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History of the Moon: how do we know when things happened?.

Although the average age of Earth's surface is only about 500 million years, the oldest rocks geologists have found on our planet formed 3.8 billion years ago. Earth had a rocky crust for 700 million years before then, but it is long gone. Erosion and plate tectonics have erased all of the landforms from the earliest periods of Earth's history. But you can see another world from your backyard where you can identify features dating from its infancy up to the most recent times.

The Moon, like the Earth, Sun, and all other objects within our solar system, ultimately formed from materials that condensed out of the solar nebula roughly 4.5 billion years ago. Its surface was shaped by both external energy (impact cratering) and internal energy (volcanism and faulting). Both processes were vastly more active early in lunar history than they are today. Borrowing a phrase used to describe Earth in the earliest days of its history, the Moon was dominated by "tempestuous tectonics." Impacts, eruptions, and faulting were everywhere!

The immense energy from the countless impacts that brought the stuff together that formed the Moon caused it to completely melt, forming a global magma ocean.

Low-density elements such as aluminum floated to the surface as a frothy scum, while denser materials such as iron sank deeper into the molten orb, in a process known as planetary differentiation.

The early Moon radiated away its heat rather quickly and began to solidify, though it continued to be punctured repeatedly by a torrent of projectiles--remnants from the solar system's formation. Over the first 600 to 700 million years, huge impactors collided with the Moon, producing giant basins that are hundreds to more than a thousand miles in diameter.

Over time, heating due to the radioactive decay of uranium and thorium melted parts of the lunar mantle. This fresh magma rose up through fractures under the young basins, erupting vast expanses of lava onto the surface.

Tens of thousands of eruptions built up the thick, dark flows on basin floors that we see today in Mare Imbrium, Mare Serenitatis, and Mare Crisium. Sinuous rilles, such as Hadley Rille, mark locations where rapidly flowing lavas cut winding channels. This upflow of magma from the mantle to the lunar surface caused the basins to sink, producing fractures around their margins (such as the concentric rilles bordering Mare Serenitatis), and faults in their interiors as the cooled lavas were forced to fit within smaller volumes.

About 3.8 billion years ago, impact cratering rapidly declined in both frequency and the size of craters formed. In fact, comparison of the number of impact craters on the lunar highlands with maria ages show that by the time most mare lavas erupted (between 3.8 and 2.5 billion years ago), cratering was thousands of times less frequent as it was during the earlier epoch.

Lunar scientists deduced this sequence of events based on the stratigraphic relations of different areas: features that are on top of others are younger than the underlying strata. To anchor this sequence to a specific time frame, scientists used radiometric dating to measure the absolute ages of samples returned by the Apollo astronauts four decades ago. Unfortunately, these samples were collected from only the six Apollo landing sites. Three more from Soviet Luna sample-return missions don't make the collection much bigger.

Numerous gaps in our knowledge of lunar chronology are filled with estimates of surface ages based mostly on crater counting. In general, we know that the rate of crater formation in the Moon's first half-billion years was very high and that the rate quickly declined afterwards, with bumps or spikes over the last 3 to 4 billion years. The exact shape of this cratering curve is debated, so the crater count ages are called "model ages" and are based on combined evidence from the Apollo landing sites, impact rates on Earth, and observations of many asteroids of different sizes.

Recently refined crater counts come from high-resolution images returned by NASA's Lunar Reconnaissance Orbiter (LRO). In fact, you can help contribute to our lunar knowledge by joining the crowd-sourced classification of LRO images at www.moonzoo.org. This ongoing work is leading to a more expansive list of model ages of lava flows, craters, and, most difficult of all, the major impact basins.

In truth, this scenario is all a house of cards, but it's a well-constructed house, and it's the best we can do

until we bring many more samples from various parts of the Moon back to Earth and study them in laboratories. That is the next major goal of lunar science, but we must return to the Moon to accomplish it.

In my next column, we'll tour the visible clues of lunar history that you can see with your own telescope.

Contributing editor Charles A. Wood (lpod.wikispaces.com) is coauthor of the book 21st Century Atlas of the Moon.

The Moon * April 2014

Phases          Distances

FIRST             Perigee   December
QUARTER                      10, 10h
                                  UT

April 7,    251,344 miles  diam. 29'
8:31 UT                          32"

FULL               Apogee   December
MOON                         22, 11h
                                  UT

April 15,   229,762 miles  diam. 32'
7:42 UT                          19"

LAST           Librations
QUARTER

April 22,            Mare    April 4
7:52 UT     Humboldtianum

NEW MOON          Cusanus    April 7
                 (crater)

April 29,     Lacus Veris   April 16
6:14 UT
           Vallis Bouvard   April 18
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Title Annotation:Exploring the Moon
Author:Wood, Charles
Publication:Sky & Telescope
Geographic Code:1USA
Date:Apr 1, 2014
Words:895
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