Mars was wet in its early history, but today it appears bone dry other than ice below the surface and at the poles. Astronomers have long wondered where all the water went.
The water might have been blown into space long ago by strong gusts of solar winds, new satellite observations suggest.
The findings, presented this week at the European Planetary Sciences Congress in Potsdam, Germany, also show that solar flares erupting on the far side of the sun can still affect the "space weather" of Earth.
Although dusty and dry today, evidence collected in recent years-including images of vast gullies and canyons plus chemical signatures-suggests Mars was once a water-rich world.
Other scientists have speculated that the missing water escaped from the Martian atmosphere, blown into space by the solar wind.
Unlike Earth, Mars does not have a protective magnetosphere, so the solar wind and high-energy particles directly interact with its atmosphere. Past studies have observed large quantities of charged oxygen atoms-a primary component of water-in Mars' atmosphere streaming into space. However, the observed outflows were not high enough to account for all of the planet's missing water.
Four eyes at once
In December 2006, the effects of a series of eruptions of high-energy particles, called solar flares, on the far-side of the sun were simultaneously recorded by four spacecraft scattered throughout the solar system: NASA's Mars Express, Venus Express and Earth-orbiting GEOS satellite, and the European Space Agency's SOHO solar orbiter, positioned in space near the sun.
Sensors on the four satellites were capable of detecting the charged particles, called plasma, that stream constantly from the sun.
Data collected at the red planet during the 2006 eruptions by Mars Express showed higher than average amounts of oxygen ions leaking away into space. If this occurred regularly over billions of years, it could account for Mars' missing water, speculated study team member Yoshifumi Futaana of the Swedish Institute of Space Physics.
"Our result is the first observation showing the increase of the oxygen ion flux from the Martian atmosphere during a severe condition," Futaana told space.com.
The long arm of the sun
The observations also suggest that solar flares on the far side of the sun can still impact the space weather of planets not in their direct line of sight-something that scientists had only speculated about before.
"We observed high-energy particles at Mars, Venus and the Earth simultaneously," Futaana said.
While solar flares and solar energetic particles have been reported many times by satellites around Earth and other planets, last year's observations marked the first time the sun's plasma activity had been recorded from so many vantage points at once.
Earth's magnetic field acts as a shield against solar radiation, but during times of unusually strong solar flares-as occurs during the peak of the sun's 11-year solar cycle-our planet's defense can be severely weakened. Particularly vulnerable are satellites in orbit around Earth.
"Such high-energy charged particles can cause spacecraft malfunctions by penetrating into satellite electronics boards," Futaana said.