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Unformatted text preview: Grand Tour of the Grand Tour of the Solar System, Part 1 Discussion Questions
Discussion Questions If the Moon was formed by an impact on Earth knocking some of the Earth’s mantle into orbit, why is its density ~40% less than the Earth’s? Why is Mercury’s density about the same as Earth’s What’s wrong with Pink Floyd’s album title “The Dark Side of the Moon?” What’s the chain of logic that is used to infer that there was a period of very frequent impacts on the Earth early in its history? Why is the night side of Mercury one of the coldest places in the solar system? What does the density of a planet tell you about its bulk composition? Inner Solar System Outer Solar System Planetary Sizes
The Sun and nine Planets of the Solar System are shown here to scale
in terms of their relative sizes, but not their distances. The Solar System at 1:1010 Scale The closest star system to the Sun, Alpha Centauri, is located 4.4 light years away.
At the 1:1010 scale of the Smithsonian model, it would be located in San Francisco. Planetary Categories
Planetary Categories The 9 8 major planets of the Solar System
can be divided into 2 classes: Terrestrial Planets Mercury, Venus, Earth, Mars
"Terrestrial" or Earthlike worlds Jovian Planets Jupiter, Saturn, Uranus Neptune
"Jovian" (Jupiterlike) or Giant planets Pluto is now known as a “Dwarf Planet”. Large moons, asteroids, and comets are in classes
of their own. Fundamental Differences
Fundamental Differences Property
Size & Mass
Large Density High Low
Rock + Metal
Gas Atmosphere Thin
Yes Surface Temp. Higher
Very Cold Internal Heat Little/None
Lots Mass and Density
Mass and Density Where’s mass in Solar System?
% Total Mass
All Other Planets
Moons and Rings
0.0000001 Density ρ = Mass / Volume
Volume V = 4/3 (π R3)
where R is the planet’s radius.
Clues to planet's composition:
Examples: Water: ρ = 1 g/cm3
= 1 g/cm
Rock: ρ ≈ 2.5 g/cm3 2.5 g/cm
Metal: ρ ≈ 810 g/cm3 810 g/cm
Earth: ρ ≈ 5.5 g/cm3: implies rock+metal interior 5.5 g/cm
All the inner planets have high densities: 3.95.5 g/cm3
The Moon is somewhat less dense: 3.3 g/cm3
Gas Giants much less dense: 0.71.6 g/cm3 Terrestrial Planets
Terrestrial Planets Mercury Venus Earth Mars D = 0.39 AU D = 0.72 D = 1.0 D = 1.5 ρ = 5.4 g/cm3 ρ = 5.3 ρ = 5.5 ρ = 3.9 T = 700 K
P = 0.24 years T = 740
P = 0.62 T = 290
P = 1.0 T = 215
P = 1.88 Jovian Planets
Jovian Planets Jupiter Saturn Uranus Neptune D = 5.2 AU D = 9.5 D = 19 D = 30 ρ = 1.3 g/cm3 ρ = 0.7 ρ = 1.2 ρ = 1.6 T = 125 K
P = 12 years T = 95
P = 30 T = 60
P = 84 T = 60
P = 165 The Moon The Moon: The View from Earth
The Moon: The View from Earth
The Moon is in synchronous rotation: it always shows the same side to the Earth: its
nearside. Because the Moon’s orbit is elliptical, the Earth-Moon distance varies over time.
At apogee, the Moon is farthest from Earth. At perigee, the Moon is closest to Earth.
The apparent size of the Moon can change by as much as shown below… Apogee Fig. credit: http://www.fourmilab.ch/earthview/moon_ap_per.html Perigee The Moon: Basic Properties
The Moon: Basic Properties
Value Comments Radius 1,738 km 0.27 REarth Mass 7.35x1022 kg 1/81 MEarth Density 3.3 g/cm3 < ρEarth ~ 5.5 g/cm3 Composition Rocks, metals Gravity 1.62 m/s2 0.17 gEarth Distance from Earth 384,000 km ~ 357,000 – 407,000 km range Orbit around Earth 27.32 days Sidereal period of revolution Rotation (or Spin) 29.53 days “Synchronous rotation” Surface Temperature 380 K / 120 K Day / Night range The Moon: Highlands vs Maria
The Moon: Highlands The Moon presents two main types of surface terrains: Highlands and Maria.
Highlands: light-colored, heavily cratered, topographically high, and very rugged.
Maria (plural of mare): dark-colored, sparsely cratered, low, and relatively smooth. The maria actually cover only
16% of the lunar surface.
Most are found on the nearside. The view of the Moon at left is NOT the one we typically
see from the Earth; it was taken from spacecraft. The Moon: Highlands
The Moon: Highlands
Highlands are areas on the Moon where its impact cratering record has been best preserved.
They are the oldest terrain on the Moon. Rock samples from Highlands are 3.8 - 4.4 GY old.
Much of the nearside and almost all of the farside of the Moon are made up of Highlands.
Highlands are composed mainly of anorthosite (silicate rich in plagioclase feldspar). The Moon: Maria
The Moon: The Maria are dark-colored areas on the Moon where the impact cratering record is sparser.
They are younger terrain than the Highlands. Samples of Maria are ~ 3.3 - 3.8 GY old.
Maria are concentrated on the nearside, although some occur on the farside.
Maria are composed mainly of basalt. The Moon: Craters
The Moon: Craters
• The lunar surface presents many craters.
• Originally thought to be of volcanic origin,
they became widely accepted to be of
impact origin instead only ~ 45 years
ago. The debate was finally settled when
lunar rock samples were brought back.
• Terrain on the Moon with higher crater
concentrations (Highlands) yield rock
samples ~3.8 - 4.4 GY in age.
Terrain with lower crater densities (Maria)
yield rock samples ~3.3 - 3.8 GY old.
• The big difference in impact crater densities between Highlands and Maria compared to
their relatively modest difference in age tells us that impacting rates dropped drastically
between 4.4 GY ago and 3.8. GY ago. Conclusion: there was an initial stage of Heavy
Bombardment after which meteorite impact rates dropped quickly to ~ present levels.
• This Heavy Bombardment must have been experienced by all bodies in the Solar System.
The major role of impacts in the formation and subsequent evolution of all planetary bodies
is one of the key lessons learned from the study and exploration of the Moon. The Moon:
Missions Surveyor (USA)
Luna (USSR) The Moon: Surface
The Moon: Surface
Lunar surface is uniformly gray, mantled by thick dust, and has many boulders.
Topography is relatively soft and smooth: no jagged peaks; rounded hills instead.
Distances are difficult to estimate because of lack of atmosphere and references. The Moon: Lunar Soil
The Moon: Lunar Soil
Lunar soil includes very fine-grained material and also coarser chunks of rocks.
Over time, micrometeorite and meteorite impacts comminute (break down and
pulverize) surface materials, producing a surface rubble layer called regolith.
The regolith contains impact-welded breccia rocks and glassy agglutinates.
As “weathering” is extremely slow, footprints on the Moon may last >>10 6 years. The Moon: Lunar Rocks
The Moon: Lunar Rocks
Almost all lunar rocks brought back are breccias: rocks formed by the welding together of many other rock fragments, in general under the effect of impacts.
A given breccia may contain bits and pieces that originated from different areas on the Moon.
Highland rocks are mainly anorthosites,
igneous rocks made up of plagioclase
feldspar (lightweight aluminum-rich
silicates). Highland rocks formed early in
the Moon’s history from the crystallization of
material that floated up to the top of an
initial global magma ocean.
Apollo 17 breccia 79135 Maria rocks are mainly basalts, igneous rocks made up of relatively heavierweight ferro-magnesian minerals. Mare basalts were emplaced at the surface
of the Moon via very fluid volcanic eruptions that took place hundreds of million
years after the anorthosite crustal rocks crystallized. Lunar Ice
Radar, neutron remote sensing, and infrared spectroscopic measurements all consistently point to the existence of water ice in permanently
shadowed regions near the Moon’s poles. The Moon: Interior
The Moon: Interior The Moon’s interior is poorly known but based
on gravity data from robotic orbiters, Apollo
seismic data, and theoretical models, it is
The lunar crust is asymmetric in
thickness: thin on nearside (~20-60
km), thicker on farside (>~100 km).
The Moon probably has a huge
mantle made of rocks roughly
similar to Earth’s mantle rocks.
The Moon might have a very
small iron core. A large iron core
is not possible because the Moon’s
average density is low: ~3.3 g/cm3.
Also, the absence of any active magnetic
field indicates that most of the iron in any
core is probably no longer molten. Copyright 1999 Calvin J. Hamilton Possible Origin: HeadOn Collision
Possible Origin: HeadOn Collision Outcome consistent with all data available:
The Moon forms by reaccretion of mostly mantle material from both planets. Mercury
Mercury Mercury: Closest to the Sun
Mercury: Closest to the Sun Mercury: Basic Properties
Mercury: Basic Properties
2436 km Comments
0.38 REarth Mass 3.3x1023 kg 0.055 MEarth Density 5.4 g/cm3 < ρEarth ~ 5.5 g/cm3 Composition Rocks, metals Gravity 3.7 m/s2 0.38 gEarth (same as Mars) Distance from Sun 0.39 AU Orbital Period 58x106 km (range: 46
88 Earth days Obliquity 0o Rotation (or Spin) 58.6 Earth days No seasons!
2/3 of orbital period exactly Atmosphere None (almost) Surface Temperature 150oC to +425oC Moons None ~ 3 Earth months Less hot than Venus! Mercury:
Little was known about Mercury’s surface
from Earth-based visible telescopic observations (below).
A lunar-like appearance was revealed when
Mariner 10 imaged the planet in 1974 (right). Mercury’s
Highlands. Mercury: Mariner 10
Mercury: Mariner 10 Mercury has been visited by only
2 spacecraft: Mariner 10 (US)
and Messenger (US).
Mariner 10 made three fly-bys
of Mercury from 1974 to 1975.
~ 45% of Mercury’s surface was
imaged by Mariner 10. Mercury: Messenger Mission
Mercury: Messenger Mission
New NASA mission to Mercury
Arrival: 3 years ago for a flyby
Orbiter will map Mercury for 1 Earth year
starting March 18, 2011:
Imaging, spectroscopy, altimetry,
magnetic field studies.
Spacecraft use solar power (of course!)
Spacecraft also has a sunshade…
Mission website: http://messenger.jhuapl.edu/ Mercury’s Unseen Side
Mercury’s Unseen Side
~80% now imaged Mercury: Impact Craters
Mercury: Impact Craters
Because Mercury’s surface gravity is ~twice
that of the Moon, impact craters tend to be
shallower and ejecta blankets less extensive
than on the Moon.
Mercury also bears the scars of giant impacts,
like the Caloris Basin (left). The formation of
Caloris led to chaotic terrain on the opposite
side of the planet (below)… Mercury: Volcanism
Most of Mercury’s surface is saturated with craters, suggesting most terrains
date back to the end of the Heavy Bombardment, i.e., ~3.8 BY old or more.
In some areas, Mercury seems to have been resurfaced:
Relatively dark, smooth areas with fewer impact craters are believed to be
younger lava plains, not unlike the lunar maria although much less extensive. Mercury: Interior
is high considering
the planet’s small
Large dense core.
Mercury has a (weak)
magnetic field, which
suggests this core is
It is not known whether
the magnetic field is still
active or remnant; thus,
it is not known whether
the core is still molten.
Mercury and the Moon
is only skin deep. They
are very different worlds. Mercury: Tectonics
As Mercury’s mantle and huge core cooled and solidified, they shrank significantly
As a result, the planet’s crust shriveled and got crumpled: compressional tectonics.
This did not happen as dramatically on the Moon because the amount of shrinking was
not as great there: the Moon does not have a huge metallic core like Mercury does.
Lobate scarps: Mercury:
Ice at the Poles?
Radar studies from Earth show strong reflectivity at
Mercury’s poles. Is it H2O ice trapped in permanently
shadowed craters? Alternative: sulfur deposits? Discussion Questions
Discussion Questions If the Moon was formed by an impact on Earth knocking some of the Earth’s mantle into orbit, why is its density ~40% less than the Earth’s? Why is Mercury’s density about the same as Earth’s What’s wrong with Pink Floyd’s album title “The Dark Side of the Moon?” What’s the chain of logic that is used to infer that there was a period of very frequent impacts on the Earth early in its history? Why is the night side of Mercury one of the coldest places in the solar system? What does the density of a planet tell you about its bulk composition? ...
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- Spring '11