The evolution of Venus, a global perspective — from exogenic to endogenic over time Vicki L. Hansen, University of Minnesota, MN, USA Ivan Lopez, Universidad Rey Juan Carlos, Madrid, Spain Thanks to: NASA PGG, NASA PGGURP, NASA PDAP, McKnight Foundation, and U.S. citizens and taxpayers! Sister planets: similar: size, density, composition, heat budget, solar location... Geologists ‘read’ the geologic record to gain What is the geologic insight into operative history, and hence processes. evolution of Venus, that we can glean from study of the surface geology? Similar (but not the same) at birth— How different (or similar) have their evolutionary paths been? Clues for Venus from Earth? Clues about early Earth from Venus? VENUS DATA: NASA global Magellan mapping Mission (1989-1994) *SAR (Synthetic Aperture Radar) *Altimetry (topography) *Gravity [*Emissivity] RESULTS & IMPLICATIONS: *Ultra-dry: no water no erosion, no burial *No plate tectonics; THE SETTING: therefore no massive recycling of the lithosphere T ~475 °C; 750 K; ~900 °F P ~100 bars (similar to Early Earth?) *An incredibly detailed & complex surface record is Dry! preserved; therefore many Atm: 96.5% CO2; 3.5% N2, H2O, geologic clues may be SO2, Ar, CO, Ne, HCl, HF archived Atmosphere is supercritical CO2 Surface: basalt Venus below the skin—similar to Earth? Crust (silicate) similar to Earth Mantle (silicate) similar to Earth Core (liquid iron-nickel alloy) similar to Earth... except, inner & outer core? Venus, composition of the skin — similar to average Earth Soviet Venera Mission landers returned pictures and bulk chemical analyses of the surface Venus’ surface is interpreted as dominated by basalt — common on Earth Above the skin—Venus’ run away greenhouse effect CO2 exists as a supercritical fluid... north Combined Magellan SAR The shape of the ‘skin’ & altimetry data can tell us a lot about Regional scale geodynamic processes Geomorphic Features Niobe Ganis Chasma Unique features: Planitia Artemis Ishtar Terra Lowlands Rusalka Hecate Chasma Planitia Planitia Atla Thetis Regio Mesolands Regio Corona & Chasma Parga Chasma chains Highlands Diana-Dali Chasma Artemis Volcanic Rises Contemporary features Nsomeka Crustal Plateaus Planitia Ancient features south north Combined Magellan SAR Ishtar & altimetry data Terra Regional scale Tellus Geomorphic Features Bell Regio Regio Unique features: Niobe Artemis e.Eistla Planitia Ishtar Terra Regio c.Eistla Lowlands Regio Western Planitia Ovda Ovda Regio Thetis Mesolands Regio Corona & Chasma chains Highlands Aino Artemis Volcanic Rises Planitia Contemporary features Crustal Plateaus Nsomeka Ancient features Planitia south Focus here: Shape of the skin... reference: Mean Planetary Radius Unlike Earth.... MPR = 6051.8 km Venus’ topography is unimodal Volcanic rises (contemporary) Highlands unique features: Crustal plateaus Mesolands Ishtar Terra & Artemis (ancient) Lowlands (Hansen 2014, Springer) Ribbon tessera terrain (RTT) global distribution Detailed geologic mapping of the global folds distribution (layer shortening) of ribbon ribbons (layer extension) tessera terrain outcrops and tectonic fabric patterns graben SAR image (Phillips & Hansen 1998 Science) RTT defines high-standing crustal plateaus RTT occurs in tracts across the lowlands RTT records processes of an ancient era 120°E m (+/- Mean Planetary Radius) lowland RTT -2930 11620 500 km RTT folds ribbons V11 V12 V23 V24 (Hansen & Lopez 2010 Geology) 120°E m (+/- Mean Planetary Radius) lowland RTT -2930 11620 500 km RTT folds ribbons V11 V12 V23 V24 (Hansen & Lopez 2010 Geology) 120°E m (+/- Mean Planetary Radius) lowland RTT -2930 11620 500 km RTT folds ribbons V11 V12 V23 V24 (Hansen & Lopez 2010 Geology) Globally, RTT 120°E records a rich early surface history, yet later? early to be quite revealed late? mid early? early mid late late Clues from global distribution m (+/- Mean Planetary Radius) 0°E 60°E 120°E 75°N Lakshmi Ishtar 180°E -2930 11620 Terra Fortuna 50°N Tellus Bell Beta Niobe Planitia w.Eistla 25°N 25°N e.Eistla c.Eistla 300°E Rusalka 0°E Planitia 240°E western Aphrodite Terra Atla 0°N 330°E Ovda Phoebe Thetis eastern Aphrodite Terra Alpha 25°S Dione Artemis Themis Imdr 50°S Lada 90°E (Hansen & Lopez 2010 Geology) 75°S 1. RTT occurs across much of the surface, despite much more recent burial 2. RTT (~12%) & shallowly-buried RTT occur across >35% of the surface 3. RTT occurs in some of the deepest lowland basins 4. Observations 1-3 (#3 in particular) are inconsistent with global catastrophic resurfacing hypotheses 5. RTT’s rich global surface history is difficult to reconcile with Venus ever having hosted plate tectonics RTT tectonic fabric preserves about crustal plateau formation...true stereo image ribbon & graben trends (layer extension) graben fold trends (layer shortening) 100 km SAR artifacts provide clues to structural fabric geometry Structural wavelengths provide clues about layer strength (thickness) ribbon trend fold trend fold trend 25 km We can decipher a rich geologic history, which provides clues bout RTT (and crustal plateau) formation 25 km fold trend (layer shortening) ribbon & graben trend (layer extension) layer extension & shortening, and local flooding from below... 25 km ribbons late-formed folds graben complexes lava fill The thin surface layer deforms like taffy (brittle & ductile) with liquid being locally leaked into surface lows, throughout the progressive deformation of the surface. Short-λ folds & ribbons formed broadly synchronously, deforming a layer <<1 km thick data from transects/study areas A-H (Ovda Regio) Average ribbon layer thickness Ribbon layer thickness Fortuna Tessera (Hansen & Willis 1998 Icarus) layer shortening layer extension average layer thickness (Hansen 2006 JGR) RTT history derived from wavelength analysis and X-cutting relations, with clues from experimental & theoretical modeling timetime 11:: thinthin layer (solid above melt) melt timetime 22:: orthogonalfolding of thin folding layer & extension of thin layer melt [<<1 km thick] (formed short-λ folds & ribbons) time 3: early formed short-wavelength time 3: earlier-formed fabrics carried piggy-back folds were carried piggy-back on on intermediate-λ folds melt intermediate-wavelength folds. time 4: earlier-formed fabrics were carried piggy-back on long-λ folds (with late extension along graben complexes) flooding of local lows local of flooding time 4: these earlier formed folds were carried piggy-back on even ductile solid longer-wavelength folds. •short-λ & intermediate-λ fold fabrics require an extremely high viscosity contrast — i.e., solid above and liquid below •long-λ folds (actually warps) Duringformed time by uplift 1-3 the surface layer was solid with melt below; Local lows were flooded by lava from below during the entire deformation. of crust with strong- weak-strong layer rheology (Hansen 2006 JGR) RTT is basically a rocky ‘scum’ formed on huge ‘ponds’ of lava RTT is basically a rocky ‘scum’ formed on huge ‘ponds’ of lava How could such large ponds of lava form? Re(turn) to Earth for possible mechanism... Ingle & Coffin (2002, 2004) proposed that the Ontong Java Plateau formed by bolide impact; Ontong Java Plateau is similar in size to a Venus crustal plateau Modeling indicates the viability of huge lava pond formation in this manner (e.g., Jones et al. 2002, 2005; Elkins-Tanton & Hager 2005) (Jones et al. 2005) need: thin lithosphere & large bolide Ingle & Coffin (2004 EPSL) So let’s form a huge pond of lava... Needed: A) Thin lithosphere B) A large bolide thin lithosphere mantle (Hansen 2006 JGR) Cartoon illustrating formation of ribbon tessera & crustal plateaus So let’s form a huge pond of lava... thin lithosphere massive partial melt zone large bolide impact causes massive partial melting in the mantle mantle (Hansen 2006 JGR) Cartoon illustrating formation of ribbon tessera & crustal plateaus melt rises to the At the surface.... surface forming a HUGE lava pond thin lithosphere bolide impact causes In the mantle.... massive partial melting in the mantle mantle (Hansen 2006 JGR) Cartoon illustrating formation of ribbon tessera & crustal plateaus solidification (freezing) of the lava pond forms RTT At the surface.... fabric as ‘pond scum’, driven by pond melt convection; lava leaks to the surface filling local topographic lows thin lithosphere mantle melt ‘residuum’ the melt ‘residuum’ left behind What remains in in the mantle is Mg-rich, the mantle is buoyant, dry, & strong also important! once residuum forms, it cannot easily be recycled to the deep mantle because it is so buoyant mantle (Hansen 2006 JGR) Cartoon illustrating formation of ribbon tessera & crustal plateaus At the surface.... the solidified lava pond (RTT) becomes elevated, forming a crustal plateau thin lithosphere mantle melt ‘residuum’ melt residuum leads to uplift of Mantle materials the overlying lithosphere, creating an elevated plateau decorated by play a role as the solid lava pond well... uplift results in late-stage long- wavelength warping and local extension of the lava-pond surface mantle (Hansen 2006 JGR) Cartoon illustrating formation of ribbon tessera & crustal plateaus At the surface... the solidified lava pond (RTT) is lowered (or never raised to plateau status) — forming lowland RTT inliers thin lithosphere Mantle materials mantle convection could sweep away the low density residuum play a role as well... and can in this case a plateau would not form be swept from the picture! and the strong residuum could be moved elsewhere in mantle the