This suggests to scientist Robert T. Pappalardo of Brown University in Providence, RI, that an ocean may have existed in the past and may still exist as a subsurface ocean. On Earth, abundant life has been discovered near hydrothermal vents deep on the ocean floor. Perhaps, subocean volcanic vents exist on Europa, and biological life may exist there also. Scientists have long wondered if there is life, or once was life, on Mars. These are some of the questions that scientists are studying in our own solar system.
Reconnaissance means to observe an object intently for a short period of time using a flyby mission.
A spacecraft makes one pass by a planet, radios back its data, and continues on into space. Surveillance means to observe an object intently for a long period of time using an orbiting spacecraft. The spacecraft will point its instruments at the planet and record volcanic, geological, or atmospheric activity. Orbiters usually orbit in a north-south axis over the planet's poles so that it can view a different piece of the planet with each rotation. In-depth study is the final phase of data gathering, when you, or your robots, land on the planet and explore a small area in great detail, like the Apollo missions to the moon or the Viking missions to Mars.