After Apollo interest in further scientific examination of the Moon seemed to rachet down almost to "nil". But as various nations have succeeded in establishing viable space programs, the Moon once again has been targeted for renewed exploration during the first decade of the 21st century.
The views presented so far highlight the two dominant characteristics of the lunar surface: 1) the mare/highlands dichotomy, and 2) the abundance of circular features, nearly all being impact craters and basins but some of probable volcanic (caldera) origin. This next scene emphasizes both characteristics by showing an exaggerated false-color image of the front side of the Moon, taken by the multispectral vidicon onboard the Galileo spacecraft (described later in this Section). The highlands, with their higher reflectances, appear in shades of red and orange and the lower reflectance maria are in blues and greens.
After Apollo, the Moon was not specifically revisited for 22 years, until an unmanned spacecraft, Clementine (funded by the Department of Defense), orbited it to conduct mapping studies between February 19 and April 21, 1994, using UV/Visible, Near IR, and High Resolution Cameras, Lidar (a radar altimeter), and a radar-like unit that transmits in the S-band radio frequency (2.293 GHz, or 13.19 cm wavelength).
Look first at a topographic map of the front and far sides of the Moon, in which stereo data provided elevation differences from high resolution photographs and radar altimetry data, acquired by the Clementine spacecraft as it orbited the lunar surface. In the Far Side view, note the low topography around the South Pole.
Clementine has produced topographic maps of the polar regions of the Moon. This one covers the South Pole region. It is notable for showing the largest impact basin on the Moon, and perhaps in the Solar System. This is the South Pole-Aitken Basin, with its main craters named in the map below. It is approximately 2500 km (1550 miles) in diameter and 13 km (10 miles deep). Being old, it has been much degraded by subsequent impacts.
The next image, made as a mosaic from Clementine images, also shows the South Polar region. But it is hard to associate (compare) this with the above maps, since it is centered on a different point and has a different orientation. This demonstrates the problems one can have in comparing lunar scenes that have features in common - there are so many craters that fitting one map into another can prove visually difficult.
As explained in the next paragraph, image data at various wavelengths can be used to map compositional differences in much the same manner as with multiband data obtained by Landsat and other terrestrial spacecraft. Below is an image of the 40 km wide crater Aristarchus that is found in the southeastern part of Oceanus Procellarum. The composite image is constructed from three ratio images (input bands in units of micrometers [Ám]): 0.750/0.415 = red; 0.750/1.00 = green; 0.415/0.750 = blue. The dark gray surface is mare basalt; the reddish unit is ejecta from Aristarchus; the light blue is probably anorthositic rock (common in the highlands) exposed in the crater interior:
Among specialized products were more detailed maps of lunar topography (elevations) and global maps of the distribution of several chemical elements, such as iron (Fe) and titanium (Ti), determined by analyzing reflectance variations at 0.75 m m and 0.95 m m, where these elements absorb irradiation. Most of the iron is actually in the form of FeO (reduced iron). The Clementine results when plotted as FeO are thus:
While iron is widespread, its maximum concentrations are in a broad region on the nearside, roughly coincident with the vast lava outpourings into Oceanus Procellarum and several other mare basins.
Clementine made a controversial discovery, which, if proved correct, has major implications for humans returning to the Moon. Its S-band radio unit detected abnormal reflections from the rim of a huge crater (basin) around the lunar South Pole, in areas permanently sheltered from the Sun's rays, as seen in this Clementine image:
These reflections could be due either to water ice or to some abnormal surface roughness condition. If indeed ice is present in significant quantity, then this precious material (which supplies water needed for life and also oxygen, when broken down by electrolysis) might allow future astronauts to establish a manned base on the Moon. Transport of sufficient water and oxygen for long stays is presently beyond the space program's technical capability.
Because the South Pole region is a candidate for an eventual lunar base, radar units from Earth have returned high resolution images of the polar terrain, without so far having confirmed the presence of ice. Here is an image made at 13.2 cm wavelength, from the Arecibo radar dish in Puerto Rico (no ice was identified at the 20 m resolution of this system).
The observation of possible ice, and other intriguing results of Clementine's compositional mapping, has led to a follow-on mission. For the first time in 25 years, NASA has returned to the Moon with a small, but versatile orbiting satellite, called Lunar Prospector. The entire mission including data analysis is another effort by NASA to achieve high scientific returns at relatively low cost (for LP, $65 million). Launched on January 6, 1998, by an Agena rocket, Prospector now is operating in a 100-km high circumlunar polar orbit, from which it can map the entire Moon over a 3-year lifetime in more detail than Clementine provided. Here is an artist's sketch of the spacecraft:
The spacecraft, just 1.4 m (4.5 ft) high and 1.2 m (4 ft) in diameter, weighing 300 kg (660 lbs), receives its power from solar cells that surround its exterior. An S-band radio sensor designed to measure lunar gravity employing a Doppler effect procedure, sits on top of a conical communications antenna (top). At the end of the 8-ft boom or mast extending to the front left, a Magnetometer/Electron Reflectometer will conduct improved measurements of the Moon's magnetic and particle fields. At the end of the left rear mast is the Gamma Ray Spectrometer, which can detect these elements: U, Th, K, Fe, Ti, O, Si, Al, Mg, and Ca. On the right boom are the Alpha Particle Spectrometer that will measure radon gas to assess lunar radioactivity as a clue to volcanic and other current events, and the Neutron Spectrometer that will determine the presence of hydrogen and can detect water ice (its confirmation from Clementine results is a major goal).
A plot of the varying thermal neutron flux, as determined by the Neutron Spectrometer, show a wide area of low neutron counts (resulting from high neutron capture) associated with the maria on the frontside and near the North Pole and higher counts in the highlands.
Compare the distribution of Fe as determined by Lunar Prospector (below) with the same coverage by Clementine shown above:
Information on the distribution of radioactivity on the lunar surface was one goal of Lunar Prospector. This map shows that the element thorium is highest on the front side of the Moon, mainly in the highlands south of Mare Imbrium. The correspondence with the Imbrium Basin suggests that the basaltic lavas that filled it were enriched in Th. Note that corresponding highland surfaces on the farside are lower.
The first results on Lunar Prospector's detection of ice were released during an exciting press conference, held on March 5, 1998. Around both poles, the neutron spectrometer has indeed detected neutrons, released from hydrogen by natural cosmic ray bombardment of water ice in craters that have permanently sheltered shadow zones. The drop in neutrons emanating from the Moon is clearly maximal around the poles as seen in this plot.
The initial estimate of the amount, to be determined more accurately with later observations, is 30 to 300 million metric tons (recent thinking has raised the upper limit to perhaps as high as 3 billion tons). If melted, this larger number would fill a "lake" 10 square kilometers in area (3.1 x 3.1 km) to a depth of 10 meters. Surprisingly, the North Pole region contains about 50% more ice than its southern counterpart. The source of the water ice is probably residues from cometary bodies that impacted the polar regions, forming craters but allowing much of the comet mass to survive embedded in the target. The implications are encouraging for future exploration of the Moon, to the extent that we can establish and occupy a manned base facility over extended time because of the availability of vital water (for consumption and as a source of hydrogen, suitable as a fuel). Landing in polar regions is technically more difficult but doable. The dream of a permanent observation post on our satellite is now much more feasible.
More details on Lunar Prospector are given at the National Space Science Data Center Web site and the Mission Management Home Page at NASA Ames Research Center. As NASA accrues and releases data and maps, we will place them in the Web version of this Tutorial and in later CD-ROM versions.
An important mission to the Moon is ESA's SMART-1 spacecraft. Launched September 27, 2003 as Europe's first venture in exploring beyond Earth, the spacecraft, using a novelion (Xenon gas) propulsion system, proceeded slowly to the Moon and then arrived in November of 2004.
SMART-1 is Europe's first lunar mission and has provided some significant advances to many issues currently active in lunar science, such as our understanding of lunar origin and evolution. The mission also contributes a step in developing an international program of lunar exploration. The spacecraft was launched on 27 September 2003 on an Ariane 5, as an auxiliary passenger to Geostationary Transfer Orbit (GTO), performed a 14-month long cruise using the tiny thrust of electric propulsion alone, reached lunar capture in November 2004, and lunar science orbit in March 2005. SMART-1 carries seven hardware experiments (performing 10 investigations, including three remote sensing instruments, used during the cruise, the mission's nominal six months and one year extension in lunar science orbit). The remote sensing instruments will contribute to key planetary scientific questions related to theories of lunar origin and evolution, the global and local crustal composition, the search for cold traps at the lunar poles and the mapping of potential lunar resources.This low cost satellite orbited the Moon for nearly two years gathering information about surface composition. Its instruments are: AMIE: A miniaturised color camera with a resolution of 40 meters for lunar surface imaging. SIR: A near-infrared spectrometer for lunar mineralogy studies. D-CIXS: A compact X-ray spectrometer to perform fluorescence spectroscopy and map the Moon's surface elemental composition. It also performed observations of celestial X-ray sources while en route to the Moon. XSM: An X-ray monitor to support D-CIXS with measurements of solar X-ray emission for calibration. It also observed solar flares while en route to the Moon.
Here are two images made by AIME of the lunar surface.
Below are an AIME view of the central peak of Crater Zucchius and ejecta around Mare Orientale:
Here is a SIR mineral map of a cratered area on the Moon:
The D-CIXS detects X-ray fluorescence of minerals that are being excited by X-rays from the Sun. This is a map of Calcium distribution within a small area of the Moon around Mare Crisium:
SMART-1 spent a lot of time looking at the polar regions, partly to search for evidence of possible water ice. In so doing, it provided information on the Aitken impact basin, some 2500 km in diameter, reputedly the largest such structure in the Solar System. Here is a map of the South Polar region showing this structure:
SMART-1 ended its mission by being deliberately impacted onto the lunar surface on September 3, 2006.The Chinese and Japanese have sent probes to the Moon in 2007 and 2008 respectively.
This is an artist's view of the China's Chang'e-1 spacecraft, which was launched on October 24, 2007:
The first image from Chang'e-1 is shown here:
This panel shows the types of products coming from Chang'e-1:
The contrast in this Chang'e-1 image of the craters at the lunar South Pole is interesting:
Chang'e-1 was sent to the lunar surface (crashed) on March 1, 2009.
Japan's spacecraft, Kaguya (Selene), is primarily an imager but it does have 13 separate instruments. Here is how it looks:
These are typical images of the lunar surface as captured by Kaguya:
Here are Kaguya images of the rim and the central peak of the crater Tycho:
This is Kaguya's image of Mare Muscovensis on the lunar far side:
Unlike the earlier U.S. orbiting imagers, Kaguya can produce quasi-color images:
An interesting Kaguya image shows the disturbed soil around the site where the Apollo 15 LM took off more than three decades earlier:
One of Kayuga's task was to search for water, especially in the polar regions. Its instruments dedicated to that search came up with no positive evidence of water. At face value, that may seem discouraging but perhaps the sensors were inadequate for that purpose. The question remains moot.
Kayuga ended its mission on June 10, 2009 by crashing onto the lunar surface. The crash was observed from Earth. This is a sequence seen through the Anglo-Australian telescope:
The impending crash itself was monitored by Kayuga as its approached the surface. Here are two of several images sent back just before contact:
India launched its first outer space probe towards the Moon on October 22, 2008. Named Chandrayaan-1, the spacecraft has multiple instruments from several nations. NASA's contribution are the MiniSAR and M3 (Moon Mineralogy Mapper, which will gather data in both the Visible and IR parts of the spectrum). Data from Chandrayaan (shown below) started to be received by mid-November:
NASA's JPL has an instrument onboard Chandrayaan-1, called the Moon Mineralogy Mapper (m3). One of the first products released to the public is this image in which iron minerals are shaded green:
In September of 2009, NASA and several scientific groups made headlines with the announcement that water has been detected over most of the lunar surface. The amounts are miniscule by earth standards. The soil spread over any area comparable to a football field in size hold a cumulative amount of water sufficient to fill an ordinary drinking glass. But the discovery is important since it indicates that water does occur in non-polar latitudes. This illustration, based on Chandrayaan-1 data, indicates typical distribution: