Chapter 7: Comets

1. Chapter 7: Comets

2. Comets • Coma and tail form at a distance of ~2.5-3 AU, where ice can sublimate • The sublimation consumes a lot of energy, providing an additional, effective cooling source.

3. Comet composition • Comets become visible as such at a distance of about 2.5-3 AU. What temperature does this correspond to? • At this temperature, ice can sublimate to form water vapour

4. Sublimation • The vapour pressure of a given substance at temperature T is given by : where HL is the latent heat of vaporization, and p0 is the vapour pressure at some temperature T0. • The sublimation rate (number of molecules per unit time per unit area) depends on the vapour pressure and temperature:

5. Energy Balance • Heating: radiation absorbed from the Sun, with efficiency (1-Av) • Cooling: • Reradiation in the thermal infrared, with efficiency (1-AIR) • Sublimation carries off an energy 4pR2ZHL • To calculate the temperature at radius r, and the sublimation rate Z, you have to solve the energy balance equation by setting the heating rate equal to the cooling rate.

6. Sublimation • Calculations of the gas outflow rate as a function of heliocentric distance, for different ices. • Water begins to sublimate at about 3 AU. CH4 H2O CO2 NH3 Equilibrium T without sublimation

7. Sublimation • Calculations of the gas outflow rate as a function of heliocentric distance, for different ices. • Water begins to sublimate at about 3 AU. • Sublimation requires a lot of energy, effectively cooling the surface of the comet CH4 H2O CO2 NH3 H2O NH3 CO2 CH4

8. Orbits • Most comets have orbital periods >200 year • A 1997 database for 937 comets lists only 191 short-period (P<200 yr) comets • From Kepler’s third law, the semimajor axis of these long-period comets must be >34 AU: halfway between Neptune and Pluto

9. Kuiper Belt • Small objects detected in the region of Neptune, in 1992 • Currently several hundred are known • Expect there are at least ~70,000 objects with diameters of 100km or more. • Kuiper belt believed to extend from 40-400 AU • Flattened, in the plane of the rest of the solar system

10. Comet Orbits • Distribution of semi-major axes has a peak at a~104 AU • Orbits are highly eccentric, so aphelion is ~2a. • Originate in the very distant solar system • Very high orbital energy. Bound to the solar system… but just. 500 AU 40 AU

11. Oort cloud • Long-period comets come from all directions: not confined to the ecliptic • Therefore it was postulated that a huge, spherical shell of cometary material surrounds the solar system. This is the Oort cloud. • Outer edge expected to be at about 105 AU, where gravitational influence of Alpha Centauri will begin to dominate.

12. Meteor showers • Meteor showers appear at predictable times of year • meteors from a given shower all radiate from the same region of space and move with similar velocities • These are due to the Earth passing through debris from cometary tails.

13. Cometary meteors • From measurements of deceleration, we can tell that these meteors are tiny, low density dust particles • No meteor from a shower has ever been known to make it to Earth • Rockets and high-alititude aircraft have collected examples of this dust

14. Orbit changes • Cometary orbits can be perturbed by gravitational interactions (somewhat predictable) • However, mass loss can also change the orbit in unpredictable ways. • Mass ejected from the tail gives rise to a rocket effect that can change the orbit. • Calculate the change in period caused by a small change in velocity as a comet approaches the Sun.

15. Orbit changes • Cometary orbits can be perturbed by gravitational interactions (somewhat predictable) • However, mass loss can also change the orbit in unpredictable ways. • Mass ejected from the tail gives rise to a rocket effect that can change the orbit. • E.g. the comet Swift-Tuttle (P=120 y) was predicted to appear in 1982, but did not appear until 1992. • Comet is associated with the Perseid meteor shower, and therefore losing mass

16. Break

17. Coma composition • Spectrum of the coma shows bright emission lines due to small molecules (2-3 atoms). • These emisison lines dominate the light • Atoms in the coma absorb solar photons, then re-emit them in all directions.

18. Coma • Coma can begin to appear at distances as great as 5 AU • Indicates significant fractions of volatiles: methane, ammonia, carbon dioxide, nitrogen • From the heating rate and the chemical composition, we can calculate the amount of mass lost to sublimation.

19. Sublimation of comets • Consider a hypothetic comet, with a pure water-ice nucleus 1 km in radius. If the sublimation rate is ~1022 molecules/m2/s, how many passages will the comet be able to make through the inner solar system?

20. Tails • Tails extend for millions of kilometers • Always point away from the Sun • Two types (often both are visible at once) • Ion tail: straight, bluish-coloured tail • Dust tail: broad, curved, and yellowish

21. Plasma (ion) tail • Straight, but complex: with rays, streamers and knots • Spectra dominated by ionized molecular emission lines • Pushed away from the sun by the solar wind

22. Dust tail • Smooth, featureless • Spectrum nearly identical to the solar, absorption spectrum • Made up of dust particles less than about 1 micron in size • Radiation pressure forces the dust particles steadily farther from the Sun

23. Comet Nuclei • Halley (1986) • Borrelly (2001) • Wild (2004) • Deep Impact (2005)

24. Visiting comets • Need to know orbit accurately • Comets have large velocities relative to Earth (10-70 km/s) • Thus visiting spacecraft launched from Earth will face debris of small particles flying at very high velocities • E.g. Halley’s comet has a retrograde orbit, so the relative velocity is about 70 km/s • European Giotto probe passed within 600 km of Halley’s nucleus • Discoveries: • Comet abundances are very near solar • Very low albedo, only 4% (darker than a lump of coal). • Most of the surface is covered with a thick dust crust, through which gas cannot escape. • Gas evaporating from the comet comes from vents or jets, on only about 10% of the surface • Density is low, only 300 kg/m3, indicating that it is loosely bound icy material.

25. Wild • The spacecraft Stardust visited comet Wild2 in 2004 • Collected samples of dust, which were jettisoned back to Earth in Jan 2006 • Nucleus is covered with numerous craters and hills • At least 10 active gas vents

26. Tempel-1 • Impacted by Deep Impact probe in 2005 • Impact created a crater no more than about 50 m deep – only scratched the surface • Demonstrates that nucleus is not a loose agglomeration of material • Surface is more dusty than icy: and finer than normal sand.

27. Collisions Sun • This “Sun-grazing” comet was observed by the SOHO spacecraft a few hours before it passed just 50,000 km above the Sun's surface. • The comet did not survive its passage, due to the intense solar heating and tidal forces. • Shoemaker-Levy collided with Jupiter in 1994 • Was previously tidally disrupted into a string of fragments • Each fragment hit Jupiter with the energy of a 10 megaton nuclear bomb explosion

28. Summary

29. As expected, comets are warmer on their sun-facing side, as this temperature map from the Deep Impact mission shows (comet Tempel 1) • Sublimation occurs more rapidly on one side than the other.

30. Asteroid and comet sources

31. Short-period comets • Jupiter-type comets are those with P<20 yr • Small inclinations, relatively small eccentricities • E.g. Encke, Tempel2 • Likely originate in the Kuiper belt. Perturbed by Neptune or Uranus? • Halley-type comets have 20<P<200 yr • More eccentric, and higher inclinations • E.g. Halley has P=76 yr but e=0.97, and a retrograde orbit with i=162 deg • These probably originate from the Oort cloud, but have had their orbit perturbed.