Asteroids Terra For Saturn Mac OS

Asteroids Terra For Saturn Mac OS

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Asteroids, sometimes called minor planets, are rocky, airless remnants left over from the early formation of our solar system about 4.6 billion years ago.
The current known asteroid count is: .
Asteroids are small, rocky objects that orbit the Sun. Although asteroids orbit the Sun like planets, they are much smaller than planets. There are lots of asteroids in our solar system. Most of them live in the main asteroid belt —a region between the orbits of Mars and Jupiter. For example, back during the Mac OS X 10.5.7 beta test in March, AppleInsider reported that the point release was being referenced in some internal circles as 'Juno' or project Juno.
Most of this ancient space rubble can be found orbiting the Sun between Mars and Jupiter within the main asteroid belt. https://savers-free.mystrikingly.com/blog/flightcontrol-mac-os. Asteroids range in size from Vesta — the largest at about 329 miles (530 kilometers) in diameter — to bodies that are less than 33 feet (10 meters) across. The total mass of all the asteroids combined is less than that of Earth's Moon.
Go farther.Explore Asteroids In Depth ›
Key Science Targets
Did You Know?Did You Know?The total mass of all the asteroids in the main asteroid belt combined is less than that of Earth's Moon.
Kid-Friendly Asteroids Greyhound manager 2 mac os.
Kid-Friendly AsteroidsAsteroids are small, rocky objects that orbit the Sun. Although asteroids orbit the Sun like planets, they are much smaller than planets. 
There are lots of asteroids in our solar system. Most of them live in the main asteroid belt—a region between the orbits of Mars and Jupiter.
Some asteroids go in front of and behind Jupiter. They are called Trojans. Asteroids that come close to Earth are called Near Earth Objects, NEOs for short. NASA keeps close watch on these asteroids.
Asteroids are left over from the formation of our solar system. 
Visit NASA Space Place for more kid-friendly facts.
NASA Space Place: All About Asteroids ›Dyls destiny pre-alpha mac os. Neoteria (itch) mac os. Resources
Additional ResourcesChapter IV: Asteroids, Jupiter, Saturn,UranusA) Meteors and Asteroids
1. History
1.1 In 1766 Johann Titius noticed an interesting arithmeticregularity in the distances of the planets. This was presented inan astronomy book published in 1772 by Johann Bode, and has been calledBode'slaw ever since (although more careful citations now call it theTitius-Bodelaw). The law is presented in the table below:1.2 In fact the Titius-Bode law is really an exponential relation,and can be written as 0.3 x 2n + 0.4 n = -∞ 0, 1, 2, .
The question is whether this near exponential relation is real or coincidental. There are many cases of physical processes that give exponentials as aresult, and there have been attempts to see this relation as the resultof an actual physical process associated with the formation of the solarsystem. One example is the vortex theory of von Weizacker,another is the ring theory of Prentice. Similar relationshipscan be found for the orbits of the moons of the giant planets.
1.3 In any case, this relation led to the discovery of Ceres(radius = 512 km), which was later called a minor planet orasteroid. Its distance from the Sun is 2.8 AU; just what would be expected for themissing planet. Other asteroids were soon found and now we know ofseveral classes of asteroids.
2. The asteroid belt
2.1 There is a band at around 2.8 AU between Mars and Jupiterwhere asteroids are found. 
2.2 They range in size from Ceres at 512 km to bodies much smaller. There are about 20 bodies larger than 125 km, some 200 larger than 50 km,and around 4000 larger than 5 km that have been discovered and named. The total mass of these asteroids is less than the mass of the Moon.
2.3 Measuring the size of an asteroid is tricky. Theyare too small for us to resolve a disk, so we must estimate theiralbedo. This is done by measuring the temperature in the infrared and assumingthat the albedo is independent of wavelength. In this case (and assumingan efficiency of 1) the temperature is related to the albedo viaA measurement of the temperature will give the albedo, and this canthen be used to relate the observed brightness to the radius of the object.
2.4 In addition to their albedo, we can measure their brightnessin different wavelength bands. This is a way of defining the asteroidcolor, and is related to their composition.
2.5 There are several composition classes of asteroids distinguishedby their albedo and color. More recently radar reflection studiesprovide information at much longer wavelengths, and even give some informationregarding the structure of theregolith. 
2.6 C-type asteroids are the most common, representing about 75%of the asteroids seen from Earth. They are believed to be made ofcarbonaceous material with many volatiles (water of hydration is observed),and are expected to be among the least evolved objects in the solar system.
2.7 S-type asteroids make up about 15% of the population, andappear to be related to stony-iron meteorites.
2.8 M-type asteroids may be related to nickel-iron meteorites,and may represent the core of a differentiated body.
2.9 Vesta (radius 277km) seems to show an igneous surface, and may have undergone melting inthe past. For such a small body to melt there must have been a ratherintense heat source available (26Al?).
2.10 Asteroids are no longer regarded as remains of a planetthat was destroyed, but rather as a planet that never got a chance to form. They collide with each other and most meteorites that hit the Earth arebelieved to originate in these collisions.
3. Other asteroid populations
3.1Trojanasteroids orbit the Sun in Jupiter's orbit, 60° in front and 60° behind. These are the L4 and L5 Lagrangepoints. We can understand the nature of these points from thefollowing considerations. Think of a small body under the influenceof two much larger bodies in orbit around each other (i.e. asteroid, Jupiter,and Sun). The energy of the small body is simply the sum of its potentialenergy in the field of each larger body, plus its own kinetic energy. It turns out to be more convenient to do this computation in a system wherethe origin is fixed on the center of mass, and the x-axis connects thetwo massive bodies (i.e. the axes are rotating). The potential energyof the small body is then given by3.2 The kinetic energy is mv2/2, but this is not all. We have to add the energy of the rotating system as well. If theframe is rotating with some mean angular velocity n, then the kinetic energyof an object at a distance R from the center will be mR2n2/2. Thus if a body is at rest in the inertial frame, it will appear to havethis energy in the rotating frame. We must therefore subtract itfrom the total. The total energy of the small body is then givenby
This total energy has to be negative for a bound orbit. Definea positive constant C by C = -2E/m. The velocity is then given by
v2 must be positive, but this will be true only if C is smallenough. For each value of C there are limits on r1, r2,and R where the small body is allowed to be. At the edges of theselimited areas, the velocity is zero. The five points of equilibriumare the five Lagrangian points. The other three are between the Sun andthe planet (L1), outside the planet on the planet-Sun line (L2), and outsidethe Sun on the same line (L3). Only L4 and L5 are stable and thisis where the Trojans are found. 
3.3 Amor asteroids have orbits intersecting the orbit of Mars. There are about 100 known.
3.4 Apollo asteroids have orbits that cross the orbit of theEarth. These are the dangerous ones! They may be extinct comets.
3.5Chiron orbitsbetween Saturn and Uranus. It may be connected with the recentlydiscovered Kuiperbelt objects.
Asteroids Terra For Saturn Mac Os 104. Meteorites
4.1 Most meteorites are believed to come from asteroids.
4.2 The most primitive meteorties arechondrites. These are stony meteorites containing millimeter-sized spheres called chondrules. These have a composition similar to that of the surrounding matrix, buthave undergone a brief intense heating event, followed by a rapid cooling. There is no good mechanism for producing the heating in a short time, ofthe right intensity, and affecting so much mass. Many not-so-goodmechanisms have been suggested. 
4.3 Chondrites, because they contain many volatiles, are believedto represent the most primitive solar system material. The abundancesfound in these meteorites are used to help determine solar abundances.
4.4 There are several classes of chondrites (carbonaceous, ordinary,and enstatite). Carbonaceous chondrites are further divided intothree classes (CI, CM, and CV). The CI are the most primitive. Ordinary chondrites are classified by iron content (H, L, and LL). Enstatite chondrites are made almost entirely of enstatite.
4.5 Irons and stony-irons make up the remaining types. There are interesting variations in isotope ratios and minerology thatcontain clues to the conditions during their formation.
4.6 There are SNCmeteorites that are believed to come from Mars. Their compositiontells us something about conditions on the Martian surface. Thereare also meteorites that are believed to come from the moon.
Asteroids Terra For Saturn Mac Os DownloadB) Jupiter
1. Data:Radius - equatorial 71,492 km, polar 66,854 km.
Mass - 1.899 x 1030 g.
Density - 1.33 g/cm3.
Albedo - 0.34
T1bar - 170K
Distance from Sun - 5.2 AU.
Rotation Period - 9 h 55 min
2. How are these measured?
 a. Radius via parallax.
 b. Mass from moons.
 c. Rotation period from:Cloud features, but theseare different at different latitudes. In addition, cloud featuresare affected by local winds, and do not necessarily describe the rotationof the body of the planet.
Magnetic field, butthis is generally only possible from spacecraft. For the case ofJupiter, there was synchrotron radiation measurable which gave the rotationperiod of the body of the planet.
Spectroscopy can also be used to measure the rotation rate. You putthe slit of the spectroscope along the equator of the planet. Thespectral line will be composed of light coming towards the telescope atsome points and going away from it at others. This will give theline some tilt. The angle of the tilt will depend on the speed ofrotation, which, in turn, gives the rotation rate of the planet.
Asteroids Terra For Saturn Mac Os X3. What does the low density tell us about the composition?
3.1 The central pressure is greater than 10 Mbar (much greater)so that the effect of pressure will raise the density of any material considerably. You can show that there must be considerable amounts of hydrogen insidethe planet.
3.2 Spectroscopy does indeed show that there is a great dealof hydrogen in the atmosphere.
3.3 We also see some NH3 and some CH4as well as small amounts of many other compounds.
3.4 The beautiful colors we see on Jupiter are due to smallamounts of possibly organic material probably in aerosols.
4. How do we get hydrogen into a planet?
4.1 One way is to gather up solids and have the hydrogen bein them like in H2O ice or NH3 or CH4 ice.The trouble is that bringing in so much heavy stuff with the hydrogen willraise the density above the observed value.
4.2 Another way is to attract gas gravitationally. Thismeans a fairly large gravitational field and it means that helium gas willbe attracted as well. This has two implications for Jupiter's structure. First there must be a core of heavy material. Second there shouldbe a solar ratio of helium to hydrogen.
4.3 Helium is hard to observe because it doesn't have any rotation- vibration bands, so its spectrum is mostly in the UV.
5. Computing the internal structure:
How can we tell if there is a heavy element core?
We need to make theoretical models of the planet. This meanssolving the equations of planetary structure.
5.1 We have equations for the pressure and the mass, and wecan get an equation of state if we make an assumption about the composition. We still need to know something about the temperature distribution.
5.2 In principle, this can also be computed if we know the methodof heat transport in the interior.If the interior is conducting, and we can compute the conductivity as afunction of pressure, temperature and composition, then we can integratethe equation for conductive heat transport to find the temperature distribution.
If the interior is radiating, and we can compute the opacity as a functionof pressure, temperature, and composition, then we can integrate the equationfor radiative heat transport to find the temperature distribution.
In the late 1960's infrared measurements of radiation from Jupiter (inthe region where the black body curve has its maximum) showed that Jupiter'seffective temperature is 125 K, whereas in thermal balance with the sun,and an albedo of 0.34, the temperature would be 110 K. The explanationis that Jupiter has an internal heat source.
The heat is from the original gravitational contraction of the planet.
Such a high flux of heat can only be transported by a gradient larger thanthe adiabatic gradient. This means that the interior is convectingthrough most of the volume.
Convection is so efficient that it keeps the gradient very close to adiabatic. This allows us to find the temperature distribution without needing tocalculate the conductivity or opacity.
5.3 Computation: 
From basic thermodynamicsdE = TdS - PdVBut for an adiabatic process, dS = 0, so dE = -PdV. If the heatcapacity of the material is C, then the internal energy can be relatedto the temperature by 
dE = CdT
Smartmemorycleaner 2 3 0 8. so
CdT = -PdVThe density is related to the volume and the mean molecular weight by
V = μ/ρ
For an ideal gas Grand fortune casino no deposit bonus codes.
Substituting givesIntegrating givesWe can define a quantitywhich is related to the number of ways energy can be deposited in themolecule. Without taking quantum mechanical details into consideration,C = N0k/2 for each degree of freedom. For pure translationalmotion, there are 3 degrees of freedom, and a = 1.5, for rotation and vibrationthere are additional degrees of freedom and a can be larger. Complicatedmolecules can have a = 3.5. What we have done here is computed anadiabat through an ideal gas. In Jupiter the material is not ideal,so the computation is more complicated, but the idea is the same. Knowledge of the equation of state allows us to compute an adiabat forthe material, and then a variation of temperature with density. Wethus have four equations for the pressure, mass, density, and temperatureas a function of radius. With the appropriate boundary conditionswe can integrate these and get the internal structure.
6. Computing the composition: 
The other problem is to decide on a composition. There are manycombinations of materials that will give the right total mass. Howcan we decide which composition is correct when our only measurements arefrom outside? 
6.1 We can only measure integral properties of the density distribution. One such integral is the mass.6.2 Another possibility is the moment of inertiaThe only problem with this is that there is no good way to measure themoment of inertia. So it is not terribly useful. It turns outthat there is a set of quantities that are related to the moment of inertiathat can give more information:
6.3 When a body is rotating it becomes oblate because in additionto the force of gravity, there is also a centrifugal force. Thisis not trivial to compute, since as the centrifugal force works, it changesthe mass distribution that, in turn, changes the gravitational field, whichaffects the centrifugal force, etc. The problem is nonlinear. There are methods to compute the shape of a rotating body, and its resultantgravitational field by successive approximations. In such a case,instead of the usual inverse distance relation, we get that the potentialcan be written as
where Re is the equatorial radius of the planet, Pnis the n'th Legendre polynomial, q is the colatitude and Jnis the n'th gravitational moment. These moments are integrals ofthe density distribution and can be determined from measurements of theexternal gravitation field of the planet (using the orbits of its satellites,for example). In this way some additional limits can be put on thedensity distribution. In practice, with the Voyager flybys, we have goodmeasurements of J2 and J4, and some information onJ6. A mass sitting on the surface of the planet will notonly feel this gravitational potential, but will also feel a centrifugalforce. This can be written in terms of a potential as well and combinedwith the gravitational term to give
Now the second Legendre polynomial is given by P2(cos θ)= (3 cos2 θ- 1)/2, so
Furthermore, since the Jn's get smaller as n increases, let us limitourselves to the first approximation where n = 1. In this case thepotential becomes
Now the planet will assume a shape such that the surface is an equipotential. In such a case, the shape of the planet can be approximated by
where f is the flattening or oblateness, given by f = (Re- Rp)/Re. If we rewrite this in terms of Legendrepolynomials we get
The potential on this surface is then given by substituting Rsfor r in the expression for the total potential. This gives a verymessy expression, but if we assume that any terms of the order of J2or f are small compared to 1, and terms of the order of J22 orf2 or J2f or higher are negligible, then we can geta much simpler (though approximate) expression which looks like
But since Rs is an equipotential, it cannot depend on θso the second bracket must be exactly zero. This gives a relationbetween J2, f, and ω that must always be satisfied:
Note that the second term is basically the ratio of the centrifugalto gravitational forces. So long as this term is small our approximationworks. In any case, if we can measure any two of these three quantities,we can compute the third.
6.4Results: What we learn is that the density distributioncorresponds to a core of around 5-10 Earth masses, surrounded by an envelopeof hydrogen and helium in the solar ratio with an admixture of about 30Earth masses of heavier material.
7. How do we determine the compositions of the envelope and core?
7.1 We have already seen that for high pressures, where thematerial behaves like a dense fluid, the density of a mixture is givenby Now, suppose I assume that the envelope is composed of a mixture ofhydrogen, helium, and water, and that the ratio of hydrogen to helium issolar. The solar ratio of hydrogen to helium is 2.7 by mass, so XH=2.7 XHe. If there is an additional mass fraction XH2Oof water, then 3.7XHe + XH2O = 1, and we get XH= 2.7 XHe = (1 - .73 XH2O). The density of the mixture then becomes
Now I can compute models with different values of XH2O inorder to find the best fit to observations. This will give the watermass in Jupiter's envelope.
7.2 The difficulty is that the hydrogen term contributes themost to the sum. It has the smallest denominator and the largestnumerator. As a result a small uncertainty in the value of the hydrogendensity will result in a large uncertainty in the water abundance. For example, if, for a given region I need a density of 0.650 at a pressureof 0.5 Mbar, then the densities of H2, He, and H2Orespectively are 0.533, 1.5, and 2.666 g cm-3. This willgive the correct density for the mixture with XH2O = 0.01. But if I made a 5% error in the density of H2, and the valueis really 0.506 then the value of XH2O goes up to 0.042. So a 5% error in the H2 density translates into a factor of4 in the H2O abundance! So you need to be really careful.
7.3 There are several pitfalls. The first is that we assumedthat the H2/He ratio is solar. This needs to be checked.The best check to date is that of the Galileo mission's probe into Jupiter'satmosphere. The results are not completely understood yet, but it seemsthat the H2/He ratio is very close to solar and may even bea bit higher. Possibly some of the helium in the outer atmospherehas sunk to lower levels. The second is that we assumed the thirdsubstance is water. This is not such a serious limitation, sincethe mass of this component will not be substantially affected. Finally,we have assumed that there are no surprises in the equations of state forthese components at high pressure. In fact hydrogen does transformto an atomic phase at a pressure of around 2 Mbar (depending on temperature),with an associated jump in density. This must be treated correctlyin order to assess the abundance of heavier materials in the envelope.
7.4 The core is harder to deal with. It does not contributesignificantly to the moment of inertia (i.e. J2), but it doescontribute to the mass. As a result, we can make a good estimateof the mass of the core, but not of its radius (i.e. density and hencecomposition). The best models for Jupiter indicate a core of about5 to 10 Earth masses. An additional 20 to 40 Earth masses of heavymaterial is mixed into the hydrogen-helium envelope.
8. Jupiter's magneticfield.
8.1 The transformation to atomic hydrogen occurs for the followingreason. The H2 molecule is held together by the electronsthat the two atoms share. When the density is high enough, an electroncan no longer tell which pair of atoms it is associated with, and movesbetween adjacent pairs. As a result the pairs are no longer bound,and the molecules break up. In addition, the electrons move morefreely and the material behaves like a metal.
8.2 This metallic hydrogen has important consequences for theplanet because it conducts electricity. This is one of the componentsof the dynamo needed to generate a magnetic field. Another componentis rotation, and Jupiter certainly rotates . about once in 10 hours. The third component is convection in the conducting region. We havealready seen that Jupiter convects, so all the conditions are there forproducing a magnetic field. Such a field is, indeed, observed.
C) Saturn
1. Data:Radius equatorial = 60,268 km, polar = 54,364 km
Mass = 5.69 x 1029 g.
Density = 0.69 g cm-3
Albedo = 0.34
Year = 29.46 yrs.
Day = 10 h 39 m
Distance from the Sun = 9.53 AU
Internal structure
2. Internal structure:
2.1 In general terms the internal structure of Saturn is verysimilar to that of Jupiter. There is a dense central core of some10 Earth masses, surrounded by an envelope of hydrogen and helium withan additional 20 Earth masses or so of heavy material mixed in.
2.2 There are some differences as well. The H2/Heratio measured for Saturn is much higher than the solar value. Heis present as only 6% of the atmosphere, compared to 24% in Jupiter. This has been explained by the fact that at high pressures hydrogen andhelium are immiscible. It is expected that the heavier helium sankout, leaving an excess of hydrogen in the envelope. This argumentis strengthened by calculations of the cooling rate for the planet. It is observed to be emitting more heat than the models predict. The additional heat source provided by helium rainout would explain thedifference. There is some evidence that the H2/He ratioon Jupiter is somewhat higher than solar. Possibly rain-out has begunthere as well, but because of Saturn's lower temperatures it has progressedfurther there.
2.3 Saturn shows fewer features on its disk. This toois because of the lower temperatures. The temperature in the interioris high, and as you go up towards the surface, you hit regions where thetemperature is low enough so that clouds can form. Since the temperaturesare lower in Saturn, these points occur deeper in the planet, so the cloudsare harder to see.
2.4 In other respects the interior structure of Saturn is similarto that of Jupiter. The core is estimated to be about 20 Earth masses,and there are about 20 to 40 Earth masses of heavy material mixed intothe envelope. Since Saturn's envelope is only 1/3 the mass of Jupiter's,the percent of additional material is higher.
3. The system of rings -themost impressive feature of Saturn. 
3.1Galileo discoveredthe rings in 1610, but it was only in 1656 that ChristianHuygens realized that they were actually rings. In 1857, J.C. Maxwell showed that they could not be solid, since tidal effectswould cause them to break up. If they were stationary, they wouldbe unstable since a small perturbation would bring one side closer to theplanet and the additional pull would cause it to continue in that directionuntil it eventually crashed into the planet. Giving it enough spinto stay in orbit doesn't help, since different parts of the ring have tomove at different speeds to stay in orbit. This is not possible fora solid ring. In fact they must be made up of many smaller bodies.
3.2 As a body approaches a planet, the part nearer to the planetfeels a slightly stronger gravitational pull than the further side. If the body is small, the difference is not large, and the material strengthof the body is enough to keep it together. For larger bodies, thedifference in force is larger, but the body's own gravity helps to holdit together. When a moon-sized body gets too close to the parent,the tidal force can overcome the moon's gravity, and rip the moon apart. This critical distance is called theRochelimit, and depends on the sizes of the interacting bodies, buta good rough guess is about 2.5 times the radius of the planet. 
3.3 The rings are very thin: Only about 2 km thick witha width of some 20,000 km. They are optically thin, and stars canbe seen through them. Spectra show that they consist mostly of waterice particles with sizes ranging from centimeters to meters.
3.4 Originally the rings were labeled A, B, C, and D, with theA ring being furthest out. Voyagerdiscovered an additional E ring, beyond the A ring, and then an F and Grings between the A and the E. Theorder is thus (going outward from the planet) D, C, B, A, F, G, E.
3.5 It was originally thought that mutual collisions betweenrings� particles would keep them spread out, but in 1675 GiovanniCassini discovered an empty region between the B and the A rings, nowcalled the Cassini division. This empty region is at a distance of about 120,000 km (about 2 Saturnradii) from the center of the planet. FromKepler'slaws, the period of an orbit varies like the distance to the 3/2 power,so the ratio of the periods of two bodies at distances R1 andR2, should vary like (R1/R2)3/2. Saturn's moon Mimas sits at a distanceof 186,000 km from the planet, so it circles Saturn in almost exactly twicethe time a Cassini division particle would complete an orbit. Sincesuch a particle would see Mimas in the same place every second orbit, itwould eventually get pulled out of the region by this 2:1 resonance.
3.6 A similar empty region, the Enckegap, was discovered in the A ring itself by Johann Encke in the19th century. This would put it into nearly a 5:3resonancewith Mimas. Until the Voyager flyby of Saturn, this was thought tobe the basic structure of the rings: a thin disk with two gaps caused byresonances with Mimas. Voyager showed that the rings had much morestructure, however, consisting of a very large number of fine ringletswith gaps between them. It was soon realized that one needed a muchmore sophisticated approach to understand their structure.

4. Some ring dynamics:
4.1 Ring particles collide with each other, not just in actualphysical collisions, but also through longer range gravitational interactions.Such collisions redistribute energy and momentum between the particles,and tend to bring all the particles in any particular region closer tosome average value of energy. This tends to circularize the orbits.
4.2 If an orbit is inclined to the equator, this averaging tendsto reduce the inclination to zero. Even those particles that arein orbits that avoid collisions don't do so for long. The oblatenessof Saturn (i.e. J2) causes the orbits to precess, so that theyeventually encounter other particles and lose their momentum perpendicularto the equatorial plane. That is why the rings are flat.
4.3 The presence of small moonlets (called shepherdsatellites) inside the rings also affects the motion ofthe ring particles, and causes them to remain in tightly restricted areas. This is a part of the explanation of the many ringlets that are seen. The rest of the explanation lies in the behavior of groups of particleswhose mutual interactions influence the motion of the group. Thisspiralwave theory, originally developed to explain the structure of spiralgalaxies, has been successfully applied to explain much of the structureof Saturn's ring system.
4.4 There are still some things not completely understood. Thebraided structure of the F-ring is one example. Electric chargingeffects on the dust, due to UV radiation or interactions with the magnetosphereis another example.
D) Uranus
1. StatisticsRadius: equatorial = 25,559 km, polar = 24,973 km
Mass = 8.66 x 1028 g
Density = 1.27 g/cm3
Albedo = 0.30
Year = 84.01 yr
Day = 17h 14m
Distance from Sun = 19.2 AU

4. Some ring dynamics:
4.1 Ring particles collide with each other, not just in actualphysical collisions, but also through longer range gravitational interactions.Such collisions redistribute energy and momentum between the particles,and tend to bring all the particles in any particular region closer tosome average value of energy. This tends to circularize the orbits.
4.2 If an orbit is inclined to the equator, this averaging tendsto reduce the inclination to zero. Even those particles that arein orbits that avoid collisions don't do so for long. The oblatenessof Saturn (i.e. J2) causes the orbits to precess, so that theyeventually encounter other particles and lose their momentum perpendicularto the equatorial plane. That is why the rings are flat.
4.3 The presence of small moonlets (called shepherdsatellites) inside the rings also affects the motion ofthe ring particles, and causes them to remain in tightly restricted areas. This is a part of the explanation of the many ringlets that are seen. The rest of the explanation lies in the behavior of groups of particleswhose mutual interactions influence the motion of the group. Thisspiralwave theory, originally developed to explain the structure of spiralgalaxies, has been successfully applied to explain much of the structureof Saturn's ring system.
4.4 There are still some things not completely understood. Thebraided structure of the F-ring is one example. Electric chargingeffects on the dust, due to UV radiation or interactions with the magnetosphereis another example.
D) Uranus
1. StatisticsRadius: equatorial = 25,559 km, polar = 24,973 km
Mass = 8.66 x 1028 g
Density = 1.27 g/cm3
Albedo = 0.30
Year = 84.01 yr
Day = 17h 14m
Distance from Sun = 19.2 AU
2. Atmosphere
2.1 Uranus atmospheredisplays very few features, and it has an essentially featureless disk. As a result it is very difficult to measure a rotation rate. We havealready seen three methods for measuring the rotation rate. Timingfeatures as they cross the disk is not practical for Uranus, since we cannotresolve any features from the ground. Instead one can use photometryto measure the overall brightness of the disk. If there are any features,even though we cannot resolve them, they will affect the total brightness,and by monitoring this over time we can hope to measure a periodic changefrom which the rotation period can be derived. Early attempts atsuch measurements around 60 years ago yielded a period of about 11h. Spectroscopic observations by Moore and Menzel in 1929 also gave a periodof 10.8h, and this was the number quoted in the literature for many years.
2.2 Above, we see still another method for measuring the rotationperiod based on measuring the flattening and J2. The difficultyis that the flattening is also difficult to measure, since it involvesthe difference between two numbers that are quite close to each other. Observations of the oblateness by stratoscope balloon-bearing telescopesindicated a period of about 18h. A number of attempts were made tomeasure the period after 1974, but the results varied widely (12h to 24h). The current value comes from the magnetospheric measurements of Voyager.
2.3 Spectroscopy has revealed that the atmosphere consists ofhydrogen and helium in solar proportions, with a rich admixture of methaneand ammonia.
3. Interior
3.1 There are no completely satisfactory models of Uranus' interior,but most models agree that there is a deep atmosphere overlying a planetmade up mostly of a mixture of compounds of C, N, O, Si, Fe, and otherelements common in the solar system. These may be roughly classifiedas ice and rock.
3.2 Like Jupiter and Saturn, we would expect Uranus and Neptuneto have internal heat sources. Neptune does, but Uranus doesn't. The reason is not completely clear, but it may be due to the fact thatUranus has cooled enough so that it is now in equilibrium with the Sun.
3.3 Uranus has a magnetic field that is tilted from the rotationaxis by 60°. The reason for this is not clear either, but may beconnected with the fact that the field-forming region is not at the centerof the planet, but in a conducting shell. The conducting materialmight be something like H3O+, NH4+,and similar ions.
3.4 Uranus has its own systemof rings (as does Jupiter). They too are narrow and mediatedby shepherd satellites. They are also of extremely low reflectivity;some tens of centimeters in radius and on average a few meters apart fromeach other.