Countdown to the Search for Life on Mars --One Day to Mars 'Curiosity' Landing

To achieve the precision needed for landing (see diagram at bottom of page) safely inside Gale Crater, the spacecraft will fly like a wing in the upper atmosphere instead of dropping like a rock. To land the 1-ton rover, an airbag method used on previous Mars rovers will not work. Mission engineers at NASA's Jet Propulsion Laboratory in Pasadena, Calif., designed a "sky crane" method for the final several seconds of the flight. A backpack with retro-rockets controlling descent speed will lower the rover on three nylon cords just before touchdown.

           

During a critical period lasting only about seven minutes, the Mars Science Laboratory spacecraft carrying Curiosity must decelerate from about 13,200 mph (about 5,900 meters per second) to allow the rover to land on the surface at about 1.7 mph (three-fourths of a meter per second). Curiosity is scheduled to land at approximately 10:31 p.m. PDT on Aug. 5 (1:31 a.m. EDT on Aug. 6).

"Those seven minutes are the most challenging part of this entire mission," said Pete Theisinger, the mission's project manager at JPL. "For the landing to succeed, hundreds of events will need to go right, many with split-second timing and all controlled autonomously by the spacecraft. We've done all we can think of to succeed. We expect to get Curiosity safely onto the ground, but there is no guarantee. The risks are real."

           

In early June, a NASA spacecraft in orbit around the Red Planet, called Mars Odyssey, suffered a malfunction on one of its reaction wheels, an instrument that helps control the probe's attitude in space.* "Odyssey looks like it may not be where we expect it to be," said Doug McCuistion, director of the Mars Exploration Program at NASA Headquarters.

The glitch does not pose a risk to Curiosity's impending arrival at the Red Planet, officials said. But Mars Odyssey's original orbit would have given it a complete view of Curiosity's landing, so the probe had been pegged to act as an orbiting outpost to relay communications and data back to mission managers on Earth.

With Odyssey in the picture, mission managers may be able to confirm Curiosity's touchdown by 10:31 p.m. PDT on Aug. 5 (1:31 a.m. EDT Aug. 6). Without Odyssey, however, mission controllers may not know the outcome of the landing maneuver until 10:35 p.m. PDT or later.

During the initial weeks after the actual landing, JPL mission controllers will put the rover through a series of checkouts and activities to characterize its performance on Mars, while gradually ramping up scientific investigations. Curiosity then will begin investigating whether an area with a wet history inside Mars' Gale Crater ever has offered an environment favorable for microbial life.

"Earlier missions have found that ancient Mars had wet environments," said Michael Meyer, lead scientist for NASA's Mars Program at NASA Headquarters. "Curiosity takes us the next logical step in understanding the potential for life on Mars."

Curiosity will use tools on a robotic arm to deliver samples from Martian rocks and soils into laboratory instruments inside the rover that can reveal chemical and mineral composition. A laser instrument will use its beam to induce a spark on a target and read the spark's spectrum of light to identify chemical elements in the target.

Other instruments on the car-sized rover will examine the surrounding environment from a distance or by direct touch with the arm. The rover will check for the basic chemical ingredients for life and for evidence about energy available for life. It also will assess factors that could be hazardous for life, such as the radiation environment.

"For its ambitious goals, this mission needs a great landing site and a big payload," said McCuistion. "During the descent through the atmosphere, the mission will rely on bold techniques enabling use of a smaller target area and a heavier robot on the ground than were possible for any previous Mars mission. Those techniques also advance us toward human-crew Mars missions, which will need even more precise targeting and heavier landers."

The chosen landing site is beside a mountain informally called Mount Sharp. The mission's prime destination lies on the slope of the mountain. Driving there from the landing site may take many months.

Because of its history, 96-mile wide Gale Crater crater with its strangely sculpted mountain --three times higher than the Grand Canyon is deep--is the ideal place for Curiosity to conduct its mission of exploration into the Red Planet's past. Researchers plan to use Curiosity to study layers in the mountain that hold evidence about wet environments of early Mars.

"This may be one of the thickest exposed sections of layered sedimentary rocks in the solar system," said Joy Crisp, MSL Deputy Project Scientist from NASA's Jet Propulsion Laboratory. "The rock record preserved in those layers holds stories that are billions of years old -- stories about whether, when, and for how long Mars might have been habitable."

An instrument on Curiosity can check for any water that might be bound into shallow underground minerals along the rover's path. Today the Red Planet is a radiation-drenched, bitterly cold, bleak world. Enormous dust storms explode across the barren landscape and darken Martian skies for months at a time. But data from the Mars Reconnaissance Orbiter suggest that Mars once hosted vast lakes and flowing rivers.

"Gale Crater and its mountain will tell this intriguing story," says Matthew Golombek, Mars Exploration Program Landing Site Scientist from JPL. "The layers there chronicle Mars' environmental history."

"Be patient about the drive. It will be well worth the wait and we are apt to find some targets of interest on the way," said John Grotzinger, MSL project scientist at the California Institute of Technology in Pasadena. "When we get to the lower layers in Mount Sharp, we'll read them like chapters in a book about changing environmental conditions when Mars was wetter than it is today.

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NASA Countdown: "The Conditions for the Emergence of Life Were Present on Mars" (Weekend Feature)

“Based on evidence, what we do have is, unequivocally, the conditions for the emergence of life were present on Mars — period, end of story,” said Michael J. Mumma, a senior scientist for NASA at the Goddard Space Flight Center in Greenbelt, Md., who led one of three teams that have made still-controversial claims of detecting methane in Mars’s atmosphere. “So life certainly could have arisen there.”

Observations over the last decade suggest that methane clouds form briefly over Mars during the summer months. The discovery has left many scientists scratching their heads, since it doesn't fit into models of the martian atmosphere. The image above shows a map of methane concentrations in Autumn (first martian year observed) overlayed on true colour map of Mars.

"The reports are extraordinary," says Kevin Zahnle of NASA Ames Research Center. "They require methane to have a life time of days or weeks in the martian atmosphere, which disagrees with the known behavior of methane by at least a factor of 1000."

                                   

Zahnle and his colleagues have expressed some serious doubts about the existence of methane on Mars in a paper that appeared in the journal Icarus.

"What we say is that the evidence is not nearly strong enough for us to suspend our trust in the known chemical behavior of methane," he says.

"We stand by our results," says Michael Mumma of NASA's Goddard Space Flight Center, who leads one of the groups that made the methane observations. "True, the measurement is difficult, but it is not impossible."

Mumma thinks the paper by Zahnle and his coauthors has been "a disservice. The community needs to understand the weakness of this argument," he says.

For over 40 years, astronomers have found various hints of methane on Mars. These reports have always generated a lot of excitement because they seem to provide some clue to the habitability of our planetary neighbor.

"Methane invokes visions of life on Mars," explains Sushil Atreya of the University of Michigan. This is because much of the methane on our planet comes from living (or once-living) things.

But solid evidence of martian methane with infrared spectroscopy only surfaced in 2003, when Mumma and his group saw signatures of methane in spectra taken with the NASA Infrared Telescope Facility in Hawaii. The methane was localized into clouds, or "plumes," over certain regions of the Martian surface, with the maximum density of methane reaching about 60 parts per billion.

In 2004, two other reports came out claiming to have seen methane. Data from the ESA's Mars Express, which began orbiting Mars in January 2004, showed signs of methane plumes, but not in the same places as Mumma's team. Another ground-based observation with the Canada–France–Hawaii Telescope detected methane, but it didn't have enough spatial resolution to see plumes.

Mumma's group performed their own follow-up observations in 2006, when the orbit of Mars again allowed methane to be detected from the ground. But in the three-year interim, all signs of the methane had disappeared, as reported in a 2009 Science paper. The implication is that the methane is a seasonal occurrence, perhaps only coinciding with summers on the red planet.

Mumma and his colleagues have estimated that the largest plumes on Mars would require a source that releases methane at rates comparable to the world's largest hydrocarbon seep, which is in California. Although thoughts turned to little methane-belching martians, geochemical processes could potentially produce these levels of methane on Mars. The jury is still out as to which of the various candidates is the most likely source.

The harder nut to crack may be the rapid disappearance of methane on Mars. Models of the martian atmosphere estimate that a methane molecule should survive on average about 300 years before being destroyed by photochemical processes.

However, analysis of the plume data suggests that methane is being removed from the atmosphere on the order of months, if not weeks.

Zahnle and his colleagues have looked into how hard it would be to include a methane "sink" big enough to swallow up the observed methane plumes over such a short time. They conclude that any chemical process that devours methane would completely mess up the planet's chemical budget, in particular the allowance of oxygen.

"Methane oxidation would exhaust Mars of its atmospheric oxygen in less than 10,000 years," Zahnle says.

"Though the details of Zahnle et al’s theoretical arguments are debatable, their basic idea of the implausibility of large abundance and short lifetime of methane is fairly sound," Atreya says.
Mumma is aware that his observations do not mesh with the established picture of Mars. "But a measurement is a measurement," he says.

It's certainly possible that Mumma and others have found a "monkey wrench" that will require revising the current models of Mars, but Zahnle and others aren't convinced this is the case due to the difficulty in observing methane on Mars.

According to their arguments, the spacecraft measurements suffer from poor spectral resolution that makes it difficult to detect the absorption lines that identify methane in a spectrum. The ground-based observations, on the other hand, have sufficient resolution, but they must contend with the Earth's atmosphere, which is full of its own absorption lines.

To explain this last point further, imagine looking for a pink flower with rose-tinted glasses: the light that you want to see is absorbed before it gets to you. The signatures of martian methane typically lie in the infrared regions of the spectrum where our own atmosphere is highly absorbing.

There are a few tricks that ground-based astronomers can use. For one, they can look at Mars when the planet is moving towards us or away from us. This causes a Doppler shift in the martian light that can make the methane lines a little easier to identify.

Still, observers must build models to try and subtract away the "foreground contamination" coming from our atmosphere. This involves a careful accounting of all the molecules that may be absorbing incoming light.

"The methane that [Mumma and others] see is not a raw measurement, but is rather a measurement filtered through a sophisticated but imperfect model of Earth's atmosphere," Zahnle says.

He and his coauthors contend that these models may be incorrectly accounting for the effect of carbon-13 methane. Most methane in our atmosphere is made with carbon-12, but a small amount contains the heavier isotope carbon-13.

The absorption lines for terrestrial carbon-13 methane just happen to lie right where Mumma's team claims to have detected the signature of martian methane. Zahnle's group doesn't think this is a coincidence. They argue that the carbon-13 methane is providing a false signal of martian methane.

Mumma disagrees. He says if they were underestimating carbon-13 methane, then this false signal should show up in all of their processed data, but it doesn't.

"You need a very sophisticated model to extract the terrestrial signal," Mumma says " "We think our multilayer models are the most advanced in the world for this region of the spectrum."

Moreover, he says that the purported methane signal changes as they look at different regions on Mars. This kind of spatial variation wouldn't be expected if the signal were due to absorption in our own atmosphere.

Mark Allen of the Jet Propulsion Laboratory has followed the debate and thinks Mumma has effectively rebutted the doubts about his group's spectroscopic analysis.

Allen's own point of view is that the methane plumes that were visible in 2003 rapidly spread out to such an extent that the gas was no longer dense enough to be detected.

"So methane doesn't disappear, à la having to have an unusually short chemical lifetime," Allen says.

Atreya has a different take on the issue. He finds the claims of spatial and seasonal variations in the methane concentration to be "quite dubious." He believes that a relatively small quantity of methane has been observed, but the present data seem consistent with it being essentially uniformly distributed over the planet.

"In the absence of rapid temporal and spatial variability, there would be no need to invoke exotic destruction or release mechanisms," Atreya says.

Most of the community is being cautious and not taking sides in the debate, according to Atreya. They are hoping upcoming missions will decide who was right after all.

A more comprehensive test is planned in 2016 with the ExoMars Trace Gas Orbiter (TGO), which will be part of the joint ESA/NASA dual mission ExoMars Program. The TGO will scan the atmosphere for exotic trace gases, such as methane.

Allen says that many people assume that the only motivation for TGO is to confirm the presence of methane, but it will perform other important science as well.

Zahnle questions whether all this effort to look for methane on Mars will be worth it in the end, "but given how the story has unfolded, it must be done."

The Daily Galaxy via Astrobio.net and The New York Times

Image credits: NASA

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