Transcript Planet Building

Planet Building Part 1
Solar Nebula’s Chemical
Composition
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Chemical Composition
• Everything astronomers know about the solar system
indicates that it began as an interstellar gas cloud.
– Cloud would have been mostly hydrogen with some
helium – small amounts of the heavier elements.
– One piece of evidence is that we see the composition of
the gas cloud of the composition of the sun.
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Chemical Composition
• The suns elemental composition revealed by
spectrum analysis indicates it is mostly
hydrogen, helium (about 25%), and 2% other
heavier elements.
– Some hydrogen has been converted to helium due
to fusion reactions.
– Astronomers draw the conclusion, looking at
other stars as well as the sun, that the solar
nebula must have had the same composition.
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Chemical Composition
• We see evidence reflected in the planets – as
we have discussed.
– Inner planets are rock and metal while outer planets are
rich in low-density gases such as hydrogen and helium.
– The chemical composition of Jupiter resembles that of the
sun.
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Chemical Composition
•
If the low-density gases were allowed to escape from a body
such as the sun or Jupiter, the remaining heavier elements
would resemble Earth’s chemical composition.
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Conversion of Matter
• Important evidence for SNT comes from how
nebular gas converted into solid matter.
From Orion Nebula,
what appears to be
disks of dust and gas
surrounding newly
formed stars.
From: http://atropos.as.arizona.edu/aiz/teaching/nats102/mario/solar_system.html
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Conversion of Matter
• The density-variations originated when the
Solar System (SS) first formed solid grains.
– The kind of matter that could condense in a
particular region depended on temperature and
the gas that was present.
– In the inner regions the temperature was around
2,240 degrees Fahrenheit.
– Only compounds with high melting points could
condense (e.g. metal oxides and pure metals).
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Conversion of Matter
• A little farther out in the nebula – it was
cooler allowing silicates (rocky material) to
condense (along with metal).
– Mercury, Venus, Earth, and Mars are composed of
a mix of metals, metal oxides, and silicates.
– Mercury is composed of more metals (closer to
sun) with less metal as you proceed outbound
from the sun.
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Ice/Frost Line
• Beyond Mars, there is the ice line, a boundary
beyond which water vapor freezes to form ice
particles.
• Farther still from the sun, compounds like
methane and ammonia condense to form
other types of ice.
• Water vapor, ammonia, and methane were
abundant in the nebula.
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Ice/Frost Line
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Ice/Frost Line
• Due to the abundance of water, methane, and
ammonia, the nebula would have been a
“blizzard” of ice particles.
– Small amounts of silicates and metal (far, far less
than in the inner SS) would also have been found
there (having condensed).
– We know this because, among other evidence, the
Jovian planets are a mix of ices and small amounts
of silicates and metal.
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Condensation Sequence
• Is the sequence in which different materials
condense from the gas as you move away
from the sun – toward lower temperatures.
– The condensation sequence implies that different
kinds of materials will condense in predictable
ways – which is exactly what we see.
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Condensation Sequence
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Common Misconceptions
• It is a common misconception that the solar
nebula was sorted by density.
– Heavier metals and rock “sinking” toward the sun
and low-density gases blown outward.
– This is not the case, the chemical composition of
the solar nebula was originally roughly the same
throughout the disk.
– The important factor was temperature. The
inner nebula was hot and the outer cold.
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Common Misconceptions
– Only rocks and metals could condense in the inner
nebula.
– The outer nebula, beyond the ice line, formed lots
of ices along with metals and rock.
– Ice line is the boundary between high-density
planets and low-density planets.
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