The Apollo missions of the 1970’s can be credited with many great discoveries–the most notable of which, were the six missions that sent twelve astronauts to the moon.  During these manned missions to the Moon, rock samples were collected and returned to the Earth.  The analysis of these samples led to the current hypothesis of lunar formation.  This hypothesis suggests that the Moon was formed by a catastrophic collision between Earth and Mars-sized planetesimal.  After this collision, the Moon would have been covered by a thick blanket of magma, which cooled to form a crust much different from the Earth’s crust.   Then 3.8 billion years ago, during the late heavy bombardment, the Moon’s surface was pounded by meteor impacts, leaving the surface deformed and heavily cratered.  New data gathered by the Japanese mission, SELENA, may offer new insights into the formation of the moon.

SELENA focused on the differences between the near and far side of the moon, such as: compositional, gravitational, topographical, and tectonic differences.   However, it is difficult for spacecrafts to relay information from the far side of the Moon, due to the fact that the Moon is tidally locked to the Earth.  SELENA was able to surmount this obstacle by using a companion satellite positioned in an elliptical orbit at a higher altitude.  This companion satellite was then able to relay information between Earth and SELENA. 

Because the Moon is a homogeneous body, there are several differences between the near (Earth facing) and far-side of the Moon.  The nearside of the Moon is covered by dark basaltic plains, (the very features that Galileo once mistook for seas).  The far side of the Moon is much more heavily cratered, and the higher elevations are composed of a bright material. These compositional differences are accompanied by differences which are intrinsic properties of the materials that make up each side of the Moon, such as crustal thickness and density.  Other differences between lunar faces include volcanic activity and surface age.

Another key difference between the lunar faces is the gravitational anomalies found on either side of the Moon.   These differences in gravitational anomalies can be used to deduce possible density differences of the interior.  Positive gravitational anomalies on the nearside of the Moon have been known about for several years and are associated with the large areas of basaltic planes.  These planes are referred to as mascons (mass concentrations).    These mascons could be the result of basaltic magma filling basins after basin formation, or they could be the result of mantle uplift that could have occurred during a large impact event.  SELENA was able to map the gravitational anomalies of the lunar far-side for the first time.  What SELENA discovered was that the far-side mascons have small central positive gravitational anomalies that are surrounded by a wide ring of negative anomalies.  These differences in gravitational anomalies observed on either side of the Moon could suggest that the far-side of the Moon may have had much cooler and rigid conditions in its early history.  

SELENA also used a Lunar Radar Sounder to map subsurface stratigraphy beneath the nearside basaltic basins.  The results of this experiment show that the thickness of the most recent volcanic flows may have been deformed compressive stresses that occurred during a period of global cooling, and not entirely because of the stresses which occurred during mascon formation.

The terrain camera onboard SELENA was able to photograph volcanic flows on the lunar far-side.  These photos were then used to estimate the age of the far-side basalts using cratering statistics. Based on the cratering statistics, the age of the lunar far side was found to be much younger than the lunar nearside, with volcanic activity continuing to make fresh surface until approximately 2.5 billion years ago.

Although the data gathered so far is not enough to paint a clear picture of lunar evolution, it has become clear that the mascons formed much differently on either side of the Moon during late heavy bombardment.  To help interpret these discoveries, new data will be on its way as China, India, and the United States all have orbiters slated for lunar observation in the next couple of years.  In the meantime we are left to wonder, are these differences due to external processes such as a giant impact, or are they due to internal processes such as core formation, and crustal differentiation?  One thing seems clear, the difference in surface age on either side of the Moon will be an important variable when devising a model for lunar evolution.

Earlier this year, India celebrated the launch of its first mission to the moon. The mission is called Chandrayaan-1, which in Hindi, translates to “trip to the moon”. The mission was launched on October 22, 2008 using India’s own launch craft which is called the Polar Satellite Launch Vehicle. On November 8^th, Chandrayaan-1 entered a lunar orbit at altitude of 100 km above the lunar surface.

Chandrayaan-1 is a multinational program with contributions from India, NASA, the Bulgarian Academy of Science (BSA), and the European Space Agency (ESA). The objective of this mission is to map out the lunar surface in greater detail than has ever been done before, by any single nation. This mission will Map out, in high resolution, the chemical and mineralogical compositions of the Moon’s North and South poles. It will also search for pockets of surface and subsurface helium, and water-ice which could be potentially used by a future Moon-base. The satellite will also map out changes in elevation, and the chemical composition of the moons interior by observing internal rock which has been exposed to the surface. It is hoped that the data gathered will help develop a better understanding to the evolution of the solar system, particularly, the origin of the Moon.

With the great success of Chandrayaan-1, the Indian Space Research Organization (ISRO) has announced plans for the launch of Chandrayaan-2 in 2011. For this relatively short mission (with a duration of approximately one month), the Russian Federal Space Agency (Roskosmos) will join the ISRO in the building of a lunar Lander/Rover. This mission will be very similar to NASA’S Martian Sample Return Mission, due to the fact that a solar powered rover would navigate across the lunar surface, collect samples and return them to an orbiting spacecraft, which will bring them back to Earth. The ISRO also has plans to launch Chandrayaan-3, 4, and 5, but the details of these missions have not yet been announced. Could India be planning a manned mission to the moon?

The reason we are still so interested in the moon is because of the fact that we still do not fully understand the processes which may have formed the moon. The leading hypothesis, developed by geologist Reginald Aldworth Daly, suggests that during the formation of our solar system, a hypothetical mars sized body called Theia smashed into proto-Earth. This collision would have caused Theia’s entire mantle, and most of Earths to explode into space, while Theia’s iron core sank into the Earth where it combined with Earths Iron core. The mantle debris, in orbit around the Earth, then would have accreted to form the current moon. This hypothesis also explains the unusually large size of Earth’s core. There is some evidence which supports this hypothesis: the lunar rocks gathered during the Apollo mission were found to contain oxygen isotopes with compositions very similar to Earth. Also, large areas of the lunar surface appear to be igneous, which means that it was once molten, and the energy produced during a large collision would be high enough to produce large scale melting of lunar rocks.

It is always exciting when other nations become more involved with space exploration, and contribute more to the collective knowledge of our surrounds. It will be interesting to see if Chandrayaan-1 will be able to produce data that will confirm the Giant Impact Hypothesis, or discover that there is enough water on the lunar surface to support a space colony. Hopefully the ISRO will be able to further expand its space program, despite critics of India’s government, who wish to have the bulk of the space exploration funds reallocated to social welfare programs.

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One-fifth of the entire world population watched the live broadcast of the first Moon-walk, so it is no surprise that we all remember or have heard those famous words spoken by Neil Armstrong in 1969. The Apollo program and lunar landings aided the advancement of many fields of engineering, and is considered by many to be the greatest achievement of mankind. Nearly forty years after the end of the Apollo missions, NASA finally plans on returning to the Moon.

Before NASA returns man to the Moon, they plan on doing extensive studies. The first mission to the Moon will be the Lunar Reconnaissance Orbiter (“LRO”), which is scheduled to launch by the end of this year or early next year. The LRO will be equipped with the most sophisticated technology ever sent to the Moon–including instruments to make detailed 3-D maps of the entire lunar surface, locate subsurface water-ice, and record radiation levels to help develop technologies which will ensure the safety of future crews.

Launching with the LRO is the Lunar Crater Observation and Sensing Satellite (“LCROSS”). In 1999, NASA’s Lunar Prospector detected the spectral signature of hydrogen in the Moon’s permanently shadowed polar craters. LCROSS will impact the Moon in one of these craters. The impact will send a plume of material into space, which will be observed by a near-infrared camera, which will analyze the plume for traces of water. Presence of water on the Moon would be an important natural resource for a future lunar colony.

NASA plans on having mankind back on the Moon by 2020. Utilizing the new equipment which is currently being developed as part of the Constellation program, four astronauts will land on the Moon aboard the new Altair Lunar Lander, which will provide life support for the initial week long mission to the Moon. The Lunar Lander will be launched into low-Earth orbit aboard an Ares V Rocket, where it will rendezvous with the Orion crew vehicle.

Returning man to the Moon is the important first step in NASA’s new Moon Mars and beyond initiative proposed by George Bush. The Lunar surface will be explored and studied in an attempt to learn how to build a successful space colony. Risks such as radiation and psychological trauma will have to be fully understood and overcome before any long-term manned missions to Mars, or elsewhere, can be pursued. Having a colony on the Moon will also help us study how the Earth and Moon were formed, and giant telescopes on the Moon will not have the atmospheric interference which is a problem on Earth. Along with the many scientific advances which will follow future lunar landings, returning to the Moon will renew the general population’s interest in space exploration.

Saturn’s sixth largest moon, Enceladus, was discovered in 1789 by British Astronomer William Herschel. With a low albedo and close proximity to Saturn, Enceladus is difficult to observe. Because of this difficulty little was known about this moon until the Voyager flybys in the 1980’s. Voyager 1 discovered that Enceladus is located in the densest part of Saturn’s E Ring, and Voyager 2 discovered that Enceladus has diverse and relatively complicated surface features.

The Voyager missions generated a number of questions about the small moon: “Is there a connection between Enceladus and Saturn’s E-ring?” “What is causing the tectonic activity which is deforming Enceladus’s surface?” The recent Cassini mission was able to answer these questions, along with generation new discoveries and new questions.

To answer the first question, Cassini discovered that Enceladus is the fourth known body in the solar system with active volcanism. The other three are Earth, Jupiter’s moon Io, and Neptune’s moon Triton. This volcanism causes icy jets, plumes of water vapor, and other materials to be shot into the atmosphere. It is this cryovolcanism which was determined to be the cause of Saturn’s E Ring. Just recently Cassini photographed the volcanic southern pole. These pictures revealed a geological feature scientists are calling “tiger stripes”. These tiger stripes are 300 meter deep fractures and are surrounded by chunks of ice, and are the source of Enceladus’s volcanism.

Cassini also discovered the cause of the tectonic activity. Enceladus, like many other moons is traped in orbital resonances, this causes tidal heating on the moons interior. Like thought to exist on Jupiter’s moon Europa, this could also cause Enceladus to have a subsurface liquid ocean. Because of the volcanic activity a subsurface ocean on Enceladus is though to be only tens of meters beneath the surface, where the oceans on Europa are thought to be 100 kilometers beneath the surface.

Does Enceladus have a subsurface ocean? If it does, is this another place to look for signs of life? With many more Enceladus flybys to come, we may yet find out if there is a subsurface ocean, but we will certainly have to wait for the right mission if we want to determine if life exists.