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

Earth Moon

WaterVolatiles

DifferentiatedAtmosphereMetallic coreComposition

Plate TectonicsMagnetic FieldTemperature

Life

YesYesYesYesYes

Rock & MetalYesYes

ModerateYes

NoNoYesNoNo

Rock onlyNoNo

Large extremesNo

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, 1726Mars’ 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 MarsDeimos• 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.

DeimosThis 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 SystemAll 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

– Small Moons• < 300 km• Potato-shaped and captured• Usually rocky• Inclined, eccentric, retrograde orbits

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

• Small outer moons (prograde)– Captured in prograde orbits– 7 km – 22 km– Substantially inclined orbits– Leda, Himalia, Elara, Lysithea– 2000 J11

• Small captured outer moons (retrograde)– Captured in retrograde orbits– 5 km – 15 km– Inclined and eccentric orbits– Ananke, Carmae, Pasiphae, Sinope– 2000 J1 – 2000 J10

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 dpsdisc@aas.org - http://dps.aas.org/education/dpsdisc/ - Released 03 December, 2009

• 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

What Process Creates the Plume?

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 dpsdisc@aas.org - 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

• 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

Tiger stripes

Prepared for the Division for Planetary Sciences of the American Astronomical Society by David Brain and Nick Schneider dpsdisc@aas.org - 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

• Oberon– Radius = 761 km– Orbital period = 13.463 days– Not very active geologically– Heavily cratered

• Umbriel– Radius = 585 km– Orbital period = 4.144 days– Not very active geologically– Heavily cratered

• Ariel– Radius = 579 km– Orbital period = 2.520 days– Once geologically active– Lightly cratered

Oberon Umbriel

Titania 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??

Moons of NeptuneHubble has discovered a newMoon: S/2004 N1

Photo taken in 2009, the moon which was noticed by Mark Showalter in 2013

It is about 12 miles wide

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 • Hydra• Kerberos• Styx

Sizes

For More Information…Press Releases

• 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.

http://www.nature.com/nature/journal/v447/n7142/full/nature05783.html• Kieffer et al., Science, 314, p.1764, 2006.

http://www.sciencemag.org/cgi/content/abstract/314/5806/1764Prepared for the Division for Planetary Sciences of the American Astronomical Society by David Brain and Nick

Schneider dpsdisc@aas.org - http://dps.aas.org/education/dpsdisc/ - Released 03 December, 2009