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Solar System – Minor Bodies (Debris)
Minor bodies (debris) – Asteroids (minor planets), meteoroids, and comets
- left over planetesimals from solar system formation
Asteroids (Minor planets)
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approximately 105 asteroids have been identified (so far)
largest (Ceres): Diameter, D  900 km
 larger than many minor moons
most have D < 100 km
composition: inner solar system composition: refractory materials
 rock and metal mineral compounds
- larger asteroids have chemically differentiated interiors (like terrestrial planets)
- due to heat of formation
Main asteroid belt
- zone lying between 2 and 3.5 AU from Sun where most (but not all) asteroids are
located
- lies between orbits of Mars and Jupiter
- individual asteroids orbit Sun  obey Kepler III (P2  a3)
 asteroids closer to inner edge of belt always overtaking those closer to outer
edge
 belt does not rotate as solid body  sheared in radial distance from Sun
Jupiter’s interaction with asteroids
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- Jupiter most massive object after Sun, is only 2.5 AU from center of main belt
 has important gravitational influence on asteroids
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- Kirkwood gaps:
- Jupiter’s gravity causes tidal forces across width of asteroid belt
- tidal force at any one place in belt constantly changing as Jupiter revolves
 prevents planetesimals from accreting into a proto-planet
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- relatively empty divisions within asteroid belt at orbits of particular a value (ie.
at particular distances from Sun)
- similar to divisions in Saturn’s ring system
- suprising; expect asteroids to be distributed randomly in orbital radius
throughout belt
- gaps occur at distances for which Porb related to Jupiter’s Porb by a whole
number ratio
- eg. gaps at a values corresponding to Porb = 1/2, 1/3, 2/5, …  Porb, Jupiter
 due to orbital resonance of asteroids with Jupiter – like Cassini division in
Saturn’s ring system
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- Trojan asteroids
-two families of asteroids in Jupiter’s orbit that precede and follow Jupiter
by 60o
- have mean orbital radius, a = aJupiter  Porb = PJupiter
 co-orbital with Jupiter (ie. revolve with Jupiter)
- Lagrange points: two points in space at which Sun’s and Jupiter’s gravity
act together to trap asteroids
- Lagrange points located 60o ahead and behind Jupiter
 Trojan asteroids are trapped at Lagrange points
Near Earth Objects:
- asteroids with very elliptical orbits
 orbits cross Mars’ orbit
- Earth crossing asteroids: Near Earth Objects with orbits that cross Earth’s orbit
- 300 known – estimated to be ~1000 more
Meteoroids
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composition: refractory material  inner solar system objects
distinction with asteroids: size – diameter, D < 100 m
smaller size  more numerous
 collisions with Earth more frequent
Meteors and Meteorites:
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if meteoroid collides with Earth:
relative velocity ~ few  104 m/s
 friction of passing though upper atmosphere heats meteoroid to 1000’s K
 meteoroid glows in visible band with Blackbody radiation
 appears as streak of light moving across sky  meteor (“shooting star”)
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- most meteoroids completely incinerated in atmosphere  converted to dust
- dust falls to Earth  300 tons of meteoroid dust added to Earth per day
- if meteoroid survives passage through atmosphere and lands on surface  meteorite
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meteorites:
- fusion crust: outer layer melted by heat of passage through atmosphere are resolidified
- allows meteorites to be distinguished from Earth rocks
- different types based on composition:
- stonies: composed of rocky, mostly non-metallic minerals
- most common: 95% of meteorites
- after fusion crust erodes away, almost indistinguishable from Earth rocks
- irons: composition: 10-20% Fe (Iron) and Ni (Nickel)
- Fe and Ni in crystalline form  not possible for rocks formed on Earth
relationship between asteroids and meteoroids:
- larger asteroids chemically differentiated
 have rocky mantle, Fe-rich core
- if asteroid pulverized in collision with other asteroid
 mantle fragments  stony meteorites
 core fragments  iron meteorites
Comets
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outer solar system planetesimals
 composition: ices and rock
- at least 50% of mass composed of ices of H2O (water), CO2 (carbon dioxide),
CH4 (methane), NH3 (ammonia)
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vast majority on orbits that stay in outer solar system
 remain far from Sun
 ices remain frozen  comet inert
a small fraction on high elliptical orbits (eccentricity, e, large)
 part of orbit within inner solar system
- at distances within 10 AU ices partially vaporized by sunlight
 escaping gas produces coma and tail
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Structure of Comets while in inner solar system:
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Nucleus: outer solar system planetesimal
- solid, composed of ices and rock
- Diameter, D, ~ 10’s km
- emits jets of gas from chaotic vaporization of ice patches
- jets on side facing Sun  turn on and off as nucleus rotates
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Coma: sphere of glowing gas surrounding nucleus
- composed of gas from vaporizing ices in nucleus
- D ~ 106 km  D >> Dnucleus
- visible head of comet is the coma, not the nucleus
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Tail:
- extends from coma
- length ~ 108 km (~ radius of Earth’s orbit)
- two tails: ion (gas) tail and dust tail
- ion tail:
- composed of gas from vaporizing ices in nucleus
- electrically charged ions and electrons  ion tail
- ions in solar wind electrically interact with ions in tail
 solar wind pushes ion tail directly away from Sun
 ion tail always straight, pointing directly away from Sun throughout
orbit
- dust tail:
- microscopic grains of rock from nucleus pulverized by explosive out-gassing
of ices
- grains pushed by radiation pressure - pressure of photons in sunlight hitting
grains
 dust tail partially pushed away from Sun
 dust tail curves due to combination of inertia (see Newton’s first
Law) and force of radiation pressure
Origin of Comets:
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important: comets are probes of outer solar system
- cometary orbits contain information about structure of outer solar system
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two classes of comets: short period and long period comets
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Short period comets:
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- Porb < 200 years
 aphelion < few  100 AU  Kepler III
 perihelion lies within inner solar system  very eccentric orbits
- eg. Halley’s Comet (Porb = 76 years)
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- Origin: Kuiper Belt
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- orbits have small inclination  orbital planes lie close to plane of ecliptic
- orbits are pro-grade  revolve counter-clockwise (CCW)
 orbits have similar properties to those of planets
 source must lie in plane of disk
- named after Astronomer Gerard Kuiper
- belt of icy planetesimals spanning 40 – 500 AU from Sun
- Pluto and Charon may be large members of the Kuiper Belt
- most Kuiper Belt objects remain in belt on circular orbits far from Sun  remain
inert
- a small percentage of Kuiper Belt objects deflected by gravity of other belt
objects onto highly eccentric orbits
 new orbit carries them from Kuiper Belt to inner solar system and back
 object develops coma and tail while in inner solar system
Long period comets:
- Porb >> 200 years, up to 106 years
 aphelion up to a few  105 AU  Kepler III
 most of orbit lies in interstellar space
 perihelion lies within inner solar system  extremely eccentric orbits
- orbital planes at any inclination  most orbits do not lie in plane of planetary
orbits
- ~ half have retrograde orbits  revolve clockwise (CW)
 source must be distributed spherically around solar system
- Origin: Oort Cloud
- named after Astronomer Jan Oort
- spherical distribution of icy planetesimals centered on Sun with radius of 105
AU
- radius = of 105 AU  cloud has huge volume  contains ~1012 planetesimals
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- most objects remain in belt on circular orbits far from Sun  remain
inert
- a small percentage of Oort Cloud objects deflected by gravity of other belt
objects onto highly eccentric orbits
 new orbit carries them from Oort Cloud to inner solar system and back
 object develops coma and tail while in inner solar system
- Origin of Oort Cloud: icy planetesimals scattered out of plane of solar system
by gravitational force of Jovian planets
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Meteor Showers and the Fate of Comets
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- during perihelion passage comet close to Sun
 sunlight vaporizes ice in nucleus AND pulverized rock ejected
 nucleus loses ~ 1 % of mass each perihelion passage
 nucleus survives ~ 100 orbits, then disintegrates
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 vaporization causes larger chunks to break off nucleus
 comets leave trail of debris along orbital path through inner solar system
 if Earth crosses orbit of comet  Earth ploughs through debris trail
 produces meteor shower on Earth
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Meteor shower
- event during which meteor rate increases significantly
- duration: < few days
- radiant: direction from which most meteors come during a shower
- the direction of meteroids’ motion relative to Earth
- Earth crosses a comets orbit at same time every year  each meteor shower is a
annual event
- eg. Perseid shower every August
Leonid shower every November
Catastrophic Impacts of Minor Bodies with Earth
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Evidence for possibility of future impacts:
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cratering record on bodies with geologically old surfaces (eg. Moon)
- shows some large impacts occurred after the heavy bombardment era
- eg. maria on surface of Moon
impact of Comet Shoemaker-Levy 9 with Jupiter in 1994 (impact was predicted)
Meteor Crater in Arizona
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- diameter = 1.2 km  caused by meteor of d ~ 100 m
Tunguska Event in Russia in 1908
Earth-crossing asteroids
Cretaceous-Tertiary (KT) boundary
- event that occurred 65 million years ago
- mass extinction (including dinosaurs)
- Chixculub Crater in Yucatan Peninsula
- age ~ 65 million years
mass extinctions throughout fossil record are quasi-periodic
- orbits of minor bodies are periodic
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Significance of an impact:
- kinetic energy of impacting body: KE = 1/2mvrel2
-where: m = mass of impacting body
vrel = velocity of impacting body relative to Earth
- before impact vrel ~ 1 ×104 m s-1  KE large
- upon impact vrel becomes 0  KE becomes 0  KE goes into causing collision
dammage
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Consequences of impact:
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- hot debris (tectites) launched onto parabolic intercontinental ballistic trajectories
 hot debris raining back down causes global forest fires
 smoke and dust cause entire atmosphere to become opaque in visible band
 surface temperature drops
 photosynthesis reduced  plants die off
 plants form base of food chain  small herbivores die off  larger
predators die off
- suppose impact of asteroid of D ≈ 10 km
- consequences do not depend on whether impact is on land or in ocean