Understanding Our Moon – NASA/SVS

The Moon has been a very influential body on life on Earth. Some argue that having a satellite like the Moon is a prerequisite for life in other planets. The Moon has a stabilizing effect on rotation axis of our planet. Without it, the axis would wobble more erratically making ice ages and inter-glacial warming periods more extreme and sudden. Understanding our Moon will help understand evolutionary history of our planet. After all, both bodies have started forming from the same materials but followed obviously very different trajectories.

Since 2009, NASA’s Lunar Reconnaissance Orbiter (LRO) has been keeping an unblinking eye on the closest celestial object accompanying our planet. LRO has generated an impressive amount of data and is still making new discoveries. The data coming from LRO is helping us understand a wide range of cosmic questions ranging from the formation of the early solar system, to the structure and evolution of the Moon itself. Moreover, if we will return to the Moon and live on it we must fully understand it.

In addition, LRO also crossed its path with other lunar missions including GRAIL (Gravity Recovery and Interior Laboratory) and LCROSS (Lunar Crater Observation and Sensing Satellite). At the end of these missions they were deliberately crashed onto the surface of the moon. LRO collected terminal data from plumes generated by both impacts (see the two videos animating their impact and data collection at the bottom of this post). LRO taught us that Moon was geologically active until more recently than thought providing evidence that there was lava plumbing and graben formation. Moon’s surface is still changing with new impact craters forming every year.

Unequal distribution of impact craters on Moon

Maps of crustal thickness derived from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission revealed more large impact basins on the nearside hemisphere of the Moon than on its farside. The enrichment in heat-producing elements and prolonged volcanic activity on the lunar nearside hemisphere indicate that the temperature of the nearside crust and upper mantle was hotter than that of the farside at the time of basin formation.

The evidence is growing that our Moon formed after a mega-impact collision of Earth with a Mars sized object. What is interesting is that models suggest that the collision may have created two moons which appear to have merged after a slow collision.

Indeed the Moon is a satellite of two distinct halves. The nearside that faces us all the time is low in altitude, flat and dark in color. Contrastingly, the farside is mountainous and deeply cratered. This lunar dichotomy might be the result of low-speed collision of two companion moons. New calculations suggest that a collision with a companion at subsonic impact velocity leads to an accretionary pile rather than a crater, resulting in a hemispheric layer consistent with the dimensions and crustal structure of the topography of the farside highlands.

Small impacts continue to bombard the Moon. Until LRO data, there was no systematical way to observe new craters. High-resolution photographs of LRO is now enabling this comparison. LRO makes a complete scan of the surface of the moon every 16 days. The first scan of the Moon forms a reference for other repetitive scans. Differences in pictures taken early in the mission with more recent images, revealed more than two-dozen new impact craters. Perhaps the most famous of them all is an 18-meter-wide crater observed as a bright flash on March 17, 2013.

Past activity on Moon surface is evident from the scars of ancient asteroid impacts and lava flows. Scientists thought the moon’s last volcanoes erupted at least 1 billion years ago. New observations by LRO have revealed dozens of geologic structures, called irregular mare patches, scattered across the lunar landscape. In these unusual areas, averaging 500 meters in diameter, smooth mounds sit next to blocky terrain. Unlike the rest of the moon, the structures have almost no craters, suggesting that they’re the remnants of recent lava flows that occurred within the last 100 million years. If true, this means the moon could be hotter inside than expected.

The moon is tidally locked to the Earth. For this reason we only see one side (the nearside). It makes one revolution around Earth and one full turn on its axis every 27.3 days. Within this period, NASA’s Lunar Reconnaissance Orbiter will have made its own journey, circling the moon 348 times. Each successive orbit differs by a single degree of longitude, resulting in a path that allows the spacecraft to survey the entire moon every two weeks. During each orbit, LRO scans the moon’s terrain using a special instrument called the Lunar Orbiter Laser Altimeter (LOLA). The data collected by the instrument not only helps scientists to create detailed elevation maps of the lunar surface, but also pinpoints LRO’s precise position in space. Watch the animation to see how LRO scans the moon.

Since the 1960’s, scientists have suspected that frozen water could survive in cold, dark craters at the Moon’s poles. While previous lunar missions have detected hints of water on the Moon, new data from the Lunar Reconnaissance Orbiter (LRO) pinpoints areas near the south pole where water is likely to exist. The key to this discovery is hydrogen, the main ingredient in water: LRO uses its Lunar Exploration Neutron Detector (LEND) to measure how much hydrogen is trapped within the lunar soil. By combining years of LEND data, scientists see mounting evidence of hydrogen-rich areas near the Moon’s south pole, strongly suggesting the presence of frozen water.

As you watch the Moon over the course of a month, you’ll notice that different features are illuminated by the Sun at different times. However, there are some parts of the Moon that never see sunlight. These areas are called permanently shadowed regions, and they appear dark because unlike on the Earth, the axis of the Moon is nearly perpendicular to the direction of the sun’s light. The result is that the bottoms of certain craters are never pointed toward the Sun, with some remaining dark for over two billion years. However, thanks to new data from NASA’s Lunar Reconnaissance Orbiter, we can now see into these dark craters in incredible detail.

Ever since getting whacked by asteroids and cooked by heat radiating from unstable elements during its violent formation, the moon has cooled. Many things shrink as they cool and the moon is no exception. But tiny valleys discovered in new images from NASA’s Lunar Reconnaissance Orbiter (LRO) indicate that the forces causing the moon to shrink were accompanied in some places by other forces acting to pull it apart. This tectonic tug-of-war taking place on the supposedly inert lunar surface surprised scientists. Not only that, it suggests the moon never completely melted in its early stages of evolution—unlike Earth and the other rocky planets—and instead was covered by an expansive ocean of molten rock. Watch the videos below to see evidence of these lunar valleys, called graben, and to learn more about the moon’s fascinating geologic past.

The Gravity Recovery and Interior Laboratory (GRAIL) mission comprises a pair of satellites launched in September, 2011 and placed in orbit around the Moon in January, 2012. The two satellites, named Ebb and Flow, used radio signals to precisely measure their separation as they flew in formation, one following the other in the same nearly circular polar orbit. These measurements allowed mission scientists to build up an accurate and detailed gravity map of the Moon.

GRAIL ended its successful mission by impacting the Moon on December 17, 2012 at approximately 5:27 p.m. EST (22:27 UT). The two spacecraft were placed in an orbit that takes them within a kilometer of the surface, so low that they will hit the side of an unnamed mountain that lies between Mouchez and Philolaus craters, near the north pole at 75°45’N, 26°11’W. Ebb striked first, followed by Flow 24 seconds later. The following animation shows the last three orbits of the two spacecraft, with views of the impact site. The impact occurs on the night side of a waxing crescent Moon. For this reason the view changes from natural color to false-color elevation map.

The Gravity Recovery and Interior Laboratory (GRAIL) mission comprised a pair of satellites that together measured the gravity field of the Moon. GRAIL ended its mission with a planned impact into the side of a lunar mountain on December 17, 2012. Lunar Reconnaissance Orbiter (LRO) maneuvered into an orbit that would allow it to observe the impact. One of LRO’s instruments, the Lyman-Alpha Mapping Project (LAMP), looked for the chemical signatures of a number of elements, including hydrogen and mercury, in the dust plume kicked up by the impact.

A two-ton Atlas Centaur rocket body, part of the Lunar Crater Observation and Sensing Satellite (LCROSS), struck the floor of Cabeus crater, near the south pole of the moon, at 11:31 UT on October 9, 2009. The purpose of the crash was to create a plume of debris that could be examined for the presence of water and other chemicals in the lunar regolith.

The Lyman-Alpha Mapping Project (LAMP) instrument aboard Lunar Reconnaissance Orbiter (LRO) observed the thin vapor cloud created by the LCROSS impact. LAMP is LRO’s “night vision.” Most of the time, it uses the ultraviolet light in starlight to peer into deep shadows on the moon’s surface. For the LCROSS impact, LAMP was pointed just above the lunar horizon to watch for the arrival of a rapidly expanding cloud of vaporized debris from the crash.

In this animation, the viewer looks down the LAMP boresight and through its narrow window. The LAMP sensor lights up as the leading edge of the expanding vapor cloud passes through its field of view. What’s shown here is actually the difference between the data recorded after the LCROSS impact and that recorded on LRO’s previous orbit.

Cabeus crater

 

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