Transcript Small solar system bodies_PM2015x

SPA
Preston Montford
Nov 2015
Small stuff in the solar system
and where to find it
Alan Longstaff
Anatomy of the solar system
Main asteroid belt
Asteroid inclinations
Kuiper belt and Oört cloud
NB: log10 scale
Oort cloud
• Existence inferred from orbits of long-period comets (average
semi-major axis, a = 50,000 AU; orbital periods, P = 103 – 107 yr)
• Extends to ~ 100,000 AU
• Inner region, Hill’s cloud, is a torus 2,000 – 20,000 AU
– contains ~ 430 objects brighter than mag 24 in red
• Estimated Oort cloud mass ~ 10 M⊕
• Bodies ejected from Kuiper belt
Classical Kuiper-Edgeworth belt
• Classical KBOs - Cubewanos
• Disk extending between 2:3 and 1:2 MMR with Neptune;
39.4 - 48 AU
• Not gravitationally perturbed by Neptune
• Resonant KBOs:
2:3 MMR Plutinos (200 worlds); 1:2 MMR Twotinos
• Kuiper belt cliff – sharp edge at ~ 48 AU
- caused by Neptune’s 1:2 MMR.
• Two populations:
- dynamically cold, e < 0.1, i < 10°, red, formed in situ
- dynamically hot, e > 0.1, i up to 30°, formed near Jupiter and
ejected out by gas giant migrations.
Scattered disk
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Scattered disc objects (SDOs)
Can be gravitationally perturbed by Neptune
Highly inclined orbits
Highly eccentric orbits; perihelion distances, q ~35 AU, aphelion
distances, Q > 200 AU
• Eris q = 28 AU, Q = 97 AU, P = 560 yr
• Source of Jupiter-family short-period comets:
SDOs → Centaurs → Jupiter-family comets
Centaurs
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Between Jupiter and Neptune (5.5 – 30 AU).
Orbits are unstable – disturbed by the gravity of the gas giants.
Come from Kuiper belt (scattered disc).
End up as Jupiter-family comets, falling into the sun, or ejected
into the Oort cloud.
Orbits are comet-like.
Chiron (discovered in 1977) had a coma at perihelion – hence
appears to be a cometary nucleus.
Spectra show water and methanol ices, amorphous carbon,
tholins, silicates – c.f. cometary nuclei.
Some (e.g. Hidalgo) are classified as (D-type) asteroids.
Extended scattered disk
• Contains detached objects
• Highly eccentric orbits with perihelion distances, q > 40 AU
• Inner region of Oort cloud (Hill’s cloud)?
• Sednitos:13 bodies with similar q, inclination and argument of
perihelion; implies a common origin.
• e.g. Sedna; q = 76 AU, Q = 936 AU, a = 524 AU, P = 11,400 yr,
diameter ~ 1,000 km, T = 12 K
• Don’t seem to be KB or inner Oort cloud
• Captured from the outer stellar system of a passing star?
Red resonant KBOs
Blue classical cubewanos
Grey scattered disc objects (SDOs)
White detached or extended SDO
Conic sections
Cometary families
• Short-period comets have orbital periods of less than 200 years;
elliptical orbits.
- Jupiter family comets have periods < 20 yr; aphelia at ~ 5AU–
scattered disc objects captured by Jupiter; 517 listed (e.g.
9P/Tempel 1, 19P/Borrelly, 81P/Wild 2, 103P/Hartley 2).
- Halley family comets have periods 20 – 200 yr (originally inner
Oort cloud objects captured by gas giants); 76 listed (e.g.
55P/Tempel-Tuttle, 109P/Swift-Tuttle).
• Long-period comets have periods longer than 200 yr, can be
many thousands of years. Aphelia in Oort cloud (e.g. C1996 B2
Hyakutake, C1995 O1 Hale-Bopp, C/2006 P1 McNaught, C/2011
W3 Lovejoy).
• Single apparition comets are in parabolic or hyperbolic orbits;
not gravitationally bound to the sun.
• Several comets have been discovered in stable orbits within the
main asteroid belt.
Composition of KBOs
• T ~ 50 K (-223°C)
• Densities ~ 1000 kg
m-3 – mostly ices
• Water, methane,
ammonium hydrate
• Grey to deep red
colour – hydrocarbons
Methane lines in IR spectra of
Eris (red) and Pluto (black)
KBOs and comets get altered
• KBOs are redder than cometary nuclei
• Both are chemically altered by:
- UV
- cosmic rays
• Comets get re-surfaced by sublimed material.
• Cometary nuclei are more elongated than KBOs - extensive mass
loss
- collisions?
Asteroids and meteorites
Asteroids
• Power law size distribution; only 200 with diameter > 100 km,
0.7 -1.7 million with diameter > 1 km
• Mass ~ 3 x 1021 kg (4% that of moon)
- half the mass due to Ceres, Vesta, Pallas and Hygiea
• Asteroids originally in orbits with mean motion resonances
(MMRs) with Jupiter cleared out to leave Kirkwood gaps (e.g.
2.5 AU = 3:1 and 3.3 AU = 2:1).
• Inner edge defined by 4:1 MMR (at 2.06 AU). Hungaria
asteroids are closer but have high inclinations
• 3 broad classes depending on albedo, spectral type, colour and
inferred composition:
S; albedo ~ 15%, stony, inner belt (17%)
M; albedo ~ 10%, metal (8%)
C; albedo ~ 2%, carbonaceous stony, outer belt (75%).
• Correspondence between asteroid and meteorite types: e.g. Cand L-type chondrites come from C asteroids.
Largest ten asteroids
Left to right; 1 Ceres, 2 Pallas, 3 Juno, 4 Vesta,
5 Astraea, 6 Hebe, 7 Iris, 8 Flora, 9 Metis, and 10 Hygiea.
Grey circle is Earth’s moon
Asteroid families
Asteroid families
• Main belt asteroids are fragments of planetesimals
that failed to accrete into a single body because
Jupiter’s gravity made collisions between them too
violent.
• Trojan asteroids are in two groups in Jupiter’s orbit
around Sun, one group 60 ahead of Jupiter, second
group 60 behind. Probably came from Kuiper Belt.
Neptune also has Trojan asteroids.
• Asteroids with orbits close to Earth’s are Near Earth
Asteroids (NEAs). If these cross Earth’s orbit they
could collide with Earth.
• Near Earth Objects (NEOs) include NEAs and Earthorbit-crossing comets.
Near Earth Asteroids
• Each year several ~ 1 kT explosions are detected in upper
atmosphere from airburst of NEOs (stony meteorites or comets)
a few metres across. A 25-kT airburst occurred over the
Mediterranean Sea on June 6, 2002. (c.f. 1MT ~ 20-m object;
Tunguska ~ 15 MT.)
• Spaceguard 1: over 75% of NEAs of about 1 km across or larger
have been identified, and many smaller ones, allowing impact
risk to be assessed.
• Spaceguard 2: NASA plans to identify >90% of the 125,000
NEAs bigger than 140-m across by 2020, using telescopic
surveys by Pan-STARRs (from 2008), LSST (from 2014) etc.
• Schemes for deflecting threatening NEAs: gravity tractor
(spacecraft so close to NEA that its gravity would perturb orbit),
kinetic impactor, sunshield or spray to alter solar radiation
pressure or, as a last resort, a nuclear bomb
• But currently no agency has responsibility for taking action!
Near Earth Asteroid impact chances
Diameter
Number
Average time
between
impacts/yr
10 m
~108
~1
100 m
~105
~1000
1 km
~1000
~100 thousand
10 km
<10
~100 million
Observing asteroids
• Vesta – the only asteroid visible to unaided eye at
magnitude 5.6
• Large number visible with binoculars or small
telescope
• Appear as star-like points
• Detected by change in position against background
stars from night to night
• Can be recorded as short trails on long-exposure
images
Meteorites
• Objects > 25 m diameter entering atmosphere are likely to survive
complete ablation and reach the ground as meteorites
• Large objects generally fragment
• Meteorites can have initial velocities 11 - 72 km/s but most slow to
~ 500 km/hr (much slower than cs = 1236 km/hr) and have time to
cool before hitting the ground
• About 31,000 meteorites collected all over the world – vast
majority derived from asteroids, 25 from moon, 30 from Mars, 2
from an unknown dwarf planet.
• Classified as iron, stony-iron or stony (achondrites and chondrites)
meteorites.
• Most meteorites are 4.5 billion years old.
Iron
Types of
meteorite
Stony
Stony-iron
Meteorites: categories %
stones
Falls
94
stonyirons
1
irons
Finds
56
4
40
Total
69
3
28
5
Classifying meteorites
Stony
Chondrites (81% of falls)
• Ordinary (87% of stone falls)
• Carbonaceous
• Enstatite
undifferentiated
Irons
Stony
achondrites
core
crust
Stony-irons
Core-mantle
boundary
differentiated
Ordinary chondrite (L6) in situ.
Dar al Gani 985.
Note how the
black fusion
crust makes it
easy
to spot against
the sand of the
Libyan desert
Carbonaceous chondrites
• CI1, CM2: chondrules absent or sparse, matrix of
serpentine, clays and organic molecules, high water
content, porous, T < 400K, highly water altered
• CV3, CO3: chondrule-rich, matrix of olivine plus
metal sulfides, low water and organic molecules,
some metal grains, modest thermal alteration (T ~
670 – 870K)
• CK4, 5, 6: poorly-defined chondrules, matrix of olivine
blackened with fine particles of Fe, Ni sulfides and
Fe3O4, no free metal, highly oxidized, shock
metamorphosed, high thermal alteration (T ~1100K)
Olivine
chondrule
Plane polarized light
Crossed polarized light
Petrologic types
1. extensive alteration by water at T ~ 50 150°C (hydrous minerals dominate)
2. water alteration at T < 20°C (some hydrous
minerals
3. modest thermal metamorphism at 400 600°C
4. thermal metamorphism between 600 - 700°C
(matrix re-crystallizing)
5. thermal metamorphism between 700 - 750°C
(loss of integrity of inclusions)
6. thermal metamorphism > 950°C (new,
metamorphic minerals formed)
CI chondritic v solar elemental
abundances
CI chondrites
should be pristine
remnants of solar
nebula
Higher in sun
Higher in
chondrite
Problem:
meteorites have
been altered by
water and heat
HED
achondrites
Eucrite. Although a basalt the
clinopyroxene pigeonite
makes it unusually pale
• Eucrites: fine-grained basalts (60% clinopyroxene (pigeonite), 40%
Ca-rich plagioclase) - rapidly cooling lava flow, upper crust
• Diogenites: coarse-grained hypersthene [(Mg,Fe)SiO3] monomict
breccia – pluton, lower crust
• Howardites: coarse-grained polymict breccia with diogenite and
eucrite clasts – regolith, surface
Canyon Diablo iron meteorite
Slice showing Widmanstatten pattern and troilite (FeS) inclusions.
Canyon Diablo is the meteorite responsible for the Barringer crater in
Arizona.
Irons: Widmanstatten pattern
taenite
taenite +
kamacite
Gibeon octahedrite
Nakhla martian meteorite
Ultramafic igneous rock 1.3 Gyr old: clinopyroxenite
- green augite with red iron-rich olivine
Comets and meteors
Recent Great Comets
C/1965 S1 Ikeya-Seki
1965
C/1975 V1 West
C/1996 B2 Hyakutake
C/1995 O1 Hale-Bopp
C/2006 P1 McNaught
C/2011 W3 Lovejoy
1976
1996
1997
2007
2011
Comets
• Irregularly shaped bodies consisting of rock, dust and
ices and organic molecules
• Orbit the sun
• As they get close to the sun, the warmth causes the
ices to sublime to form an ‘atmosphere’ or coma, and
tails
• Estimated to be one trillion cometary nuclei in the
outer solar system
Discovery of comet orbits
• First suggestion that comets obeyed Kepler’s Laws in 1610
• Isaac Newton showed that the path across the sky of the1680
comet could be explained if it moved in a parabolic orbit
• Edmond Halley (1656-1742) applied Newtonian mechanics to 23
historical records of comet appearances between 1337 and 1698
and deduced that the comets of 1531, 1607 and 1682 had similar
orbits and suggested this was the same object returning with a
period of 76 years.
• Halley predicted its return in 1758 and it appeared as expected
• Subsequently named Comet 1P/Halley. Last apparition 1986, next
will be in 2061.
• Most comets travel in elliptical or open-ended hyperbolic and
parabolic orbits highly inclined to ecliptic plane – therefore comets
can appear anywhere in sky, not just in Zodiac
Cometary structure
• Comets described by Fred
Whipple as ‘dirty snowballs’
Cometary nuclei
• Nucleus at heart of each
comet is an irregular-shaped
body 10-20 km across
• Nucleus consist mostly of
ices of water, carbon dioxide
(dry ice), carbon monoxide,
ammonia and methane,
mixed with silicate rock
• Nucleus is encased in a dark
black crust – tholins, organic
compounds formed by action
of UV on surface ices
• Cometary nuclei have very
low albedo – they reflect only
2 – 4% of incident light (c.f.
asphalt, 7%)
Nucleus of comet Halley (Giotto)
Nucleus of comet 1P/Halley - Giotto
Activating a cometary nucleus
• When a comet gets
close enough to the
sun (~2.5 AU) ices
sublime (at low
pressure ices go
straight from solid to
gas phase), and gases
and dust are ejected
through cracks in crust
• Material in these jets
forms coma and tails
• 95% of the neutral
gases in the coma are
H2O, CO and CO2
Cometary tails
• Gas or Ion tail. Neutral gas ionized by solar UV. Most
commonly: CO + hʋ → CO+ + e- . Because CO+
scatters blue light more than red the ion tail is blue.
Ions induce a magnetosphere around comet. Solar
wind magnetic field lines drape round cometary field
forming the ion tail. Hence ion tail always points
directly away from the sun.
• Dust tail created as solar radiation pressure pushes
dust particles from nucleus – shines by reflected
sunlight, appears yellow in colour and curves away
from comet and sun
• Cometary tails can be up to 3.8 AU long
C/1995 O1 Hale-Bopp; the Great Comet of 1997
Credit: Gary Becker
Comet C/2006 P1 McNaught
2007
Siding Spring Observatory
Hyperbolic trajectory
Oort cloud
Credit: Robert McNaught
Observing comets
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Typically every few years a
comet is visible to the unaided
eye
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Each year several are visible
in binoculars or small
telescopes
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Large angular size makes
comets good targets for
binoculars and low-power
telescopes
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Often best viewed at dawn or
dusk
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Tails may be too faint to see.
Comet then resembles a fuzzy
patch with bright core
•
Comets move with respect to
background stars from night to
night
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Hundreds of sun-grazers seen
in SOHO images
Comet C/2004 F4 Bradfield
Credit: Sho Endo
Meteoroids
• Meteoroids are dust and rocks in space
• Size range defined by the Royal Astronomical Society (RAS) is
from 10 μm to 10 m
• Smaller grains are micrometeoroids
• This is extended to 50 m in the context of Near Earth Objects;
rocky objects much less than 50 m across are unlikely to survive
entry into Earth’s atmosphere
• Asteroid size is poorly defined; 1 m to 1000 km?; fuzzy
boundary between meteoroid and asteroid.
• Most meteoroids are cometary dust, responsible for zodiacal
light and meteors
• Larger meteoroids are derived from asteroids and cause
fireballs (mv > - 4) on entering atmosphere
• Largest objects generate bolides (mv > -14)
Meteors
• Meteoroids travel at up to 42 km per second with respect to Earth
• On entering atmosphere air resistance slows them rapidly, they
heat up and are vaporized – high temperature ionizes surrounding
air molecules and atoms, and when the electrons recombine light
is emitted.
• Colour can provide a clue to elemental composition
• Meteors burn up between 60-120 km above surface of Earth
• Brightest meteors leave visible trail of hot gas and dust behind that
can last for many minutes
• Meteors brighter than apparent magnitude of – 4 (about the
maximum magnitude of Venus) are called fireballs. Bright fireballs
can be seen in the daytime.
• May generate sonic boom (v > cs) and other sounds (but
mechanism is complex)
Colours of meteors
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Na; orange-yellow
Fe; yellow
Mg; blue-green
Ca; violet
Atmospheric N2 and O2; red
Meteor showers
• In a dark sky around 7 meteors an hour appear randomly across
the sky – sporadic meteors
• Meteor showers are seen when Earth ploughs through dust
particles from cometary tails that have spread out along a comet’s
orbit. In an intense meteor shower tens or even hundreds of
meteors can be seen each hour
• Paths of meteors in showers appear to radiate out from a specific
point in sky (the radiant) corresponding to the direction from which
the meteoroids are entering the atmosphere; a perspective effect
• Meteor showers are named after the constellation in which radiant
found – e.g. Orionids.
• Each shower happens over the same few days each year, during
the time that the Earth crosses the comet’s orbit; shower
maximum can often be timed to within an hour or so, and the
intensity can also be predicted
Earth intersecting cometary material
Leonid meteor trails
Major meteor showers
Name
Dates
Date of
Max
Max
ZHR
r
Comet
Lyrids (LYR)
16-25 Apr
22 Apr
20
2.1
C/1861 G1 Thatcher
Eta Aquarids
(ETA)
19 Apr –28
May
6 May
60
2.4
1P/Halley
Perseids (PER)
17 Jul – 24
Aug
12 Aug
0-100
2.6
109P/Swift-Tuttle
Orionids (ORI)
2 Oct – 7
Nov
21 Oct
25
2.5
1P/Halley
Leonids (LEO)
14 – 21 Nov
17 and
18 Nov
20+
2.5
55P/Tempel-Tuttle
13 Dec
120
2.6
3200 Phaethon
Geminids (GEM) 7 – 17 Dec
Zenith Hourly Rate (ZHR), the rate of a meteor shower
seen by a single observer with a clear sky of limiting visual magnitude
mv = 6.5, and with the radiant at the zenith.
If mv < 6.5 (almost invariably the case in the UK) then:
ZHR = (Nr6.5 – mv)(1 – Cf)/sin a
where:
N is the total number of shower meteors observed per hour,
r is the population index, an estimate of the expected overall
mean magnitude of the shower; r usually ranges from 2.0 (bright)
to 3.5 (faint),
Cf is the fraction of sky covered in cloud (from 0 to 1);
in practise Cf is hard to estimate.
a is the altitude of the radiant above the local horizon.
If the sky is completely clear then:
ZHR = (N r6.5 - mv)/sin a
ZHR Example.
During a 1.5 hour observation of a Geminid meteor shower in a clear sky
with limiting visual magnitude of 5.7, an observer counted 91 meteors,
of which 86 appeared to come from the Geminid radiant.
The radiant had an altitude of 60.
This gives an hourly rate for Geminids of 86/1.5 = 57
ZHR = (N r6.5 - mv)/sin a
= 57 x 2.66.5-5.7/sin 60
= 57 x 2.60.8/0.866 (use the [xy] key on your calculator to find 2.60.8)
= 57 x 2.14/0.866
= 141.
The Great Comet of 1858,
C/1858 L1 Donati, named for
the Italian astronomer who
discovered it on 2 June 1858,
Giovanni Battista Donati
The first comet to be photographed:
William Usherwood, Walton Heath,
Surrey, possibly on 27 September 1858,
a few days before perigee.
Exposure time 7 – 9 s with an
f/3.7 portrait lens