INSIGHTS | PERSPECTIVES
By Devon Burr
Rivers develop on planetary bodies from the interplay of climate and topography. In planetary contexts, rivers and their effects are used to constrain short- and long-term cli- matemodels. However, riversarealso
controlled by the short- and long-wavelength
topography of the planetary surface. Rivers
thus provide a means to constrain the generation of topography. On page 727 of this
issue, Black et al. (1) compare topography at
a range of scales with mapped
river drainages to provide new
insights into the topography-generating mechanisms on
Earth, Mars, and Titan.
Earth is particularly suited
for comparing topography
and river drainage. Its volatile
cycle is hyperactive, relative
to others in the solar system,
and results in abundant river
drainages, which are ready-made arrows pointing the way
downhill within the channels.
The surrounding topography,
however, need not trend downhill in the same direction, for
example, when antecedent
rivers persist in their original
flow direction even during uplift of the regional topography
(2). The eastern tributaries of
the Tennessee River illustrate
discrepant drainage and topography, although the causes for this are
still under investigation (see the figure) (3).
Quantifying such discrepancies between
drainage and topography gives a new under-
standing of the mechanisms by which this
topography is generated. Plate tectonics on
Earth generates topography at plate margins,
where two plates converge to create moun-
tains, such as the Himalayas, or where sub-
duction and melting of one plate generates
volcanic orogens, such as those that formed
the South American Andes Mountains. Al-
though often extending thousands of kilo-
meters along the end-to-end dimension, the
plate-marginal topography is much shorter
in width (or wavelength), typically less than
a few hundred kilometers. Conversely, topog-
raphy generated by other processes tends to
be longer in wavelength, although exceptions
Comparison of river drainages and landscape topography by Black et al. shows that
on Earth, the downstream flow directions
of rivers are not particularly well correlated
with the landscape topography. This poor
correlation means that rivers on Earth often
drain across, or even upslope, through their
surrounding landscapes. In other words, at
locations of short-wavelength deformation,
the rivers on Earth can cut through the rock.
A finding for Mars that is similar to that of
Earth would support early plate tectonics (4,
5). However, the Martian results, in contrast
to those from Earth, show a statistically stronger correlation of river valley orientations
and the topography. This finding means that
the Martian topography preceded the rivers
and that subsequent topographic growth was
slow enough or small enough in amplitude
that erosion by the rivers kept pace. The conformity of topography and river drainage on
Mars is not perfect, but models run by Black
et al. indicate that the exceptions are attributable to impact cratering or other limited
post-fluvial-era deformation. The results for
Mars indicate that the rivers tended to flow
over, not through, the rock.
The terrestrial and Martian results can be
compared with those for Titan. The volatile
cycle on Titan, dominated by hydrocarbons
instead of water, has produced a range of flu-
vial drainages (6). The rectangular character
of many of these drainages, including a high
percentage of the polar drainages, suggests
some kind of tectonic control (7). Black et
al. found that the correlation of topography
and drainage is strong at lower latitudes but
weaker at the north pole. Thus, some long-
wavelength mechanism, such as variation
in shell-thickening, may dominate the gen-
eration of topography globally on Titan, but
with some shorter-wavelength
mechanism operating at the
poles. The low- and mid-lat-
itude rivers on Titan tend to
flow over the icy bedrock, but
at the poles, they can incise
and flow through the bedrock,
consistent with findings of
deeply incised river valleys at
the poles (8).
The technique might be ap-
plicable on other planetary
bodies with nonvolatile flow;
Black et al. suggest application
to lava channels on Venus or
hypothesized for Pluto. The
present results weigh against
the hypotheses of plate tectonics on Mars (4, 5) and lend
support for tectonic processes
operating at the poles on Titan
(7, 9, 10). However, previous
Titan work found evidence
for tectonic processes in the south polar region, whereas this new work supports short-wavelength deformation at Titan’s north
polar region. This inconsistency between the
geomorphologic mapping (9, 10) and new
analysis (1) remains to be reconciled and the
cause(s) for this asymmetric deformation on
Titan to be better understood. j
1. B. A. Black etal.,Science 356, 727 (2017).
2. L. B. Leopold et al., Fluvial Processes in Geomorphology
(W. H. Freeman & Company, 1964).
3. S. F. Gallen et al., GSA Today23, 4 (2013).
4. J. E. P. Connerney etal .,Science 284, 794 (1999).
5. A.Yin,Lithosphere 4,286(2012).
6. D.M.Burr etal., Geol.Soc.Am.Bull. 10.1130/B30612.1
7. D. M. Burr et al., Icarus 226, 742 (2013).
8. V.Poggiali etal., Geophys.Res.Lett. 43, 7887(2016).
9. Z.Y.Liu etal.,Icarus 270,14(2016).
10. S. P. D. Birch etal.,Icarus 282, 214 (2017).
Defining the topography of a planetary body
Department of Earth and Planetary Sciences, University of
Tennessee, Knoxville, TN 37996, USA. Email: email@example.com
River and landscape topography, combined, can reveal the geologic history of a planet
A geological history lesson
The map shows the Hiwassee, Little Tennessee, French Broad, Nolichucky, and Holston
rivers draining west from North Carolina to the Tennessee River, which drains to the
Mississippi River and to the Gulf of Mexico. This drainage goes through the topographically higher southern Appalachian Mountains instead of following the more
direct and downslope course to the southeast and the Atlantic seaboard.