Martian Dangers: Staring at the Sun
How planets, and any potential inhabitants, protect themseves using some analogous form of sunscreen turns out to be one of the great challenges to surviving elsewhere. While this challenge may have to be designed around in some innovative ways, any future human missions to Mars will face the Sun. It turns out also to be a recurring problem for landing rovers and orbiting probes. What style of ecosystem might be required for microbial life to survive elsewhere? Mars' thin atmosphere does little to protect the planet: the air density at martian "sea level" is roughly equivalent to that of Earth's atmosphere at 70,000 feet altitude.
The flurry of solar storms, and the largest solar flare recorded by NASA's orbiting satellites, has indeed already cost (at least temporarily) one radiation-measuring instrument on the Mars Odyssey probe. The 2001 Mars Odyssey, for example, an orbiter launched on April 7, 2001, contains an experiment to measure the amount of damaging radiation that humans travelling to Mars would need to protect themselves against. Called MARIE , for the Mars Radiation Environment Experiment, the instrument maps the radiation environment more than five million kilometers from Earth, and even at that great distance, a properly pointed solar outburst can provide challenges to protect against.
No Sunscreen? Use a Mud Pack
|Mars has huge quantities of iron, as evidenced by the rusty red color of its soil.
For life as we know it to survive, heavy sunscreens are needed to block out the full spectrum of the sun's ultraviolet (UV) rays. There are three kinds of UV rays: UV-A, UV-B, and UV-C. Modern sunscreens absorb UV-A and UV-B rays. UV-C rays are the most damaging, but they never reach the Earth because they are blocked by the ozone layer. But over 2 billion years ago, the Earth had little oxygen and no ozone layer. UV-C rays blazed down on the Earth's surface unimpeded, and would have fried to a crisp any life caught out sunbathing. Before photosynthesis led to the rise of oxygen and the formation of the ozone layer, early life protected itself by living underwater. Water can block UV light while allowing enough visible light to shine through for photosynthesis to take place. Early photosynthetic life would have had to stay below a certain water depth to take advantage of visible light while avoiding UV.
Janice Bishop of the SETI Institute has been studying this connection between iron, photosynthetic organisms, and atmospheric oxygen. She thinks iron oxide-bearing minerals like ferrihydrite and schwertmannite could have acted as a sunscreen for early photosynthetic life. The role iron may have played on early life naturally leads to questions about the potential for life on Mars. Mars has huge quantities of iron, as evidenced by the rusty red color of its soil. Mars also has silica, many volcanic features, large quantities of water ice at the poles, and perhaps even some water ice or liquid water underground in lower latitudes.
Most scientists think Mars is too cold and dry at present for life to exist, but the combination of water, heat, and sunscreen materials makes it tempting to think microbial life could have existed during a warmer, wetter period in the Red Planet's history. If life ever existed on Mars, it would not necessarily have needed sunscreen in order to survive. Life could have lived deep underground, protected from the irradiated surface, gaining energy from chemicals in the rocks. Like early life on Earth, the only organisms that would have needed to risk exposure to the sun were those that made their food from photosynthesis. As NASA Ames astrobiologist Chris McKay noted, this can even pose an underground threat over very long times: " background radiation, from natural levels of the decay of uranium,thorium and potassium, even deep below the surface, is about 0.2 rads per year on Earth and would be roughly similar on Mars. This would deliver a lethal dose, even to the most radiation-resistant organisms, in about 100 million years."
So Bright, You Have to Wear Sunglasses?
"We did some calculations a while ago", wrote the principal investigator for the science package on the Mars Exploration Rovers, Dr. Steven Squyres, "and they showed that if we were ever to point the Miniature Thermal Emission Spectrometer (or Mini-TES) at the sun we could damage the instrument". This instrument will survey the surroundings, scanning for spectral variations in the variations in the infrared radiation given off by different rocks. By analyzing these spectra - which frequencies of infrared are absorbed and which are reflected - will be able to determine the mineral composition of the rocks.
At the time of their Critical Design Review--a review of the whole spacecraft, including the instruments--, his Cornell team first addressed this challenge in June 2001, just 24 months prior to launch. "That's not bad news scientifically... there's no reason we'd want to look at the sun with Mini-TES anyway. But what if we did it accidentally? We could have a toasted spectrometer on our hands. So now we have to figure out how to keep such an accident from ever happening. It shouldn't be hard... it's easy to calculate where the sun will be in the sky, and we'll just make sure that we don't point Mini-TES in a dangerous direction. But it's one more thing to put in the software, and time is getting tight".
Their solution took a week of troubleshooting. Mini-TES sits down inside the rover and uses some mirrors near the top of the mast to get almost the same view of the world that the panoramic (Pancam) camera does. Squyres wrote: "We've figured out how we're going to keep from pointing it at the sun accidentally, which we were worried about last week. The best answer turns out to be the simplest one. The rover always knows roughly which way it's pointed, and also what time it is... which means that it can figure out where the sun is. So we keep Mini-TES pointed away from where the rover thinks the sun is, and we keep it far enough away that even if the rover's off by a little, Mini-TES will be safe anyway".
Afraid of Ghosts
Squyres' team also had to consider the effects of the Sun on any martian optics. He wrote about the design issues, by noting: "We've been worried about what are called 'ghost images'".
|Each "eye" of the Pancam carries a filter wheel that gives Pancam its multispectral imaging capabilities.
"One of the most important jobs for our Pancam camera will be to take pictures of the sun. Part of the reason we're going to do this is to figure out how dusty the martian atmosphere is: the more dust there is in the sky, the darker the sun will look," continued Squyres. "The really important reason, though, is to figure out which way is north. Mars doesn't have a magnetic field, so we can't use a compass to figure out directions. Instead, we use Pancam to see where the sun is in the sky, and then use that information to work out which way is which. Taking pictures of the sun isn't all that hard: we just have to give Pancam special dark filters. They're sunglasses for our rover, really. But even with the filters, all that sunlight can sometimes bounce around inside the camera in funny ways, creating the 'ghost images' we were worried about. But after lots of good work by the optics experts at JPL..., we've concluded that it shouldn't be a problem. So we're not afraid of ghosts any more".
Scientists from NASA, the SETI Institute and other institutions have studied microscopic life forms in some of the highest lakes on Earth atop a South American volcano to learn what life may have been like on early Mars. Scientists have conducted field tests to examine life forms in several lakes, including the Licancabur volcano crater lake, at nearly 20,000 ft. in the Andean Altiplano on the border of Bolivia and Chile. Most of the Earth is covered by a protective ozone layer that screens out higher-energy UV radiation, which is particularly damaging to life. "The best way to get high UV," explains Chris McKay, a planetary scientist at NASA's Ames Research Center in Mountain View, CA, and a member of Licancabur expeditions, "is to go to high elevations. The equatorial regions are particularly interesting because the Sun is higher in the sky," so the radiation is stronger than in regions closer to the poles.
|The Licancabur team place UV filtering plates in Laguna Blanca (above) to study how the UV affects the lake's biology.
Intense ultraviolet (UV) radiation, low oxygen, low atmospheric pressure and cold temperatures make the environment a close analog to martian lakes 3.5 billion years ago. Despite the extreme conditions at Licancabur, scientists say microscopic life is present and diverse. Its survival strategy might be very ancient, according to the project's principal investigator and expedition lead, Dr. Nathalie A. Cabrol of NASA Ames Research Center, Moffett Field, Calif., and the SETI Institute, Mountain View, Calif.
During their first expedition last year to the same area, Cabrol's science team discovered that very small plankton-like algae called diatoms had 10 times more deformities than similar algae in other lakes. UV is believed to be the 'prime suspect' that may have triggered the malformed algae, according to Cabrol.
One of the scientists' goals is to identity the species living in the high lakes and to learn how these living things cope -- or do not cope -- with UV and other stresses. "Most of the lakes we study there are shallow and do not provide substantial protection to living organisms. They have nowhere to hide from UV," Cabrol said. "We want to understand if these diatoms have developed some sort of 'sunscreen.' If not, they are probably on their way to extinction," she added. "On the one hand, we might learn more about life strategy against UV with all its implications for early planets' habitability and future astrobiological mission exploration strategies, and on the other hand, we might possibly be on our way to identifying a limit to life's adaptation on Earth," Cabrol explained.
Cabrol likens the harsh, isolated environment of Licancabur to what was perhaps the final era of Martian habitability. "High UV, low oxygen, low atmospheric pressure: These must be the conditions that were on Mars 3.5 billion years ago, when the atmosphere was not completely gone, when there was still a little bit there, when local ponds were still possible," says Cabrol.
|Recently discovered DNA rings (stained blue) in D. rad. DNA is the first part of a cell to be damaged by radiation.
Conan, the Bacterium
The champion among radiation-resistant life on Earth doesn't need sunscreen. The red bacterium, called Deinococcus radiodurans, can withstand 1.5 million rads--a thousand times more than any other life form on Earth and three thousand that of humans. Its healthy appetite has made it a reliable worker at nuclear waste sites, where it eats up nuclear waste and transforms it into more disposable derivatives. The ability to withstand other extreme stresses, such as dehydration and low temperatures, makes the microbe one of the few life forms found on the North Pole. Deinococcus radiodurans was discovered decades ago in canned food that was sterilized using radiation. Red patches appeared in the cans - colonies of the bacterium - setting off questions as to how it could have survived. Though these questions have now been answered, the tide of speculation as to how these defense mechanisms evolved - and where - is likely to continue.
Three spacecrafts are now hurtling toward the Red Planet to look for evidence that it might once have been wet enough to sustain life. Orbital projections of where Europe's Mars Express and the two NASA Mars Exploration Rovers (MER) are right now, can be continuously monitored over their half-year journeys. Experiments performed by the MERs will help to determine whether water might have once existed in volume on the red planet. The two Mars Exploration Rovers are targeting what imagery indicates might have been ancient dry lake beds and other geologically interesting sites in early 2004. Matt Golombek, who was the project scientist for the 1997 Pathfinder/Sojourner mission, believes that "the MER is exactly the next step beyond what Pathfinder did." Because it will enable a detailed study of the mineral composition of Martian rocks and soil, it will tell scientists far more than was previously known about the history of water on Mars.
Future missions to Mars will perform additional experiments to understand better the possibilities and challenges of supporting a human mission. And astronauts living aboard the International Space Station will improve NASA's understanding of the effects of long-term exposure to microgravity. But NASA's Mars-exploration roadmap for the next 20 years contains no plan to actually send human explorers there.
Related Web Pages
Mars Vistas: How Earth Will Receive Stunning, High-Resolution Views
JPL Surface Mission
The Pancam Investigation on the NASA 2003 MER Mission
Mars Exploration Rover Homepage
Extreme Environments: Licancabur Site
Michael Endl's Journey to Licancabur
What's Living in the World's Highest Lake? (Licancabur Expedition Journal: Part I)
Licancabur Expedition Journal: Part II
Hot Springs in the Andes (Licancabut Expedition Journal: Part III)