Signs of Life

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Module 13 Planets as Habitats
Activity 1 Signs of Life
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Summary
In this Activity, we will investigate (a)
Astrobiology (b) Searching for water on Mars (c)
Follow the Water! (d) A Land of Lakes? (e)
Habitable Zones (f) Searching for Signs of
Ancient Life on Mars, and (g) Life elsewhere in
the Solar System?
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(a) Astrobiology

• Astronomers, especially planetary astronomers,
  tend to need a fairly all-round knowledge, made
  up of bits and pieces of several sciences apart
  from their own - in particular physics,
  chemistry, geology and meteorology.

Until recently the one scientific area an
astronomer could safely be ignorant of was the
biological sciences, but not any more. An
astronomer who is well informed on current Solar
System research now needs to know about
astrobiology (sometimes called exobiology or
bioastronomy), the study of the possibility of
life outside Earth, and that involves at least a
smattering of palaeontology, genetics, ecology,
botany and zoology.
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• Astrobiologists have a particular interest in the
  study of when, where and how life started on
  Earth.

The recent discovery of primitive organisms
around volcanic vents deep on the ocean floor,
and bacterial contamination of samples taken
from deep insidethe Earths crust, have called
into question whether life on Earth started in
the oceans, as has been conventionallyassumed.
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While these experiments are hardly conclusive,
they dohighlight a popular premise of
astrobiology that given the raw materials and
the right conditions, life will take hold
anywhere it can.
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• That is a fairly big assumption, based on a
  sample of one one location where we know life
  has taken hold (Earth).

If life even fossilised remains of ancient
single celled life were found in another
location in the Solar System, it would make the
life will take hold wherever it can assumption
a lot more respectable.
The search for life outside Earth depends on us
being able to recognise life if and when we find
it.
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• The forms of life with which we are familiar are
  based on
• carbon molecules which are capable of forming
  long and complex chains which can store
  genetic information

• and water which has many properties vital for
  life as we know it its properties as a
  solvent, its ability to absorb a large amount
  of energy for a small temperature change
  (heat capacity), its ability to stay liquid
  over a wide temperature range, and its use in
  evaporative cooling.

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• We can theorise about lifeforms based on, for
  example, silicon, not carbon, and other liquids
  such as ammonia or methyl alcohol, but these
  alternatives are not as robust or versatile as
  carbon and water - at least to produce life as we
  would recognise it.

So astrobiologists look for conditions which
either support or have supported liquid water,
and contain or may have contained the organic,
carbon-based compounds associated with life.
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(b) Searching for Water on Mars

• The low surface pressure on Mars (only 1 of that
  on Earth) implies that there is no liquid water
  present, as any liquid water on the surface of
  Mars today would vaporise rapidly due to the low
  atmospheric pressures. Only in the very deepest
  canyons where atmospheric pressure is at a
  maximum could there possibly be liquid water.

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Last century, Giovanni Schiaparelli, an Italian
astronomer,reported that he could see what
appeared to him to be a number of dark lines
criss-crossing the Martian surface, and called
them canali, or water channels.
Translated into English, this was interpreted as
canals, and it became fashionable to assume
that, as Mars appeared to be much like Earth, it
was inhabited by intelligent life which had
built a sophisticated canal system to bring water
from the Martian polar ice caps to irrigate the
rest of the planet. (Seasonal variations in the
colour of Mars can look greenin contrast to the
prevailing red, and were misinterpreted
asvegetation.)
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By the end of the 1800s, Percival Lowell, a
wealthy American, had reported observing 160
Martian canals. In popular stories, Martians were
likely to leave theirdesert-like planet and
invade Earth, culminating in theWar of the
Worlds hoax, where the freeways of New York were
clogged by motorists attempting to escape a
Martian invasion - panic brought about by a too
realistic radio play.
Modern telescopes show no sign of canali or
canals.Imagine then, the surprise for planetary
scientists whenthe Viking and subsequent
missions sent back images of features that
looked like dried-up riverbeds!
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• The scale of these runoff channels - up to 1500
  km long with widths up to 100 km - is many times
  too small to be seen with modern Earth-based
  telescopes (much less the telescopes of
  Schiaparelli and Lowell).

They are very old the amount of cratering in
the channels suggests that they are 3 to 3.8
billion years old. No runoff channels were
observed by Viking in the younger terrain in the
north.
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The following Mars Orbital Surveyor images are of
Nirgal Vallis, one of a number of possible
runoff channels. The debate about these valleys
centres on whether they were formed by
water flowing across the surface, or by collapse
and erosion associated with groundwater
(artesian) processes.
low resolutionview
oblique view
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At the resolution of these early images from the
Mars Orbital Surveyor, it is not possible to tell
whether this is a pattern of debris resulting
from groundwater collapse, or a pattern of
drainage channels.
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The Mars Pathfinder landed in what had appeared
to be an old stream bed. Images sent back by
Pathfinder supportthis, with evidence of
terracing, and the rocks tilting in the same
direction, suggesting a massive water flow in
the past.
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The images below show a crater in Kasei Vallis
that was imaged by the Mars Orbital Surveyor in
June 1998, showing a 6 km diameter crater that
was once buried by an island of about 3 km of
Martian bedrock. Kasei Vallis is actually a
system of giant channels thought to have been
carved by catastrophic floods that occurred more
than a billion years ago.
top edge of cliff
island
bottom edge of cliff
crater
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The crater may have been formed as long as 3.5
billion years ago by meteor impact. Sometime
later it was buried by the material that
comprises Lunae Planum (the large plains unit of
which the island appears to be part). The island
is at least partly made of hard rock, however the
processes which buried the crater were gentle
enough not to destroy it. The crater is like a
giant fossil, which has apparently been exposed
by the laterfloods through the Kasei region.
island cliff
moat-like feature partially encircling the
crater, probably formed when floodwater
encountered the crater wall
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Another piece of evidence comes from meteorites
found on Earth, which isotopic analysis suggests
have come originally from Mars. Meteorites of
this type have been discovered to contain
water-soaked clay bound up inside them, which
suggests that they were exposed to liquid water
while still on Mars.
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(c) Follow the Water!

• The previous discussion summarises the
  information known and inferred about water on
  Mars, up to June 2000.
• However in that month, NASA and Malin Space
  Science systems, who are chief investigators with
  the Mars Orbital Camera (MOC) on the Global
  Surveyor, held a press conference to release
  high-resolution images taken with the MOC since
  March 1999.
• The images they released show apparently recent
  runoff features which suggest that liquid water
  has existed (and may still exist) at shallow
  depths below the surface of Mars in geologically
  recent time.

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• The planetary geologists associated with the MOC
  first suspected that liquid groundwater may seep
  out onto the surface in certain location on Mars
  when they analysed a low-resolution picture taken
  during the Orbit Insertion Phase of the mission
  in December 1997.

The image showed dark, v-shaped scars on the
western wall of a 50 kilometer-diameter impact
crater in southern Noachis Terra. The MOC
scientists interpreted the image to be similar to
that of seepage landforms on Earth that form
where springs emerge on a slope and water runs
downhill. The following images show the
increasing detail seen once the MOC started to
make high resolution images, and also shows why
these relatively small-scale features were not
visible in Viking images. The gullies are too
small to have been detected by the Mariner and
Viking spacecraft.
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Viking mission image of area
Higher resolution MOC detail of alcove
Original low resolution MOC image
Highest resolution MOC detail of alcove
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• The features that have been observed can be
  explained by groundwater seepage and runoff.
  They are mostly seen on canyon and crater walls
  facing away from the equator.

The geologists who made the discovery theorise
that there is or has recently been (geologically
speaking, this means in the last few million
years) a layer of water buried less than 500 m
below the Martian surface - an aquifer, somewhat
like the Great Artesian Basin in Australia - and
that it normally evaporates where it is in
contact with air. However on the colder sides of
canyons and craters, the evaporation cools the
water down till it forms a surface layer of ice.
The pressure behind the ice causes it to dislodge
occasionally, resulting in sudden outflows of
water down the canyon walls, causing the patterns
seen in the MOC images shown here.
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• Some of the MOC photographic evidence suggests
  that some outflows might be very recent indeed,
  because

• they contain no cratering
• they flow over wind patterns in the Martian soil
• they flow over polygonal patterns believed to be
  due to seasonal freezing melting of
  permafrost ice, and
• they contain some regions where the unusually
  (for Mars) strong contrast in surface colour
  tends to suggest that dust has not had time to
  settle

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• If the MOC scientists interpretation of these
  images is correct, Mars may contain subterranean
  liquid water today.

The existence of liquid water at Mars
temperatures and pressures is not easy to
explain. However suggestions (backed up by trace
analysis of some Martian meteorites) that the
water may be very brackish (salty) would help to
explain why it might be able to stay liquid at
low temperatures. If liquid water does exist on
Mars, or has existed in geologically recent
times, it will reopen the debate on whether Mars
ever has supported primitive forms of life, or
indeed does now. For more information, see the
Internet websitehttp//mars.jpl.nasa.gov/mgs/msss
/camera/images/june2000/index.html
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(d) A Land of Lakes?
In December 2000, the same scientists studying
MOC data released images showing what appears to
be layers of sedimentary rock, similar to
patterns seen in places on Earth where lakes once
existed. To quote Dr. Michael Malin, chief
investigator for the MOC,
We see distinct, thick layers of rock within
craters and other depressions for which a number
of lines of evidence indicate that they may have
formed in lakes or shallow seas. We have never
before had this type of irrefutable evidence
that sedimentary rocks are widespread on Mars.
These images tell us that early Mars was very
dynamic and may have been a lot more like Earth
than many of us had been thinking.
The full NASA press release is on the Internet
at ftp//ftp.hq.nasa.gov/pub/pao/pressrel/2000/00-
190.txt
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Layered Outcrops of Far West Candor Chasma

1.5 km
Low resolution MOC composite image
2.9 km
Colourised MOC high resolution image, vertical
heights exaggerated by 50
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Alternating Light- and Dark-toned Layers in
Holden Crater
Viking image
Colourised MOC high resolution image
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Sedimentary rock layers in the Grand Canyon on
Earth
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The evidence for liquid water on Mars is not
completelyconclusive, but by the end of 2000 it
was arguably the simplest explanation for the
evidence.
Some researchers, however, argue that the
features could be caused by other phenomena such
as deposits laid down by dust storms, and
pyrochlastic outflows caused by release of
hypothesized reservoirs of frozen carbon dioxide
far beneath the surface of the planet. One
version of the latter theory is called the White
Mars theory, in contrast to the arguments for
liquid water having existed or still being
present on or in Mars.
For more information about White Mars,
see http//www.earthsci.unimelb.edu.au/mars/Enter
.html http//www.spacedaily.com/news/mars-water-sc
ience-00k1.html Make your own density flow
http//www.beloit.edu/coterie/SEPM/public_html/Wat
er_Works/density_currents.html
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If liquid water has flown on the Martian surface,
where could it have come from?

• At the poles the temperature is typically at
  160K, andapart from the water ice in the polar
  caps there is probably ice frozen under the
  surface, like permafrost on Earth.
• The Martian permafrost under the Martian surface,
  if it exists, may have occasionally melted in the
  past due to meteorite impacts or volcanic
  activity, leading to ground collapse and flash
  flooding.
• The presence of permafrost as a potential source
  of both water and oxygen, on a planet similar in
  many ways to our own with soil which may support
  plant growth in greenhouses, would make Mars an
  attractive target for colonising and terraforming
  sometime in the future.

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• Repeated flash flooding is one thing, but lakes
  are another, and some astronomers believe that
  they have also identified features on Mars to be
  shorelines of oceans. To sustain oceans or even
  lakes for any period of time, ancient Mars must
  have had a much thicker atmosphere than it has
  today.

By counting craters, the proponents of Martian
oceans conclude that oceans existed up to 2.5 to
3.5 billion years ago, and propose that they
managed to remain liquid due to a greenhouse
effect caused by gases released by volcanoes.
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• Others believe that they have identified scars on
  the old southern Martian landscape due to
  glaciers. Glaciers need snow to form them, and
  the formation of snow in turn requires
  evaporation from an ocean.

Ancient Martian seas, let alone glaciers, are
highlycontroversial. The more conventional
explanation isflash flooding 13 billion years
ago when the atmosphere was denser and warmer.
This assumesthat the climate of Mars has varied
in the past, andmay still do so.
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The polar caps contain water ice, carbon dioxide
ice andaccumulated layers of dust - which are
left behind in layers when the water ice caps
recede each Martian spring.
Variation in theobserved layeringsuggests
periodicchanges in climate(perhaps due
tochanges in Marsrotational inclination or
orbit around the Sun).
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Water Ice on Mars??
In June 2002, NASA announced the findings of an
experiment on-board the Mars Odyssey satellite
which has been using a neutron spectrometer to
search for ice reservoirs just below the surface
of Mars. High-energy gamma-rays originating from
hydrogen molecules less than one metre below the
Martian surface were detected by the
spectrometre scientists currently believe that
the hydrogen is locked up in the form of ice
crystals.
The Mars Odyssey spectrometer is based upon the
same design as the Lunar Prospector which
discovered ice in the polar regions of the Moon
in 1998.
Soil enriched with hydrogen is deep blue in
colour. Smaller amounts of hydrogen are shown
light blue, green, yellow and red. The deep blue
areas in the polar regions are believed to
contain up to 50 percent water ice in the upper
one metre of the soil.
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Yes - Ice on Mars!
One of the main aims of ESAs Mars Express
mission is to discover water in any of its
chemical states.
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(e) Habitable Zones

• Could Mars have once had a thick enough
  atmosphere and warm enough temperatures to have
  supported liquid water and life on or near its
  surface?
• If we look for regions in our Solar System which
  would support life as we would recognise it
  i.e. carbon based, requiring liquid water and the
  temperatures to sustain it then we can make
  estimates of the range of distances from the Sun
  where the development of life would be possible.

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• The inner edge of the habitable zone for our Sun
  is the maximum distance from the Sun at which a
  planet would undergo a runaway greenhouse effect,
  like Venus.

The outer edge of the habitable zone for our Sun
is the distance at which a planets carbon
dioxide in its atmosphere would condense to the
ground as dry ice, causing the atmospheric
temperature to drop below freezing.
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• Calculations suggest that the Suns habitable
  zone now stretches from about 0.95 AU (just
  inside the orbit of the Earth) to about 1.4 AU
  (just inside the orbit of Mars).

The Sun is gradually brightening - the inner edge
of the zone was probably located at about 0.8 AU
approx 5 billion years ago. As the Sun ages and
becomes gradually brighter, the Earth will
eventually heat up to the point where it lies
outside the habitable zone - but not for another
5 billion years or so!
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• However there are some inconsistencies in this
  story. If the Sun is gradually becoming
  brighter, then it should have been dimmer and put
  out less energy in the early history of the Earth
  (and Moon) - but palaeontological and geological
  studies of the early history of the Earth do not
  appear to show any effects of a weaker Sun.

This makes it difficult to make definitive
statements about whether Mars was positioned
within the Suns habitable zone early in its
history. This background slide shows stills
from a Mars virtual reality movie made by the
Swinburne Centre for Astrophysics
Supercomputing, using Mars Global Surveyor data
to construct (vertical scale-enhanced) Martian
topology, then terraforms Mars by adding an
atmosphere and ocean. Click here to see the
animation.
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(f) Searching for Signs of Ancient Life on Mars

• The Viking landers carried out experiments to
  look for signs of life on the Martian soil at
  their landing sites - without success.

The current set of Martian probes are mainly
designed to look for evidence of water rather
than signs of life on Mars. However the issue of
whether life has existed on Marsin the past
became a hot topic again with the announcement
in 1996 that a meteorite, originally fromMars,
contained several forms of evidence that life
hadonce existed in cracks contained in it, while
it was stillon Mars.
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Possible Ancient Life on Mars?
Found in Antarctica in 1984
Identified as formed on Mars4.5 billion years ago
Water penetrated fractures 3.6 4 billion years
ago
16 million years ago rock ejected from Mars due
to a large impact
2kg meteorite
Rock landed in Antarcticaapprox. 13,000 years ago
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Microscopic analysis
fractured rock
carbonate mineralsfound infractures
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concentrationincreases towards interior?
In more detail
Polycyclic aromatichydrocarbons (PAHs) - from
dead organisms?
fractured
carbonate minerals
possible microscopic fossilstructures similar to
nanobacteria found in hotsprings on Earth
iron sulfides magnetite- produced by anaerobic
bacteria?
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Martian Nanobacteria?
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• So, in summary, a meteorite found in Antarctica -
  but originating on Mars - is claimed to contain
  several pieces of evidence which suggest that
  ancient life once existed in its cracks. The
  concentration of the deposits increases with
  depth of the cracks, suggesting that the deposits
  did not infiltrate the rock while on Antarctica.

The announcement of this discovery provoked a
vast amount of press coverage and interest.
However other scientific teams have since
disputed the conclusions, claiming that the
deposits did settle in the rock on Antarctica
rather than on Mars, that the deposits were
transported into the rock by a hot gas rather
than water flow, and that the claimed nanofossils
are instead crystals.
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On the principle that extraordinary claims
require extraordinary evidence, the broader
scientific community is yet to be convinced on
this claim of evidence for ancient Martian life.
However the liquid water on Mars debate (see
earlier) has now reopened the whole issue of life
on Mars, even in geologically recent times. In
December 2000 another dramatic twist in the story
occurred - a group of scientists published the
conclusions of a four-year long study of the
magnetite crystals (see earlier) found in
ALH84001. They concluded that the crystals
originated on Mars, and that a significant
proportion of the magnetite crystals are
identical to those found in aqueous bacteria on
Earth.
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Magnetite crystals act as very efficient
compasses which are believed to assist bacteria
in locating and maintaining optimal positions for
survival in environments containing, for example,
dissolved oxygen in water.
Today Mars has only localised magnetic fields of
any significant size. It was originally believed
that Mars had never had a strong magnetic field,
but instruments on the Mars Global Surveyor have
since observed magnetised strips in the Martian
crust which suggest that Mars once possessed a
strong magnetic field, perhaps at the same time
as the magnetite crystals found in ALH84001 were
formed. To find out more, see the NASA-Johnson
Space Centre press release on the Internet at
http//spaceflightnow.com/news/n0012/14marslife/
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This research, as with the original claims about
ALH84001, will be subjected to considerable
scrutiny by the scientific community. Whatever
the outcome, this and the recent Mars Express
announcement about water ice on Mars have ensured
that future (successful!) missions to the red
planet will be followed with considerable
interest.

• To follow the debates, visit the following
  Internet sites
• Mars Today
• http//humbabe.arc.nasa.gov/MarsToday.html
• Signs of Past Life on Mars?
• http//www.fas.org/mars/aaas_001.htm
• Whats New with Life on Mars
• http//www.fas.org/mars/new.htm
• Centre for Mars Exploration (NASA)
• http//cmex-www.arc.nasa.gov
• Mars Global Surveyor
• http//mars.jpl.nasa.gov/mgs/index.html

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• (d) Life elsewhere in the Solar System?

With the renewed interest in extraterrestrial
signs of life brought about by the Martian
meteorite claims, planned future NASA missions
to Mars will probe more extensively for life
signs than was possible for the Viking landers.
Mars is outside the habitable zone in the Solar
System(and probably always was) - what then, is
the point of looking for signs of ancient or
recent life there?
Surface-dwelling life on Mars, like liquid
surface water on Mars, would have depended on a
thicker atmosphere in the past, causing enough of
a greenhouse effect to raise the temperature and
pressure to a point where both could exist.
Sub-surface life forms could have tolerated a
wider range of temperatures and pressures,
however.
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Although temperatures drop steadily as we move
into the outer Solar System, there are other
local havens where life might possibly have
existed in the past, or even now.
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• Images from the Galileo probe showed that
  Ganymede and Callisto, two other large Jovian
  satellites, may also host regions of salty,
  liquid water under surface layers of ice.

Galileo ended its mission on 17 September 2003,
after 14 years of extensive investigations of
Jupiter and its Moons. Once the probes fuel was
depleted, it was put on a collision course with
Jupiter in order to avoid an impact with Europa
(thought to be one of the best candidates for
life in the solar system).
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While it is possible that there may have been an
exchange of life between the Earth and Mars
sometime in the past (via meteorites), the really
exciting thing about life of Europa is that it is
extremely unlikely that it would have been from
an exchange with the Earth or Mars, and hence
most astrobiologists believe that if there is
life on Europa, then it must have developed
totally independently of life on Earth.

• In the next Activity we will go on to model the
  evolution of Mars and Venus as compared to Earth,
  and also look at the satellites of Mars.

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Image Credits
Erosion channels on Mars http//www.anu.edu.au/Phy
sics/nineplanets/thumb/marsriver2.jpg Viking
Landing Site http//www.anu.edu.au/Physics/ninepla
nets/thumb/vlpan22.gif Nirgal Vallis Highland
Valley Network http//www-b.jpl.nasa.gov/marsnews/
mgs/images/84703b.gif http//www-b.jpl.nasa.gov/ma
rsnews/mgs/images/84702b.gif http//www-b.jpl.nasa
.gov/marsnews/mgs/images/84701b.gif http//www-b.j
pl.nasa.gov/marsnews/mgs/images/84700b.gif Nanedi
Valles runoff channel http//lunar.ksc.nasa.gov/ma
rs/mgs/msss/camera/images/top102_Dec98_rel/nanedi/
n_nanedi_8704_ICON.gif Erosion channels on
Mars http//www.anu.edu.au/Physics/nineplanets/thu
mb/marsriver2.jpg
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Image Credits
Martian southern polar region http//nssdc.gsfc.n
asa.gov/image/planetary/mars/mars_so_pole.jpg NASA
, Malin Gusev crater http//ic-www.arc.nasa.gov
80/ic/projects/bayes-group/Atlas/Mars/special/Gus
ev/plain-map-res128.gif NASA, Malin Kasei
Vallis exhumed crater http//lunar.ksc.nasa.gov/ma
rs/mgs/msss/camera/images/10_12_98_dps_release/10
_12_98_kasei_rel/34504_sub_ICON.gif http//lunar.k
sc.nasa.gov/mars/mgs/msss/camera/images/10_12_98_d
ps_release/10_12_98_kasei_rel/226a08sub_34504cntx
_ICON.gif http//lunar.ksc.nasa.gov/mars/mgs/msss/
camera/images/10_12_98_dps_release/10_12_98_kasei
_rel/kasei_region_ICON.gif Pathfinder on
Mars http//www.anu.edu.au/Physics/nineplanets/thu
mb/yogi.gif
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Image Credits
Yohkoh Soft X-ray Telescope (SXT) full-field
images from the Hiraiso Solar Terrestrial
Research Center / CRL (Japan) http//umbra.nascom.
nasa.gov/images/latest_sxt.gif NASA, Malin
Evidence of geologically-recent liquid water on
Mars http//mars.jpl.nasa.gov/mgs/msss/camera/imag
es/june2000/index.html NASA, Malin MOC West
Candor Chasma images of layered
terrain http//www.msss.com/mars_images/moc/dec00
_seds/wcandor/index.html NASA, Malin Holden
Carter images of layered terrain http//www.msss.
com/mars_images/moc/dec00_seds/holden/index.html N
ASA, Malin Layered rocks in Grand
Canyon http//www.msss.com/mars_images/moc/dec00_
seds/slides/index.html Europas
ocean? http//photojournal.jpl.nasa.gov/ Ganymede,
Calliso, Titanhttp//solarsystem.nasa.gov/
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• Now return to the Module 13 home page, and read
  more about looking for water on Mars and
  extraterrestrial life in the Solar System in the
  Textbook Readings.

Hit the Esc key (escape) to return to the Module
13 Home Page
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