Mars is Earth's inhospitable cousin. And like anybody with a wayward family member, Earthlings can't help trying to figure out what went wrong. So people throw a lot of money at the red planet, usually in the form of spacecraft. NASA's Maven mission explores Mars' upper atmosphere, puzzling over how a planet that was once wet and potentially habitable ended up a dry, cold desert world.
Metal could solve the mystery. In a new study, Maven mission scientists found that Mars' ionosphere is full of metal ions, tiny charged flecks of meteorite pulled apart by atmospheric forces. Because those ions are hardy and trackable, scientists can use them to study the mysterious dynamics of the planets' upper atmosphere, including its tendency to escape out into space. Plus, scientists have already observed similar metal ions at work in Earth's (very different) atmosphere. Scientists sorting out how these upper layers sort themselves out are rethinking their assumptions about how worlds work.
Earth and Mars get their metal ions from the same source: the constant hail of micrometeorites beating down on their atmospheres. As the flecks of meteorite burn up, charged particles in the atmosphere rip electrons away from metal atoms, turning them into positively charged ions. (Maven has found iron, sodium, and magnesium ions in Mars' atmosphere.) And that's not just interesting trivia: "It's like dropping ink in water," says Mehdi Benna, planetary scientist at NASA's Goddard Space Flight Center. "You can see which way the current moves." Before Maven detected these ions, the dynamics of Mars' upper atmosphere were darn near invisible, which makes gravity waves or a phenomenon like atmospheric escape pretty hard to study.
"We were surprised to see these metal ions in the Martian atmosphere," says Benna. That's because Mars doesn't have Earth's strong, planet-wide electromagnetic field---which protects the atmosphere from being stripped away by the solar wind. Mars used to have a magnetosphere, but the planet lost its dynamo as it cooled. Scientists didn't think metal ions would be likely to stick around.
In fact, Maven wasn't even looking for them at first. But when the spacecraft---which is in an unusually low orbit, and carries an ultra-sensitive neutral gas and ion mass spectrometer---picked up some metal ions after the comet Siding Spring grazed Mars (at a distance of 82,000 miles) in 2014, the team decided to make keep Maven's eye out for more. After two years, they've realized the ions are a permanent fixture, but behave totally differently than those in Earth's atmosphere.
On Earth, the electromagnetic field and ionospheric winds sort upper atmosphere metal ions into distinct layers. But on Mars, that only happens near super magnetic patches of the planet's crust. "In most of Mars' atmosphere, the ions are all mixed up together, and that's a surprise," says David Brain, Maven co-investigator and atmospheric scientist at the University of Colorado at Boulder. According to Brain, whatever (still mysterious) process keeping all the ions snuggled up together might affect how atmospheric particles leave the upper atmosphere in ways scientists haven't yet understood. It'll take studying these metal ions further to see if that's true.
But these ions have implications beyond Mars, too. "We should now be able to find the physics of how this works and apply it to other planets," Benna says. "Most planets with thick atmospheres should have these ions." So this is less the key to Mars' atmospheric woes and more a tool to compare and contrast different kinds of atmospheres in order to better understand all of them---including Earth's.
