Astronauts who travel on future missions to Mars would likely be exposed to their lifetime limit of radiation during the trip, not to mention time spent on the Red Planet, scientists said.
The measurements were made aboard the Mars Science Laboratory, an unmanned Nasa rover and mobile laboratory that set off for Mars in 2011 before landing 253 days later in August, the report in the US journal Science said.
"In terms of accumulated dose, it's like getting a whole-body CT scan once every five or six days," said Dr Cary Zeitlin, a principal scientist in Southwest Research Institute's Space Science and Engineering Division.
"Radiation exposure at the level we measured is right at the edge, or possibly over the edge of what is considered acceptable in terms of career exposure limits defined by Nasa and other space agencies."
Zeitlin said more study is needed to determine the actual health risks - including the likelihood of developing cancer - associated with exposure to cosmic radiation before any human trip to Mars can take place.
The US space agency has said it is aiming for the first-ever astronaut mission to Mars sometime in the 2030s. Until now, only robotic rovers have been able to tour the parched planet.
Previous radiation estimates did not benefit from the latest technology aboard the Mars Science Laboratory, which is fitted with a radiation detector shielded by a spacecraft likely similar to one that would carry humans on the 560-million-kilometer journey to Mars.
Astronauts risk radiation exposure from galactic cosmic rays and solar energetic particles from the sun's flares and coronal mass ejections.
Nasa estimates an outbound flight would take around 180 days, followed by a stint on the planet that could last 500 days, and then the trip home.
"Understanding the radiation environment inside a spacecraft carrying humans to Mars or other deep space destinations is critical for planning future crewed missions," Zeitlin said.