The Solar System

Contents of the Solar System • Sun • Planets – 9 known (now: 8) – Mercury, Venus, Earth, Mars (“Terrestrials”) – Jupiter, Saturn, Uranus, Neptune (“Jovians”) – Pluto (a Kuiper Belt object?) • Natural satellites (moons) – over a hundred • Asteroids and Meteoroids – 6 known that are larger than 300 km across – Largest, Ceres, is about 940 km in diameter • Comets • Rings • Dust

Size matters: radii of the Planets

The Astronomical Unit • A convenient unit of length for discussing the solar system is the Astronomical Unit (A.U.) • One A.U. is the average distance between the Earth and Sun – About 1.5  10 8 km or 8 light-minutes • Entire solar system is about 80 A.U. across

Mercury The Terrestrial Planets • Small, dense and rocky Mars Venus Earth

The Jovian Planets • Large, made out of gas, and low density Saturn Jupiter Uranus Neptune

Asteroids, Comets and Meteors Debris in the Solar System

Asteroids

Asteroid Discovery • First (and largest) Asteroid Ceres discovered New Year’s 1801 by G. Piazzi, fitting exactly into Bode’s law: a=2.8 A.U.

• Today more than 100,000 asteroids known • Largest diameter 960 km, smallest: few km • Most of them are named • about 20 of them are visible with binoculars

Comets - Traveling Dirty Snowballs • Small icy bodies, “dirty snowballs” • Develops a “tail” as it approaches the Sun

Comet Anatomy • Tail may be up to 1 A.U. long

Comet Tail • Two kinds of tails: • Dust • Ion (charged particles)

Shapes Comet

Giacobini Zinner

(1959) • Ion tail 500,000 km long • Coma: 70,000 km across Comet

Hale-Bopp

(1997) • Tail 40° long as seen from earth

Halley’s Comet – a typical Comet

Halley’s Comet – Now and then • Halley’s Comet in

1910

• Top: May 10, 30° tail • Bottom May 12, 40° tail • Halley’s Comet in

1986

• March 14, 1986

Meteor Showers – caused by comets Radiant Quadrantids (QUA) Lyrids (LYR) Eta Aquarids Beta Taurids Delta Aquarids Perseids (PER) Draconids Orionids (ORI) Taurids Leonids (LEO) Geminids (GEM) Duration Dec. 28-Jan. 7 Apr. 16-25 Apr. 21-May 12 June 30 July 25-31 Aug. 10-14 Oct. 6-10 Oct. 15-29 Oct.12- Dec 2 Nov. 14-20 Dec. 6-19

Meteors, Meteroids and Meteorites • A

Meteor

is a sudden strike of light in the night sky • A

Meteoroid

is a small asteroid, less than 100 m in diameter • A

Meteorite

is any piece of interplanetary matter that survives the passage through Earth’s atmosphere and lands on Earth’s surface

Meteors and Meteorites • Small particles that strike the atmosphere • Come from fragments of asteroids, Moon, Mars, comets • Strike the earth all the time (“meteorites”) – High speed means lots of energy released on impact

• Most probably caused the extinction of the dinosaurs Impact on Earth

Impact Craters • Barringer Crater, AZ 0.8 mi diameter, 200 yd deep; produced by impact about 25,000 years ago • Quebec's Manicouagan Reservoir. Large meteorite landed about 200 million years ago. The lake, 45 miles in diameter, now fills the ring.

• ~30 m body struck Siberia in 1908 • Energy equal to that of a 10 Megaton bomb!

• Detonation above ground; several craters Tunguska

Frequency of Impact Events

Formation of the Solar System • Features to explain: – planets are far apart, not bunched together – orbits of planets are nearly circular – orbits of planets lie mostly in a single plane – directions of revolution of planets about Sun is the same, and is the same as the direction of the Sun's rotation – directions of rotation of planets about their axes is also mostly in the same direction as the Sun's (exceptions: Venus, Uranus, Pluto) – most moons revolve around their planets in the same direction as the rotation of the planets – differentiation between inner (terrestrial) and outer (Jovian) planets – existence and properties of the asteroids – existence and properties of the comets

Formation of the Solar System • Condenses from a rotating cloud of gas and dust – Conservation of angular momentum flattens it • Dust helps cool the nebula and acts as seeds for the clumping of matter

Formation of Planets • Orbiting dust – planitesimals • Planitesimals collide • Different elements form in different regions due to temperature • Asteroids • Remaining gas

Structure of the Planets explained Temperature and density of materials drop with distance to sun

Cleaning up the Solar System • Small objects are forced out of the inner Solar System by gravitational pull of bigger planets • Small planetesimals collide and form planets -- or are thrown out!