Lunar South Pole

Cold traps that can hold ice

Near the poles the Sun stays low on the horizon, and the floors of some craters never see it. These permanently shadowed areas can stay cold enough to trap water molecules for very long periods. A study in Nature Astronomy by Hayne, Aharonson and Schörghofer revised the area of the Moon able to trap water efficiently upward to about 40,000 square kilometres, with about 60% of it in the south. The same study found that most cold traps lie at latitudes above 80 degrees, because shadows closer to the equator are usually too warm, and that tiny shadows down to about a centimetre across add to the total. Ice, if present, would be scattered rather than pooled in one place.

Evidence for ice at the surface

Cold traps show where ice could survive, not where it is. A 2018 paper in the Proceedings of the National Academy of Sciences by Li and colleagues used data from the Moon Mineralogy Mapper on India's Chandrayaan-1 orbiter to find several thousand pixels with signatures of water ice within 20 degrees of latitude of both poles. The authors found ice exposed in only about 3.5% of cold traps, at locations whose annual maximum temperatures stayed below 110 kelvin. That patchiness is the reason for sending robots. Orbital maps narrow the search, but only instruments on the ground can show how much ice lies in a given spot and how it is mixed with soil.

Sunlight on the high ground

The same low Sun that keeps crater floors dark can keep some nearby ridges and rims in sunlight for long periods, which matters to robots that run on solar power. A lander set down on well-lit high ground can charge its batteries for long stretches while a rover makes short trips into colder, darker terrain. JAXA's description of its Lunar Polar Exploration mission with ISRO follows this pattern, with the lander touching down near an investigation area that has long sunlit hours before deploying the rover. Planners still have to manage long shadows across the route, a Sun that skims the horizon into camera views and the need for a line of sight to Earth for direct radio contact.

Rovers sent to measure the water

The Lunar Polar Exploration mission, known as LUPEX, is a joint project of JAXA and ISRO to find out how much water the polar regions hold and in what form, so that its use can be judged. JAXA says the rover will observe up to 1.5 metres underground, analyse the surface and collect samples where hydrogen is detected. Its instruments include JAXA's Resource Investigation Water Analyzer, an ISRO ground-penetrating radar, a NASA neutron spectrometer and an ESA exospheric mass spectrometer. JAXA lists the launch as no earlier than 2028 on an H3 rocket, with the south polar region as the destination. Results from missions like this will decide whether lunar ice is a resource or a curiosity.

Sources and further reading

• Direct evidence of surface exposed water ice in the lunar polar regions (PNAS, 2018) — University of California eScholarship (Proceedings of the National Academy of Sciences).
  Study using Chandrayaan-1's Moon Mineralogy Mapper that found exposed water ice in shadowed areas near both poles.
• Micro cold traps on the Moon — Nature Astronomy.
  Estimates how much of the Moon can trap water ice, including tiny shadows, and how that area splits between the poles.
• Lunar Polar Exploration Mission (LUPEX) — JAXA Space Exploration Center.
  JAXA's page on its joint rover mission with ISRO to measure water in the Moon's south polar region.

Common questions

Is there definitely water ice at the lunar south pole?

Orbital evidence for ice in permanently shadowed areas is strong, but how much there is, how deep it lies and how it is mixed with soil remain open questions. That uncertainty is why upcoming rovers carry drills, radar and neutron instruments rather than cameras alone.

Why don't landers set down inside the shadowed craters?

Permanently shadowed floors are extremely cold, offer no sunlight for solar power and are hard to reach by radio. Most plans place the lander on sunlit ground nearby and send a rover or instrument into the shadows for shorter visits.