Early Fermi Filters
How common are exo-Earths with water?
One explanation for the Fermi Paradox
is that most exo-Earths do not have
as much water as the Earth does.
In the first in a series of articles
on Early Fermi Filters,
Jonathan Cowie looks at recent research as
to the likelihood of exo-Earths
having an Earth quantity of water.
Fermi Paradox
Fermi Filters: Early and Late
Watery planets in the habitable zone
Water delivery to the early Mars and Earth
A problem
A solution…
Exobiological implications
Fermi Paradox
So, how likely is extraterrestrial intelligence? The Fermi Paradox, proposed by Enrico Fermi, addresses this. The paradox can be summarised as so…
Given that the Galaxy is cosmologically small (150,000 to around 200,000 light years across) it can be completely crossed over 2,500 times since the Earth's formation travelling at just 10% the speed of light.
Yet paradoxically we have not seen any evidence of any extraterrestrial existence. This suggests that extraterrestrial civilisations do not exist!
Now, clearly we (technological humanity) exist, so clearly the rising of life and the subsequent evolution of a species capable of a technological, space-faring civilisation is possible. So, as per the Fermi Paradox, why do we not see any evidence of extraterrestrial civilisations? We see no distant mega-structures; do not detect extraterrestrial intelligence radio signals; and have no evidence of past alien visitations (no fossilised television sets).
One answer to this is that there is some filter – a Fermi filter – that prevents the rise of a long-lived civilisation capable of interstellar travel.
Fermi Filters: Early and Late
Fermi Filters preventing the rise of interstellar civilisations can be divided into two main classes: early and late. Late Filters took place in a civilisation's history after the time that would be equivalent to where we are today: a technological civilisation with a limited toehold in space.
The classic example of such a Late Filter would be global thermonuclear war. We simply become technologically advanced enough to develop, and implement, the means to destroy our entire civilisation. And even if there are survivors who re-build, they eventually develop to the point where they can do it again and do so. (By the way, this is the plot arc for the classic SF novel A Canticle for Leibowitz.)
Conversely, Early Filters take place on a potential life-bearing planet earlier in its history than the equivalent of our time. Early Filters include: the rise of life (life rarely gets going); the rise of multicelled life (multicellularity is a difficult evolutionary step); and the evolution of intelligence capable of developing a highly technological civilisation is very, very rare.
This brings us up to where we are now: a human technological civilisation taking its first baby steps into space.
If, as the Fermi Paradox contends, technological, interstellar space-faring civilisations are rare, then there are two Fermi Filter options: we have either already passed the Early Filters and they are now behind us (in which case we will become the first Galaxy-spanning interstellar civilisation), or alternatively, the Filter is a Late Filter and we have still to encounter it (and so fail to become a Galaxy-spanning interstellar civilisation).
Here I will focus on just one type of Early Filter: the likelihood of water bearing, rocky planets in a star's habitable zone, keeping in mind that the Earth is one such a watery planet.
Watery planets in the habitable zone
When a planetary system forms out of a cloud of mineral dust (rock powder) and ice (frozen liquid and gas), the latter condenses out far away from the system's sun where it is cooler. There are lines at certain distances in the proto-planetary system within which it is too warm for different liquids and gases (which then get blown away by Solar wind) and beyond which they exist as ices (often within rock assemblages). For water, this boundary is the frost or snow line and in our planetary system this today lies somewhere close to the orbit of Jupiter. Indeed, we can see such volatile-rich bodies from beyond the frost/snow line with in-coming comets from the outer Solar System. These objects, being very rich in volatiles, on crossing the snow line, begin to give off their frozen liquids as gases: we see this as comets' gaseous tails. The problem is that the habitable zone is within this line, where it is simply too warm for water ice to condense out.
This raises the question as to how the Earth, being in too warm a place, got its water: it must have obtained it somehow?
Up to now, the answer to that question has been that carbonaceous asteroids (and even comets from further out in the young Solar System) brought volatile compounds, such as water, to the Earth early in the Solar System's existence.
This presents another problem. The number of Earth-orbit-crossing asteroids we see today is simply too small to deliver the volume of water to the early Earth to give us the oceans we see. Fortunately, there is an easy solution to this.
Planets when they first form do not necessarily do so in the stable orbits about their star that we see today. Early on in planetary systems there is planetary migration in which planets in unstable orbits move about until they find stable orbits in which they subsequently reside. We need not go into detail, suffice to say that astronomers have a number of theories as to what happened early in our Solar System's existence. These include: the Jumping Jupiter scenario; the Nice model; and the Grand Tack hypothesis, among others. This early Solar System planetary migration disturbed comets sending them from further out in the Solar System in to the inner Solar System where Venus, Earth and Mars reside so subjecting the Earth to the Late Heavy Bombardment some 800 million years after the Earth and Moon form.
© ESA.Could asteroids bring volatiles such as water from further out in the Solar System to the primordial Earth?
Water delivery to the early Mars and Earth
Following the Late Heavy Bombardment, the inner rocky planets of Venus, Earth and Mars all received water. That there was water on Venus is evidenced by its atmosphere's deuterium/hydrogen ratio (deuterium being the heavier stable isotope of hydrogen): hydrogen was preferentially lost from Venus following its the greenhouse runaway. The Earth had (has) water and oceans: we obviously see it has today. Mars had water mainly in its northern hemisphere ocean (sea) which we can see from the remains of ancient shorelines.
However Mars was too small, with a lower gravity, and Mars did not have a protective planetary magnetic field for long (which prevented Solar wind ablation of the atmosphere). Both these meant that it could not keep its water or its atmosphere: it lost much of both over the past three billion years or so, leaving just what we see today.
Research in 2017 by the US based geographers and environmental scientists, Wei Luo, Xuezhi Cang and Alan Howard, looked at Mars examining the former oceans and valleys, looking at altitudes, literally pixel by pixel. They concluded that at the very least the volume of water on the ancient Mars was 6.86x108 km3. (1)
Having said this, compared to previous calculations, it is as much as 20 times that of some other Martian water volume estimates. This therefore might be considered the initial inventory of Martian water before it began to lose its water to space.
By comparison, the volume of Earth's water is 13.86x108 km3.(2). So the Earth has a little over twice the volume of water as the primordial Mars had.
At first this does not seem that surprising. After all Mars has a diameter a little over half that of the Earth's. But Mars' orbit is a lot closer to that of the orbits of volatile-rich asteroids and so one might expect Mars to have proportionally more water delivered to it by a perturbation of these asteroids during the period of the Late Heavy Bombardment than Earth: similarly, one might expect Earth – being further from volatile-rich asteroids – to have less.
This then begs the question as to why the Earth has so much water?
(Here, it is worth noting that even if the primordial Mars did not have as much water as this high estimate – and the true value was much lower – then the question as to why the Earth has so much more water becomes more acute.)
Of course, the frost line could have been closer to the Earth than somewhere around where Jupiter is today in the very early Solar System at the time of the late heavy bombardment, but given that by then the Sun had got going it seems unlikely to be that a significant a factor. Indeed, it is considered that the frost line in the early Solar System was never inside the orbit of Mars. So, how did the Earth get its water.
Park that notion to one side for now, as there is another issue.
A problem
There is a body of recent (2022 and 2023) research that suggests that the bulk (about 90%) of Earth's volatiles originated in the Solar System's dust cloud close to Earth's orbit and little came from asteroids originating from beyond the frost line. This body of work is based on the proportions of isotopes of elements in volatile-rich meteorites from beyond the frost or snow line. These differ from the isotopic proportions found on Earth and so suggests that Earth's volatiles came from material in orbits close to that of Earth, and not from a late veneer of volatiles provided by the Late Heavy Bombardment.(3, 4, 5)
So, given it seems as if most of Earth's volatiles did not come from beyond the frost or snow line, and given that the Earth is much further from the frost/snow line than Mars, it really is an important question to ask if the Earth got its volatiles and water another way?
As it happens, a new idea has recently been proposed, and this has implications for Earth-size (and those slightly larger) rocky planets in stars' habitable zones everywhere: both in our planetary system, across this galaxy and in others!
In 2023, this new research has provided a lifeline...
A solution…
Three planetary geologists – Edward Young, Anat Shahar and Hilke Schlichting – have provided an alternative hypothesis as to how the Early Earth got its water. They hypothesise that it did not have all its water imported via carbonaceous asteroids, but self-generated it!(6)
Their idea is this. The primordial gas and dust cloud from which the Sun and planets ultimately formed, contained a lot of hydrogen (as does the interstellar medium today as well as the Sun. Indeed today the Sun is 98% hydrogen and helium, as are the atmospheres of Jupiter and Saturn.
As the larger asteroids form (asteroids larger than tens of miles across) within this pre-Solar System cloud. the heat from the collisions – from the impacting together of billions of tonnes – temporarily melted the rock and this molten rock absorbed the hydrogen in the primordial dust and gas cloud. These large asteroids then themselves combined to form planetesimals and these in turn formed the planets.
The initial Earth was simply a giant ball of magma and this ultra hot ocean of molten rock released the stored hydrogen to form a thick hydrogen atmosphere (along with carbon dioxide, nitrogen and so forth). This ball of magma was a mix of iron and silicates: the iron core had yet to separate out.
From here, straightforward chemistry produced water. The molten blob (of iron and silicates) surrounded by hydrogen gas (and other gases) of the proto-Earth enabled a series of chemical equilibrium reactions. These can be greatly summarised as below, with iron silicate being in equilibrium with iron oxide and silicate. In turn, silicate and hydrogen are in equilibrium with silicon and water.
Once the proto-Earth's crust cooled the surface and near-surface reactions ceased and as a consequence there was surface water. Beneath the surface, the heavy elements and iron settled out into an iron-rich core. However, some other elements that iron would remain in that core and this, as we shall shortly see, indeed fits in deductions of the Earth's core's density.
The Edward Young collaboration note that the young, smaller Mars would not be molten long enough (the magma surface temperature needs to be 3,000° kelvin), nor was it large enough with sufficient gravity to be able retain a hydrogen atmosphere for any length of time for this mechanism of water creation to take place. So Mars' ancient ocean must have got its water from asteroids and comets.
Edward Young, Anat Shahar and Hilke Schlichting depict the early Earth's magma ocean and hydrogen rich atmosphere mechanism for generating water below.
A summary diagram of the process. © Young, E. D, Shahar, A. & Schlichting, H. E. Source (reference 6) used here non-commercially in the context of a science review.
One advantage of this theory is that at a stroke it explains another mystery.
It has long been known (since the 1960s) that Earth's core is not as dense as it should be if it were solid metal: their must be lighter elements in its mix.(7) This new theory of how the Earth got most of its water also explains this lower than iron Earth-core density too!
Exobiological implications
If this new theory is correct – and it is certainly logical, plausible as well as also happening to explain the low Earth core density issue – then the implications for Earth-sized planets in habitable zones, around suns are considerable. It would mean that they all would have appreciable volumes of water. Especially for stars close to our Sun's size (hence temperature and brightness) this would make life on Earth sized planets in their respective habitable zones highly likely. True, small planets the size of Mars would not have either the gravity be able to retain a dense atmosphere or have a core big enough to sustain a molten state generating a magnetic field protecting the atmosphere from Solar wind. And planets around cooler, small stars (red dwarfs) have habitable zone so close to the star that its stellar wind would likely remove the atmosphere.
However, one might reasonable conclude that around larger stars (like our Sun), planets at least three-quarters the size of the Earth all the way through to those up to one-and-a-half Earth's size would have self-generated their water in their primordial magma state. Anything bigger than one-and-a-half Earth's size would have too strong a gravity to allow hydrogen to escape and so be a gas giant.
Nonetheless, given the number of stars in the Galaxy and their size distributions that still gives us plenty that would be likely to host an Earth-sized watery world in their respective habitable zones. If there is an early Fermi filter weeding out alien intelligence, the rarity of watery Earths is probaly not it.
Jonathan Cowie
References
1. Luo, W., Cang, X. & Howard, A. D. (2017) New Martian valley network volume estimate consistent with ancient ocean and warm and wet climate. Nature Communications, vol. 8, 15,766.
2. US Geological Survey (1984) The Hydrologic Cycle – USGS pamphlet. USGS, Reston, Virginia, USA.
3. Steller, T. et al. (2022) Nucleosynthetic zinc isotope anomalies reveal a dual origin of terrestrial volatiles. Icarus, vol. 386, 115,171.
4. Nie, N. et al. (2023) Meteorites have inherited nucleosynthetic anomalies of potassium-40 produced in supernovae. Science, vol. 379, p372-376.
5. Martins, R. et al. (2023) Nucleosynthetic isotope anomalies of zinc in meteorites constrain the origin of Earth’s volatiles. Science, vol. 379, p369-372.
6. Young, E. D, Shahar, A. & Schlichting, H. E. (2023) Earth shaped by primordial H2 atmospheres. Nature, vol. 616, p306-311.
7. Birch, F. (1965) Density and composition of mantle and core. Journal of Geophysical Research. vol. 69, p4,377–4,388.
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