Download 105 Chapter 8 Review Notes • The Nebular Hypothesis, which is the

105 Chapter 8 Review Notes
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The Nebular Hypothesis, which is the currently favored idea on how our solar system
formed, involves formation of a protosun within a rotating and contracting cloud of gas and
dust that flattened into a disc and separated (by clumping mass) into the planets.
Observations of discs in nebulae elsewhere in our galaxy support this model (so far).
Angular momentum was apparently (perhaps by viscous drag in a very dense gas/dust
cloud) transferred outward, as the Sun rotates relatively slowly and the planets have most of
the angular momentum in the solar system today (this is challenging for the current model
to explain)
Much of original dust in the nebula was probably vaporized as gravitational potential
energy was converted to heat in infalling material and the protosun heated things up in the
dense cloud early on
The nebular material began to cool and condense as temperature later dropped (due to more
efficient heat release through the clearing nebula). Condensation would have proceeded
from most refractory (highest melting point) to most volatile (lowest melting point) with
time and distance from the Sun
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Aluminum oxides
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Metals
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Silicates
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Sulfur
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Water Ice
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Other ices (carbon dioxide, ammonia, methane, etc.)
When particles collided in the early solar system, they could do any of three things:
o Fragment (high velocity impact)
o Bounce off (moderate velocity impact)
o Clump together (low velocity impact)
• Clumping together is what we need to nucleate planetary embryos, which works
well for objects in similar orbits
Column mass (amount of mass from top to bottom of flattened nebula disk) decreases
outward from protosun to outer reaches of solar system, with a “bump” of suddenly
increased mass where water ice was able to condense (slightly inside orbit of Jupiter)
Pattern led to runaway growth of Jupiter, and hindrance of formation of Uranus and
Neptune (took much longer)
• Delayed formation of Uranus and Neptune until after powerful solar winds of TTauri phase of Sun (after about 1 million years) prevented them from gathering
much H and He gas because most of the remainder of these gases got blown out
of the solar system
Planetary embryos formed in inner solar system (according to computer modeling) before
the T-Tauri phase began, allowing Earth to have perhaps collected H and He into its earliest
atmosphere
• T-Tauri Sun would have blown all of this H and He away
Jupiter also largely formed prior to T-Tauri onset, allowing collection of lots of H and He
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Jupiter “kernel” - ~400 k years for form (mass of about 10X that of Earth)
Saturn kernel – (~2 Ma – after onset of T-Tauri winds)
Uranus kernel – (~10 Ma)
Neptune kernel – (~30 Ma)
Accretion of outermost planets may have continued for nearly 1 Ga
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Planets may have migrated after formation, but not clear which direction or how much
Missing planet at asteroid belt was caused by Jupiter’s influence preventing “clumping” of
mass (impacts too energetic) at that distance from the Sun
o ~99.9% of mass in that region was likely ejected by Jupiter early on
(Oort Cloud)
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Satellites of gas giants formed in protosatellite discs (similar to mini versions of the solar
system disc) around these large planets
o Evidenced by prograde orbits of all large satellites (except Triton) and
their orbital positions in the equatorial planes of the planets
o Density trend in Galilean Satellites of Jupiter (Io, Europa, Ganymede,
Callisto) suggests significant heating in inner part of disc by Jupiter’s
large gravity (gravitational potential E converted to kinetic energy by
infalling material)
o Saturn, Uranus, and Neptune generated less heat and so don’t show this
trend
Triton’s retrograde orbit, and high inclination suggest it was a Kuiper Belt planetary embryo
(as is Pluto) that was captured by Neptune later on
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All satellites (and asteroids) less than 200 km in diameter are irregular in shape and seem to
represent collisional fragments, rather than fully accreted bodies
o Ring particles around gas giants are probably debris fragmented by
impacts and Roche Limit destruction
♣ Roche Limit - the distance within which a celestial body held
together only by its own gravity will disintegrate due to a second
celestial body's tidal forces exceeding the first body's
gravitational self-attraction (Wikipedia.com)
♣ Very small bodies (~a few kms or less) can hold together within
this distance (a good thing w.r.t. spacecraft!), but larger ones (10s
of kms and up) cannot