In the continued search for life outside of our home planet, scientists have come to the conclusion that water equals life. This is a reasonable conclusion because the diverse life on Earth requires water to survive, including humanity. Without liquid water, it's safe to assume that we would not be here. Currently, we know that Earth is the only planet (or moon) with liquid water on its surface. This is due to Earth being in the habitable zone, which means that it is the right distance away from the Sun in order for liquid water to form on the surface and not immediately evaporate or freeze. However, this does not mean that water cannot exist on other planets or moons within our Solar System.
You may be thinking that if water does not exist in liquid form, then life surely cannot evolve and thrive in such an environment. However, we must look to our own planet and think about other places where live can thrive. This is where extremophiles come into the picture. Extremophiles are organisms that can survive in seemingly inhospitable environments. These can include: deep sea thermal vents, volcanoes, and permafrost.
Environments where extremophiles may be found [Credit: Frontiers in Microbiology]
If extremophiles can exist on Earth, then who's to say that they do not exist on other planets? Or even moons? It is assumed that since the universe is extremely massive in size, that we would have to look thousands or millions of light-years away to find extraterrestrial oceans and signs of life. But what if they are right here in our own planetary neighborhood?
Based on research done in our own Solar System, it has been concluded that there may be several candidates for subsurface oceans. These include: the dwarf planet Ceres, Europa, Enceladus, Titan, and Triton. There may even be a subsurface ocean on Pluto.
List of worlds in the Solar System that contain oceans [Credit: NASA]
Recently, NASA's chief scientist Ellen Stofan stated (in regards to the potential for water being plentiful in our Solar System): “I believe we are going to have strong indications of life beyond Earth in the next decade and definitive evidence in the next 10 to 20 years.” When talking about "life," even microbial life is a monumental discovery for humanity. This is why looking within our own "backyard" is crucial.
Europa:
There is an abundance of evidence that points to Jupiter's moon Europa having a subsurface ocean. Cynthia Phillips, a Europa project staff scientist at the Jet Propulsion
An artist's rendering of Europa's surface [Credit: NASA/JPL-Caltech]
Laboratory stated that: "One clue was the mass of Europa. Combined with when we measure the density, we get a figure close to one [gram per cubic centimeter] and water is the only material like that." Another clue is how Europa interacted with Jupiter's magnetic field. This was able to be measured by the Galileo Probe in the 1990s. Phillips stated that the way we are able to know that there is some liquid on Europa was by observing the probe's magnetometer."As [Europa] goes through Jupiter's field the magnetometer saw deflection exactly where you would expect if there were an induced field" (Space.com). The only way that there would be an induced field would be through salty water. Phillips additionally stated that you cannot get the same readings with isolated "pools" of water or water-ice mixtures.
Additional evidence can be seen through Europa's surface. The surface is covered in water-ice, and the topography is not very tall. The surface also does not appear to be covered in a large amount of impact craters, and seems to be "resurfacing" itself. There are still some questions, like how deep the ice and liquid layers are. While this may sound exciting, it is all just indirect evidence. The only real source of "direct" evidence we have is an observation from Hubble in 2013, when astronomers saw what they believed to be liquid water coming out in a plume from a vent on the surface, but this is not conclusive.
Dione:
With the help of Cassini, it was discovered that a subsurface ocean lies on Saturn's moon, Dione. The moon's surface is also fairly smooth, which means something has been resurfacing it. There is also evidence from Cassini which has hinted at the moon having Surface of Saturn's moon Dione, captured by Cassini [Credit: NASA/JPL-Caltech]
plume activity. One of the moon's mountains may be responsible for these plumes, where they send water gushing out into space. This is called "cryovolcanism" and takes place through a subsurface ocean. There is another instance with Cassini that also points to Dione having an ocean. If a spacecraft is flying over a solid body, the gravity will not have much of an effect on its trajectory. However, if there is a liquid ocean beneath a layer of ice on the surface of that body, then the spacecraft will exhibit small, but noticeable deviations with the pull due to the mass of the liquid. This is what Cassini experienced while flying by Dione, which adds to the previous evidence of the existence of a subsurface ocean.
Ganymede:
Scientists using the Hubble telescope have found evidence for an ocean beneath Ganymede's surface. The craziest part is that this subsurface ocean could potentially hold more water than all of the water on Earth's surface! Scientists estimated that the ocean is about 60 miles thick, which is about 10 times the depth of Earth's oceans (space.com)!
Enceladus:
Prior to Cassini, scientists had already set their sights on Enceladus, one of Saturn's moons. Images from Voyager showed that Enceladus was very smooth in some areas on its surface. It's actually the most reflective body in the Solar System. Data from Cassini's magnetometer prompted scientists to want to take a closer look at the moon. Something was pushing Saturn's magnetic field near Enceladus. "This meant that gases may have been originating from the moon's surface or interior" (NASA). Cassini was able to unravel the truth: Enceladus was an active moon with a hidden ocean of salty liquid water underneath its crust. Another interesting fact about the moon is that there are icy jets released into space from it. "The material shoots out at about 800 miles per hour (400 meters per second) and forms a plume that extends hundreds of miles into space. Some of the material falls back onto Enceladus, and some escapes to form Saturn’s vast E ring" (NASA).
This is evidence for hydrothermal vents underneath Enceladus's crust, similar to the ones found here on Earth. These findings lead to the possibility of Enceladus housing life underneath its icy crust.
Titan:
Cassini was also able to give scientists insight into another one of Saturn's most well-known moons, Titan. It was able to reveal Titan's hidden ocean underneath an icy crust. During Titan's orbit, scientists were able to study the moon's shape during different parts of it. There were points where it was more elongated, and points where it was more round. Scientists used Cassini to measure the bulges of Saturn's pull on Titan. "By studying six close flybys of Titan from Feb. 27, 2006, to Feb. 18, 2011, researchers were able to determine the moon’s internal structure by measuring variations in the gravitational pull of Titan using data returned to NASA’s Deep Space Network (DSN)" (Astrobio.net).
The depth of the ocean does not matter when it comes to the tides created by Saturn. "A liquid layer between the external, deformable shell and a solid mantle would enable Titan to bulge and compress as it orbits Saturn" (Astrobio.net). It is also most likely that Titan's ocean is made of liquid water due to the surface being mostly comprised of water ice.
The presence of a subsurface ocean does not necessarily indicate life. Scientists would need to be able to find a way to observe the bottom of Titan's ocean, depending on its depth. This is because it is believed that when water comes into contact with rock, life is more likely to arise.
Callisto:
Callisto was thought to be a "dead world" by many; however, new data could suggest otherwise. Similar to Europa, another one of Jupiter's moons, Callisto may also have a subsurface ocean. Callisto's magnetic field was the first indicator of something interesting taking place on the moon. It was discovered that the magnetic field fluctuated with Jupiter's rotation. "The best explanation was that Jupiter's powerful magnetic field was creating electrical currents somewhere within Callisto, and those currents in turn created a fluctuating magnetic field around Callisto" (NASA). Currents would be able to flow if Callisto has salty liquid beneath its icy surface, because it would have sufficient electrical currents in order to create a magnetic field.
Further evidence of a subsurface ocean comes from Galileo data, which showed that electrical currents were sometimes flowing in opposite directions, which can be explained by a salty liquid ocean. Life existing in the ocean is an entirely different question and would require further observations.
Pluto:
Last, but certainly not least, we're going to talk about everyone's favorite (dwarf) PLANET, Pluto (no, I'm not salty at all). Based on data from the New Horizons mission in 2015, Pluto may contain a subsurface ocean. Scientists were able to compare geological observations of Pluto during the flyby of New Horizons with models of its origin and evolution. It is now believed that Pluto used to be very hot, which provides explanations for the cracks on its surface, along with many ridges. Additional findings suggest that Pluto and other dwarf objects in the Kuiper Belt may have possessed subsurface oceans ever since their formation! This warm and potentially violent formation of the (dwarf) planet, may have warmed its interior enough to form a subsurface ocean! How can we infer this, you might ask. Well, we know from our own planet that when water freezes it expands, which is how cracks can get larger. Once Pluto's interior cooled off after its formation, its surface was able to stretch and create the recognizable features that we can see today from New Horizons images.
Final Thoughts: The discovery of these subsurface oceans within our own Solar System is exciting, and rapidly changing the way we think about Planetary Science. It shows that liquid water may be more common than we previously thought, although it does not imply that life resides on all of these objects. I'm not sure if we'll be able to figure out if there are extremophiles living in these subsurface oceans, but it could be possible in the near future if we were to make some sort of underwater probe that could penetrate the icy crust and make its way into the oceans of these moons (and dwarf planets) to observe the oceans. It doesn't seem entirely impossible to me. Here's an interesting article that adds to that topic if you want to learn more about NASA's plans to explore these oceans: https://www.spaceflightinsider.com/missions/solar-system/nasa-developing-ai-future-exploration-extraterrestrial-subsurface-oceans/. I highly recommend checking it out if you're interested in this topic! These discoveries really do change the way we think about habitability and the search for life elsewhere. Hopefully they lead to something extraordinary.
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