ISP205 Lecture #13, Feburary 20, 2001
Terrestrial Planets
- Review: Moon
- Mostly rock with small solid iron core
- Highlands very old (4.1-4.2 billion years old)
- Lowlands somewhat younger (3.3-3.8 billion years)
- Craters circular because of high speed of meteorites
and explosion like impact
- Highlands much more heavily cratered - impact
rate in early solar system was much higher and
leveled out around 4 billion years ago
- No atmosphere because escape velocity is too low
- Origin of the moon: most likely "Giant
Impact"
Why ? Explains compositional differences and similarities
because mostly mantle material is ejected into space
- Other theories less likely:
- Capture (requires 3rd body)
- Sister (cannot explain composition differences -
like
absence of metals)
- Fission (doesn't work - can also not explain
composition
differences)
- Overview terrestrial planets
(see pictures)
- Earth has 1 Moon
- Mars has 2 Moons - Phobos and Deimos (pics)
maybe captured asteroids deflected by Jupiter from outer
asteroid belt into Mars orbit.
- Surface features and geological activity
- Mercury
(only visited by one space craft: Mariner 10, 1974)
- Heavily cratered surface, similar to Moon
- Large basins with indication of lava floods
(some similarity to the moon, and similar ages)
but no activity since ~4 billion years
- Long scarps across surface (probably 4 billion
years old)
cracks from cooling and shrinking.
(pic)
(unique feature of Mercury)
- Radar images imply water ice just beneath polar
region surface.
- Very weak magnetic field
- Venus
- Because of clouds surface only visible with Radar
(clouds are made of sulfuric acid)
- Extensive radar mapping (98%) has been done by
Magellan
1990-1994 from orbit (see complete map)
- Some "high lands" that resemble
continents: Aphrodite
and Ishtar
- Venus is geological active with the surface
being
on average only 500 mio years old
- Main activity are volcanoes (pictures)
- It seems that most of the surface appeared in
a
short time, and that since then the geological
activity was much lower.
(for example very few craters are filled with lava)
- Cracks from tectonic forces
- No magnetic field
- Earth ( we know that one ...)
- Mars
- Young, lightly cratered lowlands mostly in
southern hemisphere
(~3 billion years old)
and older highlands (heavily cratered, ~ 4 billion years old)
(still somewhat younger than moon highlands) (picture)
- Geological activity mostly ceased 3-4 billion years
ago
but: some Volcanoes might still be active occasionally
(suggested by crater counts on volcanoe slopes)
- Largest Volcano in the solar system: Olympus
Mons
- Spectacular Canyon: Valle Marineris (7km deep and
100 km long).
- Icy polar caps - permanent caps made of water ice,
seasonal extensions of CO2 ice (dry ice)
- There are indications that Mars had liquid water at
some
point in its history:
- Curved channels that resemble water carved
canyons
on earth: Nanedi Valle
this indicates contineous flow of fluid - water ?
- These features are mainly seen in heavily
cratered
regions. What does that mean ?
- Also hints on very recent outflows:
(see recent pictures from Global
surveyor 1997)
Apparently outflows show sharp rocks not covered
at all with sediments or sand - some scientists
concluded these features are very young
(as young as a few years)
- Canals ?
- Mars Canals first claimed by 1877 by
Schiaparelli
- "Confirmed" by Percival Lowell who
observed 183
canals, many of them observed in more than 20
observing nights. 67 of Schiarpellis 79 canals "found"
(see picture)
- Today we know that canals were an optical
illusion.
- Faces ?
Also an optical illusion
- Bacteria ?
Maybe ...
- No magnetic field
- Planet interiors (Redshift
demo)
- The smaller a planet the faster it cools
- Mercury smallest terrestrial planet:
huge solid (partially molten?) iron core and thick crust
- According to one hypothesis mercury lost most of
its
rocks in a giant impact
(explains also more elliptical orbit and tilt or orbital plane of 70)
- Mercury has a weak magnetic field - probably residual
from iron
- Mars next smallest: cooled slower and had some
volcanic
activity in the distant past before crust became too thick.
- Only small solid iron core and no magnetic field
- Venus almost as big as earth - has a liquid iron core
but
still thicker crust.
Venus has active volcanoes, but less than on earth.
- Venus has no magnetic field because of the extremely
slow rotation
- Earth has the thinnest crust and the highest interior
temperatures.
- Surface conditions
(how would be life on mercury, venus, or mars ?)
- Mercury
- Siderial day: 59 days
- Year: 88 days
- Solar day: 176 days (longest day in solar system)
(see you at noon tomorrow can mean a long time)
- Until 1965 astronomers thought Mercury rotates
once a (Mercury)year. New measurements with
Doppler Radar (DEMO)
- Seasons
- Seasons entirely from variation of the
distance
to the sun (varies from 46 Mio km to 70 Mio. km)
therefore big difference in heat intensity
- No axis tilt
- Temperatures: largest temperature differences in
solar system
Night: -173oC (-280F), Day peak: 430oC (800F)
Reason: no atmosphere, close to sun and very long days and nights
- No atmosphere
- Water ? Ice has been found beneath poles (with
radar)
- Venus
- Siderial day: 243 days "clockwise"
- Year: 225 days
- Solar day: 117 days
(because Venus spins opposite to orbit, solar day gets
shortened by spin)
- No seasons:
- Axis of rotation only tilted by 3o (earth
23o)
- Almost circular orbit
- Temperatures
Constant everywhere at ~460oC (860 F)
(note that lead and zinc melt already)
because of thick atmosphere
- Atmospheric pressure: 90 bar
(= 90 kg/cm2 - or a truck on your head - 300 cm2)
- Atmosphere: 96% CO2, 3.5% N2
- Always cloudy (Sulfuric acid clouds)
therefore no direct sunlight
during day: dim, reddish illumination
(see pictures from Venerea probes)
- No wind and water and therefore very little erosion
- Mars
- Siderial day: 1 day (roughly same as earth)
- Year: 1.9 (tropical) years
- Solar day only 2 minutes shorter than siderial day
- Seasons:
- axis tilt 25o (similar to earths 23o)
- distance to the sun varies because of eccentric
orbit and influences temperatures as well
- Temperatures:
- -100oC to possibly about freezing on a hot summer
day in
a good place
- Very thin atmosphere (0.007 bar)
- Atmosphere similar to Venus: 95% CO2, 2.7%
N2)
- Some water ice and CO2 clouds and dust clouds
- Dust clouds create red sky and lead to
"inverted sunsets"
(see pictures of Mars surface)
- Atmospheres
- Why are climates so different ?
Distance to sun according to inverse square law leads to
7 times more heating on Mercury
2 times more heating on Venus
2 times less heating on Mars
this cannot explain the large differences between
Venus, Earth, and Mars
- Most likely Venus, Earth, and Mars had similar
atmospheres
in the beginning, and therefore similar greenhouse effects,
and similar surface conditions
- Venus experienced a runaway greenhouse effect:
- Slightly more heat from sun
(calculate difference from inverse square law)
- Evaporation of water and release of CO2 from rocks
- Increase of greenhouse gases and thicker atmosphere
- higher temperature - go back to 2.
(DEMO: runaway equilibrium)
- Water vapor decomposed in H and O by UV light
- O bound in rocks, H escaped - irreversible loss of
water
- Mars experienced a runaway refrigerator effect
- Contineous loss of atmosphere because of lower
gravity
- Temperatures get lower
- Water vapor freezes out, also some CO2
- Less greenhouse gases
- Goto 2
- The earths atmosphere was drastically changed by life