When scientists at the University of California, Irvine exposed rodents to doses of high-energy charged particles, the test subjects developed cognitive impairments and dementia -- bad news for Mars-bound astronauts.

Researchers are doing their best to understand the health risks facing tomorrow's pioneers of interplanetary travel. The latest tests results suggest exposure to cosmic rays poses the greatest risks.

"This is not positive news for astronauts deployed on a two-to-three-year round trip to Mars," Charles Limoli, a professor of radiation oncology in UCI's School of Medicine, said in a news release. "The space environment poses unique hazards to astronauts."

Exposure to the particles streaming through outer space can have short- and long-term effects, including anxiety, depression, memory loss and diminished cognitive performance.

The test rodents weren't sent to Mars or floated through outer space. Instead, scientists exposed the rodents to fully ionized oxygen and titanium at NASA's Space Radiation Laboratory, part of New York's Brookhaven National Laboratory.

Brain scans six months after exposure revealed lasting damage, including defects among neural dendrites and spines, key parts of brain synapses. These defects disrupt the transmission of neural signals, preventing normal function. The defects mirror those observed in a previous study, six weeks after exposure.

Tests showed exposed rodents lost their ability to get rid of negative memories, a neural mechanism called "fear extinction."

"Deficits in fear extinction could make you prone to anxiety, which could become problematic over the course of a three-year trip to and from Mars," Limoli explained.

NASA scientists are working on ways to better shield astronauts and their spacecraft from high-energy particles, but there is no perfect material.

"There is really no escaping them," Limoli said.

The research was published this week in the journal Scientific Reports.