Lecture 13 Venus Earth and Mars But first Paper 4 is posted online Paper 4 is due at beginning of class on Mar 13 No extensions over Spring Break Keep up with reading through PBD 13 Astro 202 Prof Jim Bell jfb8 cornell edu Spring 2008 Comparing Venus Earth Mars Surface and Atmosphere of Venus Radar mapping results Greenhouse gone wild Surface and Atmosphere of Mars The view from orbiters landers and rovers Once Earthlike now not How Lessons for our home world Three special readings for the Terrestrial Climate Change lecture next Tuesday are also now posted online Venus and Earth So Similar Property Diameter km Mass kg Density g cm3 Gravity m sec2 Period years Surface Composition Earth 12 756 6 0x1024 5 52 9 8 1 0 basalt Venus 12 104 4 9x1024 5 25 8 9 0 62 basalt 4 Venus Seeing is Believing Yet So different Recall The surface of Venus is Property Spin Period days Tilt of Axis Earth 1 0 23 5 Surface Temperature 25 C Surface Pressure bars 1 Venus 243 0 177 4 480 C 90 Atmospheric Comp CO2 N2 N2 O2 Ar 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 Radar mapping of Venus done by the Soviet Venera and U S Magellan space missions 5 Radar Measures Roughness Smooth surfaces scatter radio waves back to the transmitter like a mirror specular In Venus pictures Bright Rough Dark Smooth Radar image of Venus Rough surfaces scatter radio waves in all directions diffuse 7 8 The Magellan Mission Very Simple Spacecraft Named after the 16th century Portuguese explorer whose expedition first circumnavigated the Earth Launched from the Space Shuttle in May 1989 Mapped Venus from Sept 1990 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 Essentially 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 compare B1 Bomber 200 million B2 Bomber 2 2 billion 9 Geology of Venus Magellan Results 10 Venus Topography Most of the surface is at the same narrow range of elevations vast lava plains Abundant evidence of past volcanism Nearly 1000 impact craters 1 km discovered Crater density implies 500 million year age Some small areas may 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 of surface area 11 12 Volcanism on Venus Venus Global Topography Hundreds of thousands of volcanoes have Magellan data show vast expanses of low lying lava plains Two large continents Aphrodite Africa sized been discovered on Venus Volcanism is widespread and planetwide Vast lava plains like the lunar mare Many individual cones and mountains Also evidence for subsurface volcanism Ishtar Australia sized highest point 11 km Maxwell Montes 13 Pancake domes each 25 km Sapas Mons 300 km diameter 14 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 Cinder cone field each 1 km indicates a uniform surface age of 500 million years since the last resurfacing Volcano and flows 100 km across 15 16 Three large craters largest 50 km across Impact Craters on Venus About 1000 found 1 km Pretty evenly distributed around the surface uniform age Small regions of lower crater density younger Golubkina Crater diameter 30 km Dickinson crater diameter 70 km 17 18 Crater ripped apart by tectonic forces diameter 37 km Tectonism on Venus 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 Stress fractures and cracks region is 40 km across tectonics however 19 20 Wind streak 5 km volcano Erosion on Venus Evidence for erosion in Magellan images Wind streaks Dunes Long channels carved into the rock No water So erosion must indicate the action of wind and molten lava Surface winds very weak long timescales Deep long lava channels long timescales 21 Lava channel 200 km long 2 km wide Segmented channel 22 The Tick Crater Volcano size 50 km Plus oddball landforms Several categories of geologic features found in Magellan images that defy simple categorization Coronae Arachnoids Mixed terrains Impact then volcanism then tectonism Sometimes our terrestrial experience doesn t help Humbling but to be expected 23 Circular corona feature Miss Piggy s Face Volcano 200 km across 24 View from the Surface Venera landers 1978 1981 Confirmed huge 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 What is Venus made of 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 implies the existence of lower density minerals too Maybe similar to Earth like granites We just don t know 26 Critical Question Why are the Atmospheres of Earth and Venus so different What Makes Venus Hot The Greenhouse Effect Answer Earth and Venus have had divergent evolutionary histories 28 Recall the Concept Lecture 12 no greenhouse Result Greenhouse Warming with greenhouse Mars T 5 C Earth T 35 C Venus T 500 C Note Greenhouses also get warm because they prevent convective cooling by the wind so the term greenhouse effect is somewhat misleading 29 How Did Venus Get This Way Runaway Greenhouse Model Venus and Earth are assumed to have formed from essentially the same material with the same initial amounts of H2O CO2 N2 etc But divergent evolution occurred because Venus is closer to the Sun so H2O can exist only in vapor gas form At Earth s distance liquid H2O could form Model the atmospheres as time progresses 31 30 Early Earth Stage 1 Earth forms from accretion of small