QUESTION:
What kind of modem does the Pathfinder use? I am interested in knowing the
speed the modem works at. The modem manufacturer would be nice to know too.

The ANSWER comes from David Mittman on July 13, 1997:
Specifications:

  Mass: 105.9 grams 
  Dimensions: 8.13 cm (3.2") length by 6.35 cm (2.5") width by 2.3 cm (0.9") height 
  RF Connector Type: Coaxial SMA 
  DC Connector Type: 9 pin Micro-D (signal and power) 
  DC Bus Voltage: +9 Volts, Regulated 
  DC Bus Current: 28 mA Standby; 35 mA Receive; 170 mA Transmit 
  Operating Voltage: +7.5 Volts 
  DC Power: 1.7 W (includes +9V regulator efficiency) 
  RF Center Frequency: 459.7 MHz 
  RF Channel Bandwidth: 25 KHz 
  RF Signal Modulation: DGMSK (Differential Gaussian Minimum Shift Keying), 
        basically FM modulation 
  Handshaking: Half Duplex (Simplex) 
  RF Transmit Power: 100 mW 
  Computer Interface: RS232, TTL levels 
  Maximum Data Rate: 9600 BPS (Bits Per Second) ; Effective :2400 BPS 
  Temperature Range: -30C to +40C (operational), -55C to +60C (storage)

For more information, see 
http://mpfwww.jpl.nasa.gov/rovercom/rovcom.html