NASA's latest interplanetary spacecraft has settled into orbit around the Red Planet. Its year-long scientific scrutiny could reveal how and why Mars lost so much of its primordial atmosphere.

This image shows an artist concept of NASA's Mars Atmosphere and Volatile Evolution (MAVEN)  spacecraft, which reached the Red Planet on September 21, 2014. Lockheed Martin
This image shows an artist concept of NASA's Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, which reached the Red Planet on September 21, 2014.
Lockheed Martin

When it comes to interplanetary exploration, you've got to trust your hardware. That was the case this evening, when the scientists and engineers for NASA's latest deep-space sortie could do little more than wait anxiously, fingers crossed, at a Lockheed Martin control center in Littleton, Colorado.

During this tense, hands-off vigil, a computer aboard the Mars Atmosphere and Volatile Evolution spacecraft (MAVEN) — 138 million miles (222 million km) and 12½ light-minutes from Earth, too far away to control directly — fired a cluster of six braking engines. Thirty-three minutes later, MAVEN had slipped into orbit around the Red Planet.

The spacecraft didn't exactly shout "I'm here" after completing the 10-month, 442 million-mile cruise that began last November 18th. But Doppler shifts in a weak radio beacon showed that the engines had reduced the approach velocity by about 4,000 feet (1.23 km) per second, slowing the craft enough for the planet's gravity to snare the spacecraft at 10:24 p.m. EDT (2:24 Universal Time on September 22nd).

MAVEN's orbit insertion
Approaching Mars at 2,500 miles (4,700 km) per hour, the MAVEN spacecraft reduced its speed by roughly 25% in order to be captured by the planet's gravity and slip into its initial, highly elongated polar orbit.

MAVEN Readies for Work

For now, the spacecraft will follow a looping 35-hour-long polar orbit that varies from 240 to 27,700 miles (380 to 44,600 km) in altitude. Over the next 6 weeks, the engines will fire again to shrink a 4½-hour-long orbit ranging from 95 to 3,850 miles, and then small thrusters will trim that further to a final, 3½-hour loop.

Unlike NASA's other Martian explorers, which have largely focused on the state of the planet's surface and its geologic evolution, MAVEN will study the Martian atmosphere exclusively.

It carries eight instruments, six of which will measure charged particles, electromagnetic fields, and plasma waves in the solar wind as it sweeps past the planet.

An imaging ultraviolet spectrograph and a mass spectrometer, both mounted on a steerable platform at the end of a short boom, will assess the upper atmosphere's chemical makeup.

What Happened to Mars?

Over the next year, flight controllers will command MAVEN to make five "deep dips", dropping it to altitudes as low as 77 miles (125 km) to sample directly the uppermost wisps of the planet's already tenuous air. These observations hope to answer a longstanding puzzle among planetary scientists. There's ample evidence that, early in its history, the Red Planet had a much denser atmosphere. Rain fell from its sky, and water coursed across its landscape.

But then something happened to the atmosphere: it basically vanished and, with it, the brief era when Mars might have been suitable as an abode for life. Mars quickly became the desolate, frigid world we see today. Researchers led by Bruce Jakosky (University of Colorado), MAVEN's principal investigator, want to know what happened to all that gas (most of it carbon dioxide) and, especially, to the ample water that once existed on the Martian surface.

Animation of Mars and solar wind
These computer simulations show how the solar wind, encountering Mars from the left, affects the planet's ionosphere and upper atmosphere. Fast, high-density solar-wind flows can strip away large numbers of atoms and molecules. Click here to see animations of this process.

One leading theory is that the gas escaped irrevocably to space, stripped away by the solar wind rushing past. (Click here to see animations of that process.) Here on Earth, our planet's magnetosphere serves as an obstacle to the solar wind, keeping it from interacting directly with our atmosphere. But once Mars lost its global magnetic field, billions of years ago, the upper atmosphere became vulnerable.

MAVEN's spectrometers will attempt to determine if hydrogen atoms, torn from water molecules by ultraviolet sunlight, are escaping to space, and at what rate. "The stripping of gas from the atmosphere to space might have been the driving mechanism for climate change on Mars," Jakosky says.

For now, he and his team will ready the spacecraft to begin observations in early November. Results will not come quickly, he cautions, because it will take months to build up enough measurements to have a clear sense of what's going on — or going away.

However, one early, unexpected, and unprecedented opportunity will come relatively soon, when Comet Siding Spring (C/2013 A1) brushes within 82,000 miles of the Red Planet on October 19th. Because any cometary particles will strike at 35 miles (56 km) per second, there's some concern for the safety of MAVEN and other orbiters circling Mars. They'll be positioned on the back side of the planet during the time of greatest danger.

A few days before and after the comet's closest approach, MAVEN's ultraviolet spectrograph will measure both the abundance of gases within C/2013 A1's coma and also its effects on the Martian upper atmosphere (heating from cometary dust impacts or a temporary increase in water-vapor content). "We should have some pretty spectacular results," Jakosky promises.

Meanwhile, India's Mars Orbiter Mission (MOM) is joining MAVEN (as well as NASA's Mars Reconnaissance Orbiter and Mars Odyssey, plus Europe's Mars Express) in Martian orbit. Launched last November 5th, MOM was scheduled to fire its braking rocket for 24¼ minutes and slip into Martian orbit on September 24th at 7:30 a.m. India Standard Time (2:00 UT).

Our special issue, "Mars: Mysteries & Marvels of the Red Planet," is loaded with spectacular photos and a must-read for anyone interested in this intriguing neighboring world.




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September 22, 2014 at 9:46 am

That's a relief that MAVEN was successfully placed into orbit around Mars. Hopefully MOM will have similar luck in a couple of days. While a lot has been said in recent years about the white-knuckle moments associated with landing on Mars, there have been a number of planetary mission-ending orbit insertion failures over the last four decades with Mars having more than its fair share.

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Peter Wilson

September 23, 2014 at 12:29 pm

Here on Earth, our planet's magnetosphere serves as an obstacle to the solar wind, keeping it from interacting directly with our atmosphere.

It's interesting why the indirect interaction works. With no magnetic field, there is a ring around the planet that is struck tangentially by particles from the solar wind. Along this ring, atmospheric atoms are knocked into space. Ions hitting the planet face-on do not strip the atmosphere, however, because the momentum transfer is downward. The solar wind essentially “chips away” at an atmosphere around the edges. Earth’s magnetic field funnels the solar wind to the north and south poles. There, the incoming ions are heading straight down, creating beauty, but not stripping our atmosphere, because they knock atmospheric atoms down, to the ground.

Q. With no magnetic field, and being 2nd closest to the sun, why does Venus have the thickest atmosphere of the rocky planets?

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September 24, 2014 at 9:24 am

Peter, Venus has the thickest atmosphere of all the rocky planets for two reasons: First, it is massive enough (and its escape velocity high enough) to significantly slow the loss of its atmosphere to thermal loss, interactions with the solar wind, etc. compared to Mars which has just one-tenth the mass. Second, Venus has no liquid water to scrub carbon dioxide out of its atmosphere and turn it into carbonates via chemical weathering of silicates as happens on Earth. And, of course, over the last half billion years, life on Earth has also contributed significantly to the formation of carbonates which has removed large amounts of carbon dioxide from the atmosphere. If Earth had no liquid water and all of its carbonates were turned back into carbon dioxide gas via various geologic processes, it would have a carbon dioxide-rich atmosphere almost as dense as that of Venus as well.

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