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General Astronomy
Moons
Many slides are taken from lectures by Dr David Wood, San Antonio College
Inner Solar System
• Mercury and Venus have 0 moons
• Earth has 1 moon
• Mars has 2 moons
The Moon of Earth
Comparing Earth & Moon
Water
Volatiles
Differentiated
Atmosphere
Metallic core
Composition
Plate Tectonics
Magnetic Field
Temperature
Life
Earth
Moon
Yes
Yes
Yes
Yes
Yes
Rock & Metal
Yes
Yes
Moderate
Yes
No
No
Yes
No
No
Rock only
No
No
Large extremes
No
Surface Features
• Highlands
–
–
–
–
Oldest surface on Moon (> 3.9 Gyr)
Lighter in color
Less dense
Original crust
• Maria
–
–
–
–
–
Younger surfaces (3.4 ± 0.4 Gyr)
Lower in elevation
Composition of Moon’s mantle not regolith
Associated with basins
Common on near side, absent on far side
• Impact Craters form Regolith
• Ice at pole (?)
• Near side vs. Far side
– Near side is thinner than far side
– Basins all found on near side except Orientale
– Far side is more heavily cratered
Surface Features
Interior
• Radius is 1738 km
• Crust
– Regolith is a few tens of meters
thick
– 30 km (nearside)
– 100 km (farside)
• Lithosphere is 600 – 800 km deep
• Asthenosphere is 800 – 1700 km
deep
• Core?? Small if it exists at all
• How do we know all this?
– Apollo astronauts left seismic sensors
– Moon rings like a bell
– Moonquakes (mostly from impacts but
some are tidal in origin)
– Network turned off in late 1970s
The Man on the Moon
•
•
•
•
•
9 Apollo missions ventured to the Moon, 6 missions (12 men) landed on the surface
July 20, 1969…Neil Armstrong becomes first man to set foot on the Moon
– “Houston, Tranquility Base. The Eagle has landed.”
– “One small step for a man, one giant leap for mankind.”
April 11 – 17, 1970…Apollo 13 saga delays Moon launches/landings for a year
December 17, 1972…Eugene Cernan is last person to leave the Moon
Have not returned since 1972
The Moons of Mars
“…two satellites, which revolve about Mars, whereof the
innermost is distant from the center of the primary
planet exactly three of its diameters, and the
outermost five…”
Jonathan Swift, in Gulliver’s Travels, 1726
Mars’ two moons were discovered 151 years
later by Asaph Hall in 1871
Many astronomers of that era (including Kepler)
felt that there was a simple numerical sequence:
Venus = 0, Earth = 1, Mars = ?, Jupiter = 4
Moons of Mars
•Phobos
• General Properties
• Radius = 28 x 20 km
• Orbital period = 0.319 days
• Orbital radius = 9380 km
The large crater is Stickney which has
a diameter half as large as Phobos.
Phobos orbital period (7.7 hours) is
less than the Martian day (24.6 hrs);
an observer would see Phobos rise in
the west and set in the east every 11
hours.
Phobos
Moons of Mars
Deimos
• General Properties
• Radius = 16 x 12 km
• Orbital period = 1.263 days
• Orbital radius = 23460 km
Not as many craters as on Phobos.
The image looks softened – most
likely by a coating of thick dust.
Deimos
This image was
taken by the
Viking spacecraft
at a distance of
18 miles above
the surface. The
smallest objects
are about 15 feet
across.
Outer Solar System
All of the outer planets have large
numbers of satellites
We are discovering more and more as
technology improves
Moons of Jupiter
• General Properties
– Galilean Satellites
• Planet-size (> 1500 km)
• Visible to naked eye if outside Jupiter’s
glow
• Generally rock & ice composition
• Resonance and tidal heating
• Callisto does not participate in resonance
• Density decreases with distance
– Medium-sized moons
•
•
•
•
300 km – 1500 km
Spherical and natural
Rock & ice composition
Synchronous rotation
•
•
•
•
< 300 km
Potato-shaped and captured
Usually rocky
Inclined, eccentric, retrograde orbits
– Small Moons
Io
• Io
– General Properties
•
•
•
•
•
Radius = 1821 km
Orbital period = 1.769 days
Slightly larger than Earth’s Moon
“Pepperoni pizza” crust analogy
Heavy dose of X-ray radiation from Jupiter
– Volcanically active
• Tidal heating
• Pele & Loki plumes
• Umbrella shape of geysers due to low surface
gravity
• Sulfur ash and “lava” bury all impact craters
– Resonance
• Europa & Ganymede pull on one side; Jupiter pulls
on the other
• 1:2:4 resonance requires Europa & Ganymede to
line up with Io every 4 orbits
Io
Europa
• Europa
– General Properties
•
•
•
•
Radius = 1565 km
Orbital period = 3.551 days
Slightly smaller than Earth’s Moon
Heavy dose of X-ray radiation from
Jupiter
– Resurfacing
• Few impact craters if any; large cracks
• Blocks of ice appear to be shifted or
overturned
• Tidal heating produces internal ocean of
liquid water?
• Best chance for possibility of life
– Resonance
• Io & Jupiter pull on one side; Ganymede
pulls on the other
• 1:2:4 resonance requires Io & Ganymede to
line up with Europa every 2 orbits
Ganymede & Callisto
• Ganymede
– General Properties
•
•
•
•
•
•
Radius = 2634 km
Orbital period = 7.155 days
Largest moon in solar system
Larger than Mercury
Unusual ice geology
Perhaps a liquid water mantle??
• Callisto
– General Properties
•
•
•
•
•
•
Radius = 2403 km
Orbital period = 16.689 days
3rd largest moon in solar system
Larger than Mercury
Surface saturated with craters
Valhalla impact basin is largest impact basin in
the solar system
Other Jovian Moons
• Small inner moons
–
–
–
–
Probably natural moons
10 km – 135 km
Metis, Adrastea, Amalthea, Thebe
1999 J1
–
–
–
–
–
Captured in prograde orbits
7 km – 22 km
Substantially inclined orbits
Leda, Himalia, Elara, Lysithea
2000 J11
–
–
–
–
–
Captured in retrograde orbits
5 km – 15 km
Inclined and eccentric orbits
Ananke, Carmae, Pasiphae, Sinope
2000 J1 – 2000 J10
• Small outer moons (prograde)
• Small captured outer moons
(retrograde)
Moons of Saturn
• General Properties
– Titan
• Planet-size (> 2500 km)
• Only large moon of Saturn
– Medium-sized moons
•
•
•
•
•
10 moons
100 km – 800 km
Spherical and natural
Rock & ice composition
Synchronous rotation
– Small Moons
•
•
•
•
< 100 km
Potato-shaped and captured
Usually rocky
Inclined, eccentric, retrograde orbits
Titan
• Titan
– General Properties
• Radius = 2575 km
• Orbital period = 15.945 days
• Larger than Mercury
– Atmosphere
• Massive nitrogen & hydrocarbon atmosphere
• Pressure = 1.5 bars
• Why does Titan have an atmosphere
–
–
–
Outgassed NH3
UV dissociates NH3 molecule and H escapes
Callisto and Ganymede too warm for NH3 to
condense…none to outgas!
– Surface
• Appears to have methane and ethane oceans
• May have solid, liquid, and gaseous methane states
– Life??
• Right ingredients
• Chemistry too slow?
Titan’s Shoreline
Ice Volcano on Titan
Titan’s Surface
Other Saturnian Moons
• Tethys
– Radius = 530 km
– Very heavily cratered
• Enceladus
– Radius = 249 km
– Bright, lightly cratered; H2O volcanoes?
• Rhea
– Radius = 764 km
– Significant ice geology (tectonics?)
• Dione
– Radius = 559 km
– Lightly cratered; interesting ice geology
• Mimas
– Radius = 199 km
– Very heavily cratered (Death Star)
• Iaepetus
– Radius = 718 km
– One hemisphere dark; other hemisphere
bright
Dione
Hyperion
Enceladus
An Ocean Below Enceladus’ Icy Crust?
• NASA’s Cassini spacecraft has
observed plumes of material
escaping from Saturn’s small icy
moon, Enceladus
• The plume is mostly water vapor,
with tiny ice particles and other
gaseous molecules mixed in (e.g.
CO2, CH4, C2H6)
• The plume supplies ice particles
to one of Saturn’s rings
• Some ice particles contain salt,
which may indicate they originate
in an ocean deep below the icy
crust
Image mosaic of Enceladus taken by Cassini, showing
individual plumes of gas and ice escaping from the
surface. The plumes extend 100’s of km into space
from the ~500 km diameter moon.
Prepared for the Division for Planetary Sciences of the American Astronomical Society by David Brain and Nick Schneider
[email protected] - http://dps.aas.org/education/dpsdisc/ - Released 03 December, 2009
What Process Creates the Plume?
• Plumes may be material escaping
through surface cracks from an
internal salty ocean or lake
• Alternatively, ice along cracks may
sublime or melt, followed by
escape of water vapor and icy
particles
• Many scientists find the salty
ocean model most convincing, but
others favor combinations of
alternative explanations
Left: Enceladus may have a salty subsurface ocean
that releases material to space through cracks in
the moon’s icy shell. Right: The walls of icy cracks
in the surface may melt or sublime, venting gas and
icy particles to space.
Prepared for the Division for Planetary Sciences of the American Astronomical Society by David Brain and Nick Schneider
[email protected] - http://dps.aas.org/education/dpsdisc/ - Released 03 December, 2009
The Big Picture
• Enceladus is surprisingly active for
such a small body - likely a
consequence of tidal heating
• Future flybys of Enceladus by Cassini
may help to resolve whether
Enceladus joins the growing “club” of
solar system bodies believed to have
oceans
Tiger stripes
• If Enceladus has an ocean, then it
contains all of the ‘ingredients’ known
to be important for life: liquid water,
molecular building blocks, and energy
Image of Enceladus showing the ‘tiger
stripes’ region in the southern hemisphere,
where the plumes originate
Prepared for the Division for Planetary Sciences of the American Astronomical Society by David Brain and Nick Schneider
[email protected] - http://dps.aas.org/education/dpsdisc/ - Released 03 December, 2009
Mimas
Phoebe
Tethys
Tethys
Rhea
Moons of Uranus
• General Properties
– Medium-sized moons
•
•
•
•
236 km – 789 km
Miranda is triaxial; others are spherical
Icy – rock composition
Synchronous rotation
– Small Moons
• < 100 km
• Inner moons are spherical and natural
• Outer moons are potato-shaped and
captured
• Usually rocky
• Inclined, eccentric, retrograde orbits
“Large” Uranian Moons
• Titania
–
–
–
–
Radius = 789 km
Orbital period = 8.706 days
Once geologically active
Lightly cratered
–
–
–
–
Radius = 761 km
Orbital period = 13.463 days
Not very active geologically
Heavily cratered
–
–
–
–
Radius = 585 km
Orbital period = 4.144 days
Not very active geologically
Heavily cratered
–
–
–
–
Radius = 579 km
Orbital period = 2.520 days
Once geologically active
Lightly cratered
Oberon
Umbriel
Titania
Ariel
• Oberon
• Umbriel
• Ariel
Miranda
• Miranda
– General Properties
• Radius = 236 km
• Orbital period = 1.413 days
• Very small
– Surface
• Tectonically active at one time in its past
• Three large coronae
–
–
–
Arden Corona (near top)
Elsinore Corona (near bottom)
Inverness Corona (middle)
• Fault scarps are tens of kilometers tall;
takes hours for objects to fall
– How were coronae produced?
• Moon shattered and re-accreted?
• Tectonic activity and convection?
• Arden & Elsinore best modeled as diapirs (rising
plumes); Inverness best modeled as a sinker
Other Uranian Moons
• Small inner moons
–
–
–
–
–
Probably natural moons
10 km – 100 km
Located within Uranian ring system
Some may be shepherd moons
Cordelia, Ophelia, Bianca, Cressida,
Puck, Desdemona, Juliet, Portia,
Rosalind, Belinda
– 1986 U10
• Small outer moons
– Captured in both prograde & retrograde
orbits
– < 60 km
– Caliban, Sycorax, Stephano, Prospero,
Setebos
– Little known of the properties of these
moons
– Two more discovered in 2003
Puck
Moons of Neptune
• General Properties
– Triton
• Planet-size (1353 km)
• Only large moon of Neptune
• Icy composition
– Medium-sized moons
•
•
•
•
•
2 moons (Proteus & Nereid)
170 km & 218 km
Natural???
Icy composition
Synchronous rotation
– Small Moons
• 6 known (Naiad, Thalassa, Despina,
Galatea, Larissa)
• < 100 km
• Usually icy
• Located within and near Neptune’s ring
system
• Natural??
Neptune and Triton
Triton
Triton
• Triton
– General Properties
•
•
•
•
Radius = 1353 km
Orbital period = 5.875 days
Retrograde
Will eventually fall within Neptune’s Roche
limit where it will be torn apart
• Largest captured object in solar system
– Atmosphere
• Thin nitrogen atmosphere
• Why does Titan have an atmosphere
–
–
Outgassing NH3 (?) from currently active
volcanoes
UV dissociates NH3 molecule and H escapes
– Surface
• Volcanically active
• Large smooth areas are indicative of recent
lava flows (composition is NH3)
• Long trails of volcanic plumes evidence
atmosphere
The Moons of Pluto
• Pluto is in a 3:2 resonance with Neptune
• Charon is an almost identical twin to Triton
• Are Pluto and Charon escaped moons of Neptune?
– Orbits suggest probably not
– Triton is captured suggesting that Triton may once have been a Kuiper Belt
object
• Nix and Hydra
Sizes
Press Releases
•
For More Information…
Space.com - 06/14/09 - “Ocean Hidden Inside Saturn's Moon”
http://www.space.com/scienceastronomy/090624-enceladus-ocean.html
Images
•
Enceladus plume image courtesy NASA/JPL/Space Science Institute/E. Lakdawalla
http://antwrp.gsfc.nasa.gov/apod/ap091124.html
•
Europa plume cartoons adapted from image courtesy NASA/JPL
http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=2944
•
Enceladus image courtesy NASA/JPL/Space Science Institute
http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=2032
Source Articles
•
(on-campus login may be required to access journals)
Postberg et al., ‘Sodium salts in E-ring ice grains from an ocean below the surface of
Enceladus’, Nature, 459, p. 1098, doi: 10.1038/nature08046, 2009.
http://www.nature.com/nature/journal/v459/n7250/full/nature08046.html
•
Schneider et al., ‘No sodium in the vapour plumes of Enceladus’, Nature, 459, p. 1102, doi:
10.1038/nature08070, 2009.
http://www.nature.com/nature/journal/v459/n7250/full/nature08070.html
•
•
Porco et al., Science, 311, p.1393, 2006. http://www.sciencemag.org/cgi/content/abstract/311/5766/1393
Nimmo et al., Nature, 447, p.289, 2007.
•
Kieffer et al., Science, 314, p.1764, 2006. http://www.sciencemag.org/cgi/content/abstract/314/5806/1764
http://www.nature.com/nature/journal/v447/n7142/full/nature05783.html
Prepared for the Division for Planetary Sciences of the American Astronomical Society by David Brain and Nick Schneider
[email protected] - http://dps.aas.org/education/dpsdisc/ - Released 03 December, 2009