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.
In March of 2015, the Sun launched a coronal mass ejection – a bubble of energized plasma traveling at about 2 million mph – toward Mars, compressing the planet’s magnetosphere to around two-thirds of its normal size and pushing it inward by almost 600 miles, which exposed more of Mars’ atmosphere to the solar wind’s electromagnetic fields. Image credit: NASA Goddard Space Flight Center.
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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B.M. Jakosky et al. 2015. MAVEN observations of the response of Mars to an interplanetary coronal mass ejection. Science, vol. 350, no. 6261; doi: 10.1126/science.aad0210
S. Bougher et al. 2015. Early MAVEN Deep Dip campaign reveals thermosphere and ionosphere variability. Science, vol. 350, no. 6261; doi: 10.1126/science.aad0459
L. Andersson et al. 2015. Dust observations at orbital altitudes surrounding Mars. Science, vol. 350, no. 6261; doi: 10.1126/science.aad0398
N. M. Schneider et al. 2015. Discovery of diffuse aurora on Mars. Science, vol. 350, no. 6261; doi: 10.1126/science.aad0313
