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Planetary News: Mars (2008)Phoenix Mission Receives OK to Listen for Sounds on MarsBy Emily LakdawallaSeptember 18, 2008
We may soon hear the first sounds from Mars. Last night, NASA gave the go-ahead to the Phoenix mission to power on the Mars Descent Imager (MARDI) for the first time. MARDI, which was built by Malin Space Science Systems, is equipped with a tiny microphone that could have recorded sounds from Phoenix' landing, and may now be in a position to return the first sounds from the surface of Mars. MARDI is not the first microphone sent to Mars; The Planetary Society flew the first-ever planetary microphone on board the failed Mars Polar Lander. Utilizing its past experience, The Planetary Society will collaborate with Malin Space Science Systems on public outreach and understanding the sounds if MARDI is able to collect them. However, it is not at all certain that the MARDI microphone will now be able to return sounds from Mars. One concern is whether an instrument that was only intended to be used during landing still functions at all after nearly four months of being subjected to the extreme temperatures of a landing site near Mars' north pole. And, as it was intended to record the loud noises of landing, it may lack the sensitivity to record the much quieter sounds that prevail around Phoenix. Still, it's worth the attempt, says Planetary Society Executive Director Louis Friedman. "The microphone permits exploring Mars with another human sense: that of hearing. We don't quite know what to expect, but that is why we explore. The Planetary Society is proud to have first brought this idea to NASA and to help interest the public in potential sounds of Mars." What Might the MARDI Microphone Hear?Mars' thin atmosphere does not transmit sound as efficiently as Earth's. Lower-frequency (lower-pitch) sounds are carried better than higher-frequency sounds, so Mars sounds would sound quieter and deeper compared with how they would sound on Earth. Compare, for instance, two recordings of Ray Bradbury, an original and one processed to simulate how it would sound if broadcast from the surface of Mars. Other such comparisons are available on the Mars Microphone web page. The Planetary Society's Mars Microphone could have returned sounds from winds, sandstorms, dust devils, and other activities caused by local weather patterns. However, tests performed on a mockup of the MARDI microphone by the Mars Microphone's engineer (Greg Delory of the University of California, Berkeley) showed that the MARDI microphone is five times less sensitive than the Mars Microphone, so may not be sensitive enough to hear such (relatively) faint noises.
MARDI has a better chance of hearing sounds generated by Phoenix itself. Phoenix contains a large number of moving parts. Its arm has four whirring motors in its joints, and likely makes other noises as it digs and scrapes at the icy surface of the polar landscape. Some of its instruments, particularly the Thermal and Evolved Gas Analyzer (TEGA), have many internal actuators. Among the noisiest of these might be the vibrators that are part of the sample delivery mechanism within TEGA. Such sounds would be transmitted not only through Mars' thin air but also as vibrations within the spacecraft itself, which might be easier for the MARDI microphone to pick up. How Does MARDI Work?The brain of MARDI is an off-the-shelf Motorola processor, designed originally for cellular telephone applications. Because of this heritage, the processor had a built-in microphone input. Malin Space Science Systems decided to take advantage of the opportunity presented by the processor's built-in audio processing capability and attached a tiny external microphone, also an off-the-shelf device. So, unlike the very sensitive Mars Microphone provided by The Planetary Society to the Mars Polar Lander mission, the MARDI microphone was not purpose-built to record sounds on Mars, where the thin atmosphere attenuates sounds. The noises of Phoenix' dramatic descent should have been loud enough to be picked up by the relatively insensitive MARDI microphone. There is no guarantee, however, that there are any noises in Phoenix' environment that are loud enough for the MARDI microphone to hear. Still, given the fact that there is a microphone built into an active spacecraft on Mars, it would be a missed opportunity not to try to hear the first Mars sounds. With MARDI, imaging and sound go hand in hand. A digital camera works by exposing its detector to light from the environment, then closing the shutter and reading out the image into an internal buffer. MARDI only records sound while it is reading out an image; the audio data is appended to the end of each image file. A typical full-frame MARDI image would take about four seconds to read out, so the audio would come in four-second chunks. To return the four seconds of audio data to Earth would also require that the corresponding MARDI image be transmitted to Earth, with a total cost of approximately 5 Megabits of data volume per four seconds of audio.
One way of testing whether MARDI is sensitive enough to hear anything on Mars would be to take two images, each with its four-second burst of sound. One image would be taken while the spacecraft is utterly still. The other would be taken while the arm is moving or scraping, or with a TEGA vibrator operating. Sounds from within the spacecraft itself are likely to be louder than any ambient noise. All of this assumes that MARDI still functions after nearly four cold months on Mars. Phoenix' other science instruments are all kept healthy with small heaters; without their help, MARDI's electronic components might already have failed due to the daily cycling of temperature from highs of minus 30 to lows of minus 85 Celsius (minus 20 to minus 120 Fahrenheit). And given the fact that the microphone is a commercial product, not one designed for Mars, it is the most likely of all of MARDI's components to have failed. If the microphone has failed but the rest of the instrument works, images returned from the camera could be valuable; they may take in some of the interesting ice patches beneath the lander, the site dubbed "Holy Cow" by the Phoenix team. Prelaunch tests indicate that such images would be in sufficient focus to reveal interesting details of an area that is impossible to view with the mast-mounted camera, and only visible from a highly oblique angle by the robotic arm camera. Why Hasn't MARDI Been Powered on Before?MARDI was provided to the Phoenix mission by Malin Space Science Systems in order to produce images, taken from decreasing altitudes, of the Phoenix landing site as the spacecraft approached. The images would have provided valuable context, bridging the gap in resolution between what Phoenix can see with its mast-mounted camera and what is visible from space using orbiting cameras like Mars Reconnaissance Orbiter's HiRISE. Just before landing, MARDI would have captured images with resolutions as high as one centimeter per pixel, thirty times more detailed than HiRISE. Unfortunately, late in the process of Phoenix' testing and assembly, a fault was discovered within a data-handling card in Phoenix' electronics that was responsible for processing information from both MARDI and Phoenix' inertial measurement unit, or "gyro." The gyro was critical for maintaining Phoenix' orientation during its final rocket-assisted descent. The data-handling card could have lost some data from the gyro if an image was transmitted from MARDI at a critical moment when the temperature of the lander happened to be within a certain range. Although the likelihood of a failure due to this scenario playing out was considered to be extremely small, the mission decided just before Phoenix launched to take only one MARDI image rather than the initially planned 20. MARDI could hold the single image inside its internal buffer until after the safe landing, and only then transfer it to Phoenix, preventing any possibility of the fault from being triggered. With Phoenix safely launched, the mission attempted to implement this change in plans, and discovered that to do so, they would have to make very small but very late changes to the already heavily tested command sequences for the critical entry, descent, and landing period. Any such changes impose a risk. The mission elected not to take that risk and reluctantly decided not to turn on MARDI at all. However, tragedy for Phoenix MARDI was turned into relief for its sister instrument on Mars Science Laboratory (MSL), a rover mission scheduled for launch in 2009. In a negotiation among Peter Smith, Phoenix' principal investigator; Michael Malin, MARDI's developer; and NASA Headquarters, Smith agreed to release much of the funds that had been allocated to the operation of Phoenix MARDI to the MSL project to allow its own MARDI instrument to be reinstated after it had been descoped in September 2007 in response to MSL's cost overruns. MSL's MARDI is a new generation of instrument; its electronics share a design with MSL's other science cameras, MastCam and MAHLI. It will be able to provide high-definition color video of MSL's descent, and its images will be taken at a high enough rate (five times per second) for them to be used to determine the profile of wind speeds above MSL's landing site. MARDI was the first of MSL's instruments to be completed and delivered to JPL for integration onto the spacecraft. Sadly for Mars sound buffs, it contains no microphone. If Phoenix MARDI Doesn't Work, What's Next for Mars Sounds?MARDI's microphone is the best hope for sounds from Mars until The Planetary Society finds another Mars mission that is willing and able to carry its Mars Microphone to the surface. The Mars Microphone was part of the payload of France's Netlander mission, which was, unfortunately, canceled in 2004. The Mars Exploration Rovers were too mass-constrained to carry the Mars Microphone. Because Phoenix's instruments were all inherited from Mars Surveyor, it was too late once Phoenix was selected for the Mars Microphone to be included in its payload. The Society continues to approach all proposed landed Mars missions to find a new ride for the Mars Microphone. "The Planetary Society and its members have been working to obtain sounds from other worlds for more than a decade, something advocated by Carl Sagan," said Bruce Betts, the Society's director of projects. "We wish the best of luck to the MARDI and Phoenix teams, and hope they are able to obtain sounds from the Red Planet." |
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