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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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SCIENCE REPORTER, OCTOBER 2013
13
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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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 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
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SCIENCE REPORTER, OCTOBER 2013
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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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SCIENCE REPORTER, OCTOBER 2013
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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
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SCIENCE REPORTER, OCTOBER 2013
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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)