SHARAD Radar Combining Craters and RadarSHARAD (which has a resolution limit near the surface of...
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Combining Craters and Radar • The dielectric constant is related to radar propagation velocity in the equation below. • We can use depths to crater terraces with SHARAD delay times to constrain the dielectric constant and thus the properties of the ice if/when the terrace corresponds to the same interface causing the radar reflector. v = Δx Δt = c ε r SHARAD 1-way delay times depth to subsurface reflector Floor terrace depths Dielectric constant of subsurface layer Terraced Crater Number Label 10 7 2 4 5 3 9 8 1 Diameter (m) 734 631 579 391 394 461 504 625 292 Lat 46.58 42.50 47.94 43.84 47.70 53.17 39.10 44.18 45.65 Long 194.85 197.59 191.93 206.17 208.69 205.27 192.90 199.19 188.62 Wall Terrace Depth (m) 17.2 8.8 29.3 9.0 +2.83 5.8 Floor Terrace Depth (m) 42.8 56.3 41.8 26.4 26.3 23.3 23.0 25.9 17.6 Pit Depth (m) 75.7 50.3 46.3 39.1 31.5 42.1 49.9 25.4 Radar Delay Time: Closest Measurement to Crater (us) 0.2612 0.3507 0.2632 0.2201 0.2281 0.1803 0.2015 0.3788 0.2459 Distance from Crater to Closest Radar Measurement (km) 8.2 5.3 12.8 11.7 1.2 90.7 31.8 2.2 89.9 Dielectric Constant Using Closest Radar Measurement 3.35 3.49 3.57 6.24 6.77 5.40 6.93 19.19 17.57 Depth To Interface Assuming Ice Using Closest Radar Time (m) 44.2 59.3 44.5 37.2 38.5 30.5 34.1 64.0 41.6 Depth To Interface Assuming Basalt Using Closest Radar Time (m) 27.7 37.2 27.9 23.3 24.2 19.1 21.4 40.2 26.1 Number of Measurements Within 15 km of Crater 84 184 20 50 114 0 0 184 0 Weighted Mean of Radar Measurements Within 15 km of Crater (us) 0.2314 0.3085 0.2300 0.2191 0.2367 0.3336 Depth To Interface Assuming Ice Using Weighted Mean Radar (m) 39.1 52.1 38.9 37.0 40.0 56.4 Depth To Interface Assuming Basalt Using Radar Weighted Mean (m) 24.5 32.7 24.4 23.2 25.1 35.4 Dielectric Constant Using Weighted Mean Within 15 km of Crater 2.63 2.70 2.73 6.18 7.29 14.88 Table 1: Properties of the craters with DTMs, their nearby radar reflections and dielectric calculations. Dielectric Mixing Models • To determine composition of the ice, we compared our dielectric constant calculations to the dielectric behavior of a 2-component mixture (ε m ) with different volumetric fractions (v i ) of rock (ε i = 8) and air (ε i = 1) inclusions in a host material of ice (ε h = 3.15) using three different simple mixing models: General mixing model formula: ε m α = ε h α + v i (ε i α – ε h α ) where α=1 (linear), α=1/2 (refractive) Logarithmic model for α→0: log(ε m ) = v h log(ε h ) + v i log(ε i ) • The error in dielectric constant calculations is ~0.5, giving us a range of dielectric constants for the ice of 2.2 to 3.3 for the weighted mean calculations and 2.8 to 4.1 for the closest radar measurements. This means the ice layer is less than ~50% porous and a dirtiness of less than ~30%. 0 0.5 1 1.5 2 2.5 3 3.5 0.00 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Dielectric Constant of Mixture Volume Fraction of Air Inclusions in Ice Dielectric Constants of Ice-Air Mixtures Linear Model Refractive Model Logarithmic Model 0 1 2 3 4 5 6 7 8 9 0.00 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Dielectric Constant of Mixture Volume Fraction of Rock Inclusions in Ice Dielectric Constants of Ice-Rock Mixtures Linear Model Refractive Model Logarithmic Model Terraced Craters • Terraces within craters form due to changes in the strength of the target material at certain depths. • We mapped the locations of terraced craters across Arcadia Planitia by searching 311 CTX images. Several have a double-terraced structure with a weak terrace closer to the surface and a wider, deeper terrace indicative of a larger contrast in material strengths. • We obtained HiRISE stereo pairs for dozens of the craters and made Digital Terrain Models (DTMs) for 9 craters to measure the depths to terraces. Terrace Crater 10 HiRISE stereo pairs: 018522_2270 & 019010_2270 Terrace Crater 2 HiRISE stereo pairs: 033805_2285 & 034095_2285 Terrace Crater 7 HiRISE stereo pairs: 033963_2230 & 033818_2230 Group 1: Craters with the standard double crater morphology and nearby (within 15 km) SHARAD subsurface reflectors. Terrace Crater 4 HiRISE stereo pairs: 035189_2240 & 035690_2240 Terrace Crater 5 HiRISE stereo pairs: 035914_2280 & 035624_2280 Terrace Crater 8 HiRISE stereo pairs: 034108_2245 & 034174_2245 Terrace Crater 3 HiRISE stereo pairs: 035057_2335 & 034767_2335 Terrace Crater 1 HiRISE stereo pairs: 033014_2260 & 034082_2260 Terrace Crater 9 HiRISE stereo pairs: 035928_2195 & 035862_2195 Group 2: Terraced craters that lack the double terrace structure and/or nearby SHARAD subsurface reflectors. Without a second terrace at the ice-rock interface, we cannot compare the depths to the SHARAD reflector. Cartoon of standard double-terraced crater profile Floor terrace Wall terrace Pit Rock-ice interface Layering within ice Fig. 1: Diagram showing the standard double- terrace structure of the smaller, shallower “wall terrace” and the wider, deeper “floor terrace”. Conclusions Introduction • There is a lot of evidence for excess ice (ice that exceeds the pore space of the regolith) in the mid-latitudes: ice exposed in recent impact craters, scallops, thermokarstically expanded craters, and ice at the Phoenix excavation site. • Understanding the distribution and composition (pure, dirty or porous) of this ice will lead to a better understanding of the mechanism(s) and climate that emplaced this ice. • Craters and SHARAD’s radargrams are both independent mechanisms for probing the subsurface to study this ice and both show extensive layering (presumably of this ice) across Arcadia Planitia. • In this work, we combine these datasets to constrain the depth of a widespread interface. Knowledge of its depth allows the radar data to be interpreted in terms of dielectric properties and constrain the composition of the layer. SHARAD Radar Ali M. Bramson 1 , S. Byrne 1 , N. E. Putzig 2 , S. Mattson 1 , J. J. Plaut 3 , J. W. Holt 4 1 Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ (contact: [email protected]), 2 Southwest Research Institute, Boulder, CO, 3 Jet Propulsion Laboratory, Pasadena, CA, 4 Institute for Geophysics, University of Texas at Austin, Austin, TX 30 Second Summary • Terraced craters suggest a change in material strengths while subsurface SHARAD radar reflections appear due to changes in dielectric properties. • We have mapped the locations of terraced craters and SHARAD reflectors, which are widespread across Arcadia Planitia. • Comparing the delay times of these subsurface radar reflections with the depths of terraces in these craters suggests a layer of ice decameters thick that extends up to the surface. Acknowledgements This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE-1143953. • Radar reflections indicate sharp change in the dielectric constant (relative dielectric permittivity) ε r of the material. • The dielectric constant is a measure of how effectively an EM wave can move in a material. • Dielectric constants of common materials: Air ε r : ~1 Water Ice ε r : ~3.15 Basalt ε r : ~8 • We have mapped a subsurface reflector (originally detected and mapped by Plaut et al. LPSC 2009) in 277 SHARAD tracks across Arcadia Planitia by comparing the radargrams to clutter simulations (which simulate artificial reflectors due to off-nadir topography). Example Radargram with Subsurface Reflector Radar Track 949601000 delay time distance along track Surface Subsurface Example Clutter Simulation Figure 2: Map of terraced crater locations (black and white markers) plotted over the one-way delay time differences between the surface and subsurface SHARAD reflectors (red = deeper reflector; blue = shallower reflector). The presence of terraced craters in areas without a subsurface reflector suggests the thickness of the ice is heterogeneous and is too thin in some areas to be detected by SHARAD (which has a resolution limit near the surface of 15-25 m). • SeisWare International Inc. for free license of SeisWare software used in analyzing SHARAD data • CO-SHARPS for SHARAD radar data products • Charles Brothers & Jack Holt for SHARAD clutter simulations • Craters that show both wall and floor terraces and have SHARAD subsurface reflectors nearby (within 15 km) indicate the layers in the upper decameters of the surface are made of predominantly relatively pure (<50% porosity, <30% dirt) excess water ice . • The other craters, “Group 2”, give ambiguous or nonphysical results due to the crater not excavating through the whole layer. The terracing in these craters is likely due to subtler compositional differences in the ice. • Other lines of evidence (Viola et al. Mars 8, 2014) suggest this ice layer is 10’s of millions years old. An explanation is necessary for how this ice layer was emplaced and what conditions preserved it for so long. Clutter = using weighted mean radar = using closest radar measurements 10 1 2 3 4 5 7 8 9 # 10 km 10 km
Transcript of SHARAD Radar Combining Craters and RadarSHARAD (which has a resolution limit near the surface of...
Combining Craters and Radar • The dielectric constant is related to radar propagation velocity in the
equation below. • We can use depths to crater terraces with SHARAD delay times to
constrain the dielectric constant and thus the properties of the ice if/when the terrace corresponds to the same interface causing the radar reflector.
v = ΔxΔt
=cεrSHARAD 1-way delay times
depth to subsurface reflector
Floor terrace depths
Dielectric constant of subsurface layer
Terraced Crater Number Label
10 7 2 4 5 3 9 8 1
Diameter (m) 734 631 579 �391 394 461 504 625 292
Lat 46.58 42.50 47.94 43.84 47.70 53.17 39.10 44.18 45.65
Long 194.85 197.59 191.93 206.17 208.69 205.27 192.90 199.19 188.62
Wall Terrace Depth (m) 17.2 8.8 29.3 9.0 +2.83 5.8
Floor Terrace Depth (m) 42.8 56.3 41.8 26.4 26.3 23.3 23.0 25.9 17.6
Pit Depth (m) 75.7 50.3 46.3 39.1 31.5 42.1 49.9 25.4 Radar Delay Time: Closest Measurement to Crater
(us) 0.2612 0.3507 0.2632 0.2201 0.2281 0.1803 0.2015 0.3788 0.2459
Distance from Crater to Closest Radar
Measurement (km) 8.2 5.3 12.8 11.7 1.2 90.7 31.8 2.2 89.9
Dielectric Constant Using Closest Radar Measurement
3.35 3.49 3.57 6.24 6.77 5.40 6.93 19.19 17.57
Depth To Interface Assuming Ice Using Closest
Radar Time (m) 44.2 59.3 44.5 37.2 38.5 30.5 34.1 64.0 41.6
Depth To Interface Assuming Basalt Using Closest Radar Time (m)
27.7 37.2 27.9 23.3 24.2 19.1 21.4 40.2 26.1
Number of Measurements Within 15 km of Crater 84 184 20 50 114 0 0 184 0
Weighted Mean of Radar Measurements Within 15
km of Crater (us) 0.2314 0.3085 0.2300 0.2191 0.2367 -‐ -‐ 0.3336 -‐
Depth To Interface Assuming Ice Using
Weighted Mean Radar (m) 39.1 52.1 38.9 37.0 40.0 -‐ -‐ 56.4 -‐
Depth To Interface Assuming Basalt Using
Radar Weighted Mean (m) 24.5 32.7 24.4 23.2 25.1 -‐ -‐ 35.4 -‐
Dielectric Constant Using Weighted Mean Within 15
km of Crater 2.63 2.70 2.73 6.18 7.29 -‐ -‐ 14.88 -‐
Table 1: Properties of the craters with DTMs, their nearby radar reflections and dielectric calculations.
Dielectric Mixing Models • To determine composition of the ice, we compared our dielectric constant calculations to the dielectric behavior of a 2-component mixture (εm) with different volumetric fractions (vi) of rock (εi = 8) and air (εi = 1) inclusions in a host material of ice (εh = 3.15) using three different simple mixing models:
General mixing model formula: εmα = εh
α + vi (εiα – εh
α) where α=1 (linear), α=1/2 (refractive) Logarithmic model for α→0: log(εm) = vh log(εh) + vi log(εi)
• The error in dielectric constant calculations is ~0.5, giving us a range of dielectric constants for the ice of 2.2 to 3.3 for the weighted mean calculations and 2.8 to 4.1 for the closest radar measurements. This means the ice layer is less than ~50% porous and a dirtiness of less than ~30%.
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Linear Model Refractive Model Logarithmic Model
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Dielectric Constants of Ice-Rock Mixtures
Linear Model Refractive Model Logarithmic Model
Terraced Craters • Terraces within craters form due to changes in the strength of the target
material at certain depths. • We mapped the locations of terraced craters across Arcadia Planitia by
searching 311 CTX images. Several have a double-terraced structure with a weak terrace closer to the surface and a wider, deeper terrace indicative of a larger contrast in material strengths.
• We obtained HiRISE stereo pairs for dozens of the craters and made Digital Terrain Models (DTMs) for 9 craters to measure the depths to terraces.
Terrace Crater 10 HiRISE stereo pairs:
018522_2270 & 019010_2270
Terrace Crater 2 HiRISE stereo pairs:
033805_2285 & 034095_2285
Terrace Crater 7 HiRISE stereo pairs:
033963_2230 & 033818_2230
Group 1: Craters with the standard double crater morphology and nearby (within 15 km) SHARAD subsurface reflectors.
Terrace Crater 4 HiRISE stereo pairs:
035189_2240 & 035690_2240
Terrace Crater 5 HiRISE stereo pairs:
035914_2280 & 035624_2280
Terrace Crater 8 HiRISE stereo pairs:
034108_2245 & 034174_2245
Terrace Crater 3 HiRISE stereo pairs:
035057_2335 & 034767_2335
Terrace Crater 1 HiRISE stereo pairs:
033014_2260 & 034082_2260
Terrace Crater 9 HiRISE stereo pairs:
035928_2195 & 035862_2195
Group 2: Terraced craters that lack the double terrace structure and/or nearby SHARAD subsurface reflectors. Without a second terrace at the ice-rock interface, we cannot compare the depths to the SHARAD reflector.
Cartoon of standard double-terraced crater profile
Floor terrace
Wall terrace
Pit Rock-ice interface
Layering within ice
Fig. 1: Diagram showing the standard double-terrace structure of the smaller, shallower “wall terrace” and the wider, deeper “floor terrace”.
Conclusions
Introduction • There is a lot of evidence for excess ice (ice that exceeds the pore space of the regolith) in the mid-latitudes: ice exposed in recent
impact craters, scallops, thermokarstically expanded craters, and ice at the Phoenix excavation site. • Understanding the distribution and composition (pure, dirty or porous) of this ice will lead to a better understanding of the
mechanism(s) and climate that emplaced this ice. • Craters and SHARAD’s radargrams are both independent mechanisms for probing the subsurface to study this ice and both show
extensive layering (presumably of this ice) across Arcadia Planitia. • In this work, we combine these datasets to constrain the depth of a widespread interface. Knowledge of its depth allows the radar
data to be interpreted in terms of dielectric properties and constrain the composition of the layer.
SHARAD Radar
Ali M. Bramson1, S. Byrne1, N. E. Putzig2, S. Mattson1, J. J. Plaut3, J. W. Holt4
1Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ (contact: [email protected]), 2Southwest Research Institute, Boulder, CO, 3Jet Propulsion Laboratory, Pasadena, CA, 4Institute for Geophysics, University of Texas at Austin, Austin, TX
30 Second Summary • Terraced craters suggest a change in material strengths while
subsurface SHARAD radar reflections appear due to changes in dielectric properties.
• We have mapped the locations of terraced craters and SHARAD reflectors, which are widespread across Arcadia Planitia.
• Comparing the delay times of these subsurface radar reflections with the depths of terraces in these craters suggests a layer of ice decameters thick that extends up to the surface.
Acknowledgements
This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE-1143953.
• Radar reflections indicate sharp change in the dielectric constant (relative dielectric permittivity) εr of the material.
• The dielectric constant is a measure of how effectively an EM wave can move in a material.
• Dielectric constants of common materials: Air εr: ~1 Water Ice εr : ~3.15 Basalt εr: ~8
• We have mapped a subsurface reflector (originally detected and mapped by Plaut et al. LPSC 2009) in 277 SHARAD tracks across Arcadia Planitia by comparing the radargrams to clutter simulations (which simulate artificial reflectors due to off-nadir topography).
Example Radargram with Subsurface Reflector
Radar Track 949601000
delay time
distance along track
Surface
Subsurface
Example Clutter Simulation
Figure 2: Map of terraced crater locations (black and white markers) plotted over the one-way delay time differences between the surface and subsurface SHARAD reflectors (red = deeper reflector; blue = shallower reflector). The presence of terraced craters in areas without a subsurface reflector suggests the thickness of the ice is heterogeneous and is too thin in some areas to be detected by SHARAD (which has a resolution limit near the surface of 15-25 m).
• SeisWare International Inc. for free license of SeisWare software used in analyzing SHARAD data
• CO-SHARPS for SHARAD radar data products • Charles Brothers & Jack Holt for SHARAD clutter simulations
• Craters that show both wall and floor terraces and have SHARAD subsurface reflectors nearby (within 15 km) indicate the layers in the upper decameters of the surface are made of predominantly relatively pure (<50% porosity, <30% dirt) excess water ice.
• The other craters, “Group 2”, give ambiguous or nonphysical results due to the crater not excavating through the whole layer. The terracing in these craters is likely due to subtler compositional differences in the ice.
• Other lines of evidence (Viola et al. Mars 8, 2014) suggest this ice layer is 10’s of millions years old. An explanation is necessary for how this ice layer was emplaced and what conditions preserved it for so long.
Clutter
= using weighted mean radar = using closest radar measurements
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#
10 km
10 km
