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Planetary News: Mars (2005)

Mars Express: SPICAM Team Discovers Aurora

By A.J.S. Rayl
June 28, 2005

An international team of French, Russian, and American scientists has discovered an aurora on Mars for the first time in data from an instrument onboard the European Space Agency's Mars Express -- and it isn't like any other ever seen in the solar system.

The light emissions team members have interpreted as an aurora turned up in observations taken last summer with the SPICAM instrument (SPectroscopy for the Investigations and the Characteristics of the Atmosphere on Mars), a dual ultraviolet/infrared spectrometer dedicated primarily to the study of the atmosphere and ionosphere of Mars.

The same team -- Jean-Loup Bertaux, principal investigator, Francois Leblanc, and E. Quemerais (Service d’Aéronomie du CNRS/IPL, France); Olivier Witasse (ESA); Jean Lilensten (Laboratoire de Planétologie de Grenoble); Alan Stern (Southwest Research Institute, Colorado); Bill Sandel (Lunar and Planetary Laboratory, University of Arizona), Oleg Korablev (Space Research Institute-IKI, Moscow) -- reported discovering 'nightglow' on Mars in January.

Auroras are optical phenomena -- spectacular displays of light -- in the night sky, such as the famous aurora borealis or northern lights that can be seen during the winter up in Alaska and areas of northern Canada. Earth and all the giant planets have auroras because they generate global-scale magnetic fields that spread out well beyond the planet.

On Earth, auroras are usually seen at the highest latitudes, and they occur when the Van Allen radiation belts intensify with energetic particles, which travel down magnetic field lines and collide with the Earth's upper atmosphere. The most powerful auroras tend to occur after coronal mass ejections.

The auroras on the outer planets of Jupiter, Saturn, Uranus and Neptune also emanate at the foot of the planetary magnetic field lines near the poles, as charged particles -- electrons, protons or ions -- cascade down the 'lines.'

Even though Venus has no intrinsic or planetary-scale magnetic field, it still produces auroras on the night side when electrons originating from the solar wind precipitate in the atmosphere. The Venusian auroras typically appear as bright, sometimes very bright but diffuse patches of varying shapes, displayed at times across the entire planet.

Although Mars has no internally-generated, planetary-scale magnetic field right now, planetary scientists believe that it did at one time. A few years ago data obtained by the electron reflectometer, an instrument onboard Mars Global Surveyor (MGS) that maps surface magnetic fields, revealed the presence of crustal magnetic anomalies, which scientists believe are most likely the remnants of that old planetary magnetic field. Since then, there has speculation that the Red Planet would sometimes produce light shows all its own. This discovery confirms that speculation.

Both the nightglow and aurora were made in observations SPICAM recorded last August. "We performed two identical observations, at orbit 716 and orbit 734 -- about 6 days apart," Bertaux told The Planetary Society in an email interview. "We let the line of sight drifting near the horizon, in order to maximize the airglow intensity in the night that we were searching for. At orbit 734, we discovered the NO emission that is discussed in our [previous] paper [on finding the nightglow]. At orbit 716, we saw essentially exactly the same nitrogen oxide (NO) emission that [we did] at orbit 734, plus a different emission that we saw for only a few seconds. This different emission had a spectrum that you could expect in an aurora falling on CO2 [carbon dioxide]. The fact that the emission was exactly at the position of the maximum crustal anomaly as mapped by MGS was of course comforting [to] the [team's] interpretation."

The strongest magnetic anomalies, according to the MGS data, are related to Terra Cimmeria and Sirenum in the heavily cratered uplands of Mars' southern hemisphere. The anomalies or fields are nearly as strong at the surface as the Earth's magnetic field, and are laid out in east-west bands of alternating polarity, extending for more than 620 miles [1,000 kilometers] north to south, sort of like a barcode across the planet's surface.

As it turned out, the Terra Cimmeria region is where Mars Express' SPICAM detected light emissions. In analyzing the map of crustal magnetic anomalies compiled with MGS' data, tem members observed that the region of these nighttime emissions, specifically corresponding to 177 degrees east and 52 degrees south, corresponds with where the strongest magnetic field is localized.

So what did this Martian aurora look like? Although SPICAM is observing UV light, which we cannot detect with unaided eyes, if we actually happened to be standing there on Mars, we might see "some of the aurora's emissions in the violet and blue, as was observed by Mariner 9 on the dayside of the planet, as a result of solar action on the CO2 atmosphere," Bertaux said. "The aurora that we found on Mars is of smaller intensity [than those on Earth], and more localized," he added. "Instead of a long drapery that we see on Earth over several tens of degrees, if you were on the ground of Mars, it might be less extended: 30 kilometers [18.5 miles] seen from the ground would still extend over 15° in the sky if you were right at the foot of the magnetic lines. In fact, we suspect that the aurora that we captured on Mars might have been more extended along the line of sight than across the line of sight, perhaps as long as 300 km [186 miles]. But this is only a suspicion." The more conservative estimate used in the team's report suggests the emission extended about 8 km [5 miles] high.

While the detected emission is apparently pretty typical for daytime, if observed during nighttime it must indicate the excitation of the upper atmosphere by fluxes of charged particles -- probably electrons. Therefore, the correlation indicates that the origin of the light emission is probably the result of a flux of electrons moving along the crust magnetic lines and exciting the upper atmosphere of Mars.

These SPICAM observations offer insight into the Martian crustal magnetic fields play in producing these distinctive cusp-like magnetic structures that concentrate fluxes of electrons into small regions of the atmosphere. Eventually, they induce the formation of highly concentrated auroras whose formation mechanism -- a localized emission controlled by anomalies in the crust's magnetic field -- is unique in the solar system.

The team has reviewed the SPICAM data for other auroral displays, but so far, says Bertaux, "we [have] found no additional auroras since submitting our paper." That said, he noted that observing conditions have not always been optimal. "We have to realize that we share the spacecraft with six other instruments, and that the whole spacecraft has to be turned to what we want to observe with SPICAM, [and that] might not be interesting for other instruments." The report on this latest research appeared in the June 9 issue of the journal Nature.