A team of planetary scientists has used radar and a high-resolution camera to reveal the subsurface geology of Mars' northern ice cap.
The findings – based on data from SHARAD (the surface-penetrating radar) and HiRISE (the high-resolution camera) on the Mars Reconnaissance Orbiter – were published May 27 in two papers in the journal Nature.
The group studying a canyon feature called Chasma Boreale included Shane Byrne from the University of Arizona's Lunar and Planetary Laboratory. Jack Holt and Isaac Smith of The University of Texas at Austin's Institute for Geophysics are the papers' lead authors.
"The ice sheet on Mars' northern polar region is about the size and thickness of the Greenland ice sheet," said Byrne. "Just like Greenland, the layers of ice on Mars preserve a climate record that reaches back probably a few million years. Studying this ice sheet and its internal layers tells us about Martian climate and how it has varied in the past."
On Earth, large ice sheets are shaped mainly by ice flow. But on Mars, according to this latest research, other forces have shaped, and continue to shape, the polar ice caps.
The northern ice cap is a stack of ice and dust layered up to two miles deep covering an area slightly larger than Texas. Analyzing radar data on a computer, scientists can peel back the layers like an onion to reveal how the ice cap evolved over time.
Chasma Boreale is one of the most distinctive features of the northern ice cap – a canyon about as long as the Grand Canyon but deeper and wider. Some scientists have suggested Chasma Boreale was created when volcanic heat melted the bottom of the ice sheet and triggered a catastrophic flood. Others have suggested strong polar winds, called katabatics, carved the canyon out of a dome of ice.
Other enigmatic features are troughs that spiral outward from the center of the ice cap like a gigantic pinwheel. Since they were discovered in 1972, scientists have proposed several hypotheses for how they formed. One suggested that as the planet spins, ice closer to the poles moves slower than ice farther from the poles, causing the semi-fluid ice to crack. Another used an elaborate mathematical model to suggest how increased solar heating in certain areas and lateral heat conduction could cause the troughs to self-assemble.
It turns out both the spiral troughs and Chasma Boreale were created and shaped primarily by wind. Before this research, conventional wisdom held that the northern ice cap of Mars was made of many relatively flat layers like a layered cake. It was assumed some climate information would be recorded in the layers, limited to what could be gained from layer thickness and dust content.
This research, however, reveals many complex features – including layers that change in thickness and orientation, or abruptly disappear in some places – making it a virtual gold mine of climate information.
"In the past, it was thought the Chasma Boreale canyon formed by melting the bottom of the ice sheet and having the floodwater carve the canyon, or perhaps having winds erode the canyon into the ice sheet from above," said Byrne. "In this study, we've figured out that the canyon has actually always been there and the ice cap grew up on either side of it."
"The arrangement of the internal layers we see with the radar instrument and the layers exposed on cliffs that we see with HiRISE have allowed us to reconstruct the history of this feature."
"Nobody realized that there would be such complex structures in the layers," said Holt, the lead author of the paper focusing on Chasma Boreale. "The layers record a history of ice accumulation, erosion and wind transport. From that, we can recover a history of climate that's much more detailed than anybody expected."
According to Byrne, the radar data also revealed that a second, equally large canyon existed in the ice sheet in the past, but that this twin feature was later filled in with ice and no trace of it now exists.
The spiral trough results vindicate an early explanation that had fallen out of favor in parts of the Mars scientific community. Alan Howard, a researcher at the University of Virginia, proposed just such a process in 1982 based solely on images of the surface from the Viking mission.
"He only had Viking images with relatively low resolution," said Isaac Smith, doctoral student and lead author on the spiral trough paper. Holt is second author on the trough paper.
"Many people proposed other hypotheses suggesting he was wrong," he said. "But when you look at a hypothetical cross section from his paper, it looks almost exactly like what we see in the radar data."
Why are the troughs spiral shaped? First, katabatic winds are caused by relatively cold, dense air that rolls down from the poles and out over the ice cap. Second, as they blow down, they are deflected by the Coriolis force, which is caused by the planet's spinning in space. On Earth, this is what causes hurricanes to spin opposite directions in opposite hemispheres. This force twists the winds – and the troughs they create – into spiral shapes.
"These anomalous features have gone unexplained for 40 years because we have not been able to see what lies beneath the surface," said Roberto Seu, team leader for the SHARAD instrument. "It is gratifying to me that with this new instrument we can finally explain them."
SHARAD is provided to NASA by the Italian Space Agency. It has been designed and developed and is operated by a joint team formed by Sapienza University of Rome's INFOCOM Department and Thales Alenia Space Italy.
Co-authors on the paper "The Construction of Chasma Boreale on Mars" include Kathryn Fishbaugh (Smithsonian National Air and Space Museum), Sarah Christian (The University of Texas at Austin Institute for Geophysics and Bryn Mawr College), Kenneth Tanaka (Astrogeology Science Center, U.S. Geological Survey), Patrick Russell (Planetary Science Institute in Tucson, Arizona), Ken Herkenhoff (Astrogeology Science Center, U. S. Geological Survey), Ali Safaeinili (Jet Propulsion Laboratory), Nathaniel Putzig (Southwest Research Institute) and Roger Phillips (Southwest Research Institute).
Funding was provided by NASA and the Gayle White Fellowship at the Institute for Geophysics.
Daniel Stolte | University of Arizona
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