A survey of Mars — a planet whose early climate remains elusive — concludes
that its ancient rivers generated significant runoff that persisted later
into its history than previously understood, including well into the last
stages of the planet's wet days.
Evidence of persistent, strong runoff from Martian rivers even as the
planet's atmosphere was becoming less hospitable to water is published
in the March 27 issue of Science Advances. The findings complicate
the picture for scientists trying to model the climate of ancient Mars.
"It's already hard to explain rivers or lakes [on the planet] based on the
information we have," said study lead author Edwin Kite, assistant
professor of geophysical sciences at The University of Chicago. "This makes
a difficult problem even more difficult."
Mars' surface is dry today, but it was once wet. Mars' wet-to-dry climate
transition is thought to have resulted from loss of its atmosphere, a
process believed to be most intense within the first half a billion years
after Mars formed.
Strangely, more than a billion years after it formed — around 3.6 to 2
billion years ago — Mars was still home to precipitation-fed rivers and
lakes, which required more water to sustain them than could have been
produced by asteroid impacts. These late-stage wet climates present a
problem for models of planetary climate evolution.
"The most challenging observations to explain … are the evidence for
river-forming climates," said Kite.
To better understand Martian precipitation, Kite and his team undertook a
globally distributed survey of more than 200 ancient Martian riverbeds —
rich sources of clues about the water running through them and the climate
that produced them. For example, the width and steepness of riverbeds and
the size of the gravel tell scientists about the force of the water flow
As part of their survey, Kite and his colleagues studied image data of
various well-preserved paleo-river channels, alluvial fans and deltas
across the planet. They also calculated the intensity of river runoff using
multiple methods, including an analysis of the size of the river channels.
"No one has attempted a globally distributed survey of late-stage Martian
rivers' widths and wavelengths for catchments of known area before," said
In the river basins for which there was the most data, Kite and his team found
that Mars' rivers were wider than those on Earth. Between 3.6 and 1 billion
years ago, and likely after 1 billion years ago, there was intense,
persistent runoff in these channels, amounting to three to 20 kilograms per
meter squared (about .6 to four pounds per square foot) of water each day.
This would be the equivalent of "eight to 50 swimming-pools per square
kilometer per day," said Kite.
"You would expect [the rivers] to wane gradually over time, but that's not
what we see," Kite said. "The rivers get shorter — hundreds of kilometers
rather than thousands — but discharge is still strong."
The researchers say the runoff appears to have been distributed globally;
it was not a short-lived or localized phenomenon.
It's possible the climate had a sort of "on/off" switch, Kite said, which
tipped back and forth between dry and wet cycles.
If currently accepted dates for these massive rivers are correct, the
findings could suggest that Mars' late-stage atmosphere disappeared faster
than previously calculated, or that there were other drivers of snow melt or
rain under low-atmosphere conditions, the researchers say. These might
include the greenhouse effect of water-ice clouds or short-lived volcanic
greenhouse gases, said Kite.
Kite said it is also possible that currently accepted dates for the
late-stage rivers are wrong, and the rivers instead date from the early era
of rapid atmospheric escape to space.
The improved history of Mars' river runoff provides new parameters on the
unknown mechanisms that caused wet climates on Mars. Following these
results, said Kite, climate models may need to better account for the size
of Martian rivers over time, and for any effects — like a strong greenhouse
effect — that would have kept the planet warm enough to sustain
temperatures above freezing that accommodated these fast-flowing water