La missione CASSINI-HUYGENSesplorazione del sistema di Saturno

INAF- Istituto di Astrofisica Spaziale

Ritratto di Famiglia

Pioneer 11 View of Saturn’s Rings

Early Voyager Photograph

• La missione Cassini-Huygens, risultato della collaborazione tra NASA, ASI ed ESA è forse la più complessa ed ambiziosa missione mai tentata nel corso dell’esplorazione dei pianeti del Sistema Solare.

• E’ composta di due parti distinte l’Orbiter Cassini, realizzato dalla NASA con consistenti contributi italiani, e la sonda Huygens, realizzata dall’ESA.

Gian Domenico Cassini (1625 - 1712)

• Nacque in Liguria, a Perinaldo, nel 1625.

• Dal 1669 fu direttore l'Observatoire Royal, appena istituito dal Re Sole Luigi XIV.

• Scoprì quattro satelliti di Saturno e la divisione tra gli anelli che ancora oggi porta il suo nome

• Misurò inoltre la velocità di rotazione di Giove scoprendovi la "macchia rossa“ occhio di un gigantesco uragano che imperversa da secoli nell'atmosfera del pianeta.

Christiaan Huygens (1629 – 1695)

• Olandese, nel 1666, fu nominato direttore presso l'Académie des Sciences, voluta da Luigi XIV.

• Fu il primo membro onorario straniero della Royal Society.

• Nel 1655, scoprì Titano, e teorizzò che Saturno fosse circondato da un anello sottile e piatto, non collegato al pianeta, inclinato rispetto all'eclittica.

• Nello stesso anno osservò la Nebulosa di Orione. Grazie al suo telescopio fu in grado di suddividere la nebulosa in singole stelle.

300kg

3700kgS/C Height 6.7m

Il Contributo italiano• Sottosistemi della Sonda CASSINI:

– Antenna ad alto guadagno (riflettore in fibre di carbonio di 4 metri di diametro; 4 bands S,X,Ku,Ka)

– Unità di trasmissione radio (RFES and RFIS)

– Star Trackers

• Strumenti Scientifici– Radio Science Experiment– Radar Experiment– Visible and Infrared Mapping Spectrometer

(VIMS)

Una Crociera Movimentata

I f ly-bys con la Terra e Venere hanno fornito l ’equivalente di 75 tonnellate di carburante (Al lancio i serbatoi di Cassini ne contenevano 3 tonnellate)

August 17, 1999

Jupiter Flyby

Oct 2000 – Jan 2001

Jupiter & IoDecember 12 2000

Closest ApproachDecember 29 2000

Jupiter & IoJanuary 01 2001

From Jupiter to Saturn

January 2001 – July 2004

Titan & SaturnOctober 21 2002

20 mesi prima dell’arrivo di Cassini a Saturno

A 285 milioni km (quasi 2 volte la distanza Terra-Sole)

…. e finalmente dopo 6.5 anni

• Saturno osservato da Cassini nel Novembre 2003 da una distanza di 112 milioni di km

• Da Terra, grazie al telescopio spaziale si poteva ancora fare di meglio

Immagine ripresa da Hubble

Immagine ripresa da Cassini

27 Marzo 2004: il pianeta ed il suo sistema di anelli riempiono il FOV della NAC camera

Primo Luglio 2004 Inserzione in Orbita

Saturn is less brightly banded than Jupiter: Same 3 layers, but atmosphere is thicker,

so we don’t see different colored layers as well. This is due to Saturn’s lower surface gravity

Due tempeste si fondono in una

• Saturno è il pianeta più ventoso del sistema solare, con tempeste equatoriali che si spostano a velocità di 450 m/s (1600 km/ora), ovvero dieci volte le correnti a getto terrestri.

Le immagini mostrate in bianco e nero sono state riprese dalla fotocamera di Cassini mentre le due in blu e rosso sono dello strumento VIMS. Il Polo stesso è proprio nell’occhio di questo immenso vortice polare delle dimensioni di circa 1500km.

Il Polo Sud di Saturno

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Il Polo Sud di Saturno

Un Esagono al Polo Nord

Aurora sovrapposta all’Esagono polare

• Saturno è una forte sorgente di emissioni radio.

• Le onde radio emesse sono direttamente correlate con le aurore (simili alle aurore boreali sulla Terra) osservate al polo

Gli Anelli

Behind Saturn

Origin & Stability of Rings• Particles supplied by inner moons:

– Erosion by micro-meteors– Occasional bigger break-up

• Ground smaller over time– Smaller particles swept out

• Radiation or magnetic fields

• Brilliant rings may be transient– Or do moons stabilize?

Rings and the “Roche Limit”

Inside the Roche limit, the tidal stresses on a body (which grow with its size and closeness) are stronger than its internal strength (basically gravity, in the case of moons). This is generally at 2.5 planetary radii or so.

Composition of Saturn’s Rings

Rings are composed of ice particles

moving at large velocities around Saturn, but small relative velocities (all moving in the same direction).

0

Saturn’s rings are composed of numerous icy fragments.

• Doppler shift measurements confirm that inner rings are spinning faster, and are separated from, outer rings.

• The rings move following Kepler’s third law suggesting that they are composed of individual “moonlets” or ring particles.

• Ring particles range in size from 1 cm to 5 m with 10 cm being most common.

• The ring particles reflect 80% of the light that falls on them (albedo) which is consistent with water ice.

• Tidal forces pull larger ring particles apart within a critical distance from Saturn, called the Roche Limit.

• Saturn’s rings have turned out to be constantly recycling themselves: small particles sticking to one another, larger particles frequently colliding and grinding each other to dust

Artist’s impression of particles in Saturn’s ringsshowing the process of accretion and fragmentation

(William Hartmann)

• Si estendono per circa 340.000 km (quasi la distanza Terra-Luna)

• Il loro spessore è dell’ordine delle centinaia di metri• Rappresentano la struttura più piatta del Sistema Solare

• Due sono le ipotesi sulla loro origine: – che siano il risultato della distruzione di un satellite di Saturno, ad opera

di una collisione, – oppure che siano un "avanzo" del materiale da cui si formò Saturno che

non è riuscito ad assemblarsi in un corpo unico

I freddi anelli di Saturno

• Questa immagine a falsi colori, ripresa nell’infrarosso, mostra la temperatura degli anelli di Saturno, con il rosso corrisponde a 110 K ed il blu a 70 K.

• Le parti più opache del sistema di anelli sono anche le più fredde.

Ripply Rings

Onde negli anelli

• Le strie scure negli anelli sono causate dalla perturbazione gravitazionale prodotta dal satellite Pan, che orbita all’interno della divisione di Encke.

Making Waves

The “Opposition Surge”on Saturn’s rings.

This appears exactlyopposite to the directionto the Sun, at phaseangle 0° .

The size of the surge isa measure of how rough the particles are.

A Selection of Cassini Images

The Discovery of Daphnis in the Keeler Gap:The wake is highly distorted

Prometheus produces a “wake” pattern just like Pan

The aftermath of a ring impact?

“Saucer” Moons

Epimeteo

0.3 Km/px

2.1 Km/px

Febe è il satellite più lontano e si trova su di una orbita retrograda ad una distanza di 13 milioni di km da Saturno;

La densità, l’aspetto e le dimensioni di Febe sono caratteristiche dei cosiddetti planetesimi, frammenti di materiale primordiale andato poi ad accumularsi a formare i pianeti.

Simili residui del processo di accrescimento potrebbero essere sopravvissuti nelle zone più esterne del sistema solare, ma non nelle vicinanze di Saturno.

Febe: un alieno alla corte di Saturno?

Encelado: la luna bianca

Chiaro come neve fresca

• La superficie di Encelado riflette il 90% della luce solare ricevuta.

• Sulla sua superficie vi sono solchi che suggeriscono una storia tettonica complessa, nonostante i soli 505 km di diametro.

Un’occhiata da vicino

• La bassa densità di crateri conferma che la superficie è giovane.

• L’aspetto della superficie suggerisce una storia di fusione e ricongelamento.

Enceladus

• The Voyager encounters established its radius (252.1 km) and its visual geometric albedo (>0.9) which is consistent with fresh snow or ice, and higher than any other known solar system body.

• Voyager 2 also discovered that large areas of its surface have been resurfaced by endogenic forces and were estimated to be <200 million years old.

• Other parts are still heavily cratered but the craters show considerable signs of internal modification.

• All regions of the satellite, regardless of age, exhibit uniformly high albedos, implying that the entire satellite is coated with a ubiquitous fresh material

South Polar Terrain from ISS

Tiger stripes region

Ion and Neutral Mass Spectrometer ResultsAverage mass spectrum for altitudes below 500 km.

In red actual counts with 1σ errors

• 91% Water• 3.2% Carbon Dioxide• 4 % Nitrogen or CO• 1.6% Methane

• < 1% Acetylene and Propane

• Ammonia < 0.5%

ISS plume observations

Porco et al. 2006

Enceladus as the source of material for the E Ring

CIRS Observations of the South Pole region

• Cassini’s Composite Infrared Spectrometer (CIRS) detected 3 to 7 gigawatts of thermal emission from the south polar troughs at temperatures up to 145 kelvin or higher (to be compared with Io thermal emission of 105 GW)

• Enceladus is thus third known solid planetary body, after the Earth and Io, that is sufficiently geologically active for its internal heat to be detected by remote sensing.

Spencer et al. 2006

…and VIMS

Ratio of 1.2µm continuum over 1.6 µm cristalline water ice band

Cristalline ice is concentrated inside the tiger stripes

Consistent with higher emission temperatures

Enceladus: a possible thermal hystory (1)

• Enceladus is one of the most geologically dynamic bodies in the Solar System, surprisingly given the only 500km diameter.

• Cassini Doppler data have shown its density to be 1608±4 Kg/m3

• This is much larger than Mimas (1148±5 Kg/m3) and Tethys (974±4 Kg/m3) and implies a larger rocky mass fraction and consequently a larger content of radioactive elements and a greater extent of radiogenic heating

Long lived 238U 0.71Gyr, 40K 1.27Gyr, 235U 4.47Gyr, 232Th 13.9GyrShort lived 26Al 0.72 Myr, 60Fe 1.5Myr

• From density data we have about 30% to 50% the silicates mass fraction

• Inclusion of 26Al requires very early formation of Enceladus.

• 26Al found in CAI within chondrule among the oldest compounds formed in SS

The sequence of events

• 26Al heating led to full differentiation (Ice – Rock) within 2-3Myr from formation

• Long term heating kept interior warm enough over 4.5Gyr to be susceptible to tidal heating

• Liquid layers in the mantle could explain present day activity even with low eccentricity (Mimas has larger eccentricity but is a more rigid body hence no geological activity is present)

t=1.6Myr

t=3.2MyrCore 155kmMantle 90KmCrust 15km

t=0 t=0.4Myr

t=0.8Myr

Schubert et al, 2007

Titan

Titan Facts

Titan belongs to the triad of giant moons with bulk densities of 1.8 g/cm3, and radii ~ 2500 km, along with Ganymede and Callisto.

Titan has the second-densest atmosphere of the solid bodies of the solar system

Surface P,T=1.5bars, 95K. Methane 2-10% near-surface, 2% in stratosphere.

Nitrogen-methane atmospheric composition implies organic chemistry.

Methane CycleThe photolysis of methane and nitrogen leads to the formation of complex hydrocarbons, polycyclic aromatic hydrocarbon (PAH), nitriles and polymers;

The resulting compounds lead to the production of haze (Tholins), of hydrocarbons condensates and their precipitation on ground.

Methane is also key to the maintenance of the thick nitrogen atmosphere.

700km

The nitrogen atmosphere would gradually condense in the absence of warming resulting from the hydrocarbon haze

Methane is lost in the atmosphere with a lifetime of 10-100 Myr

Waite et al, 2007

Surface Methane• The methane count rate increases for 2 min after impact by about 40%.

• the behaviour of methane in the surface measurements indicates the presence of liquid methane mixed with the surface material.

Niemann et al 2005

Titan seen by VIMS• Although the VIMS spectral

range is 0.35–5.2 mm, the spectral region in which the surface can be imaged with any clarity is far more restricted.

• Due to the optically thick scattering haze particles, below 1 µm the contrast nearly vanishes.

Titan Radar Observations

• This map of Saturn's moon Titan shows the location mapped with the Cassini radar mapper using its synthetic aperture radar imaging mode on April 30, 2006.

• Cassini's radar has revealed a variety of geologic features, including impact craters, wind-blown deposits, channels and cryovolcanic features.

• This image of Saturn's moon Titan from the Synthetic Aperture Radar instrument on the Cassini spacecraft shows Shikoku Facula, a region that is bright in both radar and visible wavelengths.

• A circular feature with a radar-dark interior, probably an impact crater, is seen near the top of the image and is about 35 kilometers (22 miles) in diameter.

• Numerous linear dark features are seen running across the image, mostly on the right-hand side. These features are interpreted as dune fields.

• Narrow, sinuous, radar-bright channels, are seen on the right-hand-side of the image.

• These may be river networks that might have flowed onto the dark areas on the left of the image

• This image from the Synthetic Aperture Radar instrument on the Cassini spacecraft shows the radar-bright western margin of Xanadu, one of the most prominent features on Titan.

Titan Lakes

Titan longitudinal Dunes

• Longitudinal dunes are formed when winds tends to oscillate back and forth around the prevalent direction of the wind.

• On Titan the dominant wind has a east to west pattern altered by tidal winds

Upper Image TitanLower Image Namibia Desert

Titan Topography

• Stereo coverage resolution of about 50 m/pixel. • The perspective image is color-coded in altitude with blue lowest and red highest, with no vertical

exaggeration. • The total relief is ~250 m and the area covered is about 2.5 x 3.5 km. • The valleys exhibiting dark drainages in the brighter higher terrains have steep sides with slopes ranging

up to ~30°

Soderblom et al 2006

Iapetus

• Iapetus, Saturn’s third-largest moon (mean Radius 736km), orbits at a mean distance of 3.6 million Km (59Rs) on a slightly inclined prograde orbit.

• Iapetus one of the most intriguing objects:– Albedo asymmetry – Equatorial Ridge– Second lowest density object 1100

Kg/m3

– The farthest tidally locked satellite– Oblate spheroid

Present day Iapetus is well away from hydrostatic equilibrium shape.

With 79.33 day rotation period a deviation from spherical of no more than 10 m is expected

Actual deviation is 33 km.Hhomogeneous body 16hr rotation

period to keep shapeDifferentiated body would need about

15hrIapetus has a fossil shape frozen when

the lithosphere was strong enough to resist deformation at a time when the satellite was still despinning.

• Despinning to reach synchronicity is the result of tidal dissipation;• Iapetus’ synchronous spin was unexpected, because of its large

semi-major axis (D=60 Rs) and the very strong (D−6) dependence of the rate of despinning on distance to the planet.

Iapetus

• The leading hemisphere has a reflectance of about 6% as opposed to the trailing hemisphere which is ten times brighter

The equatorial Ridge rise more than 20 km above the surrounding plains

IAPETUS - ORSHIRES001-V1568145604Distance 45600 km - Phase 32° - VIMS 22.8 Km/pixel

VIMS-IR @ R 2.42 um (CN); G 2.05 um (H2O), B 1.94 um (OH)

ISS-WAC (raw data)

VIMS-V @ 970 nm

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