Solar System Exploration: Space Science Highlight

Featuring the Research ofSuzanne 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

9.25.2002 Space Science Highlight prepared by Richard Shope 2

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 todayBy examining radar image mapping sent back byNASA’s Magellan Mission nearly a decade ago…

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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

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Polygons formed from cooling of volcanic lava flowDiameter measured in meters

Did Giants form these polygons on Earth…?

Giant’s Causeway…Ireland

http://www.giantscausewayoffici

alguide.com

Venus Polygons…Diameter measuredIn thousands of meters

Typically ~2 kilometers

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Volcanism?Subsurface Heating?

Global Climate Change?

Evidence:Uniform size ~ 2km across

Global distribution ~ 200 sitesFits Global Climate Change Model

Venusian Polygons…What titanic forces caused them to form?

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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

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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 RESULTSPolygonal fractures on Venus indicate Global Climate Change process

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Tectonic Effects of Global Climate Change on Venus

Time0

~ 750 Million years ago

100 Million Years 100 Million Years 100 Million Years

900˚K

800˚K

700˚K

Surface

Temperature

Volatiles release, clouds form H

escapes

SO2

Clouds reduce

H2O

Climate stabilizes

ReheatingCausesExpansion

RapidCooling CausesContraction

To depth:of ~20 km

GradualCooling Results inAdditionalExtensions

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TALKING POINTS FOLLOW FROM THIS POINT

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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…

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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 H2O and SO2, which form clouds leading to a temperature rise of 90º-200º.

2. Exospheric escape of H2O 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 RESULTSFeatures Best Explained by Atmospheric and Surface Interaction

Observations tend to validate global climate change model

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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

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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.

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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