Search for life in moons - Europa Moon

Until recently, Mars was believed to be the most likely planet for life in the solar system after Earth. But after years of observation, the sending of dozens of spacecrafts, and the landing of several robots on its surface, the promise of discovering life on Mars remains elusive. Now the attention of the astronomy world is on Europa, the fourth largest moon of Jupiter, which is a better candidate for finding life than Mars.

In 1972, scientists showed that Europa's surface is more composed of water ice with spectroscopic observations by the Keith Peak National Observatory telescope in Tucson, Arizona. Thermal imaging samples also suggested that Europa's interior could contain a layer of liquid water. In the early 1970s, NASA's Pioneer 10 and 11 spacecraft flew close to Jupiter's orbit, but Voyager 1 and 2 were the first spacecraft to image the surfaces of Jupiter's moons in significant detail.

On March 4, 1979, Voyager 1 made its closest flyby of Jupiter. The spacecraft provided a complete picture of Europa from a distance of 1.2 million miles (2 million kilometers). A few months later, on July 9, 1979, Voyager 2 made its closest flyby of Europa. The images of these two Voyagers show the surface of this moon brighter than the Earth's moon, which is covered with numerous bands and ridges, but it is clear that large impact craters, high rocks, or even mountains are very rare. In other words, Europa has a very flat surface compared to other icy moons.

The researchers also noticed that some of the dark bands have opposite sides that fit together like pieces of a puzzle. These fissures were separated and dark, icy material seemed to flow into the gaping fissures. Voyager images also show only a handful of impact craters, indicating that the surface was active in the past.

We know that the planetary crust in the solar system was constantly bombarded by meteorites for billions of years, so the lack of large impact craters is a sign of the youth of this moon's surface. On the other hand, some events such as the movement of ice streams, volcanoes, and the heavy weight of the ice crust settling on the surface the soil of the moon have caused erosion and fading of craters. Also, Europa's surface has bands, ridges, fractures, and multi-ring impact structures that indicate the presence of mobile materials beneath it. This feature suggests that a mobile layer beneath Europa's crust protects the surface crust and allows it to move.

The intriguing findings led to a strong prediction for NASA's Galileo mission, which launched in 1989 and entered Jupiter's orbit in 1995. Galileo's initial mission consisted of observing each of the four Galilean moons during frequent flybys. The information that Galileo sent about Europe was so interesting that the mission was extended to a two-year trip called the Galileo Europa Mission. In total, the spacecraft made 12 close flybys of the icy moon.

One of the most important discoveries of Galileo is recording the disturbance of Jupiter's magnetic field in the space around Europa. This suggests the possibility of a huge ocean beneath Europa's crust. In proving the existence of subsurface oceans, NASA has found three conclusive pieces of evidence. Magnetometer surveys carried out by the Galileo spacecraft have discovered an induced magnetic field near Europa's surface. Scientists estimate that Europa's ice sheet is 10 to 15 miles (15 to 25 km) thick and floats on top of an ocean 60 to 150 km deep. So, while Europa is only a quarter of Earth's diameter, its ocean may have twice as much water as Earth's. Therefore, in the solar system, Europa's ocean is one of the most promising places to search for extraterrestrial life.

However, in the 1990s, the Galileo spacecraft in Jupiter's orbit did not record any clear signs. But more recent observations from the Hubble Space Telescope, as well as a reanalysis of some data from the Galileo spacecraft, have shown that thin plumes of water are likely being ejected from 100 miles (160 km) inside Europa's surface. In November 2019, NASA's international research team announced that it had directly detected water vapor above Europa's surface for the first time. The team measured the vapor using a spectrograph at the Keck Observatory in Hawaii, which emits or absorbs the chemical composition of planetary atmospheres through infrared light.

The presence of magnesium compounds on the surface of Europa indicates that water from subsurface oceans reaches Europa's surface through springs or vents. If this happens, these eruptions will bring ions and microbes (by microbes we mean very, very small enzymes effective in fermentation and effective in life, not necessarily disease-causing organisms) from the subsurface ocean to the surface.

Therefore, if there is life in Europa's subsurface ocean, it could spread to the surface of the planet where landers or rovers find it. A mission to Europa's surface might easily find evidence of life or even some microbes by sampling the surface material.

Even if Europa didn't erupt, a 2018 study concluded that Europa's ocean samples could be frozen in its ice crust, where the ice meets the ocean. As the ice sheet deforms due to tidal forces, the warmer, less dense ice rises, bringing ocean samples to the surface so the spacecraft can analyze them using infrared and ultraviolet instruments. This makes Europa a very interesting target for finding extraterrestrial life. Even some researchers believe that trying to do this is much more fruitful than searching for signs of life on Mars.

A new search for extraterrestrial life

For the existence of life, three basic factors are necessary, which are: 1) liquid water. 2) suitable chemical compounds and 3) energy source. Europe is thought to have all three. Because life takes time to survive, Europa's ocean may have existed during the roughly 4 billion years of the solar system's lifetime, which is enough time for life to arise and evolve.

1) liquid water

The surface of Europe is very cold and covered with ice. This ice itself forms a "crust" on the surface of the moon, which is thought to be several kilometers thick. It is also estimated that under the crust, there is a subsurface ocean of liquid water up to a depth of 100 km. Researchers believe that the ocean is rich in dissolved ions, especially magnesium, sodium, potassium, and chlorine. Organisms on Earth live in ion-rich solutions, so it is highly likely that these organisms also live in Europa's subsurface ocean.

2) suitable chemical compounds

Spacecraft observations showed that Europa's surface is covered with water ice. Ice and other materials on Europa's surface are bombarded with Jupiter's radiation, which can convert them into some of the chemical building blocks of life, including free oxygen (O2), hydrogen peroxide (H2O2), carbon dioxide (CO2), and Sulfur dioxide (SO2).

3) energy source

The third element of life is energy. All life forms need energy to survive. On Earth, most of this energy comes from the sun. For example, plants grow through photosynthesis (the process of converting sunlight into energy). By eating plants, energy is transferred to humans, animals, and other creatures. But the kind of life that might exist on Europa would probably only be fueled by chemical reactions, rather than photosynthesis, because possible life on Europa exists beneath the ice, where sunlight can't reach it.

Although life on Europa's surface is affected by Jupiter's strong radiation, the radiation separates the water molecules in Europa's very weak atmosphere. So that the hydrogen floats and the remaining oxygen is connected to other elements. Because oxygen is a highly reactive element, it can potentially participate in chemical reactions that release energy. If oxygen somehow finds its way into the ocean, it can react with other chemicals, possibly providing the chemical energy needed for microbial life.

In terms of its spatial position, Europe is located within the powerful gravitational field of Jupiter. This strong gravitational "pull" puts Europa in an orbit with one hemisphere constantly facing Jupiter. The elliptical orbit brings Europa alternately closer and further away from the planet. This alternating increase and decrease of the gravitational force led to long and short periods of stability of the moon with each round of the planet. The effect of this periodicity in internal motion, along with the gravitational forces exerted by neighboring moons, creates internal friction and heat in Europa.

Europa's internal heat could be the source of energy that keeps subsurface oceans from freezing and sustains any life that exists there. There may be vents of warm water on the subsurface ocean floor that deliver energy and nutrients to the planet from within. Examples of these organisms have been discovered on Earth in Antarctic sub-glacial lakes as well as in the ion-rich hot water of hydrothermal vents. Therefore, life in Europa's subsurface oceans could exist similarly.

Size and distance

With an equatorial diameter of 1,940 miles (3,100 km), Europa is about 90% the size of Earth's moon. So, if we replace the moon with Europa, it will appear in the sky almost as big as our moon, but much, much brighter! Because Europa's surface is made of water ice, Europa reflects 5.5 times more sunlight to Earth than the Moon. In other words, we will have five and a half times brighter moonlight.

Europa is 417,000 miles (671,000 km) from Jupiter, which itself orbits the Sun at a distance of 500 million miles (780 million km), or 5.2 astronomical units (AU). An AU is the distance from the Earth to the Sun. Sunlight takes about 45 minutes to reach Europe. Because of this long distance, the sunlight on Jupiter and Europa is 25 times weaker than on Earth.

Orbit and rotation

Europa orbits Jupiter once every 3.5 days and is gravitationally locked to Jupiter like the Moon, so only the hemisphere of this moon faces the planet. Jupiter takes about 4,333 Earth days (or about 12 Earth years) to orbit the Sun. Jupiter's equator (and the orbital plane of its moons) is only 3 degrees off Jupiter's orbital path around the Sun (Earth is tilted 23.5 degrees). This means that Jupiter rotates almost vertically, which means that the planet, as well as its moons, do not have seasons like Earth.

Jupiter's moons Io, Europa, and Ganymede are in a state called intensification. Thus, every time Ganymede orbits Jupiter once, Europa orbits twice and Io orbits Jupiter four times. Over time, the orbits of most moons or large planets become circular, but in the case of these three moons, accretion creates a forced eccentricity as the moons repeatedly line up at similar points in their orbits, thus aligning with each other. They exert a small gravitational pull that prevents their circuit from becoming circular.

Because Europa's orbit is elliptical (slightly more elongated than a circle), its distance from Jupiter varies, and the near side of the moon feels Jupiter's gravity more strongly than the far side. The amount of this difference creates a tide as Europa rotates around it.

Creation

Early in the history of the Solar System, Jupiter's large Galilean moons Io, Europa, Ganymede, and Calisto probably formed from material left over after Jupiter condensed from the primordial cloud of gas and dust around the Sun. These four moons are probably about 4.5 billion years old, the same age as the solar system.

The Galilean moons are sometimes called "small solar systems" because they formed from the remnants of Jupiter, similar to how Earth and the other planets formed from gas and dust left over from the formation of the Sun. The similarities do not end there. As you know, every planet in the solar system is less dense than its inner neighbor. For example, Mars is less dense than Earth, and Earth is less dense than Venus. Likewise, the density of Venus is less than that of Mercury. The Galilean moons also follow the same principle and the farther they are from Jupiter, the less dense they are.

The decrease in density at greater distances is probably due to temperature, in that the rocky and metallic materials close to Jupiter condense first (as do the planets around the Sun), while the lighter icy materials only condense at greater distances where it is colder.

The distance from Jupiter also determines how much tidal heating the Galilean moons experience Io, Jupiter's closest moon, gets so hot that it is the most volcanically active body in the Solar System. Ancient volcanic activity probably evaporated the water at the same time. Europa has a layer of ice on the surface and water is in a layer between the rocky crust and the ice subsurface, while Ganymede and Calisto have higher proportions of water ice and are less dense.

Atmosphere

Europa only has a thin atmosphere of oxygen, but in 2013, NASA announced that researchers using the Hubble Space Telescope found evidence that Europa may be actively releasing water into space (in the form of violent eruptions). This means that this moon is a geologically active planet at present.

Surface

Europa's blue ice surface is marred by dark, reddish-brown fissures. On the other hand, based on the small number of visible craters on Europa, it seems that the surface of this moon is no more than 40 to 90 million years old, which is very young in geological terms (the surface of Calisto is several billion years old). it is estimated throughout Europa's surface fractures, as well as dark patterns on its surface, a reddish-brown material whose composition is not known with certainty but is predicted to contain salt and sulfur compounds mixed with water ice and exposed to radiation. This surface composition could even be clues to show the moon's potential as a habitable sphere.

The Galileo spacecraft explored the Jupiter system from 1995 to 2003 and made several flybys of Europa. Galileo showed strange pits and domes that could be caused by the slow rotation or convective flow of Europa's surface ice sheet due to the warmth of its lower layers (colder, denser ice under, while warmer, less dense ice rises). Long, linear fractures are often only about half a mile to a mile (1 to 2 km) wide but can extend for thousands of kilometers across Europe. Some of these fractures formed in ridges hundreds of meters high, while others appear to extend into broad bands of parallel fractures.

Galileo also found areas called "disrupted areas" where sections, broken areas, and blocks were covered with red unknown material. In 2011, scientists who studied the Galileo data suggested that "melting zones" could be places where the surface is above lenticular lakes that are underlain by thinner layers of ice that surfaces collapsing into these liquid sources and different in terms of ingredients are combined.

In the end, it is concluded that although there is much evidence and reasons of the existence of an internal ocean in Europa, to definitively declare the existence of signs of life and confirm this hypothesis, we have to wait for future sent missions to check.