Chapter 7: Comets : Chapter 7: Comets
Comets : 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.
Comet composition : 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
Sublimation : 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:
Energy Balance : 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.
Sublimation : Sublimation Calculations of the gas outflow rate as a function of heliocentric distance, for different ices.
Water begins to sublimate at about 3 AU. H2O CH4 CO2 NH3 Equilibrium T without sublimation
Sublimation : 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 H2O H2O CH4 CH4 CO2 CO2 NH3 NH3
Orbits : 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
Kuiper Belt : 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
Comet Orbits : 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
Oort cloud : 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.
Meteor showers : 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.
Cometary meteors : 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
Orbit changes : 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.
Orbit changes : 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
Break : Break
Coma composition : 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.
Coma : 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.
Sublimation of comets : 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?
Tails : 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
Plasma (ion) tail : 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
Dust tail : 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
Comet Nuclei : Comet Nuclei Halley (1986) Borrelly (2001) Wild (2004) Deep Impact (2005)
Visiting comets : 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.
Wild : 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
Tempel-1 : 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.
Collisions : Collisions 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. Sun 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
Summary : Summary
Slide29 : 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.
Asteroid and comet sources : Asteroid and comet sources
Short-period comets : 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