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navigation image map
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/On this page, links to other relevant planetary Websites are emplaced.
A pitch is made to the reader to consider taking a look at the extensive
review of Astronomy/Cosmology in Section 20 as a background to this
Section - optionally switching to it here <../Sect20/A1.html> before
moving on to the planets. The definition and nature of a "planet" is
then considered. A table lists the major facts and parameters pertaining
to the solar planets. Some of the characteristics of the motions and
distribution of the planets are described in terms of the historical
contributions by Copernicus, Brahe, Kepler, and Newton. Finally, a
subsection is presented on how meteorites are used to determine
compositions of solid planetary, asteroid, and cometary bodies in the
Solar System./
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INTRODUCTION TO THE PLANETS
*
Internet Links to Planetary Sites; Book References
*
Planetary exploration has become one of incredible and vast
accomplishments, in which huge amounts of data have now accumulated.
Much of it has relied exclusively or largely on remote sensing. This
Section is one of the longest in the 27 units of the Tutorial. The
intention is to provide a thumbnail view of the major missions to the
planets. Despite the importance of learning about planetary atmospheres,
we will not say much about the results of remote sensing of these
gaseous envelopes nor do we discuss in any detail facts and conjectures
about planetary interiors.
There are many sources of additional images and descriptive information.
Among the best of these currently online is a repeat of Chapter 5:
/Planetary Geology/, by James Bell III, Bruce Campbell, and Mark
Robinson, in the 3rd Edition of the /Manual of Remote Sensing: Earth
Sciences Volume/, 1996, at Marswatch
<http://marswatch.tn.cornell.edu/rsm.html>. This lengthy and detailed
review focuses on remote sensing approaches to planetary exploration.
Its one drawback is a sparsity of images (compared with this Section 19
Overview). An excellent chronological survey of the history of space
exploration is found at the Planetscapes <http://www.planetscapes.com/>
website. A complete listing of all planetary missions can be accessed on
this NASA Goddard <http://nssdc.gsfc.nasa.gov/planetary/projects.html>
web site. Another site worth visiting is the Home Page of the Jet
Propulsion Laboratory (JPL) <http://www.jpl.nasa.gov/> where you can get
addresses to visit other sites dealing with terrestrial and planetary
space programs. Another NASA source is the National Space Science Data
Center NSSDC <http://nssdc.gsfc.nasa.gov/photo_gallery>. Two other
exceptional Home Pages are _The Nine Planets_, by Bill Arnett of the
Lunar and Planetary Laboratory, University of Arizona (LPL)
<http://seds.lpl.arizona.edu/nineplanets/nineplanets/nineplanets.html>
and _Views of the Solar System_, by C.J..Hamilton of the Los Alamos
National Laboratory (Spaceart <http://solarviews.com/eng/homepage.htm>).
Dr. J. Schombert of the University of Oregon offers three courses on
Planets, Astronomy, and Cosmology that he has put on the Web; the first
of these - _The Solar System_ - is accessed at his AST121
<http://abyss.uoregon.edu/~js/ast121/> site. The NASA Headquarters Space
Sciences Directorate maintains an excellent Site that summarizes the
major findings in both planetary and cosmological realms during the
latest 9 to 12 months that can be accessed at Space Science
<http://spacescience.nasa.gov/> (see its lists, especially News). Many
Solar System missions were managed and conducted by the Jet Propulsion
Laboratory; descriptions of Past, Current, and Future missions are
obtaining by clicking on any of interest at this Missions
<http://www.jpl.nasa.gov/missions/index.cfm> site.
Books that treat planetary remote sensing as part of a larger review of
Planetology include a now out-of-print text by this Tutorial's author
(Nicholas M. Short), /Planetary Geology/, 1975, Prentice-Hall Publ.,
still in libraries, Murray, Malin, and Greeley's /Earthlike Planets/,
W.H. Freeman & Co., 1981, and Billy P. Glass's /Introduction to
Planetary Geology/, 1982, Cambridge University, Press. More recent are
/Planetary Landscapes/ by R. Greeley, 1985, Allen & Unwin, /The
Planetary System/, by D. Morrison & T. Owen, Addison-Wesley Publ., 1988,
and /Exploring the Planets/ by W.K. Hamblin and E.H. Christiansen,
MacMillan, 1990.
The writer /strongly/ recommends /*Lonely Planets: The Natural
Philosophy of Alien Life*/, by astrobiologist David Grinspoon, Harper
Collins Publ. published in March, 2003 that covers much of what we knew
by that time about planets in the Solar System. Its main purpose,
however, is to review the conditions for and likelihood of life (from
very primitive to advanced thinking creatures) in our galaxy and beyond
- throughout the Universe. Although a bit wordy and redundant, this book
explores almost all facets of whether Earth is unique (he thinks not!!)
and under what circumstances planets containing living organisms (or
once living, now extinct) can develop.
Scientists who have spent at least part of their careers studying the
planetary bodies of the Solar System are called /Planetologists/. The
majority of these are also /Geologists/, although some are instead
/Astronomers/ and /Physicists/.
Before we start our tour of the planets, you may wish to review some of
the main principles and concepts of Astronomy. If so, please skip to
Section 20, which is a comprehensive review of this subject as it is
subsumed into the closely related field of Cosmology.
*The Nature of a Planet*
We concentrate in Section 19 almost entirely on the planetary bodies of
the Solar System. Other planetary systems, around different stars, have
been discovered discovered in recent years, as described on page 20-11.
The central body controlling these planetary bodies is the Sun. (For
background information on the Sun, check Sol
<http://seds.lpl.arizona.edu/nineplanets/nineplanets/sol.html> and/or
Sun <http://solarviews.com/eng/sun.htm>)). Despite the large size of
some of the solar planets relative, say, to Earth, these planetary
bodies orbiting the Sun taken together, plus also asteroids and comets,
contain only 0.14% of the mass of the Sun (99.86%). They do, however,
contain most of the angular momentum of all bodies (including the Sun)
in the Solar System.
To set a framework for our survey of the Solar System's inhabitants
(exclusive of the Sun - that star is treated at the top of page 20-5a
<../Sect20/A5a.html>), which we will consider as the "Planetary System"
despite the recent discovery of more than 200 extrasolar planets (again,
Section 20), look first at the illustration below, which shows the
relative sizes of the nine planets, with a small segment of the Sun
shown to the far left, so that its size relative to the planets is
scaled (the distances between them are not actual - each planet is just
placed at the same distance next to its neighbors)(this diagram dates
before the 2006 demotion of Pluto within our Solar System).
Illustration showing the relative sizes of the nine planets of our Solar
System; Pluto (at the far right) is no longer classified as a planet.
*19-1: Using their appearance, how many of the above planets can you
name? Check the answer to see a diagram that shows the size of each
named planet relative to the Sun. ANSWER <answers.html#19-1>*
In late 2003, announcement was made by a group of astronomers of
discovery of what they claim to be a possible Solar System planet, which
they named Sedna, that lies astride Neptune's orbital path. This is an
isolated small body (about 1250 to 1800 km; 750 - 1100 miles in
diameter) that is spherical. Such a shape is suggestive of melting and
reorganization into a round mass. According to the American Astronomical
Society rules, this size is below the lower limit agreed to be the
smallest a body can be to be named a planet. It appears to be a new
class - between irregular asteroids, which can be larger, and the solar
planet sizes; some rounded "moons" are in this size range which could
explain it as an escaped Neptunian satellite but a mechanism to remove
it from Neptune's orbital family into its own solar orbit is yet to be
proposed.
Actually, another planet had been imaged in 2003 but astronomers at Cal
Tech did not recognize it as such until 2006 when three images taken 90
minutes apart were inspected and analyzed. This is what they saw:
The discovery telescope images showing 2003 UB313.
The circled bright object in the left image seems to be one body. But in
the second image, two bodies are resolved. The upper body moves again in
the third image, taken 90 minutes later. The lower body proved to be a
distant star. The upper body was shown to be within our Solar System. It
was named 2003 UB313. After its acceptance as a planetary object, it was
given the name Eris. At the 2003 sighting time, Eris lay a distance of
97 Astronomical Units. (/An Astronomical Unit [A.U.] is defined as the
distance between Earth's center and the Sun's center, approximately 151
million kilometers or 93,000,000 miles./) Continuing observations show
it to have a highly elliptical orbit so that its closest approach to the
Sun will place it at a distance of 38 A.U. This orbit is inclined 44°
from the ecliptic. Eris will take 560 years to complete one revolution
around the Sun. Eris is one of several larger spherical objects that are
within the Kuiper belt of asteroids. Most of those are irregular in
shape. So, is the sphericity of Eris enough to qualify it as a planet,
along with Sedna and others.
As a momentary digression, this plot is an example of how remote sensing
- in the form of gathering reflectances as a function of wavelength - is
used in planetary studies. The plot shows the spectral reflectance curve
for Pluto and for Eris. Both are nearly identical, suggesting that Eris
has a surface composition similar to Pluto.
Spectral reflectance curves for Pluto and Eris.
The Sedna and Eris discoveries along with detection of other similar
bodies have reopened one of the great debates in Solar Planetology: Is
Pluto really a planet? Its size is the smallest (2288 km diameter) of
the 9 traditional ones stated in textbooks. Purests consider it not
worth of planet status; some believe it, like Sedna, is an escaped moon.
The controversy was the centerpiece of a debate at the 2006 meeting of
the International Astronomical Union (IAU) in Prague. A committee there
produced this simplified definition of a planet (applicable to any
planetary system, not just that of the Solar System):
*A planet is a celestial body that (1) is in orbit around a star but is
neither a star nor a satellite of a planet; (2) has sufficient mass for
its self-gravity to overcome rigid body forces so that it assumes
(usually after melting) a hydrostatic equilibrium (nearly round) shape,
and (3) has swept up most of the smaller solid bodies (e.g., asteroidal
"chunks") in the spatial region around its orbit. *
Item 3 is the new criterion that is to be used in defining a true
planet. A key word is "most". Both Earth and Mars have some
asteroidal-sized rock chunks within their orbital zones but these bodies
are in toto only a very small fraction of the mass of the parent
planets. Pluto has much more such bodies still in its orbital zone, so
it has failed this third criterion.
Item 3 leads to another parameter that clearly separates the Main
Planets from smaller bodies. This is done by calculating the ratio of
the mass of the parent planet to the mass of all small body materials
(excepting moons) in the orbital zone of the planet; this ratio is
represented by the Greek letter μ. This plots as follows:
Planet/Small Body ratios of large objects in the Solar System.
The mass ratios show that the 8 Solar System planets all have μ values
greater than about 7000; the three spherical bodies Pluto, Ceres, and
Eris all have values less 1. On this basis, the reason the IAU now
contends that there are only eight major planets is obvious.
This reclassification might seem straightforward, but exceptions may
cause confusion, e.g., Pluto's moon Charon. But Charon lies beyond the
barycenter for the Pluto-Charon pairing. The barycenter is the center of
gravity for a two body system that has one body orbiting the other. The
Moon remains a satellite despite its size because of the Earth-Moon
barycenter still residing within the larger Earth. In time (billions of
years), as the Moon recedes the barycenter will migrate outward beyond
the Earth, so that, technically, the Moon would become a planet in the
sense of Charon. The large satellites of Jupiter, Saturn, Uranus, and
Neptune all associate with barycenters located within their parent planet.
So, what did the IAU decide: They first mulled over the "strawman"
concept that as of 2006 there might be 12 planets in the Solar System,
with the three new ones (2003 UB313 is now named Eris) shown in this
diagram. They firmly decided at this meeting against these three as
'full blown' planets, keeping them as large members of the Kuiper
Asteroid Belt.
The earlier proposed 3 new planets, all much smaller than Earth (right);
2003-313UB is provisionally known as Eris.
The latest thinking is that there should be three categories: 1)
planets; 2) dwarf planets; 3) solar system objects. Dwarf planets must
not be satellites of larger planets. Dwarf planets must be round,
whereas solar system objects must be irregular in shape. Dwarf planets
that are icy can be as small as 200 km in diameter; if they are rocky,
the smallest size that is round is about 400 km. As of 2010 about 50
round dwarfs have been found in the Solar System.
A number of small round bodies have now been found in the Kuiper
(asteroidal) Belt and possibly the Oort (cometary) Cloud (page 19-22).
They are part of a vast collection of large to small objects - most
non-spherical - orbiting the Sun. According to the IAU definition, some
smaller round ones may be eventually raised to the dwarf planet level.
Here are some current candidates:
Possible future solar dwarf planets.
The IAU voted on August 24, 2006 to further redefine planets by size -
these falling into Main (large) and Dwarf (small) categories. This
enabled Pluto to retain some planet status by becoming a Dwarf Planet
along with Ceres and Xena. Pluto's moon Charon remained just that - a
satellite. But as in any democratic procedure, those on the losing side
will continue to disagree.
This diagram depicts some of the Dwarf planets in terms of their
relative sizes:
The comparative sizes of the named Dwarf planets.
Since the Dwarf planets are small, good photo images of them are not
easily acquired. Most are depicted from observations that are translated
into "artist's concept", visual representations of the 'best guess' of
their appearance. Below are some of these planets - read their captions
to determine if each one is an actual image or an artist's concept:
Sedna; this may be an actual image
Eris; artist's concept.
Haumea; artist's concept; it seems to be elliptical.
Ceres; Hubble image..
Xena and its moon Gabrielle; artist's concept
The question "What is a Planet" has been addressed in the January, 2007
Scientific American article by that name as written by Steven Soter.
There seems little doubt that Earth is still the premier planet among
those now discovered (within and beyond the Solar System). It is (so
far) unique in having life (but not organic molecules) as a main
characteristic. In the Solar System, Earth is alone in having 1)
widespread water oceans, and 2) abundant vegetation. It is the presence
of trees, grasses, and other vegetative cover that distinguishes Earth
from the other solar planets; but from the standpoint of remote sensing
this absence on the other planets makes it possible to examine them in
terms of the geologic processes that led to their development (on Earth,
extensive vegetation usually masks the underlying geology, as is typical
in the eastern United States).
This subsection ends with the illustration below, which shows the full
disc appearance of each of the four Inner or Terrestrial Planets scaled
to their actual relative sizes, of which Earth is the most important (or
you wouldn't be reading this):
The four Inner Planets (Mercury; left); Venus (with its atmosphere
removed); Earth; Mars.
* Some Planetary Parameters *
Consult the table below, which summarizes the principal characteristics
and properties of the nine planets. We list them from top to bottom in
the same sequence as those shown from left to right in the above
illustration. To simplify, we do not include the names of the principal
satellites orbiting some of these planets, but we cover them in a
listing below the table.
*PLANETARY BODY*
*DISTANCE FROM SUN (AU)*
*ORBITAL PERIOD (yrs)*
*ROTATIONAL PERIOD (days)*
*DIAMETER (km)*
*DENSITY (gm/cm)^3 *
*NUMBER OF SATELLITES*
*Mercury*
0.387
0.24
58.6
4,880
5.44
0
*Venus*
0.723
0.62
243R
12,105
5.25
0
*Earth*
1.000
1.00
1.00
12,757
5.52
1
*Mars*
1.524
1.88
1.03
6,786
3.93
2
*Jupiter*
5.203
11.86
0.41
143,797
1.34
8 R; 55 IR
*Saturn*
9.539
29.46
0.43
120,659
0.70
21 R; 26 IR
*Uranus*
19.18
84.01
0.72
51,121
1.28
18 R; 9 IR
*Neptune*
30.07
164.80
0.73
49,560
1.64
6 R; 7 IR
*Pluto**
39.44
247.68
6.4
2,288
2.06
1 R; 2 IR
* Included in the Table despite its reclassification as a Dwarf Planet.
Note: For the number of satellites; that numeral left of R refers to
those satellite that are nearly spherical - that left of IR refers to
irregular shaped satellites (see page 19-14).
AU = Astronomical Unit, which is the mean distance (approx. 150 million
kilometers, or 93 million miles) from the Sun to Earth
Names of principal satellites (smaller ones omitted):
* Earth: Moon
* Mars: Deimos; Phobos
* Jupiter: Io; Europa; Ganymede; Callisto
* Saturn: Mimas; Enceladus; Tethys; Dione; Rhea; Titan; Hyperion;
Iapetus; Phoebe
* Uranus: Miranda; Ariel; Umbriel; Titania; Oberon
* Neptune: Triton; Nereid; 1889N1
* Pluto: Charon
The above table lists the distances of the planets from the Sun. This
diagram shows these distances in terms of the orbits of the planets:
The relative distances of the planets in terms of their orbits.
As will be described later, beyond Neptune and Pluto are Sedna and the
Kuiper Belt (asteroidal bodies) and Oort Cloud (mostly comets). This
diagram indicates the extreme distances of these latter features in
relation to the nine planets:
The full extent of the Solar System.
(A brief mention is made here of an interesting hypothesis maded by Dr.
Harold Levinson of the Southwestern Research Institute: He postulates
that in the early Solar System the planets had orbits different from the
present. The Giant Planets Jupiter and Saturn had orbits such that on
very infrequent occasions they would approach each other so that their
massive gravitational interactions caused them to start a process called
resonance. This eventually forced them to readjust their positions and
also hurled Uranus and Neptune further out. About 3.9 billion years ago,
the resonance also perturbed the asteroid belt objects, forcing many of
these towards the Inner planets. The Moon, formed by then, underwent a
cataclysmic bombardment [see subsection on the Moon] during which most
of its craters were formed [extensive cratering also took place on Earth
but almost all of these have been obliterated by subsequent erosion]).
The densities of the planets is one characteristic parameter. Here is a
histogram that compares this property:
Relative densities of the Solar System planets.
The Inner planets all show rocky materials at their surfaces (all but
Mercury have atmospheres). Much of the density components depends on the
relative sizes of the core, mantle (if present), and crust. The next two
illustrations show the measured (or estimated) size of each component:
Graph showing proportions of crust, mantle, and core in each of the
Inner or Terrestrial planets.
The atmosphere thickness and size of the solid interior of each of the
Giant or Gas planets.
The next table summarizes orbital parameters and atmospheric
characteristics of the solar planets.
Orbital Parameters & Atmospheric Characteristics of the Planets.
The orbital inclination is a measure of the departure of a planet's
orbital plane, in degrees, from the orbital plane of Earth around the
Sun (Sol) which defines the *ecliptic*, a plane containing both the Sun
and the Earth's orbit (see page 20-5, near the top).
*
History of Planet Studies and Aspects of their Motions
*
The table above shows that the four planets closest to the Sun are small
compared with those beyond Mars. These are the Inner or Terrestrial
(like Earth, with rocky material at their surfaces) planets. From
Jupiter through Neptune, the planets are much larger (the Outer or Giant
group) and have surfaces that are all gas (Pluto, the exception, may be
a "maverick", possibly being an escaped satellite). Nearly all planetary
satellites are either rocky or a mix of rock and ice (one, Saturn's
Titan, has a thin atmosphere). The four inner planets and Jupiter and
Saturn were known since ancient times; Uranus was discovered in 1781,
Neptune in 1846, and Pluto in 1930. The Sun-orbiting planets are
recognized by astronomical observations because they move relative to
the background stars (the ancients called them "wanderers").
Despite the efforts of pre-Renaissance astronomers (e.g., the Greek,
Ptolemy, living in 2nd Century Alexandria, and later Arab observers) to
develop a legitimate model of the Solar System, the frame of reference
put the Earth at the center of the System (geocentric model). The
ancients perceived the "Universe" (for them, mostly the known planets,
and other points of light called stars) as a set of concentrically
nesting spheres that had the Sun on one sphere; all spheres rotated
around the Earth at different rates. The "map" below is one version of
the geocentric Solar System as envisioned in late Medieval times; note
the descriptors are in Latin:
Medieval map of a geocentric Solar System.
This was replaced in 1543 (date of publication) by the heliocentric
model, based on work by the Polish scholar and priest Nicolaus
Copernicus, who postulated that the Earth rotates and the planets
revolve around the Sun. This Copernican model was largely ignored for
decades, mainly from philosophical/theological objections, until
observations by Tycho Brahe in the 17th Century supported the
Sun-centered scheme (which, unfortunately, he rejected after conducting
a flawed experiment). Galileo also made vital observations through one
of the first telescopes; his discovery of satellites around Jupiter
confirmed the notions of bodies revolving around a central body. General
acceptance by the scientists of the times was still slow but the laws of
planetary motion enunciated by Johannes Kepler (Tycho's protege) and
motion in general by Isaac Newton finally led to such overwhelming
evidence that scientists and other thinkers and eventually the Church
acceded to this reality.
Kepler deduced from the patterns of motion that the planets revolving
around the Sun did not follow precise circles but instead followed
elliptical paths with the Sun at one of the two foci that characterize
an ellipse. The ellipses defined by him and later astronomers were only
slight departures from circularity, except for Mercury (strongly
elliptical) and Pluto (which periodically crosses the ellipse traced by
Neptune). Kepler's second law is derived as follows (see figure below):
Diagram illustrating Kepler's Second Law; see text.
Start with a line from the Sun to a planet at any locus. e.g., a, along
its orbital path. After it had moved some distance a-a' along the path,
it will define some given area A for the time in transit, For another
segment elsewhere along the orbit, a different pattern - area B - ensues
as it traverses the distance b-b'. Now, if the elapsed time between
orbital transits from positions a to a' and b to b' are specified to be
the same, the areas in the patterns will be equal (A = B). The law can
thus be stated: Imaginary lines from the Sun to any planet sweep out
equal areas in equal elapsed time intervals during different stages in
the planet's revolution. Since the distance a-a' is shorter than b-b',
it follows that the velocity (distance/time) of the planet moving
through b-b' is greater than the speed through a-a'; in other words,
planets move faster when closer to the Sun. Separate arguments based on
Newtonian mechanics show that the velocities of the planets decrease
progressively outward from the Sun.
(As an aside which applies both to the planets and to orbiting
satellites [like Landsat], the velocity needed to achieve and maintain
orbit is a balance between the forward motion vector of the moving body
and the gravity vector pulling it towards its parent body [whose mass is
assumed to be at its center]; thus the tendency to move away
tangentially is offset by gravitational force such that as the parent,
e.g., Earth, rotates such that seemingly its surface falls away from the
tangential line, in fact the satellite (or planet) is pulled downward
just enough to maintain the same distance to the center of mass,
describing a path that produces a circular orbit [or is modified to some
degree of ellipticity], even as its momentum [mv; v varies for the
elliptical case] keeps it in that orbit. Like the planets, the velocity
needed to get and keep an Earth-orbiting satellite in place decreases
outward. Landsat moves much faster [~26,600 km/hr], and with a period
[time to complete one orbit] of 103 minutes, than does a geostationary
satellite. The latter, when placed at 22,300 miles [36,235 km] above
ground, moves slower [24 hours to complete an orbit] over a much longer
orbital path at an orbital velocity of ~11000 km/hr; when inserted so as
to move parallel and over the equator, the geostationary satellite moves
forward at the same speed as its nadir point on the equator and thus is
stationary [no relative movement] with respect to that point on the
Earth's surface.)
Kepler discovered a third relationship affecting the paths of the
planets. If the orbital period P of a planet (third column in the table
above) is plotted on log-log graph paper against its distance R (second
column) from the Sun (taken as equal to the semi-major axis of the path
ellipse), then the result is as appears below. The mathematical
expression for the equation representing the resulting line is P^2 = R^3
, the mathematical statement of Kepler's third law.
Kepler's Third Law, as determined by the equation of the line shown in
this log-log plot of P vs R.
Another, rather curious relationship was put forth by Johann Titius in
1766, with later modification and promotion by Johann Bode. To formulate
it, look at this sequence:
N = 0.0 0.3 0.6 1.2 2.4 4.8 9.6 19.2 38.4 76.8
Now, add 0.4 to each N term, giving this new sequence:
0.4 0.7 1.0 1.6 2.8 5.2 10.0 19.6 38.8 77.2
The actual positions (in A.U.s) of the planets are: Mercury = 0.39;
Venus = 0.72; Earth = 1.0; Mars = 1.5; Asteroid Belt = ~2.8; Jupiter =
5.2; Saturn = 9.5; Uranus = 19.2; Neptune = 30.0; Pluto = 39.5.
For each successive planet we doubled the previous N and added 0.4; for
Mars this yields 1.6 and for Neptune this results in 38.4 + 0.4 = 38.8.
Remarkably, this set of numbers is closely matched by the actual
distances as Astronomical Units for all the planets except Neptune which
lies at 30.07 A.U. Pluto (no longer a planet), however, lies at 39.4
close to the 38.8 value. The value 77.2 suggests a "missing" planet at
that distance. No obvious physical reason has yet been found for the
Titius-Bode "rule", nor is the Neptune anomaly readily explanable (it
may relate to an interaction with Pluto). But one consequence was a
prediction that some planetary body should exist at A.U = 2.8. None was
known at that time but the later discovery of the Asteroid Belt at 2.8
fulfilled the prediction. That gap is evident in the figure above, as is
the anomalous position of Neptune.
The more general subject of celestial motions, or also referred to as
celestial mechanics, is rather complex - beyond the scope of the
Tutorial here (some aspects are treated near the top of page 20-2
<../Sect20/A5.html>). But a few ideas relating to the Sun's and the
planets' motions relative to Earth as the observing platform are
introduced here.
These motions with respect to the Earth and to the Sun's path across the
sky can be plotted on the Celestial Sphere - usually displayed as either
the hemisphere north or the hemisphere south of the Equator or one's
local horizon, for each case showing the expanse of celestial objects
above the horizon in all directions. Thus from any location on one or
the other hemispheres, there is a a local observational hemisphere on
which the motions of the Sun, the Moon, the planets, and the stars
appear to move as the Earth rotates each day, causing each body or point
of light to trace an arcuate path across the hemisphere (best seen at
night). An axis around which the heavenly bodies seem to move in
circular arcs is imagined to pass from Earth to the hemisphere, with the
North Star (Polaris) being very close to the point where the celestial
axis would penetrate the (hemi-)Sphere; by convention, the Earth's
rotational axis is placed to coincide with the celestial axis, and the
Earth' North Pole is near the celestial North Pole. The position of the
North Star, and hence all other celestial bodies, will vary with
latitude and with season of the year. At different times in a year
different distinctive groupings of stars - known as constellations
(discussed again on page 20-2) appear in the same part of the Sphere at
the same time of night, are illustrated here:
Major constellations seen in the same direction into the sky, at Summer
and Winter.
One approach to understanding celestial motions is to trace the pathway
of the Sun across the Sphere as it varies seasonally and with location
on the Earth's spherical surface. First, we introduce the concept of the
solar ecliptic. This is the plane defined by the elliptical trace of the
Earth's annual orbital path around the Sun (the orbital traces of the
other planets approximately follow the ecliptic as well; Mercury at 7°
and Pluto at 17° have the largest departures from the orbital plane as
defined by Earth's path). A corollary: the ecliptic is also the Sun's
annual path among the stars as viewed from Earth. In this frame of
reference, the Earth, and also six other planets rotate in the same
direction - counterclockwise (prograde) as viewed from above (as from
the celestial North Pole); Venus and Uranus have retrograde (clockwise)
rotations. None move in circular orbits; each follows an elliptical
path, usually with small ellipticity, such that it will be varying
distances to the Sun with points of closest and farthest approaches.
While all the planets orbit on or near the ecliptic, their rotational
axis is tilted to varying degrees relative to the ecliptic plane (or to
a line normal to [90°] the plane). Only Mercury's axis is at 0° with
respect to this normal. Venus and Pluto have their north rotational pole
tilted more than 90°, hence the top of the axis lies beyond (south of)
the ecliptic. The Earth's axis is tilted 23.5° from the a line at right
angles to (normal to) the ecliptic.
Two diagrams will indicate the seasonal and the latitudinal effects of
the Sun's movement across the celestial hemisphere:
Seasonal paths of the Sun, projected on to the Celestial (hemi)Sphere.
In the above diagram, the (unspecified) latitude of observation, along
with Earth's axial tilt, is responsible for the celestial North Pole
being about 60° down towards Earth's Equator (in this projection, an
'ellipse' marked by E, W, N, and S). The highest the Sun will reach is
at Summer Solstice (~June 21); the plot of a daily path then across the
sky at that latitude is a partial circle (arc) shown in red that has the
greatest length, thus resulting in the maximum amount of sunshine on
that day. That arc begins at sunrise (after dawn) and ends at sunset.
The smallest arc is at Winter Solstice (December 21), shown in green; at
that time the daylight is the least - several hours less than 12. At the
Spring (~March 21) and Fall (~September 21) Equinoxes - blue line - the
arc is at a median length, and the sunrise-to-sunset duration is about
12 hours.
Paths of the Sun across the Celestial (hemisphere) for observers at
latitude 41 degrees north..
The second diagram has a different orientation of the equator, and a set
of seasonal Sun paths, for a location at 41° North. Here the Sun becomes
almost vertical (would cross the zenith on the Sphere) on June 21. Thus,
the Sun path will vary systematically with latitude (the effect of the
Earth's axial tilt is built in). This can be plotted on a diagram:
Variation of the Sun's path with latitude.
Here's how to read this diagram. First consider your location at 0°
latitude, i.e., you are on the Equator. This is represented by the
bottommost curve. The starting point is September 21: the Sun is
directly overhead at Noon. By December 21 (Winter Solstice) the Sun has
migrated northward to 23° North latitude, reached when local Noon
occurs. By March 21, the Sun has 'retreated' southward to again cross
the Equator and be overhead. The Sun then continues southward into the
southern hemisphere reaching 23° South at the noontime of the Summer
Solstice. It then progresses back to the Equator to repeat this cycle.
Now, look the 90° curve (right side) which places you at the North Pole:
From March 21 to September 21, the Sun will never set - 24 hour
daylight; from September 21 to March 21 the Sun does not rise -
"perpetual" night. At high latitudes long days or long nights, but some
Sun setting, will occur. For a third case, consider the curve for 40°
North: The Sun at that latitude is at 40° North on September 21; it
migrates to about 62° North by December 21, then moves southward until
it reaches its southmost position at ~19° latitude at the Summer
Solstice, then reverses trend, going back to 40° North by Fall Equinox.
Try your ability to interpret a curve for some other latitude.
From day to day during the year, as Earth orbits the Sun in the plane of
the ecliptic and maintains a tilted axis of rotation which points in a
fixed direction in space, we see four things change:
(1) the sunrise direction changes
(2) the sunset direction changes
(3) the length of daylight changes
(4) the height of the Sun at noon above the southern horizon changes
Thus, the two Celestial Sphere diagrams show that the Sun follows
arcuate patterns (of varying lengths depending on time of year) across
the sky as seen at any location during the day. Stars, on the other
hand, will follow circular patterns, centered on the Celestial North or
South Poles, over much of the Sphere. Planets, being part of the Solar
group circuiting the Sun, will also move during a given night, but their
paths over months and the seasons will appear as though they "wander"
through the background of stars and constellations. Their shifts are not
equal per terrestrial time period and are seen from Earth as influenced
by their relative distances from the Sun and their locations in their
elliptical orbits relative to earth observers.
*Meteorites as Samples of Planetary Materials*
Prior to the space program which has led to visits of unmanned probes
past or onto planetary bodies and over one fabulous decade the landing
of humans to explore the Moon's surface, our knowledge of the planets
were largely confined to two avenues of investigation: 1) telescope
observations and selected properties measurements using accessible parts
of the EM spectrum, and 2) samples of one or more planets and smaller
solid bodies that fall to Earth as meteorites (or, as discussed in
Section 18, as large bolides - megameteorites, asteroids, and comets).
The bulk of the rest of this page will be devoted to a review of
meteorites, which continue to be a prime source of information about
some of the Sun's planetary and fragmental-bodied associates.
"Stars" falling from the skies have been known since ancient times;
rarely, stones are found that were tied to these "shooting stars". One
such rock has been venerated by Islam (in its encased shrine in Mecca)
for more than 1300 years. By the 19th Century, meteorites were
identified correctly as samples from other parts of the Solar System.
They are part of the nearly 500 tons of extraterrestrial rock material
that reaches and enters the atmosphere each day. Most of that material
is burned up by friction from the high speed of entry but meteoric dust
can remain in the air and a very few individual blocks of material
survive this passage to fall in the sea or on the ground as meteorites.
A general nomenclature has been developed to describe rocks in space
that may reach the Earth's surface. If these rocks are relatively small
(say about house-size or less), as they exist in space they are referred
to as /meteoroids/. If they reach Earth and pass through the atmosphere,
creating intense light as their outer skin is melted by friction, they
are called /meteors/. If they do not burn up completely in transit, and
land on the Earth's surface, they now are designated /meteorites/. The
largest meteorite found so far on land is about the size of an
automobile; most meteorites are much smaller. Much larger bodies moving
in space, such as asteroids and comets (page 19-22), can strike the
Earth, either as still intact bodies or broken into fragments; these
will nearly always produce impact craters (Section 18) or shock-induced
destruction on the ground (such as knocking down trees) if they explode
in the atmosphere. Another terminology distinction: Meteorites whose
passage through the atmosphere was observed and then someone soon
thereafter locates the object are referred to as /Falls/; those whose
passage was not observed but were eventually discovered (often by
serendipity) are called /Finds/.
By the start of space exploration, nearly 1900 meteorites had been
collected. That number has jumped notably (over ten thousand) when
scientists exploring the Antarctic deduced that a few of the rocks
scattered about the ice surface might be meteoritic debris. Patient
collection has since verified this, thus providing a very effective way
to find new meteorites. However, of any thousand rocks on the Antarctic
surface, only about 1 or 2 prove to be meteorites. But each year, a new
expedition (on snowmobiles) continues to add to the total.
Meteorite hunter looking down to spot meteorites on the Antarctic ice.
In searching for meteorites, two clues call attention to certain stones
as candidates to be collect and broken into to reveal indications of
their nature: 1) rocks that appear to be composed solely or largely of
iron metal; and 2) rocks that have a thin dark fusion crust, where
friction has melted the exterior. Although the classification of
meteorite varieties consists of various categories, most meteorites fall
into two types - Iron and Stony - as shown here:
A typical iron meteorite (left) and stony meteorite (right); both
specimens have a thin, dark fusion crust.
The Renazzo stony meteorite shown below as broken open reveals the
typical texture of this type:
The Renazzo stony meteorite.
The mineral composition of meteorites is distinctive. The iron
meteorites contain native iron metal alloyed with 5 to 17% nickel. The
stony meteorites are composed of minerals that are common in basic
igneous rocks: olivine, pyroxene, and plagioclase feldspar. together
with a variety of minerals (some found only in meteorites) present
usually in small quantities. Various combinations of these and some
other constituents, together with distinctive textures, provide the
basis for classifying the different meteorites. One general
classification appears below. You can examine a more detailed
classification by going online to this helpful website
<http://www.alaska.net/~meteor/type.htm>.
Classification of meteorites; note the percentages of each major type.
Iron meteorites (known as Siderites) are uncommon but quite distinctive.
(Most believe they are the core material in differentiated (melted)
asteroids. They contain one or both of the structural phases of metallic
iron: Kamacite and Taenite. The Iron types are classed by the amount of
nickel present and the nature of the iron phase(s). When an iron
meteorite's interior is exposed, usually by sawing to create slab faces
and then etched by nitric acid, some distinctive textures are often
present, such as what is termed "Widmanstatten strucure" caused by
unmixing of the two structural phases (the broader bands are Kamacite),
displayed here at two magnifications.
Widmanstatten structure in an iron meteorite, of the Octahedrite class.
Magnified close-up of interlocking iron phases in Widmanstatten
structure; this eutectic mix of Kamacite and Taenite is called Plessite.
Another planar structure in the Iron meteorites is called Neumann Bands,
which is a twinning mode induced by shock. Most likely, this shock
effect occurred during the breakup of the parent body of which the Iron
meteorite was in the interior (a core analogous to the Earth's?):
Thin Neumann Band twins in an Iron meteorite.
Transitional to the stony types are the stony irons, that include the
Pallasites and the Mesosiderites. An example of the first is the Esquel
meteorite (generally, a meteorite is named from a geographic location
where it fell and was collected:
The Esquel Pallasite; the non-metallic phase is olivine (dark or
orange); the silvery metallic phase (tinted reddish) is iron.
As the percentage of native iron decreases and silicates increase the
resulting stony-iron meteorites are called Mesosiderites.
The Lowicz Mesosiderite.
Most meteorites based on the percentage of Falls (those observed as
"shooting stars"), which may not be the same as the percentage of Finds,
since some meteorites are more likely to be destroyed by weathering,
etc.), are of the type called Chondrites which in turn are grouped into
classes depending on mineralogy and texture. Chondrites contain
generally small (millimeters up to a centimeter) spherical bodies called
/chondrules/, which most meteoriticists believe were once molten
silicate droplets produced by melting of interspatial dust by one or
more mechanisms such shock waves or heat from the forming Sun. They then
cooled and crytallized into Olivine, often accompanied by pyroxene
mineral species (Enstatite, Bronzite, Hypersthene) and Plagioclase
(calcium-rich). Most chondrites contain small crystal specks of
iron-nickel. The chondrules seem to be embedded in other dust and
isolated crystals which incorporate the chondrules as the meteoroid or
asteroid built up from the remaining materials in the dust clouds
surrounding the growing Sun, over the first few million years of the
organizing Solar System. This photomicrograph shows a texture
characteristic of chondrites, with subspherical chondrules, crystal
fragments, a few iron-nickel grains, and a fine-grained matrix:
Texture typical of a chondrite.
The next two figures depict photomicrographs (with the petrographic
microscope's Nicols in the cross-polarization mode) of individual
chondrules:
A chondrule made up mostly of Olivine, in the Brownsfield chondrite.
Two chondrules in the El Hammami chondrite; the one on the left has some
plagioclase; the one on the right has parallel crystals of Olivine,
producing a 'barred' texture.
Some chondrules show a characteristic radiating structure assumed by the
Pyroxene
Radiating Enstatite crystals in a chondrule; these appear to converge on
a common base.
Plagioclase can be conspicuous in some chondrites, as shown in this
photomicrograph.
Plagioclase in a crude chondrule and pyroxene, seen in this thin section
in polarized light.
The bulk texture typical of an Ordinary Chondrite is exemplified in this
slab cut into the Homestead meteorite:
Typical chondritic texture; most of the chondrules in this slab from the
Homestead meteorite are too small to be visible here.
Somewhat larger chondrules are present in this sample from the Brenham
meteorite:
The Brenham Chondrite.
Classification of the Chondrites is determined to some extent by the
particular mineral species present. However, the usual hierarchy (Type 1
through Type 6) is determined by the degree of water content and extent
to which the chondrule appears to have been reheated and thus
recrystallized by thermal metamorphism. Type 1 is most primitive and
contains some water; it probably was never reheated after primary
crystallization beyond about 300 °C. Type 6 is anhydrous, shows thermal
and/or shock textures, but was reheated up to about 800°C. In the next
two photomicrographs are shown 1) a ring of iron metal that accumulated
when the chondrule was thermally heated to the extent that iron was
melted; 2) a chondrule with veins of glass caused by shock heating.
A chondrule with an iron ring
Veins of blackish glass is a shocked chondrule.
We turn now to a special class of Chondrites called Carbonaceous
Chondrites. These contain up to 6% carbon, either in elemental form or
in the composition of organic (hydrocarbon compounds, including some
amino acids, but not biogenic) molecules that occur within them. Low
temperature minerals, such as clay minerals and serpentines, attest to
the conclusion that the matrix was never subjected to the high
temperatures that melted the associated chondrules. This is supported by
the variable water content; some of these meteorites contain up to 11%
H_2 O. Many meteoriticists consider carbonaceous chondrites to be the
most fundamental and primordial representatives of the solid materials
available for making up the planetary system. They are thus held to be
condensates of melted silicates that mixed with low temperature organic
and inorganic phases which grouped into asteroids and comets, or were
aggregated into the planetesimals that evolved into the planets. Here is
one of the best-studied of this class - the Murchison meteorite that
fell on Australia:
One of the pieces of the Murchison carbonaceous chondrite.
One of the most famous meteorite falls was the Allende carbonaceous
chondrite, in which nearly two tons landed in a farmer's field in
northern Mexico in 1969. It's quantity has proved to be a bonanza for
researchers. Below is one of the pieces and a thin section which shows a
carbon-rich matrix around the chondrules
A piece of the Allende meteorite
A thin section cut from the Allende meteorite; crossed nicols.
About 8% of the silicate (stony) meteorites do not contain chondrules;
the group is known as the Achondrites. There are many varieties, as
evident in the classification we pointed you to. Most members are
thought to have come from the surfaces of asteroids. Some of these are
breccias and other unusual textures may be distinctive. Eucrites are a
common class and are either similar to terrestrial basalts in texture or
are brecciated. The basaltlike Millbillillie exemplifies the first type
here;
The Millillillie meteorite.
Basaltlike texture of the Millillillie meteorite.
Two brecciated achondrites appear here:
A polymict breccia
A polymict breccia.
There is a growing realization that many of the Achondrites may be
pieces of the Moon or Mars expelled from these bodies by impact.
Lunar meteorites may directly strike the Earth after thrown off by a
lunar impact or fall after being captured in orbit. Martian meteorites
need to be thrown out beyond martian gravity into orbits that may be
perturbed or decay to allow eventual Earth-crossing encounters. Below
are three meteorite samples of probably lunar origin, as determined by
age and composition.
Lunar meteorite QUE94281,
NWA2362, a possible lunar meteorite traced to some mare site.
The Calcalong Creek Lunaite (lunar meteorite)
This last meteorite is remarkably like lunar regolith (the loose debris
on the surface). If so, it was shock-lithified by the impact that hurled
it to Earth; if not, it was probably breccia rock that was part of an
ejecta blanket later lithified.
After the Apollo moon rock returns, it became much easier to prove
certain meteorites to have come from the Moon; both chemical composition
and isotope ratios were particularly diagnostic. Here is a plot of the
composition of meteorite specimens of certain-to-probable lunar origin:
Less than a 100 meteorites are thought to Lunaites (Moon meteorites).
Most are fragmental. A neat review of Lunar Meteorites
<http://meteorites.wustl.edu/lunar/moon_meteorites.htm> is found at this
Washington University web site.
At least 35 meteorites have evidence that they came from Mars. The next
figure is of a Nakhrite type meteorite of probable martian origin
whereas the second illustration shows the texture of the Zagamil
meteorite which is considered of martian origin. We will show other
examples of these planetary meteorites on pages in this Section that
treat the Moon and Mars.
A Nakhrite meteorite, believed to come from Mars.
Photomicrograph of the texture of the martian Zagamil meteorite.
The age(s) of meteorites can be instructive. Elemental isotopes are used
to date them. The chondrites give very old ages (clustering around 4.5 -
4.6 billion years), suggesting that these formed near the beginning of
the Solar System. These ages are determined by Uranium-Lead and
Rubidium-Strontium isotopic analysis; the presence of I^129 , derived
from Xe^129 decay, which has a short halflife, confirms that at least
some of the constituents were incorporated early in the inception of the
Solar System. But there are one ot two younger ages, called exposure
ages, which indicate times when the meteorite body separated from a
larger host body and began its travel through space. Abnormalities in
the amount of Ar^40 and He gas point to a time when larger parent
asteroids may have broken up from collisions. Still younger ages deduced
from He^3 , Ne^21 , Al^26 , and A^38 contents are associated with times
when the meteoroids were traveling in their final sizes and were subject
to cosmic ray bombardment.
Genetic implications of the different meteorite types are these: For
those not of lunar or martian origin, there may have been four stages of
organization of meteorite parent bodies (most believed to be from the
asteroidal belt between Mars and Jupiter): 1) condensation of
high-temperature refractory silicates, oxides, and metals; 2) separation
of silicates from metals as granular particulates in the solar nebula;
3) condensationn of lower temperature or volatile phases; and 4) varying
degrees of remelting of the earlier condensates. From a different
persepective: 1) the Carbonaceous Chondrites are the most primitive; 2)
Chondrites formed from aggregation of chondrules (melted by shock,
thermal radiation, or other process[es] and dust into bodies that never
became large enough to melt; these bodies may, however, have experienced
collisional breakup of asteroids (thus, some of the Achondrites might be
so derived), 3) the Iron meteorites may (?) be cores of completely
melted large asteroids, or less likely, bigger planets that were
destroyed by collisional disruption, and 4) Some of the Achondrites were
made by fragmentation/reassembly of differentiated planetary or
asteroidal surfaces subjected to impact bombardment, or may be
shock-lithified surface rubble (such as the regolith deposits on the
Moon, as discussed later). Some of the general conditions that lead to
different meteorites derived from asteroids are depicted in these diagrams:
Schematics of different asteroidal histories that lead to different
types of meteorites.
At least one meteorite has been traced to a specific asteroid, Vesta,
based on strong similarities in spectral properties:
The asteroid Vesta, some 370 km in long dimension.
The Vesta meteorite, purported to come from the asteroid Vesta.
As space exploration goes on, more answers to organizational details are
forthcoming, e.g., the similarity of asteroidal material to carbonaceous
chondrites has been established by probes that approach or land on the
asteroids.
The importance of asteroids in the makeup of our Solar System is
paramount. But we will defer further discussion of these bodies until
after we have examined the major planets. However, for the curious who
would like some insight now, go to page 19-22 <../Sect19/Sect19_22.html>.
We have said nothing on this page, nor elsewhere in this Section, about
the origin of the planets and the development of a Solar System. These
topics are treated in some detail on page 20-11 <../Sect20/A11.html>,
after astronomical principles are considered. We will start our
extraterrestrial planetary tour with the Earth itself and then Earth's
sole satellite, the Moon. The geological aspects of Earth were covered
on page 2-1a <../Sect2/Sect2_1a.html> and 2-1b, to which the curious
user can refer now by clicking on this page number for a refresher
review. However, the Earth does deserve a brief overview of its general
nature and history as one of the planets within the Solar System.
navigation image map
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Primary Author: Nicholas M. Short, Sr.