Planetary scientists have known since the 1970s that a huge depression on Mars, centered on its north pole, dominates the planet's northern hemisphere. And they've mapped gigantic flood channels cut into the ruddy landscape, proof that water once gushed freely and drained into that vast lowland area.

Huge basin in northern Mars

A color-coded map of Martian topography, centered on Utopia Planitia, shows a vast lowland (blue) that dominates the planet's northern hemisphere. The high-standing northern polar cap appears near the top, and the large volcano Elysium Mons is right of center.

NASA / MOLA Science Team

So was a third of ancient Mars once covered with an ocean?

In the late 1980s, geologist Tim Parker (now at NASA's Jet Propulsion Laboratory) led a team that used high-resolution images from the Viking orbiters to identify strings of landforms encircling the northern basin. To their eyes these looked like wave-cut terraces and other features common along terrestrial shorelines, and they concluded that the big basin must have brimmed with water at least twice in the distant past. But the case for this enormous "Oceanus Borealis" was never ironclad — in fact, other scientists used the even-better views from later spacecraft raised serious doubts about the shorelines' existence.

You might think that getting a definitive answer would simply require taking more and better pictures with ever-sharper optics, or perhaps chemical analyses from landers and rovers. But some of the best recent evidence has come instead from two lesser-known instruments —a laser and a neutron spectrometer — looking down on the planet from orbit.

Each is a great detective story in itself, but for now here's a recap: The laser, at the heart of an orbiting altimeter, traced the Red Planet's highs and lows with extreme accuracy and revealed exactly where "sea level" should have been. The neutron spectrometer sensed that hydrogen, present in H2O ice, is abundant in the topmost surface layers surrounding both poles but especially so up north.

Mars Express spacecraft

Mars Express, launched by the European Space Agency in 2003, uses two whip antennas — each 65 feet (20 m) long — and a third, shorter monopole to conduct radar soundings of the surface below.

ESA / C. Carreau

The latest word comes from four researchers at the Institute of Planetology in Grenoble, France. Led by Jérémie Mouginot, they've analyzed radar soundings from the Mars Advanced Radar for Subsurface Ionosphere Sounding. MARSIS is one of several instruments aboard the European Space Agency's Mars Express orbiter, a very capable spacecraft that hasn't been in the spotlight much since its arrival in late 2003.

The radar's waves can probe the Martian surface to depths of up to 2½ miles (4 km) if the ground is loose or full of ice, or to several hundred meters if it's solid rock. During the last half of 2011, MARSIS was in the midst of a dedicated campaign to probe the vast northern basin, and Mouginot's team summarizes those findings in a recent issue of Geophysical Research Letters.

The researchers home in on a property of solid matter called the dielectric constant, essentially its ability to hold an electric charge. The MARSIS observations show that dense volcanic materials on Mars exhibit values near 10. But those surrounding the north and south poles are closer to 3 — implying that they're highly porous, chock full of ice, or both.

Interestingly, in the south, the region of low dielectic constant maps closely with the limit of where water ice near the surface should be stable in the current Martian climate (very cold and very dry).

Evidence for a Martian ocean

A view of Mars's northern hemisphere, looking down on the north pole. The color-coded map shows the dielectric constant of near-surface deposits, as found from MARSIS. Low values (blue), which are likely porous or ice-rich sediments, correspond well to a shoreline of what might have been an ancient ocean.

J. Mouginot & others / GRL

However, it's a different story up north. There MARSIS mapped a region of low values that here and there extend far beyond the presumed limits of ice's current stability — it's a poor fit. But the match is much closer to the presumed boundaries of the ancient Martian ocean, and in particular to the "Deuteronilus" shoreline traced out by Stephen Clifford and Parker back in 2001.

This coincidence falls short of outright proof, though Mouginot and his colleagues help make the case further by showing that their map of low dielectric constant also matches geologic maps of sedimentary deposits in Mars's great northern basin. Taken together, they suggest that the evidence in hand "can only be explained by the widespread deposition of (now dessicated) aqueous sediments or sediments mixed with massive ice."

It's a fascinating development in a long-running debate. You can read the European Space Agency's press release, but I recommend that you check out the excellent explanation by S&T contributing editor Emily Lakdawalla in her Planetary Society blog.


Image of Anthony Barreiro

Anthony Barreiro

February 12, 2012 at 12:19 pm

Our growing understanding of the histories of our nearest planetary neighbors, Venus and Mars, shows us just how precious and fragile are the conditions that sustain life on our own planet Earth. If one of our probes finds conclusive evidence of fossil life on Mars, that will be a very sobering day here on Earth. We will know that life can evolve and then die out on a planetary scale. I hope we will do everything we can to cherish our own home here on Earth.

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Gary Linford

February 15, 2012 at 5:19 pm

There is a hard reality about Mars that these and many other authors ignore. The incident solar intensity at Mars is 550 W/m2, only 40% that of the Earth. Even with an obliquity of 24.5 degrees, it is unlikely the polar temperature could ever have reached 273 degrees Kelvin, the melting point of water, since ice and snow reflect 90% of incident sunlight. Despite enormous quantities of frozen carbon dioxide at both poles, plus the 95% carbon dioxide atmosphere—about 30 times the CO2 in the Earth’s atmosphere—there is no “greenhouse effect,” a fact which is a problem for “anthropomorphic global warming” (AGW) supporters on Earth. Other authors mistakenly suggest AGW as a means to “terraform” Mars. It won’t work. With no magnetic field, the Solar Wind impacts Mars’ atmosphere, stripping away protons (hydrogen), desiccating Mars just as Venus (with no magnetic field) lost its H2O. However, all is not lost! This desiccation enhances the deuterium content of Martian water. Human colonization of Mars will be enabled by implementing deuterium-tritium (D/T) inertial confinement fusion power plants using plentiful lithium and deuterium supplies as fusion fuel. Captured by lithium-6, the 14.3 MeV neutron from each D/T fusion breeds a replacement tritium nucleus. Just as can be done on the Moon, glass domes can be erected over large Martian craters to provide Earth-like living conditions. Silicate glass domes really do produce a “greenhouse effect.”

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February 19, 2012 at 8:47 pm

The reports on a warm wet Mars billions of years ago continue to amaze me. Just this week other sources reported on the ongoing conflict with such a view of Mars past because of the Faint Young Sun problem - Concerning finding fossil life on Mars and a "sobering day" for Earth-how many probes do we need to send to Mars with negative results searching for life before the scientists sober up and see that life is here on Earth, not Mars? How do you falsify the belief that life evolved on Mars?

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Image of Phil


March 8, 2012 at 10:16 am

So Gary, how is the presence of FROZEN CO2 going to cause a greenhouse effect? And even with an atmosphere that's mostly CO2, it's still far thinner than Earth's, and the Sun is much dimmer. How much colder would Mars be without ANY CO2 in its atmosphere? You seem to believe that any level of CO2 should boost Mars's temperatures to Earth-like levels, and that it's colder there proves that CO2 does nothing. How do you explain the extra high temperatures of Venus, which also has a CO2-rich atmosphere? Let us know when you've figured it out, so the AGC deniers and their fossil-fuel sponsors can give you a Nobel Prize.

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