2000 March 23, SPS 1020 (Introduction to Space Sciences) - Reading: today was TNSS Ch. 22 (Icy Sats) & 23 (Small Worlds) - Read Chs. 5 and 24 (Comets) for Tuesday, March 28. - Read Chs. 25 and 26 (Asteroids and Meteorites) for Thursday, March 30. - Paper titles and summaries due: Tuesday, March 28. --------------- All 4 gas giants (Jupiter, Saturn, Uranus, Neptune) have both regular and irregular systems of satellites. Regular: in circular, equatorial orbits, e.g.: Galilean satellites. Irregular: in retrograde, eccentric orbits, e.g.: Jupiter: outer 4 Saturn: Phoebe (outermost, probably a captured comet) Uranus: Sycorax and Caliban (outermost) Neptune: TRITON (!) and Nereid Nearly all planetary satellites we have seen so far are either large or very small (e.g., shepherding satellites) Mid-Sized Icy Bodies: -------------------- For Saturn, studied only during Voyager 1 and 2 flybys; for Uranus, studied only during Voyager 2 flyby. - Have d = 400-1600 km, - Are large enough to have sufficient gravity to be spherical - Most rotationally synchronized to planets (like Earth's Moon) - Many have surprisingly active geology, i.e., show processes other than just impact cratering. Show cryovolcanism or tectonism, indicating heat flowing from interiors. For Saturn: ---------- Innermost -> outermost: (mnemonic, MET DR THIP) Innermost: any small bodies; millions, if you count ring particles! Mimas: (pronounced MEE-mas) ----- Cassini division (between A and B rings) at 2:1 resonance of Mimas r = 199 km, density = 1.1 g/cm^3, albedo = 0.8 (80%: bright!) A snowball, dominated by impact craters Largest: Herschel, d = 130 km Must have nearly shattered Mimas! Some fissures, or grooves, on opposite side of Mimas: related, like chaotic, hummocky terrain opposite Caloris, on Mercury? Enceladus: (pronounced en-CEL-a-dus: like enchiladas) --------- r = 249 km, density = 1.0 g/cm^3, albedo = 1.0 (highest in Solar System) Another snowball, but no saturation cratering. White as snow: frost on surface---from ice volcano? _Cryovolcanism_ discovered here (Voyager 2), _great_ surprise. Crater counts: crater number/area (areal density) varies by factor of 500 => Terrain varies greatly in age Surface has had several resurfacing events Ammonia acts as anti-freeze in H_2 O. At 176 K, flows like cold honey, or basaltic lava. (T = 90 K at Saturn). Floors of craters bow upward: heat escaping from interior, now? No expected radioactivity in rocks; probably tidal heating. Almost certainly, really: Enceladus in 2:1 orbital resonance with Dione Tethys: (pronounced TE-this or TEE-this) ------- r = 530 km, density = 1.0 g/cm^3, albedo = 0.8 One giant crater, Odysseus (d = 400 km) Relaxed structure, unlike Herschel: from flowing ice? Impact craters dominant Ithaca casma: a multiply fractured trench => tectonic Opposite Odysseus, though: probably from the impact. Also plains, from cryovolcanism Dione (pronounced dee-OH-nee) ----- r = 560 km, density = 1.4 g/cm^3, albedo = 0.6 Again, a near twin of its inner neighbor (Mimas and Enceladus, vs. Thethys and Dione) More extensive plains and fissures (faults?) than Tethys. Again, cryovolcanic plains Rhea: ----- r = 760 km, density = 1.2 g/cm^3, albedo = 0.7 Impact cratering dominant, _saturation_ cratered! Also parallel, linear scarps (cliffs, from global expansion and then shrinkage) Essentially no other landforms. but largest, most massive of the mid-sized satellites of Saturn! => Geological activity in icy bodies does _not_ correlate with mass, as in terrestrial planets. What matters here is _tidal_ heating, which is more extreme in _smaller_ worlds, since they're more easily gravitationally perturbed (in other words, pulled around). Titan: r = 2575 km, surface poorly known (since hazy) ----- Hyperion: (pronounced hi-PER-ee-on) -------- radius = 330 x 260 x 215 km, density = ? (probably icy) Irregularly shaped: Right on borderline, among the largest "small bodies" Tumbles _chaotically_, since perturbed by gravity of Titan, other moons. => Could not predict rotation even over the 9 months between the Voyager 1 and Voyager 2 flybys Among first known cases of _chaotic_ dynamics. Even Newtonian mechanics isn't really deterministic: small differences in initial conditions can become amplified. => "The Butterfly Effect": weather, for example, is so sensitive to small perturbations (small changes), a butterfly flapping its wings could change the weather in Russia, two weeks from now. Don't misinterpret this, though: most small perturbations dampen out, or in other words, get so lost in the background of other small perturbations, they don't matter. Still, hit the conditions just right, and it can matter. Some "clockwork Universe", huh? It's often been said that Newtonian mechanics cast the Universe as a great, predictable, clock, but Hyperion shows this model isn't right. In QM, of course, there is much true randomness, but we need not evoke it to get chaos: even plain, old Newtonian mechanics can produce chaos. Iapetus: (pronounced i-AY-pet-us) ------- radius = 720 km, density = 1.0 Leading hemisphere is black as pitch (albedo = 4%); Trailing hemisphere is white as snow (albedo = 50%). => Because one side is covered with pitch, and the other is covered by snow? Phoebe: (pronounced FEE-bee) ------ Outermost known satellite of Saturn. Irregular shape, r = 115 x 105 km Retrograde orbit: i = 175.3 degrees Very black: organic material, as seen in Halley's comet nucleus? => Captured large comet? Might explain dark material on leading hemisphere of Iapetus. Problem: on Phoebe it's neutral black; on Iapetus, it's dark red. Uranus: in equatorial orbits => tipped 97 degrees to ecliptic, with Uranus ------ Studied only by Voyager 2 flyby, so < 50% of any ever resolved! Here, outermost -> innermost (was innermost -> outermost, for Saturn): Oberon ------ r = 760 km, denisty = 1.6 g/cm^3, albedo = 0.2, dark: like Uranus's rings Heavily cratered, ancient surface Has ray craters (from more recent impacts), like Earth's Moon Dark patches in craters: like lunar maria, only C-rich icy "lava"? Titania ------- r = 790 km, denisty = 1.7 g/cm^3, albedo = 0.3 Densely cratered, with fault-bounded valleys From impact basin, on other side? Umbriel ------- r = 585 km, denisty = 1.4 g/cm^3, albedo = 0.2 Heavily cratered, ancient surface One bright crater, at pole: from frost deposit? Ariel ----- r = 581 x 578 km, denisty = 1.7 g/cm^3, albedo = 0.4 Networks of deep valleys and canyons, volcanically resurfaced Again, shows _inverse_ correlation of geological activity with size in icy moons: from tidal heating. Miranda ------- r = 240 x 233 km, denisty = 1.2 g/cm^3, albedo = 0.3 One of Solar System's oddest members. Take "all the weird geological features in the Solar System, put them together," and that's Miranda. Frozen in mid-upheaval: - Ancient cratered terrain - Deep fault valleys - 3 known polygonal "coronae" (not like Venus coronae): also called "chevrons": both tectonic and cryovolcanic. From whole moon being smashed apart (by collision), then re-accreted? Neptune has only Proteus (others are all small) ------- ------- r = 210 km (similar to Mimas, Enceladus, Miranda) No other mid-sized moons: were ejected by Triton? Small Bodies (Ch. 23): ------------ The distinction between asteroids and comets is inceasingly blurry. (Like the distinction between major and minor planets, in the case of Pluto: grrrrr....). => Depends on amount of ice a small body has If icy, it's a comet: ice vaporizes when near Sun, and makes tail. If rocky, it's an asteroid. An *old* comet, which has passed close to the Sun many times, and so has lost most of its volatiles, may therefore be classified as and asteroid: it's no doubt happened (e.g. discovery by IRAS) Chiron, "asteroid" discovered in between Saturn and Uranus Now known to have a tail => It's a comet! => It's a big one! r = 200 km Hale-Bopp was ~ 40 km, and itself was a big one. Chiron: prototype of a class of objects in outer Solar System, Centaurs -------- vs. Kuiper-belt objects (Trans-Neptunian Objects) ------------------- vs. Oort cloud objects: none yet seen in Oort cloud; will be very faint ------------------- vs. Trojan asteroids ---------------- In Jupiter's L4/L5 points, neutral gravity points +/- 60 degrees from Jupiter and Sun Telesto and Calypso: Tethys L4, L5 Trojans Helene: Dione L4 Trojan vs. Shepherding satellites of planetary rings ---------------------- vs. Cruithne: EARTH'S OTHER MOON! In Earth/Sun L4/L5 "horseshoe" orbit -------- vs. Earth-approaching, Earth-crossing asteroids: flung here by near-collisions? ------------------------------------------- vs. Main-belt asteroids: Average a = 2.5 AU, predicted by Bode's law ------------------- vs. Phobos and Deimos ----------------- Captured by Mars? But in circular, equatorial orbits!