Traces of Mars’ paleomagnetic field?

On Earth’s neighbouring planets, which have no magnetic field that can capture charged solar wind particles, aurorae manifest in a particular way. On Venus, a UV spectrometer on the Pioneer Venus probe, which orbited the planet from July, 1980, to August, 1992, identified a glow from “diffuse” aurorae, i.e. covering large areas. On Mars, “diffuse” aurorae were also observed in the Northern Hemisphere, but the ones in the Southern Hemisphere were “localised” aurorae. So how are these created? L. Soret and J.-C. Gérard investigated these light draperies or arcs, whose morphology is well defined. The phenomenon appears to be related to the solar flares that reach Mars’ atmosphere. “The sky is indeed brighter, but these aurorae are not the result of the same process as on Earth, as there is no global planetary magnetic field,” explains Lauriane Soret.

The study of the SPICAM measurements identified two dozen spots where localised aurorae appear. These are difficult to detect, and they seemingly show up at random, since they only appear temporarily, and on areas of only a few dozen kilometres. The origin of these aurorae is not clear. Professor Gérard explains that they appear in areas where ancient rocks retain traces of a past global magnetic field, trapped for nearly 4 billion years.

The study of Mars’ aurorae is difficult, as they are volatile and do not appear regularly. Many questions remain unanswered. How many aurorae are created? Do they all appear simultaneously? What energy do they contain? In order to better identify the chain of events that results in charged particles, guided by the local magnetic field, colliding with the atmosphere, researchers would need better observations on Mars’ nightside, acquired with a more sensitive instrument. In the meantime, Lauriane Soret has started working on a simulated model in order to better understand the specificity of isolated aurorae. Using this model, she determined that the altitude at which charged particles interact with Mars’ atmosphere and produce aurorae is between 125 and 135 km, with electrons being accelerated down magnetic “funnels”. In comparison, Earth’s aurorae generally manifest at altitudes ranging from 90 to 300 km.

Waiting for NOMAD

The processes resulting in “localised” aurorae appearing in specific areas on the Red Planet therefore remains to be determined. As European probe Mars Express SPICAM instrument is no longer sending data, hopes rest on NOMAD (Nadir and Occultation for MArs Discovery), an instrument carried by ExoMars TGO (Trace Gas Orbiter) satellite, whose 2016 mission is run jointly by ESA and Russia’s space agency Roscosmos. NOMADʼs advantage is that it is a triple instrument, with two IR spectrometers and a sensor for visible and UV light.

During this mission, the LPAP team will take advantage of the opportunity to observe areas of Mars during night time. These observations are not officially in the ESAʼs mission schedule. “The simultaneous detection of atmospheric phenomena in the UV, visible and IR spectra will be a first in Martian exploration,” notes LPAP’s Lauriane Soret, who counts on the ESAʼs time and resources to increase the scale and scope of her observations.