Exploring the Martian methane puzzle

 

Should India’s Mars Orbiter provide a positive result, it would revitalise the search for the gas on the Red Planet

At the beginning of next month, India’s first spacecraft for the exploration of another planet, the Mars Orbiter Mission, will blast off from Sriharikota aboard the Polar Satellite Launch Vehicle (PSLV).

The spacecraft will escape Earth’s clutches and head for Mars. After travelling 400 million kilometres, it will near its destination in September next year. The probe must then fire an onboard engine to put it into orbit around that planet. A key challenge that the Indian Space Research Organisation (ISRO) faces is in making sure that this engine operates as planned after remaining idle for so many months in the icy coldness of space.

If all goes well, the spacecraft will be in an elliptical orbit that takes it to within 400 km of Mars and then sends it swinging out to 80,000 km away. The five instruments it carries can then be switched on.

One such instrument is a sensor specifically tuned to detect methane, a gas that on Earth is largely produced by living organisms, such as bacteria in the stomachs of cows and other hoofed animals.

Telescope observations

About 10 years ago, various research groups began to report finding methane in Mars’ atmosphere using ground-based telescopes and data from two orbiting probes. Considerable excitement and speculation about Martian microbes inevitably ensued. However, as on Earth, methane can also be generated by geochemical processes in which hot rocks interact with water and carbon dioxide.

The telescope observations suggested that plumes of the gas were released on Mars only occasionally from certain locations. A big plume was seen in March 2003 by a team of scientists led by Michael J. Mumma of the NASA Goddard Space Flight Center in the U.S. That plume was estimated to contain about 19,000 tonnes of methane. But by January 2006, the total amount of methane in the Martian atmosphere was only half that amount, suggesting that it was being destroyed rapidly in some fashion. If action by sunlight was the only factor breaking it down, the gas ought to have a lifetime of about 300 years or more.

However, scientists like Kevin Zahnle of the NASA Ames Research Center in the U.S. have been deeply sceptical about the existence of methane on the Red Planet. Dr. Zahnle has questioned the methodology used to detect methane in measurements made with telescopes and satellites. In a paper titled “Is there methane on Mars?” he and two colleagues also argued that variable levels of methane on that planet were physically and chemically implausible. “For methane to vary on short timescales, much else that we thought we knew about atmospheric chemistry and the Martian atmosphere would have to be badly wrong.”

Such doubts appeared justified when results from tests carried out by America’s ‘Curiosity rover,’ which came down safely in the Gale Crater on Mars last August, were published in the journal Science recently. Methane was not detected in six samples of the Martian atmosphere analysed by a sensitive instrument aboard the rover. The upper limit for methane in the atmosphere was put at 1.3 parts per billion by volume, which was far below levels estimated by satellite and telescope observations.

But Dr. Mumma remains undaunted. He had full confidence in the measurements that his group had published, he told this correspondent. Other scientists using different instruments had claimed detection of methane as well. “The Curiosity does not refute those earlier measurements if you accept a short lifetime for methane on Mars.”

The rover was not in a location where methane release might be expected to happen, he said.

Besides, methane may not be uniformly mixed in the Martian atmosphere, he pointed out. The gas is much lighter than the carbon dioxide that formed the bulk of the planet’s atmosphere. So, when methane is released, it might rise up and only mix higher up in the atmosphere. That could explain why methane was not detected by the rover while being seen in satellite and telescope observations. However, till this idea was fully evaluated in further studies, it must be regarded as highly speculative, he cautioned in a subsequent email.

Two sides

“We don’t really understand enough about Mars yet to really know what is going on and this is why I think the Indian mission is so important,” Dr. Mumma said. The Indian spacecraft would be able to search for methane all over the planet, which the Curiosity rover cannot do.

Dr. Zahnle disagreed. “The search for methane on Mars is not likely to succeed,” he said in a brief email.

The Indian Mars orbiter’s methane sensor may be able to detect the gas only if its concentration was more than about 10 parts per billion, according to one scientist involved in planning the mission. However, by repeated imaging, it was possible that even lower levels of methane could be found.

If methane is uniformly mixed and the Martian atmosphere contains only negligible amounts of it as indicated by the Curiosity rover’s measurements, then the Indian sensor would probably draw a blank. On the other hand, if the sort of sporadic eruptions of methane seen in telescope observations were to occur while the Indian satellite is circling the planet, the sensor could pick up signals from the plume and enhanced levels of the gas in the atmosphere.

Should the Indian probe provide a positive result, it would revitalise the quest for Martian methane. The next attempt to find and map methane will be made with Europe’s ExoMars mission that is scheduled for 2016.