The year 2020 will undoubtedly be remembered by the covid-19 pandemic, caused by the SARS-CoV-2 coronavirus. But, in addition, for those interested in space exploration and astrobiology, it will go down in history as the first in which three missions were sent to Mars, almost simultaneously. Depending on the orbits that the Earth and the red planet describe around the Sun, they are at their shortest relative distance for approximately one month every two years. This is what is called a launch window, and the one that opened in mid-July of that year was very well used by three space agencies. In fact, it could have been even more so if a fourth initiative, promoted by the European (ESA) and Russian (Roscosmos) agencies – it is known as ExoMars and carries the Rosalind Franklin rover on board – had not had to be delayed until 2022.
On the 19th of that month, a newcomer to the space race, the United Arab Emirates, launched the Al Amal mission – that is, ‘hope’ – from Japan. It consists of an orbiter that contains three instruments of mainly North American manufacture with which the composition and dynamics of the Martian atmosphere are being studied.
Four days later, a rocket launched by the country that has been leading the exploration of the Moon in recent years left for Mars: China. His first attempt to reach the red planet was truly ambitious, since his Tianwen-1 mission – whose poetic name means ‘questions to the sky’ – had an orbiter, a lander and a rover.
Until now, no space agency had sent these three components together, because communication from the surface of Mars with Earth has been through probes launched previously. But the Chinese CNSA decided to go all out. Its goal is to expand what we know about Martian geology, water ice distribution, and atmosphere. To that end, the orbiter’s analytical instruments and cameras were designed and built, and all seven are housed in a 240 kg rover, with six wheels and powered by solar panels.
For its part, NASA launched the Mars 2020 mission on July 30, which incorporated the largest and most complex astromobile that it has sent to the red planet: Perseverance. It has a mass of just over a ton and its design is similar to that of Curiosity, which has been touring Gale Crater for almost a decade, although it has several improvements.
The device successfully landed on Isidis Planitia, specifically in the Jezero crater, which could have been covered by water 3.5 billion years ago. Once there, its seven instruments began to analyze the geological processes that formed an ancient river delta and to characterize the surface and subsoil of the environments over which the astromobile circulates. Its objective is also to study the most promising sedimentary rocks to try to find in them signs of molecular biomarkers, that is, of the possible existence of some kind of life in the past.
In addition, over the next few years, Perseverance will continue to collect data on the dynamics of the Martian atmosphere, a topic of great interest, given that in recent times, spikes in methane concentration have been detected that are difficult to explain. A device developed in Spain will contribute decisively to all this: the MEDA meteorological station, built by the Center for Astrobiology (INTA-CSIC) and various departments of INTA, in collaboration with some foreign institutions.
One of the novel instruments of this rover is MOXIE, with which it is studied whether it is possible to generate molecular oxygen in closed compartments, something from which astronauts will benefit in future manned missions. In addition, the Perseverance carried with it a small helicopter, called Ingenuity, which has become the first vehicle to perform controlled flights in an alien environment. The photographs that this drone has taken from several meters above the Martian soil represent another milestone in the history of space exploration.
With the aforementioned three missions underway, we could flip the plot of many movies and say that Earth has indeed invaded Mars.
The search for water on Mars
As a result of the work of the more than twenty orbiters, fixed modules and vehicles that have explored our neighboring world since the 1970s, and also thanks to the analysis of meteorites that arrived from the red planet to ours, we have a lot of data about their geological history. Everything seems to indicate that Mars, which we see today as a dusty desert with an average surface temperature close to -50 ºC, had abundant liquid water on its surface at the same time that life was emerging on Earth. Therefore, nothing prevents the transition between chemistry and biology from occurring there as well. If life prospered, could it continue to exist under the Martian surface?
To try to determine this, one of Perseverance’s tasks will be to collect up to 42 soil and subsoil samples and deposit them in a hermetically sealed container. In 2026, as part of the joint initiative of NASA and the ESA Mars Sample Return (MSR), a mission will be sent to Mars that will include a rover that will take the aforementioned container and insert it into a small rocket capable of taking off from autonomously.
Its objective is to put that cargo in Martian orbit, where it will be captured by a third spacecraft – also sent in 2026 – that will take it to Earth. Thus, if all this very complex operation is successful, in 2031 we will receive the samples and their geological, chemical and, perhaps, also biological characteristics will be analyzed. The CNSA is preparing a similar plan – although apparently simpler – to try to get ahead of the United States and Europe in this race.
Obviously, it is necessary to clean and sterilize the rovers and laboratories, so that the life of our planet does not contaminate the Martian samples. To do this, a series of strict protocols and regulations known as planetary protection must be followed, an increasingly important discipline in space exploration.
Other corners of the Solar System
But in the Solar System, the red planet is not the only target of astrobiology. Research on asteroids and the meteorites that come from them, and also on comets, have made it possible to find a wide repertoire of organic molecules.
Together, these smaller bodies contain numerous amino acids found in proteins, as well as ribose and nitrogenous bases such as those that make up DNA and RNA. Therefore, part of the biochemistry necessary for life could exist in many places in the cosmos.
Likewise, Ceres and Pluto, two dwarf planets on whose surface there are clear signs of the presence of organic matter, are carefully studied. For its part, Venus has been visited by several Soviet and American missions since the early 1960s. Thus, we know that its surface may have been welcoming to biology in the past, although the intense greenhouse effect produced by its atmosphere killed that possibility. However, some of its cloud layers are considered the environment with the greatest options for living beings to exist today in that world.
In any case, the most favorable places for extraterrestrial life, perhaps similar to ours or based on a different biochemistry, are some moons of Jupiter and Saturn. Around the largest planet in the Solar System, 79 satellites have been detected, including the four discovered by Galileo in 1610: Io, Ganymede, Callisto and Europa. Of these bodies, the last is the most interesting in this regard.
The data indicate that under its ice crust, between 15 km and 25 km thick, there is an ocean of liquid water rich in salts, whose average depth would be 100 km. In the background, the presence of hydrothermal upwellings would guarantee an energy source and the presence of organic molecules, with which some type of biology could have been developed. Due to their interest, and if the deadlines are met, two missions will depart for the Jovian system in 2022 and 2025. These will be, respectively, the Jupiter Icy Moons Explorer (JUICE), from ESA, and the Europa Clipper, from NASA . Eight years later they will reach their destination.
Among the 82 known satellites of Saturn, there are two very relevant to the possible existence of life: Enceladus and Titan. The first is an ocean world, relatively similar to Europe, although smaller. Ejections from an inland sea have been detected near its south pole that break through the ice crust and project outward.
NASA’s Cassini spacecraft flew over that moon and was able to pass through some of these geysers, which allowed it to analyze the dissolved salts in that subsurface water, characterize a good number of simple organic molecules and postulate what type of hydrothermal reactions operate in the rocky bed of your ocean.
As for Titan, it is the only satellite in our galactic neighborhood with an appreciable atmosphere; in fact, it is very dense. Between it and the surface there is a cycle equivalent to the hydrological one of the Earth, but, given its average temperature of -180 ºC, it is led by hydrocarbons. That is, liquid methane rains and there are rivers, lakes and oceans of this compound mixed with ethane. In addition, under its surface there is also an ocean of liquid water, so interesting prebiotic chemical reactions can be proposed that may end up generating some type of life.
To continue the in situ investigation of Titan, which was started by the ESA Huygens probe – in 2005, it detached itself from Cassini and landed on the surface – NASA’s Dragonfly mission has already been approved: a 450 kg helicopter provided of various instruments that from 2034 will fly over the dunes of organic matter in their equatorial latitudes.
As we see, our cosmic neighborhood has different scenarios where life could have arisen. The telescopes will continue to point towards them and, in addition, they will be the target of new missions. The study of analogous environments that exist on our planet, with physical and chemical characteristics similar to yours, will also allow us to advance in this regard.
Among them are hydrothermal springs and underwater volcanoes, ocean bottoms, liquid water lakes located under kilometers of ice, subsoil rocks, evaporation salt pans, environments with radioactive substances, deserts such as the Atacama or water courses so acidic and rich in metals like our Rio Tinto. The Extremophilic microbes that inhabit them show the extraordinary adaptability of life and allow us to venture their existence far from Earth.
Extrasolar worlds
Since the discovery, in 1995, of the first planet orbiting another star similar to ours, the number of extrasolar worlds discovered has not stopped increasing. Thanks to the use of different detection techniques, more than 4500 are recorded, in about 3300 planetary systems. They have different sizes, densities and compositions, and a good number of them are found in the so-called habitability zone of their systems, that is, at such a distance from their stars that their surface temperature would allow the existence of liquid water. Among such exoplanets, the most promising are rocky and Earth-like ones. Today, about seventy are carefully studied, such as Kepler-186f, Proxima b –the closest outside the Solar System, 4.2 light years away–, Teegarden b, and three of the seven worlds that orbit TRAPPIST-1.
They are, however, a tiny fraction of those that can exist in the observable universe, which could be as many as there are stars, some 30,000 trillion. The options for the existence of other life forms are overwhelming, but we have yet to find proof. A quote from the great science fiction author Arthur C. Clarke encourages reflection: “Sometimes I think there is life on other planets, and sometimes I think there is not. In any case, the conclusion is astonishing. “
