Don’t be surprised if you hear the god of war’s name dropped in casual conversation at the local burger joint this summer. 2018 promises to be the Year of Mars. On July 31, Mars will come closest to Earth since 2003 during its perihelic opposition. Every 15 or 16 years, Mars reaches perihelion (closest to the sun) at the same time it’s closest to the Earth. After 15 years of waiting, we’ve arrived.
Not only will be the Red Planet appear relatively large and detailed through a telescope, but it will shine as brightly as Jupiter. Just about anyone who looks up this late summer and fall will be mesmerized by the big, red “star” in the southern sky.
Telescopic observers will be watching the Martian polar cap shrink and tracking a variety of dark surface features that rotate in and out of view. Meanwhile, some Mars scientists are eager for a dust storm to materialize and grow big enough to engulf the entire planet. We’ve observed big storms from both the Earth’s surface and Martian orbit for many decades, but the variety of spacecraft presently orbiting the Red Planet would give us a much more complete understanding of the storm’s evolution.
A study published this week based on observations by NASA’s Mars Reconnaissance Orbiter (MRO) during the most recent Martian global dust storm (2007) suggests such storms may hasten the process of gas escaping from the top of Mars’ atmosphere. Loss of atmosphere is thought to play a key role in transforming what was once a wetter, warmer planet into today’s arid, frozen world.
“We found there’s an increase in water vapor in the middle atmosphere in connection with dust storms,” said Nicholas Heavens of Hampton University, Hampton, Virginia, lead author of the report in Nature Astronomy. “Water vapor is carried up with the same air mass rising with the dust.”
Once the water vapor is lofted to heights of roughly 30 to 60 miles (50 to 100 km) ultraviolet sunlight can break water molecules apart, freeing hydrogen — the H in H2O — which escapes into space. Hydrogen zips away fast because it’s lightweight and easily accelerated. The remaining oxygen, the “O” in water, either escapes or finds its way back down to the crust. There it oxidizes iron in the soil to create the iron oxides (rust) that give the planet its familiar color.
This isn’t the only way Mars lost and continues to lose its air. Without a planet-wide magnetic field to protect the atmosphere, an asset we’re fortunate to have on Earth, the stream of high speed particles from the sun called the solar wind (the ones that can create auroras on Earth) has steadily stripped away the Martian air. Without a thick atmosphere, the planet lost much of its original water (and the atmospheric pressure needed to keep water liquid at its surface) and cooled down to the freezing place it is today.
The Hubble Space Telescope and the European Space Agency’s Mars Express orbiter have detected some top-of-atmosphere water loss, but NASA’s MAVEN mission, which arrived at Mars in 2014, was built precisely to study the stripping process. NASA can’t wait to put it through its paces in the event of a global dust storm.
Not all Mars watchers are thrilled with the idea of a global dust storm, which can adversely affect ongoing surface and landing missions. Opportunity, as a solar powered rover, would have to hunker down to save energy; the upcoming InSight lander’s parameters would need to be adjusted for safe entry, descent and landing in November; and all the cameras on rovers and orbiters would need to deal with low visibility.
Decades of Mars observations show a pattern of multiple regional dust storms blowing up during the northern spring and summer. In most Martian years, which are nearly twice as long as Earth years, all the regional storms dissipate and none swells into a global dust storm. But in 1977, 1982, 1994, 2001 and 2007 local storms boiled over into regional and global storms. I well remember the 2001 storm. Just as the planet was at its best, a huge storm covered the globe, blanketing the surface from view. For weeks, the planet was a featureless orange ball. The next Martian dust storm season is expected to begin this summer and last into early 2019.
The Mars Climate Sounder on MRO can scan the atmosphere to directly detect dust and ice particles and can indirectly sense water vapor concentrations from effects on temperature. Data show a slight increase in middle-atmosphere water vapor during regional storms and a jump of more than a hundred-fold during the 2007 global storm. Before MAVEN, scientists expected Mars’ atmosphere to lose hydrogen at a steady rate. Now, we see that seasonal variations and storms can cause the amounts to vary.
2018 is bound to be a great year for Mars one way or another. We’ll have lots more to say about the Red Planet in the coming months.