How Mars lost its atmosphere decoded

Early discoveries by NASA's newest Mars orbiter have unveiled key features about the loss of the Red Planet's atmosphere to space over time, researchers say.
The findings are among the first returns from NASA's Mars Atmosphere and Volatile Evolution (MAVEN) mission, which entered its science phase on November 16.
The observations show a new process by which the solar wind can penetrate deep into a planetary atmosphere.
They include the first comprehensive measurements of the composition of Mars' upper atmosphere and electrically charged ionosphere.
The results also offer an unprecedented view of ions as they gain the energy that will lead to their escape from the atmosphere.

"We are beginning to see the links in a chain that begins with solar-driven processes acting on gas in the upper atmosphere and leads to atmospheric loss," said Bruce Jakosky, MAVEN principal investigator with the Laboratory for Atmospheric and Space Physics at the University of Colorado.

"Over the course of the full mission, we'll be able to fill in this picture and really understand the processes by which the atmosphere changed over time," Jakosky said.
On each orbit around Mars, MAVEN dips into the ionosphere - the layer of ions and electrons extending from about 75 to 300 miles above the surface.

This layer serves as a kind of shield around the planet, deflecting the solar wind, an intense stream of hot, high-energy particles from the Sun, researchers said.
Scientists have long thought that measurements of the solar wind could be made only before these particles hit the invisible boundary of the ionosphere.
MAVEN's Solar Wind Ion Analyser, however, has discovered a stream of solar-wind particles that are not deflected but penetrate deep into Mars' upper atmosphere and ionosphere.
New insight into how gases leave the atmosphere is being provided by the spacecraft's Supra-thermal and Thermal Ion Composition (STATIC) instrument.
Within hours after being turned on at Mars, STATIC detected the "polar plume" of ions escaping from Mars. This measurement is important in determining the rate of atmospheric loss.

As the satellite dips down into the atmosphere, STATIC identifies the cold ionosphere at closest approach and subsequently measures the heating of this charged gas to escape velocities as MAVEN rises in altitude.
The energised ions ultimately break free of the planet's gravity as they move along a plume that extends behind Mars.