06/12/2003
Devilish Springtime Reflections
What is so rare as a day in June? Today is one of those days with the sun shining brightly. Unfortunately, such a day has been rare here not only in June, but also in April and May. If you watched the failed effort of Funny Cide to bring home the Triple Crown last Saturday on that rain-soaked track, you’ve seen the typical weather in New Jersey this spring. And, sure enough, rain is predicted for the rest of the week. Where can we go to get away from all this? A Reuters dispatch reports that spring is arriving in the southern hemisphere of the planet Neptune. If you’re tempted to check out spring on that planet be advised that, at close to 3 billion miles from the sun, it’s a bit of a jaunt.
Furthermore, even the lousy weather we’ve been having is preferable to that on Neptune. It’s a tad too cold for one thing, in the neighborhood of minus 279 degrees Fahrenheit. Certainly not good golfing weather and the 900-mile an hour wind gusts would play tricks with even Tiger’s booming drives. Why would the arrival of spring on Neptune warrant mention in a Reuters dispatch? It’s the discovery that the planet has seasons at all that’s surprising. Being so far away from the sun, one would suspect that what little of the sun’s heat reaching the planet would have no effect on Neptune’s climate. We get roughly 900 times more heat than Neptune receives from the sun.
However, Lawrence Sromovsky and co-workers at the University of Wisconsin and at the Jet Propulsion Laboratory have followed Neptune for six years with the Hubble Space Telescope. I’ve checked their pretty pictures of the blue planet on the university’s Web site and, over the 6-year period, there is indeed a distinct brightening and widening of the cloud patterns in the southern hemisphere during that period. Clearly, there’s a change in the weather pattern that takes time to form and time to dissipate. On Neptune, spring lasts some 40 years or so! I don’t know what’s under those clouds but I’m sure that a few months of rain would look good by comparison.
Another planet closer to home has also been in the news this past couple of weeks. The European Space Agency launched the Mars Express spacecraft from the Baikonur cosmodrome in Kazakhstan on June 2. Today, June 10, NASA launched the first of two Mars rovers. Another rover is in the wings for launch this month. Let’s hope that NASA redeems itself after the disasters befalling the three previous attempts to land a craft on Mars. The European venture also includes a landing by British-built Beagle 2, designed to scout around and look for signs of life by digging into the Martian soil and sniffing the Martian air for gaseous byproducts of life.
All this activity is prompted by the fact that Mars is currently as close to Earth as it will be for a number of years. Hence the flights will be relatively short and all three rovers should be on Martian terra firma by early 2004. But another craft has been winging its way to a rendezvous with Mars ever since its launching on July 4, 1998. It’s the Japanese Nozomi spacecraft, which is scheduled to orbit the planet to study how the upper Martian atmosphere interacts with the solar wind. When it gets there, Nozomi will join two wildly successful NASA orbiters, the Mars Global Surveyor and the Mars Odyssey.
These two NASA missions have drastically changed our perception of the planet. Mars has been mapped even more thoroughly than our own planet Earth. Surveyor’s and Odyssey’s pictures and other scientific measurements have revealed the presence of vast amounts of water ice and gullies and debris that suggest there may even be current sources of liquid water that aren’t far from the surface. Water ice is now thought to cover a large portion of Mars outside the equatorial regions.
The significant amount of water is exciting for those searching for evidence of life, past or present, on Mars. If we ever travel to Mars, it’s comforting to know that water for a hot shower awaits us on landing. Yet, in spite of all this water ice, Mars is like our deserts, dry and dusty. Dust is a key factor to take into account when planning rover missions on Mars. For example, dust buildup on the solar panels is expected to limit the useful life of the rovers by cutting off the sunlight generating the power.
Surveyor and Odyssey have taken pictures of the mothers of all dust storms, storms so large that they sometimes seem like one big storm circling the Martian globe. On a smaller scale, dust devils are also a common occurrence. Dust devils are also common here on Earth, for example, on the deserts in Southwestern USA. In our cities, you may see the urban equivalent, the “trash devil”, a whirling assortment of paper, leaves or other debris. A dust devil can be frightening, as I found in an article entitled “Tiny Tornadoes” by Kathy Svitil in the July 2003 issue of Discover magazine. She cites the experience of Nilton Renno, now at the University of Michigan, but formerly at the University of Arizona where he was a glider enthusiast. The power of a dust devil was brought home to him tragically. A friend of his was killed when, while sitting beside his sailplane, a dust devil picked up his sailplane and dropped it on him!
According to Svitil, dust devils weren’t accorded much respect until scientists realized that dust devils on Mars were much bigger and “meaner” than they are in our Southwestern deserts. There, dust devils can appear out of a perfectly clear sky on a hot summer day. They usually don’t last long, maybe a few minutes, and don’t exceed more than a few hundred feet in height. Once in a while, a devil might tower a few thousand feet and swirl around at 70 miles an hour for an hour or so. These babies can pick up a small shed (or a sailplane).
A common place for a dust devil to form is where a cool patch of ground adjoins a hotter area, say at the edge of an irrigated plot bordering a desert. The cooler air will tend to slide in under the warm air and warm air will rush in to replace it. You end up with a rotating tube of air. In an updraft, the rotating tube is lifted up and you have a dust devil. The dust devil picks up dust and sand but generally will fling the sand back down to the ground, while picking up the dust.
In the process, something else happens that you might not expect. When the sand and the dust grind together in the devil, electrons are knocked off some particles and picked up on others. Typically, the heavier particles such as sand lose electrons while the finer dust particles pick up electrons. So, if the sand gets flung to the ground, the sand has a positive charge and the dust a negative charge and there’s the potential for a discharge, that is, lightning. However, for lightning to occur on earth, you need an electrical field of a few million volts per meter. In earthly dust devils the fields are only around 10,000 volts per meter - not dangerous and no lightning. On Mars, however, you only need about 20,000 volts per meter and, with much bigger dust devils, there’s a possibility of lightning. Dry thunderstorms on Mars?
If you want to see some interesting pictures of dust and dust devils on Mars, visit the June 2003 issue of Scientific American and the article “The Unearthly Landscapes of Mars” by Arden Albee. As the subtitle indicates “The red planet is no dead planet.” Let’s wish all those spacecraft bon voyage and, for the Beagle 2 and the rovers, a happy landing.
Allen F. Bortrum
|