The moving Martian bow shock

As the energetic particles of the solar wind speed across interplanetary space, their motion is modified by objects in their path.

Scientists have long been aware that a feature known as a bow shock forms upstream of a planet -- rather like the bow of a ship, where the water is slowed and then diverted around the obstacle.

A team of European scientists has investigated how and why the bow shock's location varies during the Martian year.

In a paper published in the Journal of Geophysical Research: Space Physics, the team has analysed more than five Martian years of measurements from the Mars Express Analyser of Space Plasma and EneRgetic Atoms (ASPERA-3) Electron Spectrometer (ELS) to identify 11,861 bow shock crossings.

This is the first analysis of the bow shock to be based on data obtained over such a prolonged period and during all Martian seasons.

The scientists discovered that, on average, the bow shock is closer to Mars near aphelion (the planet's furthest point from the Sun), and further away from Mars near perihelion (the planet's closest point to the Sun).

The bow shock's average distance from Mars, when measured from above the terminator (the day-night boundary) reaches a minimum of 8,102 km around aphelion, while its maximum distance of 8,984 km occurs around perihelion. This is an overall variation of approximately 11% during each Martian orbit.

However, the variations in bow shock distance also correlate with annual changes in the amount of dust in the Martian atmosphere. The Martian dust storm season occurs around perihelion, when the planet is warmer and receives more solar radiation.

Lead author Dr Benjamin Hall, who joined Lancaster University from Leicester, said: "Dust storms have been previously shown to interact with the upper atmosphere and ionosphere of Mars, so there may be an indirect coupling between the dust storms and bow shock location."

"Future investigations of links between atmospheric dust loading and the Martian upper atmosphere are needed, involving joint investigations by ESA's Mars Express and Trace Gas Orbiter, and NASA's MAVEN mission. Early data from MAVEN seem to confirm the trends that we discovered."

"Similar investigations were made by the ASPERA instrument that was flown on board the Venus Express orbiter, enabling us to compare physical processes and conditions at two very different planets that both have weak magnetic fields," said Dmitri Titov, ESA's Mars Express Project Scientist.

"This demonstrates the value of using the same instrumentation to explore different worlds."