Science payload
Mapping operations began in mid-March 1998 and will last until January 2000. During these two years, transmission rates as fast as 85,333 bits per second will allow Mars Global Surveyor's six main scientific instruments to send nearly 83 gigabytes of data back to Earth. The enormous amount of data, enough to fill over 130 CD-ROMs, will contribute to an extremely comprehensive study of the Martian atmosphere, surface features, mineral distribution, and magnetic properties.
Click here for more information regarding the specific instruments.

(From NASA)