1. The Valley Network Record on Mars (Class 1)

2. Challenges for our understanding: Climate models have a very hard time raising the Martian surface temperature significantly, even with a thick CO 2 greenhouse Standard stellar evolution models suggest that the solar output was ~70% current at 4 Gya Mineralogy of surface preserves a record of “easy to weather” volcanic minerals from Early Mars (olivine, pyroxene) But mineralogy now also reveals a bunch of weathering products! Cold today, cold tomorrow, warm 3.8 billion years ago?

3. How do valleys on Earth compare to Mars? On Earth: Erosion linked to surface precipitation and runoff is very important to the shape of the land at local-to-mega- regional scales*,** * even to the point of coupling tectonics to erosion! ** fluvial activity sets the broad form of most surfaces Drainage density is usually >>0.07 km/km 2 ; actual drainage densities are a result of a complicated competition between hillslope (diffusive) and valley- forming (advective) processes. This competition is linked to climate… (Think back to first class). Yemen

4. How do valleys on Mars compare to Earth? On Mars: Valley formation does not appear to have modified the mega- regional scale topography… Local densities can reach near- terrestrial values, but smallest tributaries are uncommon (Why?) Valleys do reach to drainage divides (groundwater alone is not a great explanation) Valleys we observe now are likely >3.5+ Gyr old…(What characteristics can we ascribe to this?) Huygens Crater East Rim Region 40E, 20S HRSC 0532 MOLA topography

5. 500 km Naktong Vallis/ Scamander Vallis Mamers Vallis system (after Irwin et al., 2005). Some basins appear integrated over length of ~3000-5000 km Numerous (200+) hydrologically ‘open’ lakes (lakes with outlets) across Mars

6. What do we want to know? Intensity Precipitation[?] rates? Discharges? Persistence Global or local activity? Long-lived climate or short excursions? Intermittency? One episode or many? Formation Time (prime topic of next class) Environmental Requirements (and/or) Implications Is an ‘ocean’ necessary to support a climate where it rains or snows? Are large lakes good enough?