1. Sea Ice Ridging and Rafting Structures on Mars and Earth Laboratory for Remote Sensing and Geoinformatics Department of Earth and Environmental Science University of Texas at San Antonio Stephen F. Ackley, Penelope Wagner, and Hongjie Xie

2. Introduction Similarities found with sea ice features on Mars and Earth (Murray et al 2005). Similarities found with sea ice features on Mars and Earth (Murray et al 2005). 2 Types Examined Further Here 2 Types Examined Further Here Pile-up behavior Pile-up behavior Ridging and Rafting Features Ridging and Rafting Features Mechanisms of transition of Ductile to Brittle Behavior Observed on Earth Mechanisms of transition of Ductile to Brittle Behavior Observed on Earth Sinusoidal Ridging: Comparison Earth and Mars Sinusoidal Ridging: Comparison Earth and Mars Compositional similarities and differences? Compositional similarities and differences?

3. Baltic Sea Pile-up  Pile up with longer flow area behind (Lepparanta, 2005)  Pileup thickness is a multiple of the ice sheet thickness, related to the area of cleared channel

4. Mars Pile-up 20m  Martian pile-up followed by clear area flow (Murray et al., 2005)  Same orientation of flow direction  Variable ratio of clear area to pile-up area up to 7:1 (HRSC, ESA, Mars Express)

5. 20m

6. Discrete Element Pile-up Simulation Model Pile-up at Earth’s gravity (~1.0)Pile-up at Mars gravity (0.38g)  Similar Mechanism – 2 Differences (Hopkins et al., 1999)  Lower gravity potential energy proportional to increased pile-up height  Lower normal force reduces friction proportional to gravity, increasing pileup height on Mars

7. Pile up Pile-up height related: Pile-up height related: Horizontal forces necessary to supply potential force Horizontal forces necessary to supply potential force Square root of sheet ice thickness Square root of sheet ice thickness Different Mars Pileup Height also due to 1/3 gravity to that of Earth affecting Potential Energy and Friction Different Mars Pileup Height also due to 1/3 gravity to that of Earth affecting Potential Energy and Friction Mars Potential Mechanism: 1 Main Event possibly from an outburst flood (single direction of the pileups) Mars Potential Mechanism: 1 Main Event possibly from an outburst flood (single direction of the pileups)

8. Transition: Ductile Brittle ~10m – 20m ductile over thrusts of adjacent sheets ~10m – 20m ductile over thrusts of adjacent sheets Lower temperature or hydraulic transformation leads to decrease in brine volume Lower temperature or hydraulic transformation leads to decrease in brine volume Precipitated salts decreases fluid volume Precipitated salts decreases fluid volume Ice sheet pushed out of fluid and onto solid surface causing drainage through porous structure Ice sheet pushed out of fluid and onto solid surface causing drainage through porous structure Loss of fluid leads to stiffening to increase brittle behavior Loss of fluid leads to stiffening to increase brittle behavior Ductile sheets create curved formations during deformation Ductile sheets create curved formations during deformation

9. Arctic Rafting Features Over thrusting sheets Over thrusting sheets Higher brine volumes related to higher ductile material Higher brine volumes related to higher ductile material Accommodate large strain w/o breaking for tens of meters Plastic deformation at low stress “Finger rafting ”

10. Arctic Ridging Features High brine volume gives high angle plasticity initially High brine volume gives high angle plasticity initially Lower brine volumes from freezing related leads to brittle material that then Fractures at low strain by own weight of sheet Lower brine volumes from freezing related leads to brittle material that then Fractures at low strain by own weight of sheet Brittle Ridges Outline outer edges of Ductile thrusts Brittle Ridges Outline outer edges of Ductile thrusts From above can look like a Sinusoidal formation From above can look like a Sinusoidal formation

11. Ridging and Rafting Features: Arctic Finger Rafting Linear Ridging Sinusoidal Ridging

12. 100m HiRise Mars images (25cm per pixel) Arctic images

13. Sinusoidal Ridging 100m 50m HiRise Mars images (25cm per pixel)Arctic images

14. Mars and Arctic Ridging Comparison  Mars values higher at lower distances  Arctic values evenly distributed

15. Mars and Arctic Ridging Comparison Similar processes Similar processes Two phase mixture required to form these features Two phase mixture required to form these features Mars Mars Lower amplitude indicates faster transition of ductile to brittle behavior Lower amplitude indicates faster transition of ductile to brittle behavior Colder weather conditions (accelerated freezing) Colder weather conditions (accelerated freezing) Water ice less saturated with brine (different composition?) Water ice less saturated with brine (different composition?) Arctic Arctic Evidence more ductile material due to finger rafting features Evidence more ductile material due to finger rafting features

16. Mars Thermal Stiffening Small temperature changes (<5C) cause large phase changes and mechanical behaviors of these features found in the Arctic Small temperature changes (<5C) cause large phase changes and mechanical behaviors of these features found in the Arctic Freezing temperatures Freezing temperatures (Mean T -63°C, range - 140° C to -20° C) Dry and thin atmosphere Dry and thin atmosphere (~6 mbar CO2, 4% Earth)

17. Additional Possibilities Composition? Composition? Spectrometer instrument necessary to determine mineral composition Spectrometer instrument necessary to determine mineral composition Observations from McMurdo area in Antarctica – Salts precipitating out of sea ice Observations from McMurdo area in Antarctica – Salts precipitating out of sea ice Temperature decrease below eutectic area to form salt precipitates “Salt Flower” Temperature decrease below eutectic area to form salt precipitates “Salt Flower” Mirabilite (NaSO·10HO) – Salt dune formations from precipitated salts Mirabilite (Na 2 SO 4 ·10H 2 O) – Salt dune formations from precipitated salts

18. Discussion and Conclusions Similar pile-up and ridging structures found on Mars and Earth indicative of sea ice formation Similar pile-up and ridging structures found on Mars and Earth indicative of sea ice formation Earth ridging features exhibits increased ductile behavior to create larger thrusting features Earth ridging features exhibits increased ductile behavior to create larger thrusting features Mars features are consistently smaller due to a faster transition following the 2-phase system from ductile to brittle behavior Mars features are consistently smaller due to a faster transition following the 2-phase system from ductile to brittle behavior Potential differences in mineralogy Potential differences in mineralogy Apparent differences in climate and gravitational potential energy Apparent differences in climate and gravitational potential energy

19. References Hopkins, Mark A. (1998) Four stages of pressure ridging. JGR vol. 103, pp. 21,883-21,891. Hopkins, Mark A. (1998) Four stages of pressure ridging. JGR vol. 103, pp. 21,883-21,891. Hopkins, Mark A. (1999) Rafting and ridging of thin ice sheets. JGR vol. 104, pp. 13,605-13,614. Hopkins, Mark A. (1999) Rafting and ridging of thin ice sheets. JGR vol. 104, pp. 13,605-13,614. Murray, J.B., Muller, J., Neukum, G., Werner, S., Gasselt, S., Hauber, E., Markiewicz, W., Head, J., Foing, B., Page, D., Mitchell, K., Portyankina, G., (2005) Evidence from the Mars Express High Resolution Stereo Camera for a frozen sea close to Mars’ equator. Nature vol. 434, pp. 352-355. Murray, J.B., Muller, J., Neukum, G., Werner, S., Gasselt, S., Hauber, E., Markiewicz, W., Head, J., Foing, B., Page, D., Mitchell, K., Portyankina, G., (2005) Evidence from the Mars Express High Resolution Stereo Camera for a frozen sea close to Mars’ equator. Nature vol. 434, pp. 352-355. Neukum, G., Jaumann, R., HRSC Co-Investigator and Experiment Team. HRSC: the High Resolution Stereo Camera of Mars Express. Neukum, G., Jaumann, R., HRSC Co-Investigator and Experiment Team. HRSC: the High Resolution Stereo Camera of Mars Express.