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- 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.
-
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- 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
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- 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.
-
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- 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.
-