Mars is Solar Groovy
Twisters ‘Not Like Kansas’
Other than predicting the weather, dust is something that means alot to engineers, meteorologists and rock-hunters–particularly those now negotiating a full day of activity around Gusev crater, or the ‘cup’.
For engineers, dust on Mars is the dominant absorber of solar radiation in the atmosphere. How long and how much the sun shines has important consequences for the transport of energy in the atmosphere and its circulation–in short, dust has alot to do with forecasting weather on the red planet. Each day, the engineering team needs to know the local dust conditions in Gusev crater, in order to gauge how easy the rover’s main power source, its solar panels, can be charged.
Engineers for the Mars rover have even adopted a catchy phrase to describe when conditions are clear. When atmospheric opacity will not interfere with their plans, the engineers have memorialized those days in their operational manual, and routinely call out flagship commands that characterize this state when the rover’s batteries and power are set for the next manuevers.
A good day for battery power is: ‘Solar Groovy’.
Television commentary for NASA has predicted that ‘solar groovy’ will eventually enter the vernacular of high schools around the country, just as other terms like ‘A-OK’ and ‘thumbs-up’ have come to indicate a condition of ‘Go’ or ‘green’.
But making surface weather maps on another planet isn’t just about predicting ‘solar groovy’ or not. Indeed on most planets and moons in our solar system, surface weather is either unknown or boring. In particular one of those weather images that so far has been hard to catch up close and on film is something actually happening on the surface. While surface pressure and temperature maps have been made in great detail, daily or hourly changes are rarely seen. Either the surface is obscured like Venus or Saturn’s moon, Titan–or the pressure is too low, as on most moons and to some extent, on Mars.
But for scientists now viewing up close what’s happening on Mars, one unique opportunity for a once-in-a-lifetime movie may be offered by a dramatic example of planetary surface weather, the dust devil. These devils, while lacking the punch of a real twister, do get very large and move across the martian landscape while marking it up like a chaotic, etch-a-sketch pad.
The tracks that these dust devils leave behind give one of the few short-term histories of the terrain. This territory on Mars may offer a chance to hear from one scientist that most people encounter daily: the weatherman.
The sheer geographical scope of a Martian storm can cover a hundred times the size of an Earth dust devil. Martian tornadoes span up to 10 km (6 miles) high with a moving and circulating center nearly 10 football fields across (one kilometer, or six-tenths of a mile).
That may seem like the ‘devil’ part of a very bad day, but the dusty part is more troublesome. Mars is a very dusty place. One illustration of this was seen immediately in the first images that were downlinked from Gusev crater, where it was impossible to resolve early on whether a large, dusty bump next to the rover was a big rock, or whether one of the airbags had gotten so dusty on impact that it looked like a rock. Martian dust is very powdery, closer to fine flour than beach sand, and includes magnetic, composite particles. Its average size is about 1/50th the width of a human hair (one micron).
|A simulated image of the Mars rover protected by descent-bracing airbag pyramid. Dust was kicked up onto the rover bags, making them ‘instant’ rocks in low resolution images.|
This dust gets picked up by swirling winds called dust devils, familiar to many in the American Southwest, but intriguing as one of the few visible kinds of surface weather on any planet in our solar system. Mars has only a faint atmosphere [less than one percent of terrestrial pressures], yet offers up its history of dust devils as swirling tracks in a remarkable landscape of wind-swept and carved terrain. These tiny twisters tend to appear in the middle afternoon on Mars — from 1 to 2 p.m.–when solar heating is maximum and when warm air rises and collides with other pressure fronts to cause circulation.
In his first press conference after the rover landed, the principal investigator for the rover’s science package, Cornell’s Steven Squyres, described one instance his team has been discussing: the intriguing possibility that at Gusev, over their three-month mission, the rover’s camera may actually be able to animate a dust devil in action.
Squyres informally proposed a mini-series of frames, or twister movie which with some meterological luck, might offer a rare example of surface weather on another planet.
What is exciting about this is new orbital pictures of the landing site now indicate that prior to touchdown, the surface weather was changing dramatically over a span as short as only a few months. In contrast to the view that Mars is a boring, rocky place, the images show a changing landscape right where the rover is thought to have come to rest.
Two particular orbital pictures illustrate these differences in their patterns of dark, squiggly streaks. These streaks are believed to have been caused by the removal of bright dust by large, passing dust devils.
Comparison of the picture from July 2003 with that of December 2003 show that a different dark streak pattern developed over a period of less than 5 months.
The rover, Spirit, landed somewhere within an approximately 83 km (~52 mi) long by ~10 km (~6 mi) wide ellipse on the floor of Gusev Crater–in a region close to the craters shown. So far the landing site has been photographed from orbit at 25 meters (82 feet) per pixel and most recently within seconds of landing, in three rapid snaps shot from the descent camera (called DIME).
|A Dynamic Spirit Site MGS MOC Release No. MOC2-596, 5 January 2004. Two Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) images acquired before the spectacular January 2004 landing of the Mars Exploration Rover (MER-A), Spirit, show the area where the lander is currently believed to have touched down.|
The banner image shows an exotic dust devil as painted by planetary scientist Bill Hartmann (left), as photographed by orbiting cameras (center) and in Arizona (right).
In images of Gusev taken by the Mars Orbital Camera(MOC) aboard the Mars Global Surveyor (MGS) spacecraft, some exposed outcrops appear to show faint layering. The prevailing scientific theory is that Gusev Crater contains sediment washed down Ma’adim Vallis from the highlands to the south nearly 4 billion years ago.
Gusev Crater, is intriguing because it appears to be the site of an ancient lakebed, where layers of sediment were deposited by a long-term flow of water into the basin formed by the crater. From orbit, the crater looks like a big lake bed with a winding riverbed feeding into it, and represents a candidate for investigating martian water history as an example of what are called paleolakes. Gusev itself is a large crater basin, some 170 kilometers (105 miles) across. Most scientists believe it once was fed by water flowing through an enormous valley channel, Ma’adim Vallis. Snaking its way northward along the Martian landscape for more than 900 kilometers (560 miles), Ma’adim Vallis is 1.5 times as long as the Grand Canyon.
|Dust devil crawls crater wall. See also image in context, full view, or storm panorama view.|
So far, MOC has acquired enough images for a very short animation of two close-up frames, although 71 pictures of the landing site have been taken over a period spanning 3 Mars years (from July 1999 through December 2003), and more than 85 pictures were acquired within Gusev Crater specifically to support the Mars Exploration Rover landing site selection process.
The identification of the area shown in the two pictures (right) is based on the pictures acquired by Spirit’s descent imaging system just before landing. The lower picture was obtained by MGS MOC on 22 July 2003, the upper picture was acquired less than a month ago on 10 December 2003.
Dust is not necessarily something that Martian rock-hunters look for. Squyres’ first impressions of the Gusev site were that it was cleaner than his expectations, and also smooth enough for their golf-cart sized rover to cruise across without frequent pauses or turns. To the delight of geologists, the Gusev region seems to have been cleaned of some wind-blown dust, because of these frequent tiny tornado tracks, or dust-devil paths.
Frequent dust devils were also found during the 1997 Pathfinder mission, with an unmistakable temperature, wind and pressure signature, and morning turbulence; at least one may have contained dust (on Sol 62), suggesting that these gusts are a mechanism for mixing dust into the atmosphere. But no high-quality pictures of the twisters in action were possible. For geologists, the dust devil tracks may help them get at clean surfaces on rocks, because fine-grains will have been swept up by wind and gusty circulations that streak the crater floor.
"At the Pathfinder site during its 83 sol mission, approximately thirty dust devils were either sensed by the pressure drop as they passed over the lander, or were imaged by the Pathfinder camera," says Peter Smith of the University of Arizona, the scientist responsible for the Imager on the July 4, 1997, Mars Pathfinder camera. "Based on these observations, one might expect to see several dust devils per hour from an active site on Mars between 10 am and 3 pm. Few, if any dust devils will be present at other times. Dust devils typically form during late spring and summer and can be found at all latitudes. Exactly, how their population density varies around the planet is currently unknown."
But chasing such a storm on Mars is something any rover will likely need to steer clear from. "Rovers and other robots must be carefully designed to withstand the sandblasting that they will endure from dust devils," said Smith. "Bearing surfaces and solar panels must be protected and dust accumulation on solar panels will lower their efficiency."
|Active Martian dust devil caught in the act of creating a sandblast track in Promethei Terra, December 11, 1999.|
Credit: NASA/JPL/Malin Space Science Systems
In addition to Pathfinder’s run-in with a dust devil, previous missions to Mars have run into very dusty days. For instance, there was a dust storm covering the Viking Lander I (VL-1) site on Martian day (1742) or sol 1742 (1 Martian year=669 Earth days). In 1971, Mariner 9 and 2 USSR missions all arrived during a dust storm. But for the first time, Spirit may have enough instrumentation to watch the action closer to when and how it happens.
High winds on Mars create more dust than any hazard one might expect from a Kansas-like tornado–the air pressure is small and atmosphere so thin (<1% or 1/150th of Earth’s) that more likely difficulties are the mess a dust storm creates than the net force other than upon descent. Even a 100-mile per hour gale on Mars packs the gentle push of a 10 mile per hour breeze on Earth.
But according to astrobiologist, Dr. Chris McKay of NASA Ames, "the key environmental factor for making Mars a better place for life, a kinder, gentler planet, is not making it warmer. The key factor is raising the pressure up from 6 to maybe 100 millibar. [One hundred millibar is one-tenth of the pressure on Earth at sea level.] Not much higher than that would be needed".
Other dust devils caught in the act by orbital cameras are highlighted below:
- Daedalia Dust Devil
- Exhumed Crater with Slope Streaks
- Peering Into A Cerberus Fossae Trough
- Buried Mid-Latitude Craters
- Terby Sedimentary Rocks
- South Polar Mesas
- Small Dust Storm in Syria/Claritas
Mars’ Artistic Temperament
Planetary scientist, Bill Hartmann, is a member of the MGS team and also a talented painter and author. Before the mission, he spoke with Astrobiology Magazine about the interesting possiblities for interpreting not just the geology of the landing site, but also atmospheric phenomena like dust devils and weather. "On my easel I have a newly started painting of the light filtering through a Martian dust storm."
"[On Mars]," continued Hartmann, "a surface gets beaten up and pulverized to depths of a meter or so by impacts (as we found on the moon), in spite of the fact that the smallest meteoroids (less than a few centimeters in size) get broken up in Mars atmosphere. If you had a deposit of 1 inch of pure salt on [the ground], that deposit would have been ground up, mixed, and probably blown away by now".
"So…a nice 3500-million-year-old lakebed deposit full of diagnostic minerals and hypothetical fossils," said Hartmann, "has been pulverized and plowed by impact, and possibly dispersed, not to mention probably covered by windblown dust and sand deposits. So the big challenge is to be lucky enough to set down in an ancient lakebed that has been preserved."
"[One] amusing [atmospheric] mistake with the first Viking 1 pictures — releasing an image with a blue sky [because of miscalibrated colors which substituted blue for a more 'true' orangish-pink light]– really was an example of what we didn’t know and why we went there and what we were learning!"
JPL, a division of the California Institute of Technology, manages the Mars Exploration Rover project for NASA’s Office of Space Science, Washington, D.C.