NASA's rover Spirit has churned up sulfate minerals in the
attempt to free itself from loose soil.
(Credit: NASA/JPL/Cornell)

Has it been six years since the twin rovers, Spirit and Opportunity, landed on Mars to begin their careers? Just about—next month is their landing anniversary.

Both rovers have shown signs of wear and tear, but Spirit, in its exploration range in Gusev Crater, has had the harder knocks. In fact, Spirit has driven backward for a couple of years now, due to a wheel that stopped working and which it was forced to drag through the Martian soil—a robot's form of limping.

About six months ago, Spirit became bogged down in loose soil, spinning its wheels but unable to break free of the trap—not unlike what happened to my car in Death Valley one time…and Spirit can't call upon the assistance of a National Park Ranger with an SUV and winch to help….

Over the months of entrapment, Spirit's handlers on Earth have continued to make measurements with the rover's instruments while trying to free it from its soil trap by manipulating its wheels in different ways (probably not unlike some of the strategies I tried to get my car out of the sand). But to no avail (either for Spirit or my car). Alas, is Spirit destined to remain a stationary explorer until its lifetime finally comes to an end?

Perhaps—but as it turns out, this doesn't mean Spirit can't still make significant discoveries–like a recent one it in fact made. All that wheel spinning and grinding and rocking back and forth have chewed up the soil in which Spirit sits—and has broken through a layer of soil to expose a surprise hiding beneath: a crust of sulfate minerals.

Sulfates—compounds containing sulfur—can be formed in the presence of water, like boiling water or steam escaping from a hydrothermal vent. It may be that these sulfates formed in the distant past when the area was active with volcanism and hydrothermal steam vents. That was then.

This is now: the layer of sulfate (calcium sulfate) Spirit's churning wheels broke through is crusty—a property that may point to more recent water activity than the original sulfate-forming steam vents. Scientists think that the crust may have been formed by the seasonal shifting of water from the Martian polar regions when it warms up in its summer, sending the water toward the equator–where it can even fall out as snow. Then, soil beneath the layer of snow warms the bottom layer of ice and causes it to melt. In turn, the melt water seeps down into the soil, dissolving and carrying away water soluble iron sulfate and leaving behind the crust of calcium sulfate.

In one fell swoop trying to escape sure peril, Spirit appears to have uncovered clues about the nature of Martian water action in the distant past as well as more recent times.

As serendipitous as Spirit's entrapment is to this accidental discovery, it's even better: the rover is stuck square on the edge of a small meteorite crater, allowing it to compare the sulfate concentrations in the sulfate-rich crust and the more typical soil medium, side by side.

Way to go! That's the spirit!

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Abengoa's solar power tower, PS10, near Seville, Spain. It is capable of supplying 11 megawatts, or approximately 5,500 households worth of power.Photo: afloresm

Comprised of one or two tall narrow towers surrounded by an enormous field of shimmering mirrors beaming sunlight back up from ground level, these power plants work by essentially the same principle you might have exploited as a child in using a magnifying glass and a hot sunny day to burn holes in the leaves of a backyard playground. A magnifying glass focuses sunlight from a round disk into a single bright dot. A solar power tower's field of mirrors focuses light onto a single water tank high in the air. The concentrated light boils the water, and the steam is used to generate electricity.

In other parts of the world the concept of the solar power tower has gained dazzling momentum as well. Last April, the Spanish company Abengoa commenced operation of a new power tower of its own, dubbed PS20. The power output is still a pittance compared to some of the largest fossil fuel or nuclear plants, but at 20 MW it is currently the largest power tower in existence.

The surge of excitement recently in solar power towers may be grounded on more than hype. Other solar technologies tend to be limited in their promise by cost. Caitlin Cieslik-Miskimena, an eSolar press contact, said that many of the components employed in the company are relatively cheap. She noted, for example, that the mirrors used to collect the Sierra Sun Tower's light are "just a step above a bathroom mirror" in quality. Because they are relatively small, they can also be manufactured to be flat, which is considerably less expensive than the parabolic mirrors used in some other designs.

Nevertheless, solar power towers are just one design in a rich assortment of ideas that people have had for harnessing solar energy. Photovoltaic cells are already used ubiquitously to energize calculators, solar-powered cars, and many satellites, and rapid advances continue to be made in this area. A less flashy form of solar thermal power known as SEGS (Solar Energy Generating Systems) uses curved mirrors to heat long troughs of water. The largest solar power plants in the world today are based on this method. Some companies are even proposing that we exploit solar energy by heating air beneath what amounts to a gigantic clear skirt. (Visit this link for a wild virtual tour of one such proposed plant.)

Time will ultimately tell which (if any) of these will turn out to be commercially viable options as the future marches toward us. Still, we are certain to have a wide array of ideas to explore.

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