Moon Revealed SR 50(10) 12-18.pdf

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SCIENCE REPORTER, OCTOBER 2013

12

VER

C O V E

R

S T O R Y

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 MOON

Age 4.5 billion years

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

Diameter 3476 km (1/4 of the Earth’s)

Revoluon 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)




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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 Jupiterand 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

OV RCOVER STORYBIMAN BASU

The moon is the most

familiar celestial body in

our night sky. From being

steeped in mystery andmythology, 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)




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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 theMoon, 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. Noneof 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 whenthe 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 flying in tandem in orbit around

the Moon to measure its gravity field 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 largebody with Earth

Ejected

debris forms

Moon Earth

Moon’s interior is

molten

Moon’s surface cools – crust

forms – smaller impactscreate craters Basins flood with lavato form maria

Large impacts create

basins

The Origin of the MoonThe Moon has been going round the

Earth for an estimated 4,500 billion yearsor 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




15

COVER STORY

towards the Earth’s equator, similarityof 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 Two

A unique feature of the Moon is the starkdifference 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 newmodel 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.

Mascons

In 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 knownabout 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)




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COVER STORY

By mapping the Moon’s gravityfield, 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 nearside 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 influenceknowledge of planetary geology beyond

that of Earth and our Moon.

Water on Moon

Despite 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 imaginaon.

Even before the first 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

celesal object.

It migh t sound lik e some thing se t

firmly 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 establish a

lunar base.

Some sciensts think humans c ould

LIVING ON THE MOON

survive 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 ternaonal Space

Staon.

Although it hasn’t been f ormally

tested, some e xperts h ypothesize tha t

the small amoun t of gr avitaonal force

put on an as tronaut’s body when on the

moon could help stem some of the adverse

effects like bone-density and muscle loss

that spaceflyers experience while living in

microgravity on the In ternaonal Space

Staon. This c ould mak e c olonizing the

moon an even more appealing opon.

A r oboc base: The fi rst s tep in

establishing a moon base might be roboc.

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 residence e xcept in the sense

of rotang crews. Just like the Internaonal

Space Staon, the lunar base would requirecrews of four to eight people to rotate in

and out of the base

3D prinng 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

praccal w ay t o es tablish an outpos t on

Earth’s nearest cosmic neighbour.

Under the E SA’s h ypothecal plan, a

roboc 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-prinngprogram aut onomously. The r obot would

use a mixture of lunar dirt and dus t, called

regolith, to cover an in flatable 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

flying price y missions t o and fr om the

moon’s surface.

Mining the moon: Once on the moon,

instead of ha ving to 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 inflatable dome shell to prepare a lunar base.

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

Infrared Reflection Blue = water absorption strength on

Infrared Reflections




17

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-1had 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 olales 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 radiaon

protecon on the e xposed lunar surf ace.

The moon has no a tmosphere, so people

would be c ompletely suscepble t o the

radiaon 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 effects.

From the moon t o Mar s: E ventually, a

base on the moon c ould lead t o human

exploraon in deeper parts of the solar

system. By developing vehicles and space

transportaon systems that can enter and

remain in the ar ea between the Earth and

the moon (called cislunar space), sciensts

can use that in-between space as a staging

and fueling ground for a mission to Mars.

The moon base c ould funcon as a

good pr oxy for these kinds of missions by

monitoring how an aut onomous habitat on

another celes al body funcons. Engineers

might be able to manufacture propellant for

deep-space travel using the natural resources

the moon has t o offer. 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 thatrevealed for the first 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, humanscan 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)



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Moon Revealed SR 50(10) 12-18.pdf