Glaciers. If you live in the northern half of the U.S. or Alaska it’s hard to go anywhere without seeing the effects of the two miles of ice that once drove slower than your grandmother over this landscape. Thousands of lakes left over from melting ice, rocks scraped and gouged by other rocks dragged by the ice and stray boulders called erratics that pop up in the middle of nowhere courtesy of the retreating ice. Digging a garden means tired arms not only from shoveling but from pitching rocks.
Last weekend I skied to an overlook on the Lester River near my home to check on the progress of two big glacier-sown boulders wedged in the dirt near the top of the river’s channel. Erosion exposed them them to view sometime in the past; this year gravity finally got the upper hand and yanked them down the slope and into the river. Was anyone there to hear this single tock of the geological clock?
The ice departed northern Minnesota some 10,000 years ago, but glaciers still populate the Arctic, Alaska and mountain ranges across the planet. One of the key features of glaciers are the junk they leave behind. The rocks and soil plucked from the landscapes they travel over and the debris that tumbles from the valley walls they squeeze through is carried forward by the moving ice. Later, as the climate warms and the ice melts back, the debris is dropped into large piles of rock called moraines.
If you’re driving across Minnesota, Wisconsin or Illinois and see a long, low ridge up ahead, there’s a fair chance it’s a moraine, or a pile of rocky debris along a lobe of a melting glacier.
Glaciers also appear to have been at work on Mars. Recently, members of the Planetary Geomorphology Working Group of the International Association of Geomorphologists described “drop moraines” in three regions of Mars created by extinct glaciers made not of water but of dry ice. These cold-based or polar glaciers are nearly frozen to the bedrock. Drop moraines form only in the coldest environments when a glacier advances and then stabilizes for a time in one spot. Rocks carried down by the glacier drop out as the ice vaporizes directly from solid to gas instead of melting first into liquid. Voila! – drop moraines.
In the top picture, the loopy ridges of extinct lobes around the central peak of the crater suggest that the glacier that left them was about a thousand feet thick. Scientists hypothesize that the glaciers formed millions to tens of millions of years ago due a shift in the tilt of Mars polar axis. Mars’ axis swings through a large range – believed to vary from 11 to 49 degrees – due to the gravitational tugging effects of the other planets over the long haul of time.
Earth’s axial tilt in relation to its orbit (called obliquity) varies only about 3 degrees thanks to the stabilizing influence of the moon. No Mars. During a time of low tilt or obliquity, scientists speculate that carbon dioxide, the main ingredient in Mars atmosphere, condensed out as snow that accumulated in the polar regions and formed glaciers.
The glaciers retreated during periods of high obliquity, when the sun traveled higher in the sky, the climate warmed and frozen carbon dioxide vaporized. Right now Mars’ axis is tipped coincidentally at 25 degrees, nearly the same as Earth’s 23.5 degrees. It seems every time astronomers take a closer look at the Red Planet, they inevitably discover a facet of its personality relating to something on our own planet. I suspect gardening on Mars is likely to be just as tedious as it is in my backyard.