Update (5 days later…) No further corrections, so the published number is still 600 Million Tons, and I still have a hard time believing it. As an engineer, I always try to put such reports into context, just to fit them into the larger pictures. So here goes:

Recently, NASA crashed the LCROSS probe into the heart of a permanently shadowed crater, to kick up some dirt and see how much water is in it. a 20 m diameter crater (300 m²) was formed, and 100 kg of water ejected. This was a direct hit to the “most shadowed” area of a crater – it stands to reason this was a good water excavation event.

The total permanently shadowed area on the moon (more on that later) is estimated anywhere to be between 50 and 15,000 km². (The larger number is based on a simulation-based prediction that leaves a lot to be desired, but let’s go with it for a moment, keeping in mind we’re way too liberal here.)

Multiplying the amount of water excavated by LCROSS, which should be a high estimate compared to the average, by the larger of the shadowed crater area estimates, we get 0.1 ton x 15,000 E6 m² / 300 m² = 3.6 million tons – not even 1% of the published estimate. If we go with the lower area estimate, we get an even larger discrepency

So what gives?

Well, the clue is in the more detailed press release on the mini-SAR web site. If you read the press release, you’ll notice it only says that: “Although the total amount of ice depends on its thickness in each crater, it’s estimated there could be at least 1.3 trillion pounds (600 million metric tons) of water ice. “ They never really even say they measured that amount of ice.

Reading further, the methodology is described pretty clearly, and it is obvious that the instrument has no ability to estimate actual amounts. It can only generate a signature which is indicative of either ice, a rough surface, or other phenomena. Since the signature is shown to be correlated with permanently shadowed surfaces, they assume it is ice, and only very indirectly make a guess as to its thickness.

The same signature also appears on fresh craters. But old craters differ from fresh craters mostly by the amount of thermal cycling they see… Could it be that permanent shadowed craters are simply preserved better? Could it be that they accumulate micro-dust instead of water? mini-SAR can’t tell.

In short, what we have here is a series of conjectures based on a radar image, mixed in with with some assumptions, and multiplied by additional enthusiasm. This is not good science. What they should have said was: “Radar measurements support the previous results of the Neutron Spectrometer Lunar Prospector, in that there is likely water ice in permanently shadowed craters.” and left it at that without the sensationalism.

See here for a summary of water-on-the-moon announcements.

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Update (9 am PST) The press release was changed quietly to say 1.3 Trillion pounds – not Million – which is now at least compatible with the 600 million tons figure. However, I still don’t like it… It need to look further into it – I’d love to see how this plays out.

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Original post:

Ok – so everyone and their cousin is echoing the latest PR from the back-to-the-moon department at NASA regarding water found on the moon.

Regrettably, as is the case with most recent lunar water announcements, the numbers don’t even add up in the context of the press release, not to mention with respect to common sense.

To start with, this priceless line:

” it’s estimated there could be at least 1.3 million pounds (600 million metric tons) of water ice. “

So first, 1.3 E6 pounds is 600 tons – not 600 E6 tons – only 6 orders of magnitude off. Not bad, whoever you are – with this sort of accuracy, an aircraft carrier weighs about 200 kg. One would think, since a ton is a lot bigger than a pound, that someone would have noticed even without doing the math… (Yet all space-related web sites reported it as is, no exceptions)

Second, enough with the passive tense already – “it is estimated” – by whom?

Then, there’s the context.

The release implies macroscopic amounts of water – stuff that makes this water “an exploration and operational destination.”

Consider this report about the permanently shadowed crater on the lunar pole. As you can see, “permanent” is a loose term. What they mean is “longer than a single lunar cycle”. This is not a billion year cryo-trap or anything, but rather a short duration condition, which means that water does not have geological time scales to accumulate there. With the lunar atmosphere being the molecular nothingness that it is, where would macroscopic amounts of water even come from?

In short – I don’t know which of the above numbers is correct, but I wish there would be more attention paid to it

There is water in the lunar Cryo-traps – there’s no arguing it. But it’s not something that’s usable for exploration – these are minuscule amounts in very low concentrations and in an extremely hostile environment. Too much spin, methinks.

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