Martian limestone

The Phoenix Mars Lander has found new evidence of ancient seas on the red planet. The probe performed a chemical analysis of surface sediments and found strong evidence of carbonate minerals, primarily calcium carbonate (limestone). Mixed in with pulverized basalt was an estimated 3 – 8% calcium carbonate by weight. This is big news, actually.

On Earth limestone accounts for about 10% of all sediments, and is formed solely by precipitation from water. Most limestone deposits on Earth are formed in the ocean, though some carbonate rocks can be laid down as a result of lakes drying up.

The key fact about limestone on Earth is that nearly all of it is made up of the shells of marine life, specifically single-celled plankton. Limestone forms as billions of generations of plankton live and die near the ocean surface where sunlight penetrates. As each old cell dies, its living matter rots away and its remaining little shell of calcium carbonate drifts down to the bottom. They mount up like snow, as millennia pass, and the older plankton snowfalls compress under the younger, eventually hardening to solid rock. Into the mix also go animals of the sea who secrete a calcium carbonate shell, including a myriad of shelled scuttlers, swimmers and floaters. Later, when the hardened rock is raised up and weathered to make a mountain, its fossil illustrations of ancient carrion are revealed.

The discovery of calcium carbonate on Mars conjures up images of red, alien seas filled with life. Perhaps if we’re lucky some of them scuttled on tripod legs. I hope for H. G. Wells’ sake they did. But the reality is probably not so romantic. Limestones on Earth are mostly from plankton, but a few formed by direct chemical precipitation from water, without the involvement of life. Such chemical limestone deposits are usually associated with evaporite rocks, which form just like their name sounds; by water drying up. Inland seas and lakes sometimes dry away as a result of changing climate, and leave behind all their salts as layers of different minerals, including rock salt, gypsum, borax, and limestone. When harsh alkali winds blow across Earth deserts, the alkali comes from windblown evaporite dust and grit.

Carbonates found by Phoenix might come from either a big wet sea, or a little dying sea. A little dying sea is more likely, especially considering that Phoenix also found evidence of perchlorates in the sand. Perchlorate is a highly oxidized form of chlorine, and is the active ingredient in laundry bleach. Perchlorates contain chlorine, which is also a component of sea salt (sodium chloride). A drying sea would lay down layers of rock salt and carbonate, more or less adjacent physically. Exposed for ages at the Martian surface, chloride salts would naturally oxidize to perchlorate under the relentless, unfiltered glare of solar and cosmic radiation. Limestones laid down by plankton don’t usually contain any chloride salts. In any event, a dry sea means an earlier wet sea. We now know beyond a reasonable doubt that long ago Mars was a wet world with lakes, seas and rivers.

But we still have no evidence that plankton grew in the wine-dark Martian sea. If limestone had been found in blocks, in layered bluffs, and with no perchlorate, well then we might have something interesting. But caliche sand mixed with perchlorate suggests an evaporite, not a fossil-bearing limestone. Life might have been there, but we still have no smoking gun to prove it.

Could anything still be alive in the sand Phoenix analyzed? Probably not. Perchlorate is bleach, remember. A conventional microbe exposed to that Phoenix sediment would sizzle to bits in an instant, its cell membrane ripped apart and its guts fizzing. No life there, unless it is very exotic. Eons of radiation oxidizing Martian salts to bleach have left the red planet with dunes saturated in poison.

Martian plankton aside, this discovery tells us something interesting about the long-ago Martian climate. The dried sea found by Phoenix is near the northern pole today, and Mars has no plate tectonics to drive it there from somewhere else. It formed there, as a liquid, on a planet where the Sun is much further away than at Earth. The Sun was substantially dimmer 3 billion years ago, too. The presence of a liquid sea at such a northern latitude suggests that it wasn’t always at such a northern latitude. Specifically, it suggests that Mars’ axial tilt has moved a lot since then. At one point in the distant past, Mars may have rolled about the Sun on its side, or at least with a tilt bigger than Earth’s.

Mars has no large moon, like the Earth does. Earth’s moon helps to stabilize our planet’s axial tilt, almost like having a giant outrigger, tethered by gravity. Without such a stabilizing influence, Mars is much more prone to tilts. The new Mars data suggest this actually happened, otherwise we’ve got an ancient sea where a permanent ice cap should have been.

But I’m still waiting on Phoenix to zoom in on a limestone bluff, and show us the fossil of a primitive tripod crab.

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~ by Planetologist on October 2, 2008.

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