This issue of Science features four studies highlighting results from the Mars Atmosphere and Volatile Evolution (MAVEN) mission, designed to study Mars' upper atmosphere, ionosphere, and magnetosphere. The novel data reveals some surprises, as well as some adjustments to previous theories and estimates. This issue's cover represents a real data visualization of Mars' magnetic field being bombarded by a powerful solar ejection, resulting in a stunning visual of fire-like tendrils emulating from the Red Planet. A special "cover-in-the-making" story will discuss how the artist and researchers created the piece.
Measurements from the upper atmosphere of Mars reveal that the escape rate of ions is enhanced during solar bursts, hinting at how substantial atmospheric loss could have occurred in early Martian history. To make this find, Bruce Jakosky and colleagues studied the effects of the Sun on Mars' atmosphere using data collected by MAVEN during an interplanetary coronal mass ejection (ICME), or burst of gas and magnetism from the Sun, occurring on 8 March, 2015. During this event, instruments on MAVEN that were monitoring Mars' magnetic field detected strong magnetic rotations that fluxed in rope-like tendrils up to 5,000 kilometers (3,107 miles) into space. Meanwhile, instruments that monitor atmospheric ionization noted dramatic spikes as this ICME struck the Red 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 - roughly ten times so - than usual. 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 solar-system history, the authors suggest that ion escape rates at that time may have been largely driven by major solar events.
A second paper by Stephen Bougher et al. highlights results from two occasions when MAVEN "dipped" into the upper atmosphere of Mars to determine the nature of the thermosphere and ionosphere. During these explorations, MAVEN 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 200 kilometers (124 miles) varied substantially as MAVEN completed each orbit, which the authors 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. The results of the study will help scientists better understand the interactions between solar wind and the atmosphere on Mars, and in particular, the physics limiting atmospheric escape through these interactions.
A third study exposes an aurora in the northern hemisphere, which dips lower into the atmosphere than any other confirmed aurora to date, at 60 km (37 mi). MAVEN's Imaging Ultraviolet Spectrograph, which depicts ultraviolet light, detected this aurora during a solar energetic particle outburst. Nick Schneider et al. note that this newly discovered Martian 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. However, where this type of aurora on Earth is driven by magnetism of the poles, the authors suspect that Mars' aurora may be driven by the remnant magnetic field of the crust, creating a more even and diffuse aurora.
A final paper by Laila Andersson and colleagues analyzes the detection of dust at altitudes ranging from 150 to 1,000 kilometers (124 - 621 mi). 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, which rules out Mars' moons as a source, the authors believe that MAVEN is detecting dust of an interplanetary origin.
Source: American Association for the Advancement of Science