Small stuff in the solar systemand where to find it

Alan Longstaff

SPA Preston MontfordNov 2015

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

• 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

• 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 KBOsBlue classical cubewanosGrey 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. C-

and 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 Earth-orbit-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.

Types of meteorite

Iron

Stony

Stony-iron

Meteorites: categories %

stones stony-irons

irons

Falls 94 1 5

Finds 56 4 40

Total 69 3 28

Classifying meteoritesStonyChondrites (81% of falls)• Ordinary (87% of stone falls) • Carbonaceous• Enstatite

Stonyachondrites

Irons

Stony-irons

undifferentiated

differentiated

crust

core

Core-mantle boundary

Ordinary chondrite (L6) in situ.Dar al Gani 985.

Note how the black fusion crust makes it easyto 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 types1. extensive alteration by water at T ~ 50 -

150°C (hydrous minerals dominate)2. water alteration at T < 20°C (some hydrous

minerals3. modest thermal metamorphism at 400 -

600°C4. 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

Higher in sun

Higher in chondrite

CI chondrites should be pristine remnants of solar nebula

Problem: meteorites have been altered by water and heat

HED achondrites

• 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

Eucrite. Although a basalt the clinopyroxene pigeonite makes it unusually pale

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

Gibeon octahedrite

taenite

taenite + kamacite

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 1976 C/1996 B2 Hyakutake 1996C/1995 O1 Hale-Bopp 1997C/2006 P1 McNaught 2007C/2011 W3 Lovejoy 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

Cometary nuclei• Comets described by Fred

Whipple as ‘dirty snowballs’

• 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 1997Credit: Gary Becker

Comet C/2006 P1 McNaught 2007Siding Spring Observatory

Hyperbolic trajectoryOort cloud

Credit: Robert McNaught

Observing comets • Typically every few years a comet is visible to the unaided eye

• Each year several are visible in binoculars or small telescopes

• Large angular size makes comets good targets for binoculars and low-power telescopes

• Often best viewed at dawn or dusk

• 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

• Hundreds of sun-grazers seen in SOHO images

Comet C/2004 F4 BradfieldCredit: 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

• 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

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

Geminids (GEM) 7 – 17 Dec 13 Dec 120 2.6 3200 Phaethon

Major meteor showers

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 forthe Italian astronomer whodiscovered 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 anf/3.7 portrait lens