Oh if mars was barren and Mercury teaming with life...

O by the by, neutrons decay in free-space, in other words, free neutrons are radioactive. With a decay time of less than 15 mins, it means 2 things: slow neutrons will be less detectable at distance because they decay, you still need to be relatively close to the source of neutrons to detect them regardless of speed. Neutrons are also the only form of radiation that will make things radioactive, meaning if you get to close and the bombardment is to intense , you can cause damage to your equipment via internal radiation of beta and gamma rays.

This is also why they use water in nuclear reactors, hydrogen, and in particular deuterium (hydrogen with a neutron) slow neutrons better than anything. Water is mostly hydrogen by mole, so it is a very good moderator, both light water (regular water) and heavy water (deuterium water).

What is happening in this particular case is known as nuclear spallation. When a high energy proton hits something like carbon or nitrogen, it will at times knock a proton or neutron loose. Those neutrons are moving at relativistic speeds in most cases, so on the flip side, when those neutrons bounce their way out back to space, if there is water in the way they get slowed way down...enough that they decay before they reach the detector.

This is the same exact effect that allows for carbon dating, sometimes, the high energy neutrons that come out via spallation will in turn knock out a proton from a nitrogen atom, it then becomes mildly radioactive carbon. This happens at a relatively predictable rate, and since the decay of carbon 14 is also predictable, dating is possible.

Science rant over