The Mars Orbiter Laser Altimeter (MOLA) did not only shed light on Martian topography, it also gave us new information about the Martian atmosphere and clouds.

MOLA is an active remote-sensing instrument - it interacts with what it observes, by sending out pulses of laser light at its target - 10 per second - and then using the light that is bounced back to determine the properties of that target. In the case of MOLA, that target is the surface of Mars.

MOLA's job was to take data on the topography or surface characteristics of Mars - which it has done very succesfully. But MOLA also ended up working as an atmospheric lidar, taking data on Martian clouds at heights up to 20 kilometers above the ground!

Dense clouds over the south polar cap. Red and black indicate relatively thin, optically dense clouds, and green and blue indicate diffuse clouds.

We have known that Mars had clouds for about a century, but their extent, composition, and distribution still contain mystery. The Viking Orbiter did observe clouds over several seasons and the Mars Orbiter Camera (MOC) has tracked regional storms and atmospheric hazes. But visual imaging is limited to daylight hours. MOLA is unique in its ability to determine the vertical structure of clouds during the polar winter night.

Clouds detected by MOLA seemed to fall into one of two categories. Bright and reflective structures, that allowed the laser altimeter to detect them, and dark, absorptive ones that were so opaque that there was no returned signal.

MOLA's observations of absorptive clouds started shortly after orbit insertion - and it started observing reflective clouds in March 1998, at the start of the Science Phasing Orbits. Global cloud measurements commenced with the primary mapping mission in March 1999, with nearly continuous coverate for 1.25 Mars years.

MOLA data show that reflective clouds begin to develop in the north polar region in northern autumn and then migrate towards the pole, dissipating right before the start of spring. During the southern winter, the reflective clouds appear high over the southern pole, with low, dense clouds flanking the pole. These clouds also disappeared by the beginning of spring. Winter reflective cloud activity was much lower in the second (Martian) year of observations, which suggests intrannual warming.

Some MOLA data suggested the presence of CO2 snow! The data at left show dry ice snow condensing out of the Martian atmosphere near the south pole. Note that the CO2 crystals (in black) extend from an altiude of 18 km all the way down to the surface.

The absorptive clouds were very different. In the northern polar regions, absorptive clouds developed during northern autumn and trace the north polar frost line. The occurance decreases during northern winter, during which time the reflective clouds dominate, and then increase again in late winter. In the southern hemisphere, a separate absorptive cloud front formed north of the polar frost line. By the end of southern winter, the cloud front had migrated southward towards the pole, clearing by southern summer soltice.

While MOLA was observing Mars, during the southern hemisphere summer, nonreflective clouds became globally distributed, and major dust storms occurred. Some reflective daylight clouds observed at low latitudes were analogous to Earth "dust devils".

Cloud locations over the North Martian Polar cap in winter. Each white encircled colored dot is a cloud.

MOLA's years of observations of cloud observations have certainly provided an interesting seasonal profile of Martian atmospheric activity!

References

This article contains information and graphics from:

Neumann, Gregory A., David E. Smith, Maria T. Zuber. "Two Mars years of clouds detected by the Mars Orbiter Laser Altimeter.", Journal of Geophysical Research, Vol 108, NO. E4, 5023, 2003.

Smith, David E., et al. "Mars Orbiter Laser Altimeter: Experiment summary after the first year of global mapping of Mars.", Journal of Geophysical Research, Vol 106, No E10, pages 689-722, October 25, 2001.

Pettengill, G. H., and P. G. Ford, Winter clouds over the north Martian polar cap, Geophys. Res. Lett., 27, 609-612, 2000.

MOLA - Clouds & Snow