The small basketball-size object is mainly made of iron and nickel, scientists say.
![](http://marsrovers.jpl.nasa.gov/gallery/press/opportunity/20050119a/Sol339B_P2581_L456-B352R1_th272.jpg)
Readings from spectrometers on the rover determined that composition. Opportunity used its panoramic camera to take the images used in this approximately true-color composite on the rover's 339th martian day, or sol (Jan. 6, 2005). This composite combines images taken through the panoramic camera's 600-nanometer (red), 530-nanometer (green), and 480-nanometer (blue) filters.