1. Solar System Exploration: Space Science Highlight Featuring the Research of Suzanne E. Smrekar, Pierre Moreels, and Brenda J. Franklin To be published in the Journal of Geophysical Research: JGR Planets Characterization and Formation of Polygonal Fractures on Venus Contact: Suzanne.E.Smrekar@jpl.nasa.gov Polygonal Plains on Venus Linked to Global Climate Change

2. 9.25.2002Space Science Highlight prepared by Richard Shope1 Greenhouse effects of global warming occur on both planets. Interplay between the atmosphere and lithosphere can stabilize--or destabilize-- global climate change over multimillion year time scales… What can we learn about Earth by exploring Venus? Planetary Scientists are exploring Venus today By examining radar image mapping sent back by NASA’s Magellan Mission nearly a decade ago… Solar System Exploration: Space Science Highlight

3. Polygons average ~2 km across. Are polygons stratigraphic markers? What play of forces best explains the magnitude and uniformity of polygon size at over 200 sites? Global Climate Change Model Fits Best! Solving the Mystery of the Polygons of Venus Solar System Exploration: Space Science Highlight

4. 9.25.2002Space Science Highlight prepared by Richard Shope3 Polygons formed from cooling of volcanic lava flow Diameter measured in meters Did Giants form these polygons on Earth…? Giant’s Causeway…Ireland http://www.giantscauseway officialguide.com Venus Polygons… Diameter measured In thousands of meters Typically ~2 kilometers Solar System Exploration: Space Science Highlight

5. 9.25.2002Space Science Highlight prepared by Richard Shope4 Volcanism? Subsurface Heating? Global Climate Change? Evidence: Uniform size ~ 2km across Global distribution ~ 200 sites Fits Global Climate Change Model Venusian Polygons…What titanic forces caused them to form? Solar System Exploration: Space Science Highlight

6. 82 km Example of gradation in size from large to small. Polygons in this region range from 6 km near top to 1 km near bottom. (Fmap 35S253, Figure 1 in Smrekar et. al. JGR-Planets, to be published.) Venusian polygons have average diameter of 1.8 ± 0.9 km Several orders of magnitude larger than on Earth Solar System Exploration: Space Science Highlight

7. Global Distribution of Polygons on Venus Model validated by new study: Anderson and Smrekar (1999) proposed Global Climate Change Model as mechanism of formation of polygonal terrain: global warming following by gradual cooling produces sufficient thermal stresses to produce strains on surface and observed fracture spacing. SIGNIFICANT RESULTS Polygonal fractures on Venus indicate Global Climate Change process Solar System Exploration: Space Science Highlight

8. Tectonic Effects of Global Climate Change on Venus Time 0 ~ 750 Million years ago 100 Million Years 900˚K 800˚K 700˚K SurfaceSurface TemperatureTemperature Volatiles release, clouds form H escapes SO 2 Clouds reduce H2OH2O Climate stabilizes Reheating Causes Expansion Rapid Cooling Causes Contraction To depth: of ~20 km Gradual Cooling Results in Additional Extensions Solar System Exploration: Space Science Highlight

9. TALKING POINTS FOLLOW FROM THIS POINT Solar System Exploration: Space Science Highlight

10. 9.25.2002Space Science Highlight prepared by Richard Shope9 Venus Polygons How did they form? THREE Proposed formation mechanisms 1.Volcanism: Cooling following lava flow emplacement Why? –Polygons typically found on surface of slowly cooled lava flows Why NOT? –To obtain large size, would need to be a lava flow 20 km thick 2.Lithospheric heating Why? Local heating events consistent with lithospheric cooling Why NOT? Inconsistent with variable size and pattern of diminishing size moving away from center of heating 3.Global Climate Change… Solar System Exploration: Space Science Highlight

11. 9.25.2002Space Science Highlight prepared by Richard Shope10 Anderson and Smrekar (1999) propose Global Climate Change Model as mechanism of formation of polygonal terrain: global warming following by gradual cooling produces sufficient thermal stresses to produce strains on surface and the observed fracture spacing. Builds on work of Bullock and Grinspoon (1996, 2001): 1.Resurfacing event ~750 M years ago triggers volatile outgassing of H 2 O and SO 2, which form clouds leading to a temperature rise of 90º-200º. 2.Exospheric escape of H 2 O and reaction w/surface minerals causes reduction in cloud cover leading to cooling. 3.Over time atmospheric albdeo and opacity return to normal and temperature equilibrates. The whole cycle lasts 300 million years. SIGNIFICANT RESULTS Features Best Explained by Atmospheric and Surface Interaction Observations tend to validate global climate change model Solar System Exploration: Space Science Highlight

12. 9.25.2002Space Science Highlight prepared by Richard Shope11 Polygon Plains are found at 204 sites all over Venus Nathan Bridges has mapped Venusian geologic regions About 50% of polygons are near shield volcanoes About 50% of polygons are in regions of varied terrain Solar System Exploration: Space Science Highlight

13. 9.25.2002Space Science Highlight prepared by Richard Shope12 Future Mapping May Reveal Polygons as Stratigraphic Markers Venus appears to have experienced a period of high resurfacing rate consistent with volcanic outgassing capable of producing global climate change--if all polygons formed in same period, mapping efforts could explore whether polygons are stratigraphic markers. Section of Fmap 77N355, with a size of 66 x 53 km. Locations “a” show polygons crossing an apparent flow boundary. At “b” a younger volcano covers preexisting polygons. Solar System Exploration: Space Science Highlight

14. 9.25.2002Space Science Highlight prepared by Richard Shope13 Lower Right Related Papers Anderson, F.S. and S.E. Smrekar, Tectonic effects of climate change on Venus, J. Geophys. Res, 104, 30, 743-30, 756, 1999. Bullock, M. A., and D.H. Grinspoon, The stability of climate on Venus, J. Geophys. Res., 101, 7521-7529, 1996. Bullock, M. A., and D.H. Grinspoon, The recent evolution of climate on Venus, Icarus, 150, 19-37, 2001. Solar System Exploration: Space Science Highlight