Lecture 16 Dwarf Planets and Comets January 8a, 2014

Transcription

1 1 Lecture 16 Dwarf Planets and Comets January 8a, 2014

2 2 Pluto -- Basic Information Discovered by Clyde Tombaugh in 1930 Period: P orb = 248 years Distance: a = 39.5 AU 3 moons (Charon, Nix, Hydra) Demoted to Dwarf Planet in 2006

3 3 General Characteristics Mass = times the Earth Determined by using General form of Kepler s 3 rd Law Radius = 0.2 Earth Determined from eclipses of Charon = 2300 kg/m 3 ice and rock Little known due to its large distance from the Sun. Pluto is tilted on its side.

4 4 Spin and Orbit Highly elliptical orbit (e = 0.25) Pluto is sometimes closer to the Sun than Neptune Orbit is tipped 17 from ecliptic Aphelion = 49.3 AU Perihelion = 29.7 AU Both Pluto and Charon are tidally locked in synchronous rotation. P spin = 6.4 days (Pluto and Charon) P orb = 6.4 days (Charon)

5 5 Surface Properties Planet predominantly water ice Frozen methane detected on surface May have thin methane atmosphere May be similar to Triton

6 6 Surface Features Pluto has never been visited by a spacecraft (the New Horizons probe is on its way and will arrive in 2015) so there are no clear images of its surface. At left are Hubble Space Telescope global maps of Pluto (smaller insets are actual images) that show bright and dark areas visible as the dwarf planet rotates. At right is a composite image in true color that is derived from eclipses by Charon.

7 7 If Pluto is sometimes closer to the Sun than Neptune, why doesn t it ever collide with Neptune? A. They do collide every few thousand years. B. Neptune is primarily made of gases, so Pluto would pass right through it. C. Pluto s orbit is steeply tilted with respect to Neptune s, so they never actually cross. D. The synchronized timing of their orbit periods ensures a collision never occurs.

8 8 If Pluto is sometimes closer to the Sun than Neptune, why doesn t it ever collide with Neptune? A. They do collide every few thousand years. B. Neptune is primarily made of gases, so Pluto would pass right through it. C. Pluto s orbit is steeply tilted with respect to Neptune s, so they never actually cross. D. The synchronized timing of their orbit periods ensures a collision never occurs.

9 9 Origins of Pluto Composition much more like a moon Other objects similar to Pluto (such as Sedna, below) are being found in the Kuiper Belt

10 10 Comparison of distant planets Object Year discovered Diameter (km) Perhelion (AU) Aphelion (AU) Pluto Quaoar Sedna Eris FY ? EL ?

11 11 Solar System Debris After formation of the Solar System, some material was left over. Asteroids, comets, and meteoroids give clues to composition of early solar system. Have undergone little processing (heating, weathering).

12 12 Comets Made of ices and some rocky material Travel in very elliptical orbits about the Sun. Comet McNaught, January Click here for more info.

13 13 Long period comets May orbit once every million years Origin in Oort Cloud -- spherical cloud up to 100,000 AU from Sun Short period comets Periods < 200 years Origin in Kuiper Belt -- disk shape AU from Sun.

14 14 Anatomy of a Comet Nucleus few km in diameter ices and rocky material ( dirty ice ball ) Only part of a comet that exists away from the Sun. Coma -- Gases evaporated off of surface of nucleus as Sun heats it.

15 15 Hydrogen envelope Tails -- Always point away from the Sun Dust tail -- small dust particles, slightly curved in direction of orbit. Ion tail -- ionized molecules pushed straight back by solar wind

16 16 Neither tail trails the comet like a jet contrail. In fact, the tail sometimes leads the comet!

17 17 Suppose the density of a comet s tail is kg/m 3 and it s a cylinder km in radius and 1.0 AU ( km) long. What is the total mass of the tail? A kg B kg C kg D. 20,000 kg

18 18 Suppose the density of a comet s tail is kg/m 3 and it s a cylinder km in radius and 1.0 AU ( km) long. What is the total mass of the tail? 2 m V r L A kg B kg C kg D. 20,000 kg m kg/m m m kg

19 19 Comet NEAT

20 20 Comet Tempel 1 Deep Impact

21 21 Comet Tempel 1 Deep Impact revealed the composition of the comet Tempel 1 Some of the expected constituents: silicates (sand), water ice and some surprises: Clay, and carbonates (how did these form without liquid water?) iron compounds aromatic hydrocarbons

22 22 Halley s Comet Nucleus almost completely dark Period: 76 years Next Visit: 2061

23 23 Halley s Comet

24 24 Comet Shoemaker-Levy 9 Comet struck Jupiter in July 1994 Original comet ~2-10 km in diameter Before impact it broke into many small pieces

25 25 Astronomers think that most comets come from A. interstellar space. B. a region in the extreme outer parts of the Solar System. C. condensation of gas in the Sun s hot outer atmosphere. D. material ejected by volcanic eruptions on the moons of the outer planets.

26 26 Astronomers think that most comets come from A. interstellar space. B. a region in the extreme outer parts of the Solar System. C. condensation of gas in the Sun s hot outer atmosphere. D. material ejected by volcanic eruptions on the moons of the outer planets.