Why is Europa considered by many to be the most likely place in the Solar System to find life?
Europa is one of the few places in the Solar System, aside from Earth, for which there is very strong evidence suggesting a liquid water ocean is present today and is in contact with rock. This is important because life as we know it requires three key basic "ingredients": liquid water, an energy source, and organic compounds to use as the building blocks for biological processes. Europa could have all three of these ingredients. Moreover, and its ocean may have existed for the whole age of the Solar System, giving life a chance the time to begin and evolve there.
Atomic particles trapped in Jupiter's powerful magnetic field crash into Europa's icy surface with great energy, producing chemical compounds that could be used by living things. This process provides a possible energy source that mimics the life-supporting energy of Earth's oxygen atmosphere. Europa's seafloor could be heated by the moon's constant flexing, driving hydrothermal activity similar to undersea volcanoes in Earth's oceans, where some of the most primordial life forms thrive on energy derived from chemical reactions between water and hot rock.
Aside from Earth and Europa, the only other only body in the solar system shows such strong evidence of all three basic "ingredients" is Saturn's tiny moon Enceladus, which also is believed to have a global subsurface ocean.
Why is Europa such an attractive place to search for life, and can we tell from orbit or flybys whether there is life there?
Europa's high-energy particle environment and the probability of a tidally heated subsurface ocean relatively close to the surface suggest that the moon could have all the conditions necessary to support life. From orbit or multiple flybys we can search for molecular signs of life that have been transported from Europa's ocean up to its surface. Such signs of life are called biosignatures, and could include byproducts of metabolism. The science instruments included on a future mission to Europa would be designed to collect the chemical fingerprints of materials on the moon's icy surface, which researchers will examine for biosignatures. Scientists around the world are working together to determine what types of biosignatures to look for and how a spacecraft's instruments could best accomplish the task. However, ultimately a future landed spacecraft would be the most suited to directly search for biosignatures on Europa.
Is there life on Europa?
Given what we know about the ingredients necessary for life, Europa is certainly a very likely place in the Solar System to look for evidence of life. A future mission to Europa would likely seek such evidence by determining whether the moon harbors an ocean of liquid water below its icy surface. Such a mission would examine Europa's surface from orbit or during multiple flybys for the chance that we might be able to identify any chemical signs of life that might have been transported from the ocean.
Why is there an ocean? Why isn't the whole moon frozen?
Tides on Europa, similar to the tidal interactions between Earth and its moon, cause Europa's ice shell and interior to flex during the course of its 85-hour orbit around Jupiter. On Io, such tidal flexing leads to intense heating, driving even more volcanic activity than occurs on Earth. The same source of heating can affect Europa's ice shell and possibly its interior. The heating is not enough to permit liquid water on the surface, but it should be sufficient to maintain a liquid water ocean beneath an outer ice shell.
What is tidal heating?
A moon that travels in an oval-shaped (or elliptical) orbit around its parent planet is repeatedly stretched and squeezed with every orbit as it moves closer to and farther from the parent planet, and this flexing produces heat. An analogy is the heat produced by repeatedly bending a paper clip back and forth. Moon orbits will tend to become less oval-shaped and more circular after a (geologically) short period of time, unless they are forced to be elliptical through gravitational interactions with other satellites. This is the case with Jupiter's moons Io, Europa, and Ganymede today.
Tides happen becasue objects orbiting each other feel more gravity from their partner on their closes side than on the side farthest away. Tidal heating falls off markedly with distance from the parent planet. This helps explain why Io is so volcanically very active today, but Europa is cooler, and Ganymede is least active today. "Warm" ice is much more easily squished than cold ice, so tidal heating concentrates in ice that is already warm; thus, there can be a feedback mechanism by which tidal heating produces warm ice, which in turn produces even more tidal heating.
Why do we think that Europa has a subsurface ocean?
There are several strong pieces of evidence that suggest an ocean exists on Europa:
- The magnetometer aboard the Galileo spacecraft detected signs of an induced magnetic field near Europa's surface, clear evidence for a conductive substance less than about 30 kilometers (about 20 miles) or so below. This is strong evidence for some amount of salty liquid below the surface. But the Galileo spacecraft was not designed specifically to test the hypothesis of a subsurface ocean on Europa. To know for sure whether Europa has an ocean, a spacecraft would need to carefully measure the tidal fluctuations of Europa's surface as it orbits Jupiter. This would require a spacecraft to either to go into orbit around the moon or make repeated flybys with the right geometries.
- Europa's surface features (including bands, ridges, chaos terrain, and multi-ringed impact structures) suggest that there is warm, mobile, glacier-like ice at relatively shallow depths that sometimes has reached the surface. The presence of an ocean would make Europa's frozen surface flex more under its daily tides, cracking and warming the ice, to help explain its strange surface geology. For example, Europa's bizarre curved fractures (called cycloids) probably owe their origin to cracking in response to the flexing of Europa's icy shell on the very rapid time scale of its 3.55-day rotation. Creating large fractures in this way this requires large tides, best facilitated by liquid water. Also, Europa's multi-ringed impact structures suggest that Europa's largest impact scars punched all the way through the ice shell into an ocean.
- Judging by the pattern of the satellite's large-scale fractures, the surface of Europa may have "slipped" relative to its interior (processes called "nonsynchronous rotation" and "polar wander"). A subsurface ocean would greatly facilitate this slipping, allowing the ice shell to slide over the fluid ocean. Such slipping would be much more difficult if the ice shell was in direct contact with rock.
Is Europa's ocean like Earth's oceans?
We don't really know, but we certainly think it is briny, with salts and minerals dissolved in it, like Earth's oceans. This hypothesis is based on theories for planet formation, which predict that Europa's water came from primordial material that is similar to some asteroids. The extraction of water from what is now Europa's rocky mantle would also have extracted minerals and salts from these rocks. Measurements by the Galileo orbiter support the likelihood of a salty ocean, because salt water is the most likely conductive material for generating Europa's induced magnetic field.
How thick are Europa's outer icy shell and its possible ocean?
Theory and observation indicate that Europa's icy shell is around 15 to 25 kilometers (10 to 15 miles) thick, overlying an ocean approximately 60-150 kilometers (40 to 100 miles) deep. Support for this hypothesis comes from observations of pits, domes, and spots on Europa's surface. The size and spacing of the features suggests that they are due to churning within the ice shell, and theory suggests that such churning (called convection) can occur only if the shell is between greater than about 15 kilometers thick. Measurements of the height of domes on Europa (up to a kilometer or just over half a mile high) also suggest that the ice shell must be fairly thick for the domes to be so tall. Some scientists have argued that the ice shell might be thinner, only a few kilometers thick. Using radio signals to track the Galileo spacecraft as it passed by the icy moon provided insight into how material is arranged inside Europa. Those results suggested an outer layer of water and/or ice between 80 and 170 kilometers (about 50 to 100 miles) thick. The rest of the interior is probably made of up of rocky mantle and a metal core.
Do "icebergs" really exist on Europa?
Close-up pictures of Europa's "chaos" region, obtained by the Galileo spacecraft, show blocks or plates of the surface, several kilometers across, which have moved about relative to one another. These have informally been referred to as "icebergs," but this term is a bit too suggestive, invoking visions of ice chunks floating in terrestrial polar seas. On Europa it would be extremely difficult (and perhaps impossible) to melt from a subsurface ocean all the way to the surface. Instead, the chaos areas may represent areas in which Europa's ice shell has partially melted above a large diapir -a blob of relatively warm ice - or an assemblage of diapirs. Understanding how chaos regions formed would be an important objective for a future mission to Europa.
How cold is Europa's surface?
Pretty darned cold. Temperature depends not only on distance from the Sun, but also on how reflective the surface is (a quality referred to as albedo), because bright surfaces are cooler and dark surfaces are warmer. On Europa, temperatures range from as high as about 140 Kelvin (about -210 degrees Fahrenheit) in dark material at the moon's equator to as low as about 50 Kelvin (-370 degrees Fahrenheit) in bright icy patches at the moon's poles.
What causes the surface of Europa to look the way it does?
Europa's fractured appearance results from flexing of the moon's surface as it circles Jupiter on its oval-shaped orbit. As the moon's surface elongates toward Jupiter and then relaxes with each orbit, fractures open at the locations where the stress is highest. The fractures open, close, and slide past each other with each orbit. Some of warm and relatively mobile icy material and/or liquid water may be squeezed to the surface to form ridges. In the past, the surface appears to have pulled open along some cracks and ridges, allowing huge tracts of warmer icy material to well up into the new gap and creating banded patterns. Other features called "chaos" probably formed as slightly warmer blobs of ice and/or water migrated upward within the ice shell, eventually breaking apart the surface.
What causes the differences in brightness and color on Europa's surface?
Most of Europa's surface consists of bright water ice. The moon has reddish non-ice material along cracks and within areas where the surface has been deformed, such as the chaos regions. This reddish substance is associated with briny material. Although brines themselves are colorless, something mixed in is reddish in color, perhaps sulfur. It appears that this material has come from the subsurface, either from the ocean or from pockets of briny material within the ice, and it might redden as a result of exposure to the radiation that affects its surface. Over time, Europa's surface brightens, possibly by radiation damage or by the deposition of frost.
How old is Europa's surface, and how do we know?
Galileo spacecraft imaging shows that Europa's surface is sparsely cratered, meaning that the surface is probably young. In other words, the longer a surface lies exposed to space, the more time it has to be hit by impacts, and the more craters it will display. Moon rocks collected by astronauts during the Apollo missions provide dates for major epochs of cratering and estimates of cratering rates in the Solar System. Simulations of comet and asteroid orbits indicate that it is primarily comets that slam into Europa and the other Galilean satellites. From the modeled and observed numbers of comets near Jupiter today, estimates can be made for the age of Europa's surface. By this method, planetary scientists estimate that Europa's surface has an average age of about 40-90 million years. (The uncertainty in their measurement indicates that the age could be between 20 million years and 180 million years, with around 65 million years being the most likely age). This is extremely young by geological standards, which are often concerned with spans of time hundreds of millions to billions of years long, and suggests that parts of Europa could still be active today.
Has Europa always looked the way it does?
Probably not. The surface may have changed its appearance through time. Features called chaos seem to be the most recent, while those called bands appear to have formed further back in Europa's history. This suggests that the type of activity that deforms Europa's surface has changed over time. Io's vigorous volcanism provides circumstantial evidence that Europa is also active in the present era. It should be noted that Europa's visible surface is only about 65 million years old on average, which is extremely young in geological terms. Because Europa's surface is so young, we can only guess at what it looked like before the most recent period of time.
Is Europa currently active (like Enceladus is)?
The surface of Europa appears to be very fresh and young by geological standards, less than 100 million years old. Because of tidal heating, Europa could have active venting of water vapor, similar to the active vents that spew vapor from the south pole of Enceladus. However, Europa is much more massive than Enceladus (about 400 times more) so its gravity would probably prevent vented material from reaching more than a few tens of kilometers above the surface.
If three of Jupiter's moons - Europa, Ganymede and Callisto - are thought to have oceans, why is Europa most significant?
Europa's ocean is relatively close to the surface, with active geological processes that might be able to transport into the ocean oxygen and other oxidizing chemical compounds (electron acceptors) generated by high-energy particles at the surface, and which could be used by living things. Water at the bottom of Europa's ocean may be in direct contact with warm rock, supplying hydrogen and other reducing chemicals (electron donors). The process, if it occurs, is similar to the energy cycle that supports life on Earth, with surface chemicals replacing direct energy input from the Sun. In contrast, Ganymede's and Callisto's oceans are around 150 km (100 miles) below the surface, and are sandwiched between ice layers. The surfaces of those moons are probably not geologically active today. This means that any chemicals generated on their surfaces that might support life cannot be transported down into their oceans. Moreover, chemical nutrients cannot easily enter from the base of their oceans, since the oceans are not in direct contact with rock.
Why does Ganymede have its own magnetic field? Why doesn't Europa?
Like Earth, Ganymede probably has a molten iron core where convection-up-and-down circulation in a fluid portion of a metal core-generates the icy moon's magnetic field. The Galileo spacecraft did not detect an internally generated magnetic field at Europa, which indicates that if Europa has a metallic core, its core is probably not molten. Even if Europa has an iron core, any melted portion of that core is too small for a dynamo to occur. The Galileo spacecraft did detect a weak magnetic field around Europa, nonetheless-one created within Europa as Jupiter's own powerful magnetic field sweeps past the moon.
Why is the moon called Europa?
Europa and the other three largest moons of Jupiter - Io, Ganymede and Callisto - are named for the lovers of Jupiter in Greek and Roman mythology. When Galileo Galilei discovered these four moons in 1610, he sought to name them the "Medicean Stars" after his wealthy patrons, the Medici family. Several years later, astronomer Simon Marius wrote of the suggestion by his friend Johannes Kepler (a very famous astronomer himself) that the moons be named for Jupiter's mythical loves. The names we use today did not truly catch on for many years, however, and most astronomers referred to them in order of their distance from Jupiter: I, II, III and IV. It wasn't until around 1850, when Saturn's growing family of newly discovered moons complicated the numbering scheme for that planet, that astronomers finally started calling the moons of both giant planets by names of figures in the myths of Jupiter and Saturn.
In the myth, Europa was a beautiful noblewoman who was abducted by Jupiter, the king of the gods. Jupiter appeared to her disguised as a white bull while she was gathering wildflowers by the sea shore. The bull appeared so calm and gentle that Europa placed flowers around its neck and climbed upon its back. At this point, the bull dove into the sea and carried her away to the island of Crete.
When was Europa discovered?
Galileo Galilei was the first person to publish an account of observing Jupiter's four largest moons, including Europa, in 1610. Io, Europa, Ganymede and Callisto are often called the Galilean moons, in honor of the great Italian astronomer. The German astronomer Simon Marius claimed to have observed Jupiter's moons at about the same time as Galileo, and is commonly credit as being the co-discoverer of the moons.
The discovery of these four moons circling Jupiter was an extremely important event in history. It provided evidence supporting the proposal by Polish astronomer Nicolaus Copernicus that all motion in the universe is not centered on our own world, and added support to the view that Earth and the other planets orbit the sun, a shift in thinking called the Copernican Revolution.
What are "convection" and "diapirs"?
Convection is the up and down circulation of material. With icy moons we are concerned with thermal convection, in which warmer (less dense) material rises and cooler (more dense) material sinks. This is the same kind of movement seen in a bubbling pot of thick soup or a liquid motion ("lava") lamp.
In the interiors of icy satellites, ice can be heated by the decay of radioactive material or by tidal activity, whereas the near-surface ice is generally very cold because the surface is far from the Sun. Even though ice is a solid material, it can actually flow slowly like a fluid. Provided that the consistency (or "viscosity") of the ice is not too stiff, warm ice tends to rise, and denser colder ice tends to sink. Warmer and thicker ice layers are more likely to undergo convection. Earth's mantle convects, with warm rock masses moving upward through colder (denser) rock over long periods of geologic time.
A rising or sinking blob of material, such as a blob of warm ice, is called a "diapir" when it pierces and moves through another material, such as a layer of cold ice. A diapir can also form when a lighter material of one composition lies beneath a denser material of different composition, which is known as compositional convection. On Earth, salt commonly rises as diapirs through overlying denser sedimentary rocks, as beneath the Gulf of Mexico or in Iran.
Radiation at Jupiter
Why is there so much radiation at Jupiter?
Radiation at Jupiter is mostly from solar wind particles, atomic particles that stream out of the sun and past the planets. Jupiter's enormous magnetic field traps belts of subatomic particles from the solar wind. These move along magnetic field lines connecting to the poles of Jupiter. Some high-energy particles strip material from the surfaces of Jupiter's satellites, adding to the trapped particles. Volcanic plumes from Io spew sulfur and other atoms into the magnetic field as well, making the space near Jupiter a dangerous mix of different types of radiation.
How do engineers design spacecraft to survive and operate in this environment?
The estimated total radiation dose that a potential future mission's electronics would receive is far more than most ordinary electronics can bear. Therefore, a mission would use parts that are radiation tolerant, along with shielding that would reduce the radiation impacting the electronics. Aluminum, tantalum, and tungsten are the most common shielding materials, and these have been used successfully on other missions such as Galileo.
What are the radiation belts at Jupiter like?
Jupiter's radiation belts are donut-shaped regions that surround the giant planet, associated with its magnetic field, and are filled with trapped atomic particles (electrons and ions) moving with great speed and energy. The tiny particles bounce back and forth within the doughnut-shaped region, creating a continuous "hail" that is collectively referred to as particle radiation. This kind of radiation can be very damaging to ordinary spacecraft electronics, and would do great harm to any living things (like astronauts) that went there unprotected.
Jupiter's most intense radiation belt is a large region of electrons between 143,000 and 787,000 kilometers (89,000 and 489,000 miles) away from Jupiter. Earth has similar radiation belts, called the Van Allen Belts. The ability of the radiation belts to trap charged particles is related to the strength of the planet's magnetic field. Therefore, Jupiter's radiation belts are much more intense than Earth's because Jupiter's magnetic field is about 20 times stronger than Earth's.
What causes the radiation belts?
Ions and electrons are atomic particles that have an electric charge. These charged particles respond to forces from the planet's magnetic field, causing them to travel in an arc toward the planet's north or south magnetic pole. At a certain point along this path, the particles feel a force that causes them to reverse direction and start moving toward the other pole. This happens again at a certain point on their journey in the opposite direction. In this way, the particles are forced to "bounce" back and forth, continually and at high speed, in an arc above the planet - essentially they are trapped in a sort of "magnetic bottle." The bouncing particles also slowly drift around the planet's middle, creating the donut-shaped region that we call a radiation belt.
What robotic missions are planned for exploring Europa?
NASA is studying mission designs that would tackle the most pressing questions about Europa:
- What are the characteristics of its ocean?
- How thick is the icy shell?
- Is there near-surface water within the ice shell?
- What is the global distribution of geological features?
- Is liquid water involved in surface feature formation?
- Is the icy shell warm and convecting?
- What does the red material on the surface tell us about ocean composition?
- How active is Europa today?
- What is the plasma and radiation environment at Europa?
- What is the specific nature of organics in salts at Europa?
NASA’s Europa Clipper mission would provide answers to many of these fundamental questions about Europa. Other missions are also being considered.
The Jupiter Icy Moons Explorer (JUICE) being planned by the European Space Agency (ESA) will address some of these questions, and conduct detailed investigations of Europa's sister moon, Ganymede.
NASA is also studying a potential Europa Lander mission concept, which would characterize the surface of Europa and directly search for biosignatures.
- More on a Potential Europa Lander Mission
Could we send astronauts to Europa?
The travel time and energy required are so large that it will be many decades before we have the technology and vehicles large enough to do the job safely. Even if we develop the ability to send people to Europa, the giant magnetic field of Jupiter traps belts of atomic particles that move at high speed and with great energy. Europa orbits Jupiter within these radiation belts, making it impractical (or perhaps impossible) for astronauts to safely work there without some very advanced form of shielding.
Why does it take so long to get to the Jovian System and Europa?
Jupiter is five times farther from the sun than Earth. It would be possible to get a spacecraft to the Jupiter system in about three years, but in such a case our current rockets would only be able to send a very small probe. Using flybys of the inner planets (Venus, Earth, and/or Mars) in a technique called a gravity assist allows us to send a much larger and more capable spacecraft to Jupiter and Europa. After the last of its gravity assists by an inner planet, a Europa-bound spacecraft would still take about three years to reach Jupiter, just as a much smaller spacecraft would on a direct route from Earth.
What are the planetary protection requirements for Europa?
The principal planetary protection requirement for Europa is that inadvertent contamination of a Europa ocean by terrestrial organisms must be avoided, to a probability level of less than 1 in 10,000.
Implementation of these requirements for missions that plan to fly by, orbit, or land on Europa is typically divided into what needs to be done on Earth before launch and what can be done after the spacecraft launches. The requirements on Earth are that the spacecraft is assembled and tested in a "cleanroom" - an extremely clean environment - and that it the number of microbes on the surface is reduced as much as possible prior to launch. All parts would have to be cleaned and/or sterilized before they are installed in the spacecraft, and the spacecraft assembly facilities would have to be monitored to understand the number and types of microbes that are present. Any problematic microbe species would have to be eradicated prior to launch. The Jupiter radiation environment would also help to eradicate microbes on the surface of the spacecraft.
Planetary Protection requirements for missions that plan to land on or impact Europa also require that an inventory of organic substances and the number and types of microbes found on the spacecraft is made prior to launch.
How would NASA meet the planetary protection requirements for Europa?
NASA calculates the probability of inadvertent contamination by conservatively estimating several factors: the number of microbes at launch; the probability of contaminating organisms surviving the lengthy trip to Europa; the probability of organisms surviving the high radiation field around Jupiter, which also affects Europa; how likely a spacecraft is to land on Europa (considered to be 100% for an orbiter that ultimately impacts the surface or for a lander); the mechanisms for transporting material from the Europa surface to its subsurface, where it might come in contact with water, and; organism survival and proliferation before, during, and after transport to the Europa subsurface.
Will a lander mission eventually be sent to Europa, and what would it do?
A lander mission is a future possibility. A mission to land a robotic probe on Europa's surface might focus on seismology and astrobiology. An instrument called a seismometer would detect Europa quakes and determine whether there is liquid water at depth and how deep that water is (in the likely case that the satellite is seismically active). Certainly it would be important to sample the chemistry of the reddish non-ice material on Europa's surface to test for its compatibility with life and for the presence of any organic materials. Understanding this material probably requires digging or drilling about 10 centimeters (4 inches) below the surface, because the radiation at Europa chemically alters materials on the surface. The possibility of a lander melting all the way through Europa's ice shell to an ocean is probably a very long time in the future.
- More on a Potential Europa Lander Mission