Testing Models for the Formation of the Equatorial …2,013 craters We have yet to perform a robust...
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A B -6 -5 -4 -3 -2 -1 0 1 2 Cumulative Number of Craters (km -2 ) Diameter (km) On Ridge Off Ridge 0% 2% 4% 6% 8% 10% 12% 14% 16% 10 20 30 40 50 60 70 80 90 Major Axis Aximuth Central Ridge 0% 2% 4% 6% 8% 10% 12% 14% 10 20 30 40 50 60 70 80 90 Major Axis Aximuth Preipheral Ridge 0% 2% 4% 6% 8% 10% 12% 14% 10 20 30 40 50 60 70 80 90 Major Axis Aximuth Off Ridge Preliminary Conclusion: The crater population in the high-resolution region is most consistent with the impact generated formation model [1] Table 1. Hypothesis Matrix Predictions 1 Early Ridge Formation Late Ridge Formation Transformed Craters E-W Elongated Craters Class Model Author(s) Exogenic Impact Generated [1] 0 1 0 1 [2] 1 0 0 1 Ancient Ring System [3] 1 0 0 1 Endogenic Despinning [4] 1 0 1 0 [5] 1 0 1 0 [6] 1 0 1 0 Upwarping of Lithosphere [7] 1 0 0 0 [8] 1 0 0 0 [9] 1 0 x 0 Cryovolcanism [10] 1 0 0 0 Planetary Contraction [11] 1 0 1 0 [12] 1 0 1 0 1. Yes (1), No (0), Either/Or (x) Testing Models for the Formation of the Equatorial Ridge on Saturn’s Moon Iapetus via Crater Counting Amanda Damptz and Andrew Dombard Department of Earth and Environmental Sciences University of Illinois at Chicago Introduction Iapetus’s equatorial ridge is unique in the solar system (Fig. 1) Formation of the ridge is likely attributed key events in the evolution of Iapetus Purpose of this study: examine the crater population on and around the ridge, test the various models of ridge formation, and assess the age of the ridge Endogenic Exogenic Despinning Impact Generated Upwarping of the Lithosphere Ancient Ring System Cyrovolcanism Planetary Contraction Methods We use two global mosaics of Iapetus: (1) Cassini and Voyager data (c/o Cassini Image Science Team), (2) Cassini data only (c/o P.M. Schenk) Craters are measured and cataloged in ArcGIS using the Crater Helper Tool An ellipse (6-point) is used to generate latitude, longitude, diameter, extent, and major and minor axis azimuth All craters counted within 8° North and South are considered “on ridge” Figure 2. A) Crater rim in high-resolution region B) Crater rim in low-resolution region Identifying craters in areas of low-resolution Crater rims in the high-resolution regions are indicated by bright pixels (Fig. 2A) Crater rims can then be identified in low-resolution regions by using the brightest pixels surrounding dark depressions (Fig. 2B) Resolution of images available for Iapetus limits further crater classification (e.g., simple, complex, and multi-ring basin, ejecta deposits, primary and secondary craters, etc.) Some features are somewhat ambiguous as to whether it is a crater or mass wasting on the ridge We make use of a recent study that identified large scale landslides [13] Formation Model Hypotheses Each model includes predictions about the crater population that, in turn, can be used to differentiate these models We compiled a list of models for ridge formation and expectant crater population (see Table 1) Testable predictions include the time frame of ridge formation, crater saturation, and elongated or transformed craters Crater saturation plays a major role in this study - if reached a lower age limit will be obtained Contact Information Preliminary Results [1] Dombard A. J. et al. (2012) JGR, 117, E03002. [2] Levinson H. F. et al. (2011) Icarus, 214, 773–778. [3] Ip W.–H. (2006) Geophys. Res. Lett., 33, L16203. [4] Porco C. C. et al. (2005) Science, 307, 1237–1242. [5] Castillo-Rogez J. C. et al. (2007) Icarus, 190, 179–202. [6] Robuchon G. et al. (2010) Icarus, 207, 959–971. [7] Giese B. et al. (2008) Icarus, 193, 359–371. [8] Czechowski L. and Leliwa-Kopystyński J. (2008) Adv. Space Res., 42, 61–69. [9] Roberts J. H. and Nimmo F. (2009) LPS XL, Abstract #1927. [10] Melosh H. J. and Nimmo F. (2009) LPS XL, Abstract #2478. [11] Sandwell D. T. and Schubert G. (2010) Icarus, 210, 817–822. [12] Beuthe M. (2010) Icarus, 209, 795–817. [13] Singer K. N. et al. (2012) Nature Geoscience, 5, 574-578. [14] Denk T. et al. (2010) Science 327, 435-439. Several hypotheses have been proposed for ridge formation, all of which fall under two categories: endogenic and exogenic Figure 1. The ridge is up to 20 km tall and 200 km wide The database, to date, contains 8,029 craters Craters range from 1 km to 591 km in diameter Total counts from leading and trailing hemispheres are 4,603 and 3,426 respectively Lower numbers in the trailing hemisphere could be an observational bias Amanda Damptz Andrew Dombard [email protected] [email protected] References Our preliminary results focus on the region of highest resolution (186,616 km 2 ) and contains 2,013 craters We have yet to perform a robust statistical analysis other than generate confidence intervals Our counts are similar to other published cumulative crater size-frequency distribution of Iapetus [14] The ridge appears to be less cratered (younger?) lt is possible that landslides effectively reset the surface producing a smaller crater population Orientation of craters located on the central ridge do not show a strong preference in the E-W direction but perhaps N-S It is unclear if this is an observational bias due to reprojection of the images of the ridge itself Off ridge, as expected, does not show any orientation preference of the long axis N-S E-W
Transcript of Testing Models for the Formation of the Equatorial …2,013 craters We have yet to perform a robust...
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Preliminary Conclusion:The crater population in the high-resolution region is most consistent with the impact generated formation model [1]
Table 1. Hypothesis Matrix Predictions1
Early Ridge Formation
Late Ridge Formation
Transformed Craters
E-W Elongated Craters Class Model Author(s)
Exogenic Impact Generated [1] 0 1 0 1 [2] 1 0 0 1
Ancient Ring System [3] 1 0 0 1 Endogenic Despinning [4] 1 0 1 0
[5] 1 0 1 0 [6] 1 0 1 0
Upwarping of Lithosphere [7] 1 0 0 0 [8] 1 0 0 0 [9] 1 0 x 0
Cryovolcanism [10] 1 0 0 0 Planetary Contraction [11] 1 0 1 0
[12] 1 0 1 0 1. Yes (1), No (0), Either/Or (x)
Testing Models for the Formation of the Equatorial Ridge on Saturn’s Moon Iapetus via Crater CountingAmanda Damptz and Andrew Dombard
Department of Earth and Environmental SciencesUniversity of Illinois at Chicago
IntroductionIapetus’s equatorial ridge is unique in the solar system (Fig. 1)
Formation of the ridge is likely attributed key events in the evolution of Iapetus
Purpose of this study: examine the crater population on and around the ridge, test the various models of ridge formation, and assess the age of the ridge
Endogenic ExogenicDespinning Impact GeneratedUpwarping of the Lithosphere Ancient Ring SystemCyrovolcanismPlanetary Contraction
MethodsWe use two global mosaics of Iapetus: (1) Cassini and Voyager data (c/o Cassini Image Science Team), (2) Cassini data only (c/o P.M. Schenk)
Craters are measured and cataloged in ArcGIS using the Crater Helper Tool An ellipse (6-point) is used to generate latitude, longitude, diameter, extent, and major and minor axis azimuth
All craters counted within 8° North and South are considered “on ridge”
Figure 2. A) Crater rim in high-resolution region B) Crater rim in low-resolution region
Identifying craters in areas of low-resolutionCrater rims in the high-resolution regions are indicated by bright pixels (Fig. 2A) Crater rims can then be identified in low-resolution regions by using the brightest pixels surrounding dark depressions (Fig. 2B)
Resolution of images available for Iapetus limits further crater classification (e.g., simple, complex, and multi-ring basin, ejecta deposits, primary and secondary craters, etc.)
Some features are somewhat ambiguous as to whether it is a crater or mass wasting on the ridge We make use of a recent study that identified large scale landslides [13]
Formation Model HypothesesEach model includes predictions about the crater population that, in turn, can be used to differentiate these models
We compiled a list of models for ridge formation and expectant crater population (see Table 1)
Testable predictions include the time frame of ridge formation, crater saturation, and elongated or transformed craters
Crater saturation plays a major role in this study - if reached a lower age limit will be obtained
Contact Information
Preliminary Results
[1] Dombard A. J. et al. (2012) JGR, 117, E03002. [2] Levinson H. F. et al. (2011) Icarus, 214, 773–778. [3] Ip W.–H. (2006) Geophys. Res. Lett., 33, L16203. [4] Porco C. C. et al. (2005) Science, 307, 1237–1242. [5] Castillo-Rogez J. C. et al. (2007) Icarus, 190, 179–202. [6] Robuchon G. et al. (2010) Icarus, 207, 959–971. [7] Giese B. et al. (2008) Icarus, 193, 359–371. [8] Czechowski L. and Leliwa-Kopystyński J. (2008) Adv. Space Res., 42, 61–69. [9] Roberts J. H. and Nimmo F. (2009) LPS XL, Abstract #1927. [10] Melosh H. J. and Nimmo F. (2009) LPS XL, Abstract #2478. [11] Sandwell D. T. and Schubert G. (2010) Icarus, 210, 817–822. [12] Beuthe M. (2010) Icarus, 209, 795–817. [13] Singer K. N. et al. (2012) Nature Geoscience, 5, 574-578. [14] Denk T. et al. (2010) Science 327, 435-439.
Several hypotheses have been proposed for ridge formation, all of which fall under two categories: endogenic and exogenic
Figure 1. The ridge is up to 20 km tall and 200 km wide
The database, to date, contains 8,029 craters
Craters range from 1 km to 591 km in diameter
Total counts from leading and trailing hemispheresare 4,603 and 3,426 respectively
Lower numbers in the trailing hemisphere could be an observational bias
Amanda Damptz Andrew [email protected] [email protected]
References
Our preliminary results focus on the region of highest resolution (186,616 km2) and contains 2,013 craters
We have yet to perform a robust statistical analysis other than generate confidence intervals
Our counts are similar to other published cumulative crater size-frequency distribution of Iapetus [14]
The ridge appears to be less cratered (younger?) lt is possible that landslides effectively reset the surface producing a smaller crater population
Orientation of craters located on the central ridge do not show a strong preference in the E-W direction but perhaps N-S It is unclear if this is an observational bias due to reprojection of the images of the ridge itself
Off ridge, as expected, does not show any orientation preference of the long axis
N-S E-W
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