Mars: Green glow detected on the Red Planet
Astronauts see this as night light when they look directly into the atmosphere
Scientists have identified green light in the Martian atmosphere.
A similar glow is sometimes seen by astronauts on the space station when they look at the limbs of the earth.
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The glow comes from oxygen atoms when they are excited by sunlight.
The phenomenon has long been predicted to occur on other planets, but the Trace Gas Orbiter (TGO) - a common European-Russian satellite on Mars - is the first to conduct the observation beyond Earth.
"It's a nice result," said Dr. Manish Patel from the Open University in the UK.
"You would never plan a mission to look for such things. Today we have to be very clear about what science we will do before we get to Mars. But when we got there we thought: 'Now let's get one Take a look at it. And it worked. "
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To be clear, this differs from classic Aurora such as the northern and southern lights.
These emissions are the result of collisions between atmospheric molecules and charged particles that race away from the sun. On Earth, this type of interaction is strongly influenced by our planet's strong magnetic field, which pulls these particles towards the poles.
Graphic: Mars glow
Aurora are not quite as focused on Mars because this world does not have a global magnetic field, but such emissions do exist and have been observed.
The green glow of the astronauts on the edge of the earth - and now the TGO on Mars - has its own origin. It is sunlight that does the job. Oxygen atoms are raised to a higher energy level and when they fall back into their rest state they produce the telltale green emission.
The earth has plenty of oxygen in its atmosphere. But on Mars it is mostly only available as a decomposition product of carbon dioxide. Sunlight releases one of the oxygen atoms in CO2 and it is the transition of this atom that glows green on the red planet.
The TGO records the excited oxygen not with an imager, but with its Nomad spectrometer package. This instrument sees oxygen at very specific heights.
In an article published in the journal Nature Astronomy, these altitudes are 80 km and 120 km above the surface. The exact heights depend on the CO2 pressure.
"And if you look at the heights that this emission is at, you can actually see the thickness of the atmosphere and its differences," said Dr. Patel.
"So if you continued to observe this phenomenon, you could see the atmospheric altitude change, such as when it warms up in dust storms. This is a problem we face when we try to to land on Mars because we are never sure how thick the atmosphere will be if we plow through it to get to the surface. "
In theory, you could therefore use green light observations to inform the models that control the entry, descent, and landing of Mars probes.
The glow on Mars was captured by the TGO's Nomad instrument, which is headed by the Royal Belgian Institute for Space (IASB-BIRA). Dr. Patel is the primary researcher of Nomad's ultraviolet and visible spectrometer.
Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos
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