Long-Lived Glaciation in the Northern Mid-Latitudes of ... · Landforms inferred to have formed as...
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Long-Lived Glaciation in the Northern Mid-Latitudes of Mars: New Constraints on Timing Caleb I. Fassett 1 , Joseph S. Levy 2 , James W. Head 3 and James L. Dickson 3 (Contact: [email protected]) 1 Dept. of Astronomy, Mount Holyoke College. 2 University of Texas Institute for Geophysics, Jackson School of Geosciences. 3 Dept. of Geological Sciences, Brown University. Landforms inferred to have formed as a result of glaciation on Mars are common in the martian mid-latitudes, including lobate debris aprons, lineated valley fill, and concentric crater fill. We have completed new mapping of the spatial extent of these landforms in the northern mid-latitudes (30-50˚N). From these data, we have derived new constraints on the timing and persistence of glaciation. During this mapping, many synglacial craters were recognized, meaning they formed after ice was already present on the surface, but subsequent deposition of ice occured in their interiors following formation. The sizable number of these features is a indicator that the glacial period during the Amazonian was long-lived. From this, we estimate that glaciation occurred for at least 600 My in the northern mid-latitudes. Glaciation need not have occurred continuously across this entire period, and may have varied in intensity spatially and temporally. Fewer post-glacial craters, stratigraphically superposed on the remnant ice, were apparent. From the number of these features, we estimate that the period of widespread ice accumulation ended relatively recently, ~100 Ma. Since accumulation of ice and its survival at the surface are presently not favored in these locations, these observations indicated that during the Middle to Late Amazonian, Mars was commonly in a different climate state than it is today. Examples of Glacial Features Post-Glacial Craters 23 craters within 1D of glacial deposits 5 craters are >2.5 km 278 lack fill, but are >1D away from glacial features Synglacial Craters 131 within 1D of glacial deposits 119 craters are >2.5 km 10 -7 10 -6 10 -5 Amazonian-Hesperian boundary Early Amazonian Mid Amazonian Late Amazonian Best Fit Age H =110 My Post-Glacial craters Crater Diameter (km) Crater Frequency (n in bin per km 2 ) 4 8 16 32 64 4 8 16 32 64 10 -7 10 -6 10 -5 Amazonian-Hesperian boundary Early Amazonian Mid Amazonian Late Amazonian Best Fit Age H =710 My Post-Glacial + Synglacial craters Crater Diameter (km) Crater Frequency (n in bin per km 2 ) Incremental crater size-frequency plots of craters compared to Hartmann 2005 isochrons. The plotted points are the number of craters in a given size bin per unit area, with the area calculated in a buffer around mapped ice-rich units in the northern mid-latitudes. The frequency of post-glacial craters (A) is consistent with the last period of glacial deposition in ending at an average age of ~110 Ma. The frequency of post-glacial plus synglacial craters (B) gives an estimate of how long the observed glacial deposits must have been present: at least ~600 My. Map of the (A) western and (B) eastern hemisphere of Mars in the northern mid-latitudes, showing glacial deposits (yellow), with the location of post-glacial (cyan) and synglacial craters (green) (not to scale). Basemap is a hillshade of Mars Laser Orbiter Altimeter data, map is in Mercator projection. Postglacial Craters Synglacial Craters Postglacial craters: We interpret these craters as (1) forming after ice was already present on the surface based on interactions of their ejecta with ice-rich deposits, and (2) experiencing no substantial later accumulation of ice in their interiors. This suggests they post-date the era when accumulation was favored. Synglacial craters: We interpret these craters as (1) forming after ice was already present on the surface based on interactions of their ejecta with ice-rich deposits, and (2) experiencing later accumulation of ice in their interiors. This implies they formed at some point after ice deposition had already occurred, and continued to occur (a glacial period). Crater Fill Lobate Debris Aprons Valley Fill
Transcript of Long-Lived Glaciation in the Northern Mid-Latitudes of ... · Landforms inferred to have formed as...
Long-Lived Glaciation in the Northern Mid-Latitudes of Mars: New Constraints on Timing
Caleb I. Fassett1, Joseph S. Levy2, James W. Head3 and James L. Dickson3 (Contact: [email protected])1Dept. of Astronomy, Mount Holyoke College. 2University of Texas Institute for Geophysics, Jackson School of Geosciences. 3Dept. of
Geological Sciences, Brown University.
Landforms inferred to have formed as a result of glaciation on Mars are common in the martian mid-latitudes, including lobate debris aprons, lineated valley fill, and concentric crater fill. We have completed new mapping of the spatial extent of these landforms in the northern mid-latitudes (30-50˚N). From these data, we have derived new constraints on the timing and persistence of glaciation.
During this mapping, many synglacial craters were recognized, meaning they formed after ice was already present on the surface, but subsequent deposition of ice occured in their interiors following formation. The sizable number of these features is a indicator that the glacial period during the Amazonian was long-lived. From this, we estimate that glaciation occurred for at least 600 My in the northern mid-latitudes. Glaciation need not have occurred continuously across this entire period, and may have varied in intensity spatially and temporally.
Fewer post-glacial craters, stratigraphically superposed on the remnant ice, were apparent. From the number of these features, we estimate that the period of widespread ice accumulation ended relatively recently, ~100 Ma.
Since accumulation of ice and its survival at the surface are presently not favored in these locations, these observations indicated that during the Middle to Late Amazonian, Mars was commonly in a different climate state than it is today.
Examples of Glacial Features
Post-Glacial Craters 23 craters within 1D of glacial deposits 5 craters are >2.5 km
278 lack fill, but are >1D away from glacial features
Synglacial Craters 131 within 1D of glacial deposits 119 craters are >2.5 km
10−7
10−6
10−5
Amazonian-Hesperian boundary
Early AmazonianMid AmazonianLate Amazonian
Best FitAgeH=110 My
Post-Glacial craters
Crater Diameter (km)
Crat
er F
requ
ency
(n in
bin
per
km
2 )
4 8 16 32 64 4 8 16 32 64
10−7
10−6
10−5
Amazonian-Hesperian boundary
Early AmazonianMid AmazonianLate Amazonian
Best FitAgeH=710 My
Post-Glacial + Synglacial craters
Crater Diameter (km)
Crat
er F
requ
ency
(n in
bin
per
km
2 )
Incremental crater size-frequency plots of craters compared to Hartmann 2005 isochrons. The plotted points are the number of craters in a given size bin per unit area, with the area calculated in a buffer around mapped ice-rich units in the northern mid-latitudes. The frequency of post-glacial craters (A) is consistent with the last period of glacial deposition in ending at an average age of ~110 Ma. The frequency of post-glacial plus synglacial craters (B) gives an estimate of how long the observed glacial deposits must have been present: at least ~600 My.
Map of the (A) western and (B) eastern hemisphere of Mars in the northern mid-latitudes, showing glacial deposits (yellow), with the location of post-glacial (cyan) and synglacial craters (green) (not to scale). Basemap is a hillshade of Mars Laser Orbiter Altimeter data, map is in Mercator projection.
Postglacial Craters Synglacial Craters
Postglacial craters: We interpret these craters as (1) forming after ice was already present on the surface based on interactions of their ejecta with ice-rich deposits, and (2) experiencing no substantial later accumulation of ice in their interiors. This suggests they post-date the era when accumulation was favored.
Synglacial craters: We interpret these craters as (1) forming after ice was already present on the surface based on interactions of their ejecta with ice-rich deposits, and (2) experiencing later accumulation of ice in their interiors. This implies they formed at some point after ice deposition had already occurred, and continued to occur (a glacial period). Crater Fill
Lobate Debris Aprons
Valley Fill
