EAS 206 Quiz 3 (Mercury/Venus)

Mercury imaged by Mariner 10

• Heavily cratered terrain

• intercrater plains and smooth plains

• Large impact basins, e.g., Caloris

Cratering on Mercury compared with the Moon

• Ejecta is discontinuous and not as extensive due to increased gravity

• Fewer craters < 50 km diameter than expected

• Large basins are more degraded

• Space weathering more severe

Intercrater Plains

Most extensive terrain on Mercury

• level to gently rolling, densely cratered surfaces between craters >30 km diameter

• Densely cratered with many 5-10 km diameter craters, resulting in a highly textured surface

• Many small (< 10 km) craters are secondary

• Many craters are degraded and partially filled with smooth material

• Most likely formed by a combination of volcanism and impact at the same time

Mercury Smooth Plains

Smooth, sparsely cratered plains that are similar to lunar maria

• Covers ~1/3 of the surface of Mercury

• More concentrated in the North, as well as in the hemisphere around Caloris Basin

• Has Flow fronts

• Fewer craters

• Some have similar colour/reflectance to Intercrater Plains, while others are different

Mercury volcanic features

• Pit craters

• Pyroclastic vents and deposits (Most common)

• Lava flows (Including flow fronts, and evidence of erosion (channels))

Mercury Hollows

Bright deposits within impact craters

• not seen on any other rocky planetary body

• brighter than surrounding material

• irregular, shallow, rimless depressions (tens of meters to km across)

• Indicate loss of volatiles – From sulfides or graphite

Mercury Caloris Basin

Largest known basin on Mercury, and represents a massive collision early in its history

• multiring basin

• Ejecta are lineated

• Filled by smooth plains that are ridged and fractured in a polygonal pattern

• Lava filled the basin as soon it was formed

• On the other side of the basin is chaotic terrain

Scarps

• Compressional tectonic stress on mercury

• lobate escarpments; steep, clifflike slopes

• Extend over entire surface

• Began forming near end of heavy bombardment

• Show that Mercury has contracted 7 km in radius

• Formed by faults pushing craters up and compression

Internal Structure of Mercury

• 70% iron, 30% silicates

• A dense, iron-rich core 3500 km in diameter (larger than Earth’s core)

• Active magnetic field

• Molten outer core

3 models why mercury has a large core

1. Accreted from a mix that included more metal

2. High temperatures after accretion

   • Predicts lower volatiles (K, Na)

3. Giant impact stripped crust and mantle away

   • Predicts lower volatiles and crustal elements (Al, Ca)

Mercury’s composition

• Has volatiles

• K is present at levels like chondrites

• Na and Cl also present

• High in sulfur – Formed from chondrite-like materials

• Crust has high Mg, low Al or Ca – Not a lunar-like feldspar-rich crust

How a magnetic field is created on Earth

• Convection within liquid outer core

• Earth’s rotation

• Geomagnetic Dynamo

Mercury Pre-Tolstojan (4.56 to ~3.9 Ga) and Tolstojan Periods (~3.9-3.7 Ga)

• Magma ocean

• Graphite-rich flotation crust

• Continued growth by emplacement of magma/eruption of lava

• Heavy bombardment

• Resurfacing by volcanism or impact ejecta

• Formation of intercrater plains

Mercury Calorian (~3.7-1.7 Ga) and Mansurian (1.7-0.3 Ga) Periods

• Formation of Caloris Basin, Mansur crater

• Smooth plains volcanism

  • Ends by ~3.5 Ga; related to global contraction

• Explosive volcanism (pyroclastic flows) throughout

Mercury Kuiperian period (~280 Ma to present)

• Formation of Kuiper crater

• Tectonic deformation (contraction) continued

• Some explosive volcanism

• Formation of hollows

  • May have formed throughout Mercury’s history, but only the youngest have survived

Are there Volatiles on Mercury?

Interiors of craters at the poles are permanently shadowed but radar shows bright observations at the poles (however could be ice or comet impact)

Venus rotation period

1 Venus year is 225 Earth days, but Venus rotates ‘backwards’ and has no seasons and more CO2

Magellan satellite

• images Venus slopes, roughness with radar with long wavelengths to see beneath clouds

• Attached to large dish, Synthetic aperture radar (SAR) and Radar altimeter (smaller “horn”; get detailed topography)

Venus plantia lowlands

• Relatively flat, smooth, probably basaltic

• Volcanic: Lava flow fronts, long sinuous rilles

• Tectonic: Ridge belts

• e.g., Atalanta, Lavinia, Sedna Planitia

Venus regio uplands

• Isolated domes and broad swells

• Volcanic: calderas, lava flows, coronae

• Tectonic: Fracture belts, troughs, rifts

• e.g., Beta, Atla, Eistla Regio

Venus terra highlands

• Plateaus, similar to Earth’s continents

• Tectonic: mountain chains, tesserae, troughs

• Volcanic: calderas

• e.g., Ishtar, Aphrodite Terra

Internal Structure of Venus

• probably differentiated

• No measurable magnetic field

• One plate

• Unknown:

  – How thick are the layers?

  – Is there an asthenosphere?

Evidence for Venus being differentiated

• Similar to Earth

• Density (5.24 g/cm3)

• Volcanic and tectonic activity

• CO2 atmosphere

• From magmatic degassing

Venus Graben

• Narrow (< 1 km) Fracture Belts

• Common in lowlands and uplands

• Modest extensional feature

• Pulls apart crust and block drops down

Venus Domes and Rift Valleys

• extensional feature

• Domical Upland

• Beta Regio

  • Rhea Mons (Tessera, intense faulting)

  • Devana Chasma (Rift valley, <6km deep)

  • Theia Mons (Shield volcano)

• Eistla Regio

  • Gula Mons (Volcanic)

  • Sif Mons (Fractured shield volcano)

Venus Ridge Belts

• compressional feature

• thousands km long, ~1 km high

• Common in lowlands

• Mantle downwelling

Venus Mountain Belts

• compressional feature

• extensive deformation

• Up to 6km high

• Border terrae

• Mantle downwelling

Venus Highland Tesserae

• Unique to venus

• Crisscrossing ridges and grooves

• 1-2 km above surroundings

• combination of extension and compression

• Extensional grabens superimposed on compressional ridges and troughs

Volcanic Features on Venus

• covers 80% of the surface

• Shield volcanoes

• Calderas

• Pancake domes

• Lava flows and sinuous lava channels

• Coronae

Venus Shield Volcanoes

• Radial lava flows on flanks

• Summit calderas

• E.g., Sapas Mons in Atla Regio (Double summit)

Venus Calderas

• Created by withdrawal of magma from huge chambers by eruption or drainage

• Concentric fractures due to collapse of chamber roof

Venus Pancake Domes

• Extrusion of viscous lava from central vent

• Small summit craters

• Radial and concentric fractures

• High amount of SiO2 (silica)

Venus Flood Lavas

• Extrusion of fluid lava from fissures

• Sometimes lava channels

• likely basalt

• Major role in resurfacing

Venus Lava channels

• Carved by basaltic lava flows

• Erodes surface

• Very similar to sinuous rilles on the Moon

Venus Coronae

• volcano-tectonic feature unique to Venus

• system of concentric fractures and ridges surrounding a central plain

• Common in uplands and highlands

• Fist stage is called novae (radial graben fractures)

• Second stage is called arachnoids (spider fractures)

• Formed by mantle plume spreading and causing crustal depression

Main components of Venus’ atmosphere

• CO2 (96.4%)

• N2 (3.5%)

• sulfuric acid (H2SO4) droplets

Atmosphere is very thick with km of clouds

Effects of Venus’ atmosphere

• High surface temperatures

• The Greenhouse Effect

• Lack of water

• Wind erosion and deposition

• Weathering (chemical reactions between the atmosphere and surface rocks)

Venus’ Greenhouse Effect

• 80% of light is reflected back to space

• Absorbed heat is trapped, distributed

• Causes High surface temperatures

• Increase CO2 compared to Earth

• Too hot for liquid water to condense, stays as a gas

Venus runaway greenhouse effect

Carbon dioxide stays as a gas and cannot condense into liquid water

Earth’s Greenhouse Effect

Carbon dioxide dissolves into oceans, precipitates as carbonate (minerals, calcite limestone), helped by Earth

Hypothesis for the lack of water on Venus

Venus’ original water has been lost, no evidence for surface water

1. Water vapour outgassed from interior

2. Too hot for liquid water to condense

3. Water vapour was lost to space

Venus Wind Erosion and Deposition

• Wind velocities of 1 – 36 km/h

• Dense atmosphere can transfer kinetic energy to particles more easily than on Earth

• Only 1/10th of the wind velocity needed to start grains bouncing and moving on Earth is required

• Impacts causing atmospheric blasts

• Causes streaks and dunes

Evidence for Weathering on Venus

radar-reflective bright and dark zones which correlate with elevation

• Thick atmosphere

• CO2 and SO2 gas reacts with minerals

• carbonates (CaCO3) not stable

Elevation differences on Venus

At higher elevation:

• Lower T (650 K)

• Lower pressure (49 bars)

• Iron and sulfur dioxide forms pyrite

Lower elevation:

• iron forms magnetite or hematite

• Pyrite is not stable

Impact Craters on Venus

not as numerous as craters on other planets

• About 1000 craters identified

• Evenly distributed over all areas, not very degraded

• Suggests average age of 0.5 Ga

• Small bolides burn up in the atmosphere and don’t hit the ground

Venus impact crater major differences from Moon and Mercury

• No craters less than 1.5 km diameter

• Diffuse splotches attest to “near hits”

• Different ejecta deposits

• Past plate tectonics

Venus ejecta deposits characteristics

• Petal-shaped, not as extensive

• Flow long distances (like pyroclastic flows)

• Arc-shaped due to blowing wind

• Ejecta mixes with atmosphere and flows downhill

Average surface age of Venus

0.5 Ga

Catastrophic vs gradual Venus geologic history

Catastrophic:

• Periodic resurfacing events on a global scale

• Change from recycling to hot-spot volcanism

Gradual:

• Resurfacing of areas ~150,000 km2 at a time

• Allows for even distribution of craters

Geologic History of Venus

Venus was Earth-like for most of its history, but a few hundred million years ago, there was a lot of igneous activity that triggered the runaway greenhouse effect; this was helped along by the Sun being brighter than it was in the early part of the solar system’s history

Venus Large Igneous Province (LIP)

Formed by mantle plume upwelling, mantle convects underneath crust compression downwelling and underneath volcano upwellings, occurs because there are no separate crust plates for subduction

Venus Express

Mission that detected high emissivity regions using an infrared imager around volcanoes and high sulfur dioxide in atmosphere in 2006, followed by a sharp decrease

• High emissivity probably from unweathered basalt (olivine)

Venus Life Finder Mission

• 2025

• Look for signs of life in the atmosphere by dropping a probe to

  look for organic compounds

• First private mission to another planet

Venus VERITAS – NASA Orbiter Misson

• 2031

• Would map the surface at high resolution using topography,

  radar and infrared imaging

Venus DAVINCI – NASA Atmospheric Probe Misson

• 2031

• Would map the surface with an orbiter

• probe would study the composition of the atmosphere during descent to the surface

• provide images of tesserae as it lands

Olivine on Venus

Shows evidence of lava flows within the past few years and atmospheric conditions

• Experiments can be done on Earth

• Heated up olivine would become hematite within days to months