Hubble Space Telescope To Monitor Changes On Mars
18 May 1991The NASA/ESA Hubble Space Telescope (HST) has begun a long-term program to monitor seasonal and interannual changes that occur on the surface and in the atmosphere of the planet Mars. This program will allow for a better understanding of the Martian climate and processes involved in surface changes, and may eventually allow scientists to characterize global weather patterns on Mars, which will be an important prerequisite for a manned expedition to the red planet.
Preliminary observations, made with the HST's Planetary Camera, yield the sharpest images (.2 arc second) of Mars ever taken from Earth. They show clearly Martian atmospheric features, as well as surface and topographic details ranging from large impact basins down to surface markings as small as 50 kilometers (31 miles) across. The HST data also provide new information about the abundance of ozone in the Martian atmosphere.
The science team undertaking this long-term program is composed of planetary scientists Philip James, University of Toledo; Steve Lee and Todd Clancy, University of Colorado; Ralph Kahn and Richard Zurek, Jet Propulsion Laboratory; Leonard Martin, Lowell Observatory, and Robert Singer, University of Arizona.
Although Mars has been studied in detail by several space missions, including the Viking Landers which made observations from the Martian surface for several years, there is still much scientific mystery in the red planet. Mars is a dynamic world because, like Earth, it possesses an active atmosphere which changes the appearance of the planet's surface over both short and long time scales. Mars goes through seasonal changes because, like Earth, its rotational axis is inclined with respect to its orbit (23.5° for Earth, 25.2° for Mars).
To understand Mars' complex meteorology and climate the planet must be continually monitored over many of its annual cycles, much as the monitoring of Earth by terrestrial weather satellites has improved our ability to understand and forecast weather on our planet. HST observations allow monitoring of the Martian surface and atmosphere, revealing the global distributions of clouds (both water and dust) in the atmosphere versus latitude, longitude, time of day, season, and year. This is an impossible job for ground based telescopes because atmospheric motions blur the telescopic image of a planet. Even under optimum observing conditions the smallest details which ground based telescopes can see on Mars are about 150 kilometres across. What is more, this resolution is only possible during a short period of time approximately every 780 days when Mars is closest to the Earth. Most of the time ground based telescopes are unable to resolve features smaller than 600-1000 kilometres so that even major events occurring on the planet are unrecognizable.
Even when Mars is at its greatest distance from Earth, the resolution of the HST Planetary Camera is comparable to that obtainable from ground based observatories only when Mars is closest to Earth (opposition). This makes HST an ideal instrument for monitoring long-term changes on Mars throughout the red planet's 1.8 year orbital period.
Since HST is located in space it can observe Mars in ultraviolet light, which is normally absorbed by ozone in the Earth's atmosphere. By comparing these images at different wavelengths the researchers can measure absorption of ozone in the Martian atmosphere, especially over the dry polar regions. Martian atmospheric ozone was identified by ultraviolet observations made by the Mariner 9 spacecraft in 1971, but HST observations provide both global and temporal ozone monitoring. Apparently water vapor initiates chemical reactions which remove ozone from the Martian atmosphere, and the team scientists hope to use the ultraviolet data to monitor the amount of water, as well as ozone, in the Martian atmosphere.
The researchers will continue to monitor Mars until its angular separation from the Sun in the sky is less than HSTs safety limit. Comparison of images obtained at different times will reveal surface, weather, and climate changes on the red planet and will hasten the day when scientists can understand and characterize weather and surface conditions on the planet in preparation for future manned and unmanned exploration.