Bruce BettsApr 03, 2007

Millions of soundings yield clues to Mars' weather

Two months after the start of Mars Reconnaissance Orbiter's primary science phase, the Mars Climate Sounder instrument has already acquired more than four million soundings, building toward a vast data set on the three-dimensional structure of Mars' atmosphere over the full Martian year of the orbiter's nominal mission.

However, an intermittent error has caused normal operations of the instrument to be suspended. The error, which first appeared in December, was a mismatch between the actual position of its scan mirror and the position that the instrument's software expected. The error caused Mars Climate Sounder it repeatedly to stow itself for safety and await further instructions from the ground, says co-investigator Tim Schofield.

"We modified our scanning and over the holidays it ran fine without any errors. We were planning to do some troubleshooting after the first of the year, but when we put the instrument back into its standard science scanning in early January we didn't get any errors. We couldn't duplicate the errors we saw before. But they returned on January 16. They escalated rapidly during testing of various kinds, and we allowed the instrument to stow automatically on January 25."

Studies of the errors revealed that the problems were occurring with one of the two motors that drive the instrument's scan mirror. Schofield continued: "On February 9, we stared at the limb of Mars, and also gathered a little space-view calibation data by scanning using the good azimuth motor during large spacecraft rolls. We are avoiding elevation scanning of any kind. This gives useful, though not optimum, science data. These observations continued uneventfully, producing good atmospheric data, until the spacecraft went into safe mode on March 14. This powered MCS off for the first time since September 24, 2006.

"The spacecraft has now recovered from safe mode and is operating nominally, although the precise cause of the entry into safe mode is not known. MCS was powered back on on March 24 and slewed back to the limb on March 27. We have been limb-staring ever since.

"The independent team investigating the MCS elevation actuator anomaly at the Jet Propulsion Laboratory has almost completed its task. Based on the data, there is no obvious cause identified, but it is likely that they will recommend a few cautious tests to see if the position errors are still present after the power cycle, and after a few changes in scanning approach. If the errors disappear, we will move back to normal scanning. If they do not, we will resume limb-staring."

Work is now underway to begin to understand what the Mars Climate Sounder data already received can tell climatologists. Each of the millions of soundings is composed of 189 samples -- 21 samples in each of nine different wavelength channels. (For more information, read about how Mars Climate Sounder Works.) The majority of these measurements are taken as Mars Climate Sounder looks forward along Mars Reconnaissance Orbiter's ground track to the limb, so that the 21 points sample the atmosphere at different elevations above the Martian surface, from -10 to +80 kilometers (-6 to +50 miles).

To begin to study what structures may be present in the data, the Mars Climate Sounder team is producing "quick look" plots of the data, each of which displays four hours worth of data, roughly two complete orbits or more than 7,000 soundings. These plots are one of the team's main methods for surveying their data for interesting features.

Here is one example quick-look plot. Let's look closer at what four of these channels tell us.

Channel A6 is the simplest to interpret. It measures visible light wavelengths, so the graph appears black for the half of the orbit over Mars' night side, and is brightest over Mars' day side. On the day side, this channel is sensitive to atmospheric haze. Most variations in brightness have to do with the viewing geometry, and in particular where the Sun was with respect to the spacecraft. The low latitudes are much more hazy than the high latitudes, with water ice and dust in the air. As the spacecraft flies over the south pole, it looks into the sunrise, the Sun ahead and roughly 45 degrees to the spacecraft's left. As it moves northward, the haze thickens and becomes brighter. Once it crosses the equator, the Sun is behind the spacecraft, and it finally sets as the spacecraft passes over the north pole. The big asymmetry in this channel between north and south is thus because forward scattering by haze is much stronger than back-scattering. However, orbit-to-orbit changes can tell the MCS team about changes in the distribution of haze in latitude and longitude.

The infrared wavelengths sampled in channel A2 are strongly absorbed by carbon dioxide, the major constituent of Mars' atmosphere. At this wavelength, Mars Climate Sounder cannot see into the atmosphere below an elevation of roughly 25 kilometers (15 miles), below which the atmosphere is thick enough to absorb all radiation. The south pole appears to be a "hole" in the atmosphere at this wavelength, but in fact it's just much colder than the rest of the atmosphere because of the perpetual darkness of winter night.

Channels A4 and B1 are sensitive to aerosol haze. The haze is more transparent in the longer wavelength B1 channel, so the atmosphere appears colder. A4, the shortest wavelength infrared channel, is more sensitive to dust and water ice than B1, and as a result shows high haze layers better. When the haze is thin, as it is at high latitudes, the surface can be seen in these channels. The hottest surface spots lie under the lowest (thinnest) haze regions. On the night side and in the south polar region, the surface is colder than the atmosphere and does not show up. Haze is thick at the equator (both day and night) and variable at high northern latitudes, hence the lumpy temperature contours. A2 never sees the surface due to CO2 absorption.

One curious feature visible in all the channels are inverted U-shaped loops. These are not Martian flares! They are vertically thin, high-elevation, latitudinally-restricted haze layers. The hazes do not actually form loops in the Martian sky. Their loopy appearance is caused by the forward limb-viewing geometry of the Mars Climate Sounder instrument combined with the curvature of the planet. As the spacecraft moves (from left to right across these graphs), the high haze layer peeks over the horizon. The layer rises in Mars Climate Sounder's field of view as the spacecraft approaches it until the instrument sees it at its true altitude, then the layer appears to fall as the spacecraft passes over it.

Three distinct loops are visible in this plot. The first lies just south of the equator on the night side, so it is visible in channels A2, A4, and B1, but not the visible channel A6. It lies at an altitude of approximately 60 kilometers (36 miles). One full orbit later -- 30 degrees of longitude to the west of the first orbit -- there is a similar layer at a slightly lower elevation, perhaps 55 kilometers (33 miles). Finally, there is another layer on the day side just north of the equator, at about 65 kilometers (39 miles), visible in channels A6 and A4 and barely discernible in B1.

These high haze layers are one manifestation of Mars' weather. The Mars Climate Sounder team will study where they occur, at what elevations, and how that varies over the Martian year. They are looking forward to discovering other surprising weather features in this entirely new kind of data set.

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