Life Leaves Subtle Signature in the Lay of the Land, UC Berkeley Researcher Report

http://www.berkeley.edu/news/media/r.../25_life.shtml

UC Berkeley Press Release

Life leaves subtle signature in the lay of the land, UC Berkeley
researchers report

By Robert Sanders
25 January 2006

BERKELEY - One of the paradoxes of recent explorations of the Martian
surface is that the more we see of the planet, the more it looks like
Earth, despite a very big difference: Complex life forms have existed
for billions of years on Earth, while Mars never saw life bigger than a
microbe, if that.

"The rounded hills, meandering stream channels, deltas and alluvial
fans
are all shockingly familiar," said William E. Dietrich, professor of
earth and planetary science at the University of California, Berkeley.
"This caused us to ask: Can we tell from topography alone, and in the
absence of the obvious influence of humans, that life pervades the
Earth? Does life matter?"

In a paper published in the Jan. 26 issue of the journal Nature,
Dietrich and graduate student J. Taylor Perron reported, to their
surprise, no distinct signature of life in the landforms of Earth.

"Despite the profound influence of biota on erosion processes and
landscape evolution, surprisingly,...there are no landforms that can
exist
only in the presence of life and, thus, an abiotic Earth probably would
present no unfamiliar landscapes," said Dietrich.

Instead, Dietrich and Perron propose that life - everything from the
lowest plants to large grazing animals - creates a subtle effect on the
land not obvious to the casual eye: more of the "beautiful, rounded
hills" typical of Earth's vegetated areas, and fewer sharp, rocky
ridges.

"Rounded hills are the purest expression of life's influence on
geomorphology," Dietrich said. "If we could walk across an Earth on
which life has been eliminated, we would still see rounded hills, steep
bedrock mountains, meandering rivers, etc., but their relative
frequency
would be different."

When a NASA scientist acknowledged to Dietrich a few years ago that he
saw nothing in the Martian landscape that didn't have a parallel on
Earth, Dietrich began thinking about what effects life does have on
landforms and whether there is anything distinctive about the
topography
of planets with life, versus those without life.

"One of the least known things about our planet is how the atmosphere,
the lithosphere and the oceans interact with life to create landforms,"
said Dietrich, a geomorphologist who for more than 33 years has studied
the Earth's erosional processes. "A review of recent research in Earth
history leads us to suggest that life may have strongly contributed to
the development of the great glacial cycles, and even influenced the
evolution of plate tectonics."

One of the main effects of life on the landscape is erosion, he noted.
Vegetation tends to protect hills from erosion: Landslides often occur
in the first rains following a fire. But vegetation also speeds erosion
by breaking up the rock into smaller pieces.

"Everywhere you look, biotic activity is causing sediment to move down
hill, and most of that sediment is created by life," he said. "Tree
roots, gophers and wombats all dig into the soil and raise it, tearing
up the underlying bedrock and turning it into rubble that tumbles
downhill."

Because the shape of the land in many locations is a balance between
river erosion, which tends to cut steeply into a slope's bedrock, and
the biotically-driven spreading of soil downslope, which tends to round
off the sharp edges, Dietrich and Perron thought that rounded hills
would be a signature of life. This proved to be untrue, however, as
their colleague Ron Amundson and graduate student Justine Owen, both of
the campus's Department of Environmental Science, Policy and
Management,
discovered in the lifeless Atacama Desert in Chile, where rounded hills
covered with soil are produced by salt weathering from the nearby
ocean.

"There are other things on Mars, such as freeze-thaw activity, that can
break rock" to create the rounded hills seen in photos taken by NASA's
rovers, Perron said.

They also looked at river meanders, which on Earth are influenced by
streamside vegetation. But Mars shows meanders, too, and studies on
Earth have shown that rivers cut into bedrock or frozen ground can
create meanders identical to those created by vegetation.

The steepness of river courses might be a signature, too, they thought:
Coarser, less weathered sediment would erode into the streams, causing
the river to steepen and the ridges to become higher. But this also is
seen in Earth's mountains.

"It's not hard to argue that vegetation affects the pattern of rainfall
and, recently, it has been shown that rainfall patterns affect the
height, width and symmetry of mountains, but this would not produce a
unique landform," Dietrich said. "Without life, there would still be
asymmetric mountains."

Their conclusion, that the relative frequency of rounded versus angular
landforms would change depending on the presence of life, won't be
testable until elevation maps of the surfaces of other planets are
available at resolutions of a few meters or less. "Some of the most
salient differences between landscapes with and without life are caused
by processes that operate at small scales," Perron said.

Dietrich noted that limited areas of Mars' surface have been mapped at
two-meter resolution, which is better than most maps of the Earth. He
is
one of the leaders of a National Science Foundation (NSF)-supported
project to map in high resolution the surface of the Earth using LIDAR
(LIght Detection And Ranging) technology. Dietrich co-founded the
National Center of Airborne Laser Mapping (NCALM), a joint project
between UC Berkeley and the University of Florida to conduct LIDAR
mapping showing not only the tops of vegetation, but also the bare
ground as if denuded of vegetation. The research by Dietrich and Perron
was funded by NSF's National Center for Earth-surface Dynamics, the NSF
Graduate Research Fellowship Program and NASA's Astrobiology Institute.