EAS
105-THE PLANETS
Prof. Robert L. Nowack
Lecture 19
Asteroids, Comets, and Meteorites - Debris
in the Solar System
Asteroids are minor planets in the solar
system. Most asteroids have orbits
between Mars and Jupiter. (This is near
a gap in the empirical Titius Bode relation near 2.8 Au). By 1980, nearly 3000 asteroids had been found
with well-determined orbits. Ceres is
the largest with a radius of 470 kilometers.
Ceres is now classified as a Dwarf planet. There are approximately 200 asteroids that
have radii 50 kilometers or more and approximately 2,000 having radii greater
than 5 kilometers.
Asteroids
were originally named after Greek and Roman goddesses. Since then, naming asteroids became almost
random. (Professor Lipschutz from
Purdue’s Chemistry Department has an asteroid named after him.)
There are many more smaller asteroids than
larger ones. This graph shows an
approximate
distribution of smaller and larger asteroids.
Several
of the largest asteroids and their distances from the Sun are shown below.
Scale
drawing shows relative sizes of some of the larger asteroids compared with the
planet Mars. Numbers next to the names
were assigned in order of discovery.
Numbers below the names are periods of rotation in hours. The horizontal scale gives the mean distance
from the Sun in Astronomical Units (AU).
The
list of asteroids is 98% complete down to objects greater than 50 kilometers
radius.
Asteroids orbit around the Sun in the same sense
as Earth – counterclockwise as seen from the North. The Main Asteroid Belt is between 2.2 to 3.3
Au.
This
view of an Asteroid Belt shows locations of more than 6000 asteroids in Feb.
1990 as seen from above. Diagram
includes orbits of Mars and Jupiter.
Diagram
demonstrates the same distribution of asteroids as seen from a position in the
plane of the Solar System.
An interesting feature of the Main Asteroid Belt
is the existence of several gaps. These
are caused by gravitational resonance with Jupiter. For example, an object at 3.3 Au would have ½
the orbital period of Jupiter. This
resulting gap marks the termination of the main part of the Asteroid Belt, since
Jupiter would ultimately perturb objects out of this orbit. In addition, there are other gaps.
Plot of the
number of asteroids with various semi-major axes (in AU). Some of the resonance are denoted by breaks,
Kirkwood Gaps, where period of orbit of an asteroid would be a simple fraction
of Jupiter’s period of orbit.
Based on reflectivity studies, a majority of
minor planets are dark, about as dark as a lump of coal. Another 15% are much brighter – about like
our Moon. Based on spectroscopic studies
of reflected light, asteroids can approximately be classified by composition. The darker asteroids are thought to be
primitive bodies composed of silicates mixed with dark organic carbon
compounds. These are classified as
"c" asteroids where c stands for carbonaceous. This includes the Ceres asteroid.
The 2nd most populous group is the
"s" asteroids, where s stands for silicate or stony. For these, the dark material is missing for
some reason.
The relationship of
compositional classes of asteroids with distance from the Sun.
The asteroid Vesta,
is the third largest and it is unusual since its reflectivity suggests that it
might have a basaltic surface composition.
Also, a group of meteorites that have fallen to Earth, called eucrites,
seem to have similar laboratory measured spectra. It
has been suggested that these meteorites are related to the asteroid
Vesta as a parent body.
These two photos from the Galileo spacecraft
shows two asteroids from space. The
Asteroid Ida even has its own small satellite; the 1.5 kilometer Dactyl. Future missions to the outer solar system
will target different asteroids in transit.
Asteroid Gaspra
Galileo
spacecraft passed within 1,600 kilometers of the stony asteroid Gaspra on
Asteroid Ida
Dactyl
Trojan asteroids are farther out than the main
asteroid belt. They are nearly in the
same orbit as Jupiter – only lagging or leading it. These asteroids are in groups plus or minus
60 degrees from Jupiter.
Trojan Asteroids and
Lagrangian Points
These
points are called "Lagrangian points" and are stable orbits with
respect to the larger planet, Jupiter.
A few asteroids are known to venture past
Jupiter. These include Pholus and
Chiron. However, the distinction between
asteroids and comets depends on the nature of the volatile compounds they
contain. In 1988, Chiron was observed to
have a tenuous atmosphere of gas - suggesting Chiron may well be a comet.
View of the Solar
System from above, showing the orbit of Comet Halley and of three of the newly
discovered objects in the outer Solar System.
Chiron (initially designated an asteroid, but subsequently seen to
display cometary activity), Photus, the
reddest known object in the Solar System, and the first object found in the
Kuiper Belt.
There are about 50 asteroids from the Main
Asteroid Belt that have highly eccentric Earth crossing orbits. The largest has a radius of 15 kilometers,
but most are 1 kilometer or less. These orbits are ultimately unstable,
resulting in either being ejected from the solar system by a near miss or a
crater forming impact.
The first asteroid found that crossed Earth’s
orbit was Apollo in 1948. The frequency
of asteroid impacts on Earth shows how the number of events increases as the
size of the object decline.
A 2nd possibility for an Earth impact
is from comets.
The
frequency of asteroid impacts on the Earth showing how the frequency increases
as the size declines. Also indicated is
the impact energy (in megatons of TNT) for asteroids of various diameters. Objects smaller than 50 meter in diameter (10
MT energy) disintegrate in upper atmosphere and do no damage. Shown for reference is the 1908
Were
the asteroids once part of a larger object? The consensus at this point is no. A majority of material is primitive less
differentiated matter.
Kring, D. (2006), Unlocking
the solar system’s past, Astronomy, August, pp. 33-37.
Comets
About one comet is visible to the naked eye
yearly and a really bright comet comes along once a decade.
Comet Halley in 1910
Edmund
Halley, was the first person to realize that comets could have highly elliptic
closed orbits with a given comet reappearing at regular intervals. The comet that bears his name has a 76 year
orbit. It appeared in 1910 and 1986 (the
next appearance will be in 2062).
Edmund Halley (1656-1742)
Short-period comets have orbit periods less than
200 years. There are 100 or more of
these. Halley’s comet has its aphelion
out near the orbit of
Diagram of Parts to a Comet
All
comets have a round, diffuse glowing head or coma and many develop long tails
that stream away from the head. The
small solid nucleus is rarely seen, but it’s the most important part of the
comet. In 1950, Fred Wipple was the
first to suggest that this nucleus was a single small "dirty
snowball" ( i.e., a mix of rock and water ice ).
The only comet nucleus to be photographed in
detail is that of Halley’s Comet in
1986 when three spacecraft flew into the center of the comet. The European spacecraft imaged the nucleus
itself. Part of the surface was covered
with a dark layer of black silicates or carbon compounds. While one part of the surface had escaping
jets of gaseous material.
(a)
Nucleus of Halley’s Comet
(b)
Diagram of Halley’s Comet
Two
major plumes of gas were photographed.
These gaseous jets are presumably caused when the comet ices, which are
normally frozen, begin to warm up as the comet moves within the orbit of Mars. Water will then begin to outgas. These outgasing
jets can even slightly change the comet’s orbit as well as cause the nucleus to
rotate. Halley’s Comet rotates every 2
days, 5 hours.
The head of the comet is called the coma and
consists of gas and dust recently ejected from the nucleus. Although the primary constituent is
presumably water vapor, many compounds can be also created by UV sunlight. These include cyanogen (CN), hydroxyl (OH),
and carbon (C2). The parent
molecules are presumed to be water (H2O), methane (CH4),
and ammonia (NH3).
On July 9, 2005, the unmanned
spacecraft Deep Impact launched a probe which impacted the nucleus of the comet
TEMPEL 1. There were a number of
features imaged including ridges, surface debris and what appeared to be an
impact crater. A plume of dust rose
after the impact imaged by the mothership.
from Astronomy magazine
(January 2006, p. 46).
Comet Tails
Many comet tails begin to form as it approaches
the Sun. The tail is an extension of the gaseous material of the coma. The plasma tail is made up of charged
particles, which as a result of the solar wind, always points away from the
Sun. In addition, most comets also have
slightly curved dust tails. Comet's
exhibit large tails as they approach the Sun, and die out as they get farther
away.
1910 Time Sequence of
Halley’s Comet
Observationally the aphelion distance of new
comets have typical values of ~ 50,000 A.U.
(Pluto gets out to 40 A.u.) This
is about one-third the average distance between stars and maybe the
"sphere of influence" of a typical star. It has been hypothesized that there is an
outer comet region, "The Oort Comet Cloud", from which comets are
perturbed from in the far outer Solar System.
A schematic
representation of a tiny fraction of the orbital paths of comets in the Oort
Comet Cloud. Center dot is much bigger
than the diameter of Pluto’s orbit at this scale.
Also
a nearer belt of short-period comets has been inferred to occur in a
disk-shaped region called the Kuiper Belt and extends outward from the orbit of
Pluto.
The Kuiper Belt
Since the first object was found beyond Pluto by
David Jewitt and Jane Luu, about two dozen more objects (all near 100
kilometers in size) have been found.
Thus, there appears to be several sources for comets, the distant Oort
Cloud and the nearer Kuiper belt. Once a
comet has been diverted into the inner Solar System, it is ultimately
unstable. It will either plunge into the
Sun, Jupiter, impact on one of the inner solar system planets, or get ejected
from the solar system by a near miss.
Also, Jupiter may be a comet "vacuum cleaner" as with the
recent Shoemaker-Levy 9 Comet.
Shoemaker- Levy 9 Comet
Impact on Jupiter
What
is the fate of the fine dust left behind as a comet passes through the inner
Solar System? A tiny fraction of this
dust strikes the Earth, burning up to produce meteors or shooting stars.
When
the Earth encounters a stream of particles in its’ path, we recognize this as a
meteor shower. A number of meteor
showers have been identified with the paths of comet. A very consistent meteor shower is on August
11. On that night one can see as many as
35 “shooting stars” per hour. This
shower has been associated with the Comet "1862 III" of over a
century ago.
Meteorites (or
large stones from the sky)
Meteorites are larger extraterrestrial objects
that survive impact with the atmosphere and reach the surface. They're not typically associated with meteor
showers, and are considered to be of various origins. Several meteorite falls are observed every
year. Most of these are stones or
metallic masses of only about 1 kilogram or so (about hand sized). For example, this meteorite crashed through
the roof of a house in
This
small meteorite crashed through the roof of a home in
Meteorites can be classified as irons which are
nearly pure metallic nickel/iron. A 2nd
group is called stones which are silicate/rocky material. A 3rd group is called stony/irons
which are made up of a mixture of stony and metallic material. Stones are the most common type.
Recently, it has been found that a number of
meteorites can be found directly on the ice in
All
of the iron and some of the stones are differentiated since they have been
chemically altered since their original formation.
Note:
The largest meteorite to fall in the
Primitive stones
are also called chondrites since
many contain small chondrules which are millimeter sized droplets of condensed
rock.
Chondrite Meteorite
This
meteorite came from
The
most primitive stones are called carbonaceous meteorites since they are
relatively rich in carbon. These are
dark in color and are very crumbly.
Meteorites can be radioactively age dated and the primitive ones have
been found to date back to the beginning of
the Solar System.
The
organic material in these early carbonaceous meteorites are not from living
processes, they just contain carbon compounds.
In 1969 the Allende meteorite was found in
The Allende Carbonaceous
Meteorite
Allende is a Type III (CV3) carbonaceous
chondrite that fell in
The early Murchison meteorite, also found in
1969, contained 16 amino acids, 11 of which are rare on Earth. These are both "right handed" and
"left handed" in their molecular arrangement, whereas on Earth, life
only has "left handed" amino acids.
Irons and stony-iron meteorites are examples of
differentiated meteorites (along with some stones). Although they may be very old, they are not
primitive in a compositional sense. This
suggests that their parent body underwent some differentiation.
Only about 4% of falls and Antarctic meteorites
are irons. They are iron with about 10%
nickel. When polished, they show crystal
patterns which indicate slow cooling.
Presumably these are fragments of the metal cores of their parent
bodies. However, based on composition,
several parent bodies are indicated (based on crystal sizes, parent bodies
about several hundred are indicated).
Widmanstätten Pattern
Meteorite
Basaltic meteorites are differentiated stones
since basalt is known to be a common form of lava. A group of these, called eucrites, are
thought to have originated on the asteroid Vesta, based on similarities in reflectance
and spectral properties.
Eucrite Meteorite from
Asteroid Vesta
Another group of basaltic meteorites, called SNC
meteorites, are thought to have originated on Mars. They have a very recent solidification age of
1.4 billion years. (In addition, some
Antarctic meteorites may have originated on the Moon.)
In 1996, a meteorite found in
At least for several meteorite falls, possible
orbits can be inferred. For several of
these, obits extend into the Asteroid Belt.
Above
schematic reconstructs the orbits of three meteorites,
Do
the differentiated meteorites indicate an exploded planet? Probably not!
They indicate a number of distinct sources. Based on cooling history, there were probably
small objects several hundred kilometers in diameter or less.