Mars is not a friendly place for humans. Dangers need to be addressed before we can send people to the planet. Cosmic radiation is one of them.

The sun emits a continuous stream of charged particles, consisting mostly of electrons and protons. We call this the solar wind. These particles move through the solar system and past the planets.

The sun occasionally produces a shotgun blast of these particles, called a coronal mass ejection, with higher density, energy and speed of the particles. These events vary in strength. Strong ones that hit the Earth can cause a dramatic increase in aurora displays, and very strong ones can disrupt communications.

Fortunately this increase in solar radiation has a minor effect on our health on Earth. That’s because our planet has a magnetic field surrounding it that directs the particles toward the poles and away from the populated areas.

It is the interaction of the particles with the upper atmosphere that creates the aurora, or Northern Lights. Increases in the density and energy of these coronal mass ejection particles cause the aurora to be brighter and visible farther south.

A very strong solar event occurred on Sept. 11 that was directed, not at the Earth, but at Mars. The Mars Atmosphere and Volatile Evolution (MAVEN) orbiter has been studying the interactions of the solar wind and the Martian atmosphere since 2014, and this event was more than 25 times brighter than any previous one seen.

An aurora on Mars can envelop the entire planet, as it has no strong magnetic field to focus the aurora near the magnetic polar regions. The September storm triggered auroras that bathed the whole planet in ultraviolet light. Sonal Jain, a member of the MAVEN imaging team, said it “lit up Mars like a light bulb.”

The storm also increased the radiation levels on the surface of Mars. The Curiosity rover has been measuring the surface radiation since its landing in 2012. The September event produced levels that were more than double any that had previously been measured. These higher levels lasted for more than two days.

The event was unexpected. The sun has an 11-year sunspot- and storm-activity cycle, with a maximum and minimum in each cycle. We are currently in the minimum part of the cycle, which should be a quiet time for solar activity. Yet this event was big enough to be detected here on Earth, even though Earth is on the opposite side of the sun from Mars.

“The current solar cycle has been an odd one, with less activity than usual during the peak,” Jain remarked, “and now we have this large event as we’re approaching solar minimum.”

Scientists are just starting to analyze the data.

“We expect to get a better understanding of how the process operates in the upper atmosphere of Mars today, and a better understanding of how storms like this may have stripped away much of the Martian atmosphere in the past,” said Bruce Jakosky, a MAVEN principal investigator,

Exposure to cosmic radiation is one of the biggest health risks to Mars explorers. It is important to measure the effects of solar weather, such as this Martian event, before sending any humans to the Red Planet. Significant levels of radiation can reach the surface of Mars from such events, and these events also interact with the atmosphere of Mars to produce secondary particles that would increase the risk to anyone on the surface.

“If you were outdoors on a Mars walk and learned that an event like this was imminent, you would definitely want to take shelter, just as you would if you were on a spacewalk outside the International Space Station,” said Don Hassler, a principal investigator with Curiosity’s radiation assessment detector.

If we ever hope to live on the Red Planet, we first need to know when and how to take cover when these events occur. They will give us some great views — as long as we are safely inside.

Marty Scott is a resident astronomer at Pacific Northwest Regional Observatory. He designs, builds, and maintains astronomical equipment. He also formerly taught astronomy at Walla Walla University. He can be reached at marty.scott@wallawalla.edu.

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