Signs of environmental changes that occurred over billions of years at a Martian crater have been recorded by Opportunity, one of NASA’s two Mars rovers.
Explorations of the rim and interior of Victoria Crater on the Red Planet, from September 2006 through August 2008, resulted in a number of key findings which reinforced and expanded data learned from previous explorations of two nearby smaller craters.
The rover revealed the effects of wind and water. Evidence show water repeatedly came and left billions of years ago. Wind persisted much longer, heaping sand into dunes between ancient water episodes. These activities still shape the landscape today. At Victoria, steep cliffs and gentler alcoves alternate around the edge of a bowl about a half a mile in diameter. The scalloped edge and other features indicate the crater once was smaller than it is today, but wind erosion has widened it gradually.
Instruments on the rover’s arm studied the composition and detailed texture of rocks just outside the crater and exposed layers in one alcove called Duck Bay. Rocks found beside the crater include pieces of a meteorite, which may have been part of the impacting space rock that made the crater.
Other rocks on the rim of the crater apparently were excavated from deep within it when the object hit. These rocks bear a type of iron-rich small spheres, or spherules, that the rover team nicknamed “blueberries” when Opportunity first saw them in 2004. The spherules formed from interaction with water penetrating the rocks. The spherules in rocks deeper in the crater are larger than those in overlying layers, suggesting the action of groundwater was more intense at greater depth.
Opportunity’s first observations showed interaction of volcanic rock with acidic water to produce sulfate salts. Dry sand rich in these salts blew into dunes. Under the influence of water, the dunes hardened to sandstone. Further alteration by water produced the iron-rich spherules, mineral changes, and angular pores left when crystals dissolved away. A rock from space blasted a hole about 2,000 feet wide and 400 feet deep. Wind erosion chewed at the edges of the hole and partially refilled it, increasing the diameter by about 25 percent and reducing the depth by about 40 percent.
The twin rovers Spirit and Opportunity continue to produce scientific results while operating far beyond their design life. The mission, designed to last 90 days, celebrated its fifth anniversary in January. Both rovers show signs of aging but are still capable of exploration and scientific discovery.