Jupiter’s Moon, Europa

Jupiter’s moon, Europa, is one of the top places in our solar system where life might exist. Europa is one of the rare places in our solar system that holds all three requirements for life.

Why Explore Europa: Key Takeaways

• A mission to Europa has a high return on investment of scientific data gathered
• Europa is the sixth largest moon in the solar system, and one of Jupiter’s 79 known moons
• The icy crust is between 20 and 180 million years old, relatively young for the planet’s age
• Beneath the crust of Europa, a global ocean exists 62 miles deep – more than twice the volume of Earth’s ocean
• The atmosphere is thin – so there is a high radiation exposure on the surface
• The temperature is −160 °C / −260 °F at the equator

Possible Life

The presence of water is exciting because it means the planet could possibly harbor life. Also, water is also an extremely important resource for humans in space.

The excitement of a subsurface ocean of liquid water and possible presence of life brings up the question – why do we think there could be life on Europa?

As far as we know, water is one of the requirements for life – as well as a source of energy and specific chemical presence (including carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur). Together, water, energy, and chemistry form the basis for life’s requirements.

Celestial bodies in the solar system often meet two out of three of these requirements. Almost nowhere has all three. Again, Europa is one of the rare places in our solar system that meets all three requirements for life.

If life were to exist within the depths of Europa’s global ocean, it may take a number of forms. To form hypotheses for what life may might look like, we can leverage what we know about the origin of life on our home planet Earth.

If anything lives in Europa’s ocean, single-celled organisms, microorganisms, and bacteria are most likely forms to be found. Given more time to evolve, there could be more complex life forms. On Earth, hydrothermal vents are home to a diverse array of life forms, which could be the same on Europa. If some hydrothermal vents exist at the bottom of Europa’s ocean, this may be a breeding ground for some life form, just like on Earth.

Life could also thrive by clinging onto the surface crust in some way, benefiting by periodic exposure to surface compounds or other non-polar molecules that accumulate near the surface.

Geological Features and Gravitational Impact

Europa’s thick ice sheet crust is beneficial for a number of reasons. For one, it protects the ocean (and any life that may inhabit it) from radiation. In addition, it allows heat from the core of the planet to stay within the, which may help the ocean maintain its liquid property by providing a layer of insulation.

• The force of gravity from Jupiter and its other moons affect Europa by causing tidal flexing, creating a constantly changing surface crust.
• The constant shifting of the large glacier crust displays:
  • geysers
  • cracks
  • craters
  • volcanic activity
• Non-synchronous rotation results in a macro movement in the liquid ocean beneath the surface.

More below on the impact that gravity has on the geological features of Europa.

Non-synchronous rotation: proof of a global ocean

Non-synchronous rotation means that the crust of the planet does not rotate at the same rate as the core. The Galileo probe found evidence of this on Europa due to the mass distribution. This rotational behavior means that there is a decoupling between the interior and exterior of the planet (between the core and the crust).

A fluidic insulation between the core and the crust means that the surface of Europa moves and changes much more than other planets.

According to Nature, because Europa spins faster than it orbits Jupiter, gravity data suggest that there may be an asymmetry in Europa’s interior mass distribution. This means that the surface of the planet is moving differently from the core of the planet. The decoupling between the rotation of the icy surface crust and the rocky core suggests that they may be separated by a layer of liquid – Europa’s global ocean. The Europan ocean is the most plausible hypothesis.

Tidal Flexing: Europa’s Forceful Energy Source

On Earth, the cause of tides is the gravitational interactions between Earth and our moon. This movement and shifting of the liquid interior mass creates tides just like in the oceans of Earth. Large movements of water within the interior of Europa cause the solid crust to crack like an eggshell, giving Europa a crust that constantly changes.

Why exactly does this happen?

Europa orbits Jupiter in an elliptical pattern, meaning that it moves nearer and further from Jupiter throughout one full orbit. When Europa is closer to Jupiter, the force of gravity between the two celestial bodies is stronger. The larger gravitational attraction causes fluidic turbulence in Europa’s liquid interior. This causes Europa to elongate like a bouncing rubber ball making impact with the floor. As Europa moves further from Jupiter, the force of gravity is lower, and the oval shape relaxes back into more of a spherical shape.

This process, called tidal flexing, is similar to how a water balloon behaves haphazardly as it is tossed through the air. Fluid moves – and water does not simply maintain a completely spherical shape.

The constant tidal flexing motion of Europa’s interior causes macro-level friction and pressure, providing a source of heat, allowing its ocean to stay liquid while affecting geological features on the surface.

Europa’s Dynamic Crust

To determine the age of a celestial body, humans observe asteroid and meteor impacts on the surface. The record of craters creates a historical map, allowing us to date the age of planets.

Because Europa’s crust is made of massive global shell of ice, these ice sheets behave similarly to glaciers in that they are moving a little bit each year. The tectonic liveliness of these ice sheets effectively erases meteor craters and other surface impacts within about a hundred million years. This means that no Europan surface features last much longer. On a geological scale of billions of years (Earth for example 4.6 billion years old), a hundred million years is a relatively short amount of time.

On a shorter time-scale, the behavior of these large glacier like ice sheets surrounding the planet is not dissimilar from the plate tectonics on Earth. The ice sheets are constantly moving and shifting, and volcanic activity occurs from within cracks and pores in the surface just like we have volcanoes on Earth.

Because of its constant shift and changing ice crust, Europa is the smoothest surface of any other object in our solar system. There are no real massive mountains, nor are there big canyons like on Earth.

Again, tidal shifting of the ocean, drives this, and the smoothness is more proof that a water ocean exists beneath the crust.

Europa’s Chemistry

Aside from the water ice crust and liquid ocean, Europa is composed mostly of silicate rock. This is most similar in core structure to the rocky planets like Mercury, Venus, Earth, and Mars. We believe that it has an iron-nickel core, which is radioactive and produces some amount of internal heat.

There is also evidence for hydrogen peroxide on the surface. This is a significant finding because hydrogen peroxide can react with water to produce an oxygen byproduct. Oxygen, of course, is yet another requirement for life as we know it, and this process could be an explanation for the presence of oxygen in the atmosphere.

Although Europa has an oxygen-based atmosphere, it is very thin and blocks almost no radiation. Based on the data we have, we still know a relatively small amount about the surface of Europa. Future missions to the moon would help us learn about the chemical composition of the surface and interior.

Craters

There are not many craters on Europa because the surface changes too quickly, removing most evidence of surface impacts. Quick, tectonically dynamic changes mean that any surface features relatively young, around 100 millions years old (as opposed to billions on planets with less dynamic surfaces).

One of the few craters that exists is the Pwyll crater, and is thought to be one of the moon’s youngest features, remaining the surface from a surface impact 26 km or 16 miles wide. Below is a picture of the Pwyll crater, taken by the Galileo orbiter.

Cracks

As tidal shifting is constantly a force of change, the slow flowing, shifting, and disruption of the solid icy crust cracks produce incredible streaking lines along the surface.

The image below is a 250 by 200 kilometer close-up photo (also taken by Galileo probe) that shows in detail some of the cracks on the surface of Europa. This image is taken about 1000km to the north of the Pwyll crater.

The reddish areas are associated with more recent internal geological activity.

The way the cracks are aligned in different directions has lead researchers to hypothesize that Europa’s axis of rotation has not been constant over time. At some point in the past, Europa may have spun around a tilted axis.

Geysers

Another unique occurrences that comes with having a global ocean beneath the surface of the moon is volcanic activity – in this case, geysers or plumes.

Hubble space telescope detected large geysers of water vapor from Europa, similar to the ones we know to exist on Saturn’s moon, Enceladus.

The volcano-like plumes of Europa are reach than twenty times as high as mount Everest. Since these periodic events expel a large amount of vapor and compounds high into the atmosphere, this provides an opportunity for future missions to capture samples and more readily analyze the constituents in the search for life.

The advantage of this sampling technique is that we don’t have to land a spacecraft on the surface to get samples. This is much less energy intensive than landing, drill through ice and rock to collect material, and then launching from the surface again. Because of the relative ease with which this sampling process can be done, a mission to Europa has a higher return on investment for the gathering of scientific data compared to other destinations in the solar system.

Missions to Europa

Humans have observed the Europa moon during flybys of space probes since the 70s.

Past Missions

• The first space probe flybys were Pioneer 10 and 11, which captured low resolution images of the icy surface in the 1970s.
• Voyager 1 and 2 have visited destinations never before seen, in addition to Europa. The data sent back show images of the ice cracks and lines on the surface. Launched in 1977, these probes are still actively transmitting data back to Earth after 40 years.
• Galileo orbiter probe orbited Jupiter for 8 years and was able to observe Europa’s surface. Galileo provided significant information about Europa’s icy surface, as well as data supporting evidence of the global ocean. It discovered important evidence for a sub-surface ocean. Overall, the total dollars invested in the probe was 1.39 billion.
• NASA’s JUNO spacecraft captured data about Europa and the other moons of Jupiter in orbit.
• Cassini-Huygens spacecraft flew by Europa on its way to Saturn and Saturn’ moon, Titan.
• New Horizons mission flew by Europa on the way towards Pluto.

Planned Missions

• ESA’s Juiper Icy Moon Explorer, which will also study Ganymede.
• The Europa Clipper will be launched by NASA in 2025.
• So far, we have never landed a spacecraft on Europa, but perhaps we will do so in the not too distant future.

This is part of a series where we discuss various Moons and Planets in our solar system, and why we might want to explore them. See more on Saturn’s moons: Titan and Enceladus.

sources:

https://www.nature.com/articles/34869
https://www.nature.com/articles/s41550-018-0450-z
life possibility on Europa: https://ui.adsabs.harvard.edu/abs/2001EOSTr..82..150S/abstract
chemistry for life on Europa: https://www.nasa.gov/topics/solarsystem/features/europa20130404.html
https://books.google.com/books?id=8GcGRXlmxWsC&pg=PA427#v=onepage&q&f=false
http://www.astronomynotes.com/solarsys/s14.htm
https://europa.nasa.gov/europa/life-ingredients/