Five (sheltered) places to look for extraterrestrial life

Humanity has embarked on a tireless search for life on other worlds in the solar system. Or is it? The truth is that this idea doesn’t quite match reality. In the history of space exploration, there has been only one mission specifically dedicated to the search for extraterrestrial life: Viking. It landed twin probes on Mars in 1976.

Still, space exploration has given us enough data to guess which planets or stars might have the best conditions for life. Let’s take a look at the five places in the solar system beyond Earth that appear to be the most habitable:

Mars

The eternal mystery of whether microbial life exists on Mars was left unanswered in 1976 by the Viking probes, those produced confusing results. Since then, other missions have attempted to determine whether the components necessary for life as we understand it are present on Mars. One of which is carbon; one in recent years fluctuating presence of methane, the simplest organic molecule, has been discovered in the Martian atmosphere. Most terrestrial methane is of biological origin, but the source of the Martian gas is still a mystery. On the other hand, the presence of more complex organic molecules used to be confirmed by the Curiosity rover in 2018and later By the Perseverance rover in 2023.

Another requirement for life is the presence of liquid water. In 2011, what They turned out to be seasonal brine flows were detected trickling down the slopes of Mars, but later studies have concluded that they were probably just dry sand. However, in July 2018 this was likely presence of a large liquid lake under the ice at the south pole of Mars was revealed, and subsequent studies have confirmed this. In short, while the chemistry of Martian soil doesn’t seem optimal for life, all bets are still off.

Europe

The smallest of Jupiter’s four large moons – slightly smaller than Earth’s moon – has been at the forefront of speculation about the existence of life elsewhere in the solar system for decades. The main reason is the large ocean of liquid water thought to lie beneath the icy crust, a hypothesis consistent with observations but yet to be confirmed. The mass of water, which can reach a depth of 100 kilometers beneath an ice surface of between 10 and 30 kilometers, could be more than twice the volume of Earth’s oceans. Europa’s ocean would be kept liquid by the heat generated by tidal friction due to Jupiter’s strong gravity.

Analysis of data obtained by NASA’s Galileo probe in 1997 confirmed that Europa’s ocean expels geysers through the ice, which would make it easier to analyze its composition without having to land a probe on its surface. The presence of chemical elements essential for life, such as carbon, oxygen, hydrogen, nitrogen, phosphorus and sulfur, have also been postulated. However, a Study from 2024 has tempered expectations of life on Europa by suggesting that the moon could produce much less oxygen than previously thought. Scientists hope that two new missions to Jupiter, ESAs Jupiter icy moon explorer (JUICE), launched in 2023, and NASA’s Europa Clipperscheduled for launch in October 2024, will provide new data on Europe’s habitability conditions.

Enceladus

The case of Enceladus is similar to that of Europa. However, on this moon of Saturn (only 500 kilometers in diameter) the presence of water vapor geysers was confirmed inside 2005 And 2008 By the Cassini ask questionswhich also detected other components such as methane, nitrogen, carbon dioxide, ammonia And complex organic molecules, all compounds that are important in biology. There is also strong evidence for the presence of a salty ocean beneath the ice sheet, which was initially thought to be limited to the Southern Hemisphere but is now thought to span the entire length of the moon.

In addition to the ocean’s favorable chemical composition, evidence of subsurface hydrothermal activity has made Enceladus a strong candidate for life. Just as recent studies may have reduced the chances of life on Europa, the chances for Enceladus have actually gotten a lot better: in 2023 it was discovered that the ocean contains phosphates, essential ingredients for biology. The scientists responsible for this discovery have developed a method that will allow NASA to launch soon Europa Clipper probe to detect the presence of cells in the geysers of Enceladus.

Titan

Saturn’s largest moon is the only world in the outer solar system where a terrestrial instrument has landed: ESAs Huygens probe in 2005. Over 90 minutes, Huygens sent back hundreds of images of Titan’s surface, covered with ice and rocks.

Titan lives up to its name as the second largest moon in the solar system, 50% larger than Earth’s satellite and possesses two unique features: it is the only moon with a dense atmosphere and the only known world other than Earth with liquid masses on the earth. surface. This is not water, but methane, and these lakes and seas could be inhabited by microbes that can consume hydrogen instead of oxygen and produce methane instead of carbon dioxide.

also the possible liquid water ocean beneath Titan’s surface could also harbor life forms more similar to those on Earth or other moons such as Europa or Enceladus. However, this hypothesis has recently lost its luster with the discovery in 2024 that the transfer of organic molecules necessary for life from the surface to the ocean beneath the ice probably very small, insufficient to support a biosphere. Unless there is an internal source of this material, such a find reduces the chances of life in Titan’s ocean. NASA plans to send a drone Dragonfly to the moon in 2028 to explore its surface and analyze its habitability.

Ganymede

Ganymede is almost the size of a planet: this satellite of Jupiter is the ninth largest object in the solar systembehind Mars and ahead of Titan and Mercury, and is the only moon with its own magnetic field; The Earth’s magnetic field protects us and all the creatures that live on it. In 2015, data from the Hubble Space Telescope on the movement of auroras caused by this magnetic field confirmed the presence of a salty ocean about 100 kilometers thick, hidden under 150 kilometers of ice.

It is now speculated that this could be the most water-rich object in the solar system, perhaps six times more than the Earth’s surface, and that its interior has a sandwich with several layers of ice and liquid water. The possibility of the inner water layer coming into contact with the inner rocky mantle increases the chances of life, because this interface provides necessary elements for biology. In 2021, Hubble data revealed the presence of water vapour in the oxygen-poor atmosphere of Ganymede, and in 2023 salts and organic compounds were reported at the surface, which would originate from the inner ocean. Ganymede will be the main target ESA’s JUICE missionlaunched in 2023 and expected to reach the Jupiter system in 2031, and will also be studied by NASA’s Europa Clipper, which will arrive a year before the European probe.

So far, these are the five worlds currently thought to be most likely to support life, but they are not the only ones; others such as Venus, Callisto (Jupiter’s moon) or even Pluto could host a habitable environment. Unfortunately, we’re far from solving these unknowns: Experts say designing experiments that fly aboard a… robotic probe to confirm the presence of life on other planets or moons is not an easy task.

Then there is another obstacle that has emerged in recent years: planetary protectiona policy set out in Article IX of the UN Outer Space Treaty, which advises against sending probes into space alien enclaves where life is most likely to exist because of the risk of contamination with terrestrial microbes. This, combined with the high cost of potential missions, means that the possibility of finding extraterrestrial life in the solar system is still a long way off.

Javier Yanes

Main photo credit: Ron Miller/Stocktrek Images/Getty Images.