Like airline pilots calling ahead for their destination's weather, controllers of two Mars-bound spacecraft are asking the Hubble space telescope what's brewing on Mars.
The answer is: "The weather's fine so far."
An early April image of the northern Martian hemisphere shows a clear atmosphere with a few water-ice clouds. That's just what the controllers want when the Pathfinder craft touches down on Mars July 4 - 21 years to the day after the first historic Viking mission landing. That will kick off a 10-year series of missions focused on the issue of whether Mars now has - or has ever had - environments that could have supported life.
The series' planners hope that, ultimately, it may find evidence of past or present life itself.
Viking 1 encountered dust storms. But there has been little sign of them during the northern Martian spring. A trio of Hubble images (left) shows the progressive evaporation of the north polar carbon-dioxide icecap under an essentially clear atmosphere as spring turns into early summer. This leaves behind a residue of water-ice that resides permanently at the pole.
Keeping a weather eye
Dust storms would be bad news for Pathfinder. Pathfinder and the tiny six-wheeled Sojourner rover vehicle it will deploy need clear air that lets in full sunshine to power their solar panels. As on Earth, however, past good behavior is no guarantee that Mars's fickle weather won't kick up a dust storm. So Hubble will literally keep a weather eye on the planet over the next few weeks.
It also will periodically check the weather for Mars Global Surveyor, which will reach the Red Planet Sept. 12.
Surveyor's controllers will use "aerobraking" to settle the craft into Martian orbit. Friction in the planet's thin outer atmosphere will slow the craft and adjust its course. This tricky maneuver depends critically on the air density at orbital altitudes. That density, in turn, depends on weather down below.
Neither the Pathfinder nor the Surveyor mission is equipped to look for any life signs.
Pathfinder is essentially an engineering test. It is prepared to survey its landing site for at least a month while the rover scurries about examining nearby rocks for at least seven days. Surveyor is to map Mars for a minimum of 687 Earth days, which is a Martian year.
Surveyor's maps, showing details only a few meters across, should sharpen the evidence of past rivers, glaciers, and lakes that has convinced many scientists that Mars has had abundant surface water in the past.
Pathfinder's landing site on a plain, bordering what looks like a water channel that may have fed a lake, could yield further evidence of past climates.
An optimistic hunt
At this stage, the search for life signs beyond Earth is a search for places where liquid water may exist or may once have existed.
Explaining this last month at a meeting of the American Geophysical Union in Baltimore, David Morrison of the NASA-Ames Research Center at Moffett Field, Calif., said that the best places to look seem to be Mars and Jupiter's moon Europa.
Everett Gibson of the Johnson Space Center in Houston added that "we now have optimism again" after many years of failing to find likely extraterrestrial sites for life.
Dr. Gibson is one of the scientists who proposed last year that microscopic bodies in a meteorite from Mars might be fossil microbes. But there was no consensus on this point at the 28th Lunar and Planetary Science Conference in Houston last March.
Gibson admits that his personal conclusion that these are fossils is up in the air. But the past or present existence of water on Mars or Europa is well established. That is what strengthens his hope, he explains.
The Galileo spacecraft now exploring Jupiter and its moons has found strong indications of water-ice and of the action of liquid water on Europa. The image shown here illustrates the kind of icy terrain that scientists believe was sculpted by water. This has caught many scientists' attention as a possible abode for life.
Volcanoes under the water
Galileo project scientist Torrence Johnson with the Jet Propulsion Laboratory in Pasadena, Calif., explains: "I think we are very certain there was liquid water on Europa ... at the time these features were formed. And it very likely was large amounts of water.
"At that time," he continues, "there were almost certainly volcanoes underneath that water.... That has been the link that leads people ... in the oceanographic and origin-of-life [scientific] communities [to be interested in Europa]."
Volcanic action supplies energy for life, and water provides a medium for it in hot vents deep beneath the sea on Earth. They could do so on Europa.
Noting this, Dr. Johnson adds: "Europa is a very good place to have had that type of environment. Now the question is, does it still have that environment?" That will be a question for an as yet unplanned successor to Galileo to answer.
Johnson said he would need a penetrating probe or craft in orbit around Europa to make the extended observations needed to detect liquid water beneath the icy crust. Meanwhile, scientists have a decade of already-authorized missions with which to search for evidence of life-nurturing environments on Mars.
Mars Pathfinder (US) Lander with mini-rover Sojourner
Launched Dec. 4, 1996.
Landing July 4, 1997.
Main mission: Seven-day rover excursion. Lander data sampling through Aug. 5.
Mars Global Surveyor (US)
Launched Nov. 7, 1996.
Arrive Mars Sept. 12, 1997.
Main mission after spacecraft checkout: Map Mars from March 1998 through January 2000.
Planet B (Japan)
Launch August 1998.
Main mission: Collect geophysical data for one Martian year.
Mars Surveyor '98 (US) Orbiter
Launch December 1998.
Main mission: Collect weather data for one Martian year.
Mars Surveyor '98 (US). Lander
Launch January 1999.
Main mission: Look for volatile materials in soil at southern high latitudes.
Mars 2001 (Russia and others?). Rover or stationary stations
Launch April 2001.
Main mission: Complement US rover missions in 2001 and 2003. Funding uncertain.
Mars Surveyor 2001 (US) Orbiter
Launch March 2001.
Main mission: Map surface chemical and mineral composition.
Details not yet firm.
Mars Surveyor 2001 (US) Lander with rover
Launch April 2001.
Main mission: Collect rock and soil samples. Experiment making rocket fuel from Martian atmospheric gases.
Mars Surveyor 2003 (US)
Nature of craft and mission in planning stage.
Mars Surveyor 2005 (US)
Nature of craft and mission in planning stage. Probably will return sample to Earth.
* Average distance from sun: 138.4 million miles.
* Equatorial diameter: 4,222 miles. (0.53 Earth's diameter)
* Orbial period: 687 Earth days.
* Length of day: 24 hours, 39 minutes.
* Average density: 0.713 Earth's average density.
* Surface gravity: 0.38 Earth's surface gravity.
* Atmospheric pressure: About 0.8 percent Earth sea level pressure.
* Average surface temperature: Wildly variable
when closest to sun: minus 20 to 10 degrees F.
when farthest from sun: minus 100 to minus 80 degrees F.
* Atmospheric composition: 95 percent carbon dioxide; 2 to 3 percent nitrogen; traces of oxygen and argon.
* Water: Seen as ground frost and clouds. Surface erosion indicates past surface water.