Polar Geology Theme Overview:
By
Shane Byrne and Ken Herkenhoff
Explanation
of the theme.
The deposits at both Martian poles can naturally be
divided into the polar layered deposits and the residual ice caps which
partially cover them. The polar layered deposits form a layered sedimentary
stack up to 3 km thick at both poles. Although their exact compositions are
unknown, both examples are thought to be primarily made up of dust and water
ice and are roughly similar in volume to the Greenland ice sheet. Mars appears to have experienced recent
climate changes, like "ice ages" on Earth. Scientists think that the Martian polar layered deposits
record climate variations over at least the last 10 to 100 million years, but
the details of how they formed and evolved remain obscure. Changes in tilt of the rotation axis
and shape of the orbit are thought to influence the climates of both Earth and
Mars, but are larger for Mars.
Many Mars researchers think that the polar layered deposits are the
result of variations in the proportions of dust and water ice deposited over
many climate cycles. The layers themselves are therefore expected to contain an
important summary of recent climate changes on Mars just as ice-cores from
Greenland and Antarctica do for the Earth.
The residual ices on top of these layered deposits
exert control over the current Martian climate (and vice versa). These ice caps
are composed of clean, large-grained water ice at the north pole and carbon
dioxide ice at the south. The northern residual ice may be the site of active
formation of the polar layered deposits and controls the global distribution of
atmospheric water vapor. The southern residual ice cap currently controls the
pressure of the atmosphere by supplying extra carbon dioxide gas when it
partially sublimates during the summer. The southern ice cap contains
populations of geomorphic features, such as groups of pits and ridges, which
change size and shape at rates of meters per year and are only a few decades
old.
Major
science questions for this theme.
Several important questions related to the residual
ices are: Are the residual caps currently accumulating or losing material? How
much exchangeable carbon dioxide and water is locked up within these deposits?
How stable are these deposits over timescales of centuries to millennia? How do the residual caps control the
formation of the polar layered deposits?
The relationship between layers within the polar
layered deposits and the climate during their formation remains uncertain. The
dust/ice ratio and how it varies from layer to layer is currently unknown; some
layers may also contain other more minor components such as volcanic fallout
and impact debris. Identification of these materials would help our
understanding of recent Martian history significantly. HiRISE is capable of observing Martian
layers at sub-meter resolution.
Such observations can be used to determine whether these layered
deposits contain even thinner layers than those that are resolved by MOC. An important goal of Mars polar science
is the association of layers with independently derived Martian orbital
changes. One of the more puzzling
questions regarding the Martian polar regions is why the southern polar layered
deposits appear to be so much older than those in the north. The difference in surface age is
inferred by counting craters, and there are many more craters on the southern
layered deposits than on the northern ones.
Relationship
to other science themes.
In contrast to other science themes polar geology
refers to a place rather than a process. Examination of these polar deposits
draws upon many other research areas including glacial and periglacial processes,
seasonal processes, stratigraphy, sedimentary geology and landscape evolution.
Polar geology spans, and can be considered a subset of, all of these broader
categories. This science theme should be used when the observational goal is
the further understanding of the current polar deposits, whether it pertains to
the stratigraphic record of the layered deposits (which represents at least
millions of years of history) or the evolving geomorphology within the residual
ice caps (which represents only a few centuries of history). Behavior of
seasonal frosts would be covered by the seasonal processes theme despite their
polar location.
Features
of interest potentially visible at HiRISE scale.
Changes in the geomorphic features within the
southern residual carbon dioxide ice will be observable over timescales
substantially less than 1 year. The northern residual ice surface has an
unusual texture at the limit of MOC resolution. HiRISE observations will allow
characterization of these small-scale landforms and their evolution rates.
Layers less than 1 meter thick are visible on the sides of mesas in the
southern residual ice. These layers can be imaged by HiRISE and may hold the
key to Mars' climatic history over the past few centuries.
High-resolution stereo imaging of exposed sequences
of polar layered deposits will allow measurements of layer properties such as
thickness and bedding attitudes. Observation of small craters on the layered
deposits will allow us to put a more accurate constraint on the timescales of
deposition and erosion operating in that area as well as providing information
about the target material and crater modification processes. HiRISE views of
how these crater shapes have relaxed and where the icy layers have broken in a
brittle way will give us a better idea of the strength of the layered deposit
material and whether it has flowed significantly (like ice caps on Earth do).
Observing the relationship between changes in the
residual ices and the current climate will allow us to better understand how
the layered deposits were formed. This information can be used as a 'Rosetta
stone' for interpreting the climatic record within the layers themselves.
Examples
of HiRISE targets
Polar geology offers a diverse set of features to
observe and puzzles for us to try and solve. Some examples of polar data from
the Mars Orbiter Camera (MOC) are shown below and serve as a guide to the kind
of features we will be targeting with HiRISE.
Southern (top panel)
and northern (bottom panel) layering have very different appearances. Layering
in the southern polar layered deposits shows staircase topography indicating
differing material strengths between layers. In contrast, layering in the north
usually crops out on smooth slopes. Both these scarps are roughly equatorward
facing with the downhill direction from top to bottom. Top panel is a sub-frame
of MOC image E11/03053, illumination from the upper right. Bottom panel is a
sub-frame of MOC image M18/01897, illumination from the lower right.
A steep scarp in the North polar layered deposits at
the head of Chasma Boreale shows the two main stratigraphic units that comprise
these deposits. The polar layered deposits (upper unit) are bright and show
continuous layers. The lower basal unit is dark and has a jumbled appearance
with some layering visible. The basal unit is thought to be comprised mostly of
sand Elevation decreases
from top to bottom. MOC image
E03/00889, illumination from the lower right.
Two views of an 8m deep pit (dubbed a 'Swiss-cheese
feature') separated by two Martian years.
The gridlines are spaced every 50 m; expansion of the pit relative to
this fixed coordinate system can be seen.
The walls retreat about 5 meters every Martian year, HiRISE will allow
us to monitor these changes over many years. The unchanging polygonal cracks on
the surrounding terrain are a mystery but allow us to precisely co-locate these
two MOC images M09/00609 (left) and R08/01050 (right). Illumination is from the
lower left.
Craters are uncommon on the polar deposits. On the
north polar layered deposits and residual cap there are only a few known
examples shown above. All four of these rare craters exhibit raised rims, and
to a lesser degree, circular outlines.
The craters on the northern residual ice are highly degraded, consistent
with a very high resurfacing rate of that unit which gives it its very young
age.
Faults give us
information about the strength of the material involved. Some examples of
brittle processes on the bounding scarp of the south polar layered deposits are
shown above. Arrows highlight offset layers indicating that faulting has taken
place. Images are from left to right M04/02455, M10/03514, M15/02058 and
E09/01926. Illumination is from the bottom right in each case and the downhill
direction is from top to bottom (except in second from left).
Return
to HiRISE Learning and Activities Page