Measurements of Mars' spin axis, coupled with similar data from the Viking missions, confirmed suspicions that Mars has a dense core much like Earth. The Mars Pathfinder, top left, is shown in this view from the rover Sojourner released by NASA in July.
Wheel tracks show the rover's progress from Pathfinder.
Mars has clouds. A molten core much like the Earth's center swirls within. Water used to flow along its surface. Long, long ago, Mars may have been much like Earth. NASA scientists this week publish their first results from the Mars Pathfinder mission, elaborating on speculations tossed out at earlier press conferences.