Astr 1050 Mon., Apr. 26, 2004

Today: Start Ch 17., Terrestrial Planets

Recall: Nice webpage your classmate provided http://www.nationalgeographic.com/solarsystem/splash.html

Chapter 17: Terrestrial Planets

Comparative Planetology

Earth

History, Interior, Crust, Atmosphere

The Moon

In particular origin

Mercury

Venus

Mars

Including water (and life ?)

“Comparative Planetology”

Basis for comparisons is Earth

Properties of Earth

Similarities and differences with Mars and Venus help us understand Earth better (e.g., life, greenhouse effect, etc.)

Won’t spend much class time on basic properties (size, gravity, orbital period, length of day, number of moons, etc.) but you should have some relative ideas about these (see “Data Files” in text).

Four Stages of Planetary Development

Timeline

Earth’s Interior

The Active Earth

Plate tectonics, volcanoes, etc., a lot of action!

Earth’s Atmosphere: Greenhouse Effect

The Moon and Mercury

No atmosphere

Cratering is evidence of final planet assembly – lots to be learned from craters

Patterns in Geologic Activity

Judge age of surface by amount of craters:
more craters Þ more ancient surface
(for some objects, have radioactive age dates)

Moon “dead” after about 1 billion years

Mercury “dead” early in its lifetime

Mars active through ~1/2 of its lifetime

Venus active till “recent” times

Earth still active

Big objects cool off slower

Amount of heat (stored or generated) proportional to Volume ( so R³)

Rate of heat loss proportional (roughly) to Surface Area          (so R²)

Heat/(Unit Area) µ R³/R² = R     so activity roughly proportional to R

Same reason that big things taken out of oven cool slower than small things     (cake cools slower than cookies)

What is a crater?

Must think of them as caused by very large explosions from release of kinetic energy of impactor

Like a mortar shell – it isn’t the size of the shell which matters,
its how much energy you get out of the explosion

DO NOT think of them as just holes drilled into surface – think EXPLOSION

Kinetic Energy E = ½ m v²

v is roughly escape speed of earth

m = mass = volume * density (Consider a 1 km asteroid)

E

This is ~4500 ´ the size of the largest (~50 Mt) hydrogen bombs ever built
and this is for a relatively small size asteroid

Formation of an impact crater

Crater caused by the explosion

Impactor is melted, perhaps vaporized
by the kinetic energy released

Temporary “transient” crater is round

Gravity causes walls to slump inward forming “terraces”

Movement of material inward from all sides (trying to fill in the hole) may push up central peak in the middle.

Final crater is typically ~10 times
the size of the impactor

Examples of craters on the moon

Images on line at
The Lunar and Planetary Institute:
http://www.lpi.usra.edu/expmoon/lunar_missions.html

Detailed record of Apollo work at:
http://www.hq.nasa.gov/office/pao/History/alsj/frame.html

Superposition
(way to get relative ages)

Newer features are superposed
on top of older ones

Large impact forms basin

Basin floods with lava

Additional impacts occur in mare lava

Over time both crater rate and volcanic activity are declining

Craters less because debris swept up

Volcanism less because moon cooling

Problems with the Condensation Model:
Why is the moon so different than the earth?

Effects of late impacts

Moon: Giant Impact Hypothesis

Explains lack of large iron core

Explains lack of “volatile” elements

Explains why moon looks a lot like earth’s mantle, minus the volatiles

Explains large angular momentum in the earth-moon system

Relative size of core in Mercury

Venus

Expect Venus to be similar to Earth
(but it isn’t!)

Venus only slightly closer to sun, so expect about same initial composition

Venus only slightly smaller than Earth, so expect about same heat flow

Venus atmosphere is dramatically different

Very thick CO₂ atmosphere

Virtually no water in atmosphere or on surface

Venus shows relatively recent volcanic activity, but no plate tectonics

Both probably related to its slightly closer position to the sun
which caused loss of its critical water

Thick atmosphere and clouds block direct view so information from:

Orbiting radar missions (Magellan in early 90’s)

Russian landers (as in previous photo)

Why does Venus have much more CO₂ in atmosphere than Earth?

Amount of CO₂ in atmosphere on Venus roughly equal to
amount of CO₂ in limestone on Earth

With no oceans, don’t have a way to get CO₂ out of atmosphere and back into rocks

Runaway effect, because high T causes faster loss of water to space.

If H₂O gets into upper atmosphere it is broken down into O, H by UV sunlight

H is so light it escapes to space

On Earth cooler T traps H₂O in lower atmosphere (it condenses if it gets to high)

Location closer to the sun pushed Venus “over the edge” compared to Earth

Surface Relief of Venus from Radar

Venus does show evidence of “recent” volcanism

It does not show linear ridges, trenches, or rigid plates

In a few spots there are weak hints of this – but clearly different

Volcanoes

Sapas Mons

Lava flows from central vents

Flank eruptions

Summit caldera

Size:

250 miles diameter

1 mile high

Lava Channels

Large!

100’s of miles long

1.2 miles wide

High Venus temperatures may allow very long flows

Composition could also be different

Pancake Domes

Pancake domes formed from very viscous lava

“Ticks”

Domes which have partially collapsed?

Corona and a possible model

Corona possibly due to upward moving plume of hot mantle which bow up surface, then spreads out and cools
(as in a “lava lamp”)

Slide 28

Lots of Martian Science Fiction

Best, most recent and scientifically accurate is probably Kim Stanley Robinson’s series:

Red Mars, Blue Mars, Green Mars

Terraforming/colonization of Mars

Mars and the Pattern of Geologic Activity
and Atmospheric Loss

Expect intermediate geologic activity based on size

R_Mars = 0.53 R_EarthR_Moon = 0.27 R_Earth

Earth still active but lunar mare volcanism ended ~3 billion years ago

Expect intermediate atmospheric loss

Smaller size will make atmospheric escape easier

Cooler temperature (farther from sun) will make astmospheric escape harder

In some ways Mars is most “Earth-like” planet

Has polar caps

Has weather patterns

Had (in past) running water

May have had conditions necessary for development of life

Why some atmospheres are lost

Compare velocity of gas atoms (V_gas) to planet’s escape velocity V_esc

If any significant # of atoms have escape speed atmosphere will eventually be lost

In a gas the atoms have a range of velocities,
with a few atoms having up to about 10 ´ the average velocity,
so we need 10 ´ V_{avg gas} < V_esc to keep atmosphere for 4.5 billion years.

In above equations R = planet radius, M = planet mass, T = planet temperature,
m = mass of atom or molecule, k and G are physical constants

Big planets have larger V_esc (i.e. larger M/RµR³/R) so hold atmospheres better

Earth would retain an atmosphere better than Mercury or the Moon

Cold planets have lower V_gas so hold atmospheres better

Saturn’s moon Titan will hold an atmosphere better than our moon

Heavier gasses have lower V_gas so are retained better than light ones

CO₂ or O₂ retained better than He, H_2, or H

Even with “heavy” gasses like we H₂O we need to worry about
loss of H if solar UV breaks H₂O apart. That is what happens on Venus.

Which planets can retain which gasses?

Mars atmosphere today

Pressure is only ~1% of Earth’s

Composition: 95% CO₂ 3% N₂ 2% Ar

Water:

Pressure too low for liquid water to exist

Water goes directly from solid phase to gas phase

CO₂ (dry ice) acts like this even at terrestrial atmospheric pressure

Water seen in atmosphere

Water seen in polar caps

Evidence of running water in past

Carbon dioxide (CO₂)

Gets cold enough for even this to freeze at polar caps

Unusual meteorology, as atmosphere moves from one pole to other each “year”

Mars dust storm

Sand Dunes on Mars

Spacecraft in Mars orbit

Mars Global Explorer

Mars Odyssey

Even though atmosphere is thin, high winds can create dust storms

Water ice clouds

Ancient River Channels?
(note channels older than some craters – by superposition)

Recent liquid water?
(water seeping out of underground “aquifer” ?)

Layered Deposits

Where is the water today?

Much may have escaped to space

Some is locked up in N Polar Cap

Much could be stored in subsurface ice (permafrost)

Mars Missions making progress this semester:

http://www.nasa.gov/vision/universe/solarsystem/mer_main.html

Location of water critical to knowing where to search for possible past life

“Comparative Planetology”

Think about how Venus, Earth, and Mars started out so similarly

Think about what properties led to the very different environments today

Think about how these issues may apply to the future of Earth, and even our prospects for terraforming (and there is a debate about whether we should terraform at all!).


	Today: Start Ch 17., Terrestrial Planets
	Recall: Nice webpage your classmate provided http://www.nationalgeographic.com/solarsystem/splash.html


Comparative Planetology
	Earth
		History, Interior, Crust, Atmosphere
	The Moon
		In particular origin
	Mercury
	Venus
	Mars
		Including water (and life ?)


	Basis for comparisons is Earth
	Properties of Earth
	Similarities and differences with Mars and Venus help us understand Earth better (e.g., life, greenhouse effect, etc.)

	Won’t spend much class time on basic properties (size, gravity, orbital period, length of day, number of moons, etc.) but you should have some relative ideas about these (see “Data Files” in text).


	No atmosphere

	Cratering is evidence of final planet assembly – lots to be learned from craters


	Judge age of surface by amount of craters: more craters Þ more ancient surface (for some objects, have radioactive age dates)
		Moon “dead” after about 1 billion years
		Mercury “dead” early in its lifetime
		Mars active through ~1/2 of its lifetime
		Venus active till “recent” times
		Earth still active

	Big objects cool off slower
		Amount of heat (stored or generated) proportional to Volume ( so R³)
		Rate of heat loss proportional (roughly) to Surface Area (so R²)
		Heat/(Unit Area) µ R³/R² = R so activity roughly proportional to R

	Same reason that big things taken out of oven cool slower than small things (cake cools slower than cookies)


	Must think of them as caused by very large explosions from release of kinetic energy of impactor
		Like a mortar shell – it isn’t the size of the shell which matters, its how much energy you get out of the explosion
		DO NOT think of them as just holes drilled into surface – think EXPLOSION

		Kinetic Energy E = ½ m v²


		v is roughly escape speed of earth


		m = mass = volume * density (Consider a 1 km asteroid)




		E





		This is ~4500 ´ the size of the largest (~50 Mt) hydrogen bombs ever built and this is for a relatively small size asteroid


	Crater caused by the explosion
		Impactor is melted, perhaps vaporized by the kinetic energy released

	Temporary “transient” crater is round

	Gravity causes walls to slump inward forming “terraces”

	Movement of material inward from all sides (trying to fill in the hole) may push up central peak in the middle.


	Final crater is typically ~10 times the size of the impactor


	Images on line at The Lunar and Planetary Institute: http://www.lpi.usra.edu/expmoon/lunar_missions.html


	Detailed record of Apollo work at: http://www.hq.nasa.gov/office/pao/History/alsj/frame.html


	Newer features are superposed on top of older ones


	Large impact forms basin
	Basin floods with lava
	Additional impacts occur in mare lava

	Over time both crater rate and volcanic activity are declining
		Craters less because debris swept up
		Volcanism less because moon cooling


	Explains lack of large iron core
	Explains lack of “volatile” elements

	Explains why moon looks a lot like earth’s mantle, minus the volatiles

	Explains large angular momentum in the earth-moon system


	Venus only slightly closer to sun, so expect about same initial composition
	Venus only slightly smaller than Earth, so expect about same heat flow

	Venus atmosphere is dramatically different
		Very thick CO₂ atmosphere
		Virtually no water in atmosphere or on surface
	Venus shows relatively recent volcanic activity, but no plate tectonics

	Both probably related to its slightly closer position to the sun which caused loss of its critical water

	Thick atmosphere and clouds block direct view so information from:
		Orbiting radar missions (Magellan in early 90’s)
		Russian landers (as in previous photo)


	Amount of CO₂ in atmosphere on Venus roughly equal to amount of CO₂ in limestone on Earth

	With no oceans, don’t have a way to get CO₂ out of atmosphere and back into rocks

	Runaway effect, because high T causes faster loss of water to space.
		If H₂O gets into upper atmosphere it is broken down into O, H by UV sunlight
		H is so light it escapes to space
		On Earth cooler T traps H₂O in lower atmosphere (it condenses if it gets to high)

	Location closer to the sun pushed Venus “over the edge” compared to Earth


	Venus does show evidence of “recent” volcanism
	It does not show linear ridges, trenches, or rigid plates
		In a few spots there are weak hints of this – but clearly different


	Sapas Mons
		Lava flows from central vents
		Flank eruptions
		Summit caldera

	Size:
		250 miles diameter
		1 mile high


	Large!
		100’s of miles long
		1.2 miles wide

	High Venus temperatures may allow very long flows

	Composition could also be different


	Corona possibly due to upward moving plume of hot mantle which bow up surface, then spreads out and cools (as in a “lava lamp”)


	Best, most recent and scientifically accurate is probably Kim Stanley Robinson’s series:
	Red Mars, Blue Mars, Green Mars
	Terraforming/colonization of Mars



Expect intermediate geologic activity based on size
	R_Mars = 0.53 R_EarthR_Moon = 0.27 R_Earth
	Earth still active but lunar mare volcanism ended ~3 billion years ago

Expect intermediate atmospheric loss
	Smaller size will make atmospheric escape easier
	Cooler temperature (farther from sun) will make astmospheric escape harder


In some ways Mars is most “Earth-like” planet
	Has polar caps
	Has weather patterns
	Had (in past) running water
		May have had conditions necessary for development of life



	Compare velocity of gas atoms (V_gas) to planet’s escape velocity V_esc

		If any significant # of atoms have escape speed atmosphere will eventually be lost

		In a gas the atoms have a range of velocities, with a few atoms having up to about 10 ´ the average velocity, so we need 10 ´ V_{avg gas} < V_esc to keep atmosphere for 4.5 billion years.




		In above equations R = planet radius, M = planet mass, T = planet temperature, m = mass of atom or molecule, k and G are physical constants

	Big planets have larger V_esc (i.e. larger M/RµR³/R) so hold atmospheres better
		Earth would retain an atmosphere better than Mercury or the Moon

	Cold planets have lower V_gas so hold atmospheres better
		Saturn’s moon Titan will hold an atmosphere better than our moon

	Heavier gasses have lower V_gas so are retained better than light ones
		CO₂ or O₂ retained better than He, H_2, or H
		Even with “heavy” gasses like we H₂O we need to worry about loss of H if solar UV breaks H₂O apart. That is what happens on Venus.


Pressure is only ~1% of Earth’s
Composition: 95% CO₂ 3% N₂ 2% Ar

Water:
	Pressure too low for liquid water to exist
		Water goes directly from solid phase to gas phase
		CO₂ (dry ice) acts like this even at terrestrial atmospheric pressure
	Water seen in atmosphere
	Water seen in polar caps
	Evidence of running water in past

Carbon dioxide (CO₂)
	Gets cold enough for even this to freeze at polar caps
	Unusual meteorology, as atmosphere moves from one pole to other each “year”


	Spacecraft in Mars orbit
		Mars Global Explorer
		Mars Odyssey

	Even though atmosphere is thin, high winds can create dust storms


	Much may have escaped to space
	Some is locked up in N Polar Cap
	Much could be stored in subsurface ice (permafrost)

	Mars Missions making progress this semester:
			http://www.nasa.gov/vision/universe/solarsystem/mer_main.html

	Location of water critical to knowing where to search for possible past life


	Think about how Venus, Earth, and Mars started out so similarly

	Think about what properties led to the very different environments today

	Think about how these issues may apply to the future of Earth, and even our prospects for terraforming (and there is a debate about whether we should terraform at all!).