The Main Point. Scattering of Radio Waves. Radar Mapping. Lecture #17: Surface of Venus:

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1 Surface of Venus: Lecture #17: Radar Mapping Concepts. The Magellan mission. Surface properties. Reading: Chapter 9.5. The Main Point Radar has been used to map the surface of Venus, and we have found a relatively young surface with abundant evidence for volcanic, tectonic, erosional, and impact processes at work. Astro 102/104 1 Astro 102/104 2 Radar Mapping Recall: The surface of Venus is obscured from view at visible to infrared wavelengths by a thick layer of clouds. But light at radio wavelengths (~1 mm to ~1 m) penetrates the clouds. Radar (an acronym for RAdio Detection And Ranging) is used to study the surface of Venus remotely. Radio sensors are either passive or active. Passive systems simply record radio energy emitted from a surface. Active systems also record the reflected radio waves they transmit. Radar mapping of Venus (and Earth) uses active systems: Arecibo radio telescope. Spacecraft like Magellan or Venera. All transmit radio waves, then measure the reflected radio energy. Smooth surfaces scatter radio waves back to the transmitter like a mirror (specular). Rough surfaces scatter radio waves in all directions (diffuse). Scattering of Radio Waves Astro 102/104 3 Astro 102/

2 Interpretation of Radar Images Radar images are different from optical images and can be somewhat counterintuitive. For most geologic targets, the strength of the radar echo depends primarily on the roughness of the surface (bright = rough, dark = smooth). Radar image of Venus Astro 102/104 5 The Magellan Mission Named after the 16th century Portuguese explorer who first circumnavigated the Earth. Launched from the Space Shuttle in May Mapped Venus from Sept to Oct. 1994: Covered 98% of Venus' surface with a resolution of ~ 1 km. Measured surface topography to ~100 m vertical resolution. Measured global gravity field to high accuracy. Provided data for insights on surface geologic processes and the nature of the planet's interior. Astro 102/104 6 A Relatively Simple Spacecraft! Geology of Venus: Magellan Results Most of it is just a large satellite dish with a transmitter and receiver. Used spare parts from Voyager and other Earth and planetary missions. Cost: about $650 million Astro 102/104 7 Astro 102/

3 Geology of Venus: Magellan Results Abundant evidence of past volcanism. Nearly 1000 impact craters > 1 km discovered: Crater density implies ~750 million year age. Some small areas could be much more recent. No "ancient" surface preserved. Abundant evidence of tectonism: But no clear evidence of plate tectonics like Earth. Some evidence for erosion: dunes, wind streaks, lava channels. Astro 102/104 9 Venus Topography: Most of the surface is at the same narrow range of elevations (vast lava plains). % of surface area Astro 102/ Venus Global Topography Magellan data show vast expanses of low-lying lava plains. Two large "continents": Aphrodite (Africa-sized). Ishtar (Australia-sized): highest point 11 km (Maxwell Montes). Astro 102/ Volcanism on Venus Hundreds of thousands of volcanoes have been discovered on Venus! Volcanism is widespread and planetwide. Vast mare-like lava plains dominate. But many individual cones and mountains. Also: evidence for subsurface magma intrusions. Astro 102/

4 "Pancake" domes each ~25 km Volcano and flows ~100 km across Cinder cone field each ~1 km Sapas Mons ~300 km diameter Astro 102/ Impact Craters on Venus Nearly 1000 craters > 1 km diameter have been discovered in Magellan images: Very few craters smaller than 10 km. Impactors smaller than 1 km in size burn up! Craters 10 km to 30 km have irregular shapes. Relative Age Dating (counting craters) suggests a ~uniform surface age of ~750 million years since the last resurfacing. Astro 102/ Impact Craters on Venus About 1000 found > 1 km. Pretty evenly distributed around the surface (uniform age?) Three large craters (largest ~50 km across) Dickinson crater (diameter ~70 km) Golubkina Crater (diameter ~ 30 km) Astro 102/ Astro 102/

5 Tectonism on Venus Stress fractures and cracks (region is ~40 km across) Numerous patterns of ridges and cracks seen in Magellan images: Evidence of tectonic forces. Probably driven by convection in the interior. Evidence for mountain belts and rift valleys. No direct evidence for Earth-like plate tectonics, however. Crater ripped apart by tectonic forces (diameter ~37 km) Astro 102/ Astro 102/ Erosion and Transport on Venus Evidence for wind transport in Magellan images: Wind streaks. Dunes. Surface winds are weak, but the atmosphere is very dense. Long, thin channels, but there s no water! Erosion by flowing lava. Astro 102/ Wind streak ~5 km volcano Lava channel ~200 km long Astro 102/104 ~2 km wide 20 5

6 Plus oddball landforms! Several categories of geologic features found in Magellan images that are not familiar on Earth Coronae. Arachnoids. Mixed terrains. Impact, then volcanism, then tectonism...??? Sometimes our terrestrial experience doesn t help. Humbling, but to be expected? The Tick A volcano? (size ~50 km) Circular corona feature (~200 km across. Formed by rising blobs of hot mantle material? Astro 102/ Astro 102/ On the Surface Venera landers returned images and other data: Confirmed surface temperature and pressure. Enough light gets through the clouds to take pictures. Surface is flat, platy, maybe layered...? Landscape is desolate and totally inhospitable to life. Surface Composition Soviet Venera 13 and 14 landers measured the chemical composition of the surface: Elemental abundances and inferred mineralogy is similar to basalt, a common low-si volcanic rock found on the Earth, Moon, and Mars. Presence of distinct "continents" may imply the existence of lower-density minerals too. Astro 102/ Astro 102/

7 Summary Radar has been used to penetrate the clouds of Venus and discover the underlying geology of the planet: Radar bright features are rough. Radar dark features are smooth. Abundant evidence for volcanism, tectonism, impact, and erosion on Venus! Entire planet appears to have been resurfaced, possibly by volcanoes, ~750 million years ago. Surface composition appears similar to Earth's volcanic rocks. Astro 102/ Venus atmosphere and climate: Properties of the atmosphere. Greenhouse Effect. Climate Implications. Reading: Chapters 9.5, 9.6, 10.5, Next Lecture... Astro 102/