mercury & venus - stony brook university€¦ · mercury & venus ¥inferior planets :...
TRANSCRIPT
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!!!