Lecture 10: Solar System Formation

Don't Panic

Sections

The Introduction

The formation of our SOLAR SYSTEM (i.e., our planetary system) involved a large random element.

The initial molecular cloud fragment from which it formed had random features---of which we know little.
There may have been random encounters with other star systems.
Collisions of proto-planetary nebula clumps and, later, of formed bodies were somewhat random.

So a detailed, exact history of our 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 solar systems in formation or fully established because we observe:

Protoplanetary disks (proplyds) which form as discussed in IAWL Lecture 21: Star Formation.
The known 111 EXTRASOLAR PLANETS as of 2004mar04 ( California & Carnegie Planet Search Almanac of Planets).

The Orion nebula is in sword of Orion about 1,500 lyr away. It is the most conspicuous star formation region on the sky. Star formation occurs in clouds of gas and dust.

This image is about 2.5 lyr wide and is only a small section of the nebula.

Hot young, massive stars are dispersing this part of the nebula and making it bright rather than dark via reflection and induced emission. The dispersal tends to turn off star formation. Star formation is compicated by these complex feedback mechanisms. In particular note the 4 Trapezium stars near the center.

Proplyds are protoplanetary disks around youngs stars. In the image there are more than about 150 of them.

The obvious ones are near the Trapezium and have tails. Light pressure from the Trapezium is blowing away some of the gas and dust that forms the proplyds.

True color? False color? The official caption doesn't tell, but I assume that the colors have been enhanced to bring out details even if they are more or less true.

Credit: NASA: image #PR95-45A.

The EXTRASOLAR PLANETARY SYSTEMS have turned out to be different from ours.

They have massive Jupiter-size planets often with very ECCENTRIC ORBITS with semi-major axes (i.e., mean distances from the star) going down to less than 0.1 AU.

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

A histogram of the extrasolar planets known as of 2004mar04 (California & Carnegie Planet Search Almanac of Planets).

The technique for finding extrasolar planets 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 such systems are clearly NOT RARE.

For more on extrasolar planets see the California & Carnegie Planet Search site. This is the site of Marcy & Butler, the top planet hunters.

Our solar system may or may not be typical---we don't yet know.

Question:

Just plain BAD LUCK that we evolved in peculiar solar system that gave such a false impression of what other solar systems are like on average.
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 the premise of the question is hypothetical.

The explanation of odd coincidences in physics or the universe by saying that if they weren't, we wouldn't be here is called the ANTHROPIC PRINCIPLE.

To give an example of ANTHROPIC PRINCIPLE which not hypothetical consider that the Earth must be a fairly rare kind of planet even if our solar system is pretty normal:

It exists just far enough from its star to have abundant liquid water.
It is massive enough to have enough gravity to hold onto an atmosphere over gigayears.
It's orbit is nearly circular, and so there are NOT wild yearly swings in climate from way below 0 degrees C to way above 100 degrees C.

RANDOM EVENTS

On that basis alone it is clear that Earth must belong 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 requires the Earth features listed above. Thus, the explanation of the Earth features listed above is the ANTHROPIC PRINCIPLE.

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.

Whatever the vagaries of other solar systems, in this lecture we will focus on OUR SOLAR SYSTEM and its story as far is it is known---and there still are many uncertainties.

But in broad outline the story seems fairly robust.

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 had posited a sort of NEBULAR HYPOTHESIS---but they were flat-Earthers (Fu-140).

In the context of the Newtonian physics, the NEBULAR HYPOTHESIS was first made in the 18th century by Immanuel Kant (1724--1804: more noted as a philosopher than an astronomer) and Simon Laplace, one of the great mathematical astronomers (No-406).

CATEGORICAL IMPERATIVE

``Act so that the maxim (determining motive of the will) may be capable of becoming a universal law for all rational beings.'']

The BASIC NEBULAR HYPOTHESIS is:

The Sun forms from a cloud (i.e., a nebula) that has collapsed under gravity and heated up from contraction to the point where hydrogen burning starts in the core.
A disk of gas and dust forms around the proto-Sun with the gas and dust all rotating the same way.
The planets form out of the disk.

Question:

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

Answer 2 is right.

The BASIC NEBULAR HYPOTHESIS explains some salient facts: here we give the facts and leave explanations to later mostly:

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.
The two main types of planets: rocky (or terrestrial) planets and gas giant (or Jovian) planets.
- 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 there are complications to the simple picture due to the nature of the extrasolar planets.
The solar system minor bodies: i.e., asteroids, meteoroids, and comets.
The common age of the solar system: about 4.6 Gyr. This is known from radioactive dating from Earth, Moon, and meteorite rocks and from modeling in the case of the Sun.

Given that we see PROPLYDS in star formation regions, there isn't much reason to doubt that in essentials the nebular hypothesis is correct.

There is, of course, a great deal of uncertainty about many aspects.

Radioactive Dating

The age of the solar system can be accurately determined from

radioactive dating

So it is worth a digression to discuss radioactive dating and RADIOACTIVE DECAY.

Radioactive nuclei are unstable: i.e., they change into other nuclei SPONTANEOUSLY. This change process is called RADIOACTIVE DECAY.

A new nucleus formed from decay is called a DAUGHTER. If it is STABLE, it will not SPONTANEOUSLY change again.

When decay occurs energy is also released. Initially in the form of dangerous ionizing radiation: i.e., gamma-rays and fast particles with large kinetic energies.

In dense environments the decay energy is often converted into heat energy.

We say the energy is THERMALIZED.

A cartoon of carbon-14 decay.

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

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

Thus, radioative material got incorporated into our solar system and there is still a lot of it around.

Also cosmic rays continuously produce some radioactive material: e.g., carbon-14 which is used in dating organic materials (SWT-644).

The decay of nucleus is a RANDOM PROCESS.

Any given nucleus may decay in a second or in a billion years. There is no way to tell even in principle according to standard QUANTUM MECHANICS THEORY.

But nuclei of a given species do have a MEAN LIFETIME.

Question:

A full-life.
A half-life.
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). The half-life for this process is 4.5 Gyr (En-225; Se-416).

[Note that there are many intermediate, unstable daughter elements in the decay process. The 4.5 Gyr is the net half-life.]

A cartoon of radioactive decay.

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.

Of course, nothing is quite that simple, but often geologists and geophysicists can deduce the original abundance of a radioactive element 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 element and its daughters that allows the time since TIME ZERO to be established.

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

The oldest known Earth rocks are zircon crystals from Western Australia that are dated to 4.3 Gyr (Cox-39). But because of geological activity older Earth rock as been mostly or nearly entirely destroyed. Most Earth rock is less than 1 Gyr old (Lissauer-132).
The oldest known Moon rock---which was brought back from the Apollo missions---is 4.48 Gyr (Se-417). Geological activity on the Moon nearly stopped a long time ago. Still 4.48 Gyr may be younger than the Moon.
The oldest known meteorite rock is 4.6 Gyr (Se-417). It is likely that this is as old as the formation of the solar system.
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.

The solar system probably took tens of millions of years to form from the primordial nebula---but that is a relatively small time compared to the time since.

Collapse to the Protoplanetary Disk or Proplyd

IAWL Lecture 21: Star Formation

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

Cartoon of disk formation around a dense/core protostar (FK-172,601--602; HI-271--272).

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

The formed 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.

But the irregularities are strongly damped out by collisions: this is just the same process that formed the disk.

Cartoon of disk relaxation to regularity.

HI-271

The disk will be hot for the same reason the proto-Sun is hot:

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.

Also EMR from the proto-Sun can heat the disk.

Question:

falls going outward from the Sun.
rises going outward from the Sun.
stays constant going outward from the Sun.

Answers 1 is right.

The disk temperature falls going outward. It heated up most closest the proto-Sun where the infall was farthest and the proto-Sun most heating.

HI-274

This temperature will evaporate REFRACTORIES like iron and silicates.

The DUST-FREE HEATED DISK GAS was probably pretty homogeneous at least in the inner disk which became the inner solar system of ROCKY PLANETS.

Condensation

As time passed, the disk cooled by emitting

EMR

CONDENSATION

Question:

Making a written passage shorter.
The evaporation of atoms or molecules from a solid or liquid.
The cohering of gas phase atoms or molecules into a solid or liquid.

Answers 1 and 3 are right---but only answer 3 is relevant to our current topic.

CONDENSATION is an atom by atom (or molecule by molecule) growth of solid or liquid phase from a gas phase.

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.

CONDENSATION of a gas depends on temperature and density.

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

For different substances there are different condensation temperatures and densities.

Blow hot air from your mouth on your glasses. The air quickly cools on the glass surface and cannot contain as much water vapor as before. Liquid water condenses out as fine drops.]

Under the density conditions of the solar disk, CONDENSATION for different substances happened at different temperatures.

The condensates were GRAINS that built up atom by atom (or molecule by molecule).

A possible temperature structure for the main condensation phase is below.

Cartoon of disk temperature structure during the main condensation phase (Se-418).

REFRACTORIES (substances with relatively high condensation temperatures) condensed out first.

VOLATILES (substances with relatively low condensation temperatures) condensed out when lower temperatures were reached.

At about 1600 K aluminum and titanium condensed out as metallic oxides: i.e., molecules of aluminum and titanium with oxygen.
At about 1400 K, the abundant metal iron condensed out.
At about 1300 K, silicates condensed out. Silicates are compounds of oxygen and silicon and other minerals. Most rocks are silicates. There are a large variety. (See SWT-558--559.)
Below about 273 K or lower, water vapor could condense as ice.
Below 100 to 200 K, ammonia (NH_3) and methane (CH_4) could condense.
References HI-274 and Se-418.

The solar wind of proto-Sun and early Sun probably blew out the disk gas within a few million years ending condensation---but not before planet formation started as we will see below.

gas giants

DID NOT

The grains were probably mainly iron and silicates (i.e., rock). We 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).

But some VOLATILES (H, C, N) were locked up in the condensates of refractories: not much by mass, but very necessary for life on Earth.

In the outer solar system (say Jupiter and beyond) icy grains and carbon-rich grains could form.

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.

Condensation gets one as far as small grains.

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

Question:

13 orders of magnitude.
9 orders of magnitude.
3 orders of magnitude.

Answer 1 is right.

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

Sticky Accretion or Collective Self-Gravity Accretion?

Condensation is an atom by atom (or molecule by molecule) growth.

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

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 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 tens of kilometers per second.

But the random relative speeds of particles in the 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.

A cartoon of a binary gravitational collision of grains.

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

How did growth of grains occur?

Two theories have been discussed.

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).
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 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 ices (water, methane, ammonia, CO_2, etc. ices).
It was once thought that turbulence in the disk would prevent this process.
But recent calculations suggest the process should happen (Youdin, A. N. 2003, astro-ph/0311191).

COLLECTIVE SELF-GRAVITY ACCRETION

Of course, 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.

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 clumps PLANETESIMALS (Se-419).

PLANETESIMAL

Gravitational Accretion

The PLANETESIMALS are big enough for gravity between two of them to pull them together and they can coalesce (

Youdin, A. N. 2003, astro-ph/0311191

Thus, binary gravitational collision accretion, which is RULED OUT for grains, is RULED IN for PLANETESIMALS.

A cartoon of gravitational accretion of planetesimals.

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

Question:

fragmentation is more likely than accretion since the bodies have a HIGH relative velocity.
fragmentation is more likely than accretion since the bodies have a LOW relative velocity.
accretion is even more likely to occur than when the planetesimals are on nearly parallel orbits.

Answer 1 is right.

Head-on collisions tend to fragment and destroy rather than build up (Se-420).

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 ices condensed during the condensation phase and the planetesimals and protoplanets probably were subtantially ices as well as iron and silicates.

Size matters.

Godzilla

PROTOPLANETS

Se-420

A cartoon of the biggest accreters eating their neighbors.

The spacing of the surviving protoplanets that 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---BODE'S LAW---that roughly gives the planet distances from the Sun:

    R = 0.4 + 0.3 x 2**n AU, 
 
       where n runs -infinity (Mercury) , 0 (Venus), 1 (Earth), 
       2 (Mars), 3 (Asteroid Belt), 4 (Jupiter), and so on.

    Reference: HI-62.

Computer simulations roughly reproduce BODE'S LAW which may work as well as it does partially by accident (HI-277).

In the inner solar system the PROTOPLANETS that ate their neighbors became the rocky or terrestrial planets: Mercury, Venus, Earth, and Mars.

But these PROTOPLANETS were never massive enough to attract or at least to retain much of the uncondensable H_2 and He gases.

rocky protoplanets

The outer solar system there are TWO THEORIES of how the gas giant planets formed (HI-277).

Theory 1
In the outer solar system protoplanets with ices as well as rock and metals could grow and become much larger than the rocky protoplanets.
These the icy protoplanets could directly attract and hold H_2 and He gas and thus grow even more massive until they become mostly H_2 and He: the gas giants: Jupiter, Saturn, Uranus, and Neptune.
Theory 2
The alternative is that the gas giants formed directly from gases like stars.
There were just cores of dense gas and gravitational runaway occurred and the gas giants formed.
But they were too small ever to become hot and dense enough under contraction to start hydrogen burning and become stars.

Theory 1

Theory 2

The main reason is that Theory 2 allows for fast formation and increasingly that seems to be necessary since protoplanetary disks (proplyds) seem to have very brief lives: recent evidence suggests less than 5 Myr (Irion, R. 2003jun06, Science, 300, 1498).

I'd guess that Theories 1 and 2 need not be exclusive---but what do I know.

extrasolar planets

gas giants

Probably the EXTRASOLAR PLANETS formed farther out and migrated inward. But how is still uncertain and we are left wondering why this did not happen in the our solar system.]

The major moons of gas giants probably formed like the rocky planets: they formed in miniature solar systems.

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 planets in detail later---here is a collage of the planets.

The Venus image is computer reconstruction of its surface from radar mapping. Venus is always swathed in thick clouds an presents a pretty bland appearance.

Note that Pluto is the only planet NEVER to have been imaged from up close. The image here is from the HST and it is about the best that can be done. Pluto's moon Charon is a bit more than half the diameter of Pluto.

Credit: NASA

The End of Planet Building

In of order a few million years after the solar nebula had become cold enough for condensation to occur, the Sun's radiation pressure and solar wind 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 crashing into planets. We will discuss this below.

Leftovers I: Asteroids

There is a relatively stable region for planetesimals and protoplanets and their fragments: the ASTEROID BELT.

The ASTEROID BELT is in the large gap between the Mars orbit (at 1.52 AU) and the Jupiter orbit (5.2 AU).

This where one finds most of the ASTEROIDS which are leftover rocky or carbonaceous planetesimals and protoplanets or fragments thereof.

It is estimated that there are a million ASTEROIDS with diameters greater than 1 km (FMW-250) of which the largest 10000 or so have been discovered (Se-559).

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.

METEOROID

cox-333

A cartoon of the Asteroid Belt (HI-257).

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

But the total mass in ASTEROIDS is only a few percent of the Moon's mass (Ze2002).

Se-565

Cox-293

Jupiter's great gravitational effect probably prevented planet formation ASTEROID BELT REGION.

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.

The 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, NOT ICY. Carbonaceous means rich in carbon materials which usually leads to a black color.

This is a false color image of 243 Ida taken on a fly-by by the Galileo probe on its way to Jupiter. North is at 1:00. Ida is 58 km along its long axis. The bright blue areas may indicated enrichment in iron-bearing minerals.

Ida like mostly smaller asteroids is not round. Its self-gravity is insufficient to pull it into a spherical shape against its electromagnetic force structure and its centrifugal force due to its rotation.

Credit: NASA: Galileo probe: Image #P-44131.

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).

Just on 2004mar18 a 30-meter asteroid zipped by with a closest approach of about 7 Earth radii. See NASA's Astronomy Picture of the Day: Asteroid 2004 FH. This is a film from the Geneva Observatory.

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.

There is a shooting star in the film too.]

Leftovers II: Icy Bodies

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

We will discuss icy bodies further in Intro Astro Lecture 17: Pluto, Icy Bodies, Kuiper Belt, Oort Cloud, and Comets. Icy bodies have several names depending on how they have been categorized: Kuiper Belt objects (KBOs), Oort Cloud objects (OCOs), trans-Neptunian objects (TNOs), and Centaurs.

We'll discuss one non-KBO, non-OCO TNO, Sedna, in Intro Astro Lecture 17: Pluto, Icy Bodies, Kuiper Belt, Oort Cloud, and Comets.]

There are two main reservoirs of icy bodies: the KUIPER BELT (pronounced koi'per belt) and the OORT CLOUD.

A lesser reservoir is that of the CENTAURS which is discussed in IAWL Lecture 17: Pluto, Icy Bodies, Kuiper Belt, Oort Cloud, and Comets.

There are also stray icy bodies elsewhere.

The two main reservoirs 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 an 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.

COMETS

We will discuss comets in Lecture 17: Pluto, Icy Bodies, Kuiper Belt, Oort Cloud, and Comets.]

Credit: Roger Lynds/NOAO/AURA/NSF.

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. See David Jewitt's Kuiper Belt site.

Kuiper Belt objects (KBOs) are icy bodies.

PLUTO is probably just the largest known KBO.

PLUTO is only about 0.2 % of the Earth's mass; only about a fifth of the Moon's mass. But the International Astronomical Union (IAU) has said Pluto will stay an official planet anyway.

Aside from PLUTO and its moon CHARON, the first KBO was discovered in 1992 (by David Jewitt and Jane Luu), and it is estimated that there are tens of thousands larger 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).

Other than PLUTO (diameter 2390 km) and its moon CHARON (1270 km), the largest KBOs are QUAOAR (officially pronounced Kwah-o-wahr) with diameter 1200+/-200 km, DW 2004 with diameter of order 1500 km, and Sedna with diameter of order 1500 km ( David Jewitt's Kuiper Belt site).

Sedna may not be KBO, but it probably is not an Oort Cloud object either. It may require some other classification.

The OORT CLOUD of icy bodies is NOT confined to near the Ecliptic Plane, but is thought to be sort of a SPHERICAL SHELL at of order 20,000--150,000 AU.

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.

OORT CLOUD

We will give more discussion on smaller bodies later in Lecture 17: Pluto, Icy Bodies, Kuiper Belt, Oort Cloud, and Comets.

What Happened to the Larger Rocky Bodies?

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.

In cartoon form, one can summarize the 4 stages that such bodies TEND to go through (Se-427).

A cartoon of the four stages of rocky body development.

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

Chemical Differentiation

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

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

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

As we discussed above, the radioactive decay energy turns into THERMAL ENERGY in dense enviroments.

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

In fluid conditions, the denser fluids sink; the less dense ones float.

Thus, there was a 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, platnum.

But silicates dominate the Earth's crust.

Cratering by the Heavy Bombardment

In the early post-nebula epoch, the Ecliptic Plane was still full of many planetesimals, planetesimal fragments, and protoplanets.

These objects were already in collision orbits or were perturbed into them.

They ended up bombarding the Sun, planets, 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 some of Jupiter's moons the early cratering has largely been erased by continuing geological activity.

However, the Moon, Mercury, the asteroids, 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.

Credit: NASA???.

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

But there is considerable uncertainty. The cratering rate cartoon below is quite problematic.

A cartoon of the estimated lunar cratering rate (Se-447).

Recently, there is some idea that the cratering rate may have fallen and then risen again before tailing off.

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.

METEOROID

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ASTEROID BELT

The small impactors (meteoroids) falling on planets or moons with atmospheres mostly burn up in descent.

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

Flooding of Basins by Lava

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.

On Earth and Venus and 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 Mars and Venus. In the case, of Venus obvious traces of this flooding has certainly been erased by later geological activity.

Continuing Geological Activity

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 RESIDUAL/RADIOACTIVE-HEAT GEOLOGY.

Residual/radioactive-heat geology causes VOLCANISM and, on Earth alone it seems, PLATE TECTONICS.

[Volcanism produced the Earth's atmosphere and oceans. Small amounts of volatiles were trapped in the refractory substances during accretion. Volcanoes outgas this material.]

Only Earth and Venus have very active RESIDUAL/RADIOACTIVE-HEAT GEOLOGY.

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 RESIDUAL/RADIOACTIVE-HEAT GEOLOGY.

Mercury, the moons, and asteroids are probably dead or close to dead in regard to RESIDUAL/RADIOACTIVE-HEAT GEOLOGY.

EROSION GEOLOGY by atmospheres occurs on Earth, Venus, and Mars, but only Earth has water erosion at present: Mars had some in the past.

Wind erosion is a weaker process than water erosion.

The airless bodies or thin-atmosphere bodies (like Mars) have continuing, but very slow meteoritic erosion. I will call this kind of geology METEORITIC GEOLOGY.

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

The Moon exhibits many small moonquakes 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, Europa and Io have more active TIDAL 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).

The colors are false I think. But Io is very colorful because of all the sulfur and sulfur compounds.

Credit: NASA.