From a Space Telescope Science Institute press release
May 19, 1999:
Astronomers using NASA's Hubble Space Telescope (HST) during
Mars's
recent close approach to Earth have
discovered an enormous cyclonic storm system raging in the
northern polar regions of the Red Planet. Nearly four
times the size of the state of Texas, the storm is composed
of water-ice clouds like storm systems on Earth, rather
than dust typically found in Martian storms.
Right: This view of the north
polar region on Mars shows the relative size and location of the
storm (smoke-ring-like structure at left) compared to the polar
cap and other classical bright and dark features in this area.
The bright surface region beneath the eye of the storm can be
seen clearly. This map covers the region north of 45 degrees
latitude and is oriented with 0 degrees longitude at the bottom.
Image credit: Space Telescope Science Institute and NASA.
The system is similar to so-called "spiral" storms
observed more than 20 years ago by NASA's
Viking Orbiter spacecraft, but it is nearly
three times as large as the largest previously
detected Martian spiral storm system. The
storm is nearly 1,100 miles across from east
to west and 900 miles from north to south. The
eye of the storm is nearly 200 miles in diameter.
The system is larger than the planet's residual north
polar ice cap, and is comparable in size to similarly
shaped terrestrial hurricanes.
The storm was detected using the Hubble's Wide Field Planetary
Camera-2 by a team of astronomers observing Mars on April 27, 1999,
just a few days after its closest
approach to Earth in nearly eight years. Jim Bell, assistant
professor of astronomy at Cornell University, principal investigator
on the team of astronomers obtaining and analyzing the Hubble images,
says the observations were somewhat serendipitous in that they
were made during the same season that the storms were first
detected by Viking.
These rapidly growing and decaying
systems do appear to be typical of the Martian
polar weather at this season, which is mid northern summer,"
says Bell. "The storm we detected from Hubble appears anomalous
because of its size, but I wouldn't be surprised if the
Mars Global Surveyor spacecraft sees other storms of this
size eventually as well."
Three days after Hubble photographed the giant storm system,
the Mars Global Surveyor spacecraft flew past the north pole
and captured the image shown here (right).
The cyclone had vanished,
leaving behind
what appeared to be normal
cloud patterns for this time of year. In
the north polar region, temperature
differences between bright areas of
year-round ice and dark areas of sand
and rock create strong winds that mix
the atmosphere and create waves of
clouds that swirl around the polar cap.
The motion of the clouds viewed in the
images is typical for this season on
Mars, and shows forms often seen on
Earth.
The April 27th storm consisted of at least three,
and perhaps more, bands of clouds organized into a spiral
structure and wrapped counterclockwise around a hollow central
core or "eye." The storm appeared in the middle of the Martian
northern hemisphere's summer season, after the planet's seasonal
carbon dioxide polar cap had completely sublimated away, leaving
only the underlying residual water-ice cap. The smaller spiral
storms seen previously by Viking were also detected during the
northern summer season, and also at high northern latitudes.
Apparently, this type of cyclonic circulation,
though rare on Mars, must be related to specific
climatic conditions unique to the planet's northern
polar regions at this season. Similar storms, some comparable
in size to the Martian storm, have been seen in Earth's polar
regions. On Earth, these polar cyclones appear to be low pressure
systems fueled by strong contrasts in oceanic versus atmospheric
temperatures. In some cases, winds within Earth's polar cyclones
can reach hurricane force.
Right:
This dramatic three-dimensional visualization of Mars' north pole is based on elevation measurements
made by an orbiting laser. During the Spring and Summer of 1998 the Mars Orbiter Laser Altimeter (MOLA) flashed laser
pulses toward the Martian surface from the Global Surveyor spacecraft and recorded the time it took to detect the reflection.
This timing data has now been translated to a detailed topographic map of Mars' north polar terrain.
More information.
The general appearance of the Martian storm
seems consistent with an intense low pressure
vortex with rising air causing cloud formation,
possibly with a small core that is cloud-free,
like the eye of a hurricane. The storm may have been
initiated by an unstable frontal system, and then
amplified by the strong temperature contrast between
the relatively warm high-latitude Martian dark regions
and the much colder and stable polar atmosphere.
"It seems that our knowledge
of the high-latitude weather on Mars
is limited by the fact that even Viking,
from orbit 20 years ago, could not observe
the polar regions very frequently," says Cornell's Bell.
"So our HST finding, and forthcoming results from the Mars
Global Surveyor spacecraft, may simply be revealing
all this activity up there because it's the first
time we've been able to look in a detailed and
dedicated way with high-resolution instruments during this season."
The team of astronomers, besides Bell,
operator of HST, included Michael Wolff and
R. Todd Clancy (Space Telescope Science Institute),
Steven Lee (University of Colorado), Philip James (University of Toledo), and Michael Ravine (Malin Space Science Systems, Inc.).
Above: Zooming in on Mars. A. A Hubble
Space Telescope View of Mars from 65 million miles distance;
B. A Martian volcano, Apollinaris Patera, with white clouds near the summit
photographed from above by Mars Global Surveyor; C The rock "Barnacle Bill" as seen
from a distance of a few meters at the Mars Pathfinder landing site;
D: Possibly the closest-ever view of Mars.
This high-resolution
electron microscope image shows an unusual tube-like
form less than 1/100th the width of a human hair. It was
found in meteorite ALH84001, believed to be of Martian
origin and is thought by some scientists to be evidence of ancient Martian life. more information.
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