Tuesday, July 12, 2016

What about the rest of the LCROSS results?

 A lot of people were excited when LCROSS returned direct evidence of water and water ice on the Moon six years ago, providing strong support for the theory of cold trap volatiles. I've seen numerous posts discussing ISRU and using that result (plus other data) as evidence of the presence of water.
 What I haven't seen is any mention of the other chemicals that were detected: carbon monoxide, mercury (!) and elemental hydrogen. Water was estimated to make up 5.6% of the crater soil with 155kg detected, meaning mercury made up another 3.5 to 4% with about 100kg detected (at larger uncertainty). Carbon monoxide represented hundreds of kilograms, meaning perhaps 4 to 8%.

{{note: Hop pointed out in comments that this paper was corrected. The reported values were overestimated by a factor of 5.5 or so, meaning these volatiles are actually perhaps 0.5-1% by mass each.}}

 Think about that for a minute... Carbon monoxide is probably more abundant in the cold traps than water, at least in the traps cold enough to freeze CO. Everything I've read suggests that the Moon is almost carbon-free and there is no hope of producing hydrocarbon propellants like methane there. At the same time, millions of tons of water are proposed to exist and to be usable for producing hydrolox propellant.

 I'm sensing a disconnect here.

 The evidence shows that both compounds are present in staggering abundance. I see no reason why we can't make methane using these resources, which in turn means there is no need for a deep cryogenic hydrogen-based lunar infrastructure. The same methane engines that will be used between here and Mars will be used on and around the Moon.

 Propellant from ISRU mining operations at the lunar poles will fuel all of the proposed chemical systems. For that matter, very simple monoxide-oxygen rocket systems work just fine (albeit at low Isp) and could be used in early exploration if methane or H2 production is not yet online. Even later on, if fuel production is constrained by available power then monoxide rockets could be used to deliver higher-value fuel to an EML2 depot.

 In addition, mercury has some very useful properties. It's a liquid at standard conditions, easy to ionize, stable, dense and with a high atomic mass. These are traits suitable for electric engine propellant. It's toxic, sure, but not as bad as hydrazine. It may not be as efficient a propellant as xenon or lithium but it's plentiful and would be accumulated anyway as a byproduct of water purification. May as well put it to use. (Here's an example engine from a family in the 2500-3600s Isp range.) There are engineering difficulties: it tends to foul the spacecraft and it's hard to feed precisely, but we built hardware in the 70's that withstood over ten thousand hours of operation. These are solvable problems.

 Other uses include as radiation shielding, for extracting native metals (via amalgamation followed by electrorefining), in fluorescent or mercury discharge lamps (including germicidal lamps) and as an electrode used in several chemical processes such as the chloralkali process for splitting sodium chloride.


  1. Volatiles in the LCROSS plume were reported in the October 2010 issue of Science. The LCROSS team also reported a lot of other exciting volatiles. Besides the carbon monoxide you mention there was 7.7% nitrogen!

    Sadly the LCROSS team published a correction in the September, 2011 issue of Science. Abundance of volatiles was overestimated by a factor of 5.5. That news was a kick to my stomach. I don't want to share it but feel compelled to.

    And you probably know, LRO's LEND data doesn't support Spudis' hopes for thick sheets of ice in the cold traps.

    I haven't completely given up hope for rich lunar volatile deposits but I'm not as excited and optimistic as I once was. Until prospector rovers check out the permanently shadowed crater floors, it's unwise to count our chickens before they're hatched.

    1. Ah. That is indeed disappointing, but thanks for setting me straight. Still, even fractions of a percent are better than a few PPM.
      Let's hope we get some samples to analyze.