Module 13:
Planets as Habitats
Activity 1:
Signs of Life
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?
(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.
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 inside
the Earth’s crust, have called into question whether life on
Earth started in the oceans, as has been conventionally
assumed.
Traditionally it has been thought that
lightning strikes and volcanic eruptions
into the primeval atmosphere and seas
of the Earth would trigger the formation
of complex carbon-based molecules in
the ancient seas which could be the
precursors to life.
Experiments have been conducted where electricity
(“lightning”) has been discharged into mixtures attempting
to replicate the primeval terrestrial atmosphere and the
“primeval soup” of Earth’s ancient seas, and the right
sort of complex molecules have indeed been formed.
While these experiments are hardly conclusive, they do
highlight a popular premise of astrobiology – that given the
raw materials and the right conditions, life will take hold
anywhere it can.
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.
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.
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.
(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.
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 green
in contrast to the prevailing red, and were misinterpreted as
vegetation.)
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 their
desert-like planet and invade Earth, culminating in the
War 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 when
the Viking and subsequent missions sent back images of
features that looked like dried-up riverbeds!
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.
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 resolution
view
oblique view
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.
The Mars Pathfinder landed in what had appeared to be
an old stream bed. Images sent back by Pathfinder support
this, with evidence of terracing, and the rocks tilting in the
same direction,
suggesting a
massive water
flow in the past.
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
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 later
floods through the Kasei region.
island cliff
moat-like feature partially encircling
the crater, probably formed when
floodwater encountered the crater wall
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.
(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.
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 lowresolution 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.
Viking mission image of area
Original low
resolution
MOC image
Highest resolution
MOC detail of alcove
Higher resolution MOC detail of alcove
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.
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:
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 website
http://mars.jpl.nasa.gov/mgs/msss/camera/images/june2000/index.html
(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
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%
Alternating Light- and Dark-toned Layers in Holden Crater:
Viking image
“Colourised” MOC high resolution image
Sedimentary rock layers in the Grand Canyon on Earth
The evidence for liquid water on Mars is not completely
conclusive, 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-science-00k1.html
Make your own density flow:
http://www.beloit.edu/coterie/SEPM/public_html/Water_Works/density_currents.html
If liquid water has flown on the Martian surface, where could
it have come from?
At the poles the temperature is typically at 160°K, and
apart 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.
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.
According to some planetary
scientists, Gusev crater may
have been an ancient lakebed.
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 highly
controversial. The more conventional explanation is
flash flooding 1–3 billion years ago when the
atmosphere was denser and warmer. This assumes
that the climate of Mars has varied in the past, and
may still do so.
The polar caps contain water ice, carbon dioxide ice and
accumulated layers of dust - which are left behind in
layers when the water ice caps recede each Martian spring.
Variation in the
observed layering
suggests periodic
changes in climate
(perhaps due to
changes in Mars’
rotational inclination
or orbit around the
Sun).
Water Ice on Mars??
In June 2002, NASA announced the findings of an experiment onboard 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.
Yes - Ice on Mars!
One of the main aims of ESA’s Mars Express mission is to
discover water in any of its chemical states.
On 18 January 2004, OMEGA,
an imaging spectrometer,
observed the southern polar cap
in three bands – optical, carbon
dioxide and water. These
observations confirm the
presence of carbon dioxide and
water ice.
H2O ice
CO2 ice
visible
(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.
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 planet’s carbon dioxide in its
atmosphere would condense to the ground as dry ice,
causing the atmospheric temperature to drop below
freezing.
Calculations suggest that the Sun’s 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!
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 Sun’s 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.
(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 Mars
in the past became a ‘hot topic’ again with the
announcement in 1996 that a meteorite, originally from
Mars, contained several forms of evidence that life had
once existed in cracks contained in it, while it was still
on Mars.
Possible Ancient Life on Mars?
Found in Antarctica in 1984
Identified as formed on Mars
4.5 billion years ago
Water penetrated fractures
3.6 – 4 billion years ago
~2kg meteorite
Click here to
see an
animation
16 million years ago rock
ejected from Mars due to
a large impact
Rock landed in Antarctica
approx. 13,000 years ago
Microscopic analysis:
fractured rock
carbonate
minerals
found in
fractures
concentration
increases towards interior?
In more detail:
fractured
carbonate minerals
iron sulfides & magnetite
- produced by anaerobic
bacteria?
Polycyclic aromatic
hydrocarbons (PAHs)
- from dead organisms?
possible microscopic fossil
structures similar to
“nanobacteria” found in hot
springs on Earth
Martian Nanobacteria?
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.
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.
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/
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
What’s 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
(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.
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.
For example, Jupiter’s satellite Europa
appears to be a cracked, icy surface
floating on liquid water - kept warm by
tidal heating (as we will see in a later
Activity).
Europa’s fluctuating magnetic field
indicates that it has a salty ocean
underneath its hard ice surface.
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.
Saturn’s largest satellite, Titan, is
also of interest to astrobiologists
because it has a smoggy 200km
deep atmosphere of nitrogen,
methane and hydrocarbons.
Ganymede
Callisto
Titan
* Galileo ended its mission on 17 September 2003,
after 14 years of extensive investigations of Jupiter
and its Moons. Once the probe’s 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).
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.
Image Credits
Erosion channels on Mars
http://www.anu.edu.au/Physics/nineplanets/thumb/marsriver2.jpg
Viking Landing Site
http://www.anu.edu.au/Physics/nineplanets/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/marsnews/mgs/images/84702b.gif
http://www-b.jpl.nasa.gov/marsnews/mgs/images/84701b.gif
http://www-b.jpl.nasa.gov/marsnews/mgs/images/84700b.gif
Nanedi Valles runoff channel
http://lunar.ksc.nasa.gov/mars/mgs/msss/camera/images/top102_Dec98_r
el/nanedi/n_nanedi_8704_ICON.gif
Erosion channels on Mars
http://www.anu.edu.au/Physics/nineplanets/thumb/marsriver2.jpg
Image Credits
Martian southern polar region
http://nssdc.gsfc.nasa.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/
Gusev/plain-map-res=128.gif
NASA, Malin: Kasei Vallis exhumed crater
http://lunar.ksc.nasa.gov/mars/mgs/msss/camera/images/10_12_98_dps_
release/10_12_98_kasei_rel/34504_sub_ICON.gif
http://lunar.ksc.nasa.gov/mars/mgs/msss/camera/images/10_12_98_dps_
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/thumb/yogi.gif
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/images/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
NASA, Malin : Layered rocks in Grand Canyon:
http://www.msss.com/mars_images/moc/dec00_seds/slides/index.html
Europa’s ocean?
http://photojournal.jpl.nasa.gov/
Ganymede, Calliso, Titan
http://solarsystem.nasa.gov/
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