Astronomy 1010-H
Planetary Astronomy
Fall_2015
Day-27
Course Announcements
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How is the sunset/sunrise observing going?
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SW-chapter 7 posted: due Fri. Oct. 30
SW-chapter 8 posted: due Wed. Nov. 4
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Exam-3 Wed. Nov. 4: Ch. 6, 7, 8
I will collect the L-T books on Monday, Nov. 23
Take more astronomy!
Registration for the Spring semester starts soon so think
about taking more astronomy.
ASTR-1010/1011: Planetary Astro & lab (Tell your friends)
ASTR-1020/1021: Stellar Astronomy & lab (Reg. + Honors)
ASTR-2020: Problems in Stellar Astronomy
ASTR-3010: History of Astronomy
ASTR-3040: Intro. To AstroBiology
PHYS-2468: Intro. To Physics Research
ASTR-3030/3031: Instrumentation & Techniques
 Earth’s Moon and the terrestrial planets
(Mercury, Venus, Earth, and Mars) have
similar and dissimilar properties.
 Must be able to explain the differences.
 Comparative planetology: studying planets
by comparing them to one another.
The Earth’s Interior
 Layers:
 Crust: continents (low density silicates) and basins
(basalt: higher iron content).
 Mantle.
 Core (iron, nickel and other dense materials).
 Produced by differentiation in the early Earth: dense
materials sink; low-density materials rise.
Geology & Habitatbility
 Earth's Interior – Seismic waves

Crust – lowest density, Al, Si, Ca

Mantle – rocky “plastic”

Outer Core – molten

Inner Core – solid, Fe, Ni
 Earth's Interior – Differentiation

Melting and sedimentation of heavy material to core

Al26 ==> Mg26 contributed a lot of early heat
Four processes
have shaped Earth:
 Impact cratering.
 Tectonism
• modifications of
the crust.
 Volcanism
• igneous activity
• magma/lava.
 Erosion.
 Material falling from
space onto a planet’s
surface create impact
craters.
 Secondary craters can
be caused by falling
ejecta from the impact.
 The Moon and all
terrestrial planets
experienced this.
 Large impacts can melt
and vaporize rock.
 Venus and Earth have relatively few craters.
 Craters on Mars suggest it was once wetter.
 Mercury and the Moon are covered with
craters.
Hadean Earth, Dawn of Life
 Late Heavy Bombardment – ~3.9 Gyr ago

Relatively quiet between formation and LHB

Since then, protected by Jupiter
 Sterilizing Impacts

350-400 km in diameter
Completely vaporize the oceans
Global surface temperature rise 2000 C (3600 F)
Last ~4.2-3.8 Gyr ago
 The number of
craters indicates the
surface’s age.
 More craters means
an older surface and
minimal geologic
activity.
 Tectonism and
erosion can erase
craters.
 Rocks returned from Moon missions (1969–
1976) give ages through radioactive dating.
 Almost all cratering happened in the first
billion years of the Solar System.
On the Moon
 Rocks returned in the Apollo missions (1969-1972) give
ages.
 Rocks from different places show rate of accretion in
the early Solar System.
 Accretion rate fell sharply after a billion years.
 Older surfaces have more craters because they were
formed when the cratering rate was higher.
Concept Quiz
The Moon Long Ago
Imagine taking a picture of the Moon about 2
billion years ago. What would you expect
to see?
A. It would have many fewer craters.
B. It would have many more craters.
C. It would have about as many craters as it
does now.
Radioactive Dating
 Some elements can decay from one to another (e.g.,
uranium to thorium).
 These changes take place at known rates.
 Parent element declines, daughter element
accumulates.
 Ratio of parent to daughter abundance gives the age of
the rock.
 Age = time since rock was last molten.