Simplify the following text for a Grade 10 native English-speaking person.
Original: The most surprising discovery in the Jovian system was the existence of volcanic activity on the moon Io , which had not been observed either from the ground , or by Pioneer 10 or 11 .
Simplified: The most surprising was the existence of volcanic activity on Io , which had not been observed from the ground or by Pioneer 10 or 11 .