The history of Martian exploration is marked by bold attempts—and some spectacular failures—to send unmanned probes to learn more about the geology, hydrology, and atmosphere of the Red Planet. The Soviet Union and United States raced to be the first to explore Mars during the height of the Cold War in the 1960s and '70s. Most of the Soviet craft and many American probes failed: some failed during launch, others lost communications, and at least one probe's landing vehicle crash-landed on Mars. But a few attempts succeeded, sending back reams of data, including evidence that water once flowed freely across the Martian surface.

While the technology of space flight has improved over the years, sending a probe to Mars remains a dicey business. That is why, when he found out that his research team had been chosen to lead NASA's next mission to the planet, CAS Professor of Astronomy John Clarke had mixed emotions.

"It felt great for about 24 hours, and then I realized we actually have to do this," he recalls. "I wondered, 'Did we ask for enough money and enough time?'"

Clarke's team of researchers, led by Bruce Jakosky at the University of Colorado at Boulder, outdid more than a dozen other teams competing for the opportunity to conduct the second Mars Scout Program voyage. The first Scout mission successfully deposited the Phoenix lander in the northern polar region of Mars in May 2008 to search for environments suitable for microbial life. The second mission, which will be overseen by NASA but designed and developed by Clarke's team, is an orbital mission that will examine the atmosphere rather than land on the surface. It is scheduled to launch in November 2013, with a budget of $485 million.

"It felt great for about 24 hours, and then I realized we actually have to do this. I wondered, 'Did we ask for enough money and enough time?'"

—John Clarke

The team's mission, dubbed MAVEN (Mars Atmosphere and Volatile Evolution), will send its probe into orbit around Mars for one Martian year—or approximately two Earth years. Armed with an array of instruments, the probe will transmit data that will allow the scientists to understand more precisely how the Red Planet's atmosphere is lost into space, and at what rate. Establishing this information is crucial if human beings are one day to live on Mars— the planet that, of all the known planets, is most similar to our own.

A Mighty Wind

Today, the Martian atmosphere is an insubstantial veil composed of 95 percent carbon dioxide with traces of other gases. It is 200 times less dense than Earth's atmosphere.

But this wasn't always the case. Evidence of erosion—streambeds, floodplains, and valleys—shows that at one time there was liquid water on the Martian surface. This implies that the average temperature was warmer then (today the temperature never exceeds the freezing point of water) and the atmosphere much thicker, or else the water would have immediately evaporated due to the low atmospheric pressure.

So what happened to the Martian atmosphere? The solar wind provides part of the answer.

The solar wind—a blast of ions and electrons ejected from the upper atmosphere of the Sun—tears through interplanetary space at 400 kilometers per second. Earth's magnetic field interacts with the solar wind to form a magnetosphere that protects us from this plasma stream by harmlessly deflecting nearly all of the charged particles into space. Mars, however, is too weakly magnetized to have a magnetosphere. Instead, it has an ionosphere that sets up a current that partially deflects the solar wind. Upon reaching Mars, the ions and electrons that are not deflected by the ionosphere smash into the planet's upper atmosphere.

They can break gas molecules into their component atoms and splinter atoms into subatomic particles. The force of these collisions knocks some particles out into space, beyond the grasp of Mars's relatively weak gravitational pull.

Clarke and his colleagues want to know precisely what these collisions look like, and how often they occur. They also want to find out how ultraviolet radiation—which also contributes to Mars's atmospheric loss—is reacting with particles in the upper atmosphere. UV light is electromagnetic radiation with a wavelength shorter than visible light but longer than X-rays. Unlike visible wavelengths of light, UV radiation from the Sun reacts easily with gases in the atmosphere. The solar UV rays transfer energy to the gases, heating them up and accelerating their motion. This increased movement over time causes gases to rise, pushing them higher up in the atmosphere.

The solar wind—a blast of ions and electrons ejected from the upper atmosphere of the Sun—tears through interplanetary space at 400 kilometers per second.

Clarke's team believes that these high-energy gases, hydrogen in particular, can escape from the upper atmosphere into the exosphere, and then out into space. The exosphere is a region beginning around 120 miles above the surface and extending many Mars radii from the planet's surface until eventually tapering away into space. In the exosphere, the gas molecules are very diffuse. With few other molecules to collide with, the energized gases can maintain their trajectory and escape into space, overcoming the pull of Mars's weak gravity.

The team is currently designing a host of instruments to tackle the phenomenon of Mars's atmospheric loss from different angles. The MAVEN probe will take an extremely elliptical orbit that will allow it to pass through the upper Martian atmosphere at certain points. During these passes, a mass spectrometer will gather up Martian air and analyze its composition by measuring the mass of its component molecules. Another package of instruments will detect ions and electrons. A magnetometer will measure the strength of the solar wind. And, finally, remote sensing equipment will view the planet from above and reveal the UV rays bouncing off the atmosphere.

The MAVEN space probe, which will orbit Mars to measure its loss of atmosphere into space.

This is where Clarke shines. He has spent most of his professional life studying other planets in our solar system by analyzing the UV radiation that is reflected by hydrogen molecules in their upper atmospheres. He has been conducting UV research regularly using the Hubble Space Telescope since it was launched in April 1990. Last year, he published his findings on the impact of the solar wind on the upper atmospheres of Jupiter and Saturn in the Journal of Geophysical Research.

The researchers hope that the upshot of this overall surveying will be a much clearer picture of how quickly the Martian atmosphere is escaping into space. By taking into account fluctuations in the strength of the solar wind over time (scientists believe it was much stronger when the Sun was younger), they should be able to extrapolate backwards in time to determine how thick Mars's atmosphere was during particular periods.

They also want to learn how the atmosphere's composition has changed with time. There is evidence that billions of years ago the Martian surface contained much more water and its atmosphere much more water vapor than is present today. Research shows that Mars has lost an entire ocean's worth of water. The evidence takes the form of deuterium, an isotope of hydrogen that is heavier than stable hydrogen molecules. In Mars's atmosphere, deuterium occurs at five times the rate that it normally does in a drop of water on Earth. The likely explanation is that there were once more water molecules in the Martian atmosphere (today there are only traces of water), but they broke apart into hydrogen and oxygen, and then the lighter hydrogen atoms escaped into space, leaving the deuterium behind.

Clarke has successfully lobbied to include on MAVEN's probe an Echelle spectrograph, a device that will measure the ratio of deuterium to hydrogen at the top of the atmosphere—the first time an Echelle spectrograph will be used while orbiting another planet. The spectrograph should be able to tell Clarke and his colleagues how much water has been lost from the atmosphere, and at what rate.

Footprints on Mars

The MAVEN orbital spacecraft will be the size of a small car, with long, thin appendages covered in solar panels, which will power the vehicle's systems (communications, steering, etc.). A battery will be used when the vehicle is in the shadow of Mars. MAVEN will be propelled toward Mars by booster rockets at approximately 10 kilometers per second. Even at this speed, it will take the MAVEN probe roughly one Earth year to reach its destination.

The MAVEN team hopes its craft becomes the next in the series of probes that have enhanced our understanding of Mars. In 1965, the U.S. probe Mariner 4 sent back the first up-close images of the Red Planet. Six years later, Mariner 9 became the first spacecraft to orbit the planet, sending back images of riverbeds and other features indicating that water once flowed across the planet's surface. Later, the U.S. Viking probes greatly expanded our understanding of Martian geology, while today the Spirit and Opportunity rovers transmit data to Earth from the planet's surface.

Someday, the spacecraft sent to Mars may no longer be unmanned. In April, President Barack Obama announced that he was jettisoning the plan approved by former President George W. Bush to return astronauts to the Moon in 10 years, replacing it with a plan to launch a manned mission to Mars by 2040.

While government leaders' long-range timelines can change, few space researchers doubt that sooner or later humans will attempt to reach and establish a presence on Mars.

If we are to survive for long on the Red Planet, though, we will probably need to thicken its atmosphere somehow—essentially reversing the effects of the past three billion or so years. A thicker atmosphere would retain more heat and help raise the temperature. It would also protect against damaging UV radiation.

Living on Mars and "terraforming" it—transforming it into a more Earth-like planet—may seem like science fiction. Indeed, realizing these goals remains hypothetical and a long way off. One of the most daunting challenges to sending humans to Mars is the need to protect the crew from the solar wind. When astronauts traveled to the Moon, they did so during a period when the Moon was safely enveloped in Earth's magnetosphere. According to Clarke, NASA is currently researching ways to shield astronauts from the most energetic solar particles; however, the technology is not there yet. The MAVEN team's research on the behavior of the Martian atmosphere and solar wind will provide data that will take us one step closer to a human venture to Mars. ■

Is Mars Our Best Hope?

The universe is vast. Just consider:

The Voyager 1 spacecraft, which will probe the far reaches of the outer solar system, is now flying away from us at approximately 38,000 miles an hour, thanks to gravity assists from Jupiter, Saturn, Uranus, and Neptune.

A craft hurtling through space at this speed would take roughly 76,000 years to reach the nearest star, Proxima Centauri, which is 4.3 light-years away. Given this vast distance, the likelihood of human beings reaching other solar systems seems remote, as does the likelihood of alien civilizations reaching ours.

Since Mars is the planet in our solar system that has an environment most like Earth's, it would appear that Mars is our best shot at finding another habitable planet.

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