45th Lunar and Planetary Science Conference (2014)
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SUBSURFACE CONTROL OF LAKES AT LOW NORTH POLAR LATITUDES ON TITAN:
IMPLICATIONS FOR FLUVIAL PROCESSES AND LAKE MORPHOLOGY. D. G. Horvath1, J. C. Andrews-Hanna1, C. E. Newman2, K. L. Mitchell3, B. W. Stiles3, 1Department of Geophysics and Center for Space Resources, Colorado School of Mines, Golden, CO, dhorvath@mines.edu, 2Ashima Research, Pasadena, CA, 3Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA.
Introduction: Lake features on Titan’s surface
exhibit various seasonal behaviors, morphologies,
and spectral characteristics. Observations of large
dendritic valleys [1,2,3], karstic lakes and valleys [4],
potential evaporite deposits [5], and ephemeral lakes
[6] indicate a diverse range of fluvial and lacustrine
behavior on Titan. In the northern polar region in
particular, large seas, fed by extensive catchment
basins [7] are present at similar latitudes to those of
karstic lakes, which are characterized by rounded,
steep-sided shorelines [4]. Dry lakebeds with similar
karstic morphologies are observed at lower polar latitudes [8] in agreement with predictions of atmospheric models for Titan, [9,10] but large seas are found at
similar latitudes implying that climate alone does not
account for the latitudinal distribution of lakes in the
northern polar region. This study uses a large-scale
hydrological model to examine the hydrological processes responsible for lakes at arid, low polar latitudes and the implications these hydrological processes have for geomorphologic processes.
Model: The catchment scale hydrological model
combines a numerical subsurface hydrology model
with an analytical surface runoff model, driven by the
outputs from a general circulation model (GCM)
[11]. The limited surface topography data available
for Titan [12] prevents direct hydrological modeling
of specific regions on Titan. Thus, we use the statistical nature of SAR topography profiles over regions
of interest to generate synthetic fractal topography.
We generate synthetic fractal topography to fill in the
gaps between topographic profiles over the northern
polar regions to generate synthetic topographic maps
over the north polar seas (Figure 1a). Previous work
[13] determined the behavior of a catchment scale
hydrological system using a domain size large
enough to encompass the individual large seas on
Titan. For this investigation, we expand the domain
to encompass multiple catchment basins. We use a
rectangular grid with dimensions of 1348 by 2022
km, to represent a portion of the surface northward of
45°N latitude. The climate in the model domain varies as a function of the distance from the pole, located
at the top edge of the grid. The latitudinal and seasonal variations in the precipitation and evaporation
rates used in the model are taken from the TitanWRF
GCM [11]. This large spatial domain allows us to
investigate the effect of latitudinal variation in climate at the north polar region on the hydrology. Pre-
vious work examined the effect of permeability [13],
but we assume a constant permeability of 10-6 cm2 for
all models presented in this study.
For ungauged catchment basins on Earth, the annual amount of precipitation that participates in the
surface hydrology is estimated using a Budyko-type
method [14], which relates the actual annually averaged evaporation to precipitation ratio to the potential
annually averaged evaporation to precipitation ratio
(the aridity index) predicted by the GCM. Terrestrial
studies found that a simple equation matched numerous gauged basins on Earth [15]. Here we use this
simple analytic relationship to determine the amount
of precipitation predicted by the GCM that does not
evaporate from the surface, which is then separated
into recharge to the aquifer and surface runoff based
on where runoff is routed in the model domain.
Results: The general circulation model predicts
increasing aridity at locations further from the pole.
Small catchment scale hydrological models using the
GCM outputs at isolated latitudes predict near zero
recharge rates and runoff at latitudes below 77°N due
to increased aridity. This results in no lake formation
below 77°N if only a single catchment is considered.
While smaller catchment scale hydrological modeling
predicts no lake formation below 77°N, the largescale multiple basin model with spatially varying
climates representing 45°N to 90°N latitude predicts
the formation of lakes down to the equivalent of
60°N latitude with decreasing lake area at lower latitudes (Figure 1). Intense recharge, direct lake precipitation, and surface runoff at wetter north polar latitudes greater than 77°N, raises the hydraulic head
Figure 1. a Synthetic topography generated for the
northern polar region. b Lakes (black) overlain on
hydraulic head at the northern polar region. Circles of
constant latitude are shown in b.
45th Lunar and Planetary Science Conference (2014)
causing ground-methane flow to latitudes below
77°N. Increased aridity at latitudes below 77°N predict high evaporation fluxes from the subsurface methane-table and the lake surfaces. The high evaporation rate at the lower latitudes acts as a sink for liquids lying above and near the surface, further driving
southward subsurface flow from latitudes greater
than 77°N.
While flow to lower latitude lakes through the
subsurface is driven by recharge at high polar latitudes, the distribution of topographic depressions also
strongly influences the distribution and formation of
lakes at lower polar latitudes. In models with large
topographic depressions at higher latitudes, or with
lower latitudes lacking any significant topographic
lows, liquid ponds at higher polar latitudes while
lakes do not form lower at latitudes. However, since
our synthetic topography is built upon the existing
topographic tracks across the region, it provides an
accurate representation of the latitudinal distribution
of lakes and seas.
While lakes form at high and low polar latitudes
(Figure 1b), the lake hydrology below and above
77°N varies significantly. Surface runoff is confined
to the northernmost latitudes where the GCM predicts
increased precipitation. However, topographic divides prevent surface runoff from reaching lower
latitude basins. This is consistent with dark (filled)
fluvial channels in the higher polar latitudes [2] and
would result in high sedimentation rates for these
lakes. Surface runoff constitutes 53% of the annual
influx to lakes above 77°N and subsurface flow constitutes 29% (Figure 2), with the remaining flux from
direct precipitation onto the lake surface. In contrast,
the lower latitude lakes located between 60° and
77°N experience little direct precipitation or surface
runoff. Subsurface flow to lakes at latitudes lower
than 77°N constitutes 99% of the annual lake influx
while surface runoff and direct precipitation constitutes less than 1% (Figure 2). Lower latitude lakes
also alternate between hydrological periods of net
loss and net influx due to the evolving balance between evaporation and subsurface influx. These lakes
experience net influx during the late summer to early
winter when the evaporation flux is low, and net loss
during late winter to early summer as evaporation
from the lake surface increases. A regional drop in
the methane table would dry these lakes out entirely.
This is consistent with observations of potential
evaporite deposits [5] located between 60° and 70°N
and karstic lake morphologies [4] concentrated below
80°N.
Conclusions: Although lakes and seas are observed down to 65°N latitude, climatic factors alone
predict no lake formation below 77°N. The
equatorward subsurface flow through an unconfined
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Figure 2. Average hydrographs for all lakes above
77°N (top) and all lakes between 60° to 77°N (bottom) over the course of one Titan year (time axis is in
Earth years).
aquifer, driven by high recharge rates to the methane
table at high northern latitudes and high evaporation
rates from lake surfaces at lower latitudes can explain
the presence of the observed lower latitude lakes. The
significant hydrological differences between the lower latitude and higher latitude lakes have implications
for the processes controlling lake morphologies at
different latitudes including sedimentation, karst development, and evaporite formation.
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Acknowledgements: Some of this research was
carried out at the California Institute of Technology
Jet Propulsion Laboratory under a contract with
NASA. We acknowledge support of the Outer Planets Research Program.