Several observations have been conducted since last September till March of this year, 1997 when Mars was at its closest point to Earth in the past two years. Further observations are scheduled for late 1998 since Mars is now nearing and will pass behind the Sun making observations by the Hubble Space Telescope impractical and dangerous to instrumentation.

Phil James (University of Toledo), Todd Clancy (Space Science Inst.,Boulder,CO), Steve Lee (University of Colorado) and NASA.

Planetary Evolution; Meteorology

Hubble's Sharpest View Of Mars

Predicting weather conditions here on Earth is a challenge, and everyone complains about the weather; but trying to predict the unpredictable weather conditions on Mars is another matter. For Mars, changes can sweep over the entire planet every week. Mars' weather conditions appear to be on a roller coaster-like ride; making conditions more chaotic than scientist first thought. Observations by the Hubble Space Telescope together with the National Radio Astronomy Observatory (NRAO) at Kitt Peak,Arizona, show that the atmosphere of Mars is more complex and variable than the picture revealed by the Viking and Mariner 9 orbits.

General Facts:	Diameter................. 0.5 Earth dia. ~ 6800 km Distance from Sun......... 1.5 AU ~ 228 x 10^6 km Rotation.................. 24.6 hrs Revolution................ 687 days ~ 1.8 yrs Temperature variations..... 17°C to -143°C Atmosphere................. thin, mostly carbon dioxide Composition................ rocky,earth-like Moons...................... Phobos (11 km), Deimos(6km)
Instrument used:	Hubble Space Telescope Wide Field and Planetary Camera 2 (WFPC2).
Date of Observations:	A series of observations taken in October,1996 and in January and March of 1997.

• Why is this information important?
  

  The Hubble Space Telescope is being used to monitor the weather conditions on Mars to support the Mars Pathfinder and Mars Global Surveyor Orbiter Missions, which are currently en route to Mars. Hubble's "weather report" from these images is invaluable for Mars Pathfinder, which is scheduled for a July 4 landing. At this time the images show no large-scale dust storm activity, which plagued a previous Mars mission in the early 1970's.

• What was known / believed before the observation?
  

  Ground based observations of the planet Mars have provided a rich if not often inaccurate picture of the Red Planet. Most famous of Martian observers was Percival Lowell who supported the idea that life existed on Mars; perhaps intelligent life as indicated by the complex system of canals Percival Lowell was so famous for mapping. It was known that Mars experienced seasonal change not unlike that here on Earth. Mars has polar ice caps that appear to grow and shrink with the seasonal change. Thin cloud cover often forms around mountains, and if observed while Mars is in perihelion--the closest point in its orbit to the Sun--planet-wide dust storms will obscure almost all features for weeks.

• What has been learned as a result of the observation?
  

  The information collected during the 1970's orbiters gave a fairly one-dimensional picture of the Martian climate, but information captured by the Hubble Space Telescope and NRAO show that Mars is more often cloudy than dusty, experiencing abrupt planet-wide swings between dusty/hot and cloudy/cold. A state of emergency would be declared on Earth if such an ice or dust storm blanketed the entire planet.

  These shifts in climate are driven by three important factors:

  1. Mars' thin atmosphere: Mars' atmosphere is so paper-thin--it weighs less than 1 percent of Earth's atmosphere--that there are no oceans of liquid water to store heat from the Sun; resulting in the planet's temperature respond occuring more quickly and intensely to surface changes and atmospheric heating by the Sun. There is also larger annual changes in sunlight falling on Mars than on Earth because Mars' distance from the Sun varies by 20 percent in its orbit around the Sun every two years.

  2. Mars' elliptical orbit:Mars' elliptical orbit leads to planet-wide changes in atmospheric and surface temperatures over the course of a Martian year which is nearly equal to two Earth years. During perihelion, when Mars is closest to the Sun (summer in the southern Martian hemisphere), the planet receives 40 percent more sunlight than during aphelion, when Mars is farthest from the Sun (summer in the northern Martian hemisphere). This annual variation in sunlight results in a 35 degree Fahrenheit increase during southern summer(perihelion), resulting in continental-size dust storms near the planet's surface. The dust is swept aloft to higher altitudes of tens of miles where it is spread globally, absorbing light from the Sun, heating the entire atmosphere by another 30 to 50 degrees Fahrenheit.

  3. Mars' climatic interactions between dust and water ice clouds: The dusty perihelion climate was observed by Viking and Mariner 9 and by NRAO in 1992,1994, and 1996. What the 1970's orbiters did not identify was the very distinctive climate of Mars at aphelion--the farthest point in its orbit to the Sun--, with its planet-wide belts of water ice clouds. It is the cold atmospheric conditions of Mars during aphelion, when the Sun's effect is much weaker, that stimulate the formation of these water ice clouds. The clouds reduce atmospheric temperatures by forming around the dust acting as condensation nuclei,reflecting sunlight back out into space and once frozen the dust falls to the ground. This competition between dust heating in the summer and cloud cooling in the winter drives the sweeping annual and short-term regional changes in Mars' climate.

• MARS FACT SHEET

• POSITIONS OF THE PLANETS

• NATIONAL SPACE SCIENCE DATA CENTER