock (including lee-side brecciation) seen on the Avalon can also be shown to have occurred in a cold subglacial environment, sometimes incorporating a component of groundwater-supported thermal brecciation. Any area on Mars that is presently glaciated would experience cold-based glaciation because of extremely cold weather and low heat flux from the interior of the planet. This assumption could reasonably be extended back in time until the Martian climate switched from permitting liquid water to flow on the surface to its present desiccated and frozen condition. Analogies can thus be drawn between the deglaciation of parts of the Avalon under cold glacial conditions and the deglaciation of areas on Mars. A key difference is that Martian deglaciation probably occurred by sublimation, whereas Avalon deglaciation occurred by fast melting and creep.
If Mars never experienced warm-based glaciation anywhere, then the analogy between Martian cold deglaciation and Avalon cold deglaciation partially breaks down. In particular, without an extended interval of thick warm-based glacial ice cover, permeable bedrock might never develop a deep reservoir of pressurized groundwater. Thermally driven subglacial bedrock brecciation observed on the Avalon required a supply of subglacial groundwater rebounding to the surface as the weight of glacial ice cover diminished. At a site like Jezero crater, a cold glacier overlying pressurized groundwater could have resulted from the freezing of a lake from top to bottom. It is also possible that the lake was always frozen over, with water draining into and out of the crater underneath thick ice. As climate cooled, a subglacial lake could freeze to the bottom. The possibility of thermal-origin subglacial bedrock brecciation at Jezero crater, analogous to that observed on the Avalon, can thus not be entirely excluded.
If Jezero crater was once occupied by a cold glacier, then evidence obtained from observations on the Avalon Peninsula can be extrapolated to infer that mechanical brecciation of bedrock by ice was a near certainty. No groundwater is needed to support the fracturing of rock by cold glacial ice. Ice is unique among minerals in its ability to undergo relatively rapid plastic deformation at cold temperatures. All forms of bedrock will, in contrast, be brittle when in contact with cold ice. As cold glacial ice undergoes creep, rock in contact with with the ice will experience shear stress, which in many contact geometries will translate to a component of tensile stress. If bedrock contains joints or planes of crystallographic weakness, then the rock will fail when exposed to sufficient deviatoric stress imparted by cold glacial ice. This principle applies to large boulders as well as to bedrock. Observations from the Avalon Peninsula show that loose boulders and small bedrock protrusions have been fragmented while embedded in deforming cold ice.
A cursory look at recent photos recorded by Perseverance at Jezero crater shows an abundance of boulders, fragmented bedrock and sharply defined cliff edges. All these features can be indicative of past cold glaciation. Many inland cliffs on the Avalon Peninsula have been cut from glacially-sculpted bedrock landforms by thermal and mechanical action beneath late-stage cold-based glaciers. Based on the presently available preliminary observations, the Martian features could be glacial or they might instead reflect subaerial brecciation by ice (the Martian equivalent of freeze-thaw weathering), fluvial erosion (catastrophic flooding), glaciofluvial erosion, erosion by wind-driven sand or even the remnants of ancient impacts or volcanic explosions. On the Avalon, it is often possible to attribute specific instances of bedrock brecciation unambiguously to cold subglacial action. For example, a joint block that has been ejected from a still-visible cavity, and transported a short distance from its point of origin, establishes a cold subglacial origin. Bedrock frost heave requires subfreezing conditions and subsequent transport strongly implies glacial ice movement.
Perhaps the most interesting paleoglacial observation that might be made at Jezero crater would be to find indications of thermal-origin subglacial bedrock brecciation. The bedrock at the Perseverance landing site is presumed to be sedimentary. On the Avalon Peninsula, tilted ancient sedimentary beds provide an optimal environment for finding instances of subglacial bedrock frost heave. Groundwater confined to a gently-dipping permeable contact between relatively impermeable beds rebounded from depth during deglaciation and disrupted frozen bedrock just below the ice-bedrock interface. Similar observations at Jezero could be tied to a sequence of climate events, beginning with groundwater saturating the bedrock. This would be followed by development of a cold glacier, freezing of the bedrock and penetration of a freezing front downward into the rock, but not too deeply. Then, the overlying glacier would need to depart via sublimation, creating a pressure imbalance whereby groundwater moved upward from depth, freezing as it approached the glacier base.
Clearly, it is unlikely that all of the above conditions were realized at the location where Perseverance is operating. Set off against this low probability of occurrence would be the relative ease of finding frost-heaved joint blocks in wide-field ground-level photography, should they exist. On the Avalon, frost-heaved joint blocks are difficult to recognize in aerial photography looking straight down, even from altitudes as low as 15 meters. However, frost-heaved joint blocks can readily be recognized in ground-level photographs taken from hundreds of meters away. Frost-heaved joint blocks stand out from other boulders when they are tall relative to their width. Most boulders lie on the ground at an orientation that approximately minimizes the height of their center of mass. Seeing a tall, thin boulder standing on edge is a clue that it might be supported by being anchored in underlying bedrock. Two examples are shown in the following screen grabs from a recent panorama released by NASA from the Perseverance rover at Jezero crater.
Two possible examples of subglacial bedrock frost heave are shown above. Without scale and views from other angles, it is impossible to exclude the very likely possibility that their size, shape and orientation are incompatible with them being frost-heaved joint blocks. They are, however, indicative of what a frost-heaved joint block might look like in a wide-angle photo taken from a distance. To confirm that a boulder was a frost-heaved joint block, a closeup look would be needed with an emphasis on seeing that the boulder extended down into bedrock and that the boulder was underlain by a cavity. Often, a frost-heaved joint block will have a small fissure alongside its base that makes observation of an underlying cavity straightforward. Sand could obstruct such observations just as snow does when observing similar features on Earth during winter.
The first picture above shows weak evidence of possible aligned tilting of rock fragments. Multiple flattened rock fragments that are tilted in similar directions could be indicative of tilted bedding planes in underlying bedrock or tilted joint sets of tectonic origin. However they can also reflect ground-level shear stress imparted by creep in a cold-based glacier. This latter interpretation is common in rocks seen in glacially-affected areas on the Avalon Peninsula. Two examples are shown below.
The first picture above shows weak evidence of possible aligned tilting of rock fragments. Multiple flattened rock fragments that are tilted in similar directions could be indicative of tilted bedding planes in underlying bedrock or tilted joint sets of tectonic origin. However they can also reflect ground-level shear stress imparted by creep in a cold-based glacier. This latter interpretation is common in rocks seen in glacially-affected areas on the Avalon Peninsula. Two examples are shown below.
Both of the above pictures show examples where subglacial bedrock frost heave has occurred under the influence of basal shear stress imparted by a cold glacier. In both cases, the bedrock is of ancient sedimentary origin, and possesses foliation resulting from regional metamorphism. The intrinsic (pre-glacial) foliation dips vertically at both locations, but the apparent foliation has been tilted by glaciotectonic action. A statistically significant asymmetry in the fracturing of bedrock or of rock fragments derived from nearby bedrock could be a clue that an area might have been affected by cold glacial ice under shear stress.
Another screen grab from the same NASA Perseverance panorama used to obtain the two preceding Mars photos in this section reveals an area of boulders that may derive from the brecciation of local bedrock. A hint of preferential alignment in the edges of boulders (dipping downward toward the right) could potentially be linked to past glaciotectonic action at this location. However, asymmetric erosion by wind or water, or a sloping joint set or sloping bedding planes in source bedrock could also account for any observed asymmetry.