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C L A S S # 2 1

Astronomy 340Fall 2007

Review/Announcements

HW #4 due nowHW #5 handed out on Thursday, due Nov 29Last Time

Moons of Jupiter Io Europa

Titan

Discovered in 1655 by Huygens2nd largest moon

R = 2575 km Density = 1.9 gm cm-3

Orbital properties 4:3 resonance with Hyperion Somewhat eccentric effect of tidal forces?

Titan’s Atmosphere

1st detected 1944Composition (pre-Cassini)

CH4 plus some C2H6, CH3D, C2H2, HCN Voyager detected N2 98% N2 Pressure comparable to Earth’s

CH4 cycle Liberated from ethane/methane oceans Some migrates upwards Dissociation & photolysis H2 + hydrocarbons

H2 may escape CxHy haze/clouds remain Possible methane/ethane snow/rain

Titan’s Atmosphere

Titan’s Clouds (Griffith et al. 2000)

Techniques Variations in brightness = variation in albedo = presence of

clouds Spectral windows = different depths

Stratosphere reflects at 2.17-2.4 μm Deep into atmosphere, but not surface 2.12-2.17 μm Surface 2.0-2.11 μm

Results Variations in brightness in 2.12-2.17 μm window Low covering factor (<5%) Altitude ~ 25-30 km

Titan’s Clouds (Samuelson et al 1983)

Effects of surface temperature variationTsurf ~90-95K, lows ~70 K, highs ~180 K

Atmospheric processes Greenhouse effect Absorption via hydrocarbons Photolysis of CH4 reservoir

Titan’s Surface

Titan’s Surface – Radar Mapping

Titan’s Surface

Titan’s Surface

Note variation in color variation in surface featuresBright spot is of particular interest CO2 ice?

Titan’s Surface

Cassini Trajectory

Surface Liquid on Titan?

Hydrocarbon lake?

Old river beds?

The surface of Saturn's largest satellite—Titan—is largely obscured by an optically thick atmospheric haze, and so its nature has been the subject of considerable speculation and discussion1. The Huygens probe entered Titan's atmosphere on 14 January 2005 and descended to the surface using a parachute system2. Here we report measurements made just above and on the surface of Titan by the Huygens Surface Science Package3, 4. Acoustic sounding over the last 90 m above the surface reveals a relatively smooth, but not completely flat, surface surrounding the landing site. Penetrometry and accelerometry measurements during the probe impact event reveal that the surface was neither hard (like solid ice) nor very compressible (like a blanket of fluffy aerosol); rather, the Huygens probe landed on a relatively soft solid surface whose properties are analogous to wet clay, lightly packed snow and wet or dry sand. The probe settled gradually by a few millimetres after landing.

Zarnecki et al. 2005 Nature 438 792

Structure of Titan

Surface landing site was firm possible liquid below

Evidence of some liquid on surface

Surface pressure is sufficient (1.5 bar)

Methane on Titan

D/H ratio 80-90 ppm higher than solar, less than ocean water, comets,

chondrites (300 ppm) Methane likely not from comets

Lifetime Photolyzed in 10 million years Requires replenishing

Surface ocean? Interior outgassing? Implies D/H ratio reflects initial composition

CO, N2 CH4, NH3 (Lewis & Prinn 1980)

CH4 present in the feeding zone at 10 AU Satellites form via accretion, methane locked in interior Requires periodic (constant?) outgassing

Outgassing Model (Tobie, Lunine, Sotin 2006)

Planetary Rings

A Little History 1610 Galileo discovered rings; they disappeared in 1612 1659 Christian Huygens discovered disk-like nature 1977 Uranus’ rings discovered during occultation 1979 Voyager 1 discovered Jupiter’s rings 1980s Neptune’s arc-like rings discovered via occulation

Cassini-Huygens Science - Rings

Study configuration of the rings and dynamic processes responsible for ring structure

Map the composition and size distribution of ring material

Investigate the interrelation of Saturn’s rings and moons, including imbedded moons

Determine the distribution of dust and meteoroid distribution in the vicinity of the rings

Study the interactions between the rings and Saturn’s magnetosphere, ionosphere, and atmosphere

Basic Properties

Solid disk vs particles Too big to rotate as solid body Maxwell rings are particles

100m thick vs 105 km wide (Saturn)Just one ring? NoAll reside within Roche limit of planet

Ring Comparison

Jupiter Main ring – bigger particles, more scattering Halo – interior component, orbits scattered by interaction with

Jupiter’s magnetic field Gossamer – very low density, farther out more inclined orbit

so its fatterUranus

6 identified rings Thickness ~10 km, width ~250000 km Very close to being in circular orbit

Rings: Uranus & Neptune

Uranus’ rings Neptune rings

Particle Size Distribution

Power law n(r) = n0r-3 number of 10 m particles is 10-9 times less than # of 1 cm particles

Total mass distribution is uniform across all binsCollisions

Net loss of energy flatten ring Fracture particles power law distribution of particle size But, ring should actually be thinner and radially distribution

should gradually taper off, not have sharp edges

Measuring Composition

Poulet et al 2003 Astronomy & Astrophysics 412 305

Ring Composition

Water ice plus impurities

Color variation = variation in abundance of impurities

What could they be?

Ring Composition

Red slope arises from complexcarbon compounds

Variation in grain size includedin model 90% water ice overall

Comparative Spectroscopy

olivine

Ring particle

Ring Dynamics

Inner particles overtake outer particles gravitational interaction

Ring Dynamics

Inner particles overtake outer particles gravitational interaction Inner particle loses energy, moves closer to planet Outer particle gains energy, moves farther from planet Net effect is spreading of the ring

Spreading timescale = diffusion timescale

Ring Dynamics

Spreading stops when there are no more collisionsIgnores radiation/magnetic effects that are linearly

proportional to the sizeExact distribution affected by

Differential rotation Presence of moons and resonances with those moons

Saturn’s Rings

D ring: 66900-74510 C ring: 74568-92000

Titan ringlet 77871 Maxwell Gap: 87491

B ring: 92000-117580Cassini divisionA ring: 122170-136775F ring: 140180 (center)G ring: 170000-175000E ring: 181000-483000

Structure in the Rings

Let’s look at some pictures and see what there is to see….

Structure in the Rings

Let’s look at some pictures and see what there is to see…. Gaps Ripples Abrupt edges to the rings Presence of small moons

Moons and Rings

Perturb orbits of ring particles Confine Uranus’ rings, create arcs around Neptune

Shepherding – two moons on either side of ring Outer one has lower velocity slows ring particle, particle

loses energy Inner one has higher velocity accelerates ring particle,

particle gains energy Saturn’s F ring is confined between Prometheus and Pandora

Shepherds in Uranus’ ring system

Moons and Rings

Perturb orbits of ring particles Confine Uranus’ rings, create arcs around Neptune

Shepherding – two moons on either side of ring Outer one has lower velocity slows ring particle, particle

loses energy Inner one has higher velocity accelerates ring particle,

particle gains energy Saturn’s F ring is confined between Prometheus and Pandora

Resonances Similar to Kirkwood Gap in asteroid belt 2:1 resonance with

Mimas