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An Odyssey of Mars Science: Part 3

Martian H20 Deposits as mapped by NASA's Mars Odyssey orbiter
by Bruce Moomaw
Sacramento - Feb 12, 2004
The new data from Mars Odyssey's GRS and THEMIS instruments, of course, are also providing information on some aspects of Mars that have nothing to do with the amount of liquid water that may have existed on its surface -- and William Boynton and Phil Christensen, during their lectures on the subject at the DPS meeting, mentioned these too.

Hydrogen isn't the only element in the upper third of a meter of Mars' surface measured by Mars Odyssey. Its gamma-ray spectrometer is making increasingly good maps of the distribution of oxygen, silicon, iron, potassium, thorium and -- most recently -- chlorine on the surface, with a peak resolution of about 300 km. In the near future, the researchers also hope to start making maps of several more elements. Once again, these have opened up new questions.

First, there's the mystery of why the 29 "SNC" meteorites that have been pretty firmly identified as being chunks of Mars rock -- thanks to several factors, including the traces of identifiable Martian atmosphere trapped inside them, their internal exposure to modest amounts of water, and the relatively recent dates at which most of them last solidified out of lava -- don't match our new surface composition maps of Mars at all well, in either the gamma-ray or the infrared wavelengths.

The spectra made by Mars Global Surveyor's "TES" medium-wavelength infrared spectrometer already indicated that most of Mars' exposed rocks fall into two general categories: "Type 1 and 2". Type 1, which makes up most of the highlands that cover Mars' southern hemisphere, seems to be basalt -- while the Type 2 rock which dominates the lowlands making up most of Mars' northern hemisphere seems to be much richer in silica (silicon dioxide). It may be either the more silica-rich volcanic rock "andesite", or else water-weathered basalt which has developed a thin surface crust of silica which TES is detecting.

But the TES spectra of neither rock matches the recovered Mars meteorites at all well. They are a different form of basalt, even poorer in silica and also lacking the aluminum-rich mineral "plagioclase feldspar" which seems much commoner in both the Type 1 and Type 2 rocks.

TES' spectral maps of Mars' surface -- and the followup maps made by Odyssey's THEMIS IR camera, which maps Mars' surface in nine spectral bands at far sharper resolution than TES -- show only a few very small patches on the surface that seem to match the mineral compositions of any of the different Mars meteorites.

The maps of seven different elements on Mars' surface made so far by Odyssey's "GRS" gamma-ray spectrometer also indicate that both the Type 1 and Type 2 rocks are considerably richer in potassium and thorium than any of the SNC meteorites.

This, however, did not come as a wild shock to scientists, who were already well aware of an even bigger peculiarity about the SNC meteorites. With only one exception -- the famous "ALH84001" meteorite that shows what may or may not be fossil evidence of ancient Martian life -- radioisotope age-dating, to determine the time at which they last solidified out of lava, shows them to be much younger than the age of most of Mars' surface, judging from its dense accumulation of impact craters over the eons.

Two groups of the Mars meteorites -- the "nakhlites" and the "Chassigny" meteorite -- solidified about 1.3 billion years ago, which seems very old until you consider that most of Mars' surface last solidified out of lava 2 to 4 billion years ago. The others -- the "shergottites" -- are even younger; they solidified between 400 and only 180 million years ago. The only older Mars meteorite found so far is ALH84001, which is fully 4.5 billion years old.

Measurements of the time the various Mars meteorites orbited the Sun after their ejection by giant impacts until they crashed on Earth -- which is possible by measuring the traces of rare isotopes that are produced in them only by exposure to cosmic rays -- shows that they've been ejected by six or seven separate giant impacts on Mars. But all but one of those impacts ejected material only from the youngest one-third of Mars' surface. Why?

The answer -- as provided by recent computer analyses -- is that the constant slow rain of various-sized meteoroids onto Mars' surface throughout its history have created a layer of pulverized rock and sand "regolith" on the older parts of Mars' surface perhaps hundreds of meters thick, while the layer on its minority of younger regions is only a few to a few dozen meters thick. And -- paradoxically -- a deeper layer of loose regolith actually makes it HARDER for the shock wave from a giant impact to accelerate solid pieces of the surface to escape velocity!

The "spall zone" -- the narrow zone around the impact in which its shock waves interact to fling rocks off Mars' surface into space at escape velocity without melting or powdering them -- is actually much narrower for a loose surface than for a solid one, because of the way the loose material absorbs the shock. And so an impact that produces a crater only a few kilometers wide on the younger and solider parts of Mars' surface can launch Mars rocks into solar orbit, while it takes one that produces at least a 20-km crater to do so on the planet's older surface.

And those much bigger impacts are much rarer. Cosmic-ray dating shows that all the Mars meteoritesthat have been found so far must have spent 15 million years or less orbiting the Sun after their launch before they crashed on Earth -- and, sure enough, simulations confirm that by that time most Mars rocks ejected into solar orbit by an impact on Mars have crashed into the Sun, one of the inner planets or an asteroid fragment, or else have flown close by Jupiter and been catapulted into the outer Solar System.

During the last 15 million years, it's been estimated that there has likely been only one impact on Mars' older surface big enough to launch any Mars meteorites -- but there have been five or six on its younger surface, despite its much smaller surface area. This matches the actual cosmic-ray dating figures perfectly.

The only place on Mars that has any really large areas of such recent surface is the "Tharsis bulge", the great rise -- the size of North America, and up to 10 km high -- on one side of Mars. The internal processes that produced Tharsis in early Mars are still debated; but it -- and the four giant shield volcanoes that top it -- have been erupting lava flows on and off from the Noachian Age up to (judging from crater counts) perhaps only a few tens of millions of years ago.

Thus we can say with near-certainty that most of the Mars meteorites found so far come from the lava flows of Tharsis. (In addition to the ancient-crust ALH 84001, however, there's another which may have been blasted off a smaller, even more recent Martian lava flow -- which we'll get to later.) The only reason the infrared maps by our Mars orbiters aren't detecting SNC-type rocks at Tharsis is simply that -- because of Mars' wind patterns -- the lava flows there are covered with a blanket of fine windblown dust from 10 cm to two meters deep.

This also explains the SNC meteorites' compositional differences with most of Mars' surface. When early Mars' crust (like Earth's) first separated chemically from its melted mantle, it took a lot of the mantle's potassium, thorium, aluminum -- and water -- with it. The lava that's been erupted from deep within the remaining mantle since then by the Tharsis volcanoes contains less of all those substances, and its smaller amount of innate water also causes its basalt to be scarcer in silica than the earlier basalt that formed Mars' original ancient crust.

Click for Part Two

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