CONCEPT SUMMARY - 7 - Moons, Debris, and Life

MOONS

•       The 6 big moons (Galilean moons of Jupiter (Io, Europa, Ganymede and Callisto), Titan at Saturn, Triton at Neptune) which are comparable to small planets in size and suffer similar 4 geological processes as the terrestrial planets.
•        Most moons are spin-synchronized with the planet so that they always have the same face towards the planet
•       Many moons are in resonant orbits - which results in tidal forces which can heat the interior of the moon - particularly the closer a moon is to the planet (e.g. Io).
•       As with all planetary objects, we learn about bulk composition from the density of the moon and the surface composition from spectroscopy.
•       If a moon is greater than ~400 km in diameter it is likely to be spherical - <400km moons have insufficient mass for their gravity to overcome the strength of the material to pull itself into a sphere - can be irregular in shape.
•       Geological Processes - cratering, tectonics and volcanism - similar in principle to the terrestrial planets (which moons show examples of each process?). Tidal heating is an additional factor that affects the internal temperature and the amount of geological processes.
•       Atmospheres - some moons have sufficient (a) volcanism to out-gas an atmosphere, (b) gravity to hold in the atmosphere. The moons with measurable atmospheres are Titan, Io, Triton.

RINGS

•       Each of the four giant planets has a ring system - many objects of widely-varying sizes that separately orbit the planet.
•       Ring particles collide and gravitationally interact with each other so that they have very circular orbits that are tighly confined to the
•       planet's equatorial plane (making a very thin disk).
•       Gravitational interactions between moons and ring particles produce gaps in the rings (at locations that are orbital resonances with a
•       moon) or shepherd the ring particles into narrow bands.
•       The rings are formed by the break up of moons or captured debris from the outer solar system when they pass within the Roche limit
•       of the planet.
•       Studies of the evolution of ring particles' orbits suggest that the rings are changing rapidly. This means that they must have formed on
•       timescales of 10s of millions of years. Through collisions with each other and the planet's atmosphere, the ring particles slowly spiral
•       inwards until they enter the planet's atmosphere.

PLUTO/CHARON

       Discovered by American astronomer Clyde Tombaugh Lowell Observatory in Flagstaff, AZ, in 1930.

      "Mis-fit" planet - small, icy, with large moon (Charon), eccentric orbit, tilted on side. Neither giant planet nor terrestrial planet.

      Pluto has a resonant orbit with Neptune so that the chances of collision are negligible, even though their orbits overlap.

      Surface patchy - ice with dark regions.

       Spectroscopy tells us that there is at least CO2, N2 and CO ice on the surface of Pluto, probably water ice on Charon.

      The bulk density of Pluto tells us that it is mostly ice with a rocky core.

       Pluto as a tenuous atmosphere

       Pluto probably captured the moon Charon in a giant impact (similar to Earth's Moon).

       Pluto and Charon are the largest of a large swarm of icy objects that form the Kuiper Belt between ~30 and ~150 AU

 

Oort Cloud

      Comets are SMALL (1-10km) chunks of ice which condensed in the outer parts of the solar system before it formed a disk.

      They form a large (~50,000 AU ) sphere around the solar system.

       Occasionally comets are perturbed and kicked into the inner solar system. When they get closer than a few AU from the Sun, the ices and volatiles in the comet heat up and vaporize. The gravity of the comet is too weak to hold in the gases so that the gas (and dust carried by the gas) escape from the comet.

      The large cloud of escaping atmosphere and dust scatters sunlight and forms a LARGE (~AU-scale) coma around the comet which can extend for degrees in the sky and seen with a naked eye.

      Broken up comets leave a trail of ice/dust along their orbit. When the Earth goes through such a trail of dust and ice, we see a shower of meteors - these happen on specific days of the year (when Earth is on particular parts of its orbit).

      Since 1992 over 200 objects have been discovered to orbit the Sun between 30 and ~150 AU.

      These Kuiper Belt objects are small (~100km), icy planetesimals that were kicked out by what became the giant planets, when the material was in a disk.

      They tend to be in resonant orbits with Neptune - so they are less likely to be perturbed.

       The Kuipter belt is thought to be the source of comets with short orbital periods of decades (rather than the millions of years of the comets in the Oort cloud).

     Asteroids are SMALL objects which orbit between Mars and Jupiter.

     The orbits of planetesimals which formed in this region were 'stirred' up by Jupiter's gravity - so they were moving too fast to accrete when they collided - they tended to break up. If the planetestimal was big enough to have differentiated, chunks of core and mantle were separated / broken off.

      The bulk density (only acturately measured for Ida - why? How?) gives us compositon - iron / metal for chunks of core, rock for chunks of mantle - a mixture for planetesimals which were not big enough to differentiate.

      There are gaps in the distribution of orbits - the locations where the orbits would be resonant with Jupiter are empty - if an object was there it would be perturbed so that it either moves in to the inner solar system or kicked out to the outer solar system

       Asteroid peices that are perturbed in to orbits that cross the earth's orbit have the potential of hitting the Earth (e.g. "Near Earth Asteroids).

       Most peices of asteroid vaporize when they hit the Earth's atmosphere - appearing as hot emitting gases - meteors. Some peices hit the ground - particularly easy to find are the peices of iron cores which are found as dense, iron meteorites.

      Definition of life - an object which adapts to the environment, feeds, reproduces, evolves.

       Earlest fossils are of bacteria about 3.8 billions old.

       Multi-cellular organisms did not appear until about 1 billion years ago.

      "Humans are but an insignificant twig on a vast, arborescent tree, which, if replanted from seed would likely not produce us

      or anything like us." Stephen J. Gould

      Early development of life was "frustrated" by impacts which changed the environment leading to mass extinctions of species and rapid evolution.

      Latest big impact occured 65 million years ago when an impact off the Yucatan coast changed the environment which lead to the extinction of the large dinosaurs. The evidence for the impact causing global climate change is a think layer of clay which is found all over the globe (at the boundary between rocks of cretaceous and tertiary ages), containing iridium - a rare metal which is not found in regular crustal rocks but is found in iron meterites.

      Extra-terrestrial life - solar system - most likely places that life COULD have evolved are Mars (probably nothing alive now - just fossil, if anything), Europa (if there is a global ocean under the ice crust) and Titan (if there are liquids on the surface).

      Possible evidence of life in meteorite from Mars - but maybe not.

      Extra-terrestial life - elsewhere in the universe - the Drake equation (see pages 695-697) - the "habitable zone" (pages 395-296)