Detailed images collected over time from Mars orbit are giving scientists more clues to work with as they decipher what’s happening on the active surface.

Data from the High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter, combined with in-situ imagery from one of the surface rovers, have produced at least a theory of why some dunes on the planet are moving and some aren’t.

One-meter-resolution images collected over the Nili Patera region of the planet’s northern hemisphere in June and October 2007 show clear shifts of as much as seven feet in ripples atop a long dune.

But in the southern hemisphere, in the Meridiani Planum area where the Mars Exploration Rover Opportunity has been operating since 2004, scientists have used impact craters to estimate the ripples haven’t moved for at least 100,000 years, and perhaps not for 300,000.

Opportunity may have spotted the reason early in its tenure on the surface — hard spherules dubbed “blueberries” as large at 3 mm across that have weathered out of softer rock and cover the ripples.

“The blueberries appear to form an armoring layer that shields the smaller sand grains beneath them from the wind,” says Matt Golommbek of the Jet Propulsion Laboratory, who presented the theory in a paper at the 41st Lunar and Planetary Sciences Conference in Houston earlier this month.

Simone Silvestro of the International Research School of Planetary Sciences at the G. d’Annunzio University in Italy, who reported on the moving ripples at the conference, says the high-resolution imagery from HiRISE “show Mars as an active world.”

Silvestro says, “The dark dunes in this part of Mars are active in present-day atmospheric conditions.”

Artist's concept of Mars Reconnaissance Orbiter: NASA