PTYS 411 Geology and Geophysics of the Solar System Shane Byrne – [email protected] Background is from Pioneer Venus History of Venus

PTYS 411 Geology and Geophysics of the Solar System

Shane Byrne – [email protected] is from Pioneer Venus

History of Venus

PYTS 411– History of Venus 2

Venus today Comparison to Earth Venusian atmosphere Water and magnetic fields

Geologic record Volcanic resurfacing Tectonic features The lack of craters Putting events in order

Resurfacing models

In this lecture

Surface activity on the Moon and Mercury mostly died off about 3 Ga

Surface history of Venus is only available from ~1.0 Ga onward (not dissimilar to Earth)

Surface activity and history of Mars spans its entire existence

…as opposed to…


81.5% of the mass of the Earth Slightly higher mean density (5230 kg m-3) Formed in a similar location – 0.72 AU

Implies a similar bulk composition

Comparisons to Earth

Earth Venus


Massive CO2 atmosphere with intense greenhouse effect

93 bars,740 K at mean surface elevation Altitude variations 45-110 bars, 650-755 K

No day/night or equator/pole temperature variations

3 distinct cloud-decks Composed of sulfuric acid droplets Produced by photo-oxidation of SO2

Effective scavenger of water vapor Layers differ in particle size Very reflective (albedo 70%) keeps surface much

cooler than it would otherwise be

100 ms-1 east-west at altitude of 65 km Drives cloud layer around planet in ~4 days Reasons for super-rotating atmosphere are

unknown True surface (243 day - retrograde) rotation

period found with terrestrial radar.

Atmosphere of Venus


Earth has obvious topography dichotomy

High continents Low ocean floors

Venus has a unimodal hypsogram

No spreading centers No Subduction zones No plate tectonics

How is this topography supported??

Topography


Earth and Venus should be the same… Venus absorbs roughly the same amount of

sunlight as the Earth. Venus has roughly the same amount of carbon

as the Earth

…but… Venus has no plate tectonics Earth’s carbon get recycled through the crust Venusian carbon accumulates in atmosphere –

regulated by ‘Urey reaction’?

CaCO3 + SiO2 = CaSiO3 + CO2

(calcite) + (silica) = (wollastonite)

log10PCO2 = 7.797 – 4456/TEquilibrium gives 92 bars at 742 K

What went wrong?

All these differences can be traced back to the lack of water on Venus


Why didn’t this happen on the Earth ? Earth has water that rains Rain dissolves CO2 from the atmosphere

Forms carbonic acid

This acidified rainwater weathers away rocks Washes into the ocean and forms carbonate rocks Carbonate rocks eventually recycled by plate tectonics

The rock-cycle keeps all this in balance Sometimes this gets out of sync e.g. snowball Earth – stops weathering


Venus started with plenty of water Temperatures were just a little too high to allow rainfall Atmospheric CO2 didn’t dissolve and form carbonate rocks

Venus and Earth have the same amount of CO2 Earth’s CO2 is locked up in carbonate rocks Venus’s CO2 is still all in the atmosphere

Same for sulfur compounds produced by volcanoes SO2 (sulfur dioxide) on Earth dissolves in the oceans SO2 on Venus stays in the atmosphere and forms clouds of sulfuric acid


Water & CO2 build up in the atmosphere A very massive atmosphere A very hot surface

No Magnetic field Slow spin

Large early impact? Solar Tides?

Little core convection Hot surface & thick lithosphere keep core hot

Water disassociated by sunlight H can thermally escape Solar wind impinges directly on Venusian ionosphere Ions can be easily stripped away

Deuterium to Hydrogen ratio: 0.024 150 times that of Earth Indicates significant loss of hydrogen

Sun was 30% fainter in early solar system Venus may once have been more Earth-like

What happened to the water?

Venus

Earth


Only glimpse of the surface Soviets had 4 successful Venera landings on

Venus Onboard experiments found basaltic surface Dark surface, albedo of 3-10% Surface winds of ~ 0.3-1.0 m/s Surface temperatures of 740 K Landers lasted 45-60 minutes

Venera 14 – 13 S, 310 E – March 1982

Landers


Spherical images can be unwraped into a low-res perspective view

Smooth-ish basaltic rock – low viscosity magmas

Baltis Vallis – 6800 km

Venera 9 – A Blockier Appearance

Venera 13


Venera 10

Venera 14


Venus rock composition Sampled in only 7 locations by Soviet landers Composition consistent with low-silica basalt Exposed crust is <1 Gyr old though

Venera 14


Surface of Venus has been imaged by radar Pioneer Venus (late 1970’s) Venera 15 and 16 (1980’s) Magellan (1992 – 1994)

Backscatter and altimetry 98% coverage

Side-looking system No shadows – observation at 0o phase Light/Dark tones don’t correspond to albedo Strong radar return from:

Terrain that has roughness on the scale of the radar wavelength

Large-scale slopes facing the spacecraft High-altitude ‘shiny’ material

High return due to unusual dielectric constant

Interpretation of Radar Data


Surface dominated by volcanic material Plenty of tectonics but no plate tectonics Over 80% of Venus made up by

Volcanic plains - 70% of surface, low-lying 9 Volcanic rises – Rift zones and major volcanoes, dynamically supported 5 Crustal plateaus – Dominated by Tesserae, isostatically compensated

Unusual lack of impact craters Very young surface 0.5 – 1.0 Gyr

Physiography


Low-lying Plains Ridged plains Smooth Plains

Highlands Crustal Plateaus Volcanic Rises


Range of volcanic styles Low viscosity plains volcanism Shield volcanism highly viscous features

Volcanism on Venus

Sinuous rills: Baltis Vallis – 6800 km


Some viscous flow features may exist…

Pancake domes – Eistla region South Deadman Flow – Long valley, CA


Shield plains Usually only a few 100 km across Fields of gentle sloping volcanic

shields Crossed by wrinkle ridges Shields usually constructed from

non-viscous lava Some shields are steep implying

more evolved lava Venera 8 lander probably sampled one of these areas


Ridged plains – 70 % Venusian surface Emplaced over a few 10’s Myr Deformed with wrinkle ridges (compressional

faults) 1-2 km wide, 100-200 km long

High-yield, non-viscous eruptions of basalt Gentle slopes and smooth surfaces Long run-out flows 100-200 km Chemical analysis – Venera 9, 10, 13 & Vega 1, 2 Total volume of lavas close to 1-2 x 108 km3 Contain sinuous channels 2-5 km wide, 100’s km long Baltis Vallis is 6800 km long, longest channel in the solar

system Thermal erosion by lava

Smooth plains cover 10-15% of Venusian surface

Superposed on ridged plains Not deformed by wrinkle ridges Consist of overlapping flows with lobate

morphology

Volcanic Plains

Sinuous rills: Baltis Vallis – 6800 km


Emplacement of plains material followed by widespread compression Solomon et al. (and some other papers) describe a climate-volcanism-

tectonism feedback mechanism Resurfacing releases a lot of CO2 causing planet to warm up Heating of surfaces causes thermal expansion resulting in compressive forces. Explains pervasive wrinkle ridge formation on volcanic plains


Morphologic term Quasi-circular raised feature Annulus of concentric fractures and ridges Radially orientated fractures in their interiors

360 Coronae identified Size ranges from 75 to 2000 Km Interiors raised about 1km Associated with large amounts of volcanism Occurred in parallel with volcanic plains

formation Typical formation sequence:

Volcanism Topographic uplift

Forming radial fractures

Withdrawal of magma Topographic subsidence

Forming concentric fractures

Coronae


Highlands Crustal Plateaus Volcanic Rises

Low-lying Plains Ridged plains Smooth Plains


Nine major volcanic rises 1000-2400km across

Containing: Rift zones Lava flows Large volcanic edifaces

Associated gravity anomalies Dynamically supported by a

mantle plume Young

Craters? Partly uplifted old plains Superposed features are young

though

Usually dominated by: Rifts Large shield volcanoes Coronae

Volcanic Rises


Rifts Extensional stress from volcanic rise uplift


Steep-sided, flat-topped, quasi-circular Isostatically compensated 1000-3000km across, raised by 0.5-4km

Dominated by Tesserae Regions of complexly deformed material Contain several episodes of both extension

and compression. Extremely rough (bright) at radar

wavelength

Origin of Tesserae Current thinking leans toward mantle plume

origin Upwelling mantle plume causes extension Crust thickens Partial collapse when plume disappears

causes compression

Crustal Plateaus


Almost 1000 impact craters on Venus

Very young surface Mean age 750 Myr 85% of the planets history is missing

All craters at >3 Km Atmosphere stops smaller impacts Craters 3-30 km in size have an irregular appearance Craters >30 km in size appear sharp

Tesserae are the old features 900 +/- 220 Ma

Volcanic plains have 2 units Old plains 975 +/- 50 Ma Young Plains 675 +/- 50 Ma

Volcanic rises have young features Rifts and large isolated shields Also contain older uplifted terrain

Cratering


Impacting bodies can explode or be slowed in the atmosphere

Significant drag when the projectile encounters its own mass in atmospheric gas:

Where Ps is the surface gas pressure, g is gravity and ρi is projectile density

If impact speed is reduced below elastic wave speed then there’s no shockwave – projectile survives

Ram pressure from atmospheric shock

Crater-less impacts

iPSi gPDei 23..

ATM

Hz

SATMram

atmosphereram

gkTHwhere

eHg

PvzP

TkvPconstTif

vP

22

2

.

If Pram exceeds the yield strength then projectile fragments If fragments drift apart enough then they develop their

own shockfronts – fragments separate explosively Weak bodies at high velocities (comets) are susceptible Tunguska event on Earth Crater-less ‘powder burns’ on venus Crater clusters on Mars


‘Powder burns’ on Venus

Crater clusters on Mars Atmospheric breakup allows clusters to form here

Screened out on Earth and Venus No breakup on Moon or Mercury

MarsVenus


Distribution of craters Appears completely random Some plains units may be older

Simulations taking in account atmospheric screening give ages of 700-800 Myr

26,000 impactors > 1011 kg to produce 940 craters

Atmosphere is very effective at blocking impacts


Low crater population Catastrophic resurfacing Continual resurfacing (like Earth)

Craters are indistinguishable from a random distribution

~80% of craters are pristine Others have superposed tectonics or volcanic material

Balch crater – 40 km

Heloise crater – 38 km

Catastrophic resurfacing?


One timeline… Tesserae form first

Most craters on them are removed by tectonics

Extensive Plains volcanism Resurfaces most of the planet

Global compression creates ridged plains

Additional volcanism makes smooth plains

Back to extension Volcanic rises Rifts

Catastrophic resurfacing?


One timeline… Volcanic rises and plains form

continuously Focused mantle plumes for rises Diffuse upwelling for plains volcanism

Volcanic rises evolve in Tesserae

Transition to thick lithosphere

~700Ma

New volcanic rises can no longer evolve into tesserae

Lack of transitional features means this occurred quite fast

Extension allows for coronae and rifts

Plains volcanism shuts off

Not so catastrophic resurfacing?


The future for Venus

Can a thick lid break? Lack of water is a problem Thermal energy builds in the mantle

Transient subduction? Happened in the past?

Venusian Geological Periods


Comparison to Earth Almost the same mass and bulk composition

Only 2 Mars-masses apart (+/- 1 giant impact)

Probably the same water budget Asthenosphere likely in early history

Basalt to eclogite transition is deeper on Venus (65 km) This could inhibit the initiation of plate tectonics

Provides more time to outgas CO2 and initiate runaway greenhouse Water outgassed and destroyed over geologic time


Venus is like the Earth in a lot of ways Size, density, composition, orbit

…but…

A runaway greenhouse atmosphere has vaporized all the water Lack of a magnetic field means that the water is easily removable No water in the mantle means no plate-tectonics or carbon cycle So the atmosphere had a profound effect on surface processes

Volcanic (low-viscosity basalt) plains dominate the surface Lengthy sinuous rills Ridged plains smooth plains, and shield plains Pancake domes might indicate some silica-rich volcanism

5 main crustal plateaus Contain extensively fractured tesserae High standing remnants, perhaps once supported by mantle plumes

9 main volcanic rises Currently supported by a mantle plume Extension creates rifts

Coronae are interpreted as collapsed upwellings Cratering record indicate a very young surface

Lack of degraded craters has been interpreted as a catastrophic resurfacing < 1Ga …OR… …surface geology can also be interpreted in terms of more gradual processes

With a transition to a thick lithosphere within the past Gyr

Summary

Documents

PTYS 411 Geology and Geophysics of the Solar System Shane Byrne – [email protected] Background is from Pioneer Venus History of Venus