Earth's subsurface may hold up to 5.6 × 10⁶ million metric tons of hydrogen
Comments
Animats
roenxi
> There's been so much well-drilling worldwide for other materials that if hydrogen was anywhere near the surface, it would have been found by now.
I'd believe it because geologists are thorough. I'd also not be that shocked if nobody was testing for hydrogen because it is a gas. I'd imagine it is possible to drill through a hydrogen deposit and not even notice it is there. Are we sure that the prospectors were checking for hydrogen? All over the globe?
I suppose if they found a real lode of the stuff it might accidentally blow up the drilling crew. That'd make headlines.
defrost
> Are we sure that the prospectors were checking for hydrogen? All over the globe?
Yep .. checking for everything really - the costs for drilling bore samples are high enough that it's commonplace to log bores to have the data to store or onsell even if specific targets aren't found.
The major explorers have petabytes of surface chemisty, seismic, EM, borehole samples and logs, radiometrics, magnetics, gravity, etc. in primary archives scattered across the globe and routinely digitised and merged into private reserve estimations.
There are many drill hole logging and interp software packages kicking about, eg: https://www.csiro.au/en/work-with-us/industries/mining-resou...
scott7ree
As a prospector myself, this is false. Assays are expensive for traditional minerals and we never assay for hydrogen as that requires a totally different set of procedures.
roenxi
Yeah I've sat on an exploration drill rig and I have a vague grasp of the physics and chemistry. That is why I'm a little sceptical - what exactly would the process be for identifying a hydrogen resource?
We're dealing with a light gas that would probably escape from core samples very quickly; especially under normal conditions. They'd need to get an accurate read during core drilling or be able to identify specific a non-magnetic gas with density of 0 underground which sounds pretty challenging - especially since it seems to have no special commercial interest for most of history. Is there a standard that you have to have a gas monitor attached to the drill hole? I don't remember anyone pointing one out to me or complaining that theirs was broken but stranger things have happened. Can hydrogen even be detected with magnets or surface chemistry analysis?
The way sound waves bounce around underground makes it quite challenging to pick things up. The geologists have put a lot of effort into this exact problem but prospecting for hydrogen sounds damn difficult and I'd be surprised if we had global coverage for it.
Animats
Right, most analyses of cores might not find small traces of hydrogen. But if someone looking for natural gas drilled into a sizable hydrogen deposit, it would be hard not to notice that the methane had way too much hydrogen.
defrost
In the drill core, even after gas escapes, there'd be specific types of capping material that can trap hydrogen under pressure, below that there'd be a reduced density of more porous material.
Hydrogen prospectors looking backwards at drill core logs would be looking for signature transitions and retesting fields, looking again at the seismic results to find ROI's in historic results.
Hence:
Geological signatures: https://academic.oup.com/jge/article/21/4/1242/7676857
Same authors, restricted access (for now): Geologic hydrogen: An emerging role of mining geophysics in new energy exploration - https://library.seg.org/doi/10.1190/image2024-4100417.1
Old people rambling:
Des FitzGerald on geophysical exploration for naturally occurring hydrogen. Des outlines the current state of exploration for natural hydrogen and discusses geological mechanisms for hydrogen generation.
etc.
roenxi
If they have to theorycraft a resource based on traces of where the hydrogen used to be, but no longer is then it is entirely possible that big hydrogen deposits have just been missed. That seems to be literally what the article today is about. For 90% of minerals they can just say what is in the drill sample is what is underground, exploration geologists aren't generally in the business of imagining what might have been in the core independently of what was directly measured.
If we need to apply specific theories to the exploration samples then the "There's been so much well-drilling worldwide for other materials that if hydrogen was anywhere near the surface, it would have been found by now" logic doesn't hold. Since the evidence has to be interpreted before we can know if there is a deposit it is quite possible that it was interpreted wrongly on a mass scale. You're linking to papers suggesting innovative novel methods for finding the stuff or talking about rechecking based on the latest theoretical understanding, suggesting we don't actually have a big historic archive to draw on.
I'm not saying geologists are ignorant, just that Animats' logic doesn't hold for hydrogen. There could be massive deposits that we technically already have the data for except nobody ever bothered to look for it.
defrost
It goes to motivation, until recently there's been a lot of talk about 'pure' hydrogen extraction but little actual pragmatic hydrogen exploration; specifically funded hydrogen targetted developable resource programs.
Now that there's growing economic justification for investing time and money (at least a decade, easily on the order of a billion (that seems low) outlay before return) in hydrogen, serious exploration starts.
As in all exploration phases the money funnel begins with prospects which means record crawling looking for patterns - actively developing prospects and mapping prospect fields expands on the current pattern knowledge and that better understanding, trained on emperical results, gets cycled back into the record crawling phase.
This goes to the original question, there is already detailed data, seventy years worth of logged geophysical, vaulted by major explorers; prospectors who look at a $50 million USD TSX prospectus as the absolute minimum low bar of any interest in capital rasing mineral development projects.
Buried in that data is almost certainly (confidence) patterns that identify most of the larger near surface deposits.
NB: the italic stresses are deliberate, across the entire globe, looking back from 50 years after today, that seems likely to stand up as a geostatistical statement of formal E(xpectation).
In the course of going forward from today a better understanding of how to read the tealeaves wrt hydrogen will develop, and this:
> You're linking to papers suggesting innovative novel methods for finding the stuff or talking about rechecking based on the latest theoretical understanding, suggesting we don't actually have a big historic archive to draw on.
will look exactly right only flipped: we have a big historic data archive, we need to develop a better prospect filter for a new resource of interest.
Years ago a similar thing happened with gold data, a big historic data archive got reprocessed with better algorithms using the latest <cough> learnings </cough> and then a few years after that watered down academic papers appeared, eg:
Towards the automated analysis of regional aeromagnetic data to identify regions prospective for gold deposits https://www.sciencedirect.com/science/article/abs/pii/S00983...
that talked about trawling already existing data archives for correlated patterns.
This is part of the ongoing grind of geophysical exploration.
lazide
Uh, if the gas had any useful quantity at all it would be under pressure and would be coming out of the borehole with noticeable speed/pressure.
Most natural gas is also hydrogen. This isn’t that unusual, in actuality.
What is unusual is ‘pure’ hydrogen, as most processes end up combining it into a denser composite molecule. Like water, or methane, etc.
ianburrell
Natural gas is methane. Methane is composed of hydrogen, but it is mostly carbon by weight. Chemicals are different than their components. Water is also made of hydrogen but it takes work to split it.
lazide
Natural gas depending on source can have a couple percent free hydrogen. Adding more is apparently becoming more popular.
In some markets, it comes from LNG which is pretty pure methane, in others it comes from wells which has more hydrogen as well as other contaminants like sulfides.
scott7ree
Water could be a key to discovery.
Orange hydrogen is a theorized method of water fracking/stimulating ultramafic rock bodies to speed up the chemical reaction creating H2.
scott7ree
Correct, testing for a gas is a lot different than traditional soil and rock sampling and assays techniques.
onlyrealcuzzo
How often are people drilling for gold or something and accidentally stumble upon oil?
I can't imagine this is a common occurrence, given how much effort people put into oil exploration...
scott7ree
Often an explorer looking for gold finds something else like copper or nickel. Oil however is generally found in a different environment. H2 is created through serpentinization in areas more prone to mineral discovery.
mnky9800n
I think what’s even worse is this paper is not connected to any kind of reality. They just make up some data from their computational imaginations and clicked submit. In places where we do have observations we don’t see much gas coming out.
For example, this paper I wrote a couple years ago that’s from a suggested hydrogen source site in Oman: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/202...
Hilift
Global production of hydrogen is about 75 million tons, about half from ammonia, half from capture during petroleum products (refining). That's a problem primarily due to it is plateaued and there isn't capability to increase supply unless someone makes ammonia crackers more efficient. The other major obstacle is natural gas has been artificially inexpensive due to the abundance of supply due to fracking. It's hard to compete with it. It's possible to build turbines that burn ammonia, but no-one wants it.
https://www.iea.org/data-and-statistics/charts/global-demand...
ianburrell
Hydrogen is used to produce ammonia, not the other way around. There is no natural source of ammonia. The first link is all about the use of hydrogen. The third is about ammonia cracker which may be useful to transport ammonia and turn back into hydrogen.
Most hydrogen is produced by steam reforming methane, called gray hydrogen.
throwaway519
We don't discover gold or diamond mines when drilling for oil but that's not to suggest we don't believe they don't exist.
The number of holes made to get oil out is quite small in comparison to the surface area of the globe.
defrost
The number of holes made to probe the dimensions of oil and gas fields greatly exceeds the number of holes made to get oil out .. and the number of holes drilled to estimate mineral reserves (copper, gold, kimberlite (diamonds), bauxite, etc. etc. etc) is large in comparision to oil wells.
The point of all those holes is to log layers, horizons, sediments, etc and to map out the geology of very large areas .. much much much larger than the combined bore hole diameter areas.
Of course boreholes are the final step in "proofing" siesmic results that map out many layers across large areas and allow geologists to rule out many areas as not having the structures required to trap gases.
sebastianmestre
The figure in the title expressed in normal units
5.6 * 10^15 kilograms
adonovan
Thanks, I didn’t notice the second million and wondered why a day’s supply of energy would be scientific news.
WorkerBee28474
I think you meant to say 1.1 trillion elephants
acchow
2.24 billion olympic sized swimming pools
peeters
They wasted an opportunity to get a third thing meaning "million" into the same number with 5.6 x 10^6 million megagrams.
fulafel
This article the crux, is this about extractable hydrogen or some proxy about it (vs just "interesting number"), to the last sentence.
The abstract is again the best summary:
"[...] Given the associated uncertainty, stochastic model results predict a wide range of values for the potential in-place hydrogen resource [103 to 1010 million metric tons (Mt)] with the most probable value of ~5.6 × 106 Mt. Although most of this hydrogen is likely to be impractical to recover, a small fraction (e.g., 1 × 105 Mt) would supply the projected hydrogen needed to reach net-zero carbon emissions for ~200 years."
oefrha
> stochastic model results predict a wide range of values for the potential in-place hydrogen resource [10^3 to 10^10 million metric tons (Mt)] with the most probable value of ~5.6 × 10^6 Mt.
As a former physicist, I find it hard to take anyone who dares to give two significant figures on such a terrible estimate seriously. At the very least tells me they don’t know shit about statistics. And whoever is clueless enough to repeat the figure in such a misleading title should be banned from scientific publishing.
phtrivier
When I started doing math seriously, I also feel strongly in love with "existence proof", where you were asked to prove that "something" existed, and any logical reasoning was considered fair game, even if you never found the "something".
Then, I started doing applied maths, where proving the existence of a solution is a nice bonus, but finding an approximate solution is the goal.
Here, we have an example of a funny proof of existence that does not tell you where to drill.
Some carbon was emitted during the publishing of this model - that will be so much more carbon to offset if we ever end up actually finding some real hydrogen.
h_tbob
CO2 has its problems but at least nature automatically recycles it and produces O2 again.
But what about hydrogen? Wouldn’t burning it consume our oxygen supply with no way to replenish without large scale electrolysis? Seems like this could be a worse disaster since nature doesn’t do that by default.
philipkglass
No, photosynthesis turns water into oxygen and hydrogen-containing organic compounds.
https://en.wikipedia.org/wiki/Photosynthesis#Refinements
Samuel Ruben and Martin Kamen used radioactive isotopes to determine that the oxygen liberated in photosynthesis came from the water.
dvh
It's only 0.000001 of the mass of Earth's atmosphere (assuming 5e18kg)
pfdietz
If extracted and fully oxidized, the water would raise ocean levels maybe 4 cm.
The Earth also includes vast quantities of reduced metals like iron, more than enough to react with all the oxygen in the atmosphere. Perhaps some way could be found to exploit that, at least a little bit.
foundart
Well there's an idea for a sci-fi disaster book: "Rustpocalypse" (if too much of that iron were to be oxidized.)
pfdietz
All it would take would be for photosynthesis to be terminated, and then wait a few million years for erosion and volcanism to expose enough reduced material to soak up the atmosphere's oxygen.
What's weird is that, as far as I know, there's no feedback mechanism that's been identified that keeps the atmosphere's O2 level stable. It may have been stable since the Cambrian just because if it hadn't been, we wouldn't have evolved, an anthropic argument.
Earw0rm
Fire must surely have a role to play there? Too much O2 and plants burn easily, too little and fires won't take hold.
And we don't know AFAIK that it's been entirely stable. There's some debate over what the level was in the Cretaceous for example.
pfdietz
If anything, fire would be a positive feedback. That's because fire produces charcoal, and charcoal doesn't decompose. Instead, it gets washed into the ocean and eventually buried. It's not photosynthesis itself that causes O2 accumulation in the atmosphere, it's the burial and sequestration of reduced material from photosynthesis.
FredPret
Apocalypse (written in Rust)
seangrogg
*rewritten in Rust
suprfsat
Rust Evangelion Strike Force
tolciho
Banded Iron Formation: the reunion tour.
ryao
If the metal is reduced, then it should not be reactive. How does this become relevant to all of the oxygen in the atmosphere?
adrian_b
Reduced substances are those that can be oxidized by the free dioxygen from the air, so they are reactive and unstable in the presence of air.
In the presence of air, oxidized substances, like silicates and the other abundant components of stones and soil, are the substances that are non-reactive and stable.
For many billions of years, even long before the evolution of the kind of phototrophy (a.k.a. photosynthesis) that produces free dioxygen by oxidizing water, the living beings had to continuously produce reduced (and reactive) forms of carbon, nitrogen and sulfur from the oxidized (and non-reactive) forms of carbon, nitrogen and sulfur from the environment.
These biological reduction processes have also used solar energy a very long time before the evolution of the variant that produces free oxygen, and before that they have used free dihydrogen, which is produced naturally by the reaction between the partially reduced iron, Fe(II), from volcanic rocks, with water, which oxidizes it to Fe(III), releasing reduced free dihydrogen as a consequence of the reaction. Here the origin of the energy that powers this process is the internal heat of the Earth, because at the higher internal temperatures the substances that are in chemical equilibrium are not the same that are in chemical equilibrium at low temperatures. So when surface rocks are created by volcanism, they are not in chemical equilibrium and the reaction between their reduced components with water can produce the energy that has fed the first forms of life until they have evolved the means for capturing solar energy.
Probably the most important development in human technology has been the discovery of how to transform the non-reactive oxidized forms of metals from the environment into reduced forms of metals, which are reactive, therefore they are easily corroded, but they are very useful materials. For many millennia, the reduced metals have been produced with the help of another reduced substance, i.e. charcoal, whose ultimate origin is in the reducing processes by which living beings produce reduced carbon from the oxidized carbon dioxide.
pfdietz
What? Carbon is reduced. Hydrogen is reduced. Any fossil fuel is reduced. All can give up electrons to oxygen and so be oxidized, liberating energy.
ryao
I went with what you wrote since it has been a while since I took general chemistry, but upon doing a simple lookup, I found that your terminology is wrong. In redox reactions, the oxidizing agent is reduced while the reducing agent is oxidized:
https://en.wikipedia.org/wiki/Redox
Here, the metal is the reducing agent. If it were somehow reduced in redox reactions, there not much chance of it being oxidized as that would make the metal an oxidizing agent that wants electrons, not a reducing agent that gives electrons.
That said, these things have already been oxidized (not reduced) and thus there is no chance to have them consume oxygen. You need the pre-oxidized material in order to be able to consume oxygen.
Finally, you failed to answer my question regarding the relevance of these metals to atmospheric oxygen. They should be inert having been oxidized long ago. That is why rocks are full of oxides, such as silicon dioxide and aluminum oxide, despite being composed of metal.
pfdietz
I think there's confusion here between "is reduced" meaning "has been reduced" vs. "and is then reduced".
Iron in the Earth is mostly in a reduced state (either Fe(+2) or even elemental iron). Upon exposure to the atmosphere it is oxidized to Fe(+3), changing from a more reduced to a more oxidized state (and similarly for other things in reduced states, such as manganese and sulfur and organics).
ryao
Plenty of iron in the earth is in the form of iron oxide, which is already oxidized and won’t oxidize further upon exposure to the atmosphere.
adrian_b
As the other poster has said, about half of the iron in the Earth is as elemental iron in the core and the other half is as ferrous Fe(II) oxide in the mantle, which both are reduced forms of iron that are unstable in the Earth atmosphere.
The fully oxidized iron, i.e. ferric Fe(III) oxide, i.e. rust, forms a negligible fraction of the iron in the Earth, being restricted almost entirely to the upper crust of the Earth. A small amount of ferric Fe(III) oxide (which forms magnetite with the remainder of ferrous oxide) is formed at great depths by the reaction with water, where this is present in the rocks, which releases some free dihydrogen, which may remain trapped in the rocks and which is the source of the hydrogen discussed in this article.
Here, on the surface of the Earth, almost all iron is oxidized as a consequence of being exposed to the air, but when iron comes from the deeper regions of the Earth, through volcanic eruptions, it is reduced and it begins to oxidize after being exposed to the air. So the volcanic rocks with iron are unstable in the air, they transform slowly into rocks with oxidized iron, in the same way as the artificial objects made of metallic iron, which rust.
In general, saying just "reduced" and "oxidized", is ambiguous for many chemical elements, because, like iron, they can have several degrees of reduction or oxidation. Most frequently, using "reduced" and "oxidized" without any other qualifier is taken to mean unstable or stable in the presence of an oxidizer. Some oxidizers are stronger than others, so a given degree of reduction/oxidation may be stable or unstable depending on the oxidizer that is used. However, without other mentions, it is understood that the implicit oxidizer is air. In the presence of air, metallic iron and Fe(II) are reduced, while Fe(III) and Fe(VI) are oxidized. Fe(VI) can be created only by a stronger oxidizer than air, but once created it can no longer be oxidized by air, like the Fe(III) of rust, so both are oxidized forms of iron that are stable in air.
pfdietz
An example of oxidation of iron in rocks can be seen in the Hawaiian island of Kauai. This island is 5 million years old and is heavily eroded. The dark black volcanic rocks typical of the younger islands have turned bright red, for example in Waimea Canyon State Park.
pfdietz
It's mostly in the form of ferrous oxide (Fe(+2)). Fully oxidized to rust, it goes into the ferric form (Fe(+3)).
It's very common in weathered rock to see the red color of ferric iron.
Ferrous iron occurs in common minerals like pyroxene, hornblende, pyrite, and olivine. Olivine is the most common mineral in the upper mantle.
Eddy_Viscosity2
IS this alot alot? Like how much hydrogen would be needed to capture all of the atmospheric oxygen and how much water would that make?
johnea
Alright! More stuff to set on fire!!!
ars
There is around 1 trillion tons of oxygen in the atmosphere, if you burned all the hydrogen you would deplete all of the oxygen on earth.
Let's not.
Although realistically we only need a tiny fraction of the hydrogen.
stouset
On the plus side if we use up all the oxygen, we’ll have solved the problem of burning fossil fuels producing CO2!
shiroiushi
If we use up all the oxygen, we'll have solved every social or political problem that currently plagues humanity. I think it's a good strategy.
dmichulke
FWIW, I consider extinction avoidance also a political problem
selimthegrim
This is like the Bojack Horseman prescription to solve America’s gun problem (Watch the show, I won’t spoil it)
m3047
Purple Earth hypothesis. The first (AFAWK) photosynthetic critters were cyanobacteria. They produced enough oxygen to kill off everything which couldn't withstand its reductive effects. Oxygen levels have been much higher than they are today, presumably this is what made e.g. 6 foot centipedes a possibility.
shwouchk
If siblings are to be believed, there is nothing we can do about it aside from being very careful not to release the hydrogen into the atmosphere (at which point it will “burn” whether we want to or not)
hgomersall
Closer to 10^15 tonnes, so a few orders of magnitude out.
blindriver
If the hydrogen gas escaped and left the atmosphere, would it affect the orbit around the sun, possibly causing the Earth to cool too much?
tzs
I'd expect not, for 2 reasons.
• I can't think offhand of any mechanism by which it would escape in some preferred direction. I'd expect to be pretty much evenly spread in all directions, so any effects on the orbit of what remains caused by the hydrogen leaving in any particular direction would be cancelled out by the effects of hydrogen leaving in the opposite direction.
• We are talking about 5.6 x 10^12 tons of hydrogen. The mass of the Earth is 5.972 x 10^21 tons. The mass of the hydrogen is about 1 billionth the mass of Earth. That's about the ratio of the mass of a grain of rice to the mass of the International Space Station.
Tossing that small of mass away, even if it was all in one direction, is not going to do anything significant to your orbit unless you tossed it away with very very very high velocity. A naive calculation just using Newtonian mechanics suggests it would have to be much faster than the speed of light to carry enough momentum away to matter. I'll leave it to others to figure out what fraction of the speed of light it would have to be going to have equivalent momentum.
Someone
> I can't think offhand of any mechanism by which it would escape in some preferred direction.
I think the sun’s heat would give it a slight preference to escape from the sunny side of earth.
hollerith
No because the escaping gas would on average have the same velocity as the (lighter) Earth does.
ngcc_hk
If two mass separated, it will affect their velocity.
But does this hydrogen escapee like a rocket gas. Or it is just a restructure or a bit move of CoG.
One also note if it not too fast escape as earth rotate the overall effect depend upon the uniformity of the “escape” as the net effect can be zero or more depends upon its location on earth. Mostly as it is not align with the tangent of travel, it will affect.
How much as point out relates also the mass. But even the minor variations make the polar star changes. It will have affect.
lazide
There is zero chance free hydrogen would exist long enough in our atmosphere for it to escape. It would convert to water long before hand.
reshlo
> Hydrogen escape on Earth occurs at ~500 km altitude at the exobase (the lower border of the exosphere) where gases are collisionless. Hydrogen atoms at the exobase exceeding the escape velocity escape to space without colliding into another gas particle.
> For a hydrogen atom to escape from the exobase, it must first travel upward through the atmosphere from the troposphere. Near ground level, hydrogen in the form of H2O, H2, and CH4 travels upward in the homosphere through turbulent mixing, which dominates up to the homopause. At about 17 km altitude, the cold tropopause (known as the "cold trap") freezes out most of the H2O vapor that travels through it, preventing the upward mixing of some hydrogen. In the upper homosphere, hydrogen bearing molecules are split by ultraviolet photons leaving only H and H2 behind. The H and H2 diffuse upward through the heterosphere to the exobase where they escape the atmosphere by Jeans thermal escape and/or a number of suprathermal mechanisms.
lazide
I can’t decide if it is a good point, or an irrelevant point! Hah.
No free/unbound hydrogen from the surface is going to escape directly that way. It will bind with oxygen or the like long beforehand and become water.
But yes, a small portion of those molecules may later be broken down and may escape the planet that way. But statistically, very few of them are likely to do so.
So, maybe technically correct?
karaterobot
This is one of those comments where I'm not sure what the downvotes meant. Do people think a downvote signifies a 'no' answer to a yes/no question, or are they trying to say "I don't appreciate it when people ask questions"?
reshlo
In this case it’s probably “this is a silly question”.
karaterobot
Maybe it is, but the person asking it will never know that unless someone takes the time to respond saying so.
The Albanian mine: "The researchers found that the gas bubbling from the pool was more than 80 per cent hydrogen, with methane and a small amount of nitrogen mixed in. It was flowing at a rate of 11 tonnes per year, almost an order of magnitude greater than any other flows of hydrogen gas measured from single-point sources elsewhere on Earth’s surface. To determine the source of the gas, the researchers also modelled different geological scenarios that could produce such a flow. They found the most likely scenario was that the gas was coming from a deeper reservoir of hydrogen accumulated in a fault beneath the mine. Based on the geometry of the fault, they estimate this reservoir contains at least 5000 to 50,000 tonnes of hydrogen. “It’s one of the largest volumes of natural hydrogen that has ever been measured,” says Eric Gaucher, an independent geochemist focused on natural hydrogen. But it still isn’t a huge amount, says Geoffrey Ellis at the US Geological Survey."
This is the second or third time someone found modest amounts of hydrogen underground, and then started making claims of vast quantities being available. There's been so much well-drilling worldwide for other materials that if hydrogen was anywhere near the surface, it would have been found by now. The "gold hydrogen" enthusiasts claim well depths of a few kilometers are enough. Oil and natural gas wells routinely go that deep.
So far, nobody has a "natural hydrogen" well producing. Even though this startup [1] said they would have one by the end of 2024. Their "news" releases are all about going to meetings, making deals, and such. Not much mention of drilling, unlike the statements they made a few years ago.
There's one well in Mali which yields enough hydrogen to run an auto engine driving a generator. That's it for actual output. That deposit been known since the late 1980s, and invested in since 2012. Exploratory wells were drilled in 2018. Results from that are, somehow, hazy.[2] Not finding followups since 2018.
The hype is strong here.[3]
[1] https://helios-aragon.com/news/
[2] https://www.sciencedirect.com/science/article/abs/pii/S03603...
[3] https://www.scopus.com/record/display.uri?eid=2-s2.0-8518695...