Geology of Terrestrial Planets
LACC §8.2, 9.1, 9.2
• Understand the Moon and Mercury’s surface
features, interiors, and origins (impact
theories)
• Understand the Venus and Mars’s surface
features and interiors
• Understand the Planet-Moon system: tides,
tide locked orbits
An attempt to answer the “big question”: what is
out there? Are we alone?
Thursday, October 1, 2009 1

Terrestrial Planets: Interiors
http://www.star.le.ac.uk/edu/Solar_System.shtml
Earth and Moon to Scale
(which amounts to 30.1 Earth diameters)
http://hyperphysics.phy-astr.gsu.edu/hbase/solar/moonscale.html#c2
Thursday, October 1, 2009 2

Terrestrial Planets:
Surface Gravity
If you weigh 100 pounds on Earth...
(These worlds are not to scale.)
http://www.exploratorium.edu/ronh/weight/
Thursday, October 1, 2009 3

Terrestrial Planets:
The Sun in the Sky
The sun is about 0.5° across as it appears from Eath
1.4° 0.7° 0.5° 0.35°
6.7x brighter 1.9x brighter 1.0x brighter 0.4x brighter
(These worlds are not to scale.)
Thursday, October 1, 2009 4

The Moon’s Surface
(red light)
http://www.lpi.usra.edu/lunar/missions/clementine/images/
Thursday, October 1, 2009 5

The Moon’s Geology
• Impact craters and radiometric dating,
indicate the Moon’s surface is as old as 4.4
billion years old at the highlands, and about
3.5 billion years old in the maria.
• The Moon’s core is unusually small. This is to
be expect though, if it formed from mantle
material (rocky--minerals w/ Si, O, other lighter
elements) after the Earth collided with a Mars-
sized object.
• Ice found in craters on the Moon’s south pole
that are always in shadow.
Thursday, October 1, 2009 6

Apollo Missions
http://www.lpi.usra.edu/lunar/missions/apollo/
Thursday, October 1, 2009 7

Earth-Moon System: Tides
The Moon is the
dominant tidal
influence because the
fractional difference in
its force across the
Earth is greater than
the fractional
difference seen from
the Sun. This difference
in force follows the
inverse square law.
http://hyperphysics.phy-astr.gsu.edu/hbase/tide.html#mstid
Thursday, October 1, 2009 8

Earth-Moon System: Tides
http://www.atlantickayaktours.com/Pages/ExpertCenter/navigation/Navigation-4.shtml
Thursday, October 1, 2009 9

Earth-Moon System:
Tide Locked Orbits
When the Moon first formed after the Giant Impact, it was much nearer to the Earth (20,000km
or 20 times nearer than present.) Both bodies exerted a pull on each other causing huge tidal
forces. ...which resulted in the slowing on both the Earth’s and the Moon’s speed of rotation. The
resultant imparting of angular momentum then caused the Moon to move further away. This
process is still ongoing, with the Earth’s days shortening by 0.0015 milliseconds per year and the
Moon moving away at about 3.8cm a year. The Moon is no longer slowing its rotation as it is in
orbital synchronicity with the Earth. That means that it takes just as long for it to rotate a full 360
degrees on its axis as it does to orbit once round the Earth. Will the Moon move so far out that
it may actually leave its orbit altogether? No worries; The Sun is set to explode long before that!
http://www.math.nus.edu.sg/aslaksen/gem-projects/hm/0102-1-phase/ORIENTATIONOFTHEMOON.htm
Thursday, October 1, 2009 10

The Moon’s Formation
the Earth was struck by a massive body, possibly as massive as the
planet Mars, shortly after its formation. The Moon was formed by
accumulating the orbiting debris through much the same processes
as the planets themselves. ...accounts for the lower proportions of
iron and volatiles in the lunar rocks. The impact may also have been
responsible for inducing the tilt in the Earth's rotational axis.
http://universe-review.ca/F07-planets.htm#Moon
Thursday, October 1, 2009 11

The Moon’s Formation
http://videos.howstuffworks.com/hsw/9998-the-moon-formation-video.htm
Thursday, October 1, 2009 12

Mercury: Cratered Surface
http://messenger.jhuapl.edu/index.php
Thursday, October 1, 2009 13

Mercury: Cratered Surface
• Mercury has a surprisingly large (~60% by
mass) core (mostly Fe and Ni).
• Impact craters imply the surface is about 4.5
billion years old. One large impact resulted in
the Caloris Basin.
• Mercury has 3+ km high cliffs that run for
hundreds of km called scarps that formed
when the planet shrank and the crust
crumpled.
Thursday, October 1, 2009 14

Mercury: Caloris Basin
The largest surface feature on Mercury is the Caloris Basin, which
resulted from a collision with an asteroid. The basin, which is more that
1000 kilometers across, is visible as the large circular feature at the
bottom of the above photograph.
http://apod.nasa.gov/apod/ap960120.html
Thursday, October 1, 2009 15

Mercury: Scarps
... a variety of intriguing surface features,
including craters as small as about 400
meters across. From such highly detailed
close-ups, planetary geologists can study the
processes that have shaped Mercury’s
surface over the past 4 billion years. One of
the highest and longest scarps (cliffs) yet
seen on Mercury curves from the top
center down across the left side of this
image. An impact crater has subsequently
destroyed a small part of the scarp near the
bottom of the image.
http://messenger.jhuapl.edu/gallery/ This image ... shows a region about 200
sciencePhotos/image.php? kilometers (about 125 miles) across.
&image_id=119
Thursday, October 1, 2009 16

Mercury: Oddities
• 3:2 spin-orbit resonance: 59-day sidereal
day, 88-day orbit. What is a solar day like on
Mercury?
• Where’s the iron?
• Mercury and special relativity
Thursday, October 1, 2009 17

Mercury:
3:2 Spin-Orbit Resonance
http://www.csulb.edu/~gordon/LectureNotes/Mercury_files/image004.jpg
Thursday, October 1, 2009 18

Venus
http://www.astrosurf.com/nunes/explor/explor_m10.htm
Thursday, October 1, 2009 19

Venus: Surface Features
Impact craters indicate a surface age of 500
million years (15% as old as lunar maria and
between the age of Earth’s younger ocean
sea-floor and older continents); yet erosion
rates are very low.
Volcanoes on Venus are about as common as
on Earth, but Venus has “pancake” volcanoes
in addition to shield, volcanoes. Coronae
(circular bulges) are more common on Venus.
Tectonic activity does occur on Venus: ridges,
cracks, mountains, the coronae; but no plate
tectonics.
Thursday, October 1, 2009 20

Venus: Pancakes & Crater
http://photojournal.jpl.nasa.gov/jpegMod/PIA00084_modest.jpg
Thursday, October 1, 2009 21

Venus: Corona
http://photojournal.jpl.nasa.gov/jpeg/PIA00202.jpg
Thursday, October 1, 2009 22

Venus: Oddities
Venus rotates backwards and slowly. A solar day on
Venus is 117 days, and the sun rises in the west and
sets in the east, not that you would ever see the sun
through the dense clouds.
Venus’s slow rotation results in low wind velocities
and low erosion rates.
Thursday, October 1, 2009 23

Mars
http://rosetta.jpl.nasa.gov/dsp_images.cfm?buttonSel=gallery&buttonSelL2=images&category=mars
Thursday, October 1, 2009 24

Mars: Surface Features
Impact craters indicate a surface age of 3 - 4
billion years (same as lunar maria) for the
Martian lowlands.
Olympus Mons, the largest volcano in the
solar system, maybe be intermittently active.
Tectonic activity does occur, but not as much
evidence for it as Earth or Venus.
Polar caps of H2O and CO2 ice.
Channels and gullies indicate liquid water
flowed, but over 3 billion years ago.
Wind erosion occurs.
Thursday, October 1, 2009 25

Mars: Tharsis Bulge
http://www.britannica.com/eb/art/print?id=70956
Thursday, October 1, 2009 26

Mars: Olympus Mons
http://www.nasm.edu/ceps/etp/mars/surface/canyons.html
Thursday, October 1, 2009 27

Mars: Oddities
In a way, Mars is so much like the Earth, it’s odd:
• Mars’s day is only 42 min. longer than Earth’s
• Mars’s axial tilt is 25°; Earths is 23.5°.
• Mars’s diameter is about 0.53 Earth diameters,
which means Mars’s total surface area is about
the same as Earth’s land surface area.
Mars has two moons: Phobos and Deimos.
Mars had liquid water at some point in its past,
and may have had life.
Thursday, October 1, 2009 28

Mars: Phobos and Deimos
• 28 x 23 x 20 km • 12 x 10 x 6 km
• 7h 39m 27s synod. • 30h 21m 16s synod.
• 5h 36m West to East • 66h East to West
http://apod.nasa.gov/apod/ap061203.html http://apod.nasa.gov/apod/ap951003.html
Thursday, October 1, 2009 29

Mars: Phobos and Deimos
• Average Distance from Mars: 9,378 km (Phobos),
23,459 km (Deimos)
• Orbital Period: 7h 39m (Phobos), 30h 18m (Deimos)
• Eccentricity of Orbit: 0.015 (Phobos), 0.0005
(Deimos)
• Rotation Period: 7h 39m (Phobos), 30h 18m (Deimos),
i.e. both moons are tide locked to Mars
• Density: 2 gm/cm3 (Phobos), 1.7 gm/cm3 (Deimos)
• Atmosphere: n/a
• Surface Temperature:
25°F Max, -170°F Min (Phobos)
http://www.solarspace.co.uk/Mars/phobosdeimos.php
Thursday, October 1, 2009 30

Mars: “Months”
Since Mars rotates a bit faster than the revolution period of Deimos, we would indeed see it rise in
the east, but it would then appear to move across the Martian sky at a very slow pace. In fact, it
would take about 33 hours to reach that point directly overhead (or very nearly so). It would then
take yet another 33 hours to descend the sky before we would see it finally set in the west.
And then, we would have to wait another 66 hours before it again reappears above the eastern
horizon.
In contrast, Phobos, takes only 7 hours and 39 minutes to rotate around Mars. So it has the
distinction of being the only natural satellite in the solar system revolving about its planet in a time
shorter than the planetary "day," running three laps around Mars each day.
As seen from the Martian equator, Phobos appears to move far more rapidly than the sluggish
Deimos. In fact, just 2 hours and 48 minutes after Phobos has risen, it is already overhead. And after
another 2 hours and 48 minutes it is setting; an astronaut on Mars could witness it rising twice
during a single Martian night.
And since Phobos west-to-east motion is much faster than Mars rotation period, it would appear to
rise in the west and set in the east.
Furthermore, about every 10 hours and 18 minutes, Phobos appears to rapidly race closely past
Deimos as they trek in opposite directions. Phobos, in fact, probably even appears to briefly eclipse
Deimos for some parts of Mars on each pass.
Try picturing this: during the 66-hours that Deimos moves ponderously in the sky toward the west,
Phobos appears to whiz rapidly in the opposite direction more than six times!
http://www.space.com/spacewatch/mars_moons_040116.html
Thursday, October 1, 2009 31

Mars: Moon Phases
Phobos goes through its entire cycle of phases in the short time it takes to go
once around Mars.
If, for example, it were rising in the west just as the Sun were setting, it would be
at its "New" phase. A little over four hours later, it will already have moved well
past the overhead point to a position roughly halfway up in the east and would
appear "Full." When it sets in the east about an hour and half later, it will have
waned to its Last Quarter phase.
As for Deimos, because the Sun appears to move across the sky more than twice
as fast, this moon would appear to go through a full set of phases more than twice
during the 66 hours that it is continuously above the horizon.
Unfortunately, because of the very small size of both satellites, we should not
expect to see the same kind of sight that we're accustomed to seeing with our
own Moon. Deimos, for example, would appear only about 1/19 the apparent
width of our Moon. It would shine at its very best when at its "Full" phase, but
because of its very small size it would probably look more like an oversized
version of Venus to the unaided eye.
http://www.space.com/spacewatch/mars_moons_040116.html
Thursday, October 1, 2009 32

Mars: Phobos and Deimos
http://www.nasaimages.org/luna/servlet/detail/nasaNAS~4~4~11828~113853:The-Night-Sky-on-Mars
Thursday, October 1, 2009 33

Mars: Phobos and Deimos
Apparent size of the Moon as seen from Earth: 0.5°
Apparent size of the Phobos as seen from Mars: 0.18°
Apparent size of Sun as seen from Earth: 0.5°
Apparent size of Sun as seen from Mars: 0.35°
Apparent size of Earth as seen from the moon: 1.9°
Apparent size of Mars as seen from Phobos: 42°
http://www.johnstonsarchive.net/astro/phobos.html
Thursday, October 1, 2009 34

Mars: Terraformed Mars
from Phobos
Book cover by Hardy for Carl Sagan's 'Pale Blue Dot'.
We see Mars at an advanced stage of terraforming
with the Tharsis volcanoes, a flooded Mariner Valley
and the Boreal Ocean.
http://www.users.globalnet.co.uk/~mfogg/gallery.htm
Thursday, October 1, 2009 35

LACC HW: Franknoi, Morrison, and Wolff,
Voyages Through the Universe, 3rd ed.
• Ch. 8, pp. 192-193: 18.
• Ch 9: Tutorial Quiz accessible from: http://
www.brookscole.com/cgi-brookscole/course_products_bc.pl?
fid=M20b&product_isbn_issn=9780495017899&discipline_number=19
Due at the beginning of the next class period.
Be thinking about the Solar System Project.
Thursday, October 1, 2009 36