Martian Paleoglaciology
Rationale
The successful deployment of robotic laboratories on Mars has enabled detailed study of the planet's geology and geomorphology at several locations. The knowledge gained from these studies can be used to infer boundaries on the Martian paleoclimate, and then to further infer whether or not conditions on Mars might once have been suitable for biological evolution. Alongside the search for life, it is of additional interest to understand how and why Mars reached its present cold, desiccated condition. Paleoglaciology provides a means to help interpret geologic and geomorphic data and to link this data to the ancient Martian climate. 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. Searching Mars for evidence of ice-modified geology and geomorphology aligns with the objective of the present analysis of ice-disrupted bedrock on the Avalon Peninsula. As with Mars, the areas of interest on the Avalon are presently unglaciated, but through their response to past glacial activity, the rocks preserve a record of ancient climate change. The most comprehensive data set for use in investigating analogies between Mars and the Avalon Peninsula is provided by the nine years of operation of NASA's Curiosity rover (Mars Science Laboratory) at Gale crater.
Scope
Possible evidence of past warm-based (basal sliding) glaciation at Gale Crater has been deliberately excluded from this analysis. Moving warm-based glaciers can shape hills and valleys by mechanical erosion and can leave large-scale deposits of glacially-transported rock fragments that are clearly indicative of past glacial action. It is less clear how geomorphic evidence of past warm-based glaciation can be linked to climate evolution on Earth or on Mars, other than to deduce that climate must have transitioned from temperate 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 an alluvial fan was formed subglacially or subaerially. The present analysis will focus solely on looking for evidence of past cold glaciation accompanied by rebounding subglacial groundwater at Gale crater. This type of evidence can potentially connect the evolution of the paleoclimate at Gale crater to the climate on the Avalon Peninsula at the end of the Younger Dryas cold period.
Cold Glaciation
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 reduced relative ice pressure, provides evidence of past ice flow and ice flow direction. Observations on the Avalon Peninsula 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 (involving groundwater), 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 (freeze-thaw weathering) rely on fluctuating temperatures and associated heat flows derived from solar heating or changes in the weather. The presence of ambient water, usually derived from precipitation, is also required for freeze-thaw weathering. Subglacial bedrock frost heave and related processes are powered by temperature gradients in bedrock and in overlying glacial ice.
Role of Groundwater
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 Gale 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 Gale crater, analogous to that observed on the Avalon, can thus not be excluded.
Ice-induced Brecciation
If Gale 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. While no groundwater is needed to support the fracturing of rock by cold glacial ice, the presence of intruding groundwater will greatly intensify the process. Ice is unique among minerals (on Earth, although on Mars, frozen CO2 must also be considered) 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.
The term "brecciation" usually refers to the fragmentation of rock by intruding magma. In a cold subglacial environment, ice plays a role similar to that of magma, opening and entering cracks in rock to form a breccia where the matrix is ice. The clasts (rock fragments) can vary in scale from millimeters to tens of meters. Unlike the more conventional version of magma-generated breccia, breccia formed by the intrusion of ice into rock disintegrates into loose clasts with no matrix present when glacial conditions end.
The successful deployment of robotic laboratories on Mars has enabled detailed study of the planet's geology and geomorphology at several locations. The knowledge gained from these studies can be used to infer boundaries on the Martian paleoclimate, and then to further infer whether or not conditions on Mars might once have been suitable for biological evolution. Alongside the search for life, it is of additional interest to understand how and why Mars reached its present cold, desiccated condition. Paleoglaciology provides a means to help interpret geologic and geomorphic data and to link this data to the ancient Martian climate. 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. Searching Mars for evidence of ice-modified geology and geomorphology aligns with the objective of the present analysis of ice-disrupted bedrock on the Avalon Peninsula. As with Mars, the areas of interest on the Avalon are presently unglaciated, but through their response to past glacial activity, the rocks preserve a record of ancient climate change. The most comprehensive data set for use in investigating analogies between Mars and the Avalon Peninsula is provided by the nine years of operation of NASA's Curiosity rover (Mars Science Laboratory) at Gale crater.
Scope
Possible evidence of past warm-based (basal sliding) glaciation at Gale Crater has been deliberately excluded from this analysis. Moving warm-based glaciers can shape hills and valleys by mechanical erosion and can leave large-scale deposits of glacially-transported rock fragments that are clearly indicative of past glacial action. It is less clear how geomorphic evidence of past warm-based glaciation can be linked to climate evolution on Earth or on Mars, other than to deduce that climate must have transitioned from temperate 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 an alluvial fan was formed subglacially or subaerially. The present analysis will focus solely on looking for evidence of past cold glaciation accompanied by rebounding subglacial groundwater at Gale crater. This type of evidence can potentially connect the evolution of the paleoclimate at Gale crater to the climate on the Avalon Peninsula at the end of the Younger Dryas cold period.
Cold Glaciation
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 reduced relative ice pressure, provides evidence of past ice flow and ice flow direction. Observations on the Avalon Peninsula 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 (involving groundwater), 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 (freeze-thaw weathering) rely on fluctuating temperatures and associated heat flows derived from solar heating or changes in the weather. The presence of ambient water, usually derived from precipitation, is also required for freeze-thaw weathering. Subglacial bedrock frost heave and related processes are powered by temperature gradients in bedrock and in overlying glacial ice.
Role of Groundwater
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 Gale 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 Gale crater, analogous to that observed on the Avalon, can thus not be excluded.
Ice-induced Brecciation
If Gale 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. While no groundwater is needed to support the fracturing of rock by cold glacial ice, the presence of intruding groundwater will greatly intensify the process. Ice is unique among minerals (on Earth, although on Mars, frozen CO2 must also be considered) 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.
The term "brecciation" usually refers to the fragmentation of rock by intruding magma. In a cold subglacial environment, ice plays a role similar to that of magma, opening and entering cracks in rock to form a breccia where the matrix is ice. The clasts (rock fragments) can vary in scale from millimeters to tens of meters. Unlike the more conventional version of magma-generated breccia, breccia formed by the intrusion of ice into rock disintegrates into loose clasts with no matrix present when glacial conditions end.
A cursory look at photos recorded by Curiosity at Gale 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-rounded bedrock landforms by thermal and mechanical action beneath late-stage cold-based glaciers. Analogous 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, the collapse of undermined bedrock outcrops, or 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 of frost-heaved blocks strongly implies glacial ice movement.
Perhaps the most interesting paleoglacial observation that might be made at Gale crater would be to find indications of thermal-origin subglacial bedrock brecciation. The bedrock at the Curiosity landing site is presumed to be sedimentary in origin. On the Avalon Peninsula, tilted ancient sedimentary beds provide an optimal environment for generating 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 Gale 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 creep or sublimation, creating a pressure imbalance whereby groundwater moved upward from depth, freezing as it approached the glacier base.
It might be considered unlikely that all of the above conditions were realized at the location where Curiosity is operating. Set off against this uncertainty of occurrence is the relative ease of finding frost-heaved joint blocks in wide-field ground-level photography, should such blocks 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 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. Wind-deposited sand on Mars could obstruct such observations just as snow does when observing similar features on Earth during winter.
Gale Crater
Gale crater is an approximately 150 km diameter, 5 km deep depression in the Martian surface that was presumably formed by impact more than 3 billion years ago. After its formation, the crater is thought to have filled with sediment transported by water and wind, and then to have been hollowed out by wind erosion leaving behind a central mountain (Mount Sharp) about 5.5 km in relief. The floor of the crater is ancient sedimentary bedrock which presumably lithified in the presence of groundwater and under the weight of kilometers of overburden, now absent. The majority of the geologic history of Gale crater lies outside the scope of the present analysis. Rather, the focus here is on a potential final stage of subglacial erosion (brecciation) of the crater floor. This final stage of subglacial erosion was presumably followed by deglaciation, desiccation and then hundreds of millions of years of aeolian erosion and the associated deposition of wind-carried particles. Possible analysis of paleoglacial activity depends on evidence of this activity being incompletely erased or buried during the lengthy interval between past glaciation and the present.
Similarities and Differences: Avalon Peninsula vs. Gale Crater
-- Both sites are underlain by layered bedrock. On the Avalon, bedrock incorporating subglacial brecciation features is usually thickly bedded ancient sedimentary rock. However, ice-induced brecciation features are also observed in ignimbrite and tuff that has been deposited in a sequence of thick layers. Rock at Gale appears to be thinly or thickly bedded, depending on location, and the bedding is frequently very distinct.
-- Rock on the Avalon has undergone regional metamorphism. Metamorphic activity has oriented phyllosilicate grains imparting distinct foliation in most areas. The Avalon rock is typically well-indurated and is strongly resistant to erosion by water or by freeze-thaw weathering. The ancient rock is typically out of equilibrium in Earth's modern environment and is subject to mild chemical attack. This attack is generally limited to a thin surface layer (rind) that protects the underlying rock. The rock at Gale crater has seemingly not experienced regional metamorphism. Most (but not all) of the beds have not been tilted and the rock appears not well indurated (even friable) in many instances. Well-indurated cap rock appears to overlie weaker beds in many cases. No such large differences in quality of lithification are seen in the layered rocks that have been affected by ice-induced brecciation on the Avalon Peninsula.
-- Both the Avalon and Gale sites show abundant joints in layered rock, with failure planes frequently running parallel to bedding planes. Commonly, additional joints and associated failure planes run at steep angles to bedding planes. Planes of foliation, usually at right angles to bedding planes, typically become planes of failure in Avalon rock. There is no equivalent weakness in the rocks at Gale crater owing to the lack of regional metamorphism. Cross joints, running at steep angles (often orthogonal) to both foliation and bedding are common in Avalon rocks. Similar joints, potentially reflecting uneven response of adjacent portions of rock to hydrostatic stress also appear abundant in Gale crater bedrock. Joint blocks are commonly seen in disrupted bedrock, both at Gale and on the Avalon.
-- Erosion by wind-driven rock particles is a significant factor in shaping the rocks seen at Gale crater. No analogous erosion has occurred in rocks on the Avalon Peninsula. Basal-sliding glacial erosion is the dominant factor in shaping bedrock landforms seen on the Avalon. It is unclear whether basal-sliding glacial erosion has occurred at Gale crater. At both the Avalon and Gale crater locations, large-scale and severe bedrock brecciation is apparent. This brecciation represents an end-stage erosion event that has not been strongly overprinted by any subsequent erosion process.
-- Sand and silt deposited by wind obscure many of the details of bedrock condition at Gale crater. Glacial till, standing water, soil and vegetation play a similar obscuring role on the Avalon. Each location, however, displays abundant exposed bedrock. Overall, bedrock is better exposed at Gale than on the Avalon.
-- The rocks on the Avalon Peninsula are of Ediacaran age (~550 million years old) and are much younger than the rocks at Gale crater (~3 billion years old). Potential end-stage glacial activity at Gale is very much older than on the Avalon (hundreds of millions of years at Gale vs. 11000 years on the Avalon).
-- Carbon dioxide as liquid, solid or gas may have significantly affected the geologic evolution at Gale crater while water or ice might have played a dominant or subordinate role. On the Avalon, water and ice were dominant agents of erosion and carbon dioxide was not a significant factor.
The Curiosity Mission
The Curiosity rover landed at Gale crater on Aug. 6, 2012 and was still operating as of May 31, 2021. The rover has moved about 25 km and gained about 330 m in elevation during the 3135 Martian sols (3220 Earth days) comprising this interval. The main science cameras on Curiosity (Mastcam) have recorded more than 170,000 photos during the interval, with a significant portion of these being Martian landscapes and closeups of the ground. The two mast cameras have angular fields of view of 15 deg. (wide angle) and 5.1 deg. (narrow angle). All of the photos selected to illustrate possible glacial activity at Gale crater for the present analysis were taken from the NASA online archive of Curiosity raw images, filtered to include only Mastcam photos. Selected photos cover just the mission interval Aug. 6, 2012 - May 31, 2021.
Martian Analogue
Paleoglaciological analogies between sites on the Avalon Peninsula and sites at Gale crater are presented in the following Martian Analogue sections.
Perhaps the most interesting paleoglacial observation that might be made at Gale crater would be to find indications of thermal-origin subglacial bedrock brecciation. The bedrock at the Curiosity landing site is presumed to be sedimentary in origin. On the Avalon Peninsula, tilted ancient sedimentary beds provide an optimal environment for generating 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 Gale 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 creep or sublimation, creating a pressure imbalance whereby groundwater moved upward from depth, freezing as it approached the glacier base.
It might be considered unlikely that all of the above conditions were realized at the location where Curiosity is operating. Set off against this uncertainty of occurrence is the relative ease of finding frost-heaved joint blocks in wide-field ground-level photography, should such blocks 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 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. Wind-deposited sand on Mars could obstruct such observations just as snow does when observing similar features on Earth during winter.
Gale Crater
Gale crater is an approximately 150 km diameter, 5 km deep depression in the Martian surface that was presumably formed by impact more than 3 billion years ago. After its formation, the crater is thought to have filled with sediment transported by water and wind, and then to have been hollowed out by wind erosion leaving behind a central mountain (Mount Sharp) about 5.5 km in relief. The floor of the crater is ancient sedimentary bedrock which presumably lithified in the presence of groundwater and under the weight of kilometers of overburden, now absent. The majority of the geologic history of Gale crater lies outside the scope of the present analysis. Rather, the focus here is on a potential final stage of subglacial erosion (brecciation) of the crater floor. This final stage of subglacial erosion was presumably followed by deglaciation, desiccation and then hundreds of millions of years of aeolian erosion and the associated deposition of wind-carried particles. Possible analysis of paleoglacial activity depends on evidence of this activity being incompletely erased or buried during the lengthy interval between past glaciation and the present.
Similarities and Differences: Avalon Peninsula vs. Gale Crater
-- Both sites are underlain by layered bedrock. On the Avalon, bedrock incorporating subglacial brecciation features is usually thickly bedded ancient sedimentary rock. However, ice-induced brecciation features are also observed in ignimbrite and tuff that has been deposited in a sequence of thick layers. Rock at Gale appears to be thinly or thickly bedded, depending on location, and the bedding is frequently very distinct.
-- Rock on the Avalon has undergone regional metamorphism. Metamorphic activity has oriented phyllosilicate grains imparting distinct foliation in most areas. The Avalon rock is typically well-indurated and is strongly resistant to erosion by water or by freeze-thaw weathering. The ancient rock is typically out of equilibrium in Earth's modern environment and is subject to mild chemical attack. This attack is generally limited to a thin surface layer (rind) that protects the underlying rock. The rock at Gale crater has seemingly not experienced regional metamorphism. Most (but not all) of the beds have not been tilted and the rock appears not well indurated (even friable) in many instances. Well-indurated cap rock appears to overlie weaker beds in many cases. No such large differences in quality of lithification are seen in the layered rocks that have been affected by ice-induced brecciation on the Avalon Peninsula.
-- Both the Avalon and Gale sites show abundant joints in layered rock, with failure planes frequently running parallel to bedding planes. Commonly, additional joints and associated failure planes run at steep angles to bedding planes. Planes of foliation, usually at right angles to bedding planes, typically become planes of failure in Avalon rock. There is no equivalent weakness in the rocks at Gale crater owing to the lack of regional metamorphism. Cross joints, running at steep angles (often orthogonal) to both foliation and bedding are common in Avalon rocks. Similar joints, potentially reflecting uneven response of adjacent portions of rock to hydrostatic stress also appear abundant in Gale crater bedrock. Joint blocks are commonly seen in disrupted bedrock, both at Gale and on the Avalon.
-- Erosion by wind-driven rock particles is a significant factor in shaping the rocks seen at Gale crater. No analogous erosion has occurred in rocks on the Avalon Peninsula. Basal-sliding glacial erosion is the dominant factor in shaping bedrock landforms seen on the Avalon. It is unclear whether basal-sliding glacial erosion has occurred at Gale crater. At both the Avalon and Gale crater locations, large-scale and severe bedrock brecciation is apparent. This brecciation represents an end-stage erosion event that has not been strongly overprinted by any subsequent erosion process.
-- Sand and silt deposited by wind obscure many of the details of bedrock condition at Gale crater. Glacial till, standing water, soil and vegetation play a similar obscuring role on the Avalon. Each location, however, displays abundant exposed bedrock. Overall, bedrock is better exposed at Gale than on the Avalon.
-- The rocks on the Avalon Peninsula are of Ediacaran age (~550 million years old) and are much younger than the rocks at Gale crater (~3 billion years old). Potential end-stage glacial activity at Gale is very much older than on the Avalon (hundreds of millions of years at Gale vs. 11000 years on the Avalon).
-- Carbon dioxide as liquid, solid or gas may have significantly affected the geologic evolution at Gale crater while water or ice might have played a dominant or subordinate role. On the Avalon, water and ice were dominant agents of erosion and carbon dioxide was not a significant factor.
The Curiosity Mission
The Curiosity rover landed at Gale crater on Aug. 6, 2012 and was still operating as of May 31, 2021. The rover has moved about 25 km and gained about 330 m in elevation during the 3135 Martian sols (3220 Earth days) comprising this interval. The main science cameras on Curiosity (Mastcam) have recorded more than 170,000 photos during the interval, with a significant portion of these being Martian landscapes and closeups of the ground. The two mast cameras have angular fields of view of 15 deg. (wide angle) and 5.1 deg. (narrow angle). All of the photos selected to illustrate possible glacial activity at Gale crater for the present analysis were taken from the NASA online archive of Curiosity raw images, filtered to include only Mastcam photos. Selected photos cover just the mission interval Aug. 6, 2012 - May 31, 2021.
Martian Analogue
Paleoglaciological analogies between sites on the Avalon Peninsula and sites at Gale crater are presented in the following Martian Analogue sections.