Saturn's small moon Enceladus

As the hunt for alien life continues, top scientists have come to believe that unlike Earth - where one look at its surface makes it easy to determine that life exists on the planet - other planets and the most-prominent moons in our solar system might not have that feature and hence require below-the-surface exploration.

The lush green continents and deep blue oceans visible on our planet's surface would be a dead giveaway that water and life are already here. For astrobiologists like Chris McKay, Earth is the only place that's blooming with signs of life visible from space. If aliens exist somewhere nearby in our cosmic neighbourhood, they're going to be much harder to find than a quick fly by, reports Business Insider.

According to McKay, we need to look underground. "Things are better below the surface," McKay, who's a senior scientist with Nasa's Planetary Systems Branch and investigates where else life could exist in our solar system, told Business Insider. "And so where we really want to go is below the surface."

However, the problem with this option is that it's super expensive and its incredibly difficult. Designing and dispatching a lander to dig deep beneath the surface of a planet, to search for signs of life, is obviously a humongous task. Not impossible though, considering that we have managed to drill and collect samples beneath the surface of the Moon and Mars.

One exception where we would not need to dig and drill is on Saturn's tiny moon Enceladus, reveals McKay. This planet harbours a massive ocean underneath a thick layer of ice on its surface.

Recently, two different teams of scientists found evidence of active volcanoes lining the seafloor of Enceladus.

"Enceladus is most likely to give us an answer soonest," he said. "The reason is Enceladus has a plume coming into space."

In 2005, the Cassini spacecraft flew by Enceladus and spotted plumes of water vapour and other materials gushing out its surface. If there's life in the solar system, the first place we're likely to find it is inside of those plumes, McKay said.

But, for McKay, the excitement of the hunt is not just about discovering whether aliens exist. It's discovering unique alien life that is completely different from life on Earth, which might be quite a bit harder since the building blocks of life are so complex.

"To my mind that's the real question: Not 'is there life on these other worlds' but 'is there a second genesis of life on these other worlds'," McKay told the publication. "That's a subtlety that's not obvious until you think about it."

"A second-genesis of alien life could, in theory, have a completely different biomolecular structure from life we see on Earth," the report states.

ocean on mars
Map showing the ocean that once covered 20% of the surface of Mars Nasa

Scientists continue to debate over the theory that life might not have actually originated on Earth but on Mars and then came to Earth via a meteorite where climatic conditions favoured growth.

That is not a stretch to imagine, researchers say, since Mars was covered with liquid water around the same time that life is believed to have begun on Earth. If we do find evidence of life on Mars and it has the same DNA as us, then it's probably our cousins, McKay said.

If we want to find truly unique alien life, then we'll have to travel farther than next door.

"As we go from Mars to Europa to Enceladus to Titan, as the worlds get farther away from Earth the conditions get less and less like Earth," McKay said. "We're more likely to find life that's not related to us the farther out we go."