Sea Ice Ridging and Rafting Structures on Mars and Earth

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

2. Introduction • Similarities found with sea ice features on Mars and Earth (Murray et al 2005). • 2 Types Examined Further Here • Pile-up behavior • Ridging and Rafting Features • Mechanisms of transition of Ductile to Brittle Behavior Observed on Earth • Sinusoidal Ridging: Comparison Earth and Mars • 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 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 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: • Horizontal forces necessary to supply potential force • 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 • 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 • Lower temperature or hydraulic transformation leads to decrease in brine volume • Precipitated salts decreases fluid volume • 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 • Ductile sheets create curved formations during deformation

9. Over thrusting sheets 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” Arctic Rafting Features

10. Arctic Ridging Features • 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 • Brittle Ridges Outline outer edges of Ductile thrusts • From above can look like a Sinusoidal formation

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

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

13. Sinusoidal Ridging 50m 100m 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 • Two phase mixture required to form these features • Mars • Lower amplitude indicates faster transition of ductile to brittle behavior • Colder weather conditions (accelerated freezing) • Water ice less saturated with brine (different composition?) • Arctic • 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 • Freezing temperatures (Mean T -63°C, range -140° C to -20° C) • Dry and thin atmosphere (~6 mbar CO2, 4% Earth)

17. Additional Possibilities • Composition? • Spectrometer instrument necessary to determine mineral composition • Observations from McMurdo area in Antarctica – Salts precipitating out of sea ice • Temperature decrease below eutectic area to form salt precipitates “Salt Flower” • Mirabilite (Na2SO4·10H2O) – 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 • 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 • Potential differences in mineralogy • 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.(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. • Neukum, G., Jaumann, R., HRSC Co-Investigator and Experiment Team. HRSC: the High Resolution Stereo Camera of Mars Express.