2000 March 21, SPS 1020 (Introduction to Space Sciences) - Reading: today was TNSS Ch. 21 (Triton, Pluto, and Charon) and PBD Ch. 9 - Read TNSS Ch. 22 (Icy Sats) & 23 (Small Worlds), for Thurs, March 23. - Read Chs. 5 and 24 (Comets) for Tuesday, March 28. --------------- Triton: ------ Large (r = 1353 km) satellite of Neptune's; has atmosphere. Discovered by W. Lassell in 1846, shortly after Neptune. => Has _retrograde_ orbit (i = 156.8 degrees): large forces have acted here! Oddly, eccentricity = 0 (?!?) Rotation is synchronous with Neptune (like Earth's Moon and Earth). => from tides of Neptune Nereid, a small satellite of Neptune: highly eccentric and inclined orbit, further evidence that the Neptunian system was gravitationally disturbed. Triton was observed by _one_ spacecraft flyby: Voyager 2 (in 1989 August, a most grand finale). Determined mass; Density = 2.05 g/cm^3, icy and rock mixture, predicted for icy planetesimals in Solar nebula Only 1/3 of surface was imaged: be careful about what you generalize, especially with so many novel, poorly understood landforms Surface: ------- Albedo = 0.72 (Bright!) => covered with ice Spectra => 5 kinds of ice: CH_4, H_2 O, N_2, CO_2, CO. Only 15 impact craters seen => implies (what?) Cryovolcanic outflows seen. Has geysers! Of Liquid nitrogen! (LN2). See dark, variable streaks, 100s of km long. The rest is quite poorly understood: Smoothness suggests a period of global melting. => this and retrograde orbit imply Triton was captured by Neptune; formed elsewhere in Solar System, probably in Kuiper belt. Enigmatic "canteloupe" terrain: probably tectonic? Canteloupe: roughly circular depressions called cavi (25-30 km across), randomly scattered. Double-ridged cracks, reminiscent of Europa, cross it. Maculae: have dark structures (guttae) surrounded by brighter aureoles: poorly understood; 100-200 km diameter (Cryovolcanic?) Atmosphere: ---------- Nearly all N_2, 0.01% CH_4. Triton is tipped on its side (like Uranus), Obliquity of its equator to its orbital period varies from +52 degrees over -52 degrees => extreme seasonal changes Triton is presently undergoing global warming. Not difficult, since T = 38 K, coldest in Solar System. Still, climate cycles over 680 years, because of changes in latitude of Sun; reminiscent of Earth and Mars (climate changes due to changes in orbital parameters, thought to have caused ice ages on Earth; Mars climate changes not well understood, but definitely more extreme). Triton's atmosphere is near vapor-pressure equilibrium: small changes in T can result in large changes in P. => Can snow its own atmosphere, like Mars (and unlike Earth). Has (methane?) frost at poles. Also wind-blown streaks: a surprise, given the thin atmosphere! Pressure = 16 microbars; pressure scale height = 15 km. Has thin haze layer: photochemical hydrocarbon smog, like Titan. => Surface probably has rich organic chemistry Pluto: THE NINTH MAJOR PLANET OF THE SUN. ----- Discovered in 1930 by Clyde W. Tombaugh, with the Lowell Observatory 13" astrographic telescope (later used for Giclas proper motion survey and Burnham's Celestial Handbook) (Tell the mountain lion story.) => Never visited by any spacecraft. One planned mission, with two flyby spacecraft, the Pluto-Kuiper Express: Launch 2004; Jupiter gravity assist; arrival 2012; extended mission to one or more Kuiper-belt objects? Review on the Kuiper-Belt Objects, also called Trans-Neptunian Objects or Plutoids: Since 1992, over 50 small, icy bodies orbiting the Sun outside of Neptune's orbit have been found (largely by David Jewitt and Jane Luu). These were predicted by Gerard Kuiper => the Kuiper Belt. They are essentially the condensed outer edge of the Solar Nebula, and are the source of short-period comets. Long-period comets (millennia) come from the Oort cloud, inferred to be roughly spherical since long-period comets are observed to have random orbital inclinations. The comets were ejected here during the early history of the Solar System by gravitational scattering (stirring) by Uranus and Neptune, near the orbits of which they condensed. Back to Pluto: Orbital period (around Sun) = 248 years. Pluto's orbit much more eccentric (e = 0.249) and inclined (i = 17 degrees) than that of any other planet. => Also disturbed, perhaps related to Triton and Nereid? Pluto is in a 3:2 orbital resonance with Neptune, must have had some relation in past. Orbit crosses that of Neptune's, unique among major planets: Neptune was most distant major planet between 1979 January 21 and 1999 Febrauary 11. Planets don't collide, though: closest approach ~ 17 AU, because of 3:2 resonance. T = 40 K Atmosphere is only present near perihelion: should fly by it, soon! Had been inside Neptune's orbit since 1989; won't be again for > 200 years Size, albedo, colors, spectrum, contrast ratio of surface markings all resemble Triton _and_ Kuiper-Belt objects. Pluto definitely an in-between case: Australia is the smallest continent, but can be thought of as the largest island. Pluto may therefore be the smallest major planet, but also the largest Kuiper-Belt Object. Pluto really *was* a red herring, though. (A red herring = something unexpected that misleads people.) It has insufficient mass to cause the gravitational perturbations inferred for the orbits of Uranus and Neptune. This became more obvious after the discovery of Charon its its series of mutual events the 1970s and 1980s, which improved mass estimates for Pluto. However, re-analysis of the measurements of the orbits of Uranus and Neptune, along with tracking of the Pioneer 10 and 11 and Voyage 1 and 2 spacecraft leaving the Solar System, led Miles Standish (JPL) to conclude in 1993 that there is no Planet X. => That the search for Pluto found it, therefore, was pure luck! (Good thing Tombaugh found it, after blinking all those plates...) Charon - Pluto's small satellite, discovered in 1978 by Jim Christy ------ Charon is pronounced "Sharon" - to honor Jim Christy's wife. The prototype Centaur, Chiron, is "KI-ron." Orbital period = 6.4 days, whole system synchronized; (Pluton & Charon spins and orbit); orbit highly inclined to ecliptic (122 degrees) Charon's rotation axis also highly inclined: 97 degrees => Uranus not only planet "on its side": also Pluto and Charon (again, from gravitational disturbance?) r = 625 km for Charon, versus 1150 km for Pluto (625/1150 = 0.54) => Definite double planet, like Earth and Moon (1738 km / 6378 km = 1/4). Handout on 3-body exchange interactions: stars do this; might Pluto and Charon have done it, too? Barely resolved, even by Hubble Space Telescope. Same for Pluto: only a few arcseconds across. (Good exercise: what is the angular diameter of Pluto?) => Can't directly see much detail. BUT: Pluto and Charon eclipse each other often, since double planet. Series of mutual events, between 1978 and 1983, recur every 124 years. Good thing it was discovered in 1978! => Photometry (brightness measurements) of these over time revealed differences in contrast (light and dark) over the surface, by turning time-resolved photometry into spatially resolved maps. Maximum-entropy mapping: one constructs the smoothest map possible that can still fit the data. Now also: Hubble Space Telescope images. Still only barely resolved, though: Charon is barely 2 resolution elements across. => Can infer radii, densities. Pluto density = 1.8 - 2.0 g/cm^3 Charon density = 1.5 g/cm^3 High albedo (= 0.7), implying an icy surface. Spectra => it's H_2 O ice Internal structure: inferred how?