Dark Dune Spots: Possible
current habitats on Mars
Eörs Szathmáry
Collegium Budapest
szathmary@colbud.hu
Eötvös University Budapest
Mars: a dead planet (?)
•Very dry: no water
•Very cold (minus
130 C)
•High UV radiation
(above 190 nm)
•Thin atmosphere
(6 mbar)
There may have been plenty of
water in the past…
Water was lost due to radiation decay and
loss of hydrogen to space
 A large oxygen sink must be present
 More than 3 billion years ago conditions for
the origin of life seem to have been
favourable
 Could anybody have survived till today?
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An ancient lake and outflow
lake
Loire
Vallis
Cataclysmic flood channels
Cyclic and episodic changes
Viking lander: no evidence for
life
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It is easy to land in the equatorial desert, BUT
It is very hard to find even traces of life there
The soil seems oxidising
BUT some oxidized organic compounds may have escaped
detection
Claims of past life remain
controversial
Is this magnetite biogenic?
Dark Dune Spots (DDSs): a
candidate habitat (2001)?
Analyzed DDS sites in the south
polar region (Mars Global
Surveyor data)
MOC images
Dark dunes
Malin Space
Science Systems
DDSs stick to the dunes…
…and have inner structure
Mere frosting-defrosting of the
CO2 ice cover
Albedo decreases as frost thins
 Positive feedback: faster sublimation
MUST mean:
 Start spot formation at sites with strong
exposure to insolation
 Otherwise at random sites (wind, etc.)
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Simple frosting-defrosting does
NOT work, because…
Spot formation begins at the bottom, not at the top!
Simple frosting-defrosting does
NOT work, because…
Spots do not develop on exposed sites!
Simple frosting-defrosting does
NOT work, because…
There is annual recurrence (>75%) at the same sites!
Simple frosting-defrosting does
NOT work, because…
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On slopes flows
originate from the
DDSs
Which always flow
downwards
From elongated spots
Gravitation is a
formative cause
Flows may be due to water runoff
The dry planet is much wetter than thought
 There is plenty of water:
 In both ice caps
 In the upper layer of the polar region
(permafrost)
 In liquid form in the gullies

Mars Odyssey HEND
measurements
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High Energy Neutron Detector
Deficit of high-energy neutrons
Hydrogen is concentrated in the subsurface
Water-rich layers tens of centimetres thick
Frozen water in polar regions
Recent gullies by melting snow
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 North
Occur in clusters on slopes
Between 30 and 70 latitude in
both hemispheres
Consist of alcoves several
hundred metres wide,
Channels up to several
kilometres long and several tens
of metres deep
Typically originate within
several hundred metres of the
slope crest,
Can occur on crater walls that
are raised above the
surrounding terrain or near the
summit of isolated knobs.
Christensen’s mechanism (2003)
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(1) Water is transported from the poles to mid-
latitudes during periods of high obliquity, forming
a water-rich snow layer
(2) Melting occurs at low obliquity as mid-latitude
temperatures increase, producing liquid water that
is stable beneath an insulating layer of overlying
snow.
(3) Gullies form on snow-covered slopes through
erosion by melt water or as a result of melt water
seeping into the loose slope materials and
destabilizing them.
Christensen’s mechanism II
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(4) Gullies incised into the substrate are observed
where the snow layer has been completely
removed.
(5) Patches of snow remain today where they are
protected against sublimation by a layer of
desiccated dust/sediment
(6) Melting could be occurring at present in
favourable locations in these snowpacks.
Clow’s (1987) model for melting
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Melting occurs beneath the surface at temperatures
well below freezing, because sunlight is absorbed
at depth rather than at the surface, and this
absorption is substantially increased by the
incorporation of minor amounts of dust.
Can occur for a wide range of snow properties and
atmospheric pressures, and occurs under current
conditions in mid-latitudes if dust abundances are
greater than 1,000 parts per million by mass.
Clow’s model II
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Meltwater moving downwards under gravity will
encounter lower temperatures and refreeze.
Conduction and latent heat transfer will gradually
warm the snow and substrate, allowing liquid
water to accumulate and be available for erosion
Subsurface erosion and collapse of the snow
mantle will occur, with liquid water potentially
reaching and eroding the substrate as the snow
layer continues to melt.
Clow’s model III
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Liquid water will begin to be generated 100 d after
the spring equinox under current conditions for
snow with a dust content of 1,000 p.p.m.m.
Will reach a depth of 20 cm approximately 25 d
later.
Up to 0.33 mm of snowmelt runoff is produced
each day for 50 d each martian year.
Compare HEND with DDS sites!
DDSs are different from gullies,
because…
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They reappear annually
Frost/snow cover above them is re-established
each year and disappears by midsummer
Crater slopes and alcoves are NOT necessary
BUT the dune material IS
Channels from DDSs on slopes are also thought to
form below the snow
Slow melting, NOT gas outbreak: complete lack of
explosive formations
Salinity unknown, could be important
Layered frost on the dunes (2002)
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Water ice, clathrate
and CO2 ice are
deposited in that order
Dunes are the first to
frost and the last to
defrost
Total frost between
0.2-1 m (laser
altimeter)
Dark spots are transformed to
summer grey spots
The biological hypothesis
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Annual reactivation and
growth of photosynthetic
organisms
Ice: excellent shield
against cold, UV and
dryness
Organisms must go to
dormancy before water ice
shield melts through
(‘adaptive sporulation’)
Looking for partial analogues on
Earth (extremophiles)
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The Dry Valleys of
Antarctica
Cold, dry
High UV due to
continuous solar
exposure
Ozone depletion
Bacterial activity in lake ice
These bacteria are permanently buried in ice!
Photosynthetic microorganisms
At the centre of a rich
consortium
Ice and snow lake covers in
mountains (Alps, etc.)
UV protection by snow
Thermal tolerance on Earth
Temperatures go down to –70 °C in the
Antarctic valleys
 Spores can be cooled down arbitrarily
 Photosynthesis is possible under the snow
down to –20 °C, when the temperature
above is a lot colder
 You do not need liquid water, only positive
water potential
 Extremely fast rehydration in cyanobacteria
 Many cyanobacteria are halophilic
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Snow and ice UV protection
More on UV resistance
There are bacteria (e.g. Deinococcus
radiodurans) on Earth, extremely resistant
to radiation and dryness
 Martian organisms must have undergone
billions of years of adaptation
 Dead cells in the upper layer efficiently
protect viable cells in the lower layer
 Viable stock may also endure as endoliths
 Efficient external and internal shields (e.g.
black in cyanobacteria: “sunglass”)
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A Mars chamber simulation is
being prepared…
at Centro di Astrobiologia (CAB) in Madrid
Objective of the chamber
experiments
1.
2.
3.
To prove that there is a layered structure
of the frost under simulated Martian
conditions
To see whether spots form by simple
frosting-defrosting or not
To introduce biological material (e.g.
cyanobacteria) into the simulation
Summary
DDSs are a potential habitat for life on
Mars today
 They may be actual habitats
 Earthly analogues are encouraging
 Chamber simulations have to be carried out
 Looking for pigments by appropriate
spectroscopy (resolution, wavelengths!)
 Sending landers to interesting sites!
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Some further thoughts
Those who consider this story scandalous
must also ultimately think that life is
improbable!
 If life is probable, this story is not against
parsimony at all
 We are faced with NON-abundant life
 We are in a lucky period, because the
phenomenon may disappear in the future
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Main collaborators
András Horváth, astronomer and
planetologist
 Tibor Gánti chemical engineer and
theoretical biologist (Principles of the
Living State OUP, in a week)
 Susanna Manrubia physicist (CAB) to
coordinate the Spanish group (incl.
chamber)
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Reading
Horváth, A., et al. (2001) Probable evidence of recent
biological activity on Mars: Appearance and growing
of dark dune spots in the South Polar Region, Lunar
Planet Sci. XXXII, #1543,
Horváth, A., et al. (2002) Morphological Analysis of the
Dark Dune Spots on Mars: New Aspects in Biological
Interpretation, Lunar Planet. Sci. XXXIII, #1108.
Horváth, A., et al. (2002) The “Inca City” Region of
Mars: Test field for Dark Dune Spots Origin, Lunar
Planet. Sci. XXXIII, #1109.
Gánti et al. (2003) Origins of Life and Evolution of the
Biosphere in press