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Dr. Mike Reynolds
Oct 1 - Exploration
Welcome to Week 2!
Table of Contents
Welcome to Week 2!
The Geology of the Moon
Missions of the 21st Century
More on the South Pole
This week we will concentrate on two things - the geology of the Moon and the past and future spacecraft missions that provide the basis for our growing understanding of the lovely world in our sky. And its all on this page.
The Geology of the Moon
The Moon has a scientific importance far in excess of its small size. It preserves a history of the early solar system, allowing us to examine processes of planet formation and modification. And because it is nearby we can study it much more easily than any other place in the universe. The Earth and Moon are a perfect pair - we see the beginning of the solar system, and how worlds evolve and become more complex. We have the whole shebang right here.
There are many details that we now understand about the Moon. As an educator, rather than a researcher, you probably don't need to know them, but you should know the main points. They are summarized in
Top 10 Scientific Discoveries from Apollo
. There is actually one other critically important understanding that is not specifically stated in the Top 10 and that is that there are only two geologic processes that are important in sculpting the Moon's surface. These are impact cratering and volcanism. In fact, these are the only processes that occur everywhere there are solid surface bodies; other more familiar processes such as wind and water erosion, ice sheets movements, and plate tectonics, are simply local aberations due to local circumstances.
I suggest you skim the
Teacher's Guide to the Moon
. It is clearly written and an up to date overview of our understanding of the formation and evolution of the Moon. These are the topics that you should know something about:
Big whack formation of the Moon
The magma ocean - one of the neatest ideas to spring unexpectedly from the first Apollo samples!
Two terrains - highlands and lowlands
South Pole - Aitken Basin
Imbrium Impact Basin
Formation of Copernicus and names of its parts (rim, terraces, central peaks, rays and secondary craters).
Hundreds of millions of years of flooding by mare lavas
Fall off in cratering rate and size of projectiles with time.
Some of these are also discussed in a lecture I gave this summer -
are videos and the powerpoints from it.
Lunar Orbiter and Apollo provided most of what we know about the Moon until the mid-90s. Surveyor and Ranger were Apollo precursor missions whose discoveries were soon extended and replaced by what we learned from Orbiter and Apollo; this might be considered a revisionistic view, but seems true.
Lunar Orbiter missions I, II and III photographed at high resolution (max about 1-10 m) all likely Apollo landing sites to aid site selections. Orbiter missions IV and V imaged at about 100 m resolution (nearside) and 300-500 m (much but not all of farside) to provide the photos we still use to map the surface; since 1994 Clementine provide an additional multi-spectral view.
Apollo missions are why we understand the Moon and solar system as well as we do. The return of many samples from 6 landing sites provided the absolute information necessary to stop speculating about basic aspects of lunar history. In the weeks before Apollo 11, Gene Shoemaker and Ralph Baldwin - the giants of lunar science at the time - published papers concluding that lunar maria were 600 million years old. They were completely wrong! The samples allowed us to date the maria to about 3.5 billion years ago. And the samples demonstrated that the maria were lava flows, not sediments, coral or other cock-eyed ideas.
The samples also showed absolutely no evidence for water or life - the Moon was dry and dead, at least biologically.
The samples also demonstrated that lunar craters were almost certainly formed by violent impact, not volcanism, because of the prevalence of breccias - rocks fragmented, shocked and melted by a hyper-violent process.
Combining the dated ages of samples with the number of impact craters in the sample area allowed calibration of the impact cratering curve that related number of craters to the age of the surface they are on. We used that curve to estimate ages for other parts of the Moon, and extrapolated it - holding our breath! - to Mars, Venus and Mercury.
The 1994 Clementine mission provided the first new orbital imaging since Apollo with 100 m resolution, but more importantly, images were taken through different color filters so we could identify likely rock compositions from their brightness at different wavelengths. This permitted mapping the global abundance of iron and the radioactive element Th which demonstrated that the Moon has three large scale terrain chemistries - the iron-rich maria, the iron-poor highlands, and the Th-rich Procellarum terrain - a broad area of the nearside centered on Oceanus Procellarum and Mare Imbrium - that is unique from the rest of the Moon.
Although Lunar Propspector greatly improved the magnetic, gravity and elemental (composition) mapping of the Moon, its main discovery from the point of view of popular awareness was excess hydrogen at the lunar poles, which may represent ice. This ice would have been deposited at the poles from a temporary water atmosphere following comet impacts. Every place on the Moon except the permanently shadowed - and hence very cold - crater floors near the poles would be so hot that the water atmosphere would quickly escape to space. But at the poles the water would condense out onto the surface and freeze. The H detected could be from ice, or perhaps it is just H from the solar wind. But because water ice is possible, NASA has selected the south pole as the likely location for a lunar base when Americans return to the Moon about 2020.
Missions of the 21st Century
So far this millenium there have been two missions to the Moon - SMART-1 from Europe and Japan's SELENE, which is enroute to the Moon right now. And China, India and the USA have probes that will be launched in the next 1 to 12 months. Since both the US and the Soviets already had lunar orbiters in the 1960s why are we doing it again?
There are three major reasons. First, during the 60s the only thing we could do from orbit was take photographs of the surface (and measure lunar gravity by how it affected spacecraft orbits). Since the 60s there have been great developments in measuring the spectral reflectivity of the surface in wavelengths from the UV through the IR. This allows identification of the elements and minerals that compose the rocks and soils of the lunar surface. Also, we also now routinely use lasers to measure very accurately the topography of planetary surfaces. Basic data files of elevations can be displayed like images with the direction of illumination from any angle we want.
The second reason orbiters are back in vogue is that we need more sophisticated data to answer practical and scientific questions. Practically, we'd like maps of landing sites that show boulders and craters that would interfer with landings - Apollo 11 almost failed because of unknown dangers in the landing area; Neil Armstrong flew to a safe spot. We also want to set up our lunar base where there are resources that can be harvested, rather than expensively bringing everything from the Earth. The main resources we need are water and energy. Water - which we drink, and break down into H and O to use as rocket fuel - may be found as ice deep in dark polar craters - we are betting the farm on that. H is also implanted by solar wind everywhere in lunar soils, and O is about 50% of lunar rocks, but it takes a lot of energy to unlock it. Some volcanic ashes that are rich in the mineral ilmenite are rich in oxygen and it is much easier to extract it from an ash particle than from a solid rock. We can detect ilmenite from orbit using multi-spectral images.
So the new generation of lunar orbiters have many more capabilities than the old ones. Here is a
of the instruments on the 21st century lunar orbiters (and the two most recent from the last century).
The third reason for more orbiters on the way to the Moon, remember I said there were three, is that a growing number of nations see space exploration as a way to advance their technology, and new powerhouse nations want to become leaders in a futurist pursuit. China and India may well be the the dominant nations during this century and they want to assume leadership roles. And many people are begining to think there may be new fortunes to be made in space.
More on the South Pole
Solar energy is abundantly available everywhere on the lunar surface for 14 days, and then is totally unavailable for the next two weeks. Except at the poles, where the Sun would make a 28 day circle around the horizon. Some people say that the most important reason to go to the poles is energy, rather than ice. Taking small nuclear reactors to the Moon would solve the energy problem, but there is great revulsion in some Americans and Europeans to the idea of possibly polluting another world with our most enduring waste. I imagine that the Russians, and probably the Chinese and Japanese, will not think twice about it, for their countries rely heavily of nuclear power here on Earth.
Finally, since we are planning on going to the lunar south pole, what are some of the issues there? First, two are the putative ice and constant solar energy. But the deep craters that are permanently shadowed are quite rugged. The US is considering landing on the rim of a 20 km wide crater called Shackelton because it is very near the pole. But we don't have accurate knowledge of the topography of Shackelton - that will be provided by the new orbiters - however, it looks like the rim of the crater has a slope of 20 degrees - far too steep for routine operations. And it would probably be nearly impossible to scale the steeper inner walls to reach the crater's dark floor. And being at a pole is the maximum distance from most of the Moon's interesting geology (and ilmenite), which is associated with the impact basins nearer the equator. Going back to the Moon will be hard, going to a pole will be really hard.
Apollo Surface Journals
: Despite it's name this is the place for detailed info on many aspects of Apollo lunar activities and samples.
Last week (in Assignment 2) you looked at the nearly full Moon with your eyes, and also were asked to learn a dozen prominent features by looking at an image. The Moon won't be available convienently for another 10 days (unless you look for it early in the morning), but I'd like you, sometime during this course, to look at the Moon with binoculars, with your map in hand, to see if you can identify the 12 features. Write your impressions of what you saw (in general, not for each of the 12). What was the day and time, the approximate phase of the Moon? Describe how the details appeared near the terminator (the sunrise/sunset line) compared to further from it.
Ask me something you'd like to understand better about the Moon's geology or history. And look at the questions other participants have asked and take a cut at answering any of them.
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