Mars’ ionosphere mystery explained

Red Planet’s two layers of charged particles sometimes merge, analysis suggests

Mars Express illustration

CLEARING THE AIR  Disagreements between Mars-bound spacecraft (such as the Mars Express, illustrated) and the Viking landers might have to do with the time of day they conducted their investigations of the Red Planet’s ionosphere. 

ESA, C. Carreau

Probing a planet’s ionosphere can be all about the timing. Confusion about how many layers of charged particles exist in the Martian atmosphere might be due to what time of day measurements were made.

Several spacecraft have, over the years, detected two ionosphere layers on the Red Planet. But both Viking landers noticed only one layer during their 1976 descents. The other spacecraft probed the atmosphere at sunrise and sunset when conditions favor the formation of two layers, space physicist Majd Mayyasi and astronomer Michael Mendillo report online September 10 in Geophysical Research Letters. The Vikings, however, landed on Mars around 10 a.m. and 4 p.m. (local time), when the midday sun blends the layers together.

“The ionosphere is critical to understanding what goes on in the atmosphere of Mars,” Mayyasi says. Harsh ultraviolet light and X-rays from the sun strip electrons from, or ionize, molecules in the atmosphere, creating a layer of charged particles. Mayyasi and Mendillo, both at Boston University, simulated how the ionosphere changes during a Martian day.

MYSTERY SOLVED The Viking landers (one shown here on Mars’ surface) measured only one ionosphere layer in the Red Planet’s atmosphere. A new simulation suggests why the landers missed the second layer. NASA

UV light forms one layer that blankets the planet’s dayside. Higher-energy X-rays, the researchers found, burrow deeper along Mars’ flank and, combined with subtle changes in atmospheric chemistry, develop a second layer where day turns to night. Spacecraft pick up this lower layer when they beam radio waves through the atmosphere toward Earth. The alignment between probe, Mars and Earth limits the radio signal to slice through a part of the Martian sky where it’s either dawn or dusk.

“It’s a very clever model and very likely to be correct,” says Jasper Halekas, a physicist at the University of Iowa in Iowa City.

Simulations such as these can help researchers unravel the fate of Mars’ water. The ionosphere not only receives the brunt of the sun’s energy, Halekas notes, but also is a gateway through which molecules — such as H2O — must pass as they leak into space.

NASA’s MAVEN probe, currently orbiting Mars, might be able to directly measure both layers (SN: 4/18/15, p. 15). Seasonal dust storms can raise the ionosphere to an altitude that MAVEN could reach during one of its dips into the Martian atmosphere, Halekas says. “But we have to get a little bit lucky.” 

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