NASA's Mars Exploration Rover Opportunity captured a view of wispy
afternoon clouds, not unlike fair weather clouds on Earth, passing
overhead on the rover's 956th sol, or Martian day (Oct. 2, 2006). With
Opportunity facing northeast, the clouds appear to drift gently toward the
west in this movie taken with the rover's navigation camera.
The 10 frames, taken 32 seconds apart, show the formation and evolution of
what are likely mid-level, convective water clouds. Such clouds are common
near Mars' equator at this time of the Martian year. They have been
observed by both of NASA's Mars Exploration Rovers, by satellites orbiting
Mars, and by the Hubble Space Telescope. In this case, the clouds appear
to develop at a fixed location, in the center of the frame about 25
degrees above the horizon. This style of origin suggests that a thermal
plume is rising over a surface feature. In spite of apparent winds aloft,
the thermal plume appears to remain stationary for the 5-minute duration
of the movie.
Though scientists have determined from the images that the wind bearing is
east-northeast, approximately 80 degrees, it is not possible on the basis
of the movie to unambiguously determine the height and speed of the
clouds. Scientists estimate, based on models of atmospheric wind profiles
and the apparent displacement of the clouds, that all of the clouds in the
movie are at about the same height somewhere between 5 kilometers and 25
kilometers (3 to 20 miles) above the surface. The clouds are estimated to
be moving at 2.5 meters per second, if they are low, to 12.5 meters per
second, if they are high (8 feet per second to 41 feet per second).
Like clouds on Earth, these Martian clouds are probably composed of ice
crystals and possibly supercooled water droplets. They are similar in
appearance to terrestrial cirrocumulus or high altocumulus clouds. On
Earth, such clouds are relatively transient and consist of small,
individual cloudlets arranged in rippled patterns. They usually form 6
kilometers to 12 kilometers (4 to 7 miles) above Earth's surface by a
process known as convection, during which warm air rises and cools, with
clouds condensing from the moist air once it has cooled sufficiently.
These Martian clouds appear to be associated with a broader layer of
ice-crystal clouds fanning out toward the upper right of the frames at
the end of the movie. This is similar to the occurrence of terrestrial
cirrocumulus and altocumulus clouds within layers of cirrus or
cirrostratus clouds on Earth. Also apparent in this movie are prominent
waves in the clouds, a result of the effect of gravity waves on cloud
thickness, as on Earth.
Though both rovers now have the ability to autonomously detect clouds,
these images were taken prior to the first use of the new abilities. The
images shown here were stored on Opportunity and were transmitted to Earth
on sol 1056 (Jan. 12, 2007) during a routine communications pass.
