A gap left by a particularly strong solar wind caused the Martian atmosphere to burst outwards. The Mars-orbiting MAVEN spacecraft captured a sudden and sharp drop in solar particles in the blowing wind on December 26, 2022, while simultaneously recording a strange and dramatic change in the Martian atmosphere. The planet’s magnetosphere and ionosphere have expanded by thousands of kilometers, more than tripling in size.

We last observed this phenomenon in 1999; The sudden collapse of the solar wind caused Earth’s magnetosphere to bulge outward, increasing its volume by a factor of 100. This is a rare glimpse into how Mars behaves when conditions in the Solar System suddenly change, and could give us some insight into how Solar System-like planets interact with their environments around different types of stars.

It’s also the kind of measurement that can only be done in situ, and shows the value of putting spacecraft into orbits around various worlds in our solar system to see how they interact with everything else.

“When we first saw the data and how dramatic the drop in solar wind was, it was almost unbelievable,” says astronomer Jasper Halekas of the University of Iowa and lead author of a new study on the event. “We formed a working group to examine the event and found that this period was rich in incredible finds.”

The solar wind is more or less constant in the solar system, including all planets. This is a stream of particles constantly hurtling away from the Sun in all directions, creating an ambient pressure throughout the Solar System, weakening as distance increases, and eventually vanishing at a limit known as the heliopause.

However, the strength of this flow can fluctuate depending on solar activity. For example, regions of weakened solar magnetic fields on the Sun’s surface can bombard the system with stronger, larger numbers of solar particles, unleashing a stronger solar wind.

The wind blowing on Mars on December 26, 2022 was different again. It consisted of two solar winds; the slower one was picked up and absorbed by the faster wind coming from behind, and then continued outward as a single superwind.

MAVEN recorded the higher particle density of this binary solar wind. As the wind passed by the orbiting observatory, a significant drop in particle density (a rare gap of extremely weak and low-density solar wind) was detected, along with a drop in solar wind pressure. Density decreased 100 times and pressure decreased 10 times.

The solar wind often pushes against a planet’s magnetosphere; On Earth-like planets, it is mostly formed by the churning of the molten interior. Mars no longer has its own global magnetosphere; but it has something called an induced magnetic field, which is created by magnetic fields embedded in the solar wind mixing with Mars’ ionosphere.

This ionosphere and the induced magnetic field within it expanded greatly outward, just as the Martian atmosphere was loosening a too-tight corset. The induced magnetic field became demagnetized and the layer between it and the solar wind (the main shock) became much calmer than normal.

According to the researchers, this could provide valuable information about how the atmosphere was lost on Mars and similar planets in other parts of the galaxy. This could be an important piece of the puzzle in determining what makes a planet habitable.

“We’re really seeing how Mars responds when the solar wind is effectively cut off,” Halekas says. “This provides an excellent study of what Mars would be like if it orbited a less ‘windy’ star.”