IAL 10: Solar System Formation

Don't Panic


  1. Introduction
  2. The Nebular Hypothesis
  3. Radioactive Dating
  4. Collapse to the Protoplanetary Disk
  5. Condensation
  6. Growing Planetesimals and the Streaming Instability
  7. Gravitational Accretion
  8. The Formation of the Gas Giants
  9. The End of Planet Building
  10. Leftovers I: Asteroids
  11. Leftovers II: Rocky-Icy Bodies
  12. What Happened to the Larger Rocky Bodies?
  13. Chemical Differentiation
  14. Cratering by the Heavy Bombardment
  15. Flooding of Basins by Lava
  16. Continuing Geological Activity
  17. Solar System Timeline

  1. Introduction

  2. The Solar System is our planetary system---the planetary system of the Sun which is Sol in Latin.

    The Solar System formed long, long ago, but in galaxy very, very nearby---our own Milky Way.

    To summarize the story of Solar System formation, we can start with a look at a Timeline of Solar System evolution.

    And as a preview of protoplanetary disks, see the figure below (local link / general link: protoplanetary_disk_beta_pic_2.html).

    1. Understanding Solar System and Evolution:

      Now the Solar System formation and evolution involved a large random element that CANNOT be predicted from general understanding of planetology:

      1. The initial molecular cloud fragment from which it formed had random features---of which we know little.

      2. There may have been random encounters with other stars.

      3. Collisions of nebula clumps and, later, of formed bodies were somewhat random.

      4. We CANNOT postdict the random element completely either. Much of evidence has been erased by the random element itself.

      So a detailed, exact history of the Solar System is NOT likely ever.

      On the other hand, there are lots of clues about how things occurred and a PLAUSIBLE SCENARIO and many more details are being worked out all the time.

      We can also learn something by comparison to other planetary systems in formation or fully established because we observe:

      1. Protoplanetary disks which form as discussed in IAL 21: Star Formation.

        See the figure below (local link / general link: nasa_orion_nebula_001.html) showing numerous protoplanetary disks.

      2. The confirmed 3992 planetary systems (of exoplanets) as of 2023 Jun23 (see The Extrasolar Planets Encyclopaedia).

    2. Planetary Systems in General:

      First, planet formation is a robust process. In star formation, planet formation happens of order half the time??? or more often. This has become clear in the age of planetary system discovery beginning with the discovery of 51 Pegasi b (discovered 1995), the first discovered ordinary exoplanet (i.e., exoplanet NOT a pulsar planet).

      Second, the exoplanet planetary systems have turned out to be a diverse and most rather different from the Solar System.

      They have massive Jupiter-size planets (which are also gas giant planets like Jupiter) often with very ECCENTRIC ORBITS with semi-major axes (i.e., mean orbital radius about their parent star) going down to less than 0.1 AU.

      The very small semi-major axes shows that some exoplanets have orbits of only a few days.

      The technique for finding exoplanets is strongly BIASED toward finding large planets close to their stars, and so the sample discovered so far may NOT be average---but on the other hand planetary systems unlike the Solar System are clearly NOT RARE.

      Some of these gas giant exoplanets are in the habitable zone where liquid water can exist---and so life as we know it can exist---but probably NOT on gas giant exoplanets---but maybe on their exomoons. See the artist's conception of exomoons in the figure below (local link / general link: exomoon_habitable.html).

      Earth-like exoplanets in the habitable zone exist and will probably be definitively discovered soon. There is a Wikipedia: List of potentially habitable exoplanets some of which may well be habitable planet, but is NOT the same has being habitated: i.e., having extraterrestrial life. Proving extraterrestrial life exists is a more difficult problem than merely showing potentially habitable.

      We will discuss exoplanets in IAL 18: Exoplanets & General Planetary Systems.

      For more on exoplanets, see the following sites:

      1. Interactive Extra-solar Planets Catalog (The Extrasolar Planets Encyclopaedia): There are histograms of the statistics of exoplanets: number per year of discovery, number per mass, number per semi-major axis, number per period, etc.
      2. Exoplanets.org: the California Planet Search site: This is the site is NOT being updated regularly since 2018 Jun30.

    3. Our Rare Solar System:

      The distribution of planetary systems is now known to be very broad and multi-dimensional. So although in a sense there are average planetary systems, very few planetary systems are actually much like those averages and all types planetary systems are probably rare in the sense that they are only a small fraction of the whole distribution.

      Our planetary system, the Solar System is probably rare in the sense just given, maybe very rare.

        Question: If the Solar System is rare, maybe very rare, what may explain some of the peculiarities?

        1. Just chance.
        2. If some of those peculiarities didn't exist, complex life in Solar System would have been precluded.

        Answers 1 and 2 may both be right I think.

        But remember, we know the Solar System is rare in a sense, but NOT how rare.

      The explanation of odd coincidences in physics or the universe by saying that if they were NOT, we would NOT be here is called the anthropic principle (see IAL 0: A Philosophical and Historical Introduction to Astronomy: The Anthropic Principle for more on the anthropic principle).

      To give an example of anthropic principle which is NOT hypothetical consider that the Earth is probably a fairly rare kind of planet even if the Solar System is a pretty normal planetary system:

      1. It exists just far enough from its star to have abundant liquid water: i.e., it is in the habitable zone.

      2. It is massive enough to have enough gravity to hold onto an atmosphere over gigayears.

      3. Its orbit is nearly circular, and so there are NOT wild yearly swings in climate from way below 0°C to way above 100°C.

      It's NOT surprising that we on this rare kind of planet since we wouldn't exist here if it weren't this rare kind of planet---O Rare Earth!

      As we will discuss below many RANDOM EVENTS went into determining the planets.

      On that basis alone, it is clear that Earth probably belongs to a fairly rare class of planets in the universe: maybe very rare.

      Is it just chance that we evolved on this rare kind of planet?

      No. Complex life as we know it probably requires all the Earth features listed above. Thus, the explanation of the Earth features listed above is the anthropic principle.

      The anthropic principle is discussed at greater length in IAL 0: A Philosophical and Historical Introduction to Astronomy: The Anthropic Principle. Recall some people think the anthropic principle is profound; some think it is trivial; some that it has scientific merit; some that it does NOT: many never think of it at all---but that's true of a lot things in this course.

    4. Our Focus:

      Whatever the vagaries of exoplanet planetary systems, in this lecture we will focus on OUR Solar System and its story as far as it is known---and there still are many uncertainties.

      But in broad outline the story seems fairly robust.

  3. The Nebular Hypothesis

  4. The nebular hypothesis is the basic theory of planet formation both for the Solar System and other planetary systems.

    1. The History of the Nebular Hypothesis:

      The idea that the Solar System formed from a cloud in space (i.e., a nebula) probably has been suggested in myth: I sort of vaguely think I've read this.

      Certainly, the ancient Greek atomists posited a sort of nebular hypothesis---but they were flat Earthers (Fu-140).

      See the imaginative portrait of the atomist Democritus (c.460--c.370 BCE) in the figure below (local link / general link: democritus.html).

      In the context of the
      Newtonian physics, the nebular hypothesis was first made in the 18th century by Immanuel Kant (1724--1804) (see figure below: local link / general link: immanuel_kant.html) and Pierre-Simon Laplace (1749--1827), one of the great mathematical astronomers (see Wikipedia: Nebular hypothesis: History; No-406).

    2. The Basic Nebular Hypothesis:

      The basic nebular hypothesis is:

      1. The Sun forms from a cloud (i.e., a nebula) that has collapsed under self-gravity and heated up from contraction to the point where hydrogen burning starts in the core.

      2. A protoplanetary disk of gas and dust forms around the protostar Sun with the gas and dust all rotating the same way.

      3. The planets form out of the protoplanetary disk.

        Question: What clue suggests that the protoplanetary disk of gas and dust was all orbiting in the same direction?

        1. The fact that the Sun has a magnetic field.

        2. The fact that the planets all revolve in the same direction and mostly rotate in that direction too: the direction is counterclockwise as view from the north ecliptic pole.

        Answer 2 is right.

    3. Salient Facts Explained by the Basic Nebular Hypothesis:

      The basic nebular hypothesis explains some salient facts---here we give the facts and leave explanations to later mostly:

      1. Why the planets, moons, and asteroids are nearly confined to a plane and most rotate and revolve counterclockwise as viewed from the north ecliptic pole. The plane is, of course, the ecliptic plane.

        There are some clockwise rotators and revolvers and bodies with odd inclinations of rotation and/or revolution. These can be explained by random collisions late in the formation process or in the course of Solar System. Halley's comet is an example a clockwise revolver (see the video Halley's Comet Orbital Path).

      2. Why there are two main types of planets: rocky planets (or terrestrial planets) and gas giant planets.

          The COMPOSITION DIFFERENCE between the types is reflected in their average densities.

        • The rocky planets have densities of 3--6 g/cm**3.

        • The densities of the gas giants are less than or about 1.7 g/cm**3.

        • Remember water has density 1 g/cm**3 and iron has density of about 8 g/cm**3.

        We'll see how the planet types follow from the nebular hypothesis and admit that planetary migration could change the simplest case prediction, but did NOT in the case of the Solar System.

        Planetary migration is needed to explain many planetary systems and probably played a role in the Solar System too. We will largely skirt planetary migration since it's a complex business and is far from fully understood.

        Below are Planetary system formation videos (local link / general link: planetary_system_formation_videos.html) that give illustrations of planetary migration.

      3. Why and where are the Solar System minor bodies---the main ones being asteroids, meteoroids, Centaurs, trans-Neptunian objects (TNOs), and comets.

      4. Why there is a common age of the Solar System which is about 4.6 Gyr. This is known from radioactive dating (which we discuss just below in the section Radioactive Dating) from Earth, Moon, and meteorite rocks and from modeling in the case of the Sun.

      Given that we see jillions protoplanetary disks in star formation regions, we know that the basic nebular hypothesis is correct for the Solar System and throughout the observable universe.

      There is, of course, a great deal of uncertainty about many aspects: qualitative and especially quantitative aspects.

      See the ALMA images of protoplanetary disks in the figure below (local link / general link: protoplanetary_disks_alma.html).

    4. Planetary System Formation Videos:

      See Planetary system formation videos below (local link / general link: planetary_system_formation_videos.html):


  5. Radioactive Dating

  6. The age of the Solar System can be accurately determined from radioactive dating (AKA radiometric dating).

    It's NOT as exciting as it sounds.

    But it is worth a digression to discuss radioactive dating and radioactive decay.

    1. Radioactive Nuclides:

      Radioactive isotopes (AKA radionuclides, AKA radioactive nuclides) are unstable atomic nuclei are unstable: i.e., they change into other nuclei SPONTANEOUSLY. This change process is called radioactive decay.

      A new nucleus formed from a radioactive decay is called a DAUGHTER. If it is STABLE, it will NOT itself SPONTANEOUSLY decay.

      When a radioactive decay occurs energy is also released. Initially, this energy is mostly in the form of dangerous ionizing radiation: i.e., gamma rays (very high energy photons) and fast particles with large kinetic energies. Ionizing radiation can damage organic material.

      The fast particles are usually beta particles and alpha particles.

      The names beta particle and alpha particle are just traditional from the early history of research into radioactivity.

      Beta particles are electrons or positrons. They are dangerous because they are energetic enough to be ionizing radiation. Also the positrons sooner or later annihilate with electrons in the surroundings to create gamma rays.

      The beta particles are NOT from the electrons bound to atoms. They originate in radioactive decay in atomic nuclei.

      Alpha particles are the nuclei of atoms of Helium-4 (He-4).

      A cartoon illustrating radioactive decay is shown in the figure below

      In dense environments, the radioactive decay energy is usually rapidly converted into heat energy.

      We say the energy is THERMALIZED.

      Thermalized radioactive decay energy helps to drive the geology of the Earth as we'll discuss in Intro-Astro Lecture 11: The Earth.

    2. The Origin of Radioactive Nuclides:

      Supernovae (giant explosions of massive stars) produce radioactive isotopes during the explosion phase and then spew them out into interstellar space where they can be incorporated in star formation regions.

      See Supernova videos below (local link / general link: supernova_videos.html):

      Thus from supernova remnants, long-lived radioactive isotopes got incorporated into the Solar System and there is still a lot of them around (i.e., those that live for gigayears).

      Also cosmic rays continuously produce radioactive isotopes: e.g., carbon-14 which is used in dating organic materials (SWT-644). See the figure above.


    3. Radioactive Decay is a Random Process:

      The radioactive decay of radioactive isotopes is a RANDOM PROCESS.

      Any given radioactive nucleus may decay in a second or in a billion years. There is no way to tell even in principle according to quantum mechanics.

      But radioactive isotopes of a given species do have a MEAN LIFETIME (which comes in two versions that differ by a constant).

        Question: What is the standard version of mean lifetime used in radioactive decay work called?

        1. A full-life.
        2. A half-life.
        3. A quarter-life.

        Answer 2 is right: half-life.

      Lets consider what happens to a sample of uranium-238 (U-238) as it decays to stable lead-206 (Pb-206) and how the sample can be used in radioactive dating. See the figure below.


        Caption: A cartoon of radioactive decay of a sample uranium-238 (U-238) as it decays to stable lead-206 (Pb-206). We explicate below how the sample is used in radioactive dating.


        1. The half-life for this process is 4.468 Gyr (see Wikipedia: Uranium-238: Side table).

          Note that there are many intermediate, unstable daughter nuclides in the radioactive decay process. The 4.468 Gyr is the net half-life.

        2. Emphasis: the radioactive decay an intrinsically random process as quantum mechanics. A given radioactive isotope nucleus can decay in a fraction of a second or in a gigayears. There is NO way of telling in principle.

        3. However, a sample of radioactive isotope will have an average radioactive decay behavior. After every half-life, half the surviving nuclei at start of the half-life period will have decay---but only on average.

          If there are many nuclei, the actual amount of decay is very nearly the average behavior

          If there are few, then the could be large fluctuations from the average behavior.

        4. What happens when the predicted remaining number of radioactive nuclei is less than 1?

          Say you started with a large number of identical samples of the radioactive nuclei.

          At the time when the predicted number for each is less than 1, then some samples will have zero radioactive nuclei, some will have 1 radioactive nucleus, and some will have 2 or more.

          The predicted number of surviving radioactive nuclei never reaches zero, but eventually in overwhelmingly most samples there will be zero remaining radioactive nuclei.

          But a small and ever decreasing set samples will continue to have undecayed nuclei---in principle forever, if the theory of radioactive decay is exactly right which it may NOT be---but it is very accurate---but there are always limits beyond which any theory is untested.

        5. The radioactive decay process just outlined is subject to some perturbations (e.g., that can alter the half-life), but those are usually negligible.

        6. If you knew your sample was pure U-238 at some TIME ZERO, then just by measuring the current Pb-206/U-238 ratio you know the time since the TIME ZERO.

          Determing that time is radioactive dating.

        7. Of course, nothing is quite that simple, but often geologists and geophysicists can deduce the original abundance of a radioactive nucleus in a rock sample.

          The original abundance being the abundance when the rock formed out lava or magma. In molten environments, there is often some chemical separation between a radioactive nucleus and its daughter nuclide that allows the time since TIME ZERO to be established.

          Nevertheless, some of the daughter nuclides are often present at the time of formation of the rock sample.

          Somehow that can be accounted for.

          Credit/Permission: © David Jeffery, 2003 / Own work.
          Image link: Itself.

    4. Class Exercise:

      Everyone take a coin (or pen) out their pocket and stand up while the lights are turned up. For a pen, heads is when tip is away from you, tails is when its toward you. It is now TIME ZERO:

      1. It is now 1 half-life later. Everyone flips their objet: heads keep standing, tails sit down.
      2. 2 half-lives Everyone still standing flips their objet: heads keep standing, tails sit down.
      3. 3 half-lives Everyone still standing flips their objet: heads keep standing, tails sit down.
      4. 4 half-lives Everyone still standing flips their objet: heads keep standing, tails sit down.
      5. 5 half-lives Everyone still standing flips their objet: heads keep standing, tails sit down.

      Enough. After 5 half-lives, only about 1/2**5=1/32 of the class would be standing.

      It usually won't be exactly that because of statistical fluctuations.

      But if we repeated the demonstration many times, after 5 half-lives the average number standing would be 1/2**5=1/32 of the class---if the coin flips had exactly 50 % probability of being heads or tails---which isn't exactly true, but is probably pretty darn close.

    5. The Age of the Solar System:

      U-238 and other very long-lived species can be used for dating the Solar System.

      Here is a summary of such radioactive dating work:

      1. The oldest known Earth rocks/minerals are zircon crystals from Western Australia that are dated to 4.4 Gyr (Wikipedia: Oldest dated rocks: Oldest terrestrial material).

        Because of geological activity older Earth rock/minerals have been mostly or nearly entirely destroyed. Most Earth rock is less than 1 Gyr old (Lissauer-132).

      2. The oldest known Moon rock---which was brought back from the Apollo missions---is 4.48 Gyr (Wikipedia: Moon rock; Se-417).

        Geological activity on the Moon nearly stopped a long time ago. Still it did happen once, and so 4.48 Gyr may be younger than the Moon.

      3. The oldest known meteorite rock is 4.55 Gyr (Wikipedia: Oldest meteorites). It is likely that this is as old as the formation of the Solar System.

      4. The theoretical age of the Sun from modeling is somewhat uncertain, but is consistent with 4.6 Gyr.

      The upshot is that we think the age of the Solar System is very nearly 4.6 Gyr = 4600 million years (Wikipedia: Formation and evolution of the Solar System: Timeline of Solar System evolution).

      The Solar System probably took of order 50 million years to form from the primordial nebula---but that is a relatively small time compared to the time since (Wikipedia: Formation and evolution of the Solar System: Timeline of Solar System evolution).

    6. Planetary System Formation Videos Redux:

      See Planetary system formation videos below (local link / general link: planetary_system_formation_videos.html):


  7. Collapse to the Protoplanetary Disk

  8. In IAL 21: Star Formation, we discussed the collapse of a molecular cloud fragment a stars and protoplanetary disk.

    We recapitulate a bit here, but go further.

    1. Why Protoplanetary Disks?

      The rotational kinetic energy of the original cloud is primarily responsible for the protoplanetary disk.

      Secondarily responsible that clumps of gas and interstellar dust have large cross sections for body-on-body interactions---unlike stars which super-rarely undergo stellar collision though they interact gravitationally at long distance.

      Protoplanetary disk formation is illustrated in the figure below (local link / general link: protoplanetary_disk_formation.html).

      To recapitulate: there are swirling in streams/clumps of gas and dust.

      Collisions of gas streams or clumps tend to cancel the opposing momenta since kinetic energy is lost to heat energy on such collisions.

      The loss of kinetic energy means the streams or clumps can't just reverse their momenta on leaving the collision. Consequently, the streams or clumps tend to leave such collisions on more similar orbits or perhaps even stuck together.

      The collisions keep happening until nearly all the material is moving in the same direction in circular orbits. When this happens collisions are minimized.

      We can say the material has relaxed to a very regular form---the protoplanetary disk. See the cartoon of a protoplanetary disk in the figure below.

        The relaxation of messy, turbulent flow can be demonstrated by a cup of coffee (HI-271). Stir black coffee vigorously, but randomly, and then wait a bit and add milk. The milk will usually show a smooth rotation in only one direction. The random eddies and flows have been damped out.

        Well this never seems to work well for yours truly, but textbooks assure yours truly that it is convincing.

      The exact orientation of the protoplanetary disk is determined by the overall angular momentum of the material that formed it. Angular momentum is a vector that points perpendicularly to the overall rotation sense. That direction is largely maintained by collapse to a protoplanetary disk.

      Actually only a fraction of the infalling material ends up in the protoplanetary disk: most ends up in the protostar or is expelled from the system by strong winds from the protostar or young star.

      The formed protoplanetary disk will probably NOT be perfectly regular. There will be clumps of gas moving in irregular orbits: i.e., eccentric and somewhat non-planar orbits.

      Further collisions and self-gravity probably act to smooth out irregularities. On the other hand, there are effects that prevent flattening to extreme thinness.

      One needs to remark that axisymmetric gravitational perturbations due to the central source of gravity and magnetic field effects can can also help in disk formation????.

      Protoplanetary disks would form without those effects, but their qualitative nature would be different.

    2. Protoplanetary Disks are Hot:

      The protoplanetary disks will be hot for two reasons:

      1. The gravitational potential energy of the primordial solar nebula was partially converted into macroscopic kinetic energy by collapse and this was then converted heat energy by the nebula material crushing together.

        This heat energy from infall is an intial condition.

      2. The EMR from the protostar Sun will heat the protoplanetary disk.

        This effect is continuing although somewhat varying as the young star settles onto the main sequence.

        Recall the protostar Sun is actually more luminous than the zero-age main-sequence (ZAMS) Sun due to the heat energy from gravitational contraction.

      At some time much of the solar nebula was probably over 2000 K (HI-274).

      At temperatures above 2000 K refractories like iron and silicates will mostly in the gas phase.

      All volatiles will be in gas phase at these temperatures, of course.

      Thus, the PRIMORDIAL DUST of the primordial solar nebula was evaporated at least in the inner region of the protoplanetary disk.

      The DUST-FREE HEATED DISK GAS was probably pretty homogeneous at least in the inner protoplanetary disk which became the inner Solar System of rocky planets.

  9. Condensation

  10. As time passed, the protoplanetary disk cooled from its early hot state by emitting as electromagnetic radiation (EMR) the heat energy from of gravitational contraction and because the Sun became less luminous as it settled onto the zero-age main sequence (ZAMS).

    Then condensation started in the protoplanetary disk.

    1. Condensation:

      We need some more thermodynamics to understand condensation.

      Condensation is an atom by atom or molecule by molecule growth of solid or liquid phase sample from a gas phase sample.

      For reference, the figure below (local link / general link: phases_major.html) illustrates the nomenclature of the main phases of matter and the main phase transitions.

      liquid phase is actually a rather delicate matter phase and only occurs under rather limited conditions of pressure and temperatures as illustrated by water in the phase diagram in the figure below (local link / general link: phase_diagram_water.html).

      everyday life example of a substance without a liquid phase at ordinary pressures (see Wikepdia: standard ambient temperature and pressure (SATP or STP, T=298.15 K=25 C, P=100 kPa=14.504 psi)) is carbon dioxide (CO_2) as illustrated in the figure below (local link / general link: co2_ice.html).

      Actually, most
      astronomers, except planetologists, never encounter the liquid phase in their research.

      Probably for that reason, astronomers have simplified the phase change terminology and use condensation and vaporization/evaporation for, respectively, deposition and sublimation in addition to their conventional meanings in thermodynamics given in the figure above (local link / general link: phases_major.html).

      Hereafter, yours truly follows that simplified terminology---except maybe when yours truly has be more precise.

    2. When Is Condensation?

      Condensation is only possible when the atoms or molecules DO NOT have enough thermal kinetic energy to free themselves from chemical bonds with each other: i.e., only when the temperature is low enough.

      Condensation also depends on density in the sense that the rate of condensation increases with increasing density.

        If the density is too low, then even at very low temperature condensation won't happen because the atoms can't find each other to cohere.

      So condensation depends temperature and density: temperature has to be low enough; density has to be high enough. There is a trade-off between the two controlling parameters, of course.

      For different substances, there are different condensation temperatures and densities.

    3. Condensation in the Protoplanetary Disk:

      In a protoplanetary disk, the condensates are GRAINS that build up atom by atom or molecule by molecule.

      Note the pressures in the protoplanetary disk are too low for the liquid phase. So the condensation is direct gas-phase-to-solid-phase condensation.

      A cartoon of the temperature structure for a representative protoplanetary disk (which could be the Solar System protoplanetary disk) near the end of the condensation phase is given in the figure below.

    4. Condensation in the Inner Solar System:

      The inner Solar System (i.e., here meaning inward ∼ 2.7 AU in the asteroid belt and NOT Jupiter as is usually meant) NEVER got cold enough for volatiles to condense in space into grains (see Wikipedia: Frost line (astrophysics): Current snow line versus formation snow line). After all it's NOT cold enough now in this region.

      Thus, the volatiles did NOT contribute much to planet formation in the inner Solar System and only exist as traces on the inner planets.

      The grains in the inner Solar System were probably mainly iron and silicates (i.e., rock). We have samples of grains that probably formed in the primordial solar system in carbonaceous chondrites (primitive meteorites). These grains are are sub-micron in size scale (HI-274--275).

      Some volatiles (e.g., H, He, N) were locked up in the condensates of refractories: NOT much by mass, but very necessary for life on Earth.

    5. Condensation in the Outer Solar System:

      From in the asteroid belt at ∼ 2.7 AU outward is the frost line (AKA snow line) as it was in the planet formation era (see Wikipedia: Frost line (astrophysics): Current snow line versus formation snow line).

      Here volatiles (as well as refractories) did condense and contribute to planet formation. As we'll see in the section The Formation of the Gas Giants, the condensation of volatiles may or may NOT have been vital to the formation of the gas giant planets.

      The most volatile gases H_2 and He and other noble gases would NOT condense at all in the inner Solar System and NOT much in the outer Solar System either it is thought.

      But the hydrogen and helium gas were strongly gravitationally accreted by the gas giants and made them grow so big. The gas giants are so called, NOT because they are made of gas, but because they are largely made of up of elements (hydrogen and helium) that are gases on the surface of the Earth.

      If the volatiles were NOT important to the formation of the gas giants, probably formation beyond the snow line (AKA frost line) was important in prevent atmospheric escape of hydrogen and helium. Atmospheric escape goes up with temperature and down with molecular mass (AKA molecular weight).

    6. The End of Abundant Gas in the Solar System:

      The solar wind of the early Sun probably blew out the disk gas within a few million years ending condensation and ending gravitational accretion onto the gas giants of hydrogen and helium.

    7. From Grains to Planets:

      Condensation gets one as far as micron-size grains.

        Question: How many orders of magnitude (i.e., powers of 10) in size scale separate a micron-sized grain (one with a size of 10**(-6) m) from Earth-size body (i.e., one with a diameter of order 10**7 m).

        1. 13 orders of magnitude.
        2. 9 orders of magnitude.
        3. 3 orders of magnitude.

        Answer 1 is right.

        10**7 / 10**(-6) = 10**13.

        13 orders of magnitude of growth is needed to get from grain to Earth.

      How does one get from micron-size grains to planet-size bodies?

      We give the answer insofar as we know in the next 3 sections.

  11. Growing Planetesimals and the Streaming Instability

  12. Planetesimals are small Solar System bodies (SSSBs), that formed in the planet formation era and are of order 1 km or larger (see Wikipedia: Planetesimal: Definition of planetesimal).

    The planet formation sequence as currently understood is:
    1. condensation of gas to grains.
    2. streaming instability/collective-self-gravitation accretion of grains to planetesimals.
    3. gravitational accretion of planetesimals to protoplanets.


    1. The streaming instability is now highly favored on growth from micron-size grains to planetesimals occurs, but there are all kinds of subtleties and variations. See Wikipedia: Streaming instability: intro.

    2. See Baroclinic Instability: Figure 1 for vortices.

    3. See How Planetesimals Are Born, 2021apr19 for pebbles and still a role for sticky accretion. Wikipedia: Streaming instability: Background says pebbles by Van der Waals forces (but are they too weak?) but How Planetesimals Are Born, 2021apr19 suggest ices help stick.

    4. Selective Aggregation Experiments on Planetesimal Formation and Mercury-Like Planets, 2018aug21 has figure with pathways possible.

    5. 162173 Ryugu is NOT a planetesimal, but is a fragmented remnant of one. It is classified as an asteroid and a rocky body.

      See On Ryugu for the evolution of Ryugu

      For Ryugu itself, see the figure below (local link / general link: 162173_ryugu_rotating.html).

      See the image of
      2014 MU69 (AKA Ultima Thule) (which is thought to be Solar System planetesimal) below.

      Arrokoth (AKA Ultima Thule) is a trans-Neptunian object (TNO), rocky-icy body and a planetesimal.

      Lee Billings, 2020 Feb20, "New Horizons May Have Solved Planet-Formation Cold Case".

      For Arrokoth (AKA Ultima Thule, AKA 2014 MU69), see the figure below (local link / general link: trans_neptunian_objects_arrokoth.html).

    6. Condensation is an atom by atom or molecule by molecule growth of a solid or liquid phase sample from a gas phase sample.

      Once you have large grains some kind of accretion of grains must have occurred to get larger bodies.

        Somewhere there must be an argument as to why condensation can't make objects much larger than micron-sized grains, but I can't seem to locate it.

        Maybe the accretion by condensation is just too slow or maybe most of the metals are exhausted in grain formation.???

      The grains were typically a few centimeters apart (Se-419).

      The orbits were mostly very similar circles about the Sun in the plane of the protoplanetary disk of grains and gas.

      Still there was a lot of random motion, and so collisions were frequent.

      Orbital speeds relative to the inertial frame of the Sun are of order of a few to a few tens of kilometers per second.

        For circular orbits around the Sun, the orbital speed is given by

                 v = (GM_☉/r)**(1/2) = 29.789 km/s * (1 AU /r_AU)**(1/2)  ,
                      where G=6.67384*10**(-11) is the gravitational constant,
                      M_☉=1.9891**30 kg is the solar mass,
                      1 AU=1.49597870700**11 m is the astronomical unit (AU)
                      which is an exact value in modern convention.
                  No correction has been made for finite planet mass
                  or gravitational perturbations
                  or other kinds of perturbations. 
                  Such corrections probably cause changes of order 0.03 % since 
                  Earth's mean orbital speed is
                  29.78 km/s, NOT 29.79 km/s as our formula implies:  (0.01/30)*100 % & ≅ 0.03 %.  

        Our orbital speed formula shows that all objects in orbit around the Sun out to the orbit of Neptune (orbital radius 30.10 AU, mean orbital speed 5.43 km/s) have orbital speeds of order a few to a few tens of kilometers per second (km/s).

      But the random relative speeds of particles in the protoplanetary disk in similar orbits were probably only a few or few tens of meters per second (Youdin, A. N. 2003, astro-ph/0311191).

      Two grains of micron or millimeter size are much too small to feel any significant gravitational attraction if their relative speeds are even a few or few tens of meters per second: i.e., their relative escape speed is much smaller than their actual relative speed---the figure below illustrates this situation.


        Caption: A cartoon of a binary gravitational collision of grains.

        Credit/Permission: © David Jeffery, 2004 / Own work.
        Image link: Itself.

      So binary gravitational collision accretion of grains is RULED OUT: i.e., two grains don't coalesce under their mutual gravitational attraction.

    7. How did growth of grains occur?

      Two theories have been discussed:

      1. Sticky Accretion:

        In this theory the colliding grains stick together through some chemical bonding or cohering force.

        But there is NO obvious sticky force strong enough to bond grains as we think they were when they are bouncing off each other at tens of meters per second.

        Possibly tarry, organic compounds containing carbon helped sticky accretion (Se-419).

      2. Collective Self-Gravity Accretion:

        The grains CANNOT grow through binary gravitational collision accretion as argued above.

        But if enough grains are compacted into a thin enough layer, then their collective self-gravity might lead to a gravitational runaway to largish lumps of compacted grains that are at rest or at very low velocity with respect to each other.

        Then chemically bonding and cohering forces fuse the lumps into lumps of rock (i.e., silicates with metals) and/or astro ices (i.e., water ice (H_2O), carbon dioxide (CO_2), nitrogen (N_2), ammonia (NH_3), methane (CH_4), etc.).

        It was once thought that turbulence in the protoplanetary disk would prevent this process.

        But some calculations suggest the process should happen (Youdin, A. N. 2003, astro-ph/0311191).

      Currently, COLLECTIVE SELF-GRAVITY ACCRETION seems the favored theory.

      The two theories are NOT mutually exclusive. Both COLLECTIVE SELF-GRAVITY ACCRETION and sticky accretion may occur.

      Sticky accretion could be a secondary process. In fact, in the compaction of COLLECTIVE SELF-GRAVITY ACCRETION relative velocities are lowered and a sort of sticky accretion must happen at some point though that may be under gravitational compression.

      Actually, other processes have been considered from getting from grains to kilometer-size objects.

    8. There seem to be no consensus resolution yet? Yes, the streaming instability

      Whatever, the exact process somehow clumps of solids of order a kilometer in size scale are reached.

      It is conventional to call these kilometer-size objects planetesimals (see Wikipedia: Planetesimal: Definition ofj planetesimal).

  13. Gravitational Accretion

  14. Recall, planetesimals are small Solar System bodies (SSSBs), that formed in the planet formation era and are of order 1 km or larger (see Wikipedia: Planetesimal: Definition of planetesimal).

    They are massive enough for for the gravity between two of them to pull them together and they can coalesce (Youdin, A. N. 2003, astro-ph/0311191)---provided they don't hit so fast that they fragment. Some of their kinetic energy must become heat energy, and so the coalesced object may be rather hot as well as shocked.

    Thus, binary gravitational collision accretion, which is RULED OUT for grains, is RULED IN for planetesimals---the situation is illustrated in the figure below.

    If the two planetesimals are moving at low relative velocity (i.e., they are on nearly parallel orbits), then accretion is most likely.

    The growing clumps of planetesimals can be called protoplanets.

    In the inner Solar System, the planetesimals and protoplanets were mostly iron and silicates with some volatiles trapped in the material.

    In the outer Solar System (i.e., from in the asteroid belt at ∼ 2.7 AU outward where the frost line (AKA snow line) was it was in the planet formation era: see Wikipedia: Frost line (astrophysics): Current snow line versus formation snow line), ices condensed during the condensation phase and the planetesimals and protoplanets probably were substantially ices as well as iron and silicates.

    The figure below (local link / general link: protoplanetary_disk_beta_pic.html) gives an artist's conception of what the a protoplanetary disk may look like when there are still many planetesimals and smaller acrretions of solid matter, and a lot of protoplanetary disk dust.

    Size matters.

    Those protoplanets that started out most massive by chance attracted the smaller nearby ones and cleared out parts of the solar disk that were nearest them (Se-420). Computer simulations tell us that this scenario does indeed happen. See the cartoon of the scenario in the figure below.

    The spacing of the surviving protoplanets (which became the planets) is NOT entirely random though.

    Partially this is just that the survivors cleared out space around themselves, and so left themselves well separated.

    But there is also an analytic formula the Titius-Bode law that roughly gives the planet distances from the Sun:

        R = 0.4 + 0.3 x 2**n AU, 
           where n runs 
          -∞ (Mercury) , 
           0 (Venus), 
           1 (Earth), 
           2 (Mars), 
           3 (asteroid belt), 
           4 ( Jupiter), 
             and so on.  
    (see Wikipedia: Titius-Bode law: Formulation).

    Computer simulations roughly reproduce Titius-Bode law, but it may work as well as it does for the Solar System partially by accident (HI-277).

    In the inner Solar System, the protoplanets that ate their neighbors became the rocky planets: Mercury, Venus, Earth, and Mars.

    But these protoplanets were never massive enough to attract or at least to retain much of the volatile molecular hydrogen (H_2) and helium (He) gases.

    So the protoplanets of the inner Solar System never became gas giants.

    See the videos for planetary migration which probably happened in the early Solar System to some degree, but did NOT move the gas giants into the inner Solar System---which probably allowed us to be. And the fact that we are, via anthropic principle, explains why they didn't move into the inner Solar System sort of.

    Further discussion of the formation of the gas giants is given in the section The Formation of the Gas Giants below.

    See Planetary system formation videos below (local link / general link: planetary_system_formation_videos.html):


  15. The Formation of the Gas Giants

  16. There are TWO THEORIES of the formation of the gas giants in the outer Solar System, (HI-277).

    The two theories apply to all planetary systems and NOT just to the Solar System.

    So our description below of the two theories will be in terms of planetary systems in general.

    The two theories of gas giant formation are:

    1. The core-accretion model:

      In an outer planetary systems, protoplanets with ices as well as refractories (mostly silicates and iron) could grow and become much larger than the rocky protoplanets of an inner planetary systems.

      These rocky/icy protoplanets are the cores of the core-accretion model

      The cores are massive enough to have a strong enough gravitational field to directly attract and hold the light gases H_2 and He.

      The growing amounts of accreted hydrogen and helium further increases the gravitational field leading to more accretion, and so on in a runaway growth---a gravitational runaway growth.

      The growth ends probably because the H_2 and He gas was exhausted partially through the accretion process and partial through a strong stellar wind blowing it out of the planetary systems.

      When the growth ended, there were gas giants.

    2. The disk-instability model:

      The alternative is that gas giants formed directly from gases like stars without any rocky/icy protoplanet core.

      In the disk-instability model, the protoplanetary disk partially fragmented (due to instability) and the fragments experienced a gravitational runaway growth ending with gas giants.

      The gas giants so formed were too small ever to become hot and dense enough under contraction to start hydrogen burning and become stars.

    The core-accretion model has been the conventional one, but recently (since circa 2003) evidence has accumulated for the disk-instability model.

    On the other hand, evidence from the now abundant sample of exoplanets favors the core-accretion model (see Howard 2013, p. 11).

    I'd guess that core-accretion model and disk-instability model are NOT exclusive. They could both act in different cases or even in some combination---but what do I know.

    The major moons of gas giants probably formed like the rocky planets: they formed as a miniture planetary systems from a miniature protoplanetary disk: a circumplanetary disk.

    The smaller moons and Neptune's largest moon Triton are probably mostly captured objects (HI-283).

    Just as a preliminary glimpse---since we'll be discussing all the Solar System planets in detail in later lectures---see the collage of the planets in the figure below (local link / general link: planet_collage.html).

  17. The End of Planet Building

  18. In of order a few million years after the solar nebula had become cold enough for condensation to occur, the solar wind and Sun's radiation pressure??? blew away most of the unaccreted dust and gas into interstellar space (Se-422).

    There were still many smallish planetesimals and protoplanets around.

    But computer simulations show that the big guys (the modern planets) would have perturbed the orbits of the remaining small bodies so as to kick them out of the inner Solar System or out of the Solar System altogether or cause them to crash into and be absorbed by a planet or the Sun.

    There must have been a lot of impacting on planets---"worlds in collision" one might say. We will discuss this impacting below.

    But we must emphasis that the outline of Solar System given in this lecture is very simplified.

    There are many complications due the collisional impacts, gravitational effects, and gas and dust effects.

    These are still being sorted out, but much is known already.

    But the complicated details will always be beyond the scope of intro lecture like this. See Wikipedia: Formation and evolution of the Solar System for a more elaboration on the Formation and evolution of the Solar System.

    See also Planetary system videos below (local link / general link: planetary_system_formation_videos.html):


  19. Leftovers I: Asteroids

  20. After the dinner, there are leftovers: same after planet formation.

    The leftovers are planetesimals, protoplanets, and fragments thereof.

    The leftovers are rocky bodies at least on the surface which we call asteroids (mostly inward of Jupiter's orbit) and or rocky-icy bodies (mostly outward of Jupiter's orbit).

    1. Where are the Leftovers?

      They are in reservoirs where there they have stable orbits over gigayears.

      Stable here means NOT unchanging, but changing slowly enough that the astro-bodies leave the reservoirs only at a very slow rate.

      The main reserviors are:

      1. The asteroid belt which is in the large gap between the Mars orbit (mean radius 1.52 AU) and the Jupiter orbit (mean radius 5.2 AU).
      2. Jupiter Trojan zones. See the figure of the inner Solar System below.
      3. The trans-Neptunian object (TNO) zone which is beyond orbit beyond Neptune.
      4. Oort cloud extending from perhaps from 2000 AU to 200,000 AU (see Wikipedia: Oort cloud: Structure and composition).

      Occasionally, perturbations, gravity assists (AKA gravitational slingshot maneuvers), or fragmenting impact events will knock an astro-body out of a reservoir.

      The astro-body may then be on an escape orbit from the Solar System and leave us forever.

      Or may just be kicked out beyond Neptune's orbit quasi-eternally.

      But if it is in unstable orbit (i.e., from Neptune's orbit inward outside of the above specified reservoirs), it will on the time scale of 10 million years have an impact event with another astro-body (i.e., the Earth) or by gravity assist be sent back to a reservoir or put on an escape orbit from the Solar System.

    2. Why the Asteroid Belt?

      Handwaving, Jupiter's strong gravity somehow maintains stable orbits there for asteroids.

      But also Jupiter's great gravitational effect probably prevented planet formation in the asteroid belt region.

      But further also, from time to time, Jupiter's gravity acting as a constant perturbation or a collision of asteroids kicks an asteroid (or fragments of one) into the inner Solar System (where its dangerous to us) or to the outer Solar System or out of the Solar System altogether on an escape orbit.

    3. How Many Asteroids?

      It is estimated that there are of order 1 million asteroids with diameters greater than 1 km and of order 25 million with diameters greater than 100 m (see Wikipedia: Asteroid: Size distribution, but the table has vanished now and Table: Approximate Number of Asteroids N Larger than Mean Diameter D).

      The number of discovered asteroids is tricky to specifiy since (1) the number is a moving target since new discoveries are being made continually by automated searches; (2) what is counted as an asteroid varies between sources. For the best yours truly can do, see IAL 16: Small Bodies of the Inner Solar System and Target Earth: How Many Asteroids?.

      There are many more small asteroids. It seems the smaller you go, the more there are. So more small ones are being discovered all the time. There is no limit: one can keep going smaller and smaller until one is down to dust.

      Actually any body smaller than about 10 m in size scale is given the generic name meteoroid (Cox-333).

      Though there are a lot of asteroids in number, their total mass is only a few percent of the Moon's mass (Ze2002).

      What of total mass in asteroids?

      One estimate is that the total asteroid belt mass is 0.08 % of the Earth's mass (Se-565) and another is 0.03 % of the Earth's mass (Cox-293).

    4. Asteroid Orbits:

      The figure below (local link / general link: solar_system_inner.html) illustrates the asteroids, asteroid belt, and the Jupiter Trojan asteroids (which are in another region of stability due to Jupiter).

      The asteroids are densely packed in a sense, but they are still widely separated compared to their sizes. If you were traveling on one you would only see others as faint stars at best, unless there was an unusually close encounter (HI-257).

      The situation is quite different in
      Saturn's rings where the icy/rocky objects can be closely packed relative to their sizes. See figure below (local link / general link: saturn_rings_artist_conception.html).

    5. Asteroids are a Diverse Lot:

      Some of them may be primitive planetesimals or post-collision fragments of such planetesimals.

      Others have undergone geological activity and these ones too have in many cases been fragmented by collisions.

      The asteroids are mainly rocky and metallic and carbonaceous at least a first glance. We go more into composition in IAL 16.

      But since circa 2010 it has been thought that some of them may have significant amounts of ices in the astro jargon sense of the word: i.e., water ice (H_2O), carbon dioxide (CO_2), nitrogen (N_2), ammonia (NH_3), methane (CH_4), etc. On asteroids, the ice is probably mainly water ice since the asteroids are warm relative to farther out in the Solar System where the other ices become common.

      The ices are probably mostly sub-surface where they have avoided easy detection.

      We show some images of asteroids in the two figures below (local link / general link: 243_ida.html; local link / general link: asteroid_collage.html).

    6. Outside the Asteroid Belt:

      Asteroids outside of the asteroid belt exist too. Probably many have been kicked out of the asteroid belt by interaction with Jupiter or a collision with another asteroid. Some might be dead comets (Se-560).

      Many of these outliers are going to end up colliding with a planet (maybe the Earth!) or the Sun (Se-560).

      Small asteroids frequently pass relatively close to the Earth, but until recently we never noticed. For an example, see the figure below.


      Caption: "Timelapse of asteroid 2004 FH's flyby (NASA/JPL Public Domain). 2004 FH is the centre dot being followed by the sequence; the object that flashes by near the end is an artificial satellite. Images obtained by Stefano Sposetti, Switzerland on March 18, 2004. Animation made Raoul Behrend, Geneva Observatory, Switzerland."

      Just on 2004mar18 a 30-meter asteroid 2004 FH zipped by with a closest approach of about 7 Earth radii. This is still the 6th closest known approach by non-impacting asteroid or meteoroid (Wikipedia: Record-Setting closest approaches to Earth: Meteoroids). But what you count as asteroid or meteoroid is a bit debatable.

      The stars in the film are unresolved. Their apparent sizes just indicate their relative brightnesses. The asteroid looks like a star in being unresolved: the name asteroid means star-like: like an astra. But the asteroid moves relative to the fixed stars as you see.

      Credit/Permission: NASA, 2004 (uploaded to Wikipedia by User:Tungsten, 2005) / Public domain.
      See also: NASA's Astronomy Picture of the Day: Asteroid 2004 FH.
      Image link: Wikipedia: File:Asteroid 2004 FH.gif.

  21. Leftovers II: Rocky-Icy Bodies

  22. Rocky-icy bodies are leftover icy planetesimals or protoplanets or fragments thereof. Some might have undergone some evolution.

    We will discuss rocky-icy bodies further in Intro Astro Lecture 17: Pluto, Rocky-Icy Bodies, Kuiper Belt, Oort Cloud, and Comets.

    Rocky-icy bodies have several names depending on how they have been categorized: Kuiper Belt objects (KBOs) (located in the Kuiper belt), scattered disk objects (SDOs) (located in the scattered disk), Oort Cloud objects (OCOs) (located in the Oort cloud), and comets. These are all trans-Neptunian objects (TNOs) since they mostly orbit beyond the orbital radius of Neptune. Another kind are the Centaurs which are NOT TNOs since they mostly orbit between the orbits of Jupiter and Neptune .

    Comets can be called TNOs in that most of them spend most of their time beyond Neptune.

    They have plunging orbits that bring them into the inner Solar System---and that's why they are comets.

    For the distribution of most types of of the rocky-icy bodies as a function of mean orbital radius, see the figure below (local link / general link: trans_neptunian_objects_distribution.html).

    There are three main reservoirs of
    rocky-icy bodies: the Kuiper belt (pronounced koi'per belt), the scattered disk, and the Oort Cloud.

    A lesser reservoir is that of the Centaurs which is discussed in IAL 17: Pluto, Rocky-Icy Bodies, Kuiper Belt, Oort Cloud, and Comets.

    There are also stray rocky-icy bodies elsewhere.

    The Kuiper belt and Oort Cloud are believed to be the source of comets: the Kuiper belt for short-period comets and the Oort Cloud for long-period comets (Se-569).

    Comets don't last forever, and so must be resupplied. Eventually they hit a planet or the Sun or just become extinct when all their volatiles are gone.

    Sometimes a collision or an encounter with a passing star is believed to send a rocky-icy body or fragment thereof from a reservoir into a plunging orbit (i.e., a highly elliptical orbit).

    The object is then a comet as its volatiles are evaporated somewhat explosively by heating by solar radiation as it comes into the inner Solar System.

    See the beautiful comet shot in the figure below (local link / general link: comet_lovejoy.html).

    We can expand a bit more on the
    Kuiper belt and Oort cloud:

    1. The Kuiper Belt:

      The Kuiper belt is from about 30 AU to 100 AU.

      Recall Neptune is at about 30 AU and Pluto at about 40 AU. of Pluto's orbit.

      Pluto (discovered 1930) and its moon Charon (discovered 1978) were the only Kuiper Belt objects (KBOs) (and TNOs) until 1992.

      Pluto was the largest TNO until the discovery of Eris (an SDO) in 2005jan.

      Pluto is about 0.2 % of the Earth's mass; only about a fifth of the Moon's mass.

      Pluto was officially a planet since the 1930s. But its status as planet was doubtful because of its small size especially when it became clear that many TNOs of comparable or larger size must exist. The International Astronomical Union (IAU) once said Pluto would stay an official planet even if it was minute.

      But then it changed its mind and degraded Pluto to dwarf planet---but a faded glory lingers---it is the only ex-planet.

      It is estimated that there are tens of thousands of Kuiper belt objects larger than 100 km (PF-157).

      But the Kuiper belt was predicted before any were discovered (circa 1950) as a source for short-period comets (Se-569).

      The brightest (which are probably about the largest) known TNOs are illustrated in the figure below (local link / general link: trans_neptunian_objects_collage.html).

    2. The Oort Cloud:

      The Oort Cloud of rocky-icy bodies is NOT confined to near the ecliptic plane, but is thought to be sort of a SPHERICAL SHELL extending from perhaps from 2000 AU to 200,000 AU (see Wikipedia: Oort cloud: Structure and composition).

      The Oort Cloud is purely theoretical. No one has ever detected an Oort Cloud object.

      But to explain long-period comets, it is thought it must exist.

      The Oort Cloud is further explicated in the figure below (local link / general link: oort_cloud.html).

  23. What Happened to the Larger Rocky Bodies?

  24. Here we will just sketch a general picture. Later we will look in detail at many of the objects.

    The objects we are considering the rocky planets, the rocky moons, and the larger asteroids.

    The 4 stages that such bodies TEND to go through (Se-427) are summarized in the cartoon in the figure below (local link / general link: rocky_body_evolution_4_stages.html).

    We can look at each stage in turn in a bit of detail.

  25. Chemical Differentiation

  26. The rocky bodies were probably mostly HOT at formation or became so soon after.

    Some of the heat energy came from the kinetic energy of colliding planetesimals being converted into heat energy.

    Additionally all these bodies have included radioactive isotopes from the primordial solar nebula.

    As we discussed above, the radioactive decay energy turns into heat energy in dense environments.

    The result of the heat energy of the rocky planets is that they were MOLTEN either from formation or shortly thereafter.

    Now in fluid conditions in a gravitational field, the denser fluids sink; the less dense ones float.

    This is the familiar buoyancy effect which is illustrated and explicated in the figure below (local link / general link: buoyancy.html).

    For molten
    planets, the buoyancy effect causes chemical differentiation.

    The abundant dense substances are iron and nickel. They sunk to core of the bodies. Iron and nickel have uncompressed densities of about 8 and 9 g/cm**3, respectively.

    The less dense silicates floated to the top. Uncompressed silicates have densities of order 3 g/cm**3.

    This chemical differentiation wasn't perfect. Obviously all the denser substances are represented in the Earth's crust: e.g., iron, lead, uranium, gold, platinum.

    But silicates dominate the Earth's crust.

    The crust is continually renewed, and so the current crust isn't the original crust. Still the current crust is formed from higher level materials of the Earth.

    Chemical differentiation---in an elementary way---is illustrated in the figure below (local link / general link: buoyancy_cylinder.html).

  27. Cratering by the Heavy Bombardment

  28. In the early post-nebula epoch, the ecliptic plane was still full of many planetesimals, protoplanets, and fragments thereof.

    Many of these objects were already in collision orbits or were perturbed into them.

    The ones in collision orbits ended up bombarding the Sun, planets, moons, and other bodies including themselves.

    The SOLID BODIES were heavily cratered.

    The gas giants, of course, show no trace: the impactors made a very temporary mess in the fluid atmospheres that quickly dispersed.

    On Earth, Venus, and Jupiter's moons Io and Europa the early cratering has largely been erased by continuing non-impactor geological activity.

    However, the Moon, Mercury, the asteroids, some of the moons, and to a lesser degree Mars show the evidence of the heavy bombardment: i.e., their heavily cratered surfaces.

    Low geological activity on these bodies has NOT destroyed the early cratering.

    Typical large lunar craters are shown in the two figures below (local link / general link: crater_keeler.html; local link / general link: crater_daedalus.html).

    Mainly by studying the
    Moon, it is estimated that the heavy bombardment tailed off after 3.8 Gyr ago (Se-422, 446, 447).

    But there is considerable uncertainty. The cratering rate illustrated in the cartoon in the figure below is quite problematic.

    Recently, there is some idea that the cratering rate may have fallen and then risen again before tailing off---this temporary rise is called the Late Heavy Bombardment.

    The tailing off occurred because the original population of impactors was progressively exhausted.

    Of course, cratering continues to the present day, but at a very slow rate.

    The larger bombarding objects are asteroids or fragments thereof or comets.

    The smaller, but most common, impactors are called meteoroids which are fragments of larger bodies in most cases.

    These impactors are resupplied from the asteroid belt or the rocky-icy body reservoirs by gravitational perturbations or collisions.

    The sufficiently small impactors (meteoroids) falling on planets or moons with atmospheres burn up (i.e., evaporate) in descent.

    When meteoroids fall into an atmosphere they are called meteors. Any remnant that survives is a meteorite.

  29. Flooding of Basins by Lava

  30. On some rocky bodies there was major early flooding of basins by lava upwelling from the interior.

    On the Moon and Mercury there are still lava plains left by flooding. On the Moon, they are the conspicuous dark lunar maria which probably formed in impactor-formed basins (FK-214). On Mercury, the lava plains are NOT dark or so conspicuous.

    For a map showing of the lunar maria on the near side of the Moon, see the figure below (local link / general link: moon_map_side_near.html).

    Earth and Venus and maybe to some degree on Mars??? early lava flooding has been erased by later geological activity or never happened.

    On Earth, flooding by water occurred between the continents. This probably happened nowhere else in the Solar System, except possibly on Venus and Mars. In the case, of Venus obvious traces of this flooding have certainly been erased by later geological activity.

    In the case of Mars, there is a theory called the Mars ocean hypothesis that suggests that nearly a third of the Martian surface was covered by an ocean about 3.8 Gyr years ago. There is some geological evidence for this theory, but it is a controversial theory.

  31. Continuing Geological Activity

  32. Bodies with continuing internal heat from original formation (residual heat) and/or radioactive decay have had continuous geological activity from that source. I will call this kind of geology primordial-radiogenic heat geology (see also Wikipedia: Earth's internal heat budget: Radiogenic heat: Primordial heat).

    Primordial-radiogenic heat geology causes volcanism and, on Earth alone it seems, plate tectonics.

    Only Earth and Venus have very active primordial-radiogenic heat geology today.

    Mars has largely lost much of its internal heat and thus is much less geologically active than Earth or Venus, but it is NOT completely without primordial-radiogenic heat geology.

    Mercury, the moons, and asteroids are probably dead or close to dead in regard to primordial-radiogenic heat geology.

    EROSION GEOLOGY by atmospheres occurs on Earth, Venus, Mars, Titan (the largest moon of Saturn), but only Earth has water erosion at present. Mars had some in the past and Titan must have some erosion by liquid methane (CH_4) since it has methane lakes (Wikipedia: Titan: Liquids).

    Wind erosion is a weaker process than water erosion.

    The airless bodies or thin-atmosphere bodies (like Mars) have continuing, but very slow impact event and space weathering. I will call this kind of geology IMPACT/SPACE-WEATHERING GEOLOGY.

    What I call TIDAL-FORCE GEOLOGY is evident on some bodies.

    The Moon exhibits many small moonquakes in most cases powered somehow by tidal flexing due to the tidal force of the Earth. This force varies because of the elliptical nature of the Moon's orbit and thus causing flexing.

    Jupiter's moons Io and Europa have more active TIDAL-FORCE GEOLOGY. In this case the tidal flexing causes internal heating which drives volcanism.

    Io is the most geologically active body in the Solar System. Volcanic eruptions are ongoing: although some vents shut down at times, others become active (HI-215).

    Io probably gets nearly completely resurfaced on time scales of less than millions of years??? (Se-506).

    For more explication of Io, see the figure below (local link / general link: io_003_eruption.html).

  33. Solar System Timeline

  34. To summarize the Solar System evolution we should look again at a Timeline of Solar System evolution---ending with what we started---but with deeper, truer understanding.