For about a century, scientists and astronomers have been searching for evidence of life beyond Earth using indirect means. For the past sixty years, we have been able to look for it using direct means, using robotic spacecraft to search for biosignatures throughout the Solar System.
And while our efforts have been unsuccessful so far, we can take comfort in knowing that we have barely have scratched the surface. And it's entirely possible that we have been looking in the wrong places. As terrestrial creatures, we can be forgiven for thinking that life is likely to exist on rocky planets with plenty of water.
But as scientists have begun to suspect since the 1970s, the best bet for finding life in our Solar System may actually be beneath the surface of some its many icy moons.
What are "Ocean Worlds"?
By definition, ocean worlds are bodies that have abundant sources of water. Given that 71% of our planet's surface is covered in water, Earth is a good example of an "ocean world". Interestingly enough, Mars and Venus also had oceans on their surface as well, but these were lost as the planets underwent significant changes in their climate.
Because water is essential to life as we know it, ocean worlds like Earth have long been thought to be very rare and precious. But from the 1970s onward, robotic missions have revealed that oceans may also exist beneath the surfaces of icy moons in the outer Solar System. The first to be discovered were Jupiter's largest moons, which are also known as its Galilean moons (after their founder, Galileo Galilee).
Combined with a rich chemical environment that contains elements essential to life (oxygen, carbon, phosphorous, methane, etc.) and internal heating mechanisms, scientists began to speculate that these bodies could support life. In the past few decades, proposals have been made to send robotic missions to these bodies to search for signs of life ("biosignatures").
In 2004, NASA founded the Outer Planets Assessment Group (OPAG), which was charged with identifying scientific priorities and pathways for exploration in the outer Solar System. By 2016, OPAG founded the Roadmaps to Ocean Worlds (ROW) group, which was tasked with laying the groundwork for a mission to explore “ocean worlds” in search for life.
The objectives and of the ROW were summarized in a presentation titled “Exploration Pathways for Europa after initial In-Situ Analyses for Biosignatures", which was delivered at the “Planetary Science Vision 2050 Workshop” at NASA HQ in Washington, DC.
These objectives were published online in a 2019 study titled "The NASA Roadmap to Ocean Worlds" which was led by Amanda Hendrix of the Planetary Science Institute and Terry Hurford of NASA Goddard Space Flight Center. As they stated:
“For the purposes of ROW (Roadmap to Ocean Worlds), and to bound the extent of a future Ocean Worlds program, we define an “ocean world” as a body with a current liquid ocean (not necessarily global). All bodies in our solar system that plausibly can have or are known to have an ocean will be considered as part of this document. The Earth is a well-studied ocean world that can be used as a reference (“ground truth”) and point of comparison.”
Ocean Worlds in our Solar System:
At present, NASA has determined that there could be as many as nine ocean worlds within the Solar System, thought it is possible that there could be even more. They include Ceres, Europa, Ganymede, Callisto, Enceladus, Dione, Titan, Triton, Pluto - a combination of icy moons and icy minor planets.
These worlds are all believed to possess interior oceans which exist between the surface ice and the core-mantle boundary. An interesting feature about these worlds is just how much of them consist of water and ice. On Earth, water accounts for only 1% of the planet's total diameter. But on these moons and minor planets, it ranges from between 55% (Europa) to 90% (Ganymede and Enceladus)
In addition, on Earth, the deepest part of the ocean is located in the western Pacific; specifically, a region known as the Challenger Deep. This region is located at the southern end of the Mariana Trench, and is approximately 11,000 m (36,200 ft) deep. Compare that to oceans that can get up to around 100 km (62 mi) in depth, with more salt water than all of Earth's oceans combined.
How much more? Consider Europa, which is at the lower end of the scale. Its ice layers and ocean water have an estimated volume of about three quadrillion cubic kilometers (3 × 10^18 m³), which is slightly more than twice the combined volume of all of Earth’s oceans. At the other end of the scale is Ganymede, which has an estimated volume of ice and water that is 39 times as much as Earth.
Aside from water, these worlds have also been found to possess volatile compounds (i.e. carbon dioxide, methane, ammonia), biological molecules, and internal heating caused by geothermal activity or the decay of radioactive elements. This combination of water, biological molecules, and energy make these moons possible candidates in the search for extra-terrestrial life.
The dwarf planet Ceres is the largest object in the Main Asteroid Belt, as well as the largest object between Mars and Jupiter. In fact, when it was discovered by Giuseppe Piazzi in 1801, it was the first member of the Asteroid Belt to be observed. For the next two centuries, it would continue to be referred to as an "asteroid".
However, with the Great Planet Debate of the early 2000s, Ceres found itself being reclassified. Like Pluto and other spherical bodies that have not cleared their orbits, Ceres became designated as a "dwarf planet" (or minor planet), in accordance with the resolution passed during the 26th General Assembly of the International Astronomical Union (IAU).
Based on its size and density, Ceres believed to be differentiated between a core composed of silicate minerals and metals and a mantle composed of ice. In addition, there are multiple lines of evidence that support the existence of a liquid water ocean in Cere's interior, which would be located at the core-mantle boundary.
For instance, scientists have detected significant amounts of hydroxide ions near Cere's north pole, which could be the product of water vapor being chemically disassociated by ultraviolet solar radiation. Several sources of water vapor have also been detected around the mid-latitudes.
These may be the result of surface ice that has sublimated due to comet impacts, or of cryovolcanic eruptions resulting from internal heat and subsurface pressurization.
In addition, infrared data on the surface has indicated the presence of sodium carbonate and smaller amounts of ammonium chloride or ammonium bicarbonate. These materials may have originated from the crystallization of brines that reached the surface from below.
The presence of ammonia, a natural antifreeze that Ceres is known to have, could be how this interior ocean remains in a liquid state. It is estimated to be 100 km (62 mi) deep, and could contain as much as 200 million km³ (48 mi³) of water. This is almost three times as much fresh water that exists on Earth - 35 million km³ (8.4 million mi³).
The odds that this body could support life in its interior? Unclear at this time, but worth a check!
The outermost of Jupiter's Galilean Moons, Callisto is also believed to harbor an ocean in its interior. Like Ceres, this ocean is believed to exist as a result of there being sufficient amounts of ammonia in the interior, as well as the possible presence of radioactive elements whose decay provides the necessary heat.
The existence of this ocean is hypothesized based on the fact that Jupiter's powerful magnetic field does not penetrate beyond the Callisto's surface. This suggests that there is a layer of highly conductive fluid beneath the icy sheet that is at least 10 km (6.2 mi) in depth. However, allowing for sufficient amounts of ammonia, it could be up to 250 - 300 km (155 - 185 mi) deep.
If true, this would mean that Callisto is approximately equal parts rocky material and water ice, which ice constituting about 49-55% of the moon and water ice with volatiles (like ammonia) constituting 25-50% of its overall surface mass. Beneath this hypothetical ocean, Callisto’s interior appears to be composed of compressed rocks and ices, with the amount of rock increasing with depth.
This means that Callisto is only partially differentiated, with a small silicate core no larger than 600 km (370 mi) surrounded by a mix of ice and rock. Not a great bet for finding life, but a mission to explore the interior ocean would be invaluable nonetheless!
Europa is the moon that started it all! For decades, the scientific consensus has been that beneath the surface of this a Jovian (and Galilean) moon lies a liquid water ocean, most likely located at the core-mantle boundary. The mechanism for this is believed to be tidal flexing, where Jupiter's powerful gravitational field causes Europa's rocky, metallic core to experience geothermal activity.
This activity could lead to the formation of hydrothermal vents on the ocean floor, where heat and minerals from the interior are injected into the ocean. On Earth, such vents are believed to be where the earliest life existed (which is indicated by fossilized bacteria that are dated to ca. 4.28 billion years ago).
In a similar vein, hydrothermal vents on Europa could give rise to similar lifeforms such as extreme bacteria and possibly even more complex lifeforms.
The existence of this interior ocean is supported by multiple lines of evidence gathered by a variety of robotic missions. These include geological models that anticipate tidal flexing in the interior and images taken by probes that revealed "chaos terrain" on Europa, where the terrain is crisscrossed by bands and ridges and is remarkably smooth.
There is also the way that periodic plumes of water have been observing breaching the surface of Europa and reaching up to 200 km (120 mi) in height - over 20 times the height of Mt. Everest! These appear when Europa is at its farthest point from Jupiter (periapsis) and are caused by tidal forces.
Using this data, scientists have developed a series of models to describe Europa's interior environment. Each of these has implications for the possible existence of life and our ability to find evidence of it on the surface.
In the “thin-ice model", the ice shell is only a few km thick - 200 m (650 ft) in some places - and contact between the subsurface and surface is a regular feature. This contact would be responsible for producing Europa's famous "chaos terrain", which are thought to be thin sections of ice sitting atop vast lakes of water.
In the “thick-ice model”, which is more favored, contact between the ocean and surface are rare and only takes place beneath open ridges. Between these two models, scientists estimate that Europa's crust is anywhere between 10–30 km (6–19 mi) thick, while its liquid ocean extends to a depth of about 100 km (60 mi).
Because of this combination of liquid water, organic molecules and chemistry, and internal heating, Europa is considered to be one of the best candidates for finding life beyond Earth.
Another Jovian moon, also one of the Galileans, is Ganymede, which sets the record for being watery! Another thing that sets this moon apart is the intrinsic magnetic field - which is something no other moon (or rocky planet) possesses - and an atmosphere that experiences aurorae.
Like Europa, this moon is thought to have a core composed of metal and silicate minerals, which flexes due to interaction with Jupiter's gravity to create internal heating. This heat is what allows for a liquid water ocean located at the core-mantle boundary.
All told, Ganymede is believed to consist of equal parts rocky material and water ice, with water accounting for 46–50% of the moon’s mass, and 50-90% of the surface's mass.
In addition to other lines of evidence, the presence of an ocean inside Ganymede has been confirmed by readings obtained by robotic missions on how Ganymede’s aurora behaves. These aurorae are affected by Ganymede's magnetic field (something no other moon possesses) which is in turn affected by the presence of a large, subsurface ocean composed of salt water.
According to readings taken by robotic probes, the moon's interior is believed to be differentiated between a solid inner core measuring up to 500 km (310 mi) in radius (and composed or iron and nickel) and a liquid iron and iron-sulfide outer core. Convection in this outer core is what is believed to power Ganymede's intrinsic magnetic field.
The outer ice shell is the largest layer of all, measuring an estimated 800 km (497 miles) in radius. If these estimates are accurate, then Ganymede possesses the deepest oceans in the Solar System. As for whether or not these oceans could harbor life, that remains highly speculative.
Here we have a more recent entry to the "Ocean Worlds" club. In 2005, NASA's Cassini mission noted the existence of water jets emanating from this moon's southern hemisphere around a series of features known as the "Tiger Stripes". These stripes correspond to linear depressions in the surface ice, where cryovolcanism forces water through to the surface.
Since that time, scientists have entertained the possibility that Enceladus has a liquid water ocean beneath its icy crust. Based on gravity measurements conducted by the Cassini mission, scientists estimate that it extends to a depth of about 10 km (6.2 mi) beneath the surface and that the surface plumes extend all the way to it.
Analysis of the plumes indicated that they are capable of dispensing 250 kg (lbs) of water vapor every second at speeds of up to 2,189 km/h, which allows them to reach up to 500 km (310 mi) into space. The intensity of these eruptions varies significantly based on changes in Enceladus’s orbit.
When Enceladus is at apoapsis (farthest from Saturn), the fissures through which the eruptions travel are under less pressure, which causes them to open wider. The plumes themselves are believed to originate from subsurface chambers at the core-mantle boundary, where geothermal activity maintains the ocean.
Even more impressive is the fact that spectroscopic analysis revealed the presence of methane and simple hydrocarbons in the plumes, as well as hydrated minerals. These elements are all essential to life as we know it and could indicate that colonies of simple lifeforms exist in Enceladus' interior.
Saturn's largest moon is renowned for having a methane cycle that is very similar to Earth's water cycle - where methane exists on the surface as lakes, evaporates to form clouds, and returns to the surface in the form of hydrocarbon rains. All told, Titan contains more hydrocarbons in its atmosphere and on its surface than all of Earth’s oil deposits combined.
At the same time, Titan also has also been found to have prebiotic conditions and organic chemistry on its surface, which could be indicative of life. On top of that, Titan could have an ocean of liquid water beneath its surface that could also support life. Much like Callisto, Titan's interior is believed to be differentiated and composed of equal parts water ice and rocky material/metals.
At the center is a 3,400 km (~2100 mi) core of hydrous rocky material surrounded by layers composed of different forms of crystalized ice and deeper levels of high-pressure ice. Above this resides a liquid ocean up to 200 km (125 mi) thick and made up of water and ammonia, which would allow the water to remain in a liquid state even where temperatures are below freezing.
As with other "Ocean Worlds", the existence of this subsurface ocean is supported by multiple lines of evidence. This includes the fact that the moon's surface is very smooth and young where most features dated to between 100 million to 1 billion years old, an indication of geological activity that renews the surface.
Another indicator is evidence of cryovolcanism, which could be responsible for some of the atmospheric methane. Since the amount of liquid methane on the surface is deemed insufficient for the gaseous concentrations in Titan's hazy atmosphere, an interior source is also thought to play a role.
The case for life on Titan remains highly speculative and would involve extreme lifeforms that are very exotic by Earth standards. Nevertheless, laboratory simulations have led to the idea that there is enough organic material on Titan to start a chemical evolution analogous to what is thought to have started life on Earth.
This moon of Saturn was first studied by the Voyager 1 and 2 space probes as they passed through the Saturn system in 1980 and 1981. It was further studied by the Cassini mission, which conducted five flybys of the moon between 2005 and 2015.
What these missions revealed was a satellite with smooth terrain, which is seen as an indication of endogenic resurfacing and renewal. Combined with models constructed by NASA scientists, it is believed that Dione’s core experiences tidal heating that increases as it gets closer in its orbit to Saturn. This may mean that Dione has a liquid water ocean at its core-mantle boundary.
Neptune's largest moon has long remained a source mystery to scientists. Roughly 55% of Triton’s surface is covered with frozen nitrogen, whereas water ice comprises 15–35% while carbon dioxide ice (aka. "dry ice") forms the remaining 10–20%. Trace amounts of key volatiles have also been discovered in the crust, which includes methane and small amounts of ammonia.
Density measurements suggest that Triton's interior is differentiated between a solid core made of rocky material and metals and a mantle and crust composed of ice. It is theorized that if there are enough radioactive elements in the interior, it could provide enough energy to power convection in the mantle, which may be enough to maintain a subsurface ocean.
The presence of volatile elements further boosts this possibility, and if enough heat is provided from the core, it could be that life may exist in this interior ocean.
Based on data obtained by NASA's New Horizon mission, scientists now believe that Pluto’s internal structure could be differentiated between a core of rocky material and metal that measures about 1700 km in diameter (70% of the planet), which is surrounded by a mantle of ice composed of water, nitrogen and other volatiles.
Once again, the presence of enough radioactive elements in the core could mean that Pluto's interior is warm enough to maintain an interior ocean. As with other Ocean Worlds, this would be located at the core-mantle boundary and is estimated to be 100 to 180 km (62 to 112 mi) thick.
All of the suspects Ocean Worlds of the Solar System have all been explored in the past. Some have been explored more extensively by multiple robotic missions over the course of the past decades. Others, meanwhile, have been explored very rarely or only recently.
The exploration of Europa and other Jovian moons began with NASA's Pioneer 10 and 11 spacecraft, which conducted flybys of the Jupiter system in 1973 and 1974, respectively. These provided the first closeup photos of Europa and other Jovian moons, but in low resolution.
The two Voyager probes followed, traveling through the Jovian system in 1979 and providing more detailed images of Europa’s icy surface. These images revealed Europa's "chaos terrain" features, which triggered speculation that the moon might harbor an interior ocean. Geophysical models that looked at Jupiter's gravitational force on the moon and the resulting tidal flexing supported this interpretation.
Between 1995 and 2003, NASA's Galileo probe orbited Jupiter and provided the most detailed examing of the Galilean moons, which included numerous flybys of Europa. It was this mission that was responsible for detecting Europa's weak magnetic moment, which indicated that a layer of highly-electrically conductive material exists in Europa's interior. The most plausible explanation for this was a large subsurface ocean of liquid saltwater.
In 1979, the Pioneer 11 pass through the Saturn system and measured Titan’s mass and atmosphere. In 1980 and 1981 (respectively), Voyager 1 and 2 conducted a more detailed study of Titan's atmosphere and revealed light and dark features on its surface (which would later come to be known as the Xanadu and Shangri-la regions).
Between 2004 and 2017, the Cassini-Huygens mission would provide the most detailed and comprehensive look at Saturn and its system of moons. It was the first robotic mission to observe plumes on Enceladus in 2005, which mission scientists concluded were an indication of an interior ocean and also what was responsible for replenishing Saturn’s E-Ring with icy particles.
The Cassini orbiter also conducted multiple flybys of Titan and took the highest-resolution images ever of Titan’s surface. This allowed scientists to discern patches of light and dark terrain that were Xanadu and Shangri-La features, detect abundant sources of liquid in the northern polar region, in the form of methane lakes and seas.
The European Space Agency's (ESA) Huygens lander touched down on the surface on January 14th, 2005, which made Titan the most distant body from Earth to ever have a robotic mission land on it. While the lander was only able to transmit for 90 minutes, the data is sent back revealed a great deal about Titan's surface.
This included evidence that many of Titan's surface features appear to have been formed by fluids at some point in the past. The lander also provided information about the region it landed in, just off the easternmost tip of the bright region called Adiri. This included the “highlands” that are believed to be composed mainly of water ice and dark organic compounds.
These compounds are created in the upper atmosphere and may come down from Titan’s atmosphere with methane rain and become deposited on the plains over time. The lander also obtained photographs of a dark plain covered in small rocks and pebbles (composed of water ice) that showed additional evidence of possible fluvial activity (liquid erosion).
Only a handful of missions have explored the other Ocean Worlds of the Solar System. These include the Voyager 2 probe, which conducted a flyby of Triton in 1989 as part of its tour of Uranus, Neptune and the outer Solar System. During this flyby, Voyager 2 gathered data that revealed a great deal about the moon's surface and composition, which is still being studied today.
Between 2015 and 2018, Ceres was investigated by NASA's Dawn mission. This orbiter became the first mission to visit a dwarf planet and go into orbit around two destinations beyond Earth - Ceres and Vesta, the second-largest object in the Main Asteroid Belt. In addition to finding evidence of a possible interior ocean, the Dawn mission confirmed that a liquid ocean may have once covered much of Ceres' surface.
Last, but not least, is Pluto, which was visited for the first time in history in 2015 by the New Horizons mission. This mission provided the first clear images of Pluto's surface, revealing things about its surface features, geological history, composition, atmosphere, and hinting at its internal processes.
Future Exploration Missions:
For obvious reasons, multiple missions have been proposed to explore the Solar System's Ocean Worlds over time. Looking to the future, a number of these concepts are either in development or are approaching realization. In addition, next-generation missions that will push the boundaries of space exploration are also expected to play a role in the study of Ocean Worlds.
In 2011, a robotic mission to Europa was recommended as part of the U.S. Planetary Science Decadal Survey, a report that was requested by NASA and the National Science Foundation (NSF) to review the status of planetary science and propose missions that would advance their exploration goals between the years of 2013 and 2022.
In response, NASA commissioned a series of studies to research the possibility of Europa lander in 2012, along with concepts for a spacecraft that could conduct a flyby of Europa and one that would study the moon from orbit. Whereas the orbiter proposal would concentrate on the “ocean” science, the multiple-flyby proposal would concentrate on questions related to Europa's internal chemistry and energy.
In July 2013, NASA’s Jet Propulsion Laboratory and Applied Physics Laboratory presented an updated concept for a flyby Europa mission (called the Europa Clipper). In addition to exploring Europa to investigate its habitability, the Clipper mission would be charged with selecting sites for a future lander. It will not orbit Europa, but instead orbit Jupiter and conduct 45 low-altitude flybys of Europa.
On January 13th, 2014, the House Appropriations Committee announced a new bipartisan bill that included $80 million worth of funding to continue the Europa mission concept studies. In May 2015, NASA officially announced that it had accepted the Europa Clipper mission proposal, which would launch sometime in the 2020s.
They also revealed that this mission would rely on a suite of instruments that would include an ice-penetrating radar, a short-wave infrared spectrometer, a topographical imager, and an ion- and neutral-mass spectrometer.
In 2012, the European Space Agency (ESA) announced that they had selected the JUpiter ICy moon Explorer (JUICE) mission concept, as part of the agency's Cosmic Vision 2015-2025 program. This mission will launch in 2022 and arrive at Jupiter in 2029, where it will spend at least three years conducting detailed observations of Jupiter and the moons of Europa, Ganymede and Callisto.
The mission would conduct several flybys of Europa and Callisto, but would ultimately be more focused on Ganymede. This will be done using a suite that includes cameras, spectrometers, a laser altimeter, an ice-penetrating radar instrument, a magnetometer, plasma and particle monitors, and radio science hardware.
NASA has also made plans in recent years for a Europa Lander, a robotic vehicle that would be similar to the Viking 1 and 2 missions that explored Mars in the 1970s using an orbit and lander combination. The mission would also rely on technologies tested by the Mars Pathfinder, Spirit, Opportunity and Curiosity rovers, particularly those designed to look for signs of past life (aka. "biosignatures").
Like its predecessors, the Europa Lander would investigate Europa’s habitability and assess its astrobiological potential by confirming once and for all the existence of a subsurface ocean. It would also rely on a suite of instruments to determine the characteristics of water within and below Europa’s icy shell.
But of course, the greatest objective of this mission would be to look for evidence of life that could have made its way to the surface. For this reason, the regions where Europa experiences plume activity would be an ideal spot to land in.
While no date has been specified yet for when such a mission would launch or arrive at Europa, the mission is considered of vital importance to future exploration. In all likelihood, it would follow in the wake of the Europa Clipper mission, landing at a site selected by the orbiter.
Titan Mare Explorer/Submarine:
NASA and the astronomical community have also considered a mission to explore the methane lakes of Titan (particularly the largest lakes of Kraken and Ligeia Mare) for signs of possible aquatic life. One concept is the proposal known as the Titan Mare Explorer (TiME), a concept under consideration by NASA in conjunction with Lockheed Martin.
This mission would involve a low-cost lander splashing down in a lake in Titan’s northern hemisphere and floating on the surface of the lake for 3 to 6 months. This proposal was overruled in 2012 in favor of the lower-cost Mars InSight lander instead, which reached Mars in 2018.
Another proposal for exploring the methane seas on Titan is the Titan Submarine, a concept being explored by NASA Glenn Research Center in conjunction with researchers from Washington State University. The plan is to send this vehicle to Titan within the next 20 years, which will then explore lakes like Kraken Mare autonomously for possible evidence of life.
Titan Aerial Drones:
Multiple proposals have also been made to explore Titan's atmosphere using aerial platforms or a combination balloon and a lander. These include the Aerial Vehicle for In-situ and Airborne Titan Reconnaissance (AVIATR), which was designed by Dr. Jason Barnes and a team of researchers from the University of Idaho.
This drone would take high-definition images of Titan's surface to shed additional light on the geology. At the end of the mission, the plane will would attempt a landing on Titan's dunes in order to gather more information on these curious features as well.
There's also the Titan Saturn System Mission (TSSM), a joint NASA/ESA proposal for the exploration of Saturn’s moons. This concept envisions a hot-air balloon floating in Titan’s atmosphere and conducting research for a period of about six months.
As an Outer Planets Flagship Mission concept, the design of the TSSM consists of three elements – a NASA orbiter, an ESA-designed lander to explore Titan’s lakes, and an ESA-designed balloon to explore its atmosphere. Unfortunately, this concept lost out to the Europa Jupiter System Mission (EJSM) in 2009.
Most recently, a radical proposal was made by the John Hopkins University Applied Physics Laboratory (JHUAPL), which is known as Dragonfly. This New Frontiers-class mission would involve a dual-quadcopter robotic explorer that is capable of vertical-takeoff and landing (VTOL) and powered by a nuclear reactor.
Such a mission would be capable of exploring Titan’s atmosphere as well as conducting science on the surface, which would include exploring Titan's mysterious dunes and methane lakes.
James Webb Space Telescope:
Now scheduled for launch by 2021, the JWST will be the successor to the Hubble, Spitzer, and Kepler Space Telescope. As the most advanced space telescope to date and using its cutting-edge infrared imaging capabilities, this telescope will have no shortage of scientific objectives.
These will include probing the early Universe, examine distant stars and exoplanets, and also study the planets of the Solar System. It is this latter respect where the study of Ocean Worlds comes into play. When deployed, the JWST will dedicate some of its mission time to studying Europa and Enceladus.
Using its advanced infrared imaging capabilities, it will look for IR signatures on the surface of both moons to discern the location of “hot spots”, which correspond to of plume activity. Spectra obtained on thee plumes will help determine their composition and look for organic molecules and signs of life.
There's something exciting about the prospect of studying the Ocean Worlds that reside within in our cosmic backyard. On the one hand, these worlds may be the most likely place where we will find evidence of life beyond Earth. On the other, the various missions that are intended to explore them directly are all expected to happen within the next few decades.
For example, the JWST is scheduled to study moons like Europa and Enceladus just six months after it is deployed and has commenced its scientific operations. The Europa Clipper mission is scheduled for the mid-2020s, while missions to Titan are expected to happen by the 2030s.
In other words, if there is life locked away beneath the icy crusts of these moons and minor planets, we will be hearing about it within our lifetime!
- NASA - Ocean Worlds
- NASA - Europa Clipper
- NASA - Europa Lander
- ESA - JUpiter ICy moons Explorer (JUICE)
- NASA - Outer Planets Assessment Group (OPAG)
- Astrobiology Magazine - the NASA Roadmap to Ocean Worlds
- Lunar and Planetary Institute - Roadmaps to Ocean Worlds (ROW)
- Woods Hole Oceanographic Institution - Exploring Ocean Worlds