Mercury & Venus - Stony Brook University€¦ · Mercury & Venus ¥Inferior Planets : planets with orbits smaller than that of the Earth ¥Both planets are observed in the morning

Mercury & Venus

• Radar, doppler shift, and rotation

• General orbital properties

• Tidal effects of Mercury

• Surface composition of Mercury

• Origin of Mercury

• MESSENGER mission

• Rotation of Venus

• Clouds of Venus

• Venusian topography

• Craters

• Tectonic features

Mercury

Discussed Thus Far –

• Closest planet to the Sun

(0.4 AU)

• Negligible atmosphere

• Heavily cratered surface

• Unusually large iron core

• Weak magnetic field

Distance Determination

• Radar: radio waves are

bounced off of an object

• Distance is determined

by the time it takes for

the signal to bounce off

of the object & return to

the radar source

Speed

• The Speed is determined through doppler shift, which

relates frequency/wavelength with speed

Rotation – determined via broadening of

radar signal

Mercury & Venus

• Inferior Planets: planets with orbits smaller than that of

the Earth

• Both planets are observed in the morning or evening sky

(i.e., leading or trailing the Sun in the sky)

• Eastern/Western Elongation – the maximum distance

the inferior planets appear to get in the sky from the Sun

Tidal Effects – Mercury

• Mercury year = 88 Earth

days

• Mercury day = 59 Earth

days

• The orbit of Mercury is

eccentric

• Rotation period minimizes

the tidal dissipation near

perihelion

! Perihelion = 0.31 AU

! Aphelion = 0.47 AU

Surface Composition

• Color & reflectivity are

indicative of igneous

silicate rocks & perhaps

metallic rock

• Age of the surface is

undetermined

Surface Composition, cont.

• Polar cap: permanent deposit of water or other frozen

volatiles in the cool polar regions of the planet

• Discovered via radar mapping of Mercury. These

regions have high radar reflectivity

Surface Composition, cont.• Tilt of rotational axies with respect to the ecliptic plane =

0°, thus polar regions receive little sunlight

• But where did the volatiles come from? Impacts with

comets? Outgassing?

Surface similar in appearance to the

Earth’s Moon

Surface Features, cont.

• Caloris Basin - largest structure on Mercury (1000 km),created by an impact with an asteroid. Circumference ofMercury is 7700 km.

• A similar feature is seen on the western edge of the Moon -Mare Orientale.

Caloris Basin Mare Orientale

Surface Features, cont.

• The impact that resulted in the Caloris Basin generated

compression on the surface of Mercury

Scarp radial to Caloris Basin Jumbled Terrain Opposite

Caloris Basin on Mercury

Origin of Mercury

• Created with a

disproportionately large

core via a collision with a

large, planet-size asteroid

• If this happened after

differentiation, much of

the “crust” material could

have been lost

The Mercury Surface, Space Environment,

Geochemistry and Ranging (MESSENGER)

• Launch - August 3, 2004, Mission End Date - March 2012

• objectives - “to characterize the chemical composition ofMercury's surface, the geologic history, the nature of themagnetic field, the size and state of the core, the volatileinventory at the poles, and the nature of Mercury'sexosphere and magnetosphere over a nominal orbitalmission of one Earth year.”

• 2 flybys will gather information for planning main mission

Venus

Discussed thus far –

• Second planet from theSun (0.7 AU)

• Thick atmospherecomposed primarily ofCO2

• High surface temperature

• Clouds of Sulfuric Acid

Radar-constructed map of

Venusian Surface

Rotation of Venus

• Retrograde rotation

• Long sidereal day = -243

Earth days

• Solar day = -117 Earth days

Clouds• Highly reflective (75%)

• Cloud temperature (-35° C)

• Upper atmosphere –

Clouds rotate around the

planet in four days due to

• Lower atmosphere

UV Image

! rotation of planet

! thin upper atmosphere

! heating by solar

radiation

! Wind speed – 0-2 m/s

! Coriolis is negligible –

slow rotation of planet

! Hadley cells – air

traveling via convection

from equator to poles

Topography

• Contrast between

high & lowlands is

not as dramatic as it

is on Earth

Topography

• Contrast between

high & lowlands is

not as dramatic as it

is on Earth

Surface Features: Craters on the

Venusian Surface

• 1000 impact craters

• Sizes: 2 – 280 km

• Craters are pristine: lowerosion rate

! No large craters:

surface doesn’t date back

to heavy bombardment

period

! No small craters: they

burn up in the thick

atmosphere

The “Tick” Crater

Plate Tectonics on Earth

Surface Features: Tectonic Features

• Created via tension or

compression in the crust

of the planet

• No well-defined tectonic

plates

Radar image of Venus (Global)

Band near equator:

crustal compression

Surface Features: Pancake Domes

• 45 km in diameter

• 2 – 3 km high

• Created when lava is

belched up all at once

Surface Features: Coronae

• Circular/Oval Features

100s – 1000s km across

• Characterized by

concentric & radial

tectonic patterns &,

often, by associated

volcanic eruptions

• What are they?

! failed hot spots

! developing hot spots

Low central dome

The Surface

• Several Venera landers

lasted for up to 2 hours

on the Venusian

surface

• Loose rocks

• Basalts: produced by

rapid cooling of lava

Runaway Greenhouse Effect: Could Venus

have been more Earth-like in the past?

1) At 0.72 AU, Venus receives a higher flux of solar

radiation

2) Evaporation of Oceans ! water vapor (greenhouse

gas)

3) Higher temperature ! more evaporation

4) Water vapor rises to high elevations in a hot

atmosphere

5) Water vapor + sunlight ! Hydrogen + Oxygen

6) Light Hydrogen escapes

7) Oxygen combines with rocks

The Venusian Surface

• Crater counts = 500 million year-old surface

• Possible Explanation – about 500 million years ago,

lava broke through the thick lithosphere & covered the

planet

! the lithosphere resealed

afterward

! Note: this is very speculative!!!