SCIENCE REPORTER, OCTOBER 2013 12

CO

VER

CO

VER

STO

RY

WE all become familiar with the Moon right from our childhood days. We

see its changing phases. We see the dark markings on its face, which resemble different things to different eyes. Some people see in the dark markings a figure of a woman with the spinning wheel, while in many western countries people see the face of a man or a hare.

One of the few astronomical bodies in the sky that can be observed without any optical aid, the Moon was enshrouded in mystery for much of history. Earlier, it was revered as a reflection of gods and goddesses. It was even viewed as another planet with seas and land. Eventually, Galileo and his telescope got a picture of what we know the Moon to be today -- a natural satellite with mountains, craters and expanses of flat plains.

The coming of the Space Age has turned around our ideas about our nearest celestial neighbour. Today, the Moon no longer holds any mystery for us. Extensive exploration by space probes and astronauts has unravelled new facts not only about the Moon, but also about the past history of our Earth and the Solar System.

A Unique SatelliteOf the eight planets of the solar system two planets, Mercury and Venus, do not have any moon of their own. Of the rest, our Earth has the least number of moons – only one, compared to a total of 168 moons discovered for the remaining five planets.

But Earth’s only Moon has no parallel in the solar system. With a diameter of 3,476 kilometres, our Moon is smaller than four other moons of the solar system. Three of the Galilean moons of Jupiter are bigger than Earth’s Moon, as is Saturn’s moon Titan. But if we compare our Moon’s diameter with the Earth’s diameter of 12,756 kilometres, the Moon is just about one-third the size of the Earth. The solar system’s largest moon Ganymede, with a diameter of 5,276

The Moon is about one-third the size of the Earth

THE EARTH’S MOONAge 4.5 billion years

Average Distance from Earth 238,855 miles (384,400 km)

Diameter 3476 km (1/4 of the Earth’s)

Revolu� on period 27.3217 days

Average speed of orbit around Earth 2,288 miles per hour (3,683 kilometres per hour)

Distance traveled around Earth 1,423,000 miles (2,290,000 kilometres)

Perigee (closest approach to Earth) 225,700 miles (363,300 km)

Apogee (farthest distance from Earth) 252,000 miles (405,500 km)

SCIENCE REPORTER, OCTOBER 201313

kilometres, is only about 1/26th the size of its mother planet Jupiter.

The Earth is about 81 times as massive as its only moon, whereas the masses of the other planets are several thousand to a million times the masses

of their moons. At least in this sense, the Earth and Moon seem more like a double planet system rather than a planet and its satellite. In fact, planetary scientists consider the Earth and the Moon going round a common point called ‘barycentre’

that lies 1,710 kilometres below the surface of the Earth.

Another peculiarity of the Moon is its distance from Earth, which is far greater than is the case for some moons of the other planets relative to their radii. The mean distance of the Moon from Earth is 3,85,000 kilometres, which is sixty times the radius of Earth. This is more than double the distance between Jupiter and its outermost moon Callisto; almost three times the distance between Uranus and its outermost moon Oberon; and four times the distance from Neptune to its large moon Triton.

However, despite its large distance from Earth, the relatively large mass of the Moon exerts enough gravitational pull on Earth to cause visible effects. The main visible effects are the ocean tides, which make the ocean waters rise

COVERCOVER STORYBIMAN BASU

The moon is the most familiar celestial body in our night sky. From being steeped in mystery and mythology, it is now an astronomical body that we understand and even visit! How much do you know about the Earth’s companion?

Earth

3,85,000 km

Moon

Barycentre(Centre of mass of Earth-Moon System - about 1,700 km from the surface)

(Not to scale)

SCIENCE REPORTER, OCTOBER 2013 14

and fall twice a day under the influence of the Moon’s gravity. The gravity of the Sun also influences ocean tides, but not as strongly as the Moon, and peak tides depend on the position of the Sun, winds, and rotation of the Earth. Sometimes the gravitational pull of the Sun and Moon work together to produce very high tides (spring tides) and sometimes they work against each other to produce low tides (neap tides).

The large size of our Moon also plays a decisive role in sustaining life on Earth. On Earth we have the seasons because of the tilt of Earth’s axis, which is at present 23.5 degrees. Were it not for the Moon, the influence of the giant planets Jupiter and Saturn would have made the tilt of Earth’s axis vary wildly – between values as extreme as 0 to 80 degrees. Such variation would probably have caused extreme climatic changes that would render our planet uninhabitable. Thus, having a large Moon may be a boon for us – without it life may not have evolved on Earth.

theory proposed that the Earth and Moon were formed together as a gravitationally bound pair. The third theory suggested that the Moon formed as an independent planetary body that was later “captured” by the Earth during a close pass. None of these theories could explain all the observed facts about the Moon. A new theory was required.

In 1975 and 1976, two teams of American scientists working with Moon rocks independently made a bold suggestion – that perhaps the Moon had indeed been a part of the Earth! W.K. Hartmann and A.G.W. Cameron proposed that the Moon was formed in a catastrophic collision between the Earth and another body, one roughly the size of Mars, about 4,530 million years ago when the Earth was still not fully formed. This idea has come to be known as the ‘Giant Impact’ hypothesis.

The giant impact hypothesis could explain many features of the Moon, particularly its size. It could also explain the inclination of the Moon’s orbit

COVER STORY

Artist’s concept of the twin GRAIL spacecraft fl ying in tandem in orbit around the Moon to measure its gravity fi eld in unprecedented detail

(Credit: NASA/JPL)

Four snapshots from the computer simulation of a collision between the Moon and a smaller companion moon show most of the companion moon is accreted as a pancake-shaped layer, forming a mountainous region on one side (far

side) of the Moon. (Credit: M. Jutzi and E. Asphaug, Nature)

Diagrammatic representation of the giant impact hypothesis of the

formation of the Moon

Mars-sized object

Young Earth

Collision of large body with Earth

Ejected debris forms

Moon Earth

Moon’s interior is molten

Moon’s surface cools – crust forms – smaller impacts

create cratersBasins fl ood with lava

to form maria

Large impacts create basins

The Origin of the MoonThe Moon has been going round the Earth for an estimated 4,500 billion years or so, but planetary scientists have no clear explanation of where it came from yet. Till the mid-1970s, there were three main theories regarding the origin of the Moon. The first theory postulates an event in which the Moon broke off from a rapidly spinning Earth. The second

SCIENCE REPORTER, OCTOBER 201315

COVER STORY

towards the Earth’s equator, similarity of its chemical composition to the Earth’s mantle, and many physical characteristics. Since the Moon was formed out of material thrown out from the Earth’s outer layers after heavy elements like iron and nickel had sunk to the core, its density of 3.34 is about the same as that of Earth’s mantle and it has little iron. The giant impact hypothesis also could explain why the Earth’s axis is tilted.

One Moon from TwoA unique feature of the Moon is the stark difference between its visible near side and the invisible far side. The familiar hemisphere facing Earth is covered by low, lava-filled plains (seen as darker grey areas on the Moon’s “face”). The far side, which is never visible from Earth, is a collection of rugged, mountainous highlands. The striking differences between the near and far sides of the Moon have been a longstanding puzzle. Till recently, scientists had long held that the Earth’s gravity and impacts by foreign bodies were to blame for the variance, but they did not have any definite clue as to why it is so.

In 2011, a study published in the journal Nature suggested that the reason the Moon appears so differently from one side to the next is because at one time it was two separate objects. According to a study by planetary scientists at the University of California, Santa Cruz, USA, the mountainous region on the far side of the Moon, known as the lunar far side highlands, may be the solid remains of a

collision with a smaller companion moon, which orbited the Earth along with the Moon we see today, some 4.5 billion years ago, when the Earth was still forming.

The new study builds on the “giant impact” model for the origin of the Moon, according to which a Mars-sized object collided with Earth early in the history of the solar system and kicked up a ring of debris around the Earth that later coalesced to form the Moon. This model adequately explains the similarity in composition of the Earth’s crust and Moon rocks.

The new study suggested that this giant impact also created another, smaller body, initially sharing an orbit with the Moon. These objects then individually orbited the Earth on a slow collision course toward each other. Eventually the smaller companion fell back onto the Moon and coated one side with an extra layer of solid crust tens of kilometres thick.

Astrophysicists M. Jutzi and E. Asphaug used computer simulations of an impact between the Moon and a smaller companion (about one-third the size of the Moon) to study the dynamics of the collision and track the evolution and distribution of lunar material in its aftermath. According to them, in such a low-velocity collision, the impact does not form a crater and does not cause much melting. Instead, most of the colliding material is piled onto the impacted hemisphere as a thick new layer of solid crust, forming a mountainous region comparable in extent to the lunar far side highlands.

An attractive feature of the new model is that it neatly explains why the near and far sides of the Moon are not only different in topography but also in composition. Several sites on the near side sampled by Apollo astronauts had rocks comparatively rich in KREEP – for potassium (K), rare earth elements (REE), and phosphorus (P). But KREEP is scarce on the lunar far side, as it would be if the colliding moonlet had pushed the cooling magma ocean and its KREEP on the still-forming Moon to the lunar near side.

MasconsIn the 1960s, as NASA officials were planning for the Apollo moon missions, they discovered strange gravitational anomalies on the Moon, which came to be known as ‘mascons’. Short for mass concentrations, they are so dense they alter the Moon’s gravity field. This causes perturbations that can tug a spacecraft lower in its orbit around the Moon, or push it wildly off course, which made landing on Earth’s nearest neighbour a tricky challenge.

At that time, nothing was known about what these mascons were or how they were formed. The mystery was solved only recently from data sent back by NASA’s twin Gravity Recovery and Interior Laboratory (GRAIL) probes – a pair of spacecraft called Ebb and Flow – that meticulously mapped the Moon’s gravitational field and ended their year-long gravity-mapping mission in December 2012 by crashing into the Moon.

Artist’s impression of Chandrayaan-1 near Moon. (Credit: ISRO)

Above: GRAIL’s Gravity Map of the Moon (Credit: NASA/JPL-Caltech/MIT/GSFC)

SCIENCE REPORTER, OCTOBER 2013 16

COVER STORY

By mapping the Moon’s gravity field, the GRAIL probes uncovered the locations of lunar mascons, and offered unprecedented views of the structure of the Moon’s interior. This melting caused the material to become denser and more concentrated. This enabled scientists to

study two basins one on the lunar near side and one on the far side of the Moon – to develop sophisticated computer models for how mascons form.

They determined that ancient asteroid impacts excavated large craters on the Moon, causing surrounding lunar

materials and rocks from the Moon’s mantle to melt and collapse inward. According to NASA scientists, mascons, which are invisible on the surface but appear in gravity maps as a sort of bulls-eye, arise as a natural consequence of crater excavation, collapse and cooling following an impact. The centre of the bulls-eye has stronger gravity, with a ring of weaker gravity surrounding the bulls-eye, and then another ring of strong gravity surrounding the bulls-eye and inner ring.

This new understanding of lunar mascons is expected to influence knowledge of planetary geology beyond that of Earth and our Moon.

Water on MoonDespite more than sixty exploratory missions including six manned missions to the Moon, it was not known till 2008 that there was water on Moon. India’s first mission to Moon – Chandrayaan-1 – was launched on 22 October 2008 on a two-

The idea of living on the moon c aptures the imagina� on.

Even before the fi rst human set foot on the lunar surface during NASA’s Apollo program in 1969, people ar ound the world were dreaming about a permanent moon base t o c olonize E arth’s closes t celes� al object.

It migh t sound lik e some thing se t fi rmly in the realm of fantasy, but experts in priv ate indus try and g overnments around the world are trying to understand how f easible it w ould be t o es tablish a lunar base.

Some scien� sts think humans c ould

LIVING ON THE MOONsurvive comfortably on the moon. In some ways, the v ery minimal gr avity of the moon migh t actually be mor e c onducive to lif e than the micr ogravity as tronauts experience on the In terna� onal Space Sta� on.

Although it hasn’t been f ormally tested, some e xperts h ypothesize tha t the small amoun t of gr avita� onal force put on an as tronaut’s body when on the moon could help stem some of the adverse eff ects lik e bone-density and muscle loss that spacefl yers experience while living in microgravity on the In terna� onal Space Sta� on. This c ould mak e c olonizing the

moon an even more appealing op� on.A r obo� c base: The fi rst s tep in establishing a moon base might be robo� c. Once unmanned missions es tablish the beginnings of a base, humans c an launch to the lunar surf ace t o c onduct r esearch and main tain the habit at. Ther e is no permanent r esidence e xcept in the sense of rota� ng crews. Just like the Interna� onal Space Sta� on, the lunar base would require crews of four to eight people to rotate in and out of the base3D prin� ng of lunar laboratory: A European Space Ag ency (E SA) s tudy f ound tha t 3D prin � ng of lunar base using ma terial already available on the moon c ould be a prac� cal w ay t o es tablish an outpos t on Earth’s nearest cosmic neighbour.

Under the E SA’s h ypothe� cal plan, a robo� c mission to the moon could do most of the work before astronauts ever needed to set foot on the lunar surface.

A robot would conduct the 3D-prin� ng program aut onomously. The r obot w ould use a mixture of lunar dirt and dus t, called regolith, to cover an in fl atable dome with layers of the r obust ma terial. B y using the moon’ s indig enous ma terial, space agencies c an sa ve mone y on the c ost of fl ying price y missions t o and fr om the moon’s surface. Mining the moon: Once on the moon, instead of ha ving t o s tage c ostly missions aimed a t deliv ering o xygen and other

Artist’s rendering of a 3D printing robot pouring layer after layer of hardened lunar dirt and dust onto an infl atable dome shell to prepare a lunar base.

Chandrayaan’s M3 image of water on Moon (Credit: ISRO)

Infrared Refl ection Blue = water absorption strength on Infrared Refl ections

SCIENCE REPORTER, OCTOBER 201317

year orbital mission to the Moon. It made the first significant discovery soon after it reached lunar orbit on 14 November and dropped the Moon Impact Probe on the lunar surface.

As the probe descended, its instruments detected evidence of water, which was subsequently corroborated by other instruments on board Chandrayaan-1 and also other probes. Before its untimely demise in August 2009, fourteen months ahead of its

planned end of mission, Chandrayaan-1 had sent back heaps of valuable data and photographs that revealed for the first time that our Moon indeed has water. This was contrary to earlier beliefs that the Moon is bone dry.

Early studies on the lunar samples returned by the Apollo missions had revealed total absence of the water-bearing primary minerals such as gypsum, chalcanthite, epsomite, melanterite, etc. that are common in Earth rocks; instead all the Moon rocks examined were composed entirely of anhydrous minerals. Chandrayaan-1 gave data for the first time of the presence of water on Moon.

In October 2009, NASA’s Lunar CRater Observation and Sensing Satellite (LCROSS), which studied the dust

plume created by the impact of a Centaur upper stage rocket, further confirmed the presence of water on Moon. Analysis of data from the satellite from its crash into the permanently shadowed region of Cabeus crater near the Moon’s South Pole on 9 October 2009 showed a tell-tale signature of water.

The impact created by the LCROSS Centaur upper stage rocket created a two-part plume of material from the bottom of the crater. The first part was a high-angle plume of vapour and fine dust and the second part was a lower angle ejecta curtain of heavier material, which had not seen sunlight in billions of years. The LCROSS science team analysed data from the satellite’s spectrometers, which provide the most definitive information about the presence of water.

At a press conference on 13 November, researchers revealed preliminary data from LCROSS, indicating that water does exist in a permanently shadowed lunar crater. An infrared spectrometer on LCROSS had recorded absorption bands of water vapour at wavelengths of 1.4 and 1.85 microns. Another spectrometer registered ultraviolet emission at 309 nanometres, a tell-tale sign of hydroxyl radicals created when water molecules break apart in ultraviolet radiation from the Sun.

Subsequent analysis of the data on lunar water showed the presence of vast pockets of water ice amounting to millions of tonnes at the North Pole of the Moon, opening up another region of the lunar surface for potential exploration by astronauts and unmanned probes. According to NASA sources, the emerging picture from the multiple measurements and resulting data of the instruments on lunar missions indicates that water

COVER STORY

necessary v ola� les fr om E arth, e xperts might be able t o actually use mined lunar material to manufacture gasses needed to sustain life on the satellite.

Water could also be used for radia� on protec� on on the e xposed lunar surf ace. The moon has no a tmosphere, so people would be c ompletely suscep� ble t o the radia� on tha t w ould bombar d the r ocky satellite e very da y. W ater manuf actured on the moon c ould help shield lunar lif ers from those eff ects.From the moon t o Mar s: E ventually, a base on the moon c ould lead t o human explora� on in deeper parts of the solar system. B y de veloping v ehicles and space transporta� on systems that can enter and

remain in the ar ea between the Earth and the moon (called cislunar space), scien� sts can use that in-between space as a staging and fueling ground for a mission to Mars.

The moon base c ould func� on as a good pr oxy f or these kinds of missions by monitoring how an aut onomous habitat on another celes � al body func� ons. Engineers might be able t o manufacture propellant for deep-space travel using the natural resources the moon has t o off er. When the pr opellant is created, it can be sent to cislunar space to help fuel spaceships ready to depart for other areas of the solar system and beyond.

(Adapted from Incredible Technology: How to Live on the Moon by Miriam Kramer , Courtesy: SPACE.com)

There is plenty of oxygen on the Moon, but it is all bound up in minerals in the lunar soil.

Chandrayaan-1 had sent back heaps of valuable data and photographs that revealed for the fi rst time that our Moon indeed has water.

The fi rst step in establishing a moon base might be robotic. Once unmanned missions establish the beginnings of a base, humans can launch to the lunar surface to conduct research and maintain the habitat.

left: NASA’s LCROSS spacecraft and Centaur rocket upper stage just before crashing into the Cabeus crater near the Moon’s south pole (Credit: NASA/JPL)

SCIENCE REPORTER, OCTOBER 2013 18

COVERCOVER STORY

over three days. For their survival on this mission, they had carried all their daily needs from Earth. But if astronauts have to spend weeks or months on the Moon, they will need to make some arrangements to get some essential provisions on the Moon itself.

The main problem with setting up a permanent base on the Moon is the lack of air, water, and food. The Moon lacks light elements, such as carbon and nitrogen, although there is some evidence of hydrogen being present near the Polar Regions. There is plenty of oxygen on the Moon, but it is all bound up in minerals in the lunar soil. It would require complex industrial infrastructure, using very high energy, to extract oxygen from lunar soil. Since many other elements are needed to produce breathable air, water, food,

and rocket fuel, they would all need to be imported from Earth until cheaper sources are developed on the Moon. Till then, astronauts would be able to stay at a lunar base for short periods only.

Despite the many problems, there are many advantages of a lunar base. It can provide an excellent site for setting up astronomical observatories. The International Lunar Exploration Working Group (ILEWG), which provides a platform for coordinating lunar exploration activities by all the space agencies of the world, visualises a human base on the Moon in the next decade.

NASA has already drawn up plans to construct a solar-powered base at one of the Moon’s poles. The regions around Moon’s poles are believed to remain in near constant sunlight, which should allow for solar power generation. The current plan of NASA is to begin construction of a base beginning in 2020, with four-person crews making seven-day visits until all arrangements are complete and meet their basic requirements. The lunar base is expected to be permanently staffed by 2024!

Mr Biman Basu is a science communicator and former editor of Science Reporter . Address: C-203, Hindon Apartments, 25 Vasundhara Enclave, Delhi-110096; Email: bimanbasu@gmail.com

creation, migration, deposition and retention are occurring on the Moon.

Are Lunar Colonies possible?Mankind has dreamed of building colonies on the Moon for ages. Before the first humans set foot on the Moon in the late 1960s, such ideas were seen as part of science fiction stories. However, now some people believe lunar settlements may soon become reality, as evident from the recent resurgence in Moon missions. The finding of water is crucial for future manned missions to the Moon, which are expected to be aimed at setting up some sort of permanent bases there.

However, before bases can be set up on Moon, several hurdles need to be overcome. Till now, the longest time astronauts have spent on Moon is a little

The main problem with setting up a permanent base on the Moon is the lack of air, water, and food. The Moon lacks light elements, such as carbon and nitrogen, although there is some evidence of hydrogen being present near the Polar Regions.

top: Artist’s concept of a possible colony on the moon (Credit: NASA)left:Till now, the longest time astronauts have spent on Moon is a little over three days and for their survival they had carried all their daily needs from Earth (Credit: NASA)