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Science
by Pat Murphy & Paul Doherty

ROVER THE SUPERDOG

 
AS A SCIENCE fiction writer, Pat has a few problems with Superman. She feels the guy just has too many powers to make for interesting fiction. As the announcer at the beginning of the 1950s TV show points out, this "strange visitor from another planet" has "powers and abilities far beyond those of mortal men." He flies, he's super strong, and he has the most remarkable eyes.

Generally, human eyes sense radiation in the narrow band of the electromagnetic spectrum known as visible light. But Superman's eyes are capable of X-ray vision, infrared vision, telescopic vision, and microscopic vision. If that isn't enough, his eyes can emit radiation to heat objects.

As a physicist, Paul is intrigued by these powers—and he points out that in the latest Mars mission, the worlds of science and comic book science fiction have collided. When NASA built the Curiosity Rover (more formally known as Mars Science Laboratory), they created their own alien visitor to another planet with a ridiculous number of superpowers.

The Curiosity Rover can examine the world around it using infrared light, visible light, ultraviolet light, and X-rays. It has heat vision—that is, it can emit a pulsed infrared laser beam that can vaporize rock. It shoots out alpha particles and neutrons, something the creators of Superman somehow missed when coming up with his superpowers. And, as is appropriate given its name, the Curiosity Rover has a nose that rivals that of Superdog.

On August 5, 2012, Jet Propulsion Laboratory landed the Curiosity Rover on the Red Planet's Gale crater. Since then, it has been exploring, using all its superpowers. In this column, we're going to introduce you to this strange visitor to another planet and take you on a guided tour of those super powers. Along the way, we'll describe some of the discoveries this robotic explorer has sent back to us Earthlings.
 

THE EYES OF CURIOSITY

Curiosity has seventeen different cameras. We'll start with two that are fairly ordinary by today's standards—the Mast Cameras or MastCams, which are mounted on a human-height mast.

These work a lot like any 1.5 megapixel digital camera that you can buy here on Earth for a few hundred dollars. On Earth or Mars, a camera like this is an impressive piece of technology when you consider how it works.

A pixel is one of the tiny dots that make up a digital picture. But the same word refers to a very small light detector that generates that tiny dot. The Curiosity's MastCams have an array of pixels (of the second variety) measuring 1600 by 1200 pixels.

Each of these light-detecting devices contains a semiconductor. The word semiconductor accurately describes a characteristic of this material. A conductor, like metal, conducts the flow of electrons that we call electricity. In a conductor, some electrons are bound to their atoms so loosely that a small electrical push will shove them over to another atom.

A semiconductor, as the word implies, is not quite a conductor. It will conduct electricity only under certain very specific conditions. When light strikes one of these pixels in Curiosity's camera (or your camera, for that matter), an electron splits away from its atom, turning into a free electron. Each photon of light releases another electron. When a camera captures an image, the light that makes up that image causes free electrons to accumulate on each pixel. After the exposure these electrons are counted and determine the brightness of the light shining on each pixel.

When we talk about light, we should warn you that we are including wavelengths of light that you can't actually see with your eyes. Pat has long claimed that her favorite color is ultraviolet, a shade that's outside the visible spectrum. The pixels in the MastCams detect light in the visible spectrum, but they are also sensitive to light beyond the visible spectrum, extending from infrared light with a wavelength of 1100 nanometers (nm) through visible wavelengths (740 nm to 380 nm), and into the ultraviolet at 300 nm.

Each pixel in the MastCams is permanently covered with a filter that lets through red light, green light, or blue light. A computer assembles the signals from these filtered pixels to make a colored image similar to what is seen by the human eye.

Seeing in visible light isn't really a superpower, but images captured using visible light have often provided clues that encourage Curiosity to exercise its superpowers. Between the north rim of Gale Crater and the base of Mount Sharp, a mountain inside the crater, Curiosity's MastCams captured an image of a layer of rock made of rounded pebbles cemented together—and sent a whole lot of geologists (or whatever the Martian equivalent is) into a frenzy of analysis. Known as conglomerate, this type of rock is created when stones tumble in a flowing stream, rounding their rough edges. This image revealed there was once flowing water in Gale Crater. From the size of the rounded stones, scientists could estimate the speed of the water (about three feet or a little less than a meter per second) and its depth (somewhere between ankle and hip deep).

Images of Gale Crater from Mars orbit provided further information, showing what scientists are interpreting as an alluvial fan of material washed down from the rim, streaked by many apparent channels.
 

INFRARED VISION

Curiosity used visible light to detect evidence of an old streambed. If you or I had been there (appropriately equipped so we didn't asphyxiate, of course) we could have seen that streambed, too. Infrared vision lets Curiosity make discoveries that would be invisible to the human eye.

Before we talk about that, let's do an experiment. You'll need a digital camera, TV remote, and a friend to push a button. Point the business end of the remote at the camera and look at the camera's monitor screen as your friend pushes any button. You won't see anything when your friend pushes a button on the remote, but the camera screen may show a bright light. If it does, your camera is sensitive to the near infrared, the wavelength that the remote uses to signal to the TV.

Most digital cameras can detect near infrared, but some have been equipped with near infrared filters to block those wavelengths. That's because photos in the near infrared can be as revealing as a wet T-shirt contest. Near infrared radiation can penetrate thin white cotton clothing, showing you whatever warm body is beneath that clothing.

But Curiosity is more interested in rocks than warm bodies, and infrared vision provides it with an important tool. Viewed in visible light, most rocks are dull brown or gray, since they reflect similar wavelengths of visible light. (It's the wavelength of the reflected light that gives an object its color.) But two minerals that both look gray in visible light may reflect very different wavelengths of infrared—and look very different from each other.

The engineers who built Curiosity gave it the equivalent of infrared color vision—it can detect different wavelengths in the near infrared. This super color vision makes it easy for Curiosity to see hydrated minerals—that is, minerals that have added water to their crystalline structure. In the infrared, hydrated minerals look much brighter than their non-hydrated counterparts.

Since Curiosity is searching for possibly habitable places on Mars, it's looking for water, an important part of a habitable environment. Curiosity has already spotted hydrated minerals, a clay mineral known as smectite, in images of a rock that broke open when the Rover drove over it and in some mineral veins in the bedrock.
 

MICROSCOPIC VISION

One of the most important tools for a geologist is a hand lens—basically, a powerful magnifying glass that lets a geologist see the shape of small mineral grains in a rock. The Curiosity Rover has a lens-equipped camera that functions as a super hand lens. It's named Mars Hand Lens Imager (MAHLI). This full color camera can see details as small as 12.5 micrometers across, about one eighth the diameter of a human hair.

MAHLI comes with its own light sources that produce both visible light and ultraviolet (UV) light. These light sources—and the nuclear battery that powers Curiosity—let this robotic explorer take the first night-time images that have been beamed back from Mars. The UV can make certain minerals fluoresce—absorbing the UV and re-emitting a visible light. Many carbonate minerals fluoresce, and on Earth many carbonate minerals are associated with life. So the combination of the UV light source and the super hand lens has the potential to help Curiosity identify evidence of life.

As is appropriate for a hand lens, MAHLI is mounted on the end of Curiosity's arm. It can be raised above and moved around the Rover. It has taken images of the Rover on the surface of Mars. It can also be used to look underneath the Rover. As scientists have discovered from other missions, this can be useful. When the Phoenix spacecraft landed on Mars in a hunt for ice, it looked under its belly and found that the retro rockets had blown away the surface dust revealing ice. Mission accomplished.
 

VAPORIZING VISION

Pat's favorite of Curiosity's superpowers is the rock-vaporizing laser. After all, no superhero should be without the ability to vaporize rock.

Though we think that The Vaporizer would be the most appropriate name for this awesome ability, the instrument is actually called Laser Induced Breakdown Spectroscopy (LIBS). Given the imagination NASA shows in other areas, we are a bit disappointed in their instrument names.

LIBS (or The Vaporizer) emits a laser pulse that delivers ten megawatts into a target area that's just one mm square. The actual beam is about 0.5 millimeter (mm) in diameter, about the size of the head of a pin. Ten megawatts per millimeter squared is ten billion times the energy delivered by sunlight on earth.

This intense flash of laser light lasts for only five nanoseconds, or five billionths of a second. But that's long enough to turn a small bit of rock into an expanding, glowing plasma cloud.

Why vaporize rocks? Well, if you're an evil genius intent on world domination, it's an excellent way to demonstrate your powers and terrify your enemies. If you are a scientist studying Mars, you have other reasons.

The vaporized rock becomes a plasma cloud made of atoms and ions that are so hot that their electrons are in excited states. To a physicist, excited state means that these electrons have absorbed energy that they can hold onto for a short time—and when we say short, we mean very short—that is, under ten nanoseconds. When these excited electrons release the absorbed energy or decay, they give out light.

LIBS is coupled with a telescopic eye called the Remote Micro-Imager (RMI), which has an opening the size of your clenched fist. This telescope gathers light from the plasma cloud and delivers it to a digital 6144-channel spectrometer that can see across a wide spectral range from ultraviolet through visible light and into the near infrared. Bright lines of light appear in the spectrum of the plasma. The position of these lines identifies the elements making up the plasma, and the brightness of each line gives the abundance of that element.

The LIBS instrument gives the Rover super science powers. As long as Curiosity can see a rock and can move within twenty-three feet (seven meters) of it, LIBS can vaporize the rock and the RMI can identify the elements that it was made of. The laser can also vaporize surface dust, and then continue to vaporize its way deeper into the underlying rock.
 

X-RAY VISION

Curiosity does have X-ray vision, but it is inside the belly of the beast.

Curiosity's hand tool drills into a rock to create a powdered sample that is examined by the MAHLI hand lens and then picked up. Inside the robotic explorer, the powder is delivered to the robot's Chemistry and Mineralogy instrument, affectionately known as CheMin.

CheMin produces X-rays by shooting electrons into a cobalt surface, the same method used in your dentist's office. When electrons crash into the cobalt, X-rays are emitted. CheMin shoots these X-rays through the powdered sample. The crystal structure of the minerals in the sample diffracts the X-rays onto a two-dimensional X-ray detector. The pattern on the detector identifies the crystal structure of the mineral.

You can see why this happens by taking a drive through Napa, California, in the spring before the grapevines leaf out. As you drive by a vineyard, notice that the trunks of the grapevines create interesting linear patterns. Those vine trunks form an array like the atoms in a crystal, and the patterns that you see when you drive by are like the patterns revealed by X-rays passing through a mineral. These patterns reveal the spacing between the atoms in a crystal, which lets geologists identify the mineral.

CheMin's X-ray vision is particularly useful at identifying hydrogen in minerals, which is present in great quantities in hydrated minerals. When CheMin looked at samples in the area of Gale Crater known as Yellowknife Bay, it found rock that was up to twenty percent clay minerals. That's evidence that those rocks had been exposed to water. Not only that—the type of clay showed that the water was near neutral pH, drinkable by humans, unlike the acidic water-altered clays found previously on Mars.

In addition, at a site named "Rocknest," Curiosity detected mineral grains of feldspar, pyroxene, and olivine, a mixture similar to what would be found in the weathered volcanic soils of Hawaii. But don't go to Mars for the beaches. Sad to say, you are a few billion years too late.
 

ALPHA PARTICLES

Curiosity's hand packs another weapon—the Alpha Particle X-Ray Spectrometer or APXS, which shoots Martian rocks with six-mega-electron-volt helium nuclei, also known as alpha particles. These alpha particles bash into atoms in the rocks, knocking electrons from inner electron shells near the atom's nucleus. Outer electrons then fall into the resulting hole, emitting X-rays as they fall. X-rays are detected by a spectrometer and used to identify the atomic composition of the rock sample.

This doesn't make a very useful weapon, since a layer of rock that's just fifty micrometers thick (half the thickness of a hair) stops the alpha particles. But if you are fighting an armored foe and you want to know what that armor is made of, this is just the thing. The alpha particles come from the nuclear decay of Curium 244 that has a half-life of eighteen years, plenty long to last the entire planned mission.
 

A NEUTRON GUN

Curiosity also has a compact linear accelerator. That sounds impressive until you realize that most American households at the end of the twentieth century contained at least one linear accelerator—a television set with a cathode ray tube. The difference is that the accelerator on the Rover accelerates particles to ten million electron volts, giving them a thousand times more energy than the particles in that TV.

Curiosity's accelerator produces neutrons through nuclear fusion between deuterium and tritium, which are isotopes of hydrogen, and creates a beam of neutrons with fourteen million electron volts of energy. The accelerator shoots these high-energy neutrons downward into the soil and Curiosity watches to see what bounces back.

If there's a layer of water or ice under the surface, some of those alpha particles will bounce back slowly. That's because water is loaded with hydrogen—two atoms of hydrogen for every atom of oxygen. A neutron has about the same mass as the nucleus of a hydrogen atom. When one of those high-energy neutrons strikes a hydrogen nucleus, it is like a cue ball striking an eight ball—the neutron slows down and maybe bounces back. So if there is a layer of water ice beneath the surface, Curiosity will detect more slower neutrons reflected back to the surface. So far, near a location palindromically named Glenelg, Curiosity has detected four percent water down to a depth of sixty centimeters (about two feet).
 

SAM THE SUPER NOSE

Curiosity has a nose called SAM, an excellent name that stands for Sample Analysis at Mars. SAM is definitely a super nose.

Curiosity has the ability to sniff the outside air. It can also heat rock dust to 1000°C (1800°F) and sniff the resulting fumes, which are analyzed by three powerful instruments: a gas chromatograph, a quadrupole mass spectrometer, and a tunable laser spectrometer. From the names of these instruments alone you could probably guess that no interesting molecule can hide from this nose.

The gas chromatograph allows gas to flow through a long thin tube. Smaller, lighter gases pass through the tube more quickly than the larger, more massive ones. This separates out gases for further analysis.

The quadrupole mass spectrometer (QMS) ionizes gas and then shoots the ions along a tortuous twisting course through the long gap between four parallel metal rods. Only ions with the correct mass-to-charge ratio can make it through this slalom course. Those that make it through have a known molecular weight. The first results of using the QMS on the Martian atmosphere found the ratio of argon forty to argon thirty-six on Mars matched the ratio found in meteorites on Earth that are thought to have come from Mars. This was an exciting result for Paul, who owns a piece of a Martian meteorite. Since Martian meteorites are among the rarest rocks on earth, they are also expensive—so Paul owns a valuable piece of Martian real estate.

The Tunable Laser Spectroscope will be used to detect isotopes of the important organic gas methane, and to determine if the gas was produced by biological processes or by volcanic emission. This device can detect methane in concentrations as low as one part per billion of methane in the atmosphere of Mars. So far, SAM hasn't found any methane in the Martian atmosphere.

The first time SAM sniffed gases released by heated Martian sand, chief scientist John Grotzinger proclaimed in great excitement that the discovery was "one for the history books." In that first sniff, SAM had successfully detected water, carbon dioxide, and simple organic chlorinated methane molecules. Though these chlorinated methanes are organic molecules, they don't necessarily come from past living organisms, so Paul isn't ready to celebrate just yet. But he is impressed with a discovery SAM made at the Rocknest site just before this column went to press. SAM sniffed the sand (as it heated to 1000°C) and found it contained two percent water by weight. That means visitors to Mars can heat a cubic foot (a thirtieth of a cubic meter) of sand and release a couple pints of water. Before we write the history books, we'll have to wait a bit longer for a few more outgassed samples from heated rocks.

UP IN THE SKY—NOT A BIRD OR A PLANE

Superman can leap over tall buildings and fly. Paul says Curiosity behaved more like the sheep in the Monty Python skit—it plummeted more than it flew. Pat thinks that description is not quite fair.

Members of NASA landing team at Jet Propulsion Laboratory described Curiosity's landing as "seven minutes of terror." Signals from Curiosity on Mars take about fourteen minutes to reach the team on Earth. If all went well, the team knew it would take about seven minutes for Curiosity to travel from the top of the Martian atmosphere to the surface. At the moment that a signal revealed that the vehicle had touched the top of the Martian atmosphere traveling at 13,000 mph, the team knew that Curiosity had already landed safely or crashed horribly seven minutes before. That was their seven minutes of terror.

Terror because the journey from the top of the atmosphere to the surface was a very complicated sequence of actions—all of which had to work perfectly and be managed completely by the shipboard computer. The spacecraft had to maneuver following its entry into the atmosphere, deploy the largest, strongest, supersonic parachute ever made, drop off the parachute along with the heat shield that protected the craft from burning up as it sped through the upper atmosphere, move to one side to avoid hitting the parachute as it further braked its descent with rockets, and then finally hover above the surface and lower the Curiosity Rover on a cable until it touched down on the surface on its wheels. Oh, yeah—then fly away, leaving the Curiosity Rover to start exploring. For a more complete account of all this, check out the JPL video Curiosity's Seven Minutes of Terror at http://www.youtube.com/watch?v=Ki_Af_o9Q9s.

Following its dramatic landing, Curiosity's journey has proceeded at a more stately pace. It creeps along the surface at a maximum speed of 1.5 inches per second (that's three cm/s), about the width of two fingers every second. One day recently it covered 140 meters on its way to climb Mount Sharp. During its first year of exploration, it drove just over one mile. Certainly not super speed.

Its goal was to determine whether Mars has ever been capable of supporting life, and that mission has been accomplished. Curiosity found the key ingredients for microbial life: carbon, hydrogen, oxygen, phosphorus, and sulfur—along with evidence of fresh water. It has already sent back more than 70,000 images and a total of 190 gigabits of data. It has fired more than 75,000 laser shots to investigate the composition of targets.

Right now, it's headed for the base of Mount Sharp, a distance of about five miles (eight km) away. This trip will take about a year, but that's okay. Since Curiosity is nuclear powered, it can (like the Energizer Bunny) just keep going. Originally, Curiosity's mission was set to last for two years, but in December 2012 that mission was extended indefinitely. NASA intends to continue to operate Curiosity as long as it's scientifically viable, and that could be a long, long time.

Even if it can't leap tall buildings in a single bound, we think that's pretty super.
 

__________________________________

Paul Doherty works at The Exploratorium, San Francisco's museum of science, art, and human perception—where science and science fiction meet. For more on Paul's work and his latest adventures, visit www.exo.net/~pauld. Pat Murphy used to work at the Exploratorium, but now she works at Klutz (www.klutz.com), a publisher of how-to books for kids. She also writes science fiction. You can find out more about Pat's fiction at www.brazenhussies.net/Murphy.

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