There are more than 180 moons in our solar neighborhood. Of the total, few have been more intriguing to scientists than Europa, Jupiter’s icy darling, along with Saturn’s methane wonder Titan, and protein-rich Enceladus.

Through previous missions dating as far back to Voyager One/Two and Cassini, researchers have sought to send probes to explore Europa’s icy, active surface and its Earth-like ocean; study the exotic methane “lakes” of Titan; and learn more about the strange plumes erupting from Enceladus.

In 1984, planetary scientist Jonathan Lunine pitched theories to a joint U.S. and European panel of Titan’s then-undiscovered surface. Over the course of 32 years, Lunine spent much of his scientific career working on research collaborations with scientists in the U.S. and Europe.

“The top two discoveries [in my opinion] are the plumes of Enceladus, and subsequent discoveries via the plume of a habitable ocean within Enceladus. The other is the discovery of the lakes and seas of Titan.”

Currently, all three moons have missions under development—with varying degrees of support. A joint, NASA-ESA proposal to explore Titan was underway with an estimated cost of $2.5 billion in 2009, but was shelved for the recently announced Europa Multiple Flyby mission. NASA’s planned launch date for the Europa-bound mission is slated around 2022. The EMF mission had its prospective budget halved in August 2016 to expand former U.S. President Barak Obama’s initiative to study climate change on Earth.

NASA also sketched a rough mission concept for exploring the biochemistry of Enceladus with the Enceladus Life Finder mission, with a proposed mission readiness for December 2021—although the competing Europa mission is further along, having entered the formulation stage.

Europa

First discovered in 1610, the Galilean moon is slightly smaller than Earth’s closest satellite, with a cracked surface strained by streaks and cracks. The moon has the smoothest surface of any known solid object in our solar system, with a water-ice crust giving way to a deep ocean—a potential haven for life, paradoxically so close to the harsh magnetosphere of Jupiter.

The proposed ocean beneath its surface stays in liquid form due to tidal heating, which drives the ice to act similarly to plate tectonics. Salt from the ocean could coat the diverse moon, prompting scientists to believe the ocean could be interacting with the seafloor.

Proposed life could exist clustered around vents on its ocean floor, or below the seafloor, but it is unclear if the moon could sustain biological processes needed to support life. If the ocean is too cold, Earth-like conditions wouldn’t prosper; and if the ocean is too salty, only extreme variants of life could survive on the icy world.

Minerals similar to clay were recently detected on the crust of Europa. The minerals could be evidence of asteroid or comet collisions, with some  believing life on Earth could have been jettisoned into space by asteroid collisions to the distant moon.

In 2015, NASA selected nine instruments to fly on board the orbiter, and in January 2016, the space agency approved the development of a lander to accompany the mission. The spacecraft would orbit Jupiter and orbit Europa 45 times with altitudes of 16 to 1,700 miles. Varying altitudes would allow scientists to build a “medium-quality” topographic survey, including ice thickness. The lowest altitude would allow the probe to sample the subsurface ocean without having to land on the icy world.

Lunine is the co-investigator in charge of the near-infrared spectrometer for the EMF mission. The search for life on Europa is just one item on the  list of goals in studying moons in greater detail, he said.

“There is a chain of robotic missions well thought out that can tell us whether these moons have life,” Lunine added. “The near infrared spectrometer will look for deposits of organic molecules on the surface that might then be directly analyzed by succeeding landers to look for life. It will also identify salts on the surface.”

Solar cells on the spacecraft must battle the extreme radiation of Jupiter, and the panels would degrade as the mission progresses. All instruments on board would study plasma surrounding the moon, its magnetic field, the satellite’s surface for organics, camera and thermal imaging of the surface and subsurface, and ejected solid particles from the erupting plumes.

The proposed lander would be just over three feet in diameter, weighing over 500 pounds with around 65 pounds of scientific payload. The daring attempt could land near an active crevasse and operate for 10 days before losing battery power.

Titan

Saturn’s only moon to sustain a dense atmosphere, Titan is the only known object other than Earth stable capable of sustaining bodies of liquid on its surface. It is over 50 percent larger than the Moon, and is larger than Mercury, the solar system’s smallest planet.

Made primarily of rock, water ice and methane, Titan’s rich atmosphere shrouded the moon from previous missions observing its surface. In 2005, the Cassini mission discovered vast, liquid hydrocarbon lakes at its poles. Mountains and massive cryovolcanoes have been found on the strange moon, contrasted with vast desert regions.

Its atmosphere is primarily nitrogen, which leads to the formation of methane and ethane clouds giving way to an organic, hazy yellowish smog. With the active atmosphere, forecasts on the distant moon call for chances of high winds and heavy showers—methane showers. The moon is dominated by seasonal weather patterns, with seasons lasting up to 30 years as Titan orbits Saturn. The extreme environment averages a temperature of -290 degrees Fahrenheit, sustaining the proliferation of liquid methane.

“Its upper atmosphere is exposed to energetic particles and ultraviolet light that appear to make many different kinds of polymers, which then rain down on Titan’s surface,” Lunine said.

First proposed by scientists—including Lunine—following the Voyager missions, data confirmed Titan could support liquid hydrocarbon lakes. The Cassini mission confirmed the findings in 2007.

Cassini radar observations show the lakes cover only a small part of its surface, making for a much drier climate than Earth. The depths of the lakes were confirmed to be more than 100 to 660 feet deep, according to Cassini mission data.

“If chemistry in a methane liquid can evolve to build structures of molecules akin to those of life in water, maybe there can be a methane-based life,” Lunine said. “A Cornell research group found that hydrogen cyanide—abundant on Titan—might make polymers and sheets of those polymers that would help to catalyze chemistry. But we have no experience with life in a methane liquid, so for now we can only speculate.”

Ligeia, Titan’s second largest methane sea could fill Lake Michigan in the U.S. three times over with liquid methane. In 2012, Cassini’s radar detected a river at its north pole over 240 miles long, and has been compared to the Nile River due to its connection to Ligeia Mare. Channels of the methane river hold canyons up to 1,870 feet deep, filled with the frigid liquid.

Most astounding of all, Titan’s surface floats as an icy shell, atop a global ocean lying just over 60 miles below its surface. In 2014, NASA reported the methane showers could interact with the icy surface to produce ethane and propane.

Currently no mission has passed the discovery phase in NASA’s exploration timeline, and a proposed mission failed to net funding in 2013 with the Titan Mare Explorer proposal. The Titan Saturn System mission was benched for NASA’s current Europa mission.

NASA claims the mission will continue to be studied for a later launch date of 2029, but no plans have passed the discovery development phase. The mission would consist of an orbiter and two exploration probe: a hot air balloon, studying Titan’s clouds, and a lander to splash down on a designated methane sea. The mission would rely on a prototype generator to supply power, due to solar panels inability to gather energy through Titan’s dense atmosphere.

Enceladus

Of the many moons of Saturn, few prove as intriguing as the small, frigid world of Enceladus.

In 2006, Cassini discovered water plumes venting from its south pole, while its north pole hosts jets of water vapor, solid materials at a rate of 440 pounds per second. Currently, over 100 geysers have been found on its surface.

Some of the jettisoned material makes up Saturn’s E ring, and the plumes have similar compositions to some comets. In 2014, a large subsurface ocean some six miles thick was detected at its south pole—and the moon remains geologically active.

“The evidence for the ocean comes also from the large nodding motion of Enceladus as it moves in its orbit, indicating its ice shell must be decoupled from the deep interior—meaning that there’s a liquid layer in between acting as the lubricant—and also from Cassini gravity data indicating a lens of liquid water beneath the south polar region.”

Enceladus is denser than some of Saturn’s other moons, and contains greater percentages of silicates and iron. Its ocean is considered salty, and the plumes from the ocean indicate the presence of methane, nitrogen, sodium, potassium and silicates.

The diverse range of organics found on the moon could indicate the ocean is similar to Europa’s and even oceans on Earth.

Researchers have been unable to determine the nature of its heat source, estimating the elevated temperatures could be due to radioactive decay of its core, tidal heating and heat-producing chemical reactions. The heating mystery has led scientists to believe the moon could not maintain its subsurface ocean, and the liquid body must be leftover from a period of previous heating and tidal activity.

Liquid water exists on the moon due to the presence of ammonia, acting as an anti-freeze and reducing the freezing point of any liquid water in the pressurized plumes. The surface water ice sits at over -217 degrees Fahrenheit, with a surface temperature of -324 degrees Fahrenheit at noon on the frigid world.

The proposed Enceladus Life Finder mission by NASA is planned to investigate the habitability of its internal ocean, while studying the moon’s active plumes. The spacecraft would be powered by solar panels, carrying two mass spectrometers.

“We carry mass spectrometers because they proved themselves champions on Cassini,” Lunine said. “They can analyze very small amounts of material, are robust, simple and well proven in missions across the solar system.”

The ELF mission would search for key protein building blocks, while searching for fatty acids to determine if methane found in the plumes is produced by tiny living organisms. The current concept calls for 10 flybys over the course of three years through the active plumes.

“String the right combination of amino acids together—hundreds or thousands of them—and the resulting chain folds, and folds again, to make a protein,” Lunine said. “Proteins are the structures of your cells and also are the enzymes the catalysts that make life-sustaining reactions go. If you were to magically render proteins unstable you would immediately cease your life processes, and your body would dissolve into a large puddle of organic-flavored soup with a nice pile of bones in the middle.”

Mr. Jonathan Lunine is a professor of physical sciences at Cornell University. He is the co-investigator on the Juno mission currently orbiting Jupiter, and works with the radar imaging equipment and other instruments on Cassini, currently in orbit around Saturn. He is on a science team for the James Webb Space Telescope, focusing on extrasolar planets and Kuiper Belt objects. He is also the principal investigator on the proposed ELF mission to Enceladus.