A new radar map of Mars' mid-latitudes confirms that they are the remnants of a vast ice sheet hidden under the Martian rubble.
The icy leftovers have been found over a significant part of Deuteronilus Mensae, an area about halfway between the Martian equator and North Pole. The ice was mapped by the Italian Space Agency's Shallow Radar instrument on NASA's Mars Reconnaissance Orbiter.
"It's definitely a record of a different climate period," says Dr Jeffrey Plaut of NASA's Jet Propulsion Laboratory.
Today all that remains are thick ice piles buried at the foot of hillsides under rocky debris. "The debris protects them from sublimating," says Plaut, referring to the process of solid water ice evaporating into the air without any intervening liquid phase.
Plaut and his colleagues prepared their new ice map for presentation at this week's Lunar and Planetary Science Conference .
The ice is just a few metres below the rubble in some places, says Plaut, but can run a kilometre thick. The long lost ice sheet probably existed tens to hundreds of millions of years ago, based on the features of local impact craters, which are used to date landscapes on Mars.
And while that might seem like a long time, it's nothing compared to the billions of years that have passed since Mars might have been capable of supporting liquid water, he says.
"It's not all the way back to the earliest days."
That timing would seem to support the idea that Mars can have very long-term climate changes caused by the gradual wobble in the tilt of the planet's axis. The wide wobble creates periods when Mars' poles dip very low, allowing the Sun to shine more on the poles in their respective summer times.
That means warmer poles, which would cause polar ice to sublime into gas, thickening Mars' atmosphere so the water could fall as snow at lower latitudes.
This idea was put forward five years ago by Professor James Head III of Brown University, who presented numerous features in this same region that looked suspiciously like glaciated terrain on Earth.
"Essentially all of the features were known from the Viking spacecraft in the late 1970s," says veteran Mars scientist Professor Vic Baker of the University of Arizona.
Head's work made the case yet again using much higher resolution images, says Baker. "Mars was screaming at us that it had a lot of water and ice."
He says the problem was the evidence was all based on the science of geomorphology, or land forms, which is not an area a lot of physicists put much stock in.
Now that a radar instrument is backing up the geomorphology that's been known for decades, "A lot of physicists will start working on it," says Baker.
One thing that's not likely to come of this is a rover mission to dig up the ice. A few metres of debris is not exactly within the current rover job description.
"It's more of a job for a backhoe," says Plaut.