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In this mini-series to kick launch
the new blog, we’ve been exploring the theme of relating what we see on Mars to
what we know on Earth, and seeing if the two offer some sort of reciprocal
illumination into their geological development through time. Jane wrote recently about the presence of liquid water on Mars, and what the new discovery
of totally awesome ancient water from 1.5 billion year old rocks in Canada
(tasty) can reveal to us about the possibility of finding traces of primordial
life, or the conditions in which it arose here on Earth.
Now a new study has proposed evidence
of chunks of frozen carbon dioxide sandboarding (like snowboarding, but in your
shorts) down dunes on Mars, associated with the sedimentary structures we find
in the remains of impact craters. Frozen carbon dioxide forms a compact
seasonal layer that waxes and wanes with the temperature, and is strongly
dependent on latitude. The team here, a deadly combination of planetary
geoscientists and geomorphologists, propose that when the seasons change, and
with it the temperature, this dry ice layer sublimates (transforms straight
from a solid into a gas), fragments, and due to the sloped boundaries of crater
walls, blocks flake off and topple downhill, leaving their traces in the sand
before disappearing forever (well, until the next freezing season..)
Russell Crater isn’t too far from the
South Pole, hovering in the mid-latitudinal belt. The gullies we can see there
from high resolution photography overlain onto topographical maps on Mars (take
a look on Google Earth!) are long, narrow, and straight – completely different
to channels we see forming from things like rivers on Earth.
These ‘linear gullies’ are found all
over Mars, peppered around dune fields and on sandy-walls of craters, usually
within the mid-latitudinal belt and on pole-facing slopes. Some of these are
ridiculously long, growing up to 2.5km, but are extremely narrow, at about 10m
wide. They’re also really shallow, typically being only 1-2m deep, so are
pretty unusual to features we usually associate with water on Earth. The
gullies are usually flanked by little levees too, which on Earth is what you
get next to rivers as they flood and deposit sediment on the banks. Here
though, they might tell a completely different story.
Seasonal frost on Mars is mostly made up of carbon dioxide. This stuff is in the form of solid dry ice, and is what film-makers use to make that eerie smoke in movies, simply by adding liquid water to it. The grooves, or gullies, are now thought to have been created during the Martian spring as temperatures increase, and fragments or blocks of the ice break off and roll away. The scars they leave are their dying traces. The final effect is pretty much the same as dragging a stick through sand or mud – you create a gouge, and the displaced sediment spills over the sides to form the levees.
This seasonal variation is far more extreme than on Earth. The tilt of Earth’s axis, and hence the amount of surface that sunlight can hit, is regulated by the near-circular orbit of the sun, and balanced in part by our Moon. Mars is a bit nuts. It has one of the most eccentric elliptical orbits of any planet, except for Mercury and Pluto, and as such suffers much greater seasonal variations due to the range of distances from the sun each year. The result is that the seasonal climates on Mars are much more extreme than on Earth, hence why carbon dioxide regularly freezes and thaws, as indicated by the new gully analysis.
Those cool swirly lines might actually be from dust-devils, mini tornadoes like we see on Earth, but completely different due to the atmospheric differences on Mars. How cool is that! (FromJames Lewis)
The issue which the team faced is distinguishing these features from those formed by debris flows, either with or without the assistance of water. This is where things can all get a bit mathsy (i.e., boring), but the angle of gullies on the slopes is too shallow for what is needed for dry rocky material to ‘flow’ downslope through gravity, so this can effectively be ruled out as a formation mechanism. It’s actually that the dry ice is sublimating while surfing downhill that lubricates it, allowing it to travel such great distances despite how shallow the slopes are.
Other gulley structures have
fan-shaped terminals, where water has come out of the sediment or rock or ice,
and sploshed out, similar to alluvial fan structures, or ‘debris aprons’, we
see here on Earth (see image below). The reason why these new gully structures terminate so abruptly
is plausibly due to the dry ice sublimating after its finished its travels. On
occasion, the gullies do appear to meander slightly, forming the snake-like
structures of Earth rivers. The sand dunes are occupied on the surface by
little ripples (2-4m wavelength, typically), which can act to deflect the dry
ice, much like how light is refracted as it enters a new medium, ultimately
given the superficial appearance that the ice is meandering and carving a
sinuous path in its wake. So there a series of features which distinguish them
from what we’d expect to see if they formed from free-flowing fluids of some
sort, all perfectly explainable on a theoretical basis.
The fact that they’re only found on
mid-latitude on pole-ward facing slopes is pretty good evidence for this being
dry ice, as if they were at lower latitudes (closer to the poles), the climate
would be such that the ice wouldn’t be stable for long enough to form the
features (they would sublimate too fast). Also, if they were at higher
latitudes towards the poles, then the temperature is just about right for them
to melt, and remain stable enough to form fluid-based gullies instead, with the
fan-shaped tips. This provides pretty compelling evidence for a ‘seasonal
defrosting’ of carbon dioxide ice, following its freezing during the colder
seasons (in the UK, this would be summer).
One additional cool thing that the team did, was try and replicate the formation of these gullies by running experiments in similar environments on Earth. There are plenty of places on this planet that could be superficially mistaken for Mars, but the Central states of the USA are second to none when it comes to mimicking the red planet’s dead surface. They went to Utah, and started science-ing by sending blocks of dry ice down sand dunes and observing the structures they left behind. Despite the inherent physical differences in the environments between Utah and Mars (temperature, humidity, gravity, air pressure, humans), the experiments revealed structures much the same to those located on Mars. Some features of the gullies, however, still remain unexplained, such as how the meanders precisely form or the occasional large pits found at the terminal points of some of the gullies.
Linear gullies on Mars (a) on a dune in Russell crater (b) on a sandy wall of Avire crater (c) Kaiser crater - note the higher sinuousity (d) terminal pits along dune gullies in Russell crater (inset in a) (source)
So, this leaves a couple of open questions. Is it scientifically rigorous to equate experiments on Earth to what we see on Mars, or do the conditions make them incomparable? If so, what methods does this leave for the formation of these mysterious gullies? Fault planes? Debris flows? Liquid water? Some other fluid or solid phase? Dinosaurs?
By Jon, with edits by Jane. Thanks to Khalil for pointing out the story, and James Lewis for providing images and comments.
Further reading:
Guardian report: http://www.guardian.co.uk/science/2013/jun/12/grooves-mars-dry-ice-sliding
Journal article (paywalled): Diniega,
S. et al. (2013) A new dry hypothesis
for the formation of martian linear gullies, Icarus, 225, 526-537. http://dx.doi.org/10.1016/j.icarus.2013.04.006
More about seasons on Mars: http://www.universetoday.com/14719/does-mars-have-seasons/