The Starbase One Astronomy & Space Collection/STARBASE_ONE.ISO/a96/disk11/plntwthr.txt

Text File | 1996-01-19 | 8KB | 143 lines

                           SCIENCE BACKGROUNDER

              HUBBLE MONITORS WEATHER ON NEIGHBORING PLANETS


MARS: A COOLER, CLEARER WORLD

Four years, (or two Mars years') worth of Hubble observations show that
the Red Planet's climate has changed since the mid-1970's.  "The Hubble
results show us that the Viking years are not the rule, and perhaps not
typical.  Our early assumptions about the Martian climate were wrong,"
said Philip James of the University of Toledo.

"There has been a global drop in temperature.  The planet is cooler and
the atmosphere clearer than seen before," said Steven Lee of the
University of Colorado in Boulder.  "This shows the need for continuous
monitoring of Mars. Space probes provided a close-up look, but it's
difficult to extrapolate to long-term conditions based upon these brief
encounters."

The researchers attribute the cooling of the Martian atmosphere to
diminished dust storm activity, which was rampant when a pair of NASA
Viking orbiter and lander spacecraft arrived at Mars in 1976.  Two
major dust storms occurred during the first year of the Viking visits,
which left fine dust particles suspended in the Martian atmosphere for
longer than normal.  Warmed by the Sun, these dust particles (some only
a micron in diameter, about the size of smoke particles) are the
primary source of heat in the Martian atmosphere.

"Hubble is showing that our early understanding based on these visits
is wrong.  We just happened to visit Mars when it was dusty, and now
the dust has settled out," Lee said.  "We are going to have to look at
Mars for many years to truly understand the workings of the climate,"
said Todd Clancy, of the Space Science Institute, Boulder, Colorado.

Knowledge about the Martian climate has been limited by the fact that
ground-based telescopes can only see weather details when Earth and
Mars are closest -- an event called opposition -- that happens only
once every two years.  Though Hubble has observed Mars only for four
years, the observations are equivalent to 15 years of ground- based
observing because Hubble can follow seasonal changes through most of
Mars' orbit.

Though the Mariner and Viking series of flyby, orbiter and lander
spacecraft that visited Mars in the late 60's and 70's provided a
close-up look at Martian weather, these were snapshots of the planet's
complex climate.  Hubble provides the advantage of a global view - much
like the satellites that monitor Earth's weather, and can follow
martian seasonal changes over many years.  When Mars is closest to
Earth, Hubble returns near-weather satellite resolution.

MARS --  NO LACK OF OZONE

Although there has been concern about a lack of ozone (a form of
molecular oxygen created by the effects of sunlight on an atmosphere),
dubbed the "ozone hole" over Earth's poles, there are no ozone holes on
Mars.  By contrast, the planet has a surplus of ozone over its northern
polar cap, as first identified by the Mariner 9 spacecraft in 1971.
(However the Martian atmosphere is different enough from Earth's that
few parallels can be drawn about processes controlling the production
and destruction of ozone.) Hubble's ultraviolet sensitivity is ideal
for monitoring ozone levels on a global scale.  The Martian ozone is
yet another indication the planet has grown drier, because the water in
the atmosphere that normally destroys ozone has frozen-out to become
ice-crystal clouds.  Spectroscopic observations made with the Faint
Object Spectrograph (FOS) show that ozone now extends down from Mars'
north pole to mid and lower latitudes.  However, the Martian atmosphere
is so thin, even this added ozone would offer future human explorers
little protection from the Sun's harmful ultraviolet rays.


SEASONS ON MARS

The fourth planet from the Sun, Mars is one of the most intensely
scrutinized worlds because of its Earth-like characteristics.  Mars is
tilted on its axis by about the same amount Earth is, hence Mars goes
through seasonal changes.  However, because Mars' atmosphere is much
thinner than Earth's, it is far more sensitive to minor changes in the
amount of light and heat received from the Sun.  This is intensified by
Mars' orbit that is more elliptical than Earth's, so it's range of
distance from the Sun is greater during the Martian year.  Mars is now
so distant, the sun is nearly 25% dimmer than average.  This chills
Mars' average temperature by 36 degrees Fahrenheit (20 degrees
Kelvin).  At these cold temperatures, water vapor at low altitudes
freezes out to form ice-crystal clouds now seen in abundance by
Hubble.

"Clouds weren't considered to be very important to the Martian climate
during the Viking visits because they were so scarce," says Clancy.
"Now we can see where they may play a role in transporting water
between the north and south poles during the Martian year."  Seasonal
winds also play a major role is transporting dust across Mars' surface,
and rapidly changing the appearance of a region. This gave early
astronomers the misperception that Mars' shifting surface color was
evidence of vegetation following a season cycle.

As clearly seen in the Hubble images, past dust storms in Mars'
southern hemisphere have scoured the plains of fine light dust and
transported the dust northward.  This leaves behind a relatively
coarser, less reflective sand in the southern hemisphere.


VENUS:  NO EVIDENCE FOR NEW VOLCANIC ERUPTIONS

Hubble spectroscopic observations of Venus taken with the Goddard High
Resolution Spectrograph provide a new opportunity to look for evidence
of volcanic activity on the planet's surface.  Though radar maps of the
Venusian surface taken by the Magellan orbiter revealed numerous
volcanoes, Magellan did not find clear cut evidence for active
volcanoes.

Hubble can trace atmospheric changes that might be driven by
volcanism.  An abundance of sulfur dioxide in the atmosphere could be a
tell-tale sign of an active volcanos.  Sulfur dioxide was first
detected by the Venus Pioneer probe in the late 1970s and has been
declining ever since.  The Hubble observations show that sulfur dioxide
levels continue to decline.  This means there is no evidence for the
recurrence of large scale volcanic eruptions in the last few years.

Ejected high into Venus' murky atmosphere, this sulfur dioxide is
broken apart by sunlight to make an acid rain of concentrated sulfuric
acid.  This is similar to what happens on Earth above coal-burning
power plants - but on a much larger and more intense scale.


FUTURE PLANS

More Hubble observations of Mars and Venus are critical to planning
visits by future space probes.  In particular, both robotic and human
missions to Mars will need to be targeted for times during the Martian
year when there is a minimal chance of getting caught in a dust storm.
Knowing whether the atmosphere is relatively hot or cold is crucial to
planning aerobraking maneuvers, where spacecraft use the aerodynamic
drag of an atmosphere to slow down and enter an orbit around the
planet.  This reduces the amount of propellant needed for the journey.
"If the atmosphere is more extended than expected the added friction
could burn up an aerobraking spacecraft, just as Earth's atmosphere
incinerates infalling meteors," says James.

Ultimately, knowing the Martian climate will be an fundamental
prerequisite for any future plans to establish a permanent human
outpost on the Red Planet.