ASTR178Other Worlds

A/Prof. Orsola De Marco9850 4241

orsola.demarco@mq.edu.au

• If you do not make your talk day you risk not getting that side of the assessment.

• Observing is on October 6th 7:15PM and 13th 8:15PM. (Sign up in class or on my door: E7A-316).

• Assignment 2 posted, due in 17th September.• Moon practical due in 17th September.

Announcements

Sample questions for this lesson

• 1-6, 8-11, 13-36,38,41, pages 349-351

In last class: gas giants part II

• Jupiter and Saturn.• Atmospheres • Inner composition• Magnetic fields• The rings

• Introduction to the moons of Jupiter

In this class: giants moons

• Introduction to the moons of Jupiter• Io• Europa• Ganymede• Callisto• Saturn’s Titan• … and all the other moons …

Giant Moons

• Noticed by Galileo ~1610• Could be resolved by naked eye were it not for Jupiter’s glare.• Synchronous rotation.• Innermost 3 have orbital periods in a 1:2:4 resonance. Why only 3 not

all 4?• Transits and occultations used to measure diameters

The Galilean satellites

• The Pioneers and the Voyagers satellites measured the moons’ masses – how?

• From radii and masses one could infer densities – how?

The composition of the satellites• Low density Callisto and Ganymede are likely made of water ice (low

density).• Io and Europa have densities similar to rock on Earth.• All but Io have spectra indicative of some water ice on the surface.

The formation of the satellites• Density decreases as you go away from Jupiter, as is the case for the

Solar System.• Jupiter formed as a miniature Solar System, where Jupiter itself was a

source of heat: rocky materials condense close where it is warmer while rock + ice coalesce further out, where ice exists because it is colder.

Io

Not quite like the Moon!!!

Io’s heat source• Tidal forces from Jupiter + Io’s elliptical orbit.• How much heat is generated? 24 tonnes of TNT exploding every

second!• How much heat goes through the surface? 2.5 Watt/square meter –

Earth’s: 0.06 Watt/square meter!

Volcanos• Plumes 70-280 km high.• Ejected at ~1100-3600 km/h. Compare Earth’s 360

km/h!• More like geysers on Earth – but propelled by Sulfur

dioxide.

• Rapidly cooled sulfur will lead to all the colours• Sulfur dioxide leads to white flakes

• Aside from geysers there are explosive lava flows.

• Hotter lava (from IR observations).• Contain magnesium – hotter

interior.• 100 km crust floating on a fully

magma interior – explain widespread volcanism

• 300 active volcanos at any one time.

• Enough material to cover Io in 1 m of ejecta in 100 years.

• Jupiter’s field generates a current across Io (400,000 V!)

• This in turn generates a magnetic field on Io.

• moving magnetic fields induce currents• Magnetised strip in your credit card induces a current

in the card reader.

• Measured field on Io is larger than predicted from above argument - hence Io has additional magnetic field from molten interior.

• Smallest body in Solar System with its own field.• Differentiation caused a solid iron and iron sulfide

core• Core ½ the size of Io – how do we know?

Europa• The smoothest world• The density indicates rock

composition…• … but the spectrum is pure ice. –

what is a spectrum?• Almost no craters – what does it

mean?

• Cracks probably from geologic activity – stress.

• Heat source as for Io, just less so since Europa is further from Jupiter.

• Cracks may be the result of tidal flexing• Many young areas (smooth) indicate much

activity

Frozen sea nearGreenland, photoTaken by FranciscoDiego from a plane(10,000 m high)

Ice on Europataken from similarAltitude

A liquid ocean under the crust?• Evidence from rafts and cracks.• Galileo measured magnetic field induced from Jupiter –

variable, needs conducting medium – water + minerals – colour on cracks.

• 100 km ocean on rocky core on metallic core?• Water + heat = life?• (Galileo was deorbited into Jupiter not to collide with

Europa)

And the atmosphere?• O2 atmosphere (very very tenuous) from collisions of

particles with ice, liberating O (H escapes into space)

• Ganymede young icy craters …

• Older darker• Younger

lighter (unlike the Moon!)

• Old cratered terrain.• Younger (but still 1 billion yr old) lighter terrain.• Fractures – tectonic activity. Heat in this moon till recently.• Ice on the surface, come through cracks? • Strange since Ganymede is very small to have retained heat.• Magnetic field twice as strong as Mercury. Molten interior!• Highly differentiated metallic core – how do we know?• Changes in magnetic field: Layer of liquid water – why?• Source of heat???

Callisto• Craters on ice• What is all the “muck”?• Lots of large craters, where are the small ones?• Changing magnetic field – hence liquid water – who keeps it

liquid? Ammonia as antifreeze. Still….• Not differentiated – hence cold.• This CO2 atmosphere

Cassini-Huygens explores Saturn and Titan

Short animation

Saturn’s Titan

• Discovered by Huygens in 1665• Suspected to have an atmosphere in 1900• Found to have one in 1944 – Kuiper found to have NH3 atmosphere

Saturn’s Titan

• NH3 in atmosphere broken down such that 95% atmosphere is nitrogen

• Surface pressure 1.5 times Earth’s. 10 times more atmosphere.• Temperature 95 K – methane and ethane liquid.

• Cassini’s flybys mapped Titan in Infrered• Sand dunes parallel to equator made of ice and polymers: wind

action• Methane from recent (100 million year) volcanos – also white

spots.• Where does the heat come from? Possible periodic reheating of

the core due to radioactive decay.

• The Huygens probe was launched from Cassini in 2005.

• It landed safely and broadcast images for 70 minutes.

• CH4 chemistry leads to hydrocarbons and the reaction of those with N2 leads to polymers which have a reddish colour – they can be in the air as aerosols or on the ground.

Movie

• Possible liquid methane lakes only detected near the north pole – might be seasonal.

• Huygens probe vaporised methane.• Only 5 cm “rainfall” per year• Few craters - erosion

Saturn and Titan in the news10 August 2009Titan was observed to havetropical clouds forming in this infrared image taken with the 8 m Gemini telescope

Titan’s clouds are likely tomake liquid methane rain.

Star Trek

Titan’s geologic historyA proposal

• Solid core, liquid water mantle icy crust formed 4.5 billion years ago.• Methane from volcanos• Methane destroyed between 3.5 and 2 billion years ago.• Radioactive isotopes – reheating of interior, convection transports the

heat to the surface, new outgassing of methane (2 billion years ago)• Methane destroyed once again.• 500 million years ago one more reheating episode. Once more outgassing.• In the future methane will go. We are just observing Titan at a special

time…

• The 4 innermost satellites of Jupiter.

• All within Io’s orbit.• All prograde

motion: they formed with the Galilean moons and J.

• There are 55 outer ones, all outside the orbit of Callisto.

• These are likely captured (inclined orbits and 48 have retrograde motion).

• 23 discovered in 2003 alone!

• All non spherical – why?

• Saturn has Titan and 6 larger satellites (spherical).

• They orbit on the Saturn’s equatorial plane, have prograde orbits and are phase locked.

• Orbit between 3 and 59 Saturn radii.

• They all look different!• It also has 54 small satellites,

some of which might be captured others are likely fragments of impacts.

• Low average density• Mimas is old and

cratered and has a HUGE impact crater.

• Enceladus has a new surface.

• Most reflective object in the solar system

• Ice eruptions come from the cracks.

• Eruptions from Enceladus spew particles out that form a thin ring.

• Heat source? Dione 2:1 resonance… possible. (Mimas and Tethys are also in a 2:1 ratio, but no heating…)

Key Ideas• Nature of the Galilean Satellites: The four Galilean satellites orbit

Jupiter in the plane of its equator. All are in synchronous rotation.• The orbital periods of the three innermost Galilean satellites, Io,

Europa, and Ganymede, are in the ratio 1:2:4.• The two innermost Galilean satellites, Io and Europa, have roughly

the same size and density as our Moon. They are composed principally of rocky material. The two outermost Galilean satellites, Ganymede and Callisto, are roughly the size of Mercury. Lower in density than either the Moon or Mercury, they are made of roughly equal parts ice and rock.

• The Galilean satellites probably formed in a similar fashion to our solar system but on a smaller scale.

Key Ideas• Io: Io is covered with a colorful layer of sulfur compounds

deposited by frequent explosive eruptions from volcanic vents. These eruptions resemble terrestrial geysers.

• The energy to heat Io’s interior and produce the satellite’s volcanic activity comes from tidal forces that flex the satellite. This tidal flexing is aided by the 1:2:4 ratio of orbital periods among the inner three Galilean satellites.

• The Io torus is a ring of electrically charged particles circling Jupiter at the distance of Io’s orbit. Interactions between this ring and Jupiter’s magnetic field produce strong radio emissions. Io may also have a magnetic field of its own.

Key Ideas• Europa: While composed primarily of rock, Europa is covered

with a smooth layer of water ice.• The surface has hardly any craters, indicating a geologically

active history. Other indications are a worldwide network of long cracks and ice rafts that indicate a subsurface layer of liquid water or soft ice. As for Io, tidal heating is responsible for Europa’s internal heat.

• An ocean may lie beneath Europa’s frozen surface. Minerals dissolved in this ocean may explain Europa’s induced magnetic field.

Key Ideas• Ganymede: Two types of terrain are found on the icy surface

of Ganymede: areas of dark, ancient, heavily cratered surface and regions of heavily grooved, lighter-colored, younger terrain.

• Ganymede is highly differentiated, and probably has a metallic core. It has a surprisingly strong magnetic field and a magnetosphere of its own.

• While there is at present little tidal heating of Ganymede, it may have been heated in this fashion in the past. An induced magnetic field suggests that it, too, has a layer of liquid water beneath the surface.

Key Ideas• Callisto: Callisto has a heavily cratered crust of water ice. The

surface shows little sign of geologic activity, because there was never any significant tidal heating of Callisto. However, some unknown processes have erased the smallest craters and blanketed the surface with a dark, dusty substance.

• Magnetic field data seem to suggest that Callisto has a shallow subsurface ocean.

Key Ideas• Titan: The largest Saturnian satellite, Titan, is a terrestrial

world with a dense nitrogen atmosphere. A variety of hydrocarbons are produced there by the interaction of sunlight with methane. These compounds form an aerosol layer in Titan’s atmosphere and fall as a gentle rain on the surface.

• Titan’s surface shows that liquid hydrocarbons have flowed over its surface, forming streams, rivers, and outflow channels. Very little of this liquid appears to be present on Titan’s surface today.

Key Ideas• Other Satellites: As of 2006, Jupiter has a total of 63 known

satellites and Saturn has a total of 56. • In addition to the Galilean satellites, Jupiter has four small

inner satellites that lie inside Io’s orbit. Like the Galilean satellites, these orbit in the plane of Jupiter’s equator. The remaining satellites are small and move in much larger orbits that are noticeably inclined to the plane of Jupiter’s equator. Many of these orbit in the direction opposite to Jupiter’s rotation.

Key Ideas• In addition to Titan, six moderate-sized moons circle Saturn in

regular orbits: Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus. They are probably composed largely of ice, but their surface features and histories vary significantly. The other, smaller moons include shepherd satellites that control the shapes of Saturn’s rings and captured asteroids in large retrograde orbits.