ASTR 330: The Solar System

ASTR 330: The Solar System

Announcements

Dr Conor Nixon Fall 2006

• Homework assignment #6 out today.

• Extra credit term paper: due December 5th (next Tuesday).

• This lesson will have a slightly different format from usual.

• We will begin by splitting into groups to discuss one fact each about the solar system. Try to formulate an explanation in your group.

• We will then discuss the explanations to each fact in turn.


Lecture 25:

Planetary System Formation



Why study formation again?


• The idea of this lecture is to revisit the subject we broached back in Lecture 6: the theory of solar system formation.

• At this stage in the course, we have finished our ‘tour’ of the properties of the solar system in detail, so it is an appropriate time to see if we can explain how all these objects arose.

• If our theories still have problems, we should give them prominence for further research.

• I will not re-capitulate the basic formation scenario from Lecture 6: I will assume everyone is familiar with this!

• Our approach for this class will be to examine thirteen principal ‘facts’ about the solar system requiring explanation, as listed in M&O page 451. Will attempt to explain each.


1: Age


Fact 1: The oldest recorded age in the solar system is less than 4.6 billion years, even though the Galaxy that contains the solar system is much older than this. Furthermore, many meteorites share this common age, which we have called “the age of the solar system”.

• Around 100 years ago, a very different theory for the formation of the planets was in favor.

• In this theory, the Sun had existed for much longer than the planets. A chance encounter with a nearby star had gravitationally pulled material from the Sun, which then coalesced to form the planets.

• At the present time, our theories of stellar evolution predict that the Sun is about the same age as the date we derive for the planets.

• Therefore, the Sun formed together with the solar system, 4.6 Gyr ago.


Star Forming Regions


• McNeil’s Nebula, a newly discovered reflection nebula surrounding a newborn star in the region of the Orion Nebula (M78). The star is at the bottom tip of the bright nebula. The entire frame is less than 10 ly across,

• This nebula was discovered by amateur astronomer Jay McNeil on January 23rd 2004.

• Subsequent searches through older images showed that it became visible in September 2003.

Picture credit: Adam Block/KPNO/NOAA/AURA/NSF


‘Proplyds’


• This image snapped by the HST in 1993 just after the corrective optics were installed shows a portion of M42, the Orion Nebula.

• Five stars surrounded by gas and dust clouds, which we call proto-planetary disks (or ‘proplyds’ for short) are seen here.

• Four of the disks appear bright, and one appears dark, depending on how close they are to the brightest stars in the nebula.

Picture credit: C.R. O’Dell/Rice Univ/NASA


2: Rotation


Fact 2: The planets all move around the Sun in the same direction that the Sun rotates, and nearly in the plane that passes through the Sun’s equator.

• It is clear from the nebula and spinning disk model, that the central star forms out of the same material as the disk: the material from the original collapsing, rotating cloud.

• Hence, the central star preserves the same spin axis as the original cloud. This is also true of the disk.

• The disk must form in the equatorial plane of the star.

• Then, when the planets form from the disk, they also orbit the star in the same sense as the disk particles: in the equatorial plane of the star.


From Cloud To Disk


• The left-side image shows a disk (500 AU diameter) with a gap, due to the presence of an unseen planet.

• The right-side image shows a ring-shaped disk. The ring is 17 AU wide, with a 80 AU hole in the middle.


3: Angular Momentum


Fact 3: Although the Sun has 99.9% of the mass in the solar system, the planets have 98% of the system’s angular momentum.

• This is a serious problem! From our skater analogy, material accreting onto the Sun cannot have retained all its original angular momentum.

• There are two parts to the problem:

1. How does material lose angular momentum and fall into the star in the first place?

2. How does the star lose angular momentum and slow down? An interesting finding is that stars 15% more massive than the Sun rotate much more rapidly, whereas solar-type stars all rotate at about the same speed at the Sun.


3: Angular Momentum Transport


Three processes have been proposed:

1. Magnetic braking.

2. Solar Wind.

1. Turbulence.

We’ll examine the pros and cons of each in turn.


3a: Magnetic Braking


• Material in the inner part of the disk must be ionized.

• Outer layers of the Sun must spin more quickly than inner disk material, as we know from our skating analogy.

• Therefore, the magnetic field of the Sun will feel resistance as it drags through the plasma, which will slow down the rotation of the Sun.

• Main problem: predicts that more massive stars, having greater magnetic fields and ionizing radiation, should brake more strongly than solar-type stars, and so rotate more slowlymore slowly. The opposite is observed.

• Also, any angular momentum lost by the Sun must be gained by the disk material, so we are left with the problem of how the inner disk material loses angular momentum and accretes onto the Sun.


3b: Solar Wind


• In this scenario, we literally blow the angular momentum problem away.

• We know that stars like the Sun have strong T-Tauri winds in their youth, which ultimately clear away residual gas and dust in the disk.

• The particles lost from the star also carry away angular momentum.

• This theory also explains why more massive stars rotate faster: they do not have proper winds like the Sun, and therefore are not as good at losing angular momentum.


3c: Turbulence


• As well as the transfer of angular momentum from the star to the disk, we must also transport angular momentum within the disk.

• One way we can do this is by convection: computer modeling shows that turbulent eddies can develop, which can transport angular momentum.

• This scenario depends on whether the disk material cools rapidly or not: the more rapid the cooling, the less time in which convection can work.

• Finally, note that gravitational interactions between clumps of material in the disk (not to mention planets and planetesimals) can cause angular momentum to be transported outwards, while gas and dust move inwards.

• In summary, our understanding of angular momentum transport is far from complete.


4: Composition


Fact 4: The inner planets, which are composed primarily of the cosmically rare silicates and metals, are smaller and denser than the outer planets. The giant planets, in contrast, have a more nearly cosmic (or solar) composition, and their satellites, with the exception of Io and Europa, have lower densities, indicative of water ice and other volatiles.

• Our explanation for this fact is basically the temperature gradient in the disk, hotter at the center, and gradually cooling with radial distance from the center. Why?

• At the very center, gravitational potential energy of infalling material heats up the protostar and allows ignition to occur.

• Further out, in the disk, material falling from the extended cloud onto the disk (in the vertical direction) causes heating by the same way.


Evaporation and Condensation


• Towards the inner parts of the disk, temperatures would reach 2000 K, enough to vaporize ‘dust’: i.e. rock, along with ices. Hence, all the elements would be mixed together in the their original abundances.

• Subsequent cooling would sort the condensates in radial order: the ones with the highest melting temperatures condensing closest to the center.

• Inner planets: mostly rock and metal condensed in the inner solar system, where the young Sun provided enough energy to keep the lighter volatiles and ices from condensing. The gas was then removed by the solar wind before condensation could occur.

• Giant planets: in the outer solar system, temperatures were cooler, and the giant planets were able to accumulate large amounts of ices, and also H and He.


Mini-disks and Sub-nebulae


• The giant planets each formed out of a ‘mini-disk’, or sub-nebula, in which there must have been a radial decrease in temperature: a miniature solar system.

• Therefore, we would expect that in the cool outer parts of the sub-nebula, all species would condense, and we would see cosmically correct proportions of the elements, about half rock and half ice.

• This is indeed the what we find for Callisto and Ganymede, the outermost (regular) Jovian moons, with densities around 1.9 g/cm3.

• The inner (regular) moons of Jupiter, Io and Europa, show higher densities: more rock compared to ices. This again is what our theory would predict for the warmer inner parts of the nebula.


5a: Asteroids


Fact 5a: The asteroids, which represent a composition intermediate between the metal-rich inner planets and the volatile-rich outer solar system, are located primarily between the orbits of Mars and Jupiter.

• The asteroids are basically material left over from the formation of the planetary system.

• Some represent original denizens of the Mars-Jupiter gap, where the formation of an actual planet was probably inhibited by the nearby presence of massive Jupiter.

• Hence, growth of planetesimals stalled at a size of about 100-1000 km.

• Jupiter’s gravity removed most of the protoplanetary material in this region of space, perhaps also causing Mars to be smaller than would otherwise have been the case.


5b: Asteroid Variations


Fact 5b: Within the main asteroid belt, there is a distinct gradient in composition, with the outermost asteroids being richest in volatile elements and compounds.

• This is what we would expect: the same gradient as we see in the planets, with the inner planets mostly metal and rock, and the outer planets volatile rich.

• An interesting theory holds that some of the dark C-type asteroids (carbonaceous) at the outer edge of the belt might actually be captured comets, originally from the outer solar system.

• Once captured into their present orbits, much closer to the Sun, these comets would gradually lose their volatiles, until only ‘soot’ remained.

• In some respects, the asteroid belt may be a kind of ‘dumping ground’ like the central gravel on a freeway.


6: Meteorites


Fact 6: The primitive meteorites are composed of compounds representative of solid grains that probably formed in a cooling gas cloud of cosmic (solar) abundance at temperatures of a few hundred Kelvins. Some of these grains even predate the formation of the solar system.

• (In other words) We have evidence that certain microscopic grains in some meteorites and IDPs are interstellar material: from the original nebula. However, we do not have enough material to use radio-dating.

• How were these grains preserved?

• Quite simply, these grains must have been incorporated into KBOs and comets, far enough from the inner disk to avoid heating and vaporization. Hence, they are preserved intact from the time before the planets formed.


Ancient IDPs?


• IDPs (cometary dust) collected by NASA’s ER-2 high-flying aircraft appear to show a 1% composition of interstellar material, as judged by analysis of oxygen isotopic ratios.

• The Stardust mission collected dust from Comet Wild-2 in Jan 2004, and dropped a sealed capsule of comet dust to Earth in Jan 2006 now undergoing analysis.

Picture credit: (I) Washington Univ. (ii) NASA (iii) JPL/NASA


7: Comets


Fact 7: Comets, like some outer-planet satellites, appear to be composed largely of water ice, with significant quantities of trapped or frozen gases like carbon dioxide and methane, plus silicate dust and dark carbonaceous material.

• It is not a surprise that comets resemble the satellites of giant planets in composition.

• Our theory for the formation of comets is that they were largely formed in the giant planet region of the solar system, at temperatures of 30-100 K. They could not have formed out in the Oort Cloud: pressures were too low to allow solids to form.

• After formation, they must have been gravitationally scattered to the Oort Cloud by close encounters with the giant planets.


Oort Cloud


• Note that many icy planetesimals must have been ejected from the solar system entirely, or ventured too close to the Sun, and burned up.

• Some will have traversed the inner solar system before being ejected: and hence their material may have been ‘reprocessed’.

• Once in the Oort region, comets may have accumulated primitive grains, and hence be mixtures of primitive and non-primitive material.

• This image shows an artist’s impression of the Oort Cloud objects (blue points) surrounding the central Sun.


8: Isotopes


Fact 8: Specific isotope ratios established prior to solar system formation are preserved in objects formed at different distances from the Sun.

• This is a fascinating but rather complex issue.

• In short: there appear to be variations in isotopic abundances throughout the solar system which are either (i) preserved ‘fossilized’ anomalies inherent in the original nebula, or (ii) fossilized anomalies from early processes in the solar nebula.

• It is reasonable to expect that varying isotopic abundances can arise in stars, but there are many processes at work to erase such anomalies.

• So now we can explain Fact 8: although there was mixing (see Fact 9), the mixing was small enough that the variation in isotopic ratios through out the solar system has been preserved.


Oxygen Isotope Variation


• This figure shows that the isotope abundances in oxygen vary between the Earth, Mars, and meteorites and asteroids.

• The material returned by the Genesis spacecraft may answer these questions.

Figure credit: Caltech Genesis GPS


9: Inner Planet Volatiles


Fact 9: Volatile compounds (such as water) have reached the inner planets even though the bulk composition of these objects suggests formation at temperatures too high for these volatiles to form solid grains.

• This point hints at the problem of mixing, of material between the outer and inner solar system.

• Consider: how did the inner planets wind up with any volatile ices at all (e.g. the Earth’s oceans), if the disk temperatures were too high for ices to condense?

• Answer: the ices came later on from the outer solar system. The key to this was objects on eccentric orbits, which we call comets.


Late Heavy Bombardment


• One explanation for the presence of volatiles in the inner solar system, is that they arrived during the Late Heavy Bombardment.

• In the outer solar system, the distances between the planets are much greater, and hence, it would take longer to ‘clear up’ any remaining planetesimals.

• The LHB may be the time when Uranus and Neptune were scattering the last planetesimals of the outer solar system, some of which reached the inner solar system and added volatiles.

• As well as volatiles, such as water for the oceans, comets could also have brought amino acids and other raw materials which may have contributed to the formation of life.

• Many comets of course were scattered outward to form the Oort Cloud (Fact 7).


10: Retrograde Rotation


Fact 10: Despite the general regularity of planetary revolution and rotation, Venus, Uranus and Pluto all rotate in a retrograde direction (although their revolution is normal).

• Remember that, if all the planets formed out of a planar disk having a certain rotation axis, the planets should all be rotating with their axes nearly aligned with the axis of the Sun.

• The most likely explanation for these retrograde rotators is… what?

• Impacts of course!

• Besides the obvious features like impact basins and craters, impacts are probably responsible for: the Moon; the anomalously high metal content of Mars; and some planetary ring systems.


Uniformitarianism vs Catastrophism


• One of the great problems of 18th and 19th century geology was explain the shape of the Earth’s surface terrain. The prevailing view was that topology had all originated catastrophically, and that the Earth was relatively young (thousands of years).

• A different view was taken by James Hutton (1726-1797), and championed by Charles Lyell (1797-1875): that the Earth’s landforms had arisen by the action of comparatively small forces, such as wind and rain, acting over very long times (billions of years).

• So successful was uniformitarianism, that it prevented scientists from recognizing the importance of catastrophic processes, e.g. the crater density of the Moon, which is underestimated using present-day rates.

• Catastrophes (unusually rare and energetic events) are also implicated in the formation of the Moon, and the tipping of Uranus.


11: Satellite Systems


Fact 11: All the giant planets have systems of regular satellites orbiting in their equatorial planes. They resemble miniature versions of the solar system itself, …

• We envisage that, when the giant planets formed, the gravitation effects produced both a central condensation (which would become the planet) and a mini-disk (or sub-nebula).

• The mini-disk would then go on to form satellites in orbit about the parent planet, in an analogous way to the planets themselves forming around the parent star.

• The moon systems would clearly form in the same plane as the planet’s equator, and also orbit in the same direction as the planet turned.


Rings


Fact 11 (continued) …with the addition of rings, which the Sun does not possess (unless you consider the asteroid belt and the Kuiper Belt as ring systems circling the Sun).

• There are two possible explanations for ring systems:

1. Debris from original satellite formation. Material which was inside the Roche limit for the planet would not have coalesced into a satellite. This is more likely for the rings of Saturn than for the other ring systems, why?

2. A shattered moon (or moons). This seems more probable for the dark rings of the other three giant planets. Due to the expected high impact rates of inner moons, it seems likely that the inner moons could be completely destroyed (look at Mimas and Miranda!) If the resulting material fell inside the Roche limit, it could not reform into a satellite.


12: Irregular satellites


Fact 12: All four of the giant planets: Jupiter, Saturn, Uranus and Neptune; have one or more highly irregular satellites - either in retrograde orbits or with with high eccentricities.

• These moons were surely captured. Satellites forming with the planets could not assume such orbits, in such different rotational motion from the sub-nebulas which created the outer planets.

• The capture scenario may have involved a planetesimal venturing close to a giant planet, and experiencing frictional drag, thereby losing energy and falling into a capture orbit.

• Friction could come from either (i) a very extended atmosphere of the early planet, or (ii) the sub-nebular disk.

• Or, a third body must have been present.


13: Giant Planets


Fact 13a: All of the giant planets are enriched in heavy elements equivalent to 10-15 Earth masses; all have atmospheres rich in hydrogen and helium …

• The 10-15 ME enrichment in heavy elements refers to the core of the giant planets. These cores formed before the outer layers of the planet.

• After a certain maximum size (10-15 ME, apparently) the surrounding gas from the nebula could then be accreted directly onto the planet core. This nebular material formed the H- and He-dominated atmosphere.

• The current atmospheres are a mixture of the captured H and He, and also out-gassed volatiles (methane for example) from the core.

• Why did Uranus and Neptune capture less gas? Did they form too late? We do not yet know the answer.


Heat Excess


Fact 13b: .. and all (the giant planets) except Uranus are radiating substantial quantities of heat from their interiors.

• This fact takes in several processes.

• For Jupiter and Saturn, the heat excess seems to be generated by the ongoing differentiation of helium, condensing or raining out within the metallic hydrogen core.

• For Neptune, there is no metallic hydrogen core. But, Neptune is slightly heavier and denser than Uranus, and the heat seems to be heat from the original formation which is still escaping.

• Uranus, being less dense than Neptune must have been able to cool more thoroughly.


Quiz-Summary


1. Why do we see such a uniformity in ‘oldest ages’ for rocks in the solar system of 4.6 Gyr?

2. Why do we not see any rocks 10 billion years old?

3. Why do all the planets move around the Sun in the same plane and direction?

4. If angular momentum is conserved, why does the Sun not have most of the angular momentum in the solar system, when it has most of the mass?

5. Why are the inner planets smaller, denser, and depleted in volatiles compared to the outer planets?

6. Why are Io and Europa denser than Ganymede and Callisto?


Quiz-Summary


7. What caused the main asteroid belt?

8. What is the gradient in composition which exists in the asteroid belt, and what might have caused it?

9. What is an IDP? Why have we never dated a substance older than 4.6 Gyr? Where might we reasonably expect to look for such remnants?

10. Why do comets have similar compositions to outer planet satellites?

11. How did comets get where they are now?

12. What is curious about oxygen isotope ratios in the solar system?

13. How did the inner planets acquire volatiles?


Quiz-Summary


14. Which planets rotate backwards? What could have caused this?

15. How did the regular satellite systems of the outer planets form?

16. What about the irregular satellites systems?

17. How did the (i) cores (ii) atmospheres of the outer planets form?

18. Why does Uranus alone not generate more heat energy than it recieves from the Sun?

ASTR 330: The Solar System

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