an introduction to astronomy part v: the moon, our nearest neighbor lambert e. murray, ph.d....
TRANSCRIPT
An Introduction to Astronomy
Part V: The Moon, Our Nearest Neighbor
Lambert E. Murray, Ph.D.
Professor of Physics
We know more about the Moon than any object in our solar
system besides Earth.
Because we’ve been there!
The Apollo program ended in December 1972 with Apollo 17 after 6 landings (Apollo 13 excepted) and a total of 12 astronauts who
walked on the moon ‘s surface.
On July 20, 1969, Neil Armstrong was the first to step onto the moon’s surface as part of Apollo 11.
Surveyor III and Apollo 12 on the Moon View of two U.S. spacecraft on the surface of the moon, taken during the second Apollo 12 extravehicular activity (EVA-2). The Apollo 12 Lunar Module is in the background. The Surveyor III spacecraft is being inspected by an Apollo 12 astronaut in the foreground. Surveyor III was an unmanned vehicle that achieved the second successful US soft landing on the moon. The astronauts retrieved parts of the earlier spacecraft for analysis.
Lunar Boulder at Taurus-Littrow Scientist-Astronaut Harrison H. Schmitt is shown here photographed standing next to a huge split boulder during the third Apollo 17 extravehicular activity at the Taurus-Littrow landing site on the Moon. The lunar rover, which transported Schmitt and mission commander Eugene A. Cernan to this location from their Lunar Module, is seen in the background..
Lunar Surface Features
Dominant Lunar Surface Features:– Craters– Maria– Highlands– Lunar Regolith (lunar “soil” which may be 100’s of
feet thick) All these features can be explained by the effects
of massive bombardments of the Moon’s surface early in its lifetime and the subsequent continual exposure to intense solar radiations which helps to break down and sterilize the surface.
Topographical Features: Craters
Craters come in all sizes – many overlapping one another.
Main Topographical Regions of the Moon
Maria
Highlands
Highlands and Maria – Another View
Major Topographical Regions of the Moon The Moon has two topographically distinct
regions:– Maria (Latin for “sea”) – the darker regions of
Moon, which are relatively smooth and flat. Once thought to be seas, these regions are now
known to be relatively young regions formed from lava flows on the surface of the Moon.
They are found almost exclusively on the Earth’s side of the Moon.
– Highlands – the lighter regions of the Moon’s surface which are mountainous.
These regions consist of older rock, and are extensively cratered.
Topography and Age
More the 800 lbs of Moon rocks were returned to Earth by Apollo missions.
Radiological dating of these rocks showed that:– Rocks from the Highlands ranged in age from
3.8 to 4.3 billion years (the age of the oldest rocks in the crust of the Earth)
– Rocks from the Maria were all much younger
Number of Surface Craters and Age We postulate that the relative age of the surface of
many planets and moons can be determined from the number of craters present (and whether they overlap)– “Young” regions have fewer craters and few overlap.– Older regions have a large number of craters, many of
which overlap. If the planet or moon has little or no atmosphere to
promote surface changes due to erosion, the craters which were formed early in the lifetime of the planet or moon may still be preserved.
The maria on the Moon are regions where lava flowed over the surface and destroyed older craters.
Number of Craters and
Age II
Craters on the
Far Side
50 mi
Topographical Features: Rays
Rays of material ejected from craters during impacts can be easily seen on the surface.
Rays
Topographical Features: Rilles
Numerous Lunar canyons called Rilles may be seen on the surface of the Moon. These may have been formed by lava flows.
Topographical Features: Scarps
Scarps are cliffs on the surface of the Moon
Lunar Seismic Studies The Apollo missions left 4 nuclear powered
seismic stations on the moon. These instruments revealed that the moon had few
“moonquakes” of any significant size (most can be correlated with tidal forces from the earth). – Some may result from continued cooling of the lunar
interior– Some are due to meteor impacts– Several moonquakes were generated by the deliberate
crashing of a Lunar Module on the surface. The data derived from these studies are consistent
with the picture of the moon represented on the next slide.
Comparison of Near and Far Sides of Moon
• The assymetrical nature of the moon (the fact that the crust is thinner on the earth side than the farside) is believed to explain the major difference in appearances between the farside (first observed by a soviet spacecraft) and the nearside.
• The farside has almost no maria regions and is very heavily cratered.
• These differences can be clearly seen in the Clementine images shown on the next slide.
Rotation of the Moon
The Moon passes through a complete phase (from full moon to full moon) in a synodic month (29 ½ days). [A sidereal month is 27 days, 7 hrs, 43 min.]
During this time the same side of the Moon is always facing Earth. This means that a point on the surface of the Moon is in daylight for about 15 days, where the temperature reaches about +130 degrees C, and in darkness for about 15 days where the temperature can reach –110 degrees C.
The Origin of the Moon I
Before the Apollo landings, there were three principle theories about the origin of the Moon:– Capture: The Moon was formed far from Earth and
later captured by Earth’s gravity
– Fission: The Moon was separated from the material that formed the Earth.
– Co-accretion: The Moon was formed at about the same time and in the same place as the Earth (the double-planet hypothesis).
The Origin of the Moon II
The results from the Apollo missions indicate problems with all three of these theories.
The model now favored is the giant impact model– A large body impacted the Earth and material from the
outer layers (the mantle) of the Earth accreted to form the Moon. This would explain the similarity in composition between the moon and the earth’s outer crust. Both are quite deficient in heavier materials.
Computer Simulation of Moon’s Formation
Ocean Tides
• One of the most obvious influences of the moon on the earth is its connection with the tides.
• Knowledge of tidal occurrences was very important in the development of the maritime age when wind driven ships were the source of commerce.
• The first “mechanical computers” built, were built in order to predict the occurrences of tides at English harbors.
The tides result from the DIFFERENTIAL gravitational force the Moon exerts on the Earth and the oceans on its surface.
The Difference in strength between these forces
The Cause of the Tides
The Characteristics of Tides
• Two high tides and two low tides occur each day.• The high tide “leads” the moon slightly due to friction
between the water and the surface.• Tides are larger at full moon and new moon stages.
The Sun’s Influence on Tides
• While the Sun exerts a larger total gravitational force on the Earth than the Moon does, the differential force (the force on the surface minus the force on the center) is smaller than the Moon’s.
• The combination of the effect of the Moon and Sun give rise to spring tides and neap tides:
• When the Sun’s forces and the Moon’s forces act together, one gets the highest possible tides.
• When these forces act at right angles to one another, one gets the lowest tides.
Tidal forces of Sun & Moon act together.
Tidal forces of Sun & Moon oppose each other.
SpringTides
NeapTides
Variations in Times and Heights of Tides
Thus, we expect the tides at any one location on the Earth occur at roughly 12 hour intervals due to the rotation of the earth. However, the actual timing of tides at a given location, and the severity of the tides is strongly influenced by the local conditions at the coastline.
Also, the severity of the tides are influenced greatly by the relative alignment of the Sun and Moon (the spring and neap tides), but the strength of the tides also depends on the relative distance to the Moon, and the Moon’s orbit is slightly elliptical.
Tidal Effects on theEarth-Moon System
These tidal forces (which also occur in earth’s solid crust) result in a gradual slowdown of the earth’s rotational speed of about .002 seconds/century.
By Newton’s 3rd Law, the Earth also exerts a force on the Moon which accelerates the moon increasing its orbital speed, and consequently increasing its orbital radius (Kepler’s 3rd Law). The Earth-Moon distance increases about 3 cm per year.
Current technology allows the direct measurement of these two effects!
Water on the Moon
News Release: NASA finds Water on Moon: http://www.space.com/common/media/show/player.php?show_id=37&ep=1
End of Part V