Is Ganymede not Mars or Europa the best place to look for alien life?
Mars must have some really smart tourism promoters. For decades, the probes we've
sent up there keep telling us that there is no trace of life, past or present, among those sands.
Yet, in anyone's imagination, Mars continues to be the only proverbial source of alien life
in the entire Solar System. The phosphines discovered on Venus
(or maybe not), have not moved an inch from this belief. Nor have the lakes of hydrocarbons
found on Titan and the geysers that bring water from Enceladus' ocean to the surface.
Nothing, the red planet remains for all the most obvious destination for
any mission from Earth in search of life. But is it really so? Are we really sure?
Are we so fascinated by the history of Mars that we do not realize that there
are worlds in the Solar System more suitable for the emergence of life?
In short, do you want a tip? If you have two pennies to throw away on a bet,
bet them on Ganymede. And not on Mars! And now, with your permission, I would
like to explain why. Follow me.
Here on Earth, life took hold very early, and has not only survived, but thrived, ever since.
Not all worlds, however, possess the right conditions and properties
for life to arise and sustain itself. As far as we can tell, there are a few
properties that are absolutely essential, and another few properties that may be essential,
optional, or completely irrelevant when it comes to sustaining and maintaining a living world. The
essential ones - at least for the chemical-based life that we know - include elements like carbon,
oxygen, nitrogen, hydrogen, and phosphorous, configured into "building blocks" like sugars,
amino acids, and other vital molecules. However, these criteria are likely necessary,
but not sufficient, for life to arise and sustain itself. On Earth, we possess a combination of
additional factors that seem to be friendly to the kind of life we know of, but we do not yet know
whether they should be considered indispensable. What? Well, for example, a magnetic field that
protects it from cosmic rays, a deep ocean of liquid water and emerged continents,
an "important" atmosphere with a not negligible pressure on the surface,
a day/night cycle neither short nor long, a relatively large and close satellite,
capable of creating substantial but not catastrophic tidal forces on our planet.
So where are we supposed to look for life in the solar system, outside of our own planet?
There are numerous good options, including Mars, our colder, smaller sibling that seemed to have
a watery past for more than 1 billion years; Venus, which may have been Earth-like before
succumbing to a runaway greenhouse effect, and which may possess life in its clouds;
Europa and Enceladus, icy moons of Jupiter and Saturn, with liquid subsurface oceans
and geysers that bring that liquid material up through the icy crust and into direct sunlight;
Titan, Saturn’s giant moon with a thicker atmosphere than Earth and liquid methane
on its surface, or Pluto and Triton, which are large, icy worlds from the Kuiper belt,
both of which have complex geological patterns and also a subsurface liquid ocean...
The fact is that until we have a statistical sample of extrasolar planets
in which we have discovered life, we will never be able to understand which of the
characteristics possessed by Earth are really indispensable to its emergence.
However, by comparing the characteristics of the Earth’s environment with those of other bodies
in the Solar System, it is already possible to venture hypotheses and get an idea of the world
that seems to have more requirements than others. And as I was telling you, I confirm that based
on these ratings, my absolute favorite is Ganymede, the largest moon in the solar system,
Ganymede, you know, is the third of Jupiter’s four large moons, with volcanic Io and ice-rich Europa
located interior to it, and the heavily cratered Callisto orbiting beyond it. Ganymede is tidally
locked to Jupiter, meaning that its same “face” always points toward the gas giant planet,
but since it’s relatively close to Jupiter at an orbital distance of 1.07 million kilometers,
it still manages to complete a full revolution around Jupiter — and hence, a full 360° rotation
about its axis - every 7 days. An incredibly cold and
seemingly inhospitable world. A cursory look at Ganymede, in fact,
might lead you to believe it’s a world like the Moon or Mercury: a largely airless world,
stripped of its atmosphere and heavily cratered. Its bland, greyish color in
photographs makes it appear even more like those two worlds, completely unremarkable and,
you might think, completely inhospitable to life. In fact, it only has a very thin atmosphere,
and a surface pressure provided by a layer of (mostly oxygen) gas. It would take approximately
100 billion Ganymede atmospheres all piled atop one another to achieve the pressures
we find here on Earth. And you know... with an atmosphere that provides so little pressure, it’s
impossible to have liquid water on the surface. No liquid water, no life, case closed, right?
So, without an atmosphere, why would we consider Ganymede at
all as an interesting world to examine for life? Yes, it’s very very unlikely that we have
substantial life processes occurring on the surface of Ganymede. But when we look at the
atmosphere in detail — as a new study recently did with archival data from Hubble — we find that
Ganymede’s atmosphere has hydrosignatures in it: copious amounts of water vapor!
Finding water vapor and oxygen on Ganymede tell us that the frozen, icy surface of the world actually
interacts with the space weather impacting it, and this is despite Jupiter's strong magnetic field.
Molecular oxygen is produced when charged particles impact and erode the ice on the surface,
indicating that solar wind particles are getting through.
The water vapor, on the other hand, must come about through sublimation:
there must be icy regions that get warm enough so that water vapor is not only produced but is hot
enough to thermally escape into the remainder of the atmosphere. Despite Jupiter's strong,
shielding magnetic effects, and Ganymede's frozen appearance,
the puzzle pieces actually put together a tantalizing story.
When the first ultraviolet images of Ganymede were taken by Hubble in 1998 - astronomers
got a bit of a surprise: there were bands of auroral activity encircling the moon,
evidence that Ganymede isn’t just embedded in Jupiter’s magnetic field, but that it generates
a magnetic field of its own. The combination of these two fields, Jupiter’s and Ganymede’s,
can lead to particles funneling down onto Ganymede’s surface, given its thin atmosphere,
creating the oxygen atmosphere we observe. But how does Ganymede maintain a magnetic field
at all? To understand that, we have to look to its interior, and that’s where the story transforms...
Yes, Ganymede has an almost negligible atmosphere. And yes, it’s a cold world:
ranging from -200 °C at its coldest, on the night side when it’s in Jupiter’s shadow, to -120 °C,
the maximum daytime temperatures observed by the Galileo spacecraft. And there are large
amounts of ice on its surface; approximately 50% or more of the surface is icy, mostly water-ice at
that. Other compounds include ammonia, various sulfates, and sulfur dioxide. But things get
really interesting when it comes to Ganymede when we examine what must be going on inside of it.
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Ganymede's outer crust is largely made of ices, particularly water-ice that forms a
hexagonal crystal structure. Although it’s coated in clays and grooves, with polar frost caps,
those minerals are thought to have largely arrived billions of years ago, when the rate of impact
craters was very high. The magnetic fields of Ganymede protect the equatorial regions, but
allow solar plasmas to strike the poles, resulting in the observed frost at high latitudes. Over the
past 3.5 billion years or so, however, Ganymede's exterior has remained largely unchanged.
Inside, however, that crystalline ice structure extends downwards for quite a ways:
about 160 kilometers. Below that, the temperatures and pressures get high enough that the water
no longer remains in its solid phase, but becomes liquid. In other words,
there actually is a thick, deep, subsurface ocean underneath the deceptively barren terrain
coating the surface of Ganymede, extending all the way down to depths of about 800 km,
or nearly a third of the way to its center. Below that, there’s certainly another layer
of ice, and possibly multiple layers of ice and liquid in various phases, until you get all the
way down to the rocky mantle, which may itself be in contact with a layer of liquid water.
Further down, beneath the mantle, there's a liquid metal core surrounding a solid iron-nickel core,
which is thought to have a radius of 500 km, a temperature of around 1300° C,
and a density approximately three times the overall density of Ganymede.
With these interior properties, Ganymede all of a sudden transforms from a barren world,
akin to Earth’s Moon, to one with perhaps the best chances for life in its ocean deep underground,
at the interface between the lowest layer of the liquid oceans and the hot, rocky mantle.
Just as we have here on Earth a set of extremophile organisms that thrive in
the environments surrounding the hydrothermal vents, it’s also possible that something very
similar is happening 800 kilometers in the deep of Ganymede, at the ocean/mantle interface.
And so, if we run through our checklists from earlier, we find that Ganymede ticks off almost
every box. From the essential list, it has: the essential elements for life;
almost certainly possesses those elements configured into biomolecules like amino acids
and sugars, with an external energy source in the form of heat from the moon’s interior
(augmented by the Jovian-induced tides); copious amounts of liquid water in
the environment where life can thrive. In addition, among the ingredients that Earth
possesses but that may or may not be essential or even conducive to life, Ganymede shows:
a substantial internal and external magnetic field, generated by an active,
metallic core and by its proximity to Jupiter; a deep, liquid water subsurface ocean;
a liquid water/rocky mantle interface at the bottom of the ocean.
All of these properties are extremely promising as far as the potential for life - at least,
as we know it. With a great advantage compared to the much-publicized Mars,
to have a magnetic field capable of protecting from cosmic rays the possible life forms brought
on the satellite by asteroids and comets according to the theory of panspermia. That
is another of my personal fascination, which we will probably talk about another time.
However, after the discovery of water vapor, Ganymede has certainly moved up
in the ranking of worlds "suspected to host life". And in the future will certainly be
launched several missions for its exploration. Recently, Ganymede was visited by the probe June,
which sent to Earth images that we have been missing since the days of the Galileo mission.
Instead, in 2022, the JUpiter ICy moons Explorer (JUICE), a mission
of the European Space Agency, should start. JUICE will visit Ganymede, Europa, and Callisto.
All three moons in fact have fair amounts of liquid water under the surface
and are ideal candidates for the search for life. The probe will arrive in the Jupiter system in
2030 after taking advantage of four times the gravitational assistance of Earth and Venus.
After a series of flybys of Europa and Callisto,
it will enter orbit in 2032 around Ganymede for further study to be completed in 2033.