respect to long-wavelength topography (as shown
in Fig. 1B at locations where river traces deviate
from gradient arrows), in contrast to most drainages on Titan and Mars.
On Mars, the strong correlation between valley
network orientations and the present-day long-
wavelength topography requires that most large-
scale topography predates valley network forma-
tion in the Noachian era and that this ancient
topography was the dominant influence on val-
ley network orientation (Fig. 1F) (1, 2, 23). Martian
topographic conformity remains imperfect even
when shorter wavelengths are considered (fig.
S5). We attribute this persistent, moderate dis-
agreement between drainage directions and topog-
raphy to the combined effects of impact cratering,
topographic resolution, and deformation after the
era of valley network formation (24, 25). Because
%d quantifies the proportion of river segments
that flow from higher to lower topography (Fig.
2A and fig. S5), our results place bounds on the
728 19 MAY 2017 • VOL 356 ISSUE 6339 sciencemag.org SCIENCE
Fig. 1. Maps of topography referenced to the geoid and expanded
to spherical harmonic degree and order 6, overlain with the fluvial
features employed in this study. (A) Earth. (B) Enlargement of North
America. (C) Titan. Blue outlines show Cassini Synthetic Aperture Radar
(SAR) observation swaths. (D) Enlargement of eastern Shangri-La and
Xanadu regions of Titan. T values represent the Cassini SAR swath numbers
(swaths are outlined in blue). (E) Mars. (F) Enlargement of Hellas Basin on
Mars. In (A), (C), and (E), white boxes outline regions enlarged in (B), (D),
and (F), where river courses are shown in light blue, black arrows indicate
topographic gradient at each point, and indicated conformity values span
these enlarged regions, with uncertainties corresponding to the 95%
confidence interval for the median (19).