Week5

Information about Week5

Published on November 15, 2007

Author: Dixon

Source: authorstream.com

Content

ESS 298: OUTER SOLAR SYSTEM:  Francis Nimmo ESS 298: OUTER SOLAR SYSTEM Io against Jupiter, Hubble image, July 1997 Titan and Other Satellites:  Titan and Other Satellites Titan (largest moon of Saturn) Atmosphere Surface Interior Cassini Other Satellites of Saturn and Uranus Why is Titan important?:  Why is Titan important? It has a thick atmosphere (unique amongst satellites) Large (Mercury-size) Surface currently unknown (haze) Astrobiologically interesting (hydrocarbons/organics) Current exploration – Cassini/Huygens Titan Basic Parameters:  Basic Parameters The surface has been imaged only at very low resolution No data on magnetic field, MoI etc. – as yet! Rp is planetary radius, 71492 km for Jupiter, 60268 km for Saturn Why an atmosphere?:  Why an atmosphere? Titan is the only satellite to have a significant atmosphere. Why? Seems to be a combination of three factors: Local nebular temperatures sufficiently cold that primordial atmosphere was able to form (Saturn is ~ twice as far from Sun as Jupiter, and is less massive) Titan’s mass sufficiently high that it was able to retain a large fraction of this original atmosphere (and later cometary additions) (Jeans escape) Surface temperature warm enough to prevent some volatiles (e.g. N2) freezing out (c.f. Pluto, Triton) Haze layer Jeans Escape:  Jeans Escape Escape velocity ve= (2 g R)1/2 (where’s this from?) Mean molecular velocity vm= (2kT/m)1/2 Boltzmann distribution – negligible numbers of atoms with velocities > 3 x vm Nitrogen N2, 94 K, 3 x vm= 0.7 km/s Titan ve=2.6 km/s A consequence of Jeans escape is isotopic fractionation – heavier isotopes will be preferentially enriched Titan has a D:H ratio in its methane of about 6 times that of Jupiter and Saturn. Half of this is due to Jeans fractionation, but the remainder is probably due to cometary additions to the atmosphere (comets have higher D:H than solar) Atmospheric Composition:  Atmospheric Composition Surface pressure 1.5 bar, temperature 94 K Obtained from UV/IR spectra and radio occultation data Various organic molecules at the few ppm level Haze consists of ~1 mm particles, methane condensates plus other hydrocarbons (generated by photodissociation and recombination of methane; similar to Los Angeles smog) These particles eventually rain down to the surface where they will condense (ethane will be liquid) Atmospheric Chemistry:  Atmospheric Chemistry Methane gets photodissociated and H2 is lost (why?): Reactants e.g. ethane will condense and fall to the surface These two effects mean that the lifetime of methane in the present atmosphere is ~107 yrs So there must be something which is continually resupplying methane to the atmosphere One suggestion was that this source was a methane ocean at the surface Methane liquefies at 90.6 K, ethane at 101 K, c.f. surface temp. 94 K There are other possibilities e.g. comet delivery, outgassing Atmosphere is reducing because of lack of oxygen. Where is the oxygen? It’s locked up in solid H2O at the surface (c.f. Earth, Mars). This in turn means there’s little CO2 Atmospheric Processes:  Atmospheric Processes Theories suggested ethane ocean could 0.5-10km deep But current observations find very little evidence for a global ocean (see later) Hydrogen escapes Photodissociation Ethane condenses at 101 K Other reactants similarly Organic drizzle Clouds Ethane etc. ponds? Underground aquifer? Methane recharge After Coustenis and Taylor, Titan, 1999 Atmospheric Structure:  Atmospheric Structure At lowest temperature (tropopause) all constituents except N2 are condensed (clouds) For an adiabatic atmosphere we have dT/dz=mg/Cp (derived in next week’s lecture) For an N2 atmosphere, m=0.028 kg, Cp~3.5R=30 J K-1 mol-1 So the lapse rate is ~1 K/km Temperature increases above tropopause due to incoming solar radiation Particulate haze makes direct observations of clouds hard From Owen, New Solar System Lapse rate ~1 K/km Particulate haze extends to ~300 km Clouds:  Clouds Keck Adaptive Optics images, from Brown et al. Nature 2002 Predominance of clouds near S pole not yet explained but may be due to local convective columns driven by small changes in surface temperature. Clouds lie beneath the haze layer, at 10-20km, and are mainly methane crystals (bright) Cassini image of clouds near South Pole 450km Is there really a surface ocean?:  Is there really a surface ocean? Sagan and Dermott (Nature 1982) presented a very clever argument against a surface methane ocean, using Titan’s orbital evolution Titan’s current eccentricity is relatively high (0.029) This eccentricity is not (currently) being excited by anything else, so it is presumably ancient But any dissipation in Titan will tend to circularize the orbit (see Week 1), at a rate governed by 1/Q The effect of a shallow surface ocean is to greatly increase the dissipation, reduce Q, and lead to an eccentricity damping timescale much shorter than the age of the solar system So either the ocean is very deep and global, or it is small or non-existent. Is there really a surface ocean (2)?:  Is there really a surface ocean (2)? Although Titan’s clouds are opaque at most wavelengths, there are transparent “windows” (see arrows) which allow the surface to be viewed The images on the right were taken by Hubble at 0.85-1.05 microns (mm), and show variations in surface reflectivity. Clearly, there is not a global ocean The images also suggest that Titan is rotating (approximately) synchronously 1.07 mm 1.28 mm 1.6 mm 2.0 mm Is there really a surface ocean(3)?:  Is there really a surface ocean(3)? Titan is the most distant object to date imaged with radar Original results showed radar-bright and –dark regions Radar surface reflectivity is 5-25%, much more than expected ocean values (2%) but less than values for icy Galilean satellites (30-90%). Perhaps the surface is an ice-rock mixture? Recent radar results (Campbell et al. Science 2003) found both diffuse and specular reflections. They suggest the specular reflections are consistent with bodies of ethane/methane liquid, but there are other possible alternatives e.g. very smooth ice regions. Surface roughness ~ radar wavelength (13 cm) diffuse specular Surface Spectra:  Surface Spectra Clever technique – use “windows” in atmosphere (see earlier slide) to obain reflectance spectra of surface Note that even in these windows, one still has to correct for scattering by haze particles Griffith et al. Science 2003 Results strongly suggest water ice is a dominant component. “Tholins” (organic material) appear to be relatively minor leading trailing Haze optical depth Titan observations What is the surface going to be like?:  What is the surface going to be like? Predominantly icy, plus a coating of organics Cratered (small ones missing – why?) Tectonized? What are the sources of stress? Volcanism? Tempting to assume that the bright areas are high (methane snow?) Windy? Not at the surface. Thick attmosphere means small temperature gradients, so weak winds (~1 m/s?) Dark – bright moonlit night Cassini false-colour NIR composite image. Yellows are hydrocarbons, green is ice, white is methane clouds Interior Structure:  Interior Structure Essentially unknown right now – density constraint Cassini will help things Main questions: 1) Is it differentiated? (Ganymede vs. Callisto) 2) Is there an ocean? (Why will detecting an ocean be much harder at Saturn than at Jupiter?) 3) Are there volatiles (other than water) present at depth? Volatiles? Two main ones are CH4 and NH3 Clearly present in the atmosphere, but may also be present at depth – other Saturnian satellites are inferred to have them on the basis of recent geological activity Were they stable during Titan’s formation? Quite likely, but depends on poorly known details of nebula Volatile Effects:  Volatile Effects Ammonia has a dramatic effect on the melting temperature of water ice – much easier to get oceans Ammonia:methane ratio ~1:1 in solar nebula 180 230 280 0.2 0.4 0.6 0.8 1.0 ice Ice + 5% NH3 Temperature, K Pressure, GPa After Grasset et al., Planet Space Sci., 2000 Methane will form clathrate structures with water of the form NH3.6H2O. These structures are stable up to at least 10 GPa (Loveday et al., Nature 2001) and provide an efficient way of storing large volumes of NH3 in the subsurface. Similar clathrates are found on Earth. Possible Structures:  Possible Structures Undifferentiated Pro – distant from Sun and Saturn, no likelihood of tidal heating Con – incorporation of volatiles makes melting easier Differentiated but no ocean Pro – hard to avoid differentiation (Callisto?) Con – hard to freeze ocean completely if NH3 present Differentiated with ocean Pro – likely end state if NH3 present Con – no tidal heating (c.f. Ganymede), dissipation may create problems How might we test these models? Cassini:  Cassini 6 tonnes, ~$2 bn, launched in 1997, planned from 1985 Note the absence of scan platform (so what?), and the reaction wheels Trajectory included Venus and Earth flybys, and will flyby Titan 44 times Instruments:  Instruments Most interesting one is the radar (uses the same system as the communications radio – whole spacecraft has to reorient itself!) Will produce images of the surface at ~1km resolution in ~100km wide swaths Also does altimetry, ~25km spacing, and measures backscatter 55km Left-hand image is of Ganymede with resolution (1.3 km/pix) comparable to Cassini radar resolution. Right hand image is from Galileo, 75m/pix Schematic of radar coverage. Two side-looking image swaths and a central altimeter pulse. Minimum altitude of spacecraft is ~1000km. From Elachi, Proc. IEEE, 1991 Approx. altimetry footprint Huygens:  Huygens Probe launches on Dec 25, 2004 Communication problems! Main chute has to be jettisoned to prevent probe falling too slowly (and freezing) Designed to survive on surface for ~1/2 hr, and to float Instruments: Imaging system Wind measurements Aerosols PT sensors Surface package GCMS Two Afterthoughts:  Two Afterthoughts Why is Titan so exciting? One reason is that it may in some respects resemble the earliest Earth, before life was established. Obviously there are differences (e.g. temperature) but Titan may be the best example of what the “primordial soup” (more accurately, gazpacho) which gave rise to terrestrial life looked like Another reason is we have no idea (yet) what the surface will look like. Will it resemble Ganymede, Callisto, Triton, or somewhere else entirely? Cassini’s First Titan flyby – TODAY! 6:30 pm NASA TV Mid-sized satellites:  Mid-sized satellites Rogues’ Gallery:  Rogues’ Gallery Common Themes:  Common Themes Tidal heating and orbital evolution (Peale Annu. Rev. Astron. Astrophys. 1999 is a good reference) Role of volatiles (ammonia, methane) Size-related effects Impact crater populations and effects Effect of distance from primary Lack of simple explanations . . . Things to Notice:  Things to Notice Jupiter has 4 large (>1500 km) moons, Saturn 1, and Uranus and Neptune none. Why? Neptune appears to be moon-poor in general. Why? All are synchronous, except Hyperion (chaotic) Densities are all close to 1 g/cc, suggesting mainly volatile ices (see next slide). Uranian satellites are denser. Uranus satellite densities increase (roughly) with distance. Why? Several of the periods are close to (or actually in) resonance e.g. Mimas-Tethys, Iapetus-Titan. May have had significant effects earlier in history. Uranian system has no resonances (at present day) Eccentricity Damping:  Eccentricity Damping Several of the satellites have eccentricity damping timescales t much less than the age of the solar system: *Here we are assuming Q=100; figures from Dermott et al. Icarus 1988 This is problematic: either their eccentricities were recently excited, or the damping timescales (and thus the assumed interior structures) are incorrect. See later. Densities/Radii:  Densities/Radii From Morrison et al., in Satellites, 1986 Theoretical lines Model density increases with increasing radius (why?) Saturnian satellites are probably >60% ice Uranian satellites are denser on average, and Triton,Pluto and Charon are denser again (why?) Condensation sequence (Week 1) favours CO, N2 (volatile) at high temps, CH4, NH3 (ice-forming) at lower temps. But if cooling is too rapid, CH4 and NH3 may not have time to form (kinetics). So where cooling is slower, more ices form, resulting in lower overall density. Albedos:  Albedos Callisto and Uranian satellites are dark, Saturnian satellites bright (except parts of Iapetus) If anything, albedo decreases with radial distance (why?) Uranian satellites are denser on average than Saturnian Cratering and Ages:  Cratering and Ages Cratering rate increases with decreasing distance to primary (grav. focusing), e.g. x2 at Rhea, x20 at Mimas compared with Iapetus (Smith et al. Science 1982) Size of crater caused by particular object increases with decreasing distance to primary So observed crater density is a strong function of distance to primary as well as surface age This makes even relative cratering ages hard to determine and model-dependent, never mind absolute cratering ages (see Zahnle et al. Icarus 2003) A consequence of gravitational focusing is that objects near the primary may have been disrupted once or several times by impacts (Mimas, Enceladus, Miranda, Ariel) Data and Models:  Data and Models Note that model impact rate decreases with increasing distance – high crater density can still mean young surface if the satellite is close to the primary Considerable scatter in observed crater densities Only highest densities are plotted here Open circles denote extrapolations Observed crater density Model impact rate Non-synchronous rotation (?):  Non-synchronous rotation (?) The satellites of Uranus and Neptune are expected to show large (6-35 times) variations in crater density from leading to trailing hemisphere if they have rotated synchronously for 4 Gyrs None of them show such a signature. Why not? Possible that large impacts (~20 km diameter crater) are sufficient to break the synchronous lock (see Chapman and McKinnon, in Satellites, 1986) There should still be an asymmetry in recent (small) impacts, but these are not visible with Voyager images Note that Iapetus does show a leading/trailing hemisphere asymmetry in albedo, suggesting that it is synchronously locked at the present day Absolute Ages (?):  Absolute Ages (?) From Zahnle et al. Icarus 2003 Uncertainties in absolute fluxes mean surface ages are very uncertain. Iapetus, Oberon, Titania and Umbriel are undoubtedly very old Mimas and Enceladus are at least slightly, and perhaps much, younger Parts of Miranda are very young Several satellites show a wide spectrum of ages (Enceladus, Rhea, Ariel) Activity (?):  Activity (?) Crater counts showing surface age diversity Kargel and Pozio Icarus 1996 Tectonic activity is relatively easy to infer Cryo-volcanic activity is much less easy to identify (e.g. Galilean satellites post-Galileo) Crater counts provide relative levels of activity Crater relaxation is an indication of increased heat flux 670 km Close-up of Miranda rift, showing large fault scarp (~5km high) scarp Expansion(?):  Expansion(?) As with Galilean satellies, almost all tectonic activity appears to be extensional – why? If satellites started cold (slow accretion) then release of radiogenic heat could generate heating and expansion (~1%) Tidal heating could also similarly generate extension Alternatively, as an ocean freezes and converts to less dense ice I it will generate extension (NB this does not work if it forms higher density ice phases, so only applicable to small satellites (P<200 MPa)) Volatiles:  Volatiles There is currently no direct evidence of ices such as ammonia or methane on the satellites of S,U,N But there are reasonable theoretical grounds for expecting them to be there Likely nebular temperatures consistent with their formation Presence of ammonia (especially) helps explain observed geological activity Titan’s atmosphere does have N2 and methane Methane forms a clathrate structure with H2O when the latter is present at the correct P,T conditions. Such clathrates may form a reservoir e.g. for Titan’s atmosphere. Ammonia:  Ammonia From Kargel, in Solar System Ices, 1998 This ammonia-rich liquid is usually denser than pure ice, but less dense than NH3.2H2O, so that it is likely to be able to ascend and erupt - cryovolcanism A mixture of ammonia and water doesn’t completely freeze until 178 K As freezing continues, the remaining liquid becomes more ammonia-rich The low temperature of this liquid may prevent convection (DT small) Saturn Observations:  Saturn Observations Small (<500 km), inactive Small, active Medium, inactive Medium, active Slide41:  Uranus Observations Small (<500 km), inactive Small, active Medium, inactive Medium, active Activity Summary:  Activity Summary “Metamorphic grade” of planet, based on cratering observations and tectonic history (Johnson, in Solar System Ices, 1998) I=“Unmodified”, II=“Intermediate”, III=“Heavily modified” M E T D R T H I P I I I II II II III M A U T O I I II II/III P T N III I Tidal Heating (1):  Tidal Heating (1) Enceladus is small but active, and currently in a resonance with Dione – differential orbital expansion similar to Io (?) So likely that tidal heating is responsible, but details are unclear (Squyres et al. Icarus 1983). In particular why did Enceladus melt if Mimas didn’t? (Mimas is in a 2:1 resonance with Tethys) Mimas is also puzzling because its eccentricity is high (how?) while at the same time it shows no sign of tidal deformation Ariel (also small and active) is not in a resonance now, but may have been (e.g. with Umbriel) in the past. How? The same also goes for Miranda (tiny and active). The fact that Miranda’s orbit is inclined at 4o is also suggestive of an ancient resonant episode (Tittemore and Wisdom, Icarus 1989) As with Ganymede, orbital evolution may explain present-day features . . . Tidal Heating (2):  Ariel’s orbit expands faster than Miranda’s because Ariel is so much more massive Tidal Heating (2) Theoretical evolution of orbits (from Murray and Dermott; c.f. Dermott et al. Icarus 1988) Note that various resonances may have been encountered on the way to the present-day configuration (e.g. Miranda:Umbriel 3:1) Passage through resonance will have led to transient eccentricities and heating Note that diverging paths do not allow capture into resonance (though they allow passage through it), while converging paths do. This may help to explain why there are no examples of resonance in the Uranian system. 3:1 resonance responsible for Miranda’s present-day inclination (?) Other effects?:  Other effects? Tides can’t be the only answer e.g. Umbriel not resurfaced, though it likely went through resonances Titania is resurfaced, but no resonance has ever been identified Some of the resonances do not generate much tidal dissipation e.g. Ariel:Umbriel 2:1 resonance One suggestion is that inner bodies were catastrophically disrupted by impacts, and then reaccreted. The energy of this reaccretion might help to explain early activity. How then do we explain Mimas? Close in, but not active. What about gradients in the initial nebula? Might expect more geological activity at smaller distances, where more volatiles had time to condense. Not really borne out by observations (e.g.Umbriel v. Titania) Example - Enceladus:  Example - Enceladus Looks like a miniature Ganymede, including relaxed craters and extensional faulting Wide variety of surface ages, some <~107 yrs May be the source of the E ring, which has a lifetime of only ~104 yrs High albedo, perhaps suggestive of recent activity and frosting? Tectonized crater Extensional faulting 50km Cassini, July 2004 Enceladus – cont’d:  Enceladus – cont’d Early deformation could be due to initial freezing and expansion, but there has been much more recent activity Current eccentricity generates ~0.1mWm-2, comparable to radiogenic, insufficient to account for activity Increase in e by ~10 times would be sufficient to explain activity. What could have caused such an increase? Not clear – current resonance with Dione insufficient Increase in eccentricity must be relatively recent: eccentricity damping timescale ~108 yrs. Mimas also presents a problem – why does it show no signs of activity when it’s closer to Saturn? See Squyres et al., Icarus 1983 for a lucid discussion Other Oddities 1 – Miranda’s Coronae:  Other Oddities 1 – Miranda’s Coronae What’s the naming theme? From Pappalardo et al., JGR 1997 Roughly circular, large tectonic features with extensional faulting on their margins Topographic profiles suggest flexure and Te~2 km What are they? Maybe upwellings, but no-one really knows . . . 480km Other Oddities 2 – Iapetus Dichotomy:  Other Oddities 2 – Iapetus Dichotomy Albedo varies from ~0.5 (ice) to ~0.05 from one hemisphere to the other The dark side is centred on and symmetrical about the leading hemisphere. Why? Two explanations: 1) Impacts on Phoebe generate dust which eventually spirals in and impacts on the leading hemisphere 2) Dark material is produced internally and then concentrated on the leading hemisphere e.g. by impacts removing a bright frost covering Symmetry suggests NSR is not happening 1400km Cassini, 2004 Voyager, 1981 dark bright Other Oddities 3 - Phoebe:  Other Oddities 3 - Phoebe Small (D~200km), dark, retrograde, eccentric (0.16) and far (215 Rp) Most likely a captured object (from where?) Albedo (~0.05) comparable to dark side of Iapetus Where does the dark material come from? High-res images suggest dark and light layering Cassini images Conclusions:  Conclusions There is a surprising amount of activity for such small satellites The energy source for this activity must be tidal heating (though the details are usually obscure) The presence of low-melting temperature species like ammonia is almost certainly required to allow the activity to happen, though there is little evidence of cryovolcanism Impacts have had significant effects in disrupting, spinning and eroding satellites Distance from primary seems to be a secondary control on satellite characteristics Extension is dominant Student Presentations:  Student Presentations Sign-up sheet – my office (4642), today, 2pm List of topics on the website (or you can pick your own – anything controversial and current) If you like you can do it in pairs – pro and con 70% of final grade Weeks 8 & 9 – 20 mins. + 5 mins. discussion Come and talk to me if you’re not clear on what’s required End of Lecture:  End of Lecture Supplementary material follows Measuring the Winds (from Earth!):  Measuring the Winds (from Earth!) This is a very clever technique, which involves serendipity (an unexpected binary star), adaptive optics, and some (fairly) simple theory The basic technique is to use an occultation (i.e. when a star (or a spacecraft) passes behind a planet as viewed from the Earth): atmosphere planet Earth star The occultation has three effects: The intensity of the light decreases (this is easy to measure) The angular deflection of the star increases (this is usually impossible to measure) The refraction leads to a “central flash” as the star passes behind the centre of the planet. If the atmosphere is not perfectly axisymmetric, other refraction effects are observed and can be used . . . Binary Occultation:  Binary Occultation When the 20 Dec 2001 occultation of Titan was observed (using adaptive optics), it was realized the star was binary. Why was this helpful? 1) It allowed the angular deflection to be measured, and the location of the refracted starlight to be tracked 2) It allowed the changes in intensity to be measured very accurately (by referencing to the unobscured star) Figures from Antonin Bouchez thesis, Caltech Modelling:  Modelling The intensity and angular deflection of the refracted starlight allow the shape of a surface of constact refractivity (=pressure) in Titan’s atmosphere to be established – the atmosphere acts as a lens Assuming an isothermal atmosphere, variations in the height of a constant pressure surface have to be balanced by zonal (horizontal) winds The inferred winds are not symmetric about the equator – this is probably due to seasonal variations in heating as a result of Titan’s obliquity (27o). Wind velocities are also high (~100 ms-1) Atmosphere (schematic) Atmos. surface altitude varying with latitude Resulting winds Winds Slide57:  “ . . . A reddish colour dominated everything, although swathes of darker, older material streaked the landscape. Towards the horizon, beyond the slushy plain below, there were rolling hills with peaks stained red and yellow, with slashes of ochre on their flanks. But they were mountains of ice, not rock . . .” Stephen Baxter, Titan Size effects:  Size effects Radiogenic heat flux goes as R Cooling rate also decreases as R increases But tidal heating is more affected by e than R Central pressure = 2GpR2r2/3 (why?) Ice converts from I-II at 200 MPa so critical radius for this conversion to occur is ~800 km cooling Cooling of large satellite will lead to ice I-II transformation, which causes large change decrease in radius and thus global compression Size Effects - Examples:  Size Effects - Examples Compression effect of Ice I-II transformation may explain why Iapetus and Rhea (slightly larger) are less active than Dione and Tethys – compression suppresses volcanism and extension. Some evidence for compressive features on Rhea. More difficult to explain the difference between Umbriel (inactive) and Titania and Ariel (active) Titania is dense, so less ice and more rock means May have evaded Ice I-II phase transformation More radiogenic heating, so more likely to be active Ariel’s activity requires a different explanation . . . Radiogenic elements:  Radiogenic elements Chondritic heat production (present day) Hr~3.5 pW/kg Over 4.5 Gyr, this generates ~1.8 MJ/kg C.f. water latent heat of fusion 0.33 MJ/kg In conductive equilibrium the temperature difference DT required to get rid of the radiogenic heat scales as Here k is the thermal conductivity (~3 W/mK). Note that DT scales as radius2. E.g. for a 500 km radius satellite at the present day, DT~100 K – borderline for melting water in interior Thermal expansion strain =aDT ~ 1% - quite a lot What complications affect this (simplified) analysis? Where from? Ammonia (cont’d):  Ammonia (cont’d) Viscosity of erupted material likely to be comparable to basaltic-intermediate lavas Evidence for such cryovolcanic lavas is currently not very strong From Kargel, in Solar System Ices, 1998 1200km Close-up view of Ariel showing flat-floored graben. It has been suggested the flat floors are due to cryovolcanic flooding. Two populations (?):  Two populations (?) Population I consists of largest craters, is associated with the heavy bombardment period, and has a slope (~ -2) similar to populations on Ganymede and the Moon Population II consists of smaller craters, with a steeper slope, and post-dates the heavy bombardment Why the difference? Possibly II is debris from a disrupted satellite, which might explain the unusually steep slope Plescia and Boyce Nature 1983 Cratering on Tethys

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