Covered with water billions of years ago, Mars has long been one of Earth’s most intriguing planetary neighbors, according to astronomers. The red planet is also a favorite destination of science fiction when imagining life outside of our home world.
The more scientists learn about Mars, the more likely it appears that it may have once been habitable, based on geological evidence. In ancient times, the planet was likely much warmer compared with the frozen desert it is today. NASA’s groundbreaking Pathfinder mission, which landed on Mars in 1997, made this discovery.
Since then, a number of missions have ventured to the red planet. These robotic explorers have been following a trail of evidence to understand more about the mysterious Martian past — and determine if life ever existed there.
Mars may be a hotspot in the search for ancient life outside Earth, but future missions to other destinations in our solar system could illuminate someplace else. Ocean worlds, like Saturn's moons Titan and Enceladus and Jupiter's moon Europa, may be the most likely places to find life beyond Earth, NASA revealed in 2017.
Where there is water, life is often present. Is the same true for these icy ocean moons?
Planning for multiple missions is underway to explore these intriguing worlds. The findings could show whether these celestial objects host life within their subsurface oceans.
Jupiter’s moon Europa has a thick shell of ice — which could conceal an ocean beneath its surface. The ice shell is between 10 and 15 miles (15 and 25 kilometers) thick, and it likely sits on top of an ocean that’s estimated to be 40 to 100 miles (60 to 150 kilometers) deep.
Previous missions have spied plumes of water vapor erupting through the ice shell. If these emissions are releasing water into space, there is a chance that a spacecraft could fly through the plumes to learn about the composition of the ocean — but they could be hard to detect.
Europa is so tantalizing because it could contain all of the ingredients necessary for life as we understand it.
The icy moon has twice as much water as Earth’s global ocean as well as some essential chemical building blocks necessary for life, scientists have predicted. It also has energy, like the heat provided by the tug of Jupiter’s gravity on the moon, and has remained pretty stable for about 4 billion years.
Titan is a strange world where life could exist in a way that we don’t yet understand. Saturn’s largest moon is also the only moon in our solar system with bodies of liquid on its surface and a thick atmosphere that’s four times denser than Earth's. This atmosphere is mostly made of nitrogen, along with a little bit of methane.
The moon is also rich in organic material, which falls like snow on the plains due to chemical reactions that occur in the atmosphere. Components of the compounds create Titan’s signature orange haze, which obscures its surface from our view.
Across Titan’s surface are rivers, lakes and seas of methane and ethane, extending for hundreds of miles. Previous observations by the Cassini-Huygens mission, a joint effort of NASA, ESA and the Italian Space Agency, also detected a liquid water ocean beneath Titan’s thick, icy crust. Holes in the ice shell suggest that organics from the surface could be interacting in the water.
There are building blocks of life on Titan that are very similar to what was once present on Earth before life began. Scientists know complex organics are present on Titan, but how far did this prelife chemistry progress?
On our home planet, life has expanded everywhere for billions of years, erasing just where life originated exactly. Titan is different: We could potentially learn how the initial chemistry for life comes into being.
With its striped surface and impressive plumes, Saturn’s tiny bright moon Enceladus is a dynamic place to search for life.
Like Europa, Enceladus is also an icy world with a global ocean beneath its crust, scientists believe. But Europa’s gravity keeps its plumes closer to the surface, while Enceladus’ plumes rise up in massive columns that continuously generate a field of ice particles around the moon and even contribute to one of Saturn’s rings.
Observing Saturn in 2005, the Cassini spacecraft spied plumes of icy water and gas blasting up at 800 miles per hour (1,287.5 kilometers per hour) through warm cracks in the ice crust, called “tiger stripes.”
The detection of molecular hydrogen in one of Enceladus’ plumes was a highlight of Cassini’s closest flyby of the moon in 2015. The molecular hydrogen forms as a result of the interaction between water and rocks when it’s in a hydrothermal environment, scientists believe.
Previous detection of complex organic molecules in the plumes has further suggested that the moon could support life as we know it. Enceladus likely has hydrothermal vents that expel hot mineral-rich water into the subsurface ocean.
Amino acids are the building blocks of life. Organic compounds are the byproduct of the reactions that create amino acids. In Earth’s oceans, vents on the ocean floor create ideal conditions for these reactions to occur. Researchers think this same process could be unfolding on Enceldaus.
Measurements of methane, molecular hydrogen and carbon dioxide in the global ocean show that this body of water has the chemical energy necessary for microbes to produce methane — if there are microbes.
The future of exploration
Visiting these fascinating ocean worlds is just the first step. Concepts in development by scientists at NASA’s Jet Propulsion Laboratory in Pasadena, California, play with the idea of investigating the ice shells and oceans themselves.
The Buoyant Rover for Under-Ice Exploration, or BRUIE, would navigate on its own by rolling along on two wheels, upside down beneath the ice. Buoyancy would help the 3-foot-long (1-meter-long) robot remain anchored to the ice.
The BRUIE prototype has already been tested in Antarctica. Using cameras to collect data and take photos, the rover could autonomously search for signs of life at the boundary between the ice shell and alien ocean of another world.
Another robotic concept is called SPARROW, or the Steam Propelled Autonomous Retrieval Robot for Ocean Worlds. The soccer ball-size robot could use its thrusters to leap across hazardous terrain, like blades of ice and crevasses, on Europa and Enceladus.
A lander would support the robot and supply SPARROW with ice to create steam. The bot could hop around the surface and collect samples — and multiple SPARROWs may be better than one to explore alien environments.
A third prototype would send a flock of smartphone-size robots swimming through ocean moons. These Sensing With Independent Micro-Swimmers, or SWIM, would release from a probe that melts a tunnel through the ice.
Unleashing so many little robots (about 5 inches or 12.7 centimeters long each) into an alien ocean could increase the chance of finding evidence of life while also exploring how habitable these moons are beneath the icy surface. Four dozen of them could fit inside the ice probe, researchers have estimated.
These concepts certainly look like creations from science fiction right now, but they could result in bots that one day hop across icy terrain or swim in extraterrestrial oceans.
Evidence for life outside of Earth hasn’t been found yet, but the very things that make us human — creativity, curiosity and ingenuity — could reveal it in the decades to come.