Understanding how the Martian atmosphere has evolved is important in our search for life beyond Earth, and the latest research suggests that the Red Planet's atmosphere is even more fragile than we thought.

According to NASA’s MAVEN (Mars Atmosphere and Volatile Evolution) spacecraft, when comet C/2013 A1 (Siding Spring) had a close encounter with Mars in October 2014, it sent the Martian magnetosphere into disarray, and gases were able to escape into space.

“Comet Siding Spring plunged the magnetic field around Mars into chaos,” said Jared Espley, a MAVEN science team member at NASA’s Goddard Space Flight Center, in a statement. 

“We think the encounter blew away part of Mars’ upper atmosphere, much like a strong solar storm would.”

The magnetic field that surrounds Mars, its magnetosphere, is very weak. It is generated by plasma in the upper atmosphere interacting with the solar winds. Earth’s magnetic field, for comparison, is generated by the internal rotation of molten iron, and it is strong enough to protect us and the atmosphere. 

Comet Siding Spring is also surrounded by a plasma-induced magnetic field. As the nucleus of the comet is heated by the Sun, it releases a large amount of charged particles that create the atmosphere-like coma, which extends to over a million kilometers (more than 600,000 miles) from the tiny 500-meter-wide (1,640-foot) nucleus.

At the closest approach, the comet was just 140,000 kilometers (roughly 87,000 miles) from Mars. The comet’s coma enveloped the entire planet, with the more dense regions nearly reaching all the way to the planet's surface.

^{The comet's magnetosphere enveloped the entire planet. NASA/Univ. of Colorado}

This research, published in Geophysical Research Letters, looked at the consequence of this encounter, highlighting how the magnetosphere was affected. As the comet was approaching the magnetic field began to realign towards it, but as the two objects got closer and closer the interaction was too intense for the flimsy Martian magnetic field. Without a stable magnetosphere, gases was able to flee the outer layers of the atmosphere.

“The main action took place during the comet’s closest approach,” said Espley, “but the planet’s magnetosphere began to feel some effects as soon as it entered the outer edge of the comet’s coma.”

MAVEN has already established that solar storms and solar winds have been responsible for turning Mars into a dead planet, by steadily blowing the atmosphere away.

“With MAVEN, we’re trying to understand how the Sun and solar wind interact with Mars,” said Bruce Jakosky, MAVEN’s principal investigator from the University of Colorado’s Laboratory for Atmospheric and Space Physics in Boulder.

“By looking at how the magnetospheres of the comet and of Mars interact with each other, we’re getting a better understanding of the detailed processes that control each one.”