• Aryan Shah

Where did the water come from?



Upfront, right here, right now - Thermal imaging has the potential to detect porosity in asteroids and the likelihood of bringing water to Earth.


If I say the word 'Asteroid', what comes to mind? Maybe the Chixulub impact crater in Mexico, the one that killed the dinosaurs?


Something bad, negative on the whole, right?


What if I told you that asteroids are a good thing. What if someone (that someone being me) told you that they could have been the reason for your very existence, the reason you are reading these words at this moment in time and space?


Well, what I hope to achieve in this article is to show you why asteroids are a good thing and how that is. What is their astronomical significance and what can we learn from them to implement into the future?


[enter Asteroid]


There exists a theory which suggests that extra-planetary satellites or mini planets (asteroids) bombarded our early planet, which resulted in the formation of water on Earth and therefore life that we see today.


The asteroids that are believed to have brought water here are the C-type asteroids, the oldest that we are aware of. These date right back to the start of our Solar System 4.6 Ga (Giga annum - billion years). They have the potential to have the essential conditions and elements for water formation due to the aftermath of the Big Bang 14 Ga and the abundance of Hydrogen distributed throughout the universe.


But before we investigate the likelihood of this theory, let's analyse some of the other ways water could have been formed on Earth:

  1. Planetary cooling - volatile substances cool enough to condense into water

  2. Extra-planetary sources - asteroids bringing those key elements and conditions

  3. Hydrate minerals - leaking of water stored in rocks on Earth

  4. Volcanic activity - water vapour from volcanoes cooling, condensing into rain

  5. Role of Organisms - water may have had some terrestrial origin to some extent = CO2 +2H2S→CH2O+H2O+2S = sulphide dependent chemoautotrophic bacteria fix Carbon dioxide into water as a by-product of a photosynthetic pathway utilising CO2 and H2S (hydrogen sulphide)



At the end of the day, it is completely up to you as to which one you believe to be the origin of water on Earth but remember we don't have enough conclusive evidence to definitively select one of these options as the direct origin.


And this is why it is so important you continue on with this article and understand why I think asteroids were the possible cause of water formation on Earth.


Ryugu asteroid gets a test done...


C-type asteroids can be analysed by physical surface properties that they display, but because of high pressure, temperature and speed of atmospheric entry, such significant properties are often lost as these asteroids do not survive entry onto Earth. This suggests that this asteroid could have been made from less consolidated materials (as its surface properties are lost to atmospheric entry). But we know just how important C-types are to this discipline, but if we cannot measure and analyse them, then are we just extrapolating and making arbitrary assumptions about the origin of water on Earth?


No, not today!


One space agency, JAXA (Japanese Aerospace Exploration Agency) have managed to do the unthinkable and use in situ measurements, especially a thermal infrared imager to explore the possibility and extent of porosity and C-type asteroid formation. They examined one particular asteroid called Ryugu, which was analysed to have a rough surface filled with boulders and a bulk density of 1190+/- 20 kgm^-3. Keep in mind, that most asteroid boulders have temperatures ranging from 300-310 Kelvin in reality - this is important!




Thermal imaging


But why would you want to use thermal imaging? What is the point?


Because thermal imaging can allow us to understand more about the 'physical state' of the asteroids, specifically 'particle size, porosity, boulder abundance and surface roughness'. All of these can be derived from thermal inertia (the degree of slowness of an objects internal temperature becoming equal to its' external temperature) which is 'remotely sensed' without the need for physical contact with the asteroid itself.


And we need to learn more about its physical condition because we don't really have enough evidence to make any conclusive statements. We've been to the moon and we've sent rovers to Mars (most recent being the successful Perseverance rover landing YESTERDAY!) which have identified for us that the surface is comprised largely of Regolith (unconsolidated solid material covering the bedrock of a planet) due to high speed meteoritic impacts. However, for smaller satellites such as the Ryugu asteroid, it is surrounded by low gravity so any loose particles that comprised its surface originally would have been lost to deep space - so it's really important we take a closer look at such asteroids to find out its composition and therefore make more informed decisions about the past and future of our planet.


After this point can we really start to answer the question in the title!


Results, results, results


In this investigation, inertial measurements gave JAXA some promising results. The many boulders on Ryugu are evenly distributed across its surface, allowing there to be a similar temperature inside and outside of this boulders - owing to a low thermal inertia. The average tiu(thermal inertia unit) for Ryugu (especially its boulders accumulated) was 282 tiu. But if you remember what I said earlier, most boulders have typical temperatures of 300–310 Kelvin, corresponding to highly porous materials with a thermal inertia of 200–300 tiu. Furthermore, with regards to Ryugu, JAXA measured it's boulders to have average of 300-340 Kelvin temperature and a bulk porosity of 50-60%. Such high porosity for an asteroid, right?


This means that its' parent body must have been very porous too for its' 'child' asteroid to have been so. I also stated in the "Ryugu' section above, that it would have been made from less consolidated materials. Well, what do we know of in space that is less consolidated and has high porosity?


Any takers?


No? Well, it could be cosmic dust - click here if you want to read more about cosmic dust (fluffy).

Now if we couple this with the low thermal inertia that it has, then we can assume that low tiu correlates to higher micro or bulk porosity present in Ryugu or any other C-type asteroid for that matter.

Therefore, by using thermal imaging on a C-type asteroid, we can identify its thermal inertial unit, micro and bulk porosity percentages - which in my eyes bodes well in the effort to understand more about the origin of water on our planet.


Whilst this was one example (detailed at that) of how thermal imaging can be utilised to our advantage in the Earth Sciences, you can start to see the wider implications of this study in the future.


Back to the Future


C-type asteroids are known to have hydrogen and nitrogen isotope levels that are pretty similar to that of Earth's seawater. This is a random sentence, but it matters in the grand scheme of things as you'll see in a minute. If we can compare the ratio of specific elements between two objects or things, we can get a better idea of how 'related' they are - this is similar to the kinship of VNTR's in genetic fingerprinting (which is a whole other topic for another day)!


So, if we compare the deuterium (stable hydrogen isotope with a neutron as well as one proton and electron) to hydrogen isotope (only proton and electron present in the atom) ratio on asteroids with the same ratio on Earth, it should be fine, right?



Well, you must be wondering why I just sprung Deuterium on you just here, it's because Earth's current Deuterium to hydrogen ratio matches ancient Chondrite asteroids which actually originate from an outer asteroid belt in our solar system; and our seas contain a beautifully large amount of Deuterium.


After reading a paper from Caltech where Dr. Renyu Hu (Jet Propulsion Lab, Caltech) discusses how he and his team made a model simulating the diurnal exchange of Deuterium-Hydrogen isotopes at the surface of Mars due to several processes occurring in the Regolith (surface material) such as adsorption of its particles, I thought that the methods he discusses are key in understanding the origin of water on our planet.


Boundary level problems


'We construct a one-dimensional model to simulate transport of isotopic water in the Martian Regolith and boundary layer. It has a thermal diffusion module, a water transport module, and a boundary layer module. The model includes the isotope fractionation effects of adsorption, condensation, and molecular diffusion.' Whilst you guys don't need to worry about this last sentence here, the key is that we can utilise boundary layers here.


Boundary level problems (in which a problem such as a differential equation is limited by a specific set of parameters which restrict the area and extent to which a BL problem can be solved), are problems where we can actually make such simulations and models as realistic as possible by adding the most life-like limitations to the rate of change to water and D-H exchange in Regolith - because nothing in life will follow a single equation to the T, so it's key to keep that in mind when applying such Mathematics to real scenarios. So, if you're doing A level maths, then the limits on the Integral symbol are the limits which define the area of the graph you can solve.



I think if we have the capacity to investigate the Regolith on Mars, then why can't we do the same on Earth? Earth too was bombarded by meteorites and extra-planetary satellites over billions of years to form the planet that we can see today, maybe we can simulate (similar to Hu's paper) the extent of water exchange in our own Earth and how this would have changed. This is where Dr. Hu's paper comes into play and where we can use such models and BL problems coupled with differential equations to answer this grand question in the title.


We could sample rocks from the pre-Cambrian period (such as in the Yarabubba impact crater in Australian outback) and observe changes in rock composition or the onset of glassy spherules too to indicate an asteroid impact. In addition, we could use a 'Tunable Laser Spectrometer (TLS)' which was used on the Curiosity rover ('the first in-situ measurement of the isotopic composition of water in the Martian atmosphere (Webster et al. 2013)'). It could be used to understand such composition of regolith or boulders on C-type asteroids such as Ryugu in the future (perhaps looking for Deuterium : Hydrogen ratios) and therefore the likelihood of water maintenance.


So, by sampling the past, we would gain a further insight into our and all other life's origins. It would be a huge task, but it would be worth it at the end of the day, because the end goal is to understand as much as we can about our planet to help it and us progress into an uncertain future.


Conclusion


So, by using thermal imaging, we can gain a deeper insight into the porosity of satellites such as asteroids and how they were formed. Therefore, by using this new knowledge, we can dig deeper into the origin of Earth's water supply and the mechanisms by which the water arrived. I think that asteroids are one of the most, if not the most viable candidates for this theory and thermal imaging really has the potential to pave the way for Earth Sciences in the future.


Where do you think water came from? Leave a comment down below of what you think!


Bibliography:


'Highly porous nature of a primitive asteroid revealed by thermal imaging' - https://hal.archives-ouvertes.fr/hal-03053282/document - accessed 5th February 2021


'Predicted Diurnal Variation of the Deuterium to Hydrogen Ratio in Water at the Surface of Mars Caused by Mass Exchange with the Regolith' - https://arxiv.org/pdf/1905.03882.pdf - accessed 10th February 2021

'Terrestrial deuterium-to-hydrogen ratio in water in hyperactive comets' - https://www.aanda.org/articles/aa/pdf/2019/05/aa35554-19.pdf - accessed 12th February 2021


Origin of water on Earth - https://en.wikipedia.org/wiki/Origin_of_water_on_Earth - accessed 9th February 2021


Where did Earth's water come from? - https://en.wikipedia.org/wiki/Origin_of_water_on_Earth - accessed 11th February 2021

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