NASA’s MAVEN Team: Solar Storms Stripped Away Mars’ Atmosphere
Published on 8th November 2015
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.
Billions of years ago, Mars was a very different world. Liquid water flowed in long rivers that emptied into lakes and shallow seas. A thick atmosphere blanketed the planet and kept it warm.
Artist's illustration depicting the terraforming of Mars.
Thanks to NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) orbiter, scientists have learned more about what happened to the Martian climate since Mars was a warm and watery planet about four billion years ago
Today, Mars is a global desert with an atmosphere far too thin to support liquid water, but evidence shows that the planet was wetter in the past. Planetary researchers think that climate change on the planet was caused by the loss of an early, thick atmosphere.
In a paper published today in the journal Science, members of the MAVEN team announced that they have determined the rate at which Mars’ atmosphere currently is losing gas to space via stripping by the solar wind.
The team studied the effects of the Sun on the planet’s atmosphere using data collected by the MAVEN spacecraft during an interplanetary coronal mass ejection (ICME) – burst of gas and magnetism from the Sun – occurring on March 08, 2015.
During this ICME event, instruments on the orbiter that were monitoring Mars’ magnetic field detected strong magnetic rotations that fluxed in rope-like tendrils up to 3,100 miles (5,000 km) into space.
Meanwhile, instruments that monitor atmospheric ionization detected dramatic spikes as the March 8 ICME struck the planet, where planetary ions spewed into space, concentrated along the flux ropes of the affected magnetic field.
“The velocity of these flux ropes is estimated to be much faster – around 10 times so – than usual,” the scientists said.
“Analysis of ion composition found O2+ and CO2+ ions, which is not surprising, but it also revealed that O+ ions were flung higher up in the atmosphere than would be expected.”
Given the likely prevalence of ICME-like conditions early in the history of Solar System, the authors suggest that ion escape rates at that time may have been largely driven by major solar events.
“The removal of atmosphere from Mars by episodic extreme events may have been very important over Mars’ history, just as a single tsunami can remove a portion of the ocean shore that would have taken millennia to erode by the steady lapping of the tides,” said Dr Jasper Halekas of the University of Iowa, principal investigator of MAVEN’s Solar Wind Ion Analyzer.
The measurements indicate that the solar wind strips away gas at a rate of about 100 grams every second.
“Like the theft of a few coins from a cash register every day, the loss becomes significant over time. We’ve seen that the atmospheric erosion increases significantly during solar storms, so we think the loss rate was much higher billions of years ago when the Sun was young and more active,” said Dr Bruce Jakosky, a scientist at the University of Colorado and MAVEN principal investigator.
In a separate study published in the same issue of the journal Science, the team included results from two occasions when MAVEN dipped into Mars’ upper atmosphere to determine the nature of the thermosphere and ionosphere.
During these explorations, the orbiter observed a large vertical temperature gradient. Data indicate a steady mixing of carbon dioxide, argon, and nitrogen dioxide, as well as higher amounts of oxygen than previously estimated.
The density of these elements near 125 miles (200 km) varied substantially as the spacecraft completed each orbit, which the scientists suggest may be caused by gravity wave interactions with wind and small-scale mixing processes occurring below.
Furthermore, variations in the magnetic field and ion layers suggest that, in addition to the magnetic field induced by solar wind, the crust of Mars also contributes to the magnetic field.
A third paper by the team, in the same issue of Science, analyzes the detection of dust at altitudes ranging from 93 – 621 miles (150 – 1,000 km).
No known processes can lift significant concentrations of particles from a planetary surface to such high altitudes. Based on the size of the grains (1 to 5 nanometers) and the even distribution of these particles, the scientists believe that the orbiter is detecting dust of an interplanetary origin.
“A comparison with laboratory measurements indicates that the dust grain size ranges from 1 to 12 micrometers, assuming a typical grain velocity of 40,265 mph (18 km per second),” the researchers said.
“These direct observations of dust entering the Martian atmosphere improve our understanding of the sources, sinks, and transport of interplanetary dust throughout the inner solar system and the associated impacts on Mars’s atmosphere.”
A fourth paper in Science reports the discovery of low-altitude, diffuse auroras spanning much of Mars’ northern hemisphere, coincident with a solar energetic particle outburst.
This aurora falls into the same category as Earth’s Northern Lights, where acceleration of particles in or out of the atmosphere along electromagnetic fields creates a stunning visual, according to the team.
However, where this type of aurora on our planet is driven by magnetism of the poles, the researchers suspect that Martian aurora may be driven by the remnant magnetic field of the crust, creating a more even and diffuse aurora.
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