Is there really life on Earth?
Obviously, the answer is yes. But if we compare topographic maps of Earth and Mars, the answer is less evident, according to researchers from the University of California at Berkeley. They've found that landscapes on the two planets are often very similar, from rounded hills to meandering stream channels or alluvial fans. And they've concluded that life leaves only a subtle signature in land. This raises very interesting questions: does life really matter? and what is its influence on the landscapes surrounding us?
Here are some parts of the introduction of this UC Berkeley article about the research done by William Dietrich, professor of Earth & Planetary Science and graduate student Taylor Perron.
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.
"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.
What would Earth look like if all life suddenly disappeared and the human artefacts had never been constructed? Below are two shaded relief images derived from airborne laser swath mapping (ALSM) data of the Angelo Coast Range Reserve along the South Fork Eel River, California. The top image includes vegetation, with colours based on canopy heights while the bottom one has been filtered to an approximate bare-earth surface. (Credit: UC Berkeley/Nature)
It's interesting to note that Dietrich started to think about life effects on planets when a someone from NASA told him that "he saw nothing in the Martian landscape that didn't have a parallel on Earth."
So what are the visible effects of life on Earth's topography?
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.
But erosion also sculpted landscapes on Mars, so there is nothing conclusive here. So the scientists started to look at riverbeds.
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.
So can we conclude that life on Earth had no influence whatsoever? Not really. The researchers simply think that there would be a different distribution of rounded and angular landforms on a lifeless Earth. But to validate their hypothesis, they need "elevation maps of the surfaces of other planets at resolutions of a few meters or less," which they will not get before a while.
This research work has been published by Nature under the title "The search for a topographic signature of life" (Volume 439, Number 7075, Pages 411-418, January 26, 2006). Here is a link to the Editor's summary, which concludes that "a lifeless Earth would look different, but the difference would lie in the frequency distribution of certain landform properties."
And here are two other links to the abstract and to the full paper (PDF format, 8 pages, 484 KB), from which the above images have been extracted.
Sources: Robert Sanders, UC Berkeley, January 25, 2006; Nature, January 26, 2006; and various web sites
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