ASTRONOMY 1010

ASTRONOMY OF THE SOLAR SYSTEM

OUTLINE OF COURSE OF STUDY


FUNDAMENTAL CONCEPTS

Prologue:  Powers of Ten

 









I. The Celestial Sphere, Cycles & Seasons

A. Astronomy: "The world’s oldest profession."

B. Cycles and seasons, the sky as seen from Earth

 1. celestial sphere
a. north celestial pole (NCP) and south celestial pole (SCP)
c. ecliptic line = Sun’s yearly path around the celestial sphere as seen from Earth = intersection of Earth's orbital plane with the celestial sphere
d. vernal and autumnal equinox points = intersection of the celestial equator and the ecliptic line
e. summer and winter solstices - point of most northern and southern travel by the Sun, respectively
2. position on the celestial sphere I (celestial longitude and latitude)
a. constellations
i. origin of the constellations
ii. star names and catalogues
b. celestial coordinates (RA and Dec)
i. right ascension = celestial longitude units = hours, minutes, seconds (1 h = 15o) (see 3b & 3c above for connection with solar and sidereal time)
ii. declination = celestial latitude units = degrees, arc-min., arc-sec.
iii. origin (0 h RA, 0oDec) = vernal equinox point
3. position on the celestial sphere II (the sky relative to the horizon)
a. horizon line
b. zenith, nadir
c. local meridian
d. altitude and azimuth coordinates
4. daily motion (Earth’s spin)
a. daily motion of celestial objects (northern hemisphere descriptions)
i. north circum-polar objects = celestial objects whose angular distance from the NCP is less than the observer's latitude never set below the northern horizon
ii. objects north of the celestial equator = rise N of E, cross local meridian at high altitude, set N of W
iii. objects on the celestial equator = rise due E, cross local meridian at an altitude equal to 90o - latitude angle, set due W
iv. objects south of the celestial equator = rise S of E, cross local meridian at low altitude, set S of W
v. south circum-polar objects = celestial objects whose angular distance from the SCP is less than the observer's latitude never rise above the southern horizon
b. solar time = position of Sun relative to local meridian
i. a.m. = ante meridian (ante = Latin for "before" as in ante-bellum "before the war")
ii. p.m. = post meridian
iii. 1 solar day = meridian to meridian crossing time of Sun (solar day period is slightly different for different times of the year)
iv. 1 mean solar day = average meridian to meridian crossing time over the whole year = meridian to meridian crossing time of the "mean" Sun
iv. local apparent solar time (LAST) = "sundial time" = solar time defined by the position of the real Sun's position relative to an observer's local meridian
v. local mean solar time (LMST) = solar time defined by the position of the "mean" Sun's position relative to an observer's local meridian (related to apparent solar time by the "Equation of Time" which quantitatively describes how "fast" or "slow" the real Sun is to the "mean" Sun for through out the year)
vi. standard time zones (eg:  eastern standard time EST) = time within a standard time zone defined by the LMST at the center "standard meridian" of the zone (each time zone is roughly 15 degrees of longitude wide ( angle Earth rotates in 1 hour )
vii. various regions around the world also define "daylight savings time" (USA) or "summer time" (GB) by adding an hour beginning in the spring and going back to standard time in the fall (eg:  eastern daylight time EDT)
c. sidereal time = position of vernal equinox point relative to local meridian (see alternate definition below)
i. only 24 hr time, "a.m." and "p.m." not used
ii. meridian to meridian crossing time of vernal equinox = 1 sidereal day
iii. 1 sidereal day = actual spin period of Earth = 23 hr 56 min 4.092 sec (solar time)
d. sidereal time (alternate defn.) = the RA (in hours, min, sec) that is lined up with the local meridian is the local sidereal time at that moment in time
5. calendars and seasons
a. week = not based on a celestial cycle, however 7 days are approximately ¼ of the lunar phase cycle (29.5 days) and there were 7 "planets" known to the ancients, each day is named for one of the "planets"   e.g.:  Sunday – Sun (to the ancients, the word "planet" meant wanderer, which included anything that moved relative to the "fixed stars" of the celestial sphere such as the Sun and the Moon)
Sunday – Sun (see above)
Monday – (French: Lundi) - Moon (see above)
Tuesday – (French: Mardi) – Mars
Wednesday – (French: Mercredi) – Mercury
Thursday – (French: Jeudi) – Jupiter
Friday – (French: Vendredi) – Venus
Saturday – (French: Samedi) – Saturn
b. month = approx. 1 lunar phase cycle = 29.5 days the modern calendar has designated months of between 28 and 31 days (some ancient calendars were based on the lunar phase period rather than the yearly solar period)
c. year = period of time for the Sun to (apparently) travel once around the celestial sphere on a great circle path called the ecliptic line = the orbital period of the Earth around the Sun
i. the year is divided into four seasons, the seasons are caused by the tilt of the Earth's rotation axis by 23.5 degrees to a line perpendicular to the Earth's orbital plane (i.e.:  the Earth's equatorial plane by 23.5 degrees to the ecliptic plane {or orbital plane}, see I.B.1 above), this tilt causes the rays of the Sun to be more direct at midday during mid-latitude Summer and much less direct during mid-latitude Winter, also the opposite hemisphere experiences the opposite season, the radiant power delivered to the Earth's surface at some point on the Earth is called the Solar Insolation at that point [NOTE:  the seasons are NOT caused by the elliptical nature of the Earth's orbit, the Earth's orbit is just barely elliptical and the difference between its closest approach and its maximum distance is not significant, in fact the Earth's closest approach occurs about two weeks after northern hemisphere winter begins]


ii.  the beginning of each season is defined by when the Sun reaches a particular position on the celestial sphere as it moves along the ecliptic line, the following list gives the Sun's position along the ecliptic line on the celestial sphere for the first day of each season, each line has the following format:
[~date of 1st day of season] season N hemisphere (season S hemis.) position of Sun on celestial sphere
[~Mar. 21] spring (fall) vernal equinox point crossing equator south to north
[~June 21] summer (winter) summer solstice point furthest north of equator
[~Sep. 21] fall (spring) autumnal equinox point crossing equator north to south
[~Dec. 21] winter (summer) winter solstice point furthest south of equator

Seasons and Ecliptic Simulator  (Univ. Nebraska-Lincoln)

d. precession = Earth’s rotation axis "wobbles" like top (period of wobble = 26,000 years) -> no unit of time, season, or calendar based on this period, but the effect of precession is to "slide" the equinox points along the ecliptic over time changing the position of the origin of the celestial coordinate system (note: there are several effects of precession including the fact that Polaris has not always and will not always be the North Star)


C. Eclipses and ancient astronomy

1. lunar phases 
a. new
b. waxing crescent
c. 1st quarter
d. waxing gibbous
e. full
f. waning gibbous
g. last or 3rd quarter
h. waning crescent
2. eclipses
a. solar eclipses (new moon)
i. total (umbra)
ii. partial (penumbra)
iii. annular (umbra doesn’t reach Earth)
b. lunar eclipses (full moon)
i. total (umbra)
ii. partial (partially in umbra)
iii. penumbral (penumbra only)
 
c. eclipse frequency
i. the line of nodes (= intersection between the Moon’s orbital plane and the Earth’s orbital plane) and the eclipse year (= time period for the same lunar node to line up with the Sun = 346.6 days)
ii. 5 total solar eclipses possible in one calendar year (however seems rarer than lunar eclipses)
iii. 3 lunar eclipses possible in one calendar year (however more people witness lunar eclipses)
d. eclipse prediction and the Saros cycle
i. eclipse year and lunar cycle combined
ii. 1 Saros cycle = 223 lunar phase periods = 19 eclipse years = 18 years 111/3 days = period time in which conditions for similar eclipses occur (ie: similar location, max. time of totality, etc.)

Total Solar Eclipse, August 21, 2017

Aug 21 2017 path of umbra shadow    Timings of eclipse

Note that the small dot (umbral shadow) passes over N. GA.  Dahlonega is NOT in totality but moving less than 15 miles north of Dahlonega will put you in the path of totality

 

 

COPERNICAN REVOLUTION

II. Ancient Astronomy

A. Earliest scientific astronomy

1. Early "Greek world" astronomy (~600 BC)
a. applied logic and observation to learn about the physical world
b. eclipse prediction (Saros cycle), stars = balls of fire, speculation on distances to Sun, Moon, other planets, shape of Earth, etc.
2. The Pythagoreans (540 BC – 510 BC)
a. founded by Pythagoras
b. Earth spherical, Earth at center, true cause of eclipses
3. Plato and Aristotle (384 BC – 322 BC)
a. Aristotle = Plato’s most famous student
b. credited with founding modern scientific investigation
c. Earth at center – became "final authority", later lead to rejection of Sun centered universe in Middle Ages over a millennium later!
d. other ideas that were correct: Moon and Earth spherical, Sun farther than Moon

B. Other Greek achievements in astronomy

1. Aristarchus and relative distances ~(310 BC - 230 BC)
a. used lunar eclipses, geometry of lunar phases to estimate diameters and distances from Earth of the Sun and Moon
b. large quantitative errors, but estimated that the Sun was much further than Moon
2. Eratosthenes and the size of earth ~(276 BC – 192 BC)
a. researcher and librarian at Alexandria
b. completed a catalog of 675 brightest stars, measured the tilt of the Earth’s rotation axis = 23½o
c. most famous for measuring the Earth diameter and getting it correct!
3. Hipparchus, mags and precession ~(162 BC – 125 BC)
a. completed a catalogue of 850 stars, listed by magnitude – as defined by him, later used in modern definition of magnitude
b. discovered precession by using ancient Babylonian star catalogs


III. The Copernican Revolution

A. Ptolemy (~140 AD) and geocentric theory = Earth centered universe

1. retrograde motion = apparent "backwards" or westward motion of superior planets around opposition
2. epicycles and deferents = Ptolemy’s explanation for retrograde motion using the geocentric model = planets actually go backwards at certain times as they orbit the Earth
3. The Mathematical Collections = al-Megiste (The Greatest) = Almagest = 17 volume work incorporating earlier Greek thought and original models on the motions of the solar system, became the unquestioned authority on Astronomy for over 1400 years

B. Copernicus (1473 AD – 1543 AD) and heliocentric theory = Sun centered universe

1. retrograde motion explained using the heliocentric model:  
    (After the in-class graphical demo, click here for a static graphical summery of the
      in-class demo)                                                    
2. planetary configurations
a. superior planets
i. opposition
ii. conjunction
iii. quadrature (E and W)
b. inferior planets
i. inferior conjunction
ii. superior conjunction
iii. greatest elongation (E and W)
3. mapping the solar system, the relative orbits
4. De Revolutionibus (pub. 1543)

C. Tycho's (1546 AD – 1601 AD) measurements; comets, novae & parallax

1. greatest pre-telescopic observer
2. measured parallax of Moon
a. measured parallax of Moon, ie: distance
b. no measurable parallax for stars
c. determined comets were NOT atmospheric phenomena ie: much farther than the Moon
d. observed a supernova, determined it to be as distant as the stars, ie: no measurable parallax
3. hired Kepler (1600) and bequeathed all his observational data to Kepler on his (Tycho’s) deathbed

D. Kepler's (1571 AD – 1630 AD) laws of planetary motion

1. using the heliocentric model and Tycho’s observations he was able to determine the true shapes and motions of planetary orbits - Kepler’s 3 laws of planetary motion:
a. 1.) law of ellipses: each planet moves in an ellipse with the Sun at one focus
b. 2.) law of areas: the line between the Sun and the planet sweeps over equal areas in equal time intervals
c. 3.) harmonic law: the ratio of the cube of the semimajor axis (a) to the square of the orbital period (P) is the same for every planet which orbits the Sun:  [ a3 = P2 ]
2. laws 1 & 2 published in The New Astronomy (1609), the 3rd law was published 10 years later in The Harmony of the Worlds (1619)

E. Galileo's (1564 AD – 1642 AD) observations  (Click here for an unusual link related to Galileo.)

1. 1st person to use a telescope to observe the heavens and publish his observations (did NOT invent the telescope, but did improve the design)
a. phases of Venus – proved Venus orbited the Sun
b. moons of Jupiter – proved the not all heavenly bodies orbited the Earth
c. mountains on the Moon, spots on the Sun – showed that heavenly bodies were not perfect, unblemished spheres
2. contrasted the heliocentric and geocentric models in his work: Dialogue of the Two Chief World Systems where he presented his observations which overwhelmingly favored heliocentric theory
3. brought to trial at Rome by the Inquisition for heresy for his ideas, forced to recant and put under house arrest for the rest of his life
4. known also for his experiments with motion and light:  discovered a version of the law of inertia, which later became Newton’s 1st law and showed that the acceleration due to gravity on the surface of the Earth was the same value, no matter what the mass of an object was.  The following you-tube video is from the Apollo 15 mission and confirms that Galileo was correct!

 

F. Newton's (1642 AD – 1727 AD) synthesis: 3 laws of motion published in the Principia (1686) (see below) (Click here for images relating to Woolsthorpe Manor, Newton's birthplace and site where his laws of optics and gravity were inspired.)


IV.Classical Mechanics: Newton’s Synthesis, Culmination of the Copernican Revolution

A. Fundamental concepts

1. vector quantity = direction & magnitude (amount) (symbol in outline = bold italic; eg: force = F, force has a direction and an amount of force)
2. scalar quantity = magnitude (amount) only (symbol in outline = normal lettering; eq: mass = m, mass only has an amount, not a direction)
3. kinematic definitions 
a. r = position vector
b. Δr = displacement vector = r2 – r1
c. v = velocity vector = Δr/Δt= (r2– r1)/(t2– t1)
d. a = accel. vector = Δv/Δt= (v2– v1)/(t2– t1)
4. centripetal acceleration for uniform circular motion
a. uniform circular motion = motion with constant speed (mag. of velocity) around a circular path
b. changing direction, constant speed = changing velocity = acceleration = for this case (uniform circular motion), accel. called: centripetal
c. magnitude of centripetal acceleration: ac = v2/r where v2 = speed, r = radius of circular path
d. direction of centripetal acceleration: always towards the center of the circle

B. Newton’s laws of motion

1. if Fnet = 0, then v = constant (including v = 0)
2. Fnet = ma (Newton's original version:  Fnet= Δp/Δt, where p = mv = linear momentum)
      3. F1 on 2 = -F2 on 1

C. Newton's law of universal gravity (Click here to hear part of the Gravity Song by Prof. Boggs.)     

1. universal law of gravity (always between 2 or more objects)
2. inverse square law: Fgrav inversly proportional to the square of the distance between the 2 objects
3. F proportional to the product of the 2 masses
4. magnitude of force between 2 point or spherical mass distributions: 
Fgrav = Gm1m2/r2 , direction – on a line between objects (always force of attraction), 
where: 
m1 = mass of object 1, 
m2 = mass of object 2,  
r = distance from center of mass 1 to center of mass 2, 
G = universal grav. const. 
G = 6.67 x 10-11 Nm2/kg2

D. Kepler's laws from Newton's laws

1. K1: law of ellipses – shape of orbits  (properties of ellipses)
a. net force on planet = gravity force from Sun
b. use Newton’s 2nd law to determine a(t), from a determine v(t), from v determine r(t) = path as a function of time -> possible paths are conic sections, including ellipses
c. conic sections = circle, ellipse, parabola, hyperbola; circle, ellipse = closed orbits; parabola, hyperbola = open orbits
d. Newton's cannon
i. "weightlessness" = freefall, falling around the curve of the Earth
ii. circular orbit velocity = velocity needed to fall around Earth (or any other planet) in a circular path of radius r from the center: vcir = (GM/r)½ where G = univ. grav. Const., M = mass of planet, r = radius of orbit
iii. escape velocity = velocity needed (starting from a distance r from the center of the planet) to "escape" from a planet on an open ended parabolic trajectory and never return; also -> velocity that an object would be traveling at a distance r from the center of a planet if it were "dropped" from infinity: vesc = 2½ vcir where vcir is the circular orbit velocity at a distance r from the center (see ii above)
2. K2: law of areas – speed in the orbit
a. angular momentum (symbol = L), for the case of an object of mass m moving with a velocity v perpendicular to the position vector r, the magnitude of the angular momentum L = mrv
b. angular momentum is conserved for the case of one object orbiting another, therefore L must be a constant value everywhere in the orbit: let rp be the distance of a planet at perihelion (closest approach to Sun), vp = velocity at perihelion, and let ra be the distance at aphelion (furthest dist.), va = velocity at aphelion, then by conservation of L: mrpvp = mrava, however since rp < ra then  vp > va : in other words the closer to the Sun the faster the planet moves, the law of areas is a result of conservation of angular momentum
3. K3: harmonic law – period as a function of distance from the Sun
a. derivation of the harmonic law for circular orbits:
i. net force = force of gravity from the Sun on the planet, provides the centripetal accel. to keep the planet moving in a circular path; use Newton’s 2nd law with gravity as net force and the expression for centripetal accel. as the acceleration; replace the orbital speed v with the equivalent expression 2πr/P
ii. result: Newton’s version of Kepler’s 3rd law GM/4π2 = r3/P2
b. a more detailed analysis taking into account motion of 2 bodies around a common center of mass gives:  G(M+m)/4π2 = r3/P2 where m is the mass of the smaller body and M is the mass of the larger body, for the case of the planets around the Sun m << M and this equation reduces to the result given in a.ii above

 


EXPLORATION OF THE SOLAR SYSTEM

 

 

 

V. Space Age Exploration of the Solar System

A. Navigating the solar system 

1. navigating to the planets (Hohmann transfer orbit)
a. elliptical orbit where perihelion point touches the Earth’s orbit and aphelion point touches target planet’s orbit – by Kepler’s 1st law the Sun defines where one of the focal points must be, thus setting the size and eccentricity of the elliptical trajectory: aHoh = (aEarth + aPlanet)/2
b. use Kepler’s 3rd law to calculate period of transfer orbit, then time of flight between Earth and the target planet is ½ this period: for the Hohman trajectory between Earth and Mars the time of flight is 0.7 years = 8.4 months; between Earth and Jupiter the time of flight is 2.6 years
2. sling-shot (or gravitational assist) trajectories: a spacecraft which follows a parabolic or hyperbolic path by orbiting planets can gain or lose energy to the planet: several robot spacecraft have used this technique to travel to the outer planets and beyond (eg: Pioneers 10 & 11 and Voyagers 1 & 2 all used a "sling-shot" around giant Jupiter to achieve escape velocity from the Sun, Voyager 2 flew by all the other gas giants as well) [Galileo gravity assist trajectory]   [Cassini gravity assist trajectory]

B. Rocket engines and advanced propulsion for the exploration of the Solar System (and beyond?)

1. Newton's 3rd law and rocket propulsion
a. F1 on 2 = -F2 on 1: object 1 "pushes" on object 2, then object 2 "pushes" on object 1
b. rocket engines: high pressure gas generated in "combustion" chamber escapes through nozzle (is "pushed" out the nozzle), gas "pushes" back on rocket body producing thrust = force on rocket
c. the rocket equation: FR = vex(ΔM/Δt) where: FR = thrust (force on rocket body),  vex = exhaust vel., (ΔM/Δt) = fuel/oxidizer consumption rate = mass expulsion rate
d. specific impulse: units of seconds – no. of seconds engine could thrust with a force of 1 pound while using 1 pound of fuel/oxidizer, or, measure of total energy available for propulsion per unit mass of fuel (engine efficiency)
2. chemical energy rocket propulsion
a. solid fuel rocket engine: fuel and oxidizer combined in solid form, molded solid fuel in the rocket body becomes the combustion chamber, exothermic chemical reaction touched off by addition of energy (light the fuse!) – less control than liquid fuel, once burning – can’t stop the reaction and no real time control
b. liquid fuel rocket engine: fuel and oxidizer stored in liquid state in tanks, high speed pumps combine fuel and oxidizer in combustion chamber – may be "throttled" up or down by controlling fuel/oxidizer flow; highest specific impulse liquid fuel is liquid hydrogen (LH2) and liquid oxygen (LOX)
3. advanced/future types of propulsion
a. SCRAMJET = supersonic RAMJET:  air breathing engine does not carry oxidizer, but can only be operated within atmosphere – SCRAMJET/rocket combination may be used for Shuttle replacement as a vehicle that could fly from a runway directly to orbit
b. nuclear rocket engines: combustion chamber replaced by a nuclear reactor inside a chamber (fission or fusion when available), reactor heats working fluid (water or any stable liquid) which is expelled out a nozzle to produce thrust
c. ion rocket engines: charged electrodes accelerate charged particles (ions) to relativistic speeds, magnetic field controls direction, powered by any electrical energy source (solar panels, nuclear reactor, etc.) – very high specific impulse, but very low thrust, can achieve extremely high speed over long thrust period
d.  Variable-Specific-Impulse Magnetoplasma Rocket (VASMIR):  uses magnetic confinement technology from fusion research to expel a plasma at high speed - can have higher thrust with lower specific impulse or low thrust with very high specific impule; would be powered by nuclear power (fission at first, then later fusion when developed)
d. other advanced concepts: fusion pulse rocket, light sail powered by giant lasers, Bussard fusion "ramjet"…

VI. Solar System Characteristics and Its Formation

Saturn occultation by Moon
GIF movie showing a grazing occultation of Saturn by the Moon:  Observed in I (near infrared filter) with Spectrasource CCD camera system using 16" B&C at old NGC Observatory, Fall Quarter 1997

A. Solar system overview

1. general characteristics
a. major planets orbit (approx.) in a single plane (same as Sun’s equatorial plane) in approx. circular orbits and all in prograde direction (prograde = counterclockwise from above N pole)
b. composition of major and minor planets vary roughly from refractatory materials (rock, metal) near the Sun to more volatiles (gas, ices) away from the Sun
c. most major planets rotate prograde (exceptions:  Venus, Uranus, Pluto) with and all medium to large moons orbit prograde; planets contain most of the angular momentum of solar system
d. rocky-metallic minor planets (asteroids) are concentrated in region between Mars & Jupiter (asteroid belt), icy outer system asteroids are found near Lagrangian points of gas giants and a thick disk beyond Neptune (Kuiper belt), comet nuclei orbits define a spherical halo around Sun (Oort cloud)
2. table of particular characteristics (click here for graphical version):

Object
Distance (AU)
Diam. (Earth=1)
Classification
Mercury
0.4
0.4
Terrestrial
Venus
0.7
~1
"
Earth
1
1
"
Mars
1.5
0.5
"
asteroids
~2.5
<0.08
Minor Planet
Jupiter
5
11
Jovian gas-giant
Saturn
10
9
"
Uranus
20
4
"
Neptune
30
4
"
Pluto
40
0.2
Kuiper belt (Demoted to dwarf planet status - Pluto's not a planet anymore.)
comets
50,000
<0.01
Oort cloud
3.  Click here for a digital orrery of the solar system.  (Programmed by Sean Frankum as a computer science senior project.)
4.  Many of the "exo-planet" planetary systems discovered over the last 10 - 15 years have gas giant planets (volatile material compositions) orbiting extremely close to their parent star in extreme contrast to our planetary system where planets with volatile material compositions are found far away from the Sun (see 1.b. above)
Note:  the reason many of the exo-solar planetary systems discovered over the past 10 -15 years are these kind of "hot Jupiter" systems with gas giants orbiting extremely close to their parent stars is a "selection effect"; a Jupiter size exo-solar planet that orbits close to its parent star, say with a period of a few days to a week,  can be confirmed by observations in just a few weeks; but a Jupiter size planet orbiting its parent star at a Jupiter distance (5 AU) will take more than 10 years (12 y for Jupiter) to orbit the parent star and so it might take more than a decade to confirm the existence of such a planet around a star

B. Formation model of the solar system

1. proplyds = proto-planetary disks = solar nebula:  gravitational collapse of dense molecular cloud leads to proto-star with accretion disk containing gas (mostly hydrogen) and dust
a. near proto-star, high temps mean refractatory materials condense, volatiles remain gaseous
b. away from proto-star, low temps mean volatiles also condense (freeze to surface of dust)
2. magnetic braking transfers angular mom. to disk from proto-star, gas molecules and dust particles orbit in approx. circular orbits with small relative velocities
a. planets form in circular orbits with prograde revolution & rotation, all in approx. same plane
b. planets contain more angular momentum than the Sun
3. accretion process: low relative velocities allow colliding particles to "stick together" into larger particles more often than fragmenting into smaller particles
a. average size of particles grow from dust size grains to mountain size planetesimals through the process of accretion
b. larger bodies form and begin to accrete mass at the expense of the smaller bodies ("runaway" process where more mass -> more gravity -> more planetesimals accreted -> more mass, and so on…
c. over ~100 million year time span, most planetesimals have either been swept up into several proto-planets or have been ejected into high eccentricity or parabolic orbits; larger bodies are molten due to heat from accretion and decay of radioactive material, these worlds become differentiated (dense material sinks to center, low density material rises to surface)  Proto-planets beyond the "frost line" (see 1.b. above) grow larger, faster, due to the extra frozen volatile material available and since the hydrogen gas has not been cleared out by the new Sun's solar wind at that distance, the massive proto-planets pull in the gas directly increasing their mass even more producing the so-called "gas giants" beyond the "frost line" (many exo-solar planetary systems that have been discovered have large "gas giant" type planets extremely close to their parent stars, the modification to the model of general planetary system formation to explain these so-called "hot Jupiters" is that the hydrogen gas in the proto-planetary disk is not cleared out by the stellar wind as fast as for the solar system, such that the gas giants that form beyond the "frost line" then spiral inward as they accumulate the hydrogen gas due to fluid resistance from the gas, until they wind up very close to the parent star)
d. interaction of proto-planets shape solar system into final configuration and explains some of the irregularities such as Earth’s large moon, large core of Mercury, etc.
e. gravity of proto-Jupiter "stirs up" planetesimals between proto-Mars and Jupiter causing more fragmentation than accretion -> asteroid belt; the proto-giant planets also eject many (perhaps ½) of the icy planetesimals at their distance from the Sun into high eccentricity, random inclination (not in the ecliptic plane) orbits -> Oort cloud, source of comets; larger icy planetesimals near Neptune are ejected into a "thick" disk of fairly eccentric orbits beyond Neptune  -> Kuiper belt  (Pluto was the largest known Kuiper belt body, but was still considered a "major planet"; more recently another Kuiper belt body was found that is larger than Pluto (see section 5 - Eris) and Pluto has been demoted to a new designation:  dwarf planet,   so Pluto is no longer considered a major planet, and is now the 2nd largest Kuiper belt object known)
WORLDS OF THE INNER SOLAR SYSTEM

VII. The Terrestrial Planets: Mercury, Venus, Earth, Mars, and Earth’s Moon

A. Structure and interiors of the terrestrial planets

1. general structure:
a. chemical layering
i. dense nickle-iron core
ii. mantle of dense rock
iii. thin crust of low density rock
b. physical layering, details depend on planet
i. rigid upper mantle and crust: lithosphere
ii. "plastic" layer of mantle beneath lithosphere: asthenosphere
iii. molten outer core (Earth only?)
c. surface/volume ratio higher for small worlds (goes as 1/radius)-> small worlds have lower heat capacities and lose heat at a faster rate, larger worlds retain the heat of accretion and radio-active decay longer
2. observational evidence for general structure:
a.  mean mass density = average mass density of the planet = (mass of planet)/(volume of planet); combined with assumption that all differentiated terrestrial planets have the same general structure, the relative sizes of the core, mantle, and crust can be estimated using mean density
b.  analysis of seismic waves: only used on Earth and Moon, can provide detailed knowledge of internal structure (see basic properties of waves)
i.  P waves = primary-fastest wave speed, longitudinal waves, propagates through solid or fluid
ii.  S waves = secondary-slower wave speed, transverse waves, does not propagate through fluid
c.  spacecraft flybys: some details of the internal structure may be inferred by analysis of trajectory perturbations
d.  planetary magnetic field strength (Dynamo Model): planetary magnetic fields generated by a) rapid rotation, b) electrically conducting layers in the core or inner mantle, c) sufficient internal heat energy for the conducting layers to be molten or partially molten (liquid state), – some conditions within core may be inferred by presence/absence, strength of planetary magnetic field
3. specific characteristics: largest to smallest
a. Earth: most detailed knowledge from analysis of seismic waves, outer core molten (S wave shadow zones, strong planetary magnetic field), residual heat flows from core to surface by convection currents in outer core, mantle and asthenosphere (plastic part of mantle) driving the interaction of many rigid lithospheric plates at the surface (see plate tectonics below…)
b. Venus: approx. same (slightly less) size and mean density as Earth presumably with similar interior structure, however no evidence of global plate tectonics from surface landforms and virtually no magnetic field (prob. due to slow rotation)
c. Mars: mean density indicates similar relative core to planet size as Earth, size and surface features show arrested development of plate tectonics indicating little residual heat left and very thick lithosphere; also no magnetic field indicates solid core
d. Mercury: mean density indicates extremely large nickel-iron core and little mantle, size and surface indicates little residual heat left in core, however reccent measurements by the Messenger spaceprobe of an asymetric dipole field (3X stronger at the N pole) indicates a still partially molten core, with a high sulfur content giving a lower "freezing" point and creating conditions for the axymetric field;  the large core may be explained by a direct impact of another proto-planet early in its formation process, similar to the formation model of the Moon (see below)
e. Moon: mean density and some seismic study (made possible by the Apollo Moon program) indicates little or no nickel-iron core and 800-900 km thick lithosphere, also surface features indicate geologically dead world for over 4 billion years;  the most favored model of lunar origin is called the Giant Impact Hypothesis which can explain the small or absent nickel-iron core and the volatile depleted surface rocks found by the Apollo astronauts.

B. Terrestrial world surface features: internal tectonic forces verses external impacts

1. general "rule of thumb" on surface feature development for the planets: large worlds retain heat of formation and radioactive decay longer which drives internal tectonic forces that shape the surface long after initial intense cratering rate dies out; small worlds lose their heat early on before the cratering rate falls off and the surfaces of those worlds are shaped more by external impacts
2. determining planetary surface ages (and solar system age):
a. radiometric dating = radioisotopic dating:  using known half-lives of certain radioactive elements, the formation age (time solidified) of rocks can be determined (results using oldest Moon rocks and undifferentiated meteorites:  Solar system age = 4.6 billion years)
b. crater counts = no. craters per unit area: gives relative ages of planetary surfaces; more craters per unit area = older surface  (Note: This link is to a site where you can download an executable screen saver that simulates (and nicely illustrates) this process.  I have downloaded the screen saver from this site and executed it on my system and no problems or viruses have been detected.)
3. specific characteristics: largest to smallest
a. Earth: surface shaped mostly by action of plate tectonics and also by erosion and sedimentation
i. plate tectonics: unifying geological theory which explains almost all geological processes by the interaction of lithospheric plates driven by convection currents in the mantle which are in turn driven by the heat flow from the Earth’s still hot interior; in effect the Earth’s surface in recycled over geologic time periods, especially the ocean crust which is generally less than 200 million years old
ii. plate tectonics and surface features:
mid-oceanic ridge – basaltic (low silica) lava welling up from the mantle causes seafloor spreading
rift valley – usually when continental plates split and begin to move apart, this process leads to the formation of ocean crust and a mid-oceanic ridge
subduction zone – one plate forced beneath another where it melts into the asthenosphere, the re-melted oceanic crust then rises to the surface producing explosive strato-volcanoes emitting more viscous (more silica) andesitic lava; this process repeated produces magma with even more silica content, eventually producing granite which makes up the continental crust
folded mountain ranges – produced by two continental plates converging, nether is subducted and a "train wreck" occurs
fault line – two plates sliding past one another (eg: San Andreas fault)
b. Venus: radar studies of surface indicate no global plate tectonics, small "proto-continent" regions, many volcanoes scattered over most of the surface, and evenly distributed impact craters giving an estimated age of ~500 million years for the entire surface; Russian landers report mostly basaltic rock except near the proto-continents where the composition is more grantitic
c. Mars: one hemisphere heavily cratered with maria similar the Moon, the other hemisphere boasts the largest shield volcano and rift valley in the entire solar system; indicates arrested development of plate tectonics, planet cooled quickly which resulted in a thick lithosphere which could not be broken up into plates by the residual heat
d. Mercury: almost all surface features derived from external impacts, however some volcanism and tectonic activity (rupes = fault cliffs) lasted past the initial intense bombardment
e. Moon: all surface features are the result of external impact, geologically dead for more than 4 billion years; age of solar system derived from radio-isotopic dating of lunar rock samples returned by Apollo astronauts and also from undifferentiated meteorite samples = 4.6 billion years

C. Terrestrial world atmospheres

1. 1st marginal atmospheres may have been H2 from solar nebula, but the 1st robust atmospheres were derived from volcanic out-gassing and icy planetesimal impacts (sometimes called the secondary atmospheres), these so called  secondary atmospheres are thought to have had the same composition for all terrestrial planets:  ~½ water vapor, ~½  carbon dioxide, lesser amounts of nitrogen and other volatiles; modification of this original atmospheric composition through several loss processes has produced the present atmospheres of the terrestrial planets
2. thermal escape: atmospheric "evaporation" due to molecular velocities achieving escape velocity
a. temperature of a gas is proportional to the mean energy of motion (kinetic energy) per molecule or atom of gas: higher temps. mean faster speeds, at a given temp. lower mass molecules/atoms move at faster speed
b. low escape velocity, high surface temp. means rapid loss of planetary atmospheres
c. low mass molecules/atoms lost faster than higher mass molecules/atoms
3. greenhouse effect: planets are in radiation equilibrium, total radiated energy intercepted and absorbed from Sun = total thermal radiation (energy radiated due to temperature) lost to space by the planet – planetary equilibrium temperature is that temperature at which the above equality is true; for typical equilibrium temperatures, the thermal radiation peaks in the IR (infrared); for planets with "greenhouse" gases (molecules which absorb in the IR) in their atmospheres, a large percentage of the IR radiation is "trapped" near the surface which heats the lower atmosphere and surface to much higher temperatures which in turn causes more IR radiation until the surface temperature reaches an equilibrium value such that the fraction of the IR radiation which does escape is equal to the radiation absorbed from the Sun (note: even though this is called the "greenhouse effect", this is not the mechanism which traps heat in a greenhouse!)
4. generic planetary atmosphere structure
a. Troposphere: greenhouse gases trap IR radiation; heated from below by surface warmth, temperatures decrease with height, convection causes "weather"
b. Stratosphere: heated by UV light (Earth’s ozone layer) temperature constant or rise with height eliminating convection (no convection); no UV absorbing molecules = no stratosphere
c. Thermosphere and Ionosphere: X rays heat and ionize gases causing increasing temperature with height
d. Exosphere: heated by solar UV and X rays, high speeds and long free paths (due to very low density) make this the region of greatest thermal escape; Moon, Mercury with no real atmospheres (some atoms due to radiation bombardment of surface and capture of solar wind particles) have only exospheres
      5. specific characteristics, evolution of present atmosphere: largest to smallest
a. Earth: the average surface temperature was cool enough for water vapor to condense into liquid oceans of H2O, the carbon dioxide (CO2) was absorbed (dissolved) into the oceans where it was precipitated out of solution by chemical and life processes and was locked into carbonate rock (e.g.: limestone); this reduced the greenhouse effect (from both water vapor and CO2 – both greenhouse gases) such that liquid water could still exist, leaving a mostly nitrogen (N2) atmosphere; later a sustained level of oxygen (O2) was introduced into the atmosphere by the action of early life-forms, some of which entered the stratosphere where it formed a layer of the triple atomic form of the oxygen molecule known as ozone (O3) which absorbed sterilizing UV radiation and allowed life to colonize land
b. Venus: the average surface temperature was too hot for water to condense and the water vapor and CO2 caused a "runaway" greenhouse effect with surface temperatures soaring higher than even the present 740K average temperature; the water vapor was disassociated ("broken apart") into oxygen which chemically combined with surface rocks and hydrogen which was lost by the process of thermal escape leaving a massive, mostly CO2 atmosphere with a surface pressure of 90 bar (1 bar = surface presure on Earth) and an intense greenhouse effect giving Venus the hottest surface temperature in the solar system
c. Mars: the average surface temperature was warm enough with the greenhouse effect caused by the water vapor and CO2 for water to condense (perhaps into ocean sized regions?); as the CO2 was absorbed (dissolved) into the oceans where it was precipitated out of solution by chemical (and life?) processes and was being locked into carbonate rock the greenhouse effect was reduced, also as Mars cooled internally its nickel-iron core solidified and the planet lost its protective magnetosphere - allowing the solar wind to directly interact with the upper atmosphere slowly stripping the gas away by "bombardment", both these processes continued to reduce the greenhouse effect until the average temperature dropped below the freezing point of water; the atmospheric pressure also dropped to the point that liquid water could not exist and the water vapor left in the atmosphere was disassociated into oxygen which chemically combined with surface rocks (hence the "red" surface rocks of Mars) and hydrogen which was lost by the process of thermal escape leaving a thin, mostly CO2 atmosphere, a portion of which seasonally "freezes out" to form the "dry ice" polar caps over top of the permanent water ice polar caps.  Evidence from the Mars Exploration Rovers have confirmed that Mars once had bodies of liquid water on its surface.  Click here for an archived article on the first evidence confirming liquid water on Mars in the ancient past.
d. Mercury: the high surface temperature and low surface gravity combined with (perhaps) an early major impact event which may have denuded the planet of its 1st robust atmosphere and surface volatiles caused it to lose any atmosphere it had left due to thermal escape, leaving virtually no atmosphere
e. Moon: the warm surface temperature and very low surface gravity combined with (perhaps) its giant impact origin which denuded the bulk material of the Moon of all of its volatiles causing it to lose any atmosphere it had (or acquired later by bombardment of icy planetesimals) due to thermal escape, leaving virtually no atmosphere

 


WORLDS OF THE OUTER SOLAR SYSTEM & SPACE DEBRIS

VIII. The Jovian Planets: Jupiter, Saturn, Uranus, and Neptune

A. Structure and interiors of the Jovian planets

1. general structure:
a.  dense cores of rock, metal and hydrogen compounds; approx. 10 earth masses for all four worlds, prob. molten
b.  mantles of liquid and metallic hydrogen for Jupiter and Saturn, water with a mixture of other hydrogen compounds for Uranus and Neptune (also significant amounts of helium for all the jovian worlds-less than 25% by mass)
c.  gaseous atmospheres of mostly hydrogen with less than 25% helium (by mass), also visible cloud layers made from ice particles or liquid droplets of various hydrogen compounds
      2.  observational evidence for general structure:
a.  mean mass density; combined with assumption that all the jovian worlds have similar cores and bulk compositions, the mantle size, "state" and probable composition can be calculated
b.  spacecraft flybys (see VI.A.2.c.)
c.  planetary magnetic field strength (see VI.A.2.d.)
      3.  specific characteristics: closest to furthest
a.  Jupiter: largest mass and diameter of all major planets, pulled in the most H, He from the solar nebula before the solar wind cleared it out (~300 Earth masses); massive enough for a large portion of the mantle to be metallic hydrogen (metallic hydrogen = liquid hydrogen under extreme pressure takes on the characteristics of metals such as conduction of electricity and heat); liquid hydrogen mantle transitions smoothly to gaseous hydrogen atmosphere (ie: no surface as we know it!); extremely strong magnetosphere due to rapid spin rate, molten conducting material in core and inner mantle, intense interior heat source (gives off ~twice as much energy as received from the Sun)- most likely due to liquid He separating out of the liquid H and residual gravitational contraction
b.  Saturn: least dense of all major planets (0.7 g/cm3, normal density of water on Earth is g/cm3); pulled in only ~75 Earth masses of H, He from the solar nebula, enough for inner mantle to be metallic hydrogen, but gaseous hydrogen atmosphere extends much deeper before becoming liquid hydrogen (texts are unclear as to the presence or absence of a "surface" at the transition point); 2nd strongest magnetosphere for the same reasons as Jupiter (gives off more than twice as much energy as received from the Sun-probably by the two mechanisms mentioned for Jupiter)
c.  Uranus: mean density about the same as Jupiter’s but much less massive indicates that it captured only a few Earth masses of H & He from the solar nebula; its mantle is probably composed of water mixed with other hydrogen compounds (no metallic hydrogen), above this "ocean" surface is a gaseous, mostly hydrogen atmosphere; intense magnetosphere indicates molten conducting core but Uranus only gives off a small percentage more heat than it receives from the Sun (this is expected since the mechanism of He separation is not available for Uranus), also the magnetic dipole axis is far removed from the spin axis (i = 98o, "tilted on its side")
d.  Neptune: interior structure thought to be the same as Uranus’; Neptune also has an intense magnetosphere with a severely offset magnetic axis, however Neptune’s spin axis is more perpendicular to its orbital plain as is the case for most of the other major planets and (surprisingly since the mechanism of He separation is not available), it gives off nearly twice as much energy as it receives from the Sun, perhaps due only to gravitational contraction?

B. Jovian planets’ atmospheres
      1.  all Jovian worlds have clear atmospheres of about 75% hydrogen, 25% helium derived from the solar nebula with visible cloud layers composed of hydrogen compounds which may be "colored" (for Jupiter and Saturn) by trace organic elements produced in the atmospheres
      2.  the "light weight" hydrogen atmospheres of the gas giants have been retained against thermal escape due to the cold temperatures and large escape velocities for these worlds; and while there is greenhouse heating due to trace greenhouse gases in the atmospheres, the main source of atmospheric heating comes from the internal sources mentioned in V.A.3. above; the general temperature structure of the atmospheres are similar with deep tropospheres heated from below mostly by the internal heat of the planets’ cores (rather than solar heating of a surface as is the case with the terrestrial planets), stratospheres where a few minor ingredients in the atmospheres absorb solar ultraviolet photon producing a smog-like haze which makes the color and sharpness of the cloud layers below (this is more pronounced for Saturn and Uranus than for   Jupiter and Neptune), extended thermospheres, ionospheres and exospheres; the scale of the Jovian planet atmospheres is much, much larger than the terrestrial planets, for instance the gaseous hydrogen atmosphere of Jupiter is thought to extend down perhaps 7000 km (10% of the radius) before it transitions to liquid hydrogen while on Earth the highest clouds form on the order of 10 km above the surface
      3.  Coriolis effect: an apparent acceleration relative a reference frame caused by the actual acceleration of the reference frame (for our purposes, the "reference frame" = the spinning "surface" or "layer of atmosphere" of a planet); for a planet spinning counter-clockwise as viewed from above the north pole the Coriolis effect diverts the path of moving particles or parcels of atmosphere to the "right" in the northern hemisphere and to the "left" in the southern hemisphere, the further from the axis of rotation and the faster the rotation of the planet the greater the effect; the Coriolis effect is responsible for surface wind bands on all the terrestrial planets (with atmospheres) and the direction of winds around low and high pressure areas; the extreme Coriolis effect on the gas giants shapes the visible clouds into bands which stretch completely around the planets
      4.  specific characteristics of Jovian planet atmospheres:
a.  Jupiter: strong convection and a pronounced Coriolis effect produce intense wind bands which stretch clouds into light "zones" and dark "belts" with different altitudes and compositions; rising currents in the "high pressure area" zones form white ammonia (NH3) ice clouds complete with ammonia "snow"; the middle layer of "brownish-red"** ammonium hydrosulfide (NH4SH) clouds is visible as the dark belts seen through the clear descending currents off the edges of the zones; the lowest cloud layer hidden below the hydrosulfide clouds is composed of water (H2O) ice or liquid droplets similar the Earth clouds; there are numerous cyclonic type storms visible in Jupiter’s atmosphere including the long lived (>300 year) Great Red Spot, these storms may be driven by wind shear between the belts and zones or perhaps by "hot spots" in the liquid mantle; temperatures at the cloud tops allow for relatively efficient production of trace organic compounds which color some of the cloud layers and storms   **note:  the ammonium hydrosulfide ice crystals are white when produced in the "lab", so the brownish-red color must come from trace elements produced in this deeper cloud layer
b.  Saturn: belts, zones and cloud layer are essentially the same as Jupiter, however colder temperatures reduce the production of trace organic compounds believed to color the clouds and a thicker haze layer mutes the colors and sharpness of the cloud tops that are visible; storms erupt occassionally, but there is no single long-lived storm like the GRS on Jupiter
c.  Uranus: wind bands similar Jupiter and Saturn exist, but the extremely cold temperatures at the cloud tops don’t allow for the production of trace organic compounds to color the clouds; while it is thought that the three cloud layers found in Jupiter’s and Saturn’s atmospheres also exist in Uranus’ atmosphere, they are hidden from view by a layer of methane ice clouds above them; extra methane gas in the atmosphere above these clouds produce the bluish green color associated with Uranus by selectively absorbing red light as sunlight reflects off the deep layer of methane clouds, also (perhaps due to the long seasons that each hemisphere undergoes) there is a thicker photochemical haze which gives Uranus a "bland" look with no visible belts or zones and an overall lighter blue color than Neptune
d.  Neptune: very similar to Uranus’ atmosphere except the haze layer is not as thick giving Neptune a deeper blue coloration; Voyager II images showed a deep (dark) cyclonic storm very similar in relative size and position to Jupiter’s GRS, it was called the Great Dark Spot, but later Hubble Space Telescope images revealed that it had dissipated; Neptune’s atmosphere seems to be more active that Uranus’ (more like Jupiter and Saturn) perhaps due to the more normal spin axis inclination and a similar ratio of heat output to heat input as Jupiter and Saturn (see V.A.3.d above)

    C.  Planetary satellite and ring systems

      1.  Overview:
a.  satellites can be classified as small (< 300 km diam), medium (between 300 km and 1500 km), large (> 1500 km diam) with more than half of all known moons falling in the "small" category; small moons sometimes have unusual orbits (eg: retrograde or very elliptical), are usually non-spherical (potato shaped) and are thought to be captured asteroids, medium and large moons probably co-accreted during the formation of their parent planet (Earth’s large Moon is thought to have formed from a glancing impact of a large planetesimal just after differentiation); almost all moons are tidally locked in a 1:1 spin-orbit resonance
b.  summary:
 
Earth  [1] – 1 large moon
Mars  [2] – 2 small moons
Jupiter [67] – 63 small moons, 4 large moons (as of Oct 2012, most moons in solar system)
Saturn  [62] – 55 small moons, 6 medium moons, 1 large moon
Uranus  [27] – 22 small moons, 3 medium moons, 2 borderline medium-large moons (just bigger than 1500 km)
Neptune  [13] – 10 small moons, 2 medium moons, 1 large moon
Dwarf Planet Pluto  [5] – 4 small moons, 1 medium moon

[Note:  This summary updated Oct 2012.  See this link for the most up to date numbers.  (Check the 3rd row from bottom of the linked fact sheet.)]
      2.  Tidal forces:
a.  differential gravity force – gravity forces on an extended body produce "stretching" and "squeezing" forces relative to the actual "center of force" or point where the overall gravity force can be considered to be acting
b. in terms of orbital mechanics – an extended body orbiting another body must orbit at the orbital speed required at the distance its center of mass is from the center of the body it orbits, however the point on the extended body that is closest to the body it’s orbiting is going slower than necessary and "wants" to fall inward, while the point furthest is orbiting faster than necessary and "wants" to move outward producing the "stretching" force; note that tidal forces tend to elongate a body and then produce a torque on the elongated body that slows its rotation until its rotation period is synchronous with its orbital period (1:1 spin-orbit resonance)
c.  examples of tidal force effects: synchronous rotation of Earth’s Moon, slowing of Earth’s rotational speed, ocean and body tides on the Earth, Mercury’s 3:2 spin-orbit resonance causing "hot poles" on its equator
      3.  specific characteristics of satellite systems:
a.  Jupiter:
i.  Small moons: 8 outer small moons are captured asteroids, 4 inner small moonlets orbit just outside its dust ring, some colored by material ejected from Io’s volcanoes
ii.  Galilean satellites (closest to furthest):
Io – ~size of Earth’s Moon, mostly rocky material, most volcanically active body in the solar system, powered by tidal heating from interaction with next 2 Galilean moons and Jupiter (elliptical orbit caused by interactions with other moons results in constant "squeezing" and "stretching" due to body tides from Jupiter, this gravitationally driven mechanical energy is converted into heat energy in its interior, which is probably completely molten), no impact craters since surface is "repaved" in a short geological time scale;
Europa - ~size of Earth’s Moon, surface is water ice with rocky core, recent Galileo probe images indicate a liquid water ocean below ice crust, relatively smooth ice crust with linear "healed" cracks (probably due to tidal forces), tidal heating keeps interior warm enough for liquid water ocean;
Ganymede – largest moon is solar system, bigger than Mercury, ice crust with large "young" regions devoid of impact craters, Galileo probe detected a magnetosphere indicating a molten nickel-iron core (due to tidal heating?)
Callisto – ~size of Mercury, saturated with impact craters, surface material ice-rock mixture, no tidal heating, Galileo probe measurements indicate it didn’t even undergo differentiation
b.  Saturn:  medium size moons help prove the importance of impact cratering in shaping planetary surfaces; 1 large moon, Titan - ~size of Mercury, only moon with thick atmosphere – mostly nitrogen gas, 1.6 x surface pressure on Earth, dense haze or organic particles and thin cloud layers hide surface but measurements from Voyagers indicate possibility of liquid methane or ethane oceans or seas and organic particulate "rain", Cassini probe will radar map the surface and release a landing probe (2004)
c.  Uranus:  mostly small and medium sized moons show evidence of extreme impact events, some moons may have be completely fragmented and then gravitationally reassembled, perhaps related to possible cataclysmic impact event that caused Uranus’ spin axis to be "tilted on its side"
d.  Neptune:  only 2 moons known before Voyager II flyby (Triton & Nereid), 1 large moon, Triton - ~size of Pluto, thought to be similar in density and composition, strange retrograde, high inclination orbit is evidence that it was captured into orbit, evidence of "ice volcanoes" spewing dark material into a thin nitrogen atmosphere indicates some internal heating perhaps caused by tidal heating after capture as tidal forces circularized its originally (probably) elliptical orbit, can be described as a captured Kuiper belt object
      4.  ring systems:
a.  overview: all Jovian planets have some form of ring system; observational evidence showed that the rings undergo Keplerian rotation and therefore are made up of individual particles, particle sizes range from dust to the size of large buildings and are thought to be composed of ice or ice covered rocky material; all ring systems are within the Roche zone (Roche zone – distance from center of a planet in which the tidal forces are strong enough to overcome the mechanical strength which holds a body together, within the zone a moon or asteroid would fragment
b.  origin and fate of ring systems: ring systems dissipate in a much shorter time than the age of the solar system, so while rings may have formed from material in the Roche zone that could not accrete to form moons, those rings would have dissipated by now; ring particles may be replenished occasionally by close passes of wandering asteroids, but Saturn’s spectacular ring system may be a rare event that we have the pleasure of observing just by chance
c.  specific characteristics of the Jovian ring systems:
Jupiter: dust rings, not visible from Earth, detected by forward scattering of sunlight by the Pioneer and Voyager space probes
Saturn: broad spectacular ring system easily seen from Earth; 6 rings and 2 gaps had been detected from Earth based telescopes, but during the flyby of the Pioneer and Voyager probes hundreds of thousands of ringlets and millions of particles were detected and theorists still can’t explain much of the detailed structure; the ring system is 270,000 km in diameter but only a few meters thick, collisions between ring particles scatter some out of the plane but Saturn’s oblate shape ensures its gravity field has a component which brings most particles back into the plane
Uranus: dark, thin rings with dust between held in place (perhaps) by small "shepherd satellites" whose gravity keeps the particles from spreading out; first detected by a stellar occultation
Neptune: dark, thin rings like Uranus except that they are not uniform, they are denser on one side; also detected by stellar occultation

IX. Pluto, Kuiper belt objects, asteroids, comets and dust

A. Pluto and Charon
      1.  discovery: Uranus, Neptune and Pluto are too faint to be detected by the unaided eye and were all discovered after the advent of the telescope
a.  Uranus - actually, Uranus is visible in dark skies to the eye and was catalogued several times as a faint star before William Herschel discovered it had a disk and determined it orbited the Sun in 1781;
b.  Neptune – after 60 years of observations it became clear that Uranus’ orbit was being gravitationally perturbed by another body, ~1840 John Adams (24 year old student) predicted by calculation the existence of another planet beyond Uranus but he was unable to convince British astronomers to look for it; 1846 Urbain Leverrier (French astrophysicist) independently made the same calculations and was able to convince some German astronomers to look, Johann Galle at the Berlin Observatory pointed his telescope to the predicted coordinates on the night of September 23, 1846 and discovered Neptune within 1O of the predicted position;
c.  Pluto – by the early 20th century, Percival Lowell (and others) thought Uranus was showing effects beyond that caused by Neptune and that Neptune was showing orbital perturbation as well (this was in error, further study has shown no detectable effects) and Lowell predicted the existence of a planet beyond Neptune; after Lowell’s death, the search continued from Lowell Observatory with the task assigned to a young observer named Clyde Tombaugh who discovered Pluto in 1930; as observations continued it was realized that Pluto was much too small to have caused any gravitational perturbations of the giant planets Uranus or Neptune and today it is not considered a major planet; 1978 Pluto’s moon Charon was discovered as a "bump" on the side of Pluto’s star-like image and it was determined that it would begin a series of eclipses with Pluto in the late 1980s; Dr. Jones and two colleagues at CFHT measured the separation distance between Pluto and Charon and the relative brightness of the two bodies to help establish some baseline characteristics for the eclipse measurements; In 2005, 2 other "captured asteroid" type moons , Nix & Hydra were discovered.  Two more moons have been detected since then as well.
2. characteristics of Pluto and Charon: a stellar occultation pinned down the diameter of Pluto and observations of the series of eclipse from 1985 to 1990 determined the diameter of Charon and produced a map of surface markings; Pluto and Charon’s mean densities suggest they are mostly ice with some rock, Pluto is slightly denser than the average outer system small body while Charon is slightly less dense suggesting an impact origin for Charon similar to Earth’s Moon.  In 2005 two more small moons were confirmed to be orbiting Pluto.  See APOD for Nov. 3, 2005.  Recently two more small moons have been detected orbiting Pluto for a total of 5.  See APOD July 22, 2011 for the discovery of the first of the most recent two moons.

NEWS FLASH:  Another Kuiper Belt object (designated 2003 UB313) has been discovered that is probably larger than Pluto.  No official name has been designated for this object and astronomers will now have to decide if it will be considered a 10th major planet or if Pluto will be demoted to a minor planet.  See APOD for Aug. 1, 2005.  (2003 UB313 has been named Eris and is also considered a "Dwarf Planet" and along with Pluto also has the sub-classification of "Plutino")

NEWS FLASH:  It's official, Pluto's not a planet anymore (click here for the song).  The IAU (International Astronomical Union) has designated Pluto as a "Dwarf Planet", but many astronomers are unsatisfied with this definition and plan to take up the debate the next time the IAU meets.  Click here for an article about the demotion of pluto.

    B.  minor planets of the solar system:

      1.  Kuiper Belt: Pluto is best described as the largest known member of a "donut" shaped belt of large icy asteroids which orbit beyond Neptune; these bodies were the larger icy planetesimals of the outer solar nebula and were thrown into their present orbits by perturbations from the proto-Uranus and Neptune
      2.  Oort Cloud: smaller outer system planetesimals were thrown into highly elliptical orbits oriented in a spherical "halo" around the solar system with an average aphelion of 50,000 AU; these asteroid sized bodies (~10 km diam) are the nuclei of comets we observe when one of these objects is perturbed into an orbit which passes close to the Sun

      Oort Cloud diagram

      3.  Asteroids: the inner system planetesimals were more rocky-metallic in composition; in the region between Mars and Jupiter the proto-Jupiter’s gravity prevented those planetesimals from accreting into a planet and caused more fragmentation than accretion, this was the origin of the asteroid belt; asteroids are found from the orbit of Mercury on outward and their compositions become more icy with distance from the Sun as expected from their origin as planetesimals in the solar nebula; debris from the asteroid belt is the source of most meteorites which fall to the surface of Earth as bright fireballs

    C.  Comets, dust and meteor showers:

      1.  Comets: a comet is formed when one of the Oort cloud bodies’ orbits is gravitationally perturbed (even by the tiny force of a nearby star) which causes this icy comet nucleus to fall into the inner solar system; these comet nuclei are made up of ~½ water ice and ~½ CO2 ice (dry ice), plus other frozen gases mixed with dark dust particles and organic materials; when the comet nucleus approaches the Sun, the ices begin to melt directly to their gaseous state (sublime, like dry ice does on Earth) and a large gas (plasma – gas ionized by UV solar radiation) cloud begins to form around the nucleus, the gas erupts in discrete jets rather than subliming evenly over the surface; the dust is liberated as the frozen gases melt and also form a dust cloud around the nucleus, this cloud of dust and gas is called the coma or head of a comet; the solar wind (stream of protons from the Sun) interacts with the gas (ions) and pushes the gas into a long straight tail which points away from the Sun (sometimes called the ion tail, plasma tail or gas tail), the dust is pushed back by the photons of light from the Sun, this dust tail curves gently away from the direction of the Sun (note that even on its way out from closest approach, a comet’s tail always points away from the Sun – NOT opposite the direction of travel of the comet); as a comet heads back out toward its aphelion, the frozen gases stop subliming and the nucleus goes back into deep freeze after losing as much as 1 meter of surface material during its close pass around the Sun; the jets act like steering rockets and can randomly effect the comet’s orbit, sometimes making accurate predictions of its reappearance difficult
      2.  meteor showers and comet dust: in general there is a concentration of dust in the orbital plane of the solar system, much of it left over from the original solar nebula, this materials can be detected from backscattered light from the Sun as the Zodiacal light seen after sunset (in the northern hemisphere) around the beginning of spring and just before sunrise around the beginning of fall, a faint glow of backscattered light can sometimes be detected at the anti-solar point in the sky as well; the dust that is released by comet nuclei as they pass through the inner solar system continues in almost the same orbit but eventually spreads throughout the comets’ orbit; if a comet’s orbit passes relatively close to the Earth’s orbit the Earth may encounter the debris cloud at a certain position in its orbit (ie: a certain day of the year), more than the usually number of dust particles collide at high speed with the Earth’s atmosphere and vaporize on those days producing a meteor shower; the Leonids are a good example - the parent comet is comet Temple-Tuttle which has a 33 year period, the dust is not evenly spread out around the orbit, but is thought to be bunched in denser ribbon-like strands the densest of which follow the comet by about one or two years; every 33 years it is possible for the Leonid shower to produce a meteor storm with rates approaching 100,000 meteors per hour as it did in 1833 and 1966! On off years the Leonid shower is relatively weak with a rate of only 10 to 20 meteors per hour; the comet dust that produces meteor showers is very small and rarely if ever survives the passage through Earth’s atmosphere, usually only debris from asteroids might be large enough to survive atmospheric passage and land on the surface; meteoroid – particle while in space, meteor – the flash of light caused by the passage of the particle at high speed through the atmosphere, meteorite – the rock or particle if it makes it to the surface

      (Check out NASA's Fireball Camera Network for more info about meteors:  http://fireballs.ndc.nasa.gov/  NGAO (UNG's Observatory) hosts one of the cameras, check for station no. 8 in the archive.)

Back to 1st Section