The waning crescent Moon. The darkside is illuminated by earthshine.

    The Moon is our nearest neighbor in space. It is the only world for which real geological features are evident in detail with backyard telescopes. Note that additional information on images displayed on this page can be obtained in the "Moon Image Gallery" page. 

LUNAR GEOLOGY

     The Moon can be divided into two major terrains: the terrae - bright and ancient Lunar highlands and maria - dark lowlands composed of basalt-covered impact basins.  The highlands are composed of a heavily cratered crust composed largely of anorthosite. This aluminum-rich plagioclase crystallized from a global magma ocean that once completely covered the primeval Moon. The lowlands are contained within the interiors of numerous multi-ring impact basins that scar the moon. Numerous basaltic lava flows flooded these lowlands.

    The Moon as imaged with an 203 mm f/7 newtonian and a Nikon CoolPix 4500 with afocal projection through a 40 mm Kellner on November 19, 2002.

Photomosaic of the first quarter moon.

Lunar Terrae, or lunar highlands near the Lunar terminator.

Impact Features

    The Moon is peppered with impact craters. The preserved Lunar cratering record dates back to the formation of the Lunar crust soon after the Moon coalesced. Impacts are formed by the release of kinetic energy as objects of various sizes strike the moon at high velocity. Impacts release an enormous amount of energy. All the terrestrial worlds bear scars of impact. On the Moon, craters range in size to near planet-splitting impacts to, without an appreciable atmosphere, the microscopic. The general Lunar crater morphology is directly related to the size of the crater. Small craters with diameters less than about 20 km are usually simple, bowl-shaped depressions lacking terraced walls and central peaks. Larger craters show a more complex morphology including central peaks and terraces of the crater wall due to slumping. The large impact basins can exhibit multiple concentric rings rather than a single crater wall. These impact basins are usually modified by volcanism. All of the Lunar multiring basins were formed within its first billion years. Based on age estimates from cratering density, eleven of the Moon's largest impact basins formed during a 70 million year interval between about 3.9 and 3.85 billion years ago. 

Clavius is a large and ancient impact structure with a width of about 230 km in diameter located in the bright, southern Lunar highlands. 

Tycho, a crater about 85 km in diameter, is one of the youngest large crater on the moon. Possible Tycho ejecta samples collected and returned by Apollo 17 astronauts indicate an age of 108 million years for Tycho. This coincides with the end of the Early Cretaceous Epoch on Earth. The crater was excavated in a just a few minutes following the impact that created it. Ejecta flung from the impact site extends at least one quarter of the way around the moon. The crater rim rises about 1 km above the surrounding terrain. The walls of the crater have terraces that formed as large blocks of material slumped inward within a few hours after the impact. The central peak, formed by material "splashing" inward and upward from the crater walls, rises about 2 km above the impact melt that pooled to form the crater floor. The image on the left was obtained by Lunar Orbiter IV. The center right image was taken soon after first quarter, the image on the right was taken near last quarter showing tycho under a different lighting angle.

Copernicus. An age of 1 billion years is given to the crater Copernicus based on dates of soil collected at the Apollo 12 landing site. The central peaks of large impact craters such as Copernicus may be composed of rocks that have been uplifted from deep in the crust. It has been estimated that the central peak of Copernicus may be rocks that originated from 15 - 20 km deep.

Theophilus, Cyrillus, and Catharina (from top to bottom). These three craters represent impacts of varying ages. Note the secondary impacts from Theophilis on the floor of Mare Nectaris to the right.

Aristarchus is situated on the Aristarchus Plateau, one of the youngest terrains on the moon. The image on the left is a photomosaic obtained by the Clementine spacecraft. The image in the middle was obtained by on November 16, 2002. The image on the right was obtained near the last quarter Moon on September 17, 2003 under a high sun angle. The reddish brown Aristarchus Plateau is topped by dark mantle deposits that were issued as fire fountain deposits from vents near the head of Vallis Schroteri, the large rill running west of Aristarchus.

Kepler is a small (about 19 km in diameter), relatively simple crater with lighly terraced crater walls and no distinct central peak. This image obtained on September 17, 2003 under a relatively high sun angle accentuates its ray system.

Messier A and B in Mare Fecunditatis are attributed to a low-angle impact, possibly of a "double" object. Messier A (to the lower right) is approximately 11 km in length and 8 km in width and 1,250 meters in depth. The two images on the left were obtained by the Lunar Orbiter spacecraft. The two images on the right were taken with an 8" telescope and a Philips ToUcam webcam.The smallest craters visible in the rightmost image are about 2 km in diameter.

Lunar Volcanism

    The Moon is often considered a geologically "dead" world. However, based on dating of basalt flows, it has actually been experiencing volcanic activity spanning most of its geologic history.

    Many of the largest multiring basins on the near side of the Moon were invaded numerous times by basalt that was differentiated from the Moon's upper mantle and extruded to the surface. This basalt flowed as massive eruptions of low-viscosity magma. 

Mare Orientale is the youngest multiring impact basin on the Moon. The image on the left was obtained by by the Galileo Spacecraft during  a lunar flyby. The image on the right was obtained by Lunar Orber IV which imaged the moon between May 11-26, 1967.

Mare Orientale from my backyard. During favorable librations, earth observers can catch a glimpse of this impact basin. The left image was shows portions of the outermost ring obtained on September 17, 2003 with a Philips ToUcam webcam. The middle and right images were captured with a Nikon Coolpix 4500. The outermost ring of Mare Orientale is about 900 km diameter.

    The earliest lavas erupted during the "main phase" of mare volcanism were the high titanium basalts of Maria Tranquillititas and Serenitatis (basalts returned from Apollo 11 and 17 missions, respectively) between 3.8 and 3.6 billion years ago. In general, lower titanium basalts were erupted between about 3.6 and 3.1 billion years ago in mare Crisium and Fecunditatis (Luna 16 and 24 sample-return missions), Mare Imbrium (3.3 billion years; Apollo 15), and Oceanus Procellarum (3.1 billion years; Apollo 12). The very youngest flows may be "only" about 1.5 to 2.0 billion years old from portions of Mare Imbrium and the Surveyor 1 landing site in Oceanus Procellarum (within the lava-filled crater Flamsteed P) may be as young as 1 billion years old based on crater density age estimates. The ejecta from the rayed crater Lichtenberg (20 km in diameter; west of Aristarchus) is partially covered by lavas. It is estimated to be less than 1 billion years old and so the lavas that partially bury the eject from this crater must be younger still. The differences in color between the various mare may be due to these differences in composition.

  The Moon may appear nearly gray on initial examination yet it actually exhibits very subtle color variation. Most of these subtle color differences are due to compositional differences of surface materials. For example, differences in color between basaltic flows may be due to the differing relative abundances of iron and titanium. Also color of terrains may be due to differences in ages as there is a gradual darkening of the surface materials caused by bombardment of the Solar wind.

   Color ratios of the Lunar surface were used to create this false color image of the Moon. The ratios were calculated based on images obtained by the Galileo spacecraft through six narrow-band filters [centered on the wavelengths in nanometers: 410 (violet), 560 (green), 670 (red), 756, 889, and 990]. For this image, the color ratios 756/410 are shown as red, 756/990 as green, and 415/756 as blue.    

Aristarchus Plateau and Oceanus Procellarum. This color image of a portion of the Moon was obtained with an Philips ToUcam Pro webcam. Some of these subtle color differences were detected visually, though the camera has a tendancy to exaggerate these colors. Note that the Aristarchus Plateau stands out as reddish compared to the rest of Oceanus Procellarum. This is due to the relatively high amount of iron in these young basaltic flows compared to the older, more titanium rich magmas that blanket the surrounding region.

Lunar Volcanoes, volcanic vents, and volcanic rills. 

  Schlumberger Domes. This image contains a region just north of the crater Hortensius (just out of field bottom left).  At least four domes are visible in the bottom left hand corner. Several of these domes show a summit depression. These domes are sometimes referred to as the "Schlumberger domes" and were recently detailed in ALPO's Strolling Astronomer (Winter, 2003). They are part of the Hortensius-Milichius-Tobias dome field in the region of Oceanus Procellarum.  They represent small, central-vent shield volcanoes formed from small-scale, low-volume eruption of low-viscocity basaltic lava.

Ptolemy, Alphonsus, and Arzachel. Alphonsus was the target of Ranger 9 that impacted the crater floor in 1965. Note the dark ash deposits that issued from several circular vents around the floor perimeter. 8" f/7 newtonian and a Philips ToUcam Pro webcam with televue 1.8x barlow and extension tube. August 18, 2003. 

Rima Hadley is the remains of a collapsed lava tube. This interpretation was confirmed by the crew of Apollo 15.

Rupes Recta. This feature is also known as the "Straight Range." Note Rima Birt to the west (left). The dark smudge at the north end of Rima Birt is an ash deposit from a fire fountain originating from the vent. To the right (middle image) is the same image with the color exaggerated. The mare basalts show up as purple and the volcanic ash deposits associated with Rima Birt are a deep red. The image to the far right is Rupes Recta soon after sunrise. 

Plato and Vallis Alpis. Note the craterlets in Plato and the thin rill extending down the valley floor of Vallis Alpis. 203 mm f/7 newtonian and a Philips ToUcam Pro webcam with televue 2x barlow and extension tube. August 18, 2003. 

 Hyginus and Triesnecker. 8" f/7 newtonian and a Philips ToUcam Pro webcam with televue 2x barlow and extension tube. August 18, 2003. 

Lunar Tectonic Features.

    Wrinkle Ridge are formed by surficial compression as regions, such as centers of multiring basins. Subsidence and resulting lateral compression may be caused largely by weight of overiding basalt. The lateral compression creates low-angle thrust faults 

Wrinkle ridges on the western margin of Mare Serenitatis can be seen in this image. The northern portion of the Montes Apennins define much of the eastern edge of Mare Imbrium, one of the largest of the Moon's multiring impact basins. Numerous parallel grabens caused by extensional forces are evident on the eastern margins of Mare Imbrium.

OBSERVING THE MOON

    The moon has been mapped at high resolution by orbiting spacecraft and so one is not likely to image some previously unknown or unseen lunar feature. However, orbiting spacecraft usually imaged the Moon under consistent sun angles and not at a variety of orientations and lighting conditions. However, the study of various geologic features serves as a learning experience, especially as seen under changing lighting conditions and viewing angles. CCD imaging of the Moon can compete favorably with the highest resolution earth-based photographs of the Moon published before the advent of the space age (see the Photographic Lunar Atlas, 1960, 1967).

    Features are often (but not always) best viewed when they are near the terminator. Evening and morning terminators will also give very different aspects of the same feature (consider the views of the Straight Wall at evening versus morning lighting). The Moon also varies in distance and altitude above the horizon at a given phase. For example, in the northern hemisphere, the first quarter moon is highest in altitude during the Spring. Lunar libration, the "wobbling" of the moon due primarily to the Moon's orbital eccentricity but constant rotation rate, allows changing viewing angles, especially for features near the Lunar limb. Considering all these together, the best views of a particular feature might be obtained when it is near the terminator, giving it low angle lighting to accentuate the topography, when the feature is farthest from the limb, when the Moon is high in the sky, and when the moon is near perihelion. Virtual Moon Atlas, shareware, is excellent to use to plan for imaging opportunities.

    Lighting - the terminator near the Lunar equator moves at about 9 miles per hour and can change noticeably in a matter of minutes. This can appear especially dramatic when lunar peaks capture the light of the rising sun. Low angle lighting accentuates topographic features. Some features such as mare wrinkle ridges, can look very dramatic when viewed at the terminator but actually represent relatively minor relief. Some features, such as ejecta blankets or freshly exposed highlands crustal rocks, appear especially bright at high solar angles. Therefore, examination of the moon at high solar illumination highlights younger impact features. Small fresh craters in the lunar highlands, for example, will appear as bright rings as they expose fresh anorthosite in crater walls surrounded by bright radiating splashes of ejecta. High solar angles can also accentuate certain albedo features and color differences. These might be the best times to view the younger basalt flows and ash deposits.

    Image scale is chosen according to the feature that one wants to capture. Most CCD cameras are of relatively small size and therefore are limited in the area that they can capture at high resolution. Therefore, photomosaics can be constructed to obtain high resolution images that encompass larger regions. My scope is on an altazimuth mount and suffers from image rotation. Until I construct that image de-rotator, images that are to be used to construct a photomosaic must captured in a short interval to minimize field rotation. Otherwise, images must be carefully rotated and cropped before assembly, a difficult and time-consuming exercise.

Links

The Moon

British Astronomical Association (BAA), Lunar Section

Books

"Seeing the Solar System" by Fred Schaaf, John Wiley & Sons, New York, 208 pp. 1991.

"The Once and Future Moon" by Paul Spudis, Smithsonian Institution Press, Washington, 308 pp.1996.

"The New Solar System" edited by J. Kelly Beatty, Carolyn Petersen, and Andrew Chaikin