Transcript PHAS 2B17 Physics of the Solar System

ASTEROIDS
Objects orbiting the Sun –
i) Not volatile enough to be classed as comet
ii) Too small to be planets (< few 1000 km)
iii) Too large to be meteroids (> few 10’s km) e.g. fragmented and
disrupted moons and planets + relics of the early stages of
accretion of the solar nebula
Gaspra (16 km)
Asteroid belt
A “gap” in the known planetary
system between Mars (1.5 AU)
and Jupiter (5.2 AU) –
`predicted’ by the Titius-Bode
rule! (see ealier notes)
• Discoveries: Ceres 1801, Pallas
1802, Juno 1804, Vesta 1807
• Largest is Ceres (940 km
diameter)
• …several million with diameters
of 1 km or more
• Total mass ~1/20 mass of Moon
Belt Asteroids…
• Semimajor axes 2.2–3.3 AU
• Periods 3.3–6 years
• Sub-groups of asteroids (similar orbits, surface appearance)
 may be fragments of a single asteroid produced by
collisions
• Some gaps in belt caused by resonances with Jupiter:
…ratio of orbital period at that distance
to Jupiter’s orbital period
Classification based on visual appearance –
i.e. albedo, spectral characteristics
S-type –
High albedo (A > 0.15)
Silicon rich  similar spectral characteristics to stony meteorites
Peak in main belt between 2 – 2.5 AU
C-type –
Low albedo (A < 0.03)
Carbonaceous in nature (like chondrite meteorites)
Peak further out at ~ 3 AU
~75% of asteroids are C-type
M-type –
Metal-rich (above all Iron and Nickel)
Other asteroid groups…TROJANS
• 60° ahead and behind Jupiter
• Stable orbits (see Lagrange
pts.) 
the asteroids will not be swept
up by Jupiter
• May be several thousand in
number
• Size: Most are a few km, some
are >100 km
Earth Approaching –
May be several thousand >1 km diameter
Radar images are available for several that approached Earth
Amor -Have orbits crossing Mars’s orbit
Perihelion distances between 1.017 and 1.4 AU (ie between
Earth and Mars)
Methods of study to derive properties of asteroids:
i) Occultation with stars – rare occurrence  radii
ii) Polarimetric data – light becomes polarised depending on angle of
incidence with surface  surface structure
iii) IR and visible radiometry – IR brightness due to re-emission of solar
radiation  albedo
iv) Earth-based radar  surface size, roughness
v) Since 1991, direct imaging (Galileo, NEAR,…)
(25143) Itokawa
• Target of the Japanese
Hayabusa mission
• Best-fit ellipsoid:
535294209 m
• S asteroid with ordinary
spectrum
• Density = 1.9 g/cm3
• About 40% porosity; rubble
pile structure
• Smooth terrain  Potential:
mobility of fine material
Asteroid spin rates
• Very rapid spins, i.e.,
periods shorter than 1
hour, only exist for very
small asteroids
• Interpretation: all the
larger asteroids have a
rubble-pile structure that
does not survive a rapid
spin
• Asteroid Lutetia has been revealed as a battered world of many
craters. ESA’s Rosetta mission has returned the first close-up
images of the asteroid showing it is most probably a primitive
survivor from the violent birth of the Solar System
Lutetia asteroids seen by ESARosetta
Flyby Lutetia
Comets
Comet - Structure
Comets
• Comets are icy objects that release gas and dust as they
orbit the Sun. The solid part of a comet is called the
nucleus and is mainly made of frozen water, dust and
sometimes other frozen substances such as
ammonia.Solar radiation heats the nucleus and gives it
an atmosphere of gas and dust called the coma. A
comet's distinctive tail is caused by solar radiation and a
stream of charged particles that constantly jets away
from the Sun called the solar wind.It is thought that
comets are material leftover from the formation of the
outer planets, although another theory is that many
formed outside our solar system.
Comets
• The comets are ice-rich bodies that become prominent
when heat from the sun causes their trapped volatiles to
sublimate. The most visible and distinctive features of
comets are the coma and tail. However, most of the
mass of a comet is contained within a comparatively tiny
central nucleus, and it is this body that is of the highest
scientific interest because of its likely identity as a
planetesimal from the outer regions of the solar nebula.
Nucleus
• The solid, centrally located part of the comet is known as the
"nucleus". The nucleus is a repository of dust and frozen gases.
When heated by the sun, the gases sublimate and produce an
atmosphere surrounding the nucleus known as the
• Size The sizes of cometary nuclei are mostly unknown because the
measurement is a difficult one. We have reliable measurements of
the sizes of about 10 nuclei. Most of them have diameters from a few
km to 10 or 20 km. The nucleus of comet Schwassmann-Wachmann
1 is probably one of the largest (perhaps 20 km), as is the nucleus of
comet Hale-Bopp (perhaps 40 km). Except in the special cases of
comets Halley and Borrelly, the sizes are inferred.
Nucleus
• Composition The composition of the nucleus is determined by
measuring the composition of the coma. We know nothing directly
about the interior The dominant volatile is water, followed by CO,
CO2 and a host of minor species present at the <1% level. There is
some evidence for abundance variations among comets. The
CO/H2O ratio reached 0.2 to 0.3 in Hale-Bopp but is typically 4 or 5
times smaller.
• The refractory (non-volatile) dust consists of some silicate minerals
and carbon rich CHON (Carbon-Hydrogen-Oxygen-Nitrogen) grains.
The upper layers of the nucleus are volatile depleted, consisting of a
refractory "mantle".
• The ratio of volatile mass to refractory mass is probably near 1.
Nucleus
• Lifetime The lifetimes of active comets are limited for at least two
reasons:
1. the nuclei are losing mass at rates that cannot be sustained for very
long. For example, a 5 km radius spherical nucleus would have a
mass about 4x10^15 kg. When near the sun, this nucleus might
sublimate at 10^4 kg/s (10 tonnes per second), so the sublimation
lifetime is 4x10^11 s = 1000 years. True, the comet might spend only
part of each orbit near the sun, and so might be able to keep going
for more than 1000 years, but it is simply unable to sustain mass-loss
for the 4.5x10^9 year age of the solar system.
Nucleus
• Lifetime The lifetimes of active comets are limited for at least two
reasons:
1. the active comets are under the gravitational control of the planets.
There is a finite chance that a comet will be either ejected from the
solar system, injected to the sun, or absorbed by an impact with one
of the planets (as happened in the famous case of Shoemaker-Levy
9). The "dynamical" lifetime of a typical comet is about 1/2 million
years.
Nucleus structure
COMA
The gas coma consists of molecules liberated from the nucleus by
solar heating and sublimation. Once clear of the nucleus,
molecules in the coma are exposed to direct solar radiation and
can be damaged in various ways. Most molecules are broken
apart ("dissociated") within a day of leaving the nucleus. For
example, and are both reactions in which a molecule released
from the nucleus absorbs a photon and breaks into two pieces. By
convention, the initial molecule is often referred to as the "parent
molecule" while the fragments produced by the absorption of a
solar photon are known as "daughters" (mysteriously, there are no
"sons" in coma physics, only daughters).
COMA
It turns out that the daughters are quite easy to observe because
they have strong spectral lines at optical wavelengths. In fact,
most of the light scattered from a comet at optical wavelengths is
scattered by daughters. For this reason, the daughters have
received a lot of observational attention and many are well
understood. What we would prefer to understand is the origin and
abundance of the parent molecules, but the parents generally lack
useful optical spectral features. In addition to being photodissociated, gas species in comets can also be ionised, as in .
The ions are susceptible to a magnetic force due to the solar
magnetic field carried by the solar wind. Consequently, the ions
are swept almost radially away from the sun, into a long,
distinctive tail.
TAIL
The neutral gas species in cometary comae can be ionised by
solar UV photons, as in
The ions are susceptible to a magnetic force due to the solar
magnetic field carried by the solar wind. Consequently, the ions
are swept out of the coma into a long, distinctive ion tail. Because
the most common ion, CO+, scatters blue light better than red, the
ion tail often appears to the human eye as blue. Also, the
magnetic force is very strong and produces ropes, knots and
streamers that distinguish the ion tail from the dust tail. The solar
wind sweeps past the comet at about 500 km/s, causing the ion
tail to be swept almost exactly in the anti-solar direction.
Hunt for molecules in comets (spectroscopy)
A typical optical/near-IR comet spectrum
109P/Swift-Tuttle
UV cometary spectra
HST spectra of C/1996 B2 (Hyakutake)
FUSE spectrum of C/2001 A2 (LINEAR)
Comet Holmes taken by the Spitzer Space
Telescope (NASA)
Structure of a Comet
• Solar heat vaporizes the
nucleus to produce
– Coma - Hydrogen gas
Envelope
– Dust tail
– Ion tail
Cometary nucleus
Comets
• Icy leftover planetesimals of the outer solar system.
• Today comets exist mainly in the Kuiper belt and the Oort
Cloud.
• The strong gravity of the Jovian planets cleared most of the
comets in between Jupiter and Neptune 
sent to a collision course with other planets,
or ejected to the Kuiper Belt and the Oort Cloud.
• Comets beyond the orbit of Neptune have time to grow
bigger and stay in stable orbit.
• Pluto may be (the biggest) one of them??!
• Nucleus ~ 10—20 km dia. - `dirty snowball’  conglomerate of silicate
rock/dust + ice
• Coma ~ 105 km  outgassing of water, CO2 etc. from surface in jets
• H corona – very extended (~107 km)…sparse envelope of neutral H
• Ion tail – outgassed ions driven away from Sun by solar wind e.g. CO+
flourescence and emission lines --- up to ~ 1 AU in length
• Dust tail – shorter (<0.1 AU) – micron sized dust particles – driven by
photon pressure
…fan shaped due
to different mass
distribution of
grain and ejection
velocities
IR spectroscopy
IKS/VEGA
Combes et al. (1986)
Simple species: H2O, CO, CO2, H2CO, CH3OH
3.3-3.5 mm band: CH-bearing species in gas phase
unidentified compounds at 3.42mm
3.28 mm band: Hydrocarbons
Radio spectroscopy
 OH 18cm lines (1973, comet Kohoutek)
 HCN 89 GHz (1985, comet Halley)
 19 molecules now detected
 Isotopes: HDO, DCN, H13CN, HC15N, C34S, H234S
 Radicals and ions: NS, CS, SO, CN, H3O+ ,CO+
Deuterium in comets
C/1996B2 Hyakutake
CSO
In H2O: D/H = 3 10-4
In HCN: D/H = 2.3 10-3
Atomic D detected (HST)
In CH3OH, H2CO, NH3, CH4:
upper limits of 10-2 to a few 10-2
Bockelée-Morvan et al. (1998)
JCMT
Meier et al. (1998)
Tempe 1
Tempe 1- Deep Impact
What’s new from Deep Impact ?
9P/Tempel 1, July 4, 2005
 4.9 x 7.6 km dark nucleus with low
thermal inertia, low density, negligible
strength
 smooth and rough terrains, natural impact
craters + escarpments…
 DI impact: fine dust ejected, no dramatic
increase in gas production
What’s new from Deep Impact ?
Deep Impact spectra :
large increase in the amount of organics compared to water
Comet Wild 2
(from Star Dust)
Image, taken using an electron
microscope, of a portion of a comet
particle from Comet Wild 2. The light
gray doughnut surrounding the X
consists of carbon-rich organic
material. The scale bar at the bottom
of the figure is 0.1 microns long.
Looks like the organic material produced
when we expose water-rich ices to
ultraviolet radiation under space-like
conditions in the laboratory. It suggests the
possibility that comets contain some of the
original organic products churned out by
dense interstellar nebulae when their
particle contents were bombarded by
cosmic rays and ultraviolet radiation
Open questions in comet
chemistry
 a lot of lines still unidentified
 origin of HNC : coma or nucleus product
 origin of CN ?
 nature of dust ?
 How abundances in the coma are related to
abundances in the nucleus ?
(chemical differentiation in the nucleus)
 degree of compositional uniformity in comet nuclei
What does composition tell us about
the origin of comet material?
 molecular composition present analogies with composition of star
forming regions and interstellar ices
 D/H ratios kept interstellar signatures
low-T formation
(grain surface, ion-molecule processes)
 highly processed material is present (cristalline silicates)
mixed with nebular products
Origin of chemical diversity in comets?
Comet origins
• Two kinds of comets
– Short period (<200 yrs) and long period (>200 yrs)
– Different orbital characteristics:
ecliptic
Short period: prograde, low inclination
Long period: isotropic orbital distribution
• This distribution allows us to infer the orbital characteristics of the source
bodies:
– S.P. – relatively close (~50 AU), low inclination (Kuiper Belt)
– L.P. – further away (~104 AU), isotropic (Oort Cloud)
Short-period comets
• Period < 200 yrs. Mostly close to the ecliptic plane (Jupiter-family or
ecliptic, e.g. Encke); some much higher inclinations (e.g. Halley)
• Most are thought to come from the Kuiper Belt, due to collisions or
planetary perturbations
• Form the dominant source of impacts in the outer solar system
Long-period comets
• Periods > 200 yrs (most only seen once) e.g. Hale-Bopp
• Source is the Oort Cloud, perturbations due to nearby stars (one star
passes within 3 L.Y. every ~105 years).
 Such passages also randomize the inclination/eccentricity
• Distances are >>100 A.U.
• Maybe 10-100 Earth masses in total
• Sourced from originally scattered planetesimals
• Source of periodic bio. extinctions(?)
Kuiper Belt
• The Kuiper belt is a region of the Solar System beyond the
planets extending from the orbit of Neptune (at 30 AU) to
approximately 55 AU from the Sun.
• It is similar to the asteroid belt, although it is far larger—20 times
as wide and 20–200 times as massive.
• Like the asteroid belt, it consists mainly of small bodies, or
remnants from the Solar System's formation. While the asteroid
belt is composed primarily of rock and metal, the Kuiper objects
are composed largely of frozen volatiles (termed "ices"), such as
methane, ammonia and water.
• The belt is home to at least three dwarf planets – Pluto,
Haumea, and Makemake. Some of the Solar System's moons,
such as Neptune's Triton and Saturn's Phoebe, are also
believed to have originated in the region
• ~800 objects known so far, occupying space
between Neptune (30 AU) & ~50 AU
• Largest objects incl. Quaoar (1250km
diameter),
+ Pluto and Charon???
• Two populations – low eccentricity, low
inclination (“cold”) and
high eccentricity, high inclination (“hot”)
ECCENTRICITY
Kuiper Belt
“hot”
“cold”
• Total mass small, ~0.1 Earth masses
• Difficult to form bodies as large as 1000 km – too little total mass is
available
• A large number (few percent) are in binaries
KBO
Pluto
1. Frozen nitrogen
2. Water ice
3. Rock
Pluto from Hubble
Triton – Neptune’s moon
Triton – Polar cap
Triton: Cryovolcanism
UB313 (large Kuiper belt object)
Photographed October 31, 2003 – Palomar obs.
97 AU from the Sun - now at aphelion.
At its closest, it will be 38 A.U. from the sun (cf. Pluto’s average ~ 40 AU.)
Orbital period ~ 557 years.
= Kuiper Belt object (KBO) – incl. Pluto, Triton….
Size? From Albedo 
If surface is Pluto-like methane/water ice  2850 and 3000 km in diameter. (cf
Pluto ~ 2200 km)
Even if its surface is fresh snow  ~ Pluto-sized!
Kuiper Belt Formation
Early in solar system
Ejected planetesimals (Oort cloud)
“Hot” population
J
S
U
Initial edge of
planetesimal
swarm
N
18 AU
Present day
J
30 AU
“Hot” population
Planetesimals transiently pushed
out by Neptune 2:1 resonance
S
U
48 AU
N
Neptune
3:2 Neptune
stops at
original edge resonance
(Pluto)
“Cold”
population
2:1 Neptune
resonance
Remote Kuiper belt
Oort Cloud
• The Oort cloud is a hypothesized spherical cloud of comets which
may lie roughly 50,000 AU, or nearly a light-year, from the Sun.
• The Kuiper belt and scattered disc, the other two reservoirs of transNeptunian objects, are less than one thousandth the Oort cloud's
distance.
• The outer extent of the Oort cloud defines the gravitational boundary
of our Solar System.
• The Oort cloud is thought to comprise two separate regions: a
spherical outer Oort cloud and a disc-shaped inner Oort cloud, or
Hills cloud.
• Objects in the Oort cloud are largely composed of ices, such as
water, ammonia, and methane.
• Probably the matter composing the Oort cloud formed closer to the
Sun and was scattered far out into space by the gravitational effects
of the giant planets early in the Solar System's evolution
Oort Cloud
All the planetesimals scattered out by Jupiter  end up in the Oort
cloud
• This is a spherical array of planetesimals at distances out to ~200,000
AU (=3 LY), with a total mass of 10-102 Earths
• Spherical  due to combination of initial random scattering from
Jupiter, plus passages from nearby stars
• Forms the reservoir for long period comets
Earth
1 AU
Saturn
10 AU
Pluto
Oort cloud
(spherical after ~5000 AU)
Kuiper Belt
100 AU
1,000 AU
10,000 AU
100,000 AU
Composition of the Nucleus
• Most cometary activity (outgassing) starts at around 3 AU from the Sun:
just where we’d expect water ice to reach 210 K and start evaporating.
• But other comets (e.g. HaleBopp) start activity at 5 AU
and hence must possess
other species which
evaporate more readily: N2
and CO are examples.
• From both direct measurements by spacecraft, and by
spectroscopy  a typical
composition.
Molecule
Abundance
(% by mass)
H2O
CO
CO2
CH3OH
CH4
NH3
HCN
H2S
hydrocarbons
65-80
5-20
2-10
2-10
<1
<1
<1
<1
<1
Borelly Nucleus
• The US Deep Space 1 probe made a flyby of Borelly in 2001. Borelly
is much less active than Halley, and so at 5000 km range, much
better images were obtained than the closer Giotto/Halley pass.
• Borelly is 8 km
long and 3-4 wide,
and even darker
than Halley.
• We don’t have a
mass for Halley or
Borelly, so we
don’t know the
density.
How much mass is lost?
• Obviously a comet loses mass every time it makes a
pass of the inner solar system: 
• Typically, about 1010 to 1011 kg is lost each time: less
than 0.1% of a comet’s expected mass.
• This cannot last forever: after a few thousands orbits
it will have lost all its volatile ices and dust.
Several possible comet death scenarios:
1. Total evaporation: all the mass eventually is lost by heating.
2. Dead comet: a comet core remains orbiting the Sun  nonvolatile material. These dead comets may in fact be some of
the Near-Earth ‘Asteroids’ we see.
3. Collision: with the Sun, a planet or other body.
Sun-grazing (`Kamikaze’) Comets have actually been seen
to disappear into the Sun, or melt to nothing on a close pass.
4. Gravitational ejection from the solar system.