Download comets

Comet PANSTARRS over Phoenix, March 12, 2013
COMETS
GLG-190 – The Planets
Chapter 14
LECTURE OUTLINE
 Introduction
and
history
 Orbits
 Comet Structure
 Comet Dust
 Origin
COMETS THROUGH HISTORY



Greek komitis: “long-haired” – Aristotle
(384-322 BCE)  describes hair-like tail
Found in written records (Near East and
China) to at least 1140 BCE (top right)
Believed to be bad omens…






Assassination of Julius Caesar, 44 BCE
Battle of Hastings, 1066 (below right)
Fall of Jerusalem, 70 BCE, "foretold" by comet
(4 years earlier!)
Eruption of Vesuvius, 79 CE
Conquest of Constantinople by Turks, 1456
Outbreak of Black Plague that killed 100,000
in London, 1665
When beggars die, there are no comets seen;
The heavens themselves blaze forth the death of
princes – Julius Caesar, Act II, Sc. II
EARLY STUDIES


Comets considered atmospheric
phenomenon
Great Comet of 1557 observed by
Tycho Brahe


Halley’s Comet 1910
Proved orbit was beyond Moon 
comets not atmospheric
Edmund Halley (pronounced “hawley”)
Noted Great Comets of 1456, 1531,
1607, and 1682 had almost same
retrograde orbits
 Different appearances of same comet
 orbital variation from gravitational
perturbations by Jupiter and Saturn
 Predicted comet’s return in 1758
 Died 1743, before seeing that he was
right (next appearance in 2061)

Halley’s Comet 1986
I came in with Halley's comet in 1835. It's coming again next year (1910), and I expect to go
out with it. The Almighty has said no doubt, ‘Now here are these two unaccountable freaks;
they came in together, they must go out together.’ – Mark Twain
ORBITS OF COMETS

Highly eccentric unstable orbits
Affected by gravities of Jupiter and
Saturn
 Long Period Comets (period >200 yrs)
 Short Period Comets (<200 yrs)


Long period comets comprise 90%
of total)
Enter planetary region from no
preferred direction
 50% are retrograde  consistent with
random distribution
 From Oort Cloud (e.g., Hyakutake)


Three short period comets types
Most from Kuiper Belt (e.g. Halley)
 Some appear to be icy “asteroids” 
Main Belt comets
 Some are longer period comets
thrown into closer orbits by Jupiter’s
gravity (e.g., Tempel 1)  Main Belt
comets

Origin of
short period
comets
Main Belt comets
have circular orbits
and aphelions
inside Jupiter’s
orbit
Some comets have orbits
extending far from the
sun (>1000 AU)
Most comets have very
eccentric inclined orbits
OORT CLOUD AND KUIPER BELT
LIFE OF A COMET
STRUCTURE

Nucleus




Solid, centrally located part
Made of dust and frozen gases
When heated by sunlight, gases sublimate and produce atmosphere
surrounding the nucleus
Atmosphere


Coma: gas and dust cloud around nucleus
Coma materials swept into elongated tails: plasma and dust
NUCLEUS SIZE



Size measurement is difficult  sizes mostly unknown
Reliable measurements (flybys) indicate small diameters (above)
Largest known are: Hale-Bopp (60 km), 29P/Schwassmann-Wachmann
(31 km), 109P/Swift-Tuttle (26 km), and 28P/Neujmin (21km)
NUCLEUS STRUCTURE



Nothing known directly about
interior structure
Comets sometimes split apart 
some nuclei are fragile
Nucleus has non-volatile outer
mantle



Comets sublime less gas than
expected for their sizes
Jets of material breaking through
crust observed in close-up images
Mantle origin(s)
Rubble mantle  loss of volatiles
leaves behind rocky “rind” (top)
 Irradiation mantle  high energy
particles damage molecular bonds in
outer few meters of nucleus creating
carbon-rich, hydrogen-depleted
“skin” (bottom)

GAS COMA
Molecules liberated from
nucleus by solar heating
(sublimation) are
exposed to direct solar
radiation
 Most are broken apart
(dissociated)  resulting
ions are easy to observe
using strong spectral
lines at optical
wavelengths

Green coma of
Comet 17P/Holmes
(note bluish ion tail)
Halley's
Comet
DUST COMA
Dust coma
imaged in
infrared
Dust dragged from nucleus by
sublimating gas
 Grains have all sizes…



Most grains are 1 m (0.001 mm)
Active, near-sun comets can eject
grains 10s of cm across
Grains become part of the tail if
they exceed escape velocity of
nucleus (effectively in free
orbits around Sun)
 Particles pushed out of dust
coma by solar radiation
pressure to form dust tail

Dust particle collected from
comet in Kuiper Belt by
Stardust mission
ION (PLASMA) TAIL

Gas ionized by solar radiation is
susceptible to solar magnetic
field and wind
Solar wind passes comets at 500
km/s  ion tail is swept almost
exactly in anti-solar direction
 Carried by solar wind ions are swept
into long, distinctive ion tail
 Most common ion, CO+, scatters
blue light better than red  ion tail
often appears blue


Solar magnetic force is very
strong and produces ropes,
knots and streamers that
distinguish ion tail from dust tail
Comet Hale-Bopp. White: dust tail; blue:
plasma tail (note thicker “knots”)
DUST TAIL




Dust tail is white or slightly
pink because dust grains
reflect sunlight slightly better
at longer wavelengths than
shorter wavelengths
Particles in tail are in
individual orbits around Sun
 dust tail is curved as comet
swings around Sun
Both plasma and the dust tails
can attain great lengths, up to
1 AU (1011 m) in most
spectacular cases
Most meteors are dust
particles from comets
Orionid
meteor
shower


Meteor shower occur when Earth’s orbit intersects streams
of material left along orbits of comets
Earth intersects orbit of Halley’s Comet twice…
Eta Aquarids in May
 Orionids in October


Other comet orbits intersect Earth’s orbit just once…

Perseids in August come from comet Swift Tuttle
DEEP IMPACT MISSION
Studied composition of interior by crashing impactor into
nucleus of 9P/Tempel
 Impactor hit (July 4, 2005)  plume of material released
and studied by main spacecraft instruments
 Comet more dusty than expected (ices lost over time?)

DEEP IMPACT RESULTS

Material excavated
contained more dust and
less ice than expected



Material was finer than
expected (more like talcum
powder rather than sand)
Other materials included
clays, carbonates, sodium,
and crystalline silicates
which were found by
studying the spectroscopy
of the impact (right)
Observations also
revealed that comet is
about 75% empty space
STARDUST MISSION
Launched February 1999
 Comet Wild 2 rendezvous on
January 2004

Captured material using aerogel
 Dust and carbon-rich material from
comet and samples of interstellar
dust


Earth return January 2006


Re-entry capsule parachuted to
Earth's surface
Much of dust formed close to
young sun and somehow ended
up in outer solar system
COMET COMPOSITION


Some of darkest objects in Solar
System (albedos < 0.05)
Composition from spectra of comas,
and Deep Impact and Stardust
results
Mass ratio of gases to dust and rock is
probably 1
 Dominant volatile is water, followed by
CO and CO2
 Dust consists of silicate minerals (olivine,
pyroxene) and carbon-rich CHON
(carbon-hydrogen-oxygen-nitrogen)
grains


Surprising that comets contain hightemperature silicate minerals
Similar to meteorite minerals
 Formed near to Sun and were somehow
mixed into cold comet-forming regions of
outer Solar System

Two-micrometer grain of forsterite, a
high-T mineral, collected by Stardust
spacecraft from Comet Wild 2.
ROSETTA MISSION



ESA (European Space Mission) to
rendezvous and land on Comet
67P
All previous comet missions have
been flybys at speeds >20,000
km/hour with closest approach of
240 km
Will make long term
measurements as comet
approaches Sun (from 3.5 AU to
1.4 AU for at least six months)
Map nucleus (Aug. 2014)
 Release Philæ Lander (Nov. 2014)
 Measurements on surface as comet
goes around Sun (Nov. 2014 to Dec.
2015)

COMET ORIGINS

Formed in outer part of
protoplanetary disk



Low temperatures  high volatile
content (water and carbon monoxide
in nucleii)
Contain silicate grains from hotter
parts of solar system  mixing
Rearrangement of giant planets
ejected most planetesimals into
interstellar medium


Small fraction (10%?) of ejected
bodies perturbed by passing stars
into Oort Cloud (long-period comets)
Planetesimals not ejected that
formed substantially beyond orbit of
Neptune make up Kuiper Belt (shortperiod comets)