A closer look at data from NASA's oldest functional Mars orbiter has thrown up a puzzling result scientists didn't see coming: the Red Planet is hiding water ice where there shouldn't be any.

A reanalysis of readings sourced by the Mars Odyssey probe reveals substantial amounts of water ice buried just under the Martian surface around the Red Planet's equator – but given what we know about Mars's climate, that shouldn't be possible.

A team led by planetary scientist Jack Wilson from Johns Hopkins University reevaluated previous measurements taken by the Odyssey's Neutron Spectrometer tool, which gauges the presence of hydrogen close to the surface of Mars.

One of the Odyssey's primary research objectives is to find water on the Red Planet, but the probe can't measure water directly from its lofty orbit altitude of some 3,800 km (2,400 miles).

Instead, it looks for hydrogen, which acts as a reliable indicator of where water can be found. While the orbiter also can't measure hydrogen directly, it can infer it from neutrons it detects when cosmic rays collide with atoms in the Martian soil.

This high-tech prospecting technique is what led Odyssey to detect buried water ice on the Red Planet way back in 2002, but at the time the readings indicated that the discovery was largely restricted to Mars's polar regions.

This makes sense, because scientists think that ice around the Red's Planet equator can't persist for long periods buried in the soil, as thermal conditions mean it should sublimate into the atmosphere.

But by using a Bayesian image reconstruction method to effectively up-sample the resolution of Odyssey's previous neutron detections, the team found inferred water reservoirs that hadn't been discovered before – including at latitudes near the equator.

"It was as though we took the spacecraft and lowered altitude by half, so we saw a few new things on the surface that weren't visible before," Wilson explained to Jesse Emspak at New Scientist.

"We knew in the past there was buried ice close to the poles. We saw there are also small patches near the equator."

It's an important finding, indicating the potential layout of much-needed water resources that could one day influence where human colonists set up camp when we finally make it to the Red Planet.

But before that happens, there's the little matter of resolving how this water ice can be there at all.

According to Wilson, one possibility is that Mars may sometime in its past have rotated on an axis highly inclined to its current one.

The team thinks if the Red Planet's current inclination was once tilted by 20 degrees a few million years ago, it might have been possible for water ice in polar regions to sublimate, before eventually redistributing to these newly found lower latitude reservoirs over time.

The researchers acknowledge other explanations may be more likely, such as the Martian soil somehow offering properties that enable water ice buried within it to resist sublimation.

It will take more research to figure out which of these two hypotheses holds water (ahem), but while we go about that, the latest study highlights the value of taking another look at the results you've already got from time to time – because you easily could have missed something before.

"This is a really wonderful example of how data, once collected, can be analysed with new techniques," planetary geologist Jim Head from Brown University, who wasn't involved with the study, told Science.

"When we eventually send people to Mars, we'll want to go where the water is."

The findings are reported in Icarus.