The thick cap of water ice at Mars' north pole is more than 1000 kilometres wide and up to three kilometres deep.© NASAPictures of Mars's north pole have revealed a record of the planet's climate over the past 3 million years.
The climate history is written in light and dark bands exposed on the sides of ice cliffs. Scientists now say that they can read these bands in the same way as climatologists on Earth interpret cores drilled from deep-sea sediments.
Sarah Milkovich and James Head, geologists from Brown University in Rhode Island, used a series of images from the Mars Orbiter Camera on board NASA's Mars Global Surveyor craft.
The pair combined the pictures with data from the craft's Mars Orbiter Laser Altimeter to pinpoint the precise separation of these bands to a resolution of just a few metres.
The climate changes recorded in the bands seem to be driven by wobbles in the planet's orbit, something that planetary scientists have suggested could make the ice at the poles wax and wane, or even disappear altogether. These results strongly support that theory, the two researchers argue in the Journal of Geophysical Research: Planets1.
Orbital wiggle
The darker bands on the wall of this polar trough are rich in dust whipped up by summer storms.NASA/JPL/Malin Space Science SystemsThe north pole of Mars is covered with a thick cap of water ice, more than 1,000 kilometres wide and up to 3 kilometres deep. Spiral patterns of deep troughs cut deep into the ice, in places exposing 800-metre-high vertical cliffs.
The researchers saw a banding pattern of light and dark layers, recurring roughly every 30 metres through the ice. They say that this is caused by a change in the planet's orbit that happens every 51,000 years, moving it slightly closer to or further from the Sun.
They think the darker bands contain more dust, and relate to periods when Mars's northern hemisphere was getting more sunlight than the southern hemisphere. When this happens the atmosphere in the north warms, whipping up winds that carry dust to the north pole.
The increase in sunlight also melts larger amounts of the polar ice than melt in a normal summer, so the dust becomes more concentrated in the uppermost layer of ice.
Matching patterns were found in ten different troughs across about three-quarters of the ice cap, proving that the layering was caused by a regional change and not just a local event.
Missing band
The researchers also found that the banding pattern is missing from a 100-metre section of the ice that corresponds to a period between 0.5 million and 2 million years ago. That looks like strong evidence for an 'ice age' on Mars, when frozen material from the pole crept down towards the equator, like ice ages on Earth.
Unlike the shorter cycle caused by orbital wobbles, this change was probably caused by a tilt of the planet's axis, which chilled the equator by moving it away from the Sun, but relatively warmed the north pole. The much increased melting at the pole meant that the layers were disturbed enough to break up the regular 51,000-year pattern.
“Geological evidence within the cap for ice ages has been lacking.”
Sarah Milkovich
Brown University, Providence, Rhode Island.
"The concept of ice ages on Mars has been worked on by a lot of researchers," says Milkovich, "but the geological evidence within the cap has been lacking."
The 30-metre pattern resumes deeper into the ice, and indicates an 'interglacial period' between 2.1 million and 2.7 million years ago. Then, even earlier, the pattern disappears again, possibly corresponding to the end of another ice age between 3 million and 4 million years ago.
The research should help to explain how Mars's polar ice caps change, and how they distribute water around the rest of the planet, says Milkovich. The pair are now analysing pictures from the planet's south pole to recover a similar climate record.
Brown University, Providence, Rhode Island.
© 2019 Nature is part of Springer Nature. All Rights Reserved.
