太阳风“吹”走了火星大气（英文）

摘要：火星曾经也有和地球差不多的大气层，包围着一个温暖湿润的星球。火星大气为什么快速流失？NASA科学家最新研究揭示答案：是太阳风暴喷射的粒子不断冲击，让火星大气越来越稀薄。

The air on Mars ― what there is of it ― is leaking away, about half a pound a second sputtering into space, scientists announced on Thursday.

The planet’s early atmosphere is thought to have been as thick as or thicker than Earth’s today, and even over the 4.5-billion-year history of the solar system, that slow leak would not explain how it atrophied to its current wisps.

But new readings from NASA’s Mars Atmosphere and Volatile Evolution mission ― Maven, for short ― show that when Mars is hit by a solar storm, the ferocious bombardment of particles from the sun strips away the upper atmosphere much more quickly.

That could help explain the disappearance of the atmosphere. The sun during its youth was more unsettled, with many more solar storm eruptions, and it shone brighter in the ultraviolet wavelengths that also help knock atoms out of Mars’ atmosphere.

“What this tells us is loss through space has been an important process,” said Bruce M. Jakosky, a scientist at the Laboratory for Atmospheric and Space Physics at the University of Colorado and the principal investigator for the Maven mission.

The answer to what happened to the Martian air is key to understanding how Mars might have once been a warm, habitable planet with lakes and maybe an ocean covering the northern hemisphere. When the air disappeared, liquid water largely disappeared, too.

Dr. Jakosky and other scientists reported their findings from Maven in four scientific papers published on Thursday in the journal Science. More than 40 additional papers by the Maven team appear in the journal Geophysical Research Letters.

“We’ve been trying to piece together its upper atmospheric physics from a bunch of incomplete views from other spacecraft,” said Michael W. Liemohn, a professor of atmospheric, oceanic and space sciences at the University of Michigan who is not directly involved with Maven. “These are great stories that they’ve put together from the initial data sets.”

The Maven spacecraft, which entered orbit around Mars in September last year, carries a suite of instruments to analyze the solar wind and its effects on the atmosphere.

The air disappears in mainly two ways. Sometimes an electron is knocked off an atom in the upper atmosphere, and then the charged atom is accelerated away by the electric and magnetic fields of the solar wind. Particles of air can also be knocked into space through collisions with incoming solar wind particles, like billiard balls.

Dr. Jakosky said the two phenomena are roughly equal in importance. The current papers focus on the effect on the charged atoms, usually escaping at a rate of about 100 grams, or almost a quarter of a pound, per second. During a solar storm on March 8, the rate of charged atoms flying into space was 10 to 20 times as high, as much as five pounds a second. That gave the team the first good measurements of what happens when a solar storm hits Mars. “This is hopefully going to help us fill in many pieces of that puzzle,” Dr. Jakosky said.

Jasper Halekas, a professor of physics and astronomy at the University of Iowa and a member of the Maven team, said the energy hitting the Martian atmosphere during the storm was equivalent to a million tons of TNT an hour. “That’s one large nuclear weapon per hour, if you like,” he said.

Such solar storms are not everyday events, but they are also not rare, happening perhaps a few times a year, Dr. Halekas said. He gave an analogy of a geologist studying beach erosion, wondering whether more sand is washed away by the steady, daily effects of waves and tides or by one or two big tsunami.

The solar storm, Dr. Halekas said, “is the equivalent of the tsunami at Mars.”

Maven’s instruments also captured, to the surprise of the scientists, occasional ultraviolet auroras glowing in the Martian atmosphere ― one episode lasting several days last December and then three briefer episodes in February and March.

On Earth, the planet’s magnetic field channels the solar wind toward the poles, and the nighttime light shows are seen mostly at the higher latitudes and rarely near the Equator.

Mars does not possess a global magnetic field, and scientists thought Maven might observe auroras near some ancient magnetic fields that persist in hardened lava flows. “That’s what we expected, naïvely, on Mars,” said Nick Schneider, a planetary scientist at the University of Colorado who is also on the Maven team.

Instead, during a few periods of intense solar wind, Maven spotted diffuse auroras over much of Mars. “What we in hindsight were pretty foolish about was, what Earth’s magnetic field really does is prevent auroras from happening everywhere on Earth,” Dr. Schneider said. “Mars, without a global magnetic field, should have auroras everywhere, certainly when solar conditions are right. We sort of did this dope slap, saying, ‘Well, of course, what’s going to prevent those particles from the sun from slamming into Mars’ atmosphere anywhere and maybe everywhere?’”

Another set of observations measured dust in the Martian upper atmosphere, so high and so evenly distributed that the scientists concluded the grains came from interplanetary space and not the surface or Mars’ moons.