Mars: Cool Place, Not Much Atmosphere

The history of Martian exploration is marked by bold attempts—and some spectacular failures.

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

John Clarke, a College of Arts & Sciences professor of astronomy, and his team of researchers want to find out how Mars is losing its atmosphere. Today, the Martian atmosphere, 200 times less dense than Earth’s, is an insubstantial veil composed of 95 percent carbon dioxide with traces of other gases.

But this wasn’t always the case. Evidence of erosion—streambeds, floodplains, and valleys—shows that there was once liquid water on the Martian surface, implying that the average temperature was warmer then (today the temperature never exceeds the freezing point of water) and the atmosphere much thicker, or the water would have immediately evaporated because of the low atmospheric pressure.

So what happened to the Martian atmosphere? Part of the answer is the solar wind—a blast of ions and electrons ejected from the upper atmosphere of the sun. It 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, too weakly magnetized to have a magnetosphere, has an ionosphere that sets up a current that partially deflects the solar wind. 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’ relatively weak gravitational pull.

Clarke and his colleagues want to know what these collisions look like and how often they occur. They also want to find out how ultraviolet radiation—another contributor to Mars’ 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, transferring energy to the gases, heating them up, and accelerating their motion. Over time this causes gases to rise, pushing them higher up in the atmosphere.

Clarke’s team believes that these high-energy gases, hydrogen in particular, can escape from the upper atmosphere into the exosphere, a region beginning about 120 miles above the surface and extending several thousand miles from the surface, eventually tapering away, and then out 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’ weak gravity.

The team is currently designing a host of instruments to tackle the phenomenon of Mars’ atmospheric loss from different angles. Their mission, dubbed MAVEN (Mars Atmosphere and Volatile Evolution), is part of NASA’s Mars Scout Program. The team will send a robotic spacecraft known as a probe (above) into orbit around Mars for approximately two years. The probe will take an extremely elliptical orbit that will allow it to pass through the upper Martian atmosphere at certain points and will carry instruments that will measure characteristics of Mars’ atmospheric gases, upper atmosphere, solar wind, and ionosphere. All that information will then be sent back to Earth for analysis.

This is where Clarke shines. For most of his professional life he has studied other planets in our solar system by analyzing the UV radiation that is reflected by hydrogen molecules in their upper atmospheres. He has regularly used the Hubble Space Telescope since its 1990 launch for his UV research. 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 the upshot 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 determine how thick Mars’ 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 had much more water and its atmosphere much more water vapor than is present today—an entire ocean’s worth of water is gone. The evidence takes the form of deuterium, an isotope of hydrogen heavier than stable hydrogen molecules, which in Mars’ atmosphere occurs at five times the rate as in a drop of water on Earth. The likely explanation is that there were once more water molecules in the Martian atmosphere than today’s traces of water, but they broke apart into hydrogen and oxygen, with the lighter hydrogen atoms escaping into space, leaving the deuterium behind.

Clarke has successfully lobbied for an Echelle spectrograph, which will measure the ratio of deuterium to hydrogen at the top of the atmosphere, to be included on MAVEN’s probe—the first time such a device will be used while orbiting another planet. It should be able to tell the scientists how much water has been lost from the atmosphere, and at what rate.

Image of Mars courtesy of nasaimages.org, NASA/JPL Malin Space Science Systems

Footprints on Mars
The MAVEN orbital spacecraft will be the size of a small car, with long, thin appendages covered in solar panels powering the vehicle’s systems (a battery will be used when it’s in the shadow of Mars). The spacecraft will be propelled toward Mars by booster rockets at approximately 10 kilometers per second, but even at this speed, it will take roughly one Earth year to reach its destination.

The team hopes MAVEN will join the series of probes increasing our understanding of Mars. In 1965, the U.S. probe Mariner 4 sent back the first up-close images, six years later images of riverbeds and other features indicating Mars once had water came from Mariner 9, the first to orbit the planet. Later, the U.S. Viking probes added to the understanding of Martian geology, and today the Spirit and Opportunity rovers transmit data to Earth from the planet’s surface.

A computer rendering of the MAVEN space probe, which will orbit Mars to measure its loss of atmosphere into space.

Someday, the spacecraft sent to Mars may no longer be unmanned. In April, President Obama announced that he was replacing the Bush plan to return astronauts to the moon in 10 years 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 establish a presence on Mars.

To survive on the Red Planet, 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. Clarke says NASA is currently researching ways to shield astronauts from the most energetic solar particles, a technology not yet available. 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.

Jeremy Schwab can be reached at jschwab@bu.edu.

A version of this story originally appeared in the fall 2010 issue of Arts & Sciences.