Life in the Solar System and Beyond

One of the most difficult problems that science still needs to solve is the question of the origin of life. We have very strong evidence that life emerged on Earth around 3.7 billion years ago, when our planet was less than 1 billion years old. It is very exciting to notice that in the first billion year, the Earth resembled very closely what might be considered hell, with a molten surface, lava and very active volcanoes. Indeed that geologic era is known as “the Hadean” which takes its name from Hades, the Greek god of the underworld. Life emerged almost immediately after the surface of the Earth solidified and once the heavy asteroid and comet bombardment of the Earth ended.

Earth_formation
The Hadean Earth, 4 billion years ago. The surface is still partially molten and constantly hit by a heavy flow of comets and asteroids.

The heavy bombardment, which was produced by the migration of the giants planets in the Solar System, is also thought to have brought most of the water that exists on our planet (the asteroids being the primary source rather than comets). If it took such a relatively short time for life to emerge, does it mean that we were just lucky but life is a very complex and unlikely event or that life does indeed create itself very easily? Or perhaps life was brought to Earth from space (the so-called panspermia)?

It is well known that several kinds of microbes (mostly named “extremophiles”) can survive in acidic environments which are more corrosive than sulfuric acid. Some others can survive extreme temperatures well above 100 degrees Celsius or below -200 degrees Celsius. The same goes for high pressures, with bacteria found thriving at 400 times the pressure we’re used to (which would crush you in an instant).

The Tardigrade is a micro-animal that can withstand extreme conditions (although it is not an extremophile). It can survive in space, radioactive environments, boiling water, freezing temperatures close to absolute zero and can survive without a food/water source for more than 10 years.
The Tardigrade is a micro-animal that can withstand extreme conditions (although it is not an extremophile). It can survive in space, radioactive environments, boiling water, freezing temperatures close to absolute zero and can survive without a food/water source for more than 10 years.

However, one common mistake that is often made is to confuse the fact that life has spread basically everywhere on Earth, even in the most extreme environments, with the fact that life can form everywhere. Indeed the fact that we find microbes in such extreme environments on Earth means that life has an immense power to adapt to those conditions. Adaptation is an evolutionary process and has nothing to do with the origin of life which is an entirely different question. Indeed the theory of evolution is a well known and established scientific theory whereas the origin of life still lacks a complete explanation and there are only a number of hypotheses that still need experimental confirmation. There are more than 20 proposed scenarios which attempt to explain the origin of life on Earth. Many of those build upon the “primordial soup” idea of Haldane and Oparin, two scientists that in the 1920s proposed that the atmosphere of the early Earth, when exposed to an energy source (e.g., lightnings), can produce organic molecules that accumulate in the sea forming a “soup”. These molecules then start a chain of chemical reactions which ultimately lead to the formation of life. A famous attempt to verify this idea was made by Miller (and Urey) in 1953 who created several amino-acids (the building blocks of proteins and thus life) in a “soup” when injecting energy in a simulated early atmosphere of the Earth.

A scheme of the apparatus used by Miller to produce organic compounds and amino-acids in his experiment.
A scheme of the apparatus used by Miller to produce organic compounds and amino-acids in his experiment.

The Miller experiment has been criticized since the simulated conditions of the early atmosphere were probably not the correct ones as it emerged in later studies. Thus if Miller had used a simulated atmosphere with the right composition, his experiment might have failed to produce amino-acids, although more recent experiments have succeeded in producing some. Another interesting fact is that when Miller performed his experiment  he reported that about 10% of the carbon present in the simulated atmosphere was transformed into organic compounds. About 1 milligram of a few different types of amino-acids were created. After the death of Miller in 2007, the sealed vials of the experiment were opened and re-examined. It was found that there were way more amino-acids than those reported by Miller in the his work in 1953, so this experiment was even more successful than originally reported.  In any case, amino-acids are just organic compounds and in the past few years they have also been found in space and even on meteorites landed on the Earth’s surface. Such meteorites had experienced extreme temperatures of more than 1000 degrees Celsius while burning during their descent in the Earth’s atmosphere. Therefore amino-acids might possibly be quite widespread and resist to extreme physical conditions, at least in the Solar System. Could the amino-acids present in the “primordial soup” on Earth (provided there was really a primordial soup!) have been brought here by asteroids/comets/meteorites?  Whatever the answer is, it is worth noticing that there’s a gigantic leap between creating organic molecules and creating life. But what is life? The most elementary forms of life are very very simple and it is difficult to establish whether such systems, viruses for example, are really alive. Today science defines a living system as a complex with a body, a metabolism (i.e., life can take resources from the environments and transform them into useful energy) and the property of having inheritable information (e.g., RNA and DNA). However, the latest scientific evidence suggests that the distinction between life and non-life is blurred and continuous rather than discrete.

How can astrophysics help to shed new light on the problem of the origin of life? In the last decade there has been an increasing interest in planetary exploration with a particular emphasis on the search for life in the Solar System. In the past, a lot of research has focused on Mars, for the reason that it is a rather close planet with a possible similar history as the Earth. We do know that large seas and a thick atmosphere were present on the ancient Mars, whereas today water is found in relatively small quantities and only in form of ice. Since Mars had liquid water and a favorable position in the Solar System, i.e., it lies within the so-called “habitable zone”, it was and still is obvious to look for the presence of life on that planet.

An artistic impression of how the ancient Mars might have looked like with its large oceans and thicker atmosphere. The close resemblance with the Earth is striking. (
An artistic impression of how the ancient Mars might have looked like with its large oceans and thicker atmosphere. The close resemblance with the Earth is striking. (“AncientMars” by Ittiz – Own work. Licensed under CC BY-SA 3.0 via Commons)

However, the whole concept of “habitable zone” is rather debated in astronomy. This zone is defined as that region in a solar system where a planet with sufficient atmospheric pressure can support liquid water on its surface. A few decades ago it was pretty clear that only rocky planets like the Earth (and Mars) can satisfy this condition, since gas giants (e.g., Jupiter and Saturn) have a huge atmosphere that sits on a thick layer of liquid hydrogen and helium. There are then the icy dwarf planets like Pluto, which, however, are too far to receive enough energy from the Sun and are too small to have a dense atmosphere so they do not have any liquid water on their surface.

However, with further planetary exploration there is now mounting evidence that not only rocky planets might be able to host life, but also large moons around gas giants, even if those are well outside the “habitable zone”. There are at least three examples of such moons in the Solar System: Titan (the largest moon of Saturn), Europa and Callisto (two of the four large moons of Jupiter).

The three gas giant moons Europa and Callisto (orbiting Jupiter) and Titan (Saturn).
The three moons Titan (orbiting Saturn),  Europa and Callisto (orbiting Jupiter).

Titan was recently explored by the Cassini-Huygens mission (NASA and ESA) with the Huygens probe that landed on its surface in 2005. The data collected revealed the presence of large amounts of organic material (organic aerosol) and the existence of large lakes of methane, ethane and propane. Europa and Callisto instead are increasingly becoming the two most striking examples of potentially habitable worlds that we know. In the late ’90s Jupiter was visited by the Galileo orbiter (an ESA mission) and Callisto was found to behave like a perfect conductor. This implies that Callisto is a highly conductive sphere which means that Jupiter’s magnetic field  – which is 10x larger than Earth’s –  cannot penetrate inside Callisto which is able to deflect it, as perfect conductors do. This was interpreted as a sign that beneath its solid surface there must be a thick layer of more than 10 km made of highly conductive liquid. Since water ice is widespread on the surface of this moon and salty water is a highly conductive material, it has been proposed that a large ocean can be enclosed within Callisto’s solid crust.

Current model of Callisto, with an outer solid crust, an inner ocean of liquid salted water (in blue) and a solid core.
Current model of Callisto, with an outer solid crust, an inner ocean of liquid salted water (in blue) and a solid core.

Even more striking is the case of Europa, another moon of Jupiter. The current idea is that a large ocean exists here too underneath the solid surface. Astronomers have estimated the content of liquid water on this moon and have found that there should be more liquid water on Europa than on the whole Earth. This water is liquid instead of frozen because the tides that Jupiter exerts on these moons are huge and the internal friction heats up the water and keeps it in a liquid form.

Estimated total amount of salted water on Europa compared to Earth
Estimated total amount of salted water on Europa compared to Earth

Unfortunately we have no probes that can visit directly these alien oceans because the liquid water is enclosed within a thick shell of solid material which would be impossible to penetrate. Some water vapor plumes are, however, regularly emitted from the surface of this moon. Therefore analyzing the chemical content of the plumes might be a smart way around penetrating the solid surface. The plumes can be used as probes of the internal ocean and help to understand how the underneath environment might look like. Indeed one recently approved mission (the JUICE mission of ESA and NASA) will explore these plumes of Europa and the whole Jupiter system to assess potential habitability of those worlds and determine whether our current ideas are sound and can be applied to other gas giants (especially the many known beyond the Solar System).

But how can life form on Europa or other moons? There is an interesting hypothesis on the origin of life that proposes hydrothermal vents in the deep oceans of the Earth as the possible location for the creation of life. Deep oceanic vents release energy and heat up the environment with water that can reach up to 450 degrees Celsius. Such water creates a gradient of temperature that slowly decreases towards the deep ocean which has an average temperature of just a few degrees Celsius. There should be a shell around hydrothermal vents which has just the right temperature and composition to trigger chemical reactions that might lead to life. Something similar might be happening on Europa and other moons too? It’s still too early to say, but it is probable that we will find out pretty soon.

I just notice that when searching info on abiogenesis and related problems on google I was literally flooded by tens of websites of creationists, christian fundamentalists and so on. They seem to have figured all out: it must have been god. Right.

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2 thoughts on “Life in the Solar System and Beyond

Add yours

  1. Very interesting read, I hope we do find some sort of life out there. I would like to point out the idea of abiogenesis does not rule out the notion of god and vice versa.

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