The Mars Orbiter Laser Altimeter (MOLA) did not only shed light on Martian
topography, it also gave us new information about the Martian atmosphere
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
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
MOLA's years of observations of cloud observations have certainly provided
an interesting seasonal profile of Martian atmospheric activity!
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.
Clouds & Snow