Overview of the Solar System

Outline General properties of planets and Solar System Spectroscopy, composition, density of planets Why do some have atmospheres? Terrestrial/Jovian planet distinction Clues to interiors and history NEAR Shoemaker (2001). Japanese too, on 25143 Itawaka, in 2005, returned in 2010, but not yet known whether sample collection worked properly.

You live in a special age Landings on Moon, Venus, Mars, Titan, an asteroid and a comet Returned rocks from the Moon Atmospheres probed on Venus, Mars, Jupiter, Titan Fly-bys past all planets and Pluto Venus and Titan surfaces revealed by radar mapping All of this in past ~45 years - and more to come! NEAR Shoemaker (2001). Japanese too, on 25143 Itawaka, in 2005, returned in 2010, but not yet known whether sample collection worked properly.

Contents of the Solar System Sun in center, contains most of the mass Planets Moons Rings Asteroids – mostly between Mars and Jupiter. Mostly rocky material Comets – High eccentricity orbits. Icy material “Trans-Neptunian Objects” Meteoroids Gas and dust

Key questions How did the Solar System and planets form? How are planets similar to and different from Earth? What are the planets made of and how do we know? What range of properties do their moons show? What is the origin and fate of rings? What can we learn from asteroids and comets?

Solar System objects to scale

The Solar System is BIG! It is difficult to make a correctly scaled model Most of the Solar System is (nearly) empty space

If the Solar System were 10 km across… Object Distance Diameter Size example The Sun 0 km 1.55 m A large beach ball Mercury 65 m 5.4 mm A pebble Venus 121 m 1.35 cm A small marble Earth 167 m 1.42 cm Earth's Moon 43 cm 3.9 mm Mars 254 m 7.6 mm A pea Jupiter 868 m 15.9 cm A softball Saturn 1.6 km 13.4 cm Uranus 3.2 km 5.7 cm A hen's egg Neptune 5 km 5.5 cm

The Sun Average-sized star (mostly H, He) 99.8% of the mass of the Solar System ~4.6 Gyr old (middle-age) Surface (photosphere) about 5800 K (emits mostly in visible, UV, IR) Hot because of nuclear fusion in core Builds He nuclei from H nuclei, a process that releases energy

Planetary orbits All planets orbit the Sun in same direction and almost same plane Orbits are close to circular Main exception is Mercury: orbital tilt 7o eccentricity 0.21 Remember e related to b and a All spin in same sense as orbit, except Venus and Uranus

Key concept: Density = mass/volume. Depends on composition, compression by gravity. Can compare to density of water: water = 1000 kg/m3 = 1 g/cm3

(Earth=1) (Earth=1) (g/cm3) Planet Diameter Mass  (Earth=1) (Earth=1) (g/cm3) ------------------------------------------------------------ Mercury 0.383 0.055 5.4 Venus 0.949 0.815 5.2 Earth 1.000 1.000 5.5 Mars 0.533 0.107 3.9 Jupiter 11.21 317.8 1.3 Saturn 9.45 95.2 0.7 Uranus 4.01 14.5 1.3 Neptune 3.88 17.2 1.6 (“Terrestrial” Planets) (“Jovian” Planets) Rocks at surface have density 3. So interiors must contain denser material. Inner four are rocky, outer four are mostly gas and liquid. Must give clue to formation.

How do we know what planets, moons, etc., are made of? Chemical composition - determined by spectroscopic observations or sometimes direct chemical analysis. The spectrum of a planet with a thick atmosphere reveals the atmosphere’s composition If there is no atmosphere, the spectrum indicates the composition of the surface. To a large extent, we must infer what the interiors are made of. direct analysis – Venus, Moon, Mars (also asteroid, comet tail, interplanetary space).

Visible light from planets, moons, comets, etc. is dominated by Can learn much from reflected spectrum, but must understand what features are due to Sun’s incident spectrum and Earth’s atmosphere. Compare with lab spectra. Wien’s law (for 10’s-100’s K, where is peak?) Visible light from planets, moons, comets, etc. is dominated by reflected sunlight. In IR, might see emitted blackbody radiation.

near IR spectrum

For context, consider overall abundances of elements in our part of the Milky Way

Where in the Solar System is this kind of abundance found? Sun, Jupiter, Saturn, Uranus, Neptune (except planet cores) Where is it NOT found? Mercury, Venus, Earth, Mars, Moons, comets, asteroids

H and He abundant on the Jovian planets H and He abundant on the Jovian planets. The terrestrial planets are composed mostly of heavier elements (e.g. Fe, Si, Mg, O, Ca, Al). Can see solid surface of Mars. Color due to iron.

Planetary Atmospheres Why do some planets have atmospheres, and others don't? Why do they have different atmospheric compositions?

Now recall escape speed from a planet V is the typical speed of particles of mass m in a gas of temperature T Now recall escape speed from a planet Particles colliding all the time. Some gain energy, some lose, so speeds change. If a particle is moving too fast, the planet can't retain it Rule of thumb: a gas will be retained in the atmosphere if Vesc > 6V

Example: For room temperature (293 K) VH2 = 1.9 km/s, and 6 times 1.9 km/s = 11.4 km/s VO2 = 0.5 km/s, and 6 times 0.5 km/s = 3 km/s The escape speed from surface of Earth is 11.2 km/s.  Molecular oxygen is easily retained, but hydrogen is not.

Guess which planets have atmospheres: Planet Vesc (km/s) __________________________________ Mercury 4.3 Venus 10.4 Earth 11.2 (Moon) 2.4 Mars 5.0 Jupiter 59.5 Saturn 35.5 Uranus 21.3 Neptune 23.5

Two Kinds of Planets "Terrestrial" Mercury, Venus, Earth, Mars "Jovian" Jupiter, Saturn, Uranus, Neptune Close to the Sun Small (D=5000-13000 km) Far from the Sun Large (D=50,000-143,000 km) Mostly Rocky High Density (3.9 -5.5 g/cm3) Mostly Gas and Liquid Low Density (0.7 -1.6 g/cm3) Venus, Uranus retrograde Slow Rotation (1 - 243 days) Fast Rotation (0.41 - 0.72 days) Few Moons No Rings Main Elements Fe, Si, Mg, O Many Moons Rings Main Elements H, He

Also in the Solar System Seven giant moons: size ~ size of Mercury Many smaller moons Comets Asteroids (most orbit between Mars and Jupiter) Meteoroids “Trans-Neptunian” objects Some moons were made in place, others captured asteroids. Asteroids in ecliptic, between Mars and Jupiter. Comets are generally not in the ecliptic

Chunks of rock and ice Asteroids: small, rocky objects, most orbiting between Mars and Jupiter Comets: small, dirty ice balls whose orbits bring them into inner Solar System Trans-Neptunian Objects – icy bodies beyond Neptune’s orbit, including Pluto and Eris Kuiper belt – zone 30-50 AU from Sun containing most of the TNOs All debris left over from planet making process

Trans-Neptunian Objects Orbit of Eris

More definitions! A planet is a spherical object orbiting a star that is not a star itself, and has swept out its path A dwarf planet is a spherical object orbiting a star that has not swept out its path (Pluto, Eris, Ceres, a few other TNOs), and is not a satellite. Note Pluto and Eris are also TNOs, and Ceres is an asteroid.

Cratering on terrestrial planets Result of impacts from interplanetary debris (but some are volcanic) Geologic activity => Many craters means old surface and low geological activity Smaller objects lose heat faster: more cratered Complications: external drivers of heat; water and wind erosion; atmospheric burn-up; gravitational attraction of impactors

Olympus Mons on Mars – largest volcano in Solar System. Volcanic flows can fill impact craters. Its crater is a caldera.

Magnetic Fields – a direct indication of interior The presence of a global, regular, magnetic field indicates a liquid, conducting interior Need circulating currents to generate magnetic field, like in an electromagnet

The global magnetic field of the Earth is produced by metals, mainly iron, in the liquid state The stronger fields of the Jovian planets are generated by liquid metallic hydrogen or by water with ionized molecules dissolved in it Liquid metallic H: high pressures, electrons can move from one H atom to another. Venus, no field. Mars and Moon, weak fields in rocks at surface. Indicates they were earlier in molten state, and only field left is in rocks that were permanently magnetized when they solidified. B field period = rot rate of interior

Solar system formation All objects formed from the same cloud of gas and dust Composition determined by cosmic history Different objects formed in different environments depending on their distance to the Sun

Problem 6.36 The four largest moons of Jupiter are roughly the same size as our Moon and are about 628 million km from Earth at opposition. What is the size in km of the smallest surface feature that the HST can detect (resolution 0.1")? How does this compare with the smallest feature that can be seen on the Moon with the unaided human eye (resolution 1')? 300 km ( (0.1”/206265)x6.28e8), 110 km ( (60”/206265)x384,400).

Copernicus crater on the Moon. Ejecta extends to 800 km away