Scientists wanted to move further ahead and check up the entire solar system. Like in the present times, probes had been sent to various planets in the solar system.

The habitability of any natural satellites is considered after seeing whether it has the environment which would be hospitable to life. At that time the strongest satellites were the icy satellites of Jupiter called Castillo, Julie, Vance, and Steve. Then there were those of Saturn called Europa, Enceladus, and t.i.tan. But life in most would only exist at the subsurface level. The moons of Mars called Phobos and Deimos. Hence, there were only three natural satellites—the Moon of Earth, and moons of Mars called Phobos and Deimos which were slightly outside the habitable zone

Though even today, life on Earth is considered to be at surface level the fact is that half of the Earth's bioma.s.s lives below the surface. Europa and Enceladus exist outside the circ.u.mstellar habitable zone which has historically defined the limits of life within the Solar System as the zone in which water can exist as a liquid at the surface.

Then detecting Exomoons ( moons outside our solar system) was extremely difficult, it still is. This is because the current methods are limited to transit timing. Some scientists estimate that there are as many habitable exomoons as habitable exoplanets. Given the general planet-to-satellite ma.s.s ratio of 10,000, large Saturn or Jupiter sized gas planets in the habitable zone are thought to be the best candidates to harbor Earth-like moons.

Most astrobiologists consider liquid water to be an essential prerequisite for extraterrestrial life.

A stable orbit is also essential. A moon having an orbital period less than about 45 to 60 days will remain safely bound to a ma.s.sive giant planet.

An atmosphere is important in developing prebiotic chemistry, sustaining life and for surface water to exist. Most natural satellites in the Solar System lack significant atmospheres, the sole exception being the moon of Saturn called t.i.tan.

Then there is the sputtering, a process whereby atoms are ejected from a solid target material due to the bombardment of the target by energetic particles, presents a significant problem for natural satellites. All the gas giants in the Solar System, and likely those orbiting other stars, have magnetospheres with radiation belts potent enough to completely erode an atmosphere of an Earth-like moon in just a few hundred million years. Strong stellar winds can also strip gas atoms from the top of an atmosphere causing them to be lost to s.p.a.ce.

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To support an Earth-like atmosphere for about 4.6 billion years (Earth's current age), a moon with a Mars-like density is estimated to need at least 7% of Earth's ma.s.s. One way to decrease loss from sputtering is for the moon to have a strong magnetic field of its own that can deflect stellar wind and radiation belts. Large moons can have magnetic fields; it has been found on Ganymede which has its own magnetosphere, even though its ma.s.s is only 2.5% of Earth's. Alternatively, the moon's atmosphere may be constantly replenished by gases from subsurface sources, as though by some scientists to be the case with t.i.tan

The effects of tidal acceleration need to be relatively modest on planets, it can be a significant source of energy for natural satellites and an alternative energy source for sustaining life.

Tidal effects could also allow a moon to sustain plate tectonics, which would cause the volcanic activity to regulate the moon's temperature and create a geodynamic effect which would give the satellite a strong magnetic field.

Axial tilt and climate are equally essential. Provided gravitational interaction of a moon with other satellites can be neglected, moons tend to be tidally locked with their planets. In addition to the rotational locking mentioned above, there will also be a process termed 'tilt erosion', which has originally been coined for the tidal erosion of planetary obliquity against a planet's...o...b..t around its host star. The final spin state of a moon then consists of a rotational period equal to its...o...b..tal period around the planet and a rotational axis that is perpendicular to the orbital plane.

The ma.s.s of the moon is not too low compared to the planet, it may, in turn, stabilize the planet's axial tilt, i.e. its obliquity against the orbit around the star. On Earth, the Moon has played an important role in stabilizing the axial tilt of the Earth, thereby reducing the impact of gravitational perturbations from the other planets and ensuring only moderate climate variations throughout the planet. On Mars, however, a planet without significant tidal effects from its relatively low-ma.s.s moons Phobos and Deimos, axial tilt can undergo extreme changes from 13° to 40° on timescales of 5 to 10 million years.

We had explored Europa, Ganymede, and Lo of the planet Jupiter. We also explored Enceladus, Dione, and t.i.tan of the planet Saturn. Finally, we explored Triton of the planet Neptune and Charon of the planet of Pluto.

t.i.tan, the satellite of Saturn was considered the most suitable. Its atmosphere was similar to that of the early Earth, although somewhat thicker. The surface had some lakes and others were hydrocarbon lakes. Some parts of t.i.tan had cryovolcanos, as well as methane rain and snow. t.i.tan is s.h.i.+elded from the solar wind by a magnetosphere, like on Earth.