For a long time it was thought that Mars, like Venus, had no magnetosphere. The Russians first made measurements which suggested that there was a magnetic field present.
Scientists found a magnetic field surrounding Mars using an instrument on the Mars Surveyor probe. The magnetic field was found on September 15, 1997, just 4 days after Surveyor plunged into Martian orbit.
The Mars Surveyor discovered the outermost boundary of the magnetic field, known as the bow shock. The Martian magnetic field is weak compared to that of Earth's magnetic field, only 1/800 the strength.