THE MARTIAN NORTH POLAR CAP SPIRALS ARE THE TRACES OF AN ANCIENT ICE SHEET COLLAPSE A. A. Kostrikov, Laboratory for Comparative Planetology, Vernadsky Institute, 19, Kosygin St, 117975, Moscow, Russia
The surface of north polar cap of Mars is essentially heterogeneous unlike
flat terrestrial ice sheets . Troughs up to one kilometer deep with gently
(no more 10-15°) sloping are seen all over the ice cap. The unique feature of
the trough system is its helical appearance (Fig. 1).
1. The north polar cap of Mars.
Analogs of ice
spiral structures are not known. The troughs have been attributed to the
action of aeolian erosion [2-3], sublimation  or to “accublation”
hypothesis (glacial flow + sublimation + accumulation) [5-7]. It is supposed
that an ice mass transfer occurs by sublimation from equatorward-facing slopes
and subsequent accumulation on pole-facing slopes. No ideas on origin of
spiral pattern have been moved forward with the exception of an attempt to
explain trough revolving by combined effects of accublation and ice movement
Analysis made in [9-10] suggests that Chasma Boreale - the greatest trough
of north pole ice sheet is a giant scour generated by subglacial outflow. This
mass of water (maybe subglacial lake) can arise in consequence of a number of
reasons, as a result of intensive ice sheet melting provided geothermal flux
increasing, for example.
is natural that water lubrication comes to sufficient decreasing of bed
friction. In view of this cause the ice sheet will spread radially with high
speed. Augmentation of radial stress over breaking point generated ice
entirety breaking, came to emergence of crevasses. Its trajectories depended
on physical characteristics of ice and Coriolis force.
experiment: A simple laboratory experiment has been run to prove the
A disk made of wet clay that measures 14 centimeters across and two tenths of a centimeter at the center (tapering down to 0 at the outer edge) had been thrown counter clockwise. In two minutes of rotating and subsequent drying spiral cracks made their appearance (see Fig. 2). There is a long crack on the
right part of the figure, a shorter and S-like cracks in the center. Every crack has a clockwise cockling. One can see a qualitative resemblance of the spiral structure of Martian north polar cap and this clay model. Model: A simple model of crack progression in viscous ice sheet on rotating planet has been developed. Model crack trajectory depends on Coriolis parameter f, initial coordinates, initial velocity (u0, v0) and ice resistance coefficient с.
Results: An example of model trajectory for x0=0, y0=600 km, u0=60 m/sec, v0=0 m/sec, f=10-4 sec-1 с=2 10-5 sec-1, visualized on Fig.3. One can see the model crevasse looks like a spiral, coiling around center. Qualitative resemblance of helical troughs is evident. It makes sense to verify if they fit quantitatively. An analysis of trough pattern (Fig.1) has been made in order to take angles (α) between its tangent directions and local meridian lines – “spiral inclines” (see Fig.3). Fig.4 shows these data, lines of polynomial interpolation and modeling dependency α(φ). One can see the scatter of points is sufficiently large at first sight. Trend line says that spiral inclines are above mean (67°) on the sheet periphery, approaching 90°. They diminish near pole and close to 70° in the wide enough latitude range (82,5°-86°). It is interesting that the model line loops a sufficient large loop.
Model curve behavior is defined by starting conditions and parameters of
angular rotation velocity of Mars practically did not change since
hypothetical collapse of polar sheet, and so used value of Coriolis parameter
deserves credit. As a matter of fact the size of ice sheet could be different,
though, one can hope, its distinction from present one is a little. The used
values of initial speed and resistance coefficient c,
for their parts, depend on value of ultimate stress, kinematic coefficient of
ice viscosity and ice density. Terristrial ice investigations show, that ice
density varies in sufficiently close limits and only a little smaller of the
used value 103 kg/m3, but the ranges of ultimate stress
and kinematic coefficient of ice viscosity are wide . Mean values have
been used in this research. Generally speaking, on account of faint maturity
of the cracking dynamics theory application of simple models seems justified.
examined trough pattern closely, one can notice that no trough traces from
sheet margin to the pole continuously, that they, as rule, consist of several
sections. This says that cracking happened step by step. Continued spread of
ice sheet resulted in a rise of stretching stress in the vicinity of the crack
vertex that, in its turn, sooner or later, set going spasmodic initiation of a
Flatness of trough slopes denotes that collapse
accompanied by creation of immense crevasses, took place a long time ago.
After reduction of geothermal flux to the previous level bed friction rose,
ice sheet spreading dropped down, cracking stopped, and accumblation began to
play a key role in sheet surface modification. Thus, regardless of the fact
that the ice spreads away slowly (order of speed magnitude is mm/year )
smoothing irregularities of its surface, accumblation process drives helical
troughs irrepressiblly north. If accumblation process did not start up, deep
troughs would close in 105-106 years .
the way, absences of spiral troughs on the surface of terrestrial ice sheets
can stands for remoteness or lack of their collapses. However one can think
that being disappeared to the end of last glacial period Laurentian and
Fennoscandian ice sheets could left footprints (moraineas?) of their helical
structure. One must take into account that underlying topography could
disfigure this structure substantially.
Thus, it follows from this investigation that being defreezed at bed the
Martian north pole ice sheet began to transform, as a matter of fact, to an
ice body resembling ice shelf. This transformation was accompanied by drastic
amplification of radial tension that came to breaking of ice entirety, to
emergence of deep crevasses all over the sheet. This planetary scale process
was so intensive that being influenced by Coriolis force crack trajectories
deviated to the right, forming spirals. After bed temperature fell down and
sheet collapse ceased, obtained relief began to undergo a smoothing owing to
continuous slow ice spreading and mass transfer from the warmed by sun north
crack slope to the shady south one. This process transformed the helical
structure of crevasses to the helical structure of troughs.
Acknowledgments: The author met with support from Dr A. Basilevsky.
Thank to Mr. Ken Turner, who run a laboratory experiment with the clay slab,
very much. Discussions with M. Balme, V. Bogush, K. Fishbaugh, J. Head, T.
Hughes, J. Hutchinson, L. Ingel, M. Krass, M. Kreslavsky, R. Kuzmin, E.
Lomakin, S. Netreba, M. Roberts, A. Rodin,
T. Scambos were helpful.
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