PURDUE UNIVERSITY

EAS 105-THE PLANETS

Prof. Robert L. Nowack

 

Lecture 8

 

 

This shows some illustrations from the Jules Verne Novel “From the Earth to the Moon” published at the end of the 19th Century.

 

 

 

 

A little over a century later, we heard Neil Armstrong say from the Moon's surface:

"One small step for man..."

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The Moon is Earth's only natural satellite and is really a small planet.  With a diameter of 3,476 km, its diameter is less than a flight from New York to Los Angeles.  Its total surface area is about as large as North America.  The mass is about 1/80th that of Earth at 7.3 x 1022 kg.  The Moon revolves around the Earth at an orbital distance of about 30 Earth diameters away.  With respect to the "fixed stars", the period of revolution is 27.32 days (the Sidereal month).  However, the Synodic month, (the time period between a given phase of the Moon) is 29.53 days.

 

The Moon has the amazing property that its period of rotation about its own axis is also 27.32 days.  This means only one side faces toward Earth at any particular time.  Most of the far side of the Moon is never seen from Earth!  (prior to Apollo!)

 

This is an excellent example of gravitational phase locking which will be studied further with respect to the planet Mercury.

 

 

 

 

Galileo was the first to look at the Moon through a telescope in the year 1610.  It became clear from seeing the Moon this way that it was rugged and mountainous, similar to Earth.

 

 

 

 

Apollo 11: Sea of Tranquillity           Apollo 12: Ocean of Storms          Apollo 14: Fra Mauro

Apollo 15: Hadley-Apennine            Apollo 16: Decartes                      Apollo 17: Taurus-Littrow

 

Photograph of lunar surface showing major features:  Dark Maria covered by younger basalt lava flows and light-colored Highlands, whose dense cover of craters indicate their great age.  White circles show the Apollo landing sites.

 

 

 

What would it feel like to stand on the Moon?

 

 

 

 

First thing one would notice is that there is no air!  You would have to live in a space suit.  It would have to be heated and cooled because temperature can reach 100o C during the day and –153o C at night.  In fact, most Apollo missions landed during "early morning" to avoid the temperature extremes.  Also the Moon is bone dry, it has no water!  The pull (or force) due to gravity would be about 1/ 6th that of Earth's.  It would be much easier to jump off the ground, and one would seem to float before falling back.  This reduced force of gravity explains why the Moon could not retain an atmosphere.

 

 

Footprint of Neil Armstrong or Edwin Aldrin in the Lunar Soil

 

 

 

 

The soil (or regolith) is powdery, somewhat like kitchen scouring powder.  There are also larger rocks.  Many of these turn out to be a conglomeration of bits of stone and sand that have been packed together, called Breccia, in boulder sized chunks.  Surrounding terrain has two distinct types:  (1) Maria (Latin for seas) and (2) Highlands.  Most Apollo missions took place on Maria regions.  These are flat plains stretching for hundreds of kilometers and appear dark when seen from Earth.  The other kind of terrain called Highlands is mountainous and densely covered with circular crater structures. 

 

Since the Moon is smaller than the Earth, the lunar horizon appears closer.  Over 30,000 craters can be seen on the Moon from Earth on just the visible (near) side.  Some Highlands have such an intensely cratered surface that the craters crowd out each other.  The following photographs show numerous craters.  These pictures are images of the far side of the Moon taken during the Apollo 8 mission.

 

 

 

 

 

 

 

Craters are more widely spread out in the Maria regions.  In contrast, the Earth's surface shows relatively few craters!   Since the Moon has no atmosphere, the sky is black and stars are visible - even during the day.

 

Most of what is now known about the Moon is derived from the Apollo Space Program.   President Kennedy originally gave the "signal" to proceed.   President Kennedy said to Congress and the Nation on May 25, 1961:

 

“I believe that this Nation should commit itself to achieving the goal, before  this decade is out, of landing a man on the Moon and returning him safely  to Earth.  No single space project in this period will be more impressive to mankind, or more important in the long-range exploration of space; and none will be so difficult or expensive to accomplish.”

 

This was basically a politically motivated decision at a time when the Superpowers were competing for world leadership.  Initially, the USSR was ahead of the U.S. in both manned and unmanned space flight.  Both nations sent animals up first.  The first manned space flights in the U.S. were Mercury and Gemini.

 

In 1959, Luna 2 became the first vehicle to reach and subsequently crash into the Moon.  By 1966, the Soviets were able to land a spacecraft, Luna 9, on the surface and transmit pictures to Earth.

 

 

Noteworthy Unmanned Spacecraft

 

Spacecraft        Date of Launch                      Significance

 

Luna 1                    1/2/59                      First flyby of the Moon

Luna 2                    9/12/59                    First manmade object on Moon; crashed 9/14

Luna 3                    10/4/59                    Flyby; 1st photographs of the Moon’s Far Side

 

Ranger 7                7/28/64                    1st close-up photos; crashed in Mare Nubium

Ranger 8                2/17/65                    Crashed in Mare Tranquillitatis

Ranger 9                3/21/65                    Crashed in Crater Alphonsus

 

Zond 3                   7/18/65                    Photographs of Far Side

Luna 9                    1/31/66                    1st soft landing; 2/3/66 in Oceanus Procellarum

Luna 10                  3/31/66                    1st space probe to go into orbit around Moon

 

Surveyor 1             5/30/66                    Soft landing in Oceanus Procellarum

Lunar Orbiter 1      8/10/66                    Photographed proposed Apollo landing sites

Luna 11                  8/24/66                    Moon orbiter

Luna 12                  10/22/66                  1st Soviet lunar orbiter to return photographs

 

Lunar Orbiter 2      11/6/66                    Photographs proposed landing sites

Luna 13                  12/21/66                  Soft landing in Oceanus Procellarum

Lunar Orbiter 3      2/5/67                      Photographs proposed landing sites

 

Surveyor 3             4/17/67                    Soft landing in Oceanus Procellarum; later visited by Apollo 12 astronauts in 1969.

Lunar Orbiter 4      5/4/67                      Photographs for mapping Near Side

Explorer 35            7/19/67                    Magnetic field studies from lunar orbit

Lunar Orbiter 5      8/1/67                      Photographs for mapping Far Side

 

Surveyor 5             9/8/67                      Soft landing in Mare Tranquillitatis

Surveyor 6             11/7/67                    Soft landing in Sinus Medi

Surveyor 7             1/7/68                      Landed near crater Tycho

 

Luna 14                  4/7/68                      Moon orbiter

Luna 16                  9/12/70                    1st automated return of soil to Earth (101 gm)

Luna 17                  11/10/70                  Automated Lunokhod (rover) went 10.5 km

Luna 19                  9/28/71                    Moon orbiter

Luna 20                  2/14/72                    Lander; returned 100 grams soil to Earth

Luna 21                  1/8/73                      Lunokhod (rover) went 35 km over surface

Luna 22                  5/29/74                    Moon orbiter

Luna 23                  10/28/74                  Moon lander; no soil sample returned

Luna 24                  8/9/76                      Lander; returned 150 grams soil to Earth

 

[“Luna” and “Zond” were Soviet spacecraft missions.]

 

 

The United Stated had a three-part program of unmanned flights.  From 1961-1965, 9 Ranger missions were sent, but only 3 returned close-up photos.  During 1966-1967, 5 Lunar orbiters were sent into orbit about the Moon to map the surface.  From 1966-1968, 7 Surveyor launches were completed.  These were the first controlled landings on the Moon that took pictures and tested soil.

 

The Apollo program was a complicated 10-year success culminating in the first man, Neil Armstrong (a Purdue Graduate), to set foot on the Moon July 20, 1969.  The Apollo program was in a hurry to succeed before the end of the 1960's decade.  Apollo 11 was only the 5th manned Apollo flight, and it landed on the Moon!  The 2nd manned Apollo flight orbited the Moon!

 

The Spacecraft had 3 Subsystems:

 

(1) The huge Saturn V rocket

(2) Command and service module

(3) Lunar module

 

(Apollo Command/Service and Lunar Modules and Saturn V Rocket on next 2 pages )

 

 

 

 

Trajectory for Apollo 8: (not to scale)

 

 

 

 

 

 

Two of the six Apollo flights landed on the Moon with Purdue graduates.  Neil Armstrong being the first man on the Moon in 1969 and Eugene Cerman being the last man.  In more recent times, most space shuttles have had Purdue graduates.

 

 

 

 

Possibly the most important scientific aspect of the space program was returning about 300 kg of Moon rocks to Earth for study.  A second important scientific aspect of the Moon program was that each mission left behind an automated lab ALSEP (Automated Lunar Surface Exploration Package) to measure solar wind, heat flow, etc.

 

Diagram of Entire ALSEP Apparatus on the Moon

 

 

 

 

ALSEP, the automatic observatory above, left behind on the surface of the Moon by Apollo astronauts.  Seismographs, magnetometers, heat-flow probes and gas detectors transmitted data back to Earth for many years.  In the active seismic experiment, a mortar shell fires an explosive charge on signal command from Earth to generate seismic waves into the lunar crust.  Geophones picked up these waves.

 

 

Magnetometers, as well as seismometers, were placed on the Moon to listen for Moonquakes.  They shut down operations in 1978 as a cost cutting measure.

 

A final scientific objective was to map the lunar surface from the command module in orbit.  Unfortunately, orbits were all near the Moon's equator giving limited coverage of the Moon's polar regions.

 

The Soviets never made a manned moon mission.  However, they achieved some unmanned landings.  It is ironic that 20 years after Apollo, neither the U.S. nor any other nation has the immediate capability for further manned exploration of the Moon.

 

 

 

Lunar Cratering

 

Craters are the dominant geological features on the Moon.  They are much rarer on Earth, but they exist.  Many unusual theories developed in the last century to try to explain what craters are.  One group of Moon observers speculated that since the common type of circular objects with middle craters on Earth are volcanoes, so should they be on the Moon also.  However, on the Moon, lunar craters are not higher in elevation than the surrounding terrain.

 

 

 

 

Craters can be huge on the Moon spanning hundreds of kilometers across.  Also, there are huge numbers of them.  In the 1890's, geologist, G.K. Gilbert, was the first to suggest that these craters resulted from large meteor impacts.

 

 

An example of a crater on Earth is meteor crater in Arizona:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

A business man, D. Barringer, bought the land that contained this crater hoping to obtain iron and nickel from outer space, but only found a few tons of ore.  So where was the iron from the meteor?

 

Another difficulty with Gilbert's theory was that all Moon craters (at least big ones) are circular.  Our experience in throwing stones in sand is that the objects tend to make oblong pits.  It seems unlikely that all meteors on the Moon hit from directly overhead.  The solution to understanding impact craters is realizing they are not the same thing as holes dug in the sand by a rock.  A large meteorite crashing into a planet is traveling at about 10 km/s and would result in a hypervelocity impact:  the meteorite traveling faster than the speed of sound of the surface material.  As a result, the surface does not have time to move out of the path of the meteorite.  Since the surface can’t yield initially, the meteorite must suddenly stop.  Energy turns to heat and the meteorite literally explodes into pieces (like an artillery shell).  The outcome is a circular crater, regardless of what direction the projectile is coming from.

 

The energy source is the raw speed of the impacting object.  From studies of craters on Earth, as well as special "gun" tests, geologists have gotten the following picture of how a crater forms:  When the projectile hits, it explodes sending a shock wave into the ground.  The meteorite is totally destroyed, although small fragments of it may be found far from the crater.  As soon as the explosion is over, the rock surface, which has been compressed, is suddenly released from compression.  This release causes a general expansion of the rock which ejects material up and out from the crater forming an ejecta blanket.

 

Rock fragments can be thrown a considerable distance from the crater.  These fragments produce a number of smaller craters (which can be oblong in shape) and streaks running hundreds of kilometers away from the crater.

 

 

Model of Impact Cratering

 

 

 

 

Stages in the formation of an impact crater:  (1) the impact; (b) the projectile vaporizes and a shock wave spreads through the lunar rock; (c) ejecta are thrown out of the crater; and (d) most of the ejected material falls back to form secondary craters, rays, and the ejecta blanket.

 

Craters come in all sizes depending on the speed and size of incoming meteorite.  Also, crater walls can be quite steep.

 

Lunar crater Tycho has a massive central peak and terraced walls.  The size of the crater is approximately 85 kilometers.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Lunar crater Taruntius has a flat floor and smooth sides.  The size of the crater is approximately 8.5 kilometers.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The great Orientale crater has a series of concentric rings instead of a central peak.  The impact was so great that the central floor is flooded with frozen lava up-welled from below.  Orientale is located on the edge between the near and far side of the Moon.

 

 

Great Orientale Crater

 

 


 

 

 

 

 

 

 

 

Lunar Volcanism

 

Lunar volcanism is essentially the story of the Maria.  Lunar Maria are immense basalt flows.  Lunar volcanic basalts are similar to terrestrial basalts on Earth except they are higher in iron content and free of water and effects related to water content.  Most lunar basalts have solidification ages from 3.2 to 3.9 billion years ago.  Lunar eruptions did not form volcanic mountains.  Instead, large volumes of fluid lava erupted from fissures resulting in thin sheets similar to flood basalts on Earth.  Among lunar landforms related to the Maria are long curving valleys called sinuous rilles. 

 

One of the largest of these sinuous rilles is Hadley Rille visited by Apollo 15.

 

 

Hadley Rille

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Apollo 15 Astronauts Near the Edge of Hadley Rille

 

 

 

 

It’s now thought these sinuous rilles are collapsed lava channels and related to volcanic activity.  Other areas of the Maria seem to have compressed lava forming so called wrinkle ridges.

 

 

Aristarchus Plateau Photo Taken by Apollo 15

 

 

 

 

We’ll find later that the Moon's outer crust is thinner on the near side of the Moon.  In 1994, the Clementine spacecraft took pictures and measured elevation and gravity of the Moon.  One of the big surprises of the mission was the existence of the large South Pole Aitken basin on the far side, one of the largest in the solar system.  It wasn't fully appreciated in earlier missions because these had mostly equatorial orbits.

 

 

Lunar Topographical Map

 

 

 

 

Map reveals prominent basins on the Lunar NEAR side including Imbrium, Crisium and Nectaris, all at least partly filled with mare basalt, accounting for the relative smoothness of the Near side.

 

Crater counts can be used to relatively age date planetary surfaces.  First, an estimate of the rate of formation of impact craters is needed based on current numbers of Earth crossing comets and asteroids.

 

 

 

 

The larger the impact indicates the longer the time between comparable impacts.  For the Moon:

 

1 km crater occurs every 50,000 years

10 km crater occurs every 5,000,000 years

100 km crater occurs every 500,000,000 years

 

These recurrence intervals will be somewhat shorter for the Earth since the Earth is a bigger target.

 

On the Moon, small craters are much more numerous than large craters.  Thus, the size distribution of craters on the Moon is directly a result of size distribution of impacting bodies.  Based on the present distribution of asteroids and comets, it would have taken several billion years to obtain the present craters on the Moon!  Also, it appears that the cratering rates have been fairly constant during the past 3.8 billion years.  However, during first 800 million years of solar system, the rate of impacts must have been much higher (otherwise, the Lunar Highlands would not be as heavily cratered compared to Maria regions).

 

 

 

 

From this, as well as from absolute age dating, we can derive a geologic history for the Moon.

 

 

A Geologic Time Scale of the Moon

 

System                Age (109 years)                                       Remarks

 

pre-Nectarian            Began: 4.60                  Includes crater and basin deposits and many

                                 Ended: 3.92                        other structures formed prior to the Nectarisbasin impact. Includes formation of Lunar crust and surfaces most heavily crater covered.

 

Nectarian                  Began: 3.92                  Defined by deposits of the Nectaris basin which

                                 Ended: 3.85                        is a large multi-ring basin on the near side of the Moon.  Includes almost four times as many large craters and basins as the Imbrium system.  May also contain some volcanic deposits.

 

Imbrian                      Began: 3.85                  Defined by deposits of the Imbrium basin, includes

                                 Ended: 3.15                        the impressive Orientale basin on the extreme western limb of the Moon and the most visible mare deposits and numerous large impact craters.

 

Eratosthenian             Began: 3.15                  Includes those craters that are slightly more

                                 Ended: @ 1.0                      degraded and have lost visible rays; also includes most of the youngest mare deposits.

 

Copernican                Began: @ 1.0                 The youngest segment in the stratigraphic

                                 Ended: (present)                 hierarchy of the Moon.  It encompasses the freshest lunar craters in which most have retained their rays.

 

 

A period of heavy bombardment occurred 3.85 billion years ago and older: Was this a unique event or was it the final stages of planetary accretion - left over debris in the early solar system?  In any event, the heavy bombardment period obliterated the early surface of the Moon that existed before. 

 

Lunar impacts indicate interaction between:

 

(1)  Catastrophism - sudden or violent geological events.  Ex)  Heavy bombardment period.

 

(2)  Uniformitarianism - very slow processes acting over long times.  Ex)  Uniform bombardment in recent geologic times.

 

There will always be interplay in planetary geology between these two viewpoints.

 

 

MOON ROCKS: A Geologic Time Scale for the Moon

 

 

 

 

The shaded areas in this above schematic representation indicate duration and intensity of the events that formed the lunar basins, maria and craters.  (F) indicates a lunar geologic landform on the far side.