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 Credit: NASA/JPL/Cornell Bottom left: Amy Acheson Bottom right: Mel Acheson |
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Aug 27, 2004 The Mars rover Opportunity discovered BB-sized spheres scattered all over Meridiana Planum, as seen in the above picture taken on Sol 19 of the rover's mission. They were nicknamed "blueberries" because of their grey-blue color and the way they are embedded in the Martian rocks "like blueberries in a muffin." After spectroscopic analysis, the Martian blueberries were identified as hematite concretions. But knowing what they are called is not the same thing as understanding how they were made. Hematite concretions are one of several types of spherical rocks that are found on Earth but are not completely understood. In the center photo above, we see the Martian blueberries. Compare these with hematite concretions from Texas (bottom right photo), and with Moqui balls from Utah (hematite spheres with sandstone cores, bottom left photo.) Other spherical formations that are difficult to explain include geodes, thunder eggs, and concretions as large as ten feet in diameter. One problem is explaining how a spherical rock forms in the first place. This problem is compounded by the fact that many of the spheres are layered or hollow or even contain a separate "nut" rattling around inside. Theories to explain the layered interiors include multiple episodes of mineralized water "leaking in" and "leaking out." This "leaky theory" is particularly hard to imagine in the case of the oil-filled geodes found in Illinois. Many are pressurized and squirt when the shell is cut. The speculations about the formation of Moqui balls range from meteorite impacts to underground fires. One popular idea is that they began under an inland sea as unstable limonite. Under pressure, limonite forms a gel, which might be rolled into balls, trapping sand from the seafloor inside. Later, the limonite might be converted to stable hematite by heat and gases from volcanic venting. Several characteristics must be addressed by any theory attempting to explain these round rocks: Most of them are clustered in zones, not randomly distributed. They are often common in one region of a particular rock formation, but absent in higher, lower, and adjacent regions of the same rock formation. In some deposits, it is obvious that there cannot have been spherical cavities while the flat surrounding sediments were being deposited. Nor could there have been spherical cavities while the sediments were being compressed into rock. Because concretions are found in the same zone, it is assumed that geodes began as concretions (or formed simultaneously with concretions.) So when did the concretions form? And why are they spherical? If they form in place from a liquid or plastic state, gravity would squash them into a dome shape. If they form while moving through a resistive medium, friction would change their shape. The forces that formed them must have been spherically symmetric. (This concern also makes one skeptical of the popular idea that hailstones, especially large ones that are spherical and radially layered, are formed in updrafts that blow the proto-stones into the cold tops of thunderheads.) All these speculations are based on chemistry and mechanics. But there is another force that commonly produces spheres -- electric discharge. This is because the spherical focus of an electric pinch is much more powerful than gravity. In the plasma lab, tiny spheres produced by electric pinches are often hollow, like the hematite concretions seen above. Electric discharge tends to produce spherical layering and a distinct equator and pole, because the pinch "squeezes" perpendicular to the current that creates it. These characteristics are also found in the "natural" spheres. The Moqui balls pictured above have both equatorial bulges and polar markings. Rock-cutters recommend that you will get a better display from a geode if you first locate the equator and poles, then cut across the poles. The layered crystalline look of a giant hailstone produced by a Midwestern thunderstorm (although very temporary) is also similar in form to the cauliflower-like shell and inward growing crystals of a geode. Very little research has been done in the field of "plasma geology." But space probes since Explorers 1 and 3 in 1958 have shown us again and again that plasma plays an important role in space. We're beginning to imagine how it affects our solar system and the galaxy beyond. Perhaps the time has come to look back at our home planet and ask if plasma played an active role in Earth's geological history, too. |
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