One story that is unfolding involves the changing thickness of the atmosphere with each Martian day, called a sol, and even with seasonal changes. Those changes impact the amount of radiation – cosmic rays and charged particles from the sun – reaching the surface.
For instance, sensors on the Rover Environmental Monitoring Station (REMS) have detected an unexpected and large day-night shift in atmospheric pressure, corresponding to changes the sun brings to the atmosphere. During the day, the atmosphere heats, expands, and grows less dense as it does so. This reduces the amount of pressure the atmosphere exerts on pressure sensors. At night, when temperatures drop to about minus 130 degrees Fahrenheit, the pressure increases as the atmosphere contracts and grows more dense.
At the same time, the rover's Radiation Assessment Detector (RAD) has found that cosmic and other forms of radiation peak during the day and drop at night as changes to the atmosphere thicken or thin this tenuous shield.
REMS's pressure sensors also are picking up a longer-term shift in pressure as the southern hemisphere moves into spring and summer, notes Claire Newman, a scientist with the Ashima Institute in Pasadena and a member of the REMS science team.
"The entire Martian atmosphere is growing in size," she says, as the south pole ices vaporize. "Each year, the Martian atmosphere shrinks and grows by about 30 percent because a portion of the atmosphere is freezing out to the poles in fall, then vaporizing again in spring. That, of course, is unlike anything we see on Earth."
Teasing out these cycles and their effect on the levels of radiation reaching the surface are important because "radiation is a life-limiting factor for habitability," says Don Hassler, a scientist with the Southwest Research Institute in Boulder, Colo., and the lead investigator for RAD. The information also is important in designing a safe mission for astronauts.