Where to Source Carbon on the Moon?

[sanman]

Where can carbon be sourced on the Moon?

Carbon may be useful in production of propellant, or for polymers, or even for production of biomass.

What are the primary or most accessible lunar sources of it?

Would it mainly be cometary ice (methane) deposits? Or could it be found deeper underground?

Carbon is quite abundant on Earth, and the Moon is supposed to share some common origin with the Earth. Are there particular types of mineral deposits which might be looked to useful sources?

[DistantTemple]

Where can carbon be sourced on the Moon?

Carbon may be useful in production of propellant, or for polymers, or even for production of biomass.

What are the primary or most accessible lunar sources of it?

Would it mainly be cometary ice (methane) deposits? Or could it be found deeper underground?

Carbon is quite abundant on Earth, and the Moon is supposed to share some common origin with the Earth. Are there particular types of mineral deposits which might be looked to useful sources?

Wikipedia says that Carbon and Nitrogen appear to only be present in trace amounts on the moon. If its age is 4.5Bn years as currently believed, that is before life on Earth and before carboniferous rocks were laid down like limestone (all the result of lifeforms). However AIUI carbon was present  on Earth then. If there was a big impact that "blew away the atmosphere" was available carbon burned and blown away as CO2? The proto Moon would have been molton, and is not big enough to keep and atmosphere, and now has 45% by weight O2... !!! Guess guess guess...

[guckyfan]

There may be CO and CO2 in the polar cold traps. We don't know for sure. Maybe even nitrogen?

[Lar]

Short and medium term I think ice traps or brought with. Long term probably cometary impact? I think we're going to consume a fair bit of the oort cloud industrializing this system...

[sanman]

Is it fair to say that it would probably be easier to produce hydrolox propellant on the Moon than to produce methalox?

For example, Musk has stated that Starship would be capable of making a return round trip to the Moon and back, all on a single tank of propellant. That had better be the case, since Starship is using methalox, which may be hard to come by on the Moon (at least the methane part of it).
Meanwhile, the more Moon-focused Blue Origin is has both hydrolox and methalox in its set of technologies. Their Moon plans seem to be more centered around the Blue Moon lander, which seems to use hydrolox as propellant. So they likely won't be carbon-constrained in terms of what they'll be flying.

Will there be a desire to produce carbon-based propellant on the Moon in the near/medium term future, due to overlap between lunar and Mars mission objectives?

[speedevil]

Is it fair to say that it would probably be easier to produce hydrolox propellant on the Moon than to produce methalox?

For example, Musk has stated that Starship would be capable of making a return round trip to the Moon and back, all on a single tank of propellant. That had better be the case, since Starship is using methalox, which may be hard to come by on the Moon (at least the methane part of it).

A very large fraction of ISRU utility is from oxygen alone.
As to hydrolox, a lightweight hydrogen tank (it is never launched full so never sees earth launch or entry loads) inside the cargo compartment of SS with ~15 tons of hydrogen, and a couple of RL10 class engines would do the bulk of the heavy lifting - with a hair of methane for the raptors to get it off the ground gets you all the way to earth with almost no methane.

This tank is launched filled with STP H2 nominally, and does not impact the other payload other than to slightly volume limit it.
You don't much care about boiloff in this tank, as it is filled, and immediately burned through TEI.

[DistantTemple]

Is it fair to say that it would probably be easier to produce hydrolox propellant on the Moon than to produce methalox?

For example, Musk has stated that Starship would be capable of making a return round trip to the Moon and back, all on a single tank of propellant. That had better be the case, since Starship is using methalox, which may be hard to come by on the Moon (at least the methane part of it).

A very large fraction of ISRU utility is from oxygen alone.
As to hydrolox, a lightweight hydrogen tank (it is never launched full so never sees earth launch or entry loads) inside the cargo compartment of SS with ~15 tons of hydrogen, and a couple of RL10 class engines would do the bulk of the heavy lifting - with a hair of methane for the raptors to get it off the ground gets you all the way to earth with almost no methane.

This tank is launched filled with STP H2 nominally, and does not impact the other payload other than to slightly volume limit it.
You don't much care about boiloff in this tank, as it is filled, and immediately burned through TEI.

No extra complication, just run a few pipes! (A three fuel rocket doesn't at first sight match EM's quest for simple design and manufacture.) But seriously this provides an alternative to a mini Raptor for Moon landing and launch! With some plumbing you get 2 for a tonne. Thrust 110KN (for RL10) each so with gravity at 1.6N/Kg 220KN lifts 140Tonnes (just) If you can mount them half way up the SS (near the H2 tank?) it also deals with rocket plume impingement on unprepared surfaces!  Trouble is if they were able to buy the engines they would probably be several times the price of any SX engine!

[sanman]

If water ice on the Moon is primarily through cometary deposits, then why should cometary methane ice deposits be so much less?

[speedevil]

If water ice on the Moon is primarily through cometary deposits, then why should cometary methane ice deposits be so much less?

Any impactor will make plumes of aerosol, it does not land intact.
This then does ~zero to a handful of 'orbits' of the moon, and sticks if it hits a very cold part.

Even at 70K, methane has ~0.1% earth atmosphere vapor pressure.
The parts of the moon where it's cold enough for methane to stick are at best considerably less than that where water can survive.

[Lampyridae]

According to observations from the LCROSS impact:

"Spectroscopic  measurements from this experiment revealed 5-7 wt% of H2O, among a  comet-like  array  of  volatiles,  including  (abundance relative  to  H2O  in  parentheses):  H2S  (16.8%),  NH3(6.0%), SO2 (3.2%), C2H4 (3.1%), CO2 (2.2%), CH3OH (1.6%),   CH4   (0.7%),   OH   (0.03%)"

Page 42 of this PDF: https://www.lpi.usra.edu/lpi/contribution_docs/LPI-002087.pdf

Carbon is there, just not in massive quantities, and mostly as ethylene, not methane. But ~5 tonnes of carbon for every 100 tonnes of water is not a bad production rate. Ethylene is particularly useful as you can go direct to polyethylene production with that just by tapping it off from the volatiles. No extra steps needed. Alternatively, you can directly use it a fuel for smaller surface hoppers etc. There will be an excess of oxygen in the production chain anyway because hydrolox runs fuel-rich to keep the temperature down, and your humans probably won't breathe through all that oxygen.

Human beings also breath out a lot of carbon. About 600kg of CO2 will be exhaled by a crew of 4 over a 6 month stay, so you will have a couple of hundred kg carbon available from human sources if you don't choose to close the life support loop.

[speedevil]

According to observations from the LCROSS impact:

"Spectroscopic  measurements from this experiment revealed 5-7 wt% of H2O, among a  comet-like  array  of  volatiles,  including  (abundance relative  to  H2O  in  parentheses):  H2S  (16.8%),  NH3(6.0%), SO2 (3.2%), C2H4 (3.1%), CO2 (2.2%), CH3OH (1.6%),   CH4   (0.7%),   OH   (0.03%)"

Carbon is there, just not in massive quantities.

If I'm doing my mental math right, the mass of carbon atoms as a total fraction of the mass of all the atoms is >>5%.
That's quite significant.

[Lampyridae]

Yup. Nitrogen is also surprisingly abundant. Ammonia is another propellant option, useful for coolant loops, can be the basis of hypergolics and you can use it for polyamide production for products like kevlar. You can combine it with oxygen and aluminium to make ALON, aka transparent aluminium.

You can also burn sulphur as a rocket propellant, albeit not a great one, and I'm sure there's a bunch of industrial processes that use it.

[KelvinZero]

According to observations from the LCROSS impact:

I remember someone here providing some links to doubts about the other constituents in the LCROSS results. We just have to go and have a look. I hope Chandrayaan-2 shines some light on this. Very happy to see someone doing something.

[Lampyridae]

CO2 has a rather low sublimation temperature of 55K compared to 110K for water so it's only found (abundantly?) in the deepest cold traps.

https://www.hou.usra.edu/meetings/lpsc2019/pdf/2628.pdf

It is possible that there are very deep reservoirs of graphite in the lunar interior, which are responsible for "fire fountain" activity.

https://pubs.geoscienceworld.org/msa/ammin/article/100/8-9/1668/104657/magmatic-volatiles-h-c-n-f-s-cl-in-the-lunar

[LMT]

IR Detection of Carbonaceous Lunar Ices

According to observations from the LCROSS impact:

I remember someone here providing some links to doubts about the other constituents in the LCROSS results. We just have to go and have a look. I hope Chandrayaan-2 shines some light on this. Very happy to see someone doing something.

Carbonaceous lunar ices are certainly of interest.  Near-term detection of appreciable carbonaceous ice content could shift lunar ISRU propellant production from hydrolox to methalox.

Notably, Chandrayaan-2 has an imaging IR spectrometer (IIRS):

IIRS has two primary objectives:

- Global mineralogical and volatile mapping of the Moon in the spectral range of ~0.8-5.0 µm for the first time, at the high resolution of ~20 nm
- Complete characterisation of water/hydroxyl feature near 3.0 µm for the first time at high spatial (~80 m) and spectral (~20 nm) resolutions

Might IIRS also detect carbonaceous ices?

Carbon Dioxide

CO2 snow has strong spectral features in the IIRS detection window.  Singh & Flanner 2016, Fig. 3.  Old CO2 ice might be harder to detect due to large grain size, but the paper notes that "CO2 snow albedo varies less with grain size [than H2O ice] because CO2 ice is inherently less absorptive in the near-IR..."  So maybe IIRS can detect it.  Hansen 1997 Fig. 11 suggests the possibility.  That CO2 absorption spectrum is for "clear, thick CO2 ice samples."  The thick-ice spectrum has similarities with the snow spectrum.

Some mixed CO2/H2O spectra are given in Bernstein et al. 2005.  Fig. 1 shows a strong fundamental CO2 absorption at ~ 4.2 µm.  Also there's a 2.13 µm feature, which is found only in mixtures.  It's considered a "sensitive indicator of CO2 intermolecular interactions". 

We highlight the enhancement of the putative CO2 (2ν3) overtone near 2.134 micron (4685 cm-1) and its potential as an observational (spectral) indicator of whether solid CO2 is a pure material or intimately mixed with other molecules. As far as we know this is by far the most sensitive indicator of CO2 intermolecular interactions.



Figure 1. The 1.75-22 micron (5700-450 cm-1) IR spectrum of an H2O/CO2 = 5 ice mixture at 15 K. In addition to the broad absorptions of amorphous solid H2O and the sharper CO2 fundamentals one sees the 'forbidden' 2ν3 overtone of CO2 at 2.135 micron (4684 cm-1). This feature is prominent in spectra of mixtures but is not seen in spectra of pure CO2 (see Fig. 3).

Carbon Monoxide

The interstellar ice modeling work of Zamirri et al. 2018 predicts a strong CO spectral feature near 4.68 µm, just inside the IIRS detection window.

Ethylene

Ethylene (C2H4) has deep absorption features between 2.2 and 2.4 µm which seem to be separable from features of H2O, CO2 and CO ices.  Merlin et al. 2008. 

Figure 11. Reflectance spectra of ethane (dotted line) and ethylene (continuous line). The reflectance spectra were shifted of +0.4 and−0.1, respectively, for clarity. The deepest and largest bands appear between 2.0 and 2.4 μm.

Q:  Which other carbonaceous ices have spectral features potentially detectable by IIRS? 

Note:  Hurley et al. 2016:

Although most scientists agree that water was positively detected in the LCROSS plume, identification of some of the organic species is more tentative, and the spectrum is not well-modeled longward of 2.1 µm.

Refs.

Bernstein, M. P., Cruikshank, D. P., & Sandford, S. A. (2005). Near-infrared laboratory spectra of solid H2O/CO2 and CH3OH/CO2 ice mixtures. Icarus, 179(2), 527-534.

Hansen, G. B. (1997). The infrared absorption spectrum of carbon dioxide ice from 1.8 to 333 μm. Journal of Geophysical Research: Planets, 102(E9), 21569-21587.

Hurley, D., Colaprete, A., Elphic, R., Farrell, W., Hayne, P., Heldmann, J., ... & King, D. (2016). Lunar Polar Volatiles: Assessment of Existing Observations for Exploration. NASA Goddard Space Flight Center.

Merlin, F., Alvarez-Candal, A., Delsanti, A., Fornasier, S., Barucci, M. A., DeMeo, F. E., ... & Schmitt, B. (2008). STRATIFICATION OF METHANE ICE ON ERIS' SURFACE. The Astronomical Journal, 137(1), 315.

Singh, D., & Flanner, M. G. (2016). An improved carbon dioxide snow spectral albedo model: Application to Martian conditions. Journal of Geophysical Research: Planets, 121(10), 2037-2054.

Zamirri, L., Casassa, S., Rimola, A., Segado-Centellas, M., Ceccarelli, C., & Ugliengo, P. (2018). IR spectral fingerprint of carbon monoxide in interstellar water–ice models. Monthly Notices of the Royal Astronomical Society, 480(2), 1427-1444.

[LMT]

Chandrayaan-2 Updates

The first Chandrayaan-2 IIRS results may be posted there.  If the team posts IIRS spectral data, we can compare against reference spectra for carbonaceous ices, such as CO2, CO, and C2H4, for armchair interpretation.

It'll be interesting to see whether carbonaceous ices are present in lunar cold traps.  IIRS data could potentially decide the question of lunar ISRU propellant production:  i.e., hydrolox vs. methalox.

Q:  What carbon concentration might justify the switch from hydrolox to methalox? 

One reference point: 

CAVoR methalox production was quantified with baseline feed concentrations of 5 wt% hydrocarbon, 15 wt% water, and 80 wt% inorganic inert matter.  Therefore baseline CAVoR carbon concentration is 4 wt%.