WHEN NASA’s Mars Reconnaissance Orbiter slipped into orbit around the Red Planet last week, it joined three other illustrious probes circling Mars and two intrepid rovers on the ground that have all been studying the planet in unprecedented detail over the past two years.
While the MRO’s increased sensitivity promises a deluge of information over the next few years, its hard-working brethren have already transformed our understanding of the planet’s geological past, not least by adding a whole new epoch to Martian prehistory. This grand reappraisal of Mars’ past, thanks to the joint efforts of NASA’s Mars Global Surveyor and Mars Odyssey, its rovers Spirit and Opportunity, and ESA’s Mars Express, was the subject of a symposium at the Lunar and Planetary Institute in Houston, Texas last week.
The basic three-part division of Martian geological history into the Noachian, Hesperian and Amazonian periods was based on images from the Mariner 9 probe taken in the early 1970s. This was refined into a total of eight sub-periods after the twin Viking orbiters provided higher-resolution global coverage in the late 1970s. This overall scheme remained unchanged until now.
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The key has been laser altimetry measurements from the Mars Global Surveyor. The probe’s detailed information on the relative elevations of topographic features on the Red Planet has revealed vast numbers of huge circular depressions that are completely invisible even in high-resolution images. Researchers think the majority of these must be ancient craters the raised rims of which have been worn down so that only subtle measurements of elevation reveal their presence. What’s more, these “quasi-circular depressions” greatly increase the number of known early craters on Mars, and this new population packs a surprise.
Herb Frey of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, has shown that many of these eroded ancient craters are partly “overwritten” by craters from the Noachian epoch – previously thought to be the planet’s first. This suggests a distinct epoch prior to the Noachian, now dubbed the pre-Noachian. “It’s pretty clear that the early impact rate was quite a lot higher than we expected,” Frey told the meeting. “Buried basins are found everywhere in the highlands, which means the cratering rate was significantly higher than previously estimated.”
The implications extend beyond Mars. It could be that the moon, too, has a hidden past. The technique of dating surfaces based on crater counts was developed for the moon and calibrated using the rocks brought back by the Apollo astronauts, but the moon has never been subjected to the kind of laser altimetry deployed on Mars. “It becomes important to know if there is a buried population of impact basins on the moon,” said Frey. We may get some answers when NASA flies a laser altimetry mission as part of the Lunar Reconnaissance Orbiter in 2008.
As for Mars’s more recent history, it now appears that the vast majority of smaller craters on its surface are most likely not primary impact craters at all, but merely secondary craters created by thousands of huge boulders thrown up by larger impacts. Last year, Alfred McEwan of the University of Arizona in Tucson discovered that huge numbers of widely dispersed smaller craters could be clearly linked to a single recent impact crater known as Zunil (żěè¶ĚĘÓƵ, 26 March 2005, p 18). The connection, invisible in previous images, was starkly apparent in the pictures provided by the Thermal Emission Infrared Spectrometer camera on Mars Odyssey, which showed distinct dark rays linking the secondaries to the main crater.
McEwan concluded that the practice of dating surfaces based on counting smaller craters was leading geologists astray. While McEwan’s startling conclusion received considerable support, a few researchers argued that secondary cratering is rare. Now new evidence presented at the meeting supports McEwan’s idea.
It seems that virtually all of the craters and “hollows” seen by the Mars rover Spirit as it crossed the basin of Gusev Crater – and there have been dozens of them, ranging from less than a metre across to over 100 metres wide – are secondaries. Matt Golombek, a member of the rover science team at NASA’s Jet Propulsion Laboratory in Pasadena, California, told delegates that all of the craters are extremely shallow, as expected for secondary craters but not for those produced by impacts from space. “Virtually every crater we’ve looked at is a secondary,” Golombek said.
That’s bad news for dating the most recently-formed surfaces on Mars, which are also those of most interest in deciphering signs of recent climatic change. Dating surfaces based on the number of accumulated craters depends on the assumption that impacts occur at a relatively constant rate. But if large enough impacts can form thousands of secondary craters that then dominate the population of small craters, it could make it virtually impossible to tell how old these surfaces are.
There is an additional complication. A study of the numbers of craters seen on the surfaces of landslides shows that apparently younger landslides, with few craters, are far more numerous than expected. One reason could be that the rate of landslides has increased steadily over time, but this is unlikely, according to Cathy Quantin of the National Air and Space Museum in Washington DC. Instead, it may be that the rate of impact cratering has declined more steeply than previously thought, by about a factor of three over the last three billion years, while the rate of landslides has remained constant. The upshot would be that many regions of Mars are actually older than had been thought.
“Dating based on counting small craters is leading geologists astray. Many regions of Mars may be older than thought”
“That could solve a lot of problems in Martian geology,” says Bill Hartmann of the Planetary Science Institute in Tucson, Arizona, a specialist in crater-count dating. “It could push young ages back.” It would explain not only why landslide activity appeared to be increasing, but also similar findings about apparent increases in volcanic flows and other features.
However, pinning down the true ages of surfaces on Mars may require precise dating of rocks returned to Earth from carefully selected sites there. And this is where the MRO could play a role: part of its mission is to identify future landing sites.
The atmosphere that was
The atmosphere of Mars, which is so thin that on Earth it would be considered a hard vacuum, was once as thick as or even thicker than Earth’s. At least that’s the accepted theory, based on the abundant evidence of river valleys on Mars’s surface, suggesting that it must have once had air thick enough to support evaporation and rainfall. However, the idea has remained controversial.
Now a study of meteorites lends further support to the theory. John Bridges and Ian Wright of the Planetary and Space Sciences Research Institute at the Open University in Milton Keynes, UK, have found that the proportion of carbonates in some of the 32 known Mars meteorites found on Earth suggests that the atmosphere originally contained carbon dioxide at pressures somewhere between 2.3 and 2.6 bars, compared with roughly 1 bar at sea level on Earth today.
The meteorites also reveal that by 670 million years ago the atmospheric pressure had come down to about 50 to 100 millibars and then fell further to today’s level, which varies from virtually zero to 30 millibars depending on variations in the shape of Mars’s orbit. Bridges and Wright presented the work at the Lunar and Planetary Sciences Conference in Houston, Texas.
Soft at heart
After years of debate, it is now clear that Mars has a liquid core similar to Earth’s, rather than a solid one like the moon or Mercury.
Pieces of the puzzle have come from a variety of sources, including precise measurements of subtle changes in the orbit of the Mars Global Surveyor (MGS) orbiter due to the tidal deformation of Mars caused by the sun, plus analysis of bands of magnetic regions of the Martian crust, and computer models of the core. But it took cutting-edge laboratory experiments to clinch the case.
Yingwei Fei of the Carnegie Institution in Washington DC and colleagues subjected compounds of iron, nickel and sulphur to intense pressure inside a “multi-anvil” cell, which concentrates forces onto the sample, and then to even higher pressure in a laser-heated diamond-anvil cell.
The experiments showed that if Mars’s core contains even minuscule amounts of sulphur, it would be molten at temperatures above 1126 °C. Models indicate that the core actually contains more than 14 per cent sulphur and is at about 1726 °C. “The Martian core is most certainly liquid,” says Fei, who presented the work at the Lunar and Planetary Sciences Conference in Houston, Texas.
