ASTR 330: The Solar System
Announcements
• Homework Assignment #2 returned today & solutions.
• Class average was 35 (70%) versus 42.5 (85%) in
HW#1.
• HW#2 was more challenging, less straightforward.
• Overall class average is currently 77%.
• If you missed handing in HW#1 or HW#2 on time, you
can make up the points by submitting the extra credit
term paper.
• Who needs more time for homeworks?
• Mid-term exam #1 is on Thursday 5th October!
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Mid-term Exam Format
•
The 75-minute exam will consist of 3 parts
totaling 200 points, equal to 20% of the course
credit.
• 5 short answer questions (50 points total).
• 3 true/false questions (50 points total).
• 2 longer, structured answer questions (100 points
total).
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Study Advice
• Read all the lecture notes on-line, and review quiz questions.
• Check out the on-line review/summary for Lectures 1-10.
• Read Chapters 1-6 of the book.
• Everything in the lectures, and Chapters 1-6 is examinable, but
core topics are identified by quiz questions.
• Try questions at the end of the book chapters.
• Exam #1 from Spring 2004 is available in class and on-line.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Exam conduct
• Closed-book, no notes or calculators allowed.
• Bring your own pens and pencils and ruler. Don’t use
correction fluid. Paper is provided.
• No talking or other communicating between students once
the papers are distributed until they are collected.
• Cheating will be not be tolerated. If you are seen/heard to
be cheating you may be asked to leave the exam room, and
the case immediately referred to the Head of Classes in the
Astronomy Department. You will lose all credit for the exam
and your case may be taken to the student honor council.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Lecture 9:
Comets
Photo credit: Dr Michael Stecker
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Comets
•
What is a comet?
• Comets are small primitive bodies like the asteroids, but mainly
composed of ices rather than rock and metals.
• They are remnants of the formation of the cold, outer solar
system, rather than the warm inner, solar system.
• When a comet approaches the inner solar system, the ices
evaporate to form a temporary atmosphere.
• Ice and dust particles stream behind the comet for millions of km,
making the characteristic long tail.
Thanks to Bill Arnett at LPL: The Nine Planets page
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Comets through history
• Do you think asteroids or comets were discovered first?
• There is approximately one naked-eye visible comet every year.
• Comets have been known since ancient times. BC writings in China,
the Middle East, Greece and other cultures all record prominent
comet appearances.
• The periodic comet Halley was observed at least 31 times over 2500
years in China, staring in 613 BC.
• Halley’s comet also put in an appearance in 1066 in the year of the
Battle of Hastings, and is recorded in the Bayeux tapestry.
• Halley’s comet has also been proposed as the ‘star’ of Bethlehem.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Bayeux Tapestry
• The appearance of Halley’s comet in 1066 around Easter proved bad
luck for the Saxon king Harold. Later in the year, the Norman duke
William invaded, and Harold was killed by an arrow to the eye.
• The events leading up
to and including the
battle are recorded on
the Bayeux tapestry.
• The words: “isti mirant
stella” are Latin,
meaning: “they
marveled at the star”.
• Comets were often
regarded as ill-omens.
Image © Musee de la tapisserie, Bayeux, France
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Appearance of a comet
• A comet consists of a nucleus, the essential core of the comet which
has traveled from the outer solar system.
• As the comet approaches the sun, gases are released, forming the
coma. The nucleus and coma together are called the head.
• Gases from the head
are ionized by the Sun,
and swept out away
from the Sun to form
the plasma or ion tail.
• At the same time, dust
left behind by the
comet forms a dust
tail, dragging in a
curved path.
Figure credit: thursdaysclassroom.com
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Tycho Brahe
• Tycho made the most accurate observations ever made prior to the
invention of the telescope. He was also a very interesting character!
• A Danish nobleman by birth, he
constructed huge quadrants which he
used to measure star positions to 1/60 of
a degree (1 arcminute).
• We discussed in Lecture 2 how his
measurements were the basis of
Kepler’s ideas of planetary motion.
• Tycho measured the parallax of a comet
in 1577, deducing it was further away
than the Moon. This overturned the
prevailing idea that comets were
atmospheric phenomena of the Earth.
Picture: Univ. St Andrews
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Halley
• Edmund Halley was an important figure in 17th century science, a friend
of Newton’s who finally persuaded him to write down his work.
• Halley’s major contribution to astronomy
was the realization that several different
comet appearances: in 1531, 1607 and
1682; were actually the same comet
coming back again and again!
• This was a major new idea in his day,
when comets were considered one-time
events.
• He successfully predicted the reappearance of the comet, now named
after him, in 1758, but did not live to see
his prognosis verified.
Picture: Univ. St Andrews
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
How big and how dense?
• Tycho’s work on comet distances showed that the head
was also very large: as big as the Earth! Public fear of
comets grew after this news.
• However, the fact that stars could be seen through the
comet’s atmosphere showed that they were also very
tenuous.
• Newton correctly guessed that the atmosphere was the
result of vapors from a solid nucleus being heated by the
Sun.
• 19th century astronomers deduced that the streaming of
the ion tail away from the Sun was due to the force of
solar radiation.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Apparition and Discovery
• Each appearance of a comet is called an ‘apparition’ (you can see the
supernatural connotations).
• 1910 was a wonderful year for comet-viewing: as well as the
anticipated return of Comet Halley, an even brighter new comet also
appeared some months earlier (the “Great Daylight Comet of 1910” –
why do you think it got this name?)
• 1986 was a disappointment for Halley watching: the comet was on
the wrong side of the Sun when at its best. However, for the first time,
spacecraft were sent to intercept the comet.
• Comets, unlike planets, are named after their discoverers. Some
teams discover many comets: e.g. ‘Shoemaker-Levy 9’.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Comet orbits
• Comets nearly all follow very highly elliptical, and often inclined orbits
around the Sun, taking many years (often millions) to complete.
• We sometimes categorize them according to orbital period:
LONG PERIOD: T >= 200 years (90% of all comets).
INTERMEDIATE PERIOD: T= 30 to 200 years.
SHORT PERIOD: T < 30 years.
• To which category does Halley’s comet belong?
• Comet orbits cross the orbits of the planets and so they are inherently
unstable. What is likely to happen eventually?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Diagram: Tim Stauffer
Dr Conor Nixon Spring 2004
ASTR 330: The Solar System
Long and Short Period Comets
• Long and intermediate period comets both originate in a spherical
region far outside the solar system… called what?
• Short period comets have a different origin: they come from the disk
of icy proto-planetesimals outside the orbit of Neptune called…?
• The Oort cloud is a vast reservoir of probably a trillion (1012) cometary
nuclei.
• The comets we see are a tiny fraction of the huge numbers which
exist in these two reservoirs.
• Most of the possible comet nuclei never enter the inner solar system.
A few however are disturbed and fall inwards. What could cause this?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
The Coma
• The coma (‘hair’) is the atmosphere of molecules surrounding the
nucleus, which have just evaporated.
• Often, the coma shows
structure: bright streamers of
material leaving the nucleus.
It is also brighter towards the
Sun.
• The images (left) show the
head of comet Hale-Bopp on
10/23/95, in a fairly
quiescent state.
• Near the surface of the
nucleus the gas density is
already 1 millionth of Earth’s.
Picture: HST/NASA/STSI
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Coma Gas Composition
• The volatile gases leaving the nucleus: CH4, H2O, NH3 are quickly
broken apart by solar UV photons, into fragments such as NH, CH
and OH.
• Much complex chemistry takes place in the coma, before the gas
densities drop too low for any interactions.
• At 100,000 km from the nucleus, the gas is mainly ionized.
• The spectroscopic detection of H2O+ plasma confirms that water ice is
the main volatile constituent, as expected. OH, H and H2O have also
been detected.
• We also see C2, C3, CO, CO2, CH4, CH3OH indicating that other
carbon-bearing ices are present. Are all these molecules stable forms
which could be found on the Earth?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Composition contd.
• Nitrogen-bearing species are also found: HCN, N2, NH3, CH3CN as
well as some sulfur compounds.
• HCN is cyanide, a gas deadly
to humans!
• The detection of HCN in the
atmosphere of Halley
frightened people in 1910,
when they learned that the
Earth would pass through
Halley’s tail!
• What is the main effect of
passing through a comet’s tail?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Hydrogen Envelope
• The hydrogen envelope is a vast region outside the coma which
can stretch more than a million km – bigger than the Sun!
• This glows blue with hydrogen emission (Hale-Bopp, below right, 4/1/97).
Photo: Peter Barvoets
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Comet tails
• As the comet
nears the Sun, the
gases and dust
ejected from the
nucleus are swept
out by the solar
wind to form the
characteristic
tail(s).
• How many tails
are there?
Picture: Cambridge University
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Plasma Tail
• The plasma tail is composed of ionized volatile gases.
• The plasma tail glows blue, mainly due to CO+
fluorescent emission. What do we mean by fluorescence?
• Fluorescence is the effect where photons emitted at one
frequency are absorbed and re-emitted at a different
frequency.
• Plasma tails are straight and point away from the Sun;
usually several different streamers or rays are seen.
• The rays are normally a few 105 km long, but can reach
108 km for a very active comet, e.g. Hale-Bopp.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Dust Tail
• The dust tail is composed of dust particles from the nucleus, and is
usually not more than 107 km long.
• Dust tails are different from
plasma tails in several respects,
including shape and color.
• Their color is yellow-white: why?
• They are also curved, showing the
past path of the comet. Hence, the
dust and gas particles separate
after leaving the nucleus.
• Measurements indicate that the
mass in the dust tail and gas tails
are comparable.
Picture: Nanjing University Astr.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Dirty Snowball or Icy Dirtball?
• Let us now consider the cometary nucleus.
• Comets were originally thought to be like meteorites: agglomerations
of rock and dust containing some volatile gases trapped inside.
• The problem was: to match the measured amount of gas released,
the comet had to be very large!
• An alternative theory was proposed by Fred Whipple in 1950: a ‘dirty
snowball’ that was about equal parts of silicate and ices, evenly mixed
together.
• However, some comets have produced more dust than ice, leading to
the terminology ‘icy dirtball’!
• Do you think these dirtballs could have formed differently, or are just
older than the snowballs?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
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, we arrive at a
typical composition shown
in the table (right).
Table from: The Planetary System, Morrison and Owen.
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
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Comet differences
• Great variations exist between comets however.
• In some comets, such as Halley visited in 1986 by the
Vega and Giotto probes, dark carbonaceous dust seems
to predominate over silicates for the dust component.
• Comet Borelly was even darker, with an average surface
albedo of 3% in the nucleus, and parts as dark as 1%...
darker than copier toner! It must be carbon-rich as
opposed to silicate.
• Borelly had no detectable surface ice: so all the ices
which went into the tail must be sub-surface.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Structure of the Nucleus
• Once the atmosphere of a comet forms, it becomes very
difficult to see the nucleus inside.
• We can try to use radar to ‘see’ through the gas and dust:
this was done for Comet Encke in 1980 and another
comet in 1983, yielding a diameter of 5-10 km.
• The best way is to go close up: this feat has been
achieved for only a few comets so far: Halley and Borelly,
and in 2004, Comet Wild-2.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Halley Nucleus
• The European
Giotto
spacecraft in
1986 made a
close flyby of
Halley’s comet
at only 500 km
distance,
imaging the
nucleus as a
dark mass (34% albedo)
16x8x8 km in
size.
Image: ESA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
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.
Images: NASA/JPL (right hand image shows topography)
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Stardust Sample Return
• The Stardust sample-return mission,
launched in 1999, was designed to
return the first ever pure samples of
cometary and interstellar dust.
• The spacecraft spent seven years
orbiting the Earth, and encountered its
target (Wild-2) in January 2004.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Wild-2 Nucleus
• Stardust flew within 230 km of the nucleus of Comet Wild-2. The
nucleus is about 5 km in size. These are the best ever pictures of a
comet nucleus.
Images: NASA/JPL
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Cometary Grains
• Stardust used a novel material called
‘aerogel’ to gently capture the primitive
coma grains travelling at 6.1 km/s
without damaging them.
• Aerogel, a silicon-based material is
1000x less dense than water!
• Particles
became
buried in the
gel, leaving
tracks which
will be used
to find the
microscopic
grains.
• Minerals identified so far
include olivine and anorthite,
which should have formed
close to the Sun in the inner
solar system.
• The question is how these
materials ended up in cold icy
comets!
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Deep Impact
• In January 2005, the Deep Impact space mission, led by Maryland’s
own Prof. Mike A’Hearn, launched towards Comet Tempel-1.
• The mission was designed to shoot a 370-kg copper mass into the
comet, with the objective of finding the true interior composition.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
July 4th Blast
• The impactor struck on
Independence Day,
2005, with a blast equal
to 4.5 tons of TNT.
• The estimated crater
size is 100-m wide,
although was not seen
due to the debris from
the blast.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Deep Impact - Results
• The impact debris contained far more dust and less ice than
anticipated. The dust was also finer (‘talcum powder’) than expected.
• By and large, the surface crust was depleted in volatiles, as expected
by multiple passages of heating in the inner solar system.
• The surface was substantially cratered,
which should ultimately provide insights
into its age.
• The spectrum (right) shows the preimpact (red) and post-impact (green)
views of the nucleus, along with a
model prediction (blue).
• Water, C-H and CO2 are all present.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Comet comparisons
• We are now in a position to make some comparisons between
different comets:
1. The 4 comets with measured sizes are all around 5-10 km in size,
although they vary in activity by a factor 100.
2. The ones we have seen close-up are extremely dark: a strange
finding, seeing as the atmospheres are so bright. We conclude
that the bright gases and dust come from inside, and the surface
‘crust’ is mainly non-volatiles.
3. Comets smaller than 1 km are rare: unlike small asteroids which
are common.
4. Comets can be large: e.g. Chiron, an expired comet is ~200 km.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
How much mass is lost?
• Clearly, a comet loses mass every time it makes a
pass of the inner solar system: but how much mass is
lost?
• 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 ‘trips’
it will have lost all its volatile ices and dust.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Comet Shoemaker-Levy 9
• In 1993 a very unusual comet was discovered by Gene and Carolyn
Shoemaker and David Levy:
• About 20 small nuclei were strung out in a line like a ‘string of pearls’.
• Calculations showed that the comet had just made a close pass of
Jupiter at 35,00 km, and been torn apart by Jupiter’s massive gravity.
• The density was hence calculated to be 1.0 g/cm3.
Picture: HST: H. Weaver (JHU), T. Smith (STScI), NASA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
On a collision
course…
• More interestingly, S-L 9
was soon calculated to be
in orbit about Jupiter, not
the Sun.
• In fact, calculations
showed that next time S-L
9 went around its orbit, it
would not miss Jupiter at
all, but dive straight into
the planet!
Picture: HST: H. Weaver (JHU), T. Smith (STScI), NASA
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
The Fate of S-L 9
• The impacts, at a speed of
60 km/s, were witnessed by
spacecraft, and telescopes
large and small on the
Earth. The fireballs were
larger than anticipated,
exploding with millions of
megatons of energy.
• This was the first ever
witnessed event of two solar
systems bodies colliding,
and yielded incredible
information about the size
and composition of both
Jupiter and S-L 9.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Comet Dust
• What happens to cometary dust when the Earth passes through the tail
of a comet?
• Answer:
a Meteor
shower!
Picture credit: Wally Paholka: Leonids 2001, Joshua Tree Nat. Park
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Prominent Meteor Showers
• We can now match up many regular meteor showers with the paths of
specific comets:
Shower
Peak Date
April Lyrids
21-Apr
Max rate /
hr
12
Associated
Comet
Thatcher 1861
Eta Aquarids
May 5
45
Halley
Perseids
Aug 12
80
Swift-Tuttle 1862 II
Orionids
Oct 22
25
Halley
Taurids
Leonids
Geminids
Ursids
Nov 3
Nov 17
Dec 13
Dec 22
12
10
80
9
Encke
Tempel-Tuttle
Phaethon
Tuttle
Sources:
1. Patrick Moore (1988) Amateur Astronomy, Bloomsbury Books, London.
2. Susan Goodman (1993) Spacefacts, Oxford University Press.
3. Valerie Illingworth (ed)(1994) Dictionary of Astronomy, HarperCollins Publishers
.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
In Literature
• Meteor showers can be very impressive. Samuel Taylor Coleridge's
famous lines from The Rime of the Ancient Mariner:
The upper air burst into life!
And a hundred fire-flags sheen,
To and fro they were hurried about!
And to and fro, and in and out,
The wan stars danced between
And the coming wind did roar more loud,
And the sails did sigh like sedge;
And the rain poured down from one black cloud;
The Moon was at its edge
may have been inspired by the Leonid meteor shower that he
witnessed in 1797.
Thanks to Bill Arnett at LPL: The Nine Planets page
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Meteoric Material
• A typical meteor is the size of a pea, but can become a much larger
streak of glowing gas when it disintegrates, seen for 100s for km.
• Millions of visible meteors happen each day: many will not be seen,
happening in remote areas.
• A meteor is very different from a meteorite: not only in the fact that it
doesn’t land on the Earth.
• Meteors are generally cometary with densities of 1 g/cm3, as opposed
to asteroid fragments of higher density (3-7 g/cm3).
• Careful analysis of cometary dust collected in the upper atmosphere
indicates that they are pristine bits of interstellar material, with
different isotopic ratios from the normal solar system matter.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Death of a Comet
•
There are 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, of the 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. 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.
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz-summary
1. What is a comet and where do they come from?
2. When and how did astronomers realize that the same comet could
come back multiple times?
3. What are the three categories of comet based on period?
4. What was Tycho Brahe’s contribution to the study of comets?
5. Give the general properties of comet orbits; are their orbits similar to
those of the planets?
6. What are the main parts of a comet; when and how do they appear?
7. How many tails does a comet have, and how are they different?
Dr Conor Nixon Fall 2006
ASTR 330: The Solar System
Quiz-summary
8. What typical volatiles compose a comet, and what happens after they
evaporate?
9. What is meant by the dirty snowball model of Fred Whipple?
10. It has been stated that comets are actually very dark. How come we
can see them?
11. How big is a typical comet?
12. How does a comet end its life?
13. Should we be worried when the Earth passes through a comet’s tail;
what effects might we expect?
14. Describe one spacecraft visit to a comet, and say briefly what was
found.
Dr Conor Nixon Fall 2006