These include incoming Solar Energetic Particles (SEPs), escape on a molecule-by-molecule basis (Jeans Escape), the effect of Coronal Mass Ejections (CMEs) and extreme solar ultraviolent radiation (EUVs).
Credit: The Lunar and Planetary Institute and LASP
Ninety kilometers over our heads, the sky is glowing. During the day, the Sun turns the top of our sky into a sea of electrons.
They flow over one another without friction, creating plasma. Radio waves that hit these electrons bounce back to Earth, allowing transmissions to literally turns corners and circle the globe.
The free electron layer conducts current and responds to magnetic fields. As a result, during solar storms this part of the atmosphere lights up, creating undulating auroras.
While liberated, the electrons devise visual spectacles and technical challenges, but the atoms they leave behind must content themselves with being ions.
For this reason, this part of our atmosphere is known as the ionosphere. It's the largest part of our atmosphere, and does a commensurately big job.
It absorbs x-rays that would otherwise destroy life on Earth. If it weren't for our atmosphere, Earth might look a good deal more like Mars.
Why Mars doesn't look more like Earth is the subject of ongoing study. The loss of most of the atmosphere is believed to have been a major factor in Mars turning away from the path of water, warmth and habitability.
Uncovering where that atmosphere went, when and why is the mission of the Mars Atmosphere and Volatile Evolution (MAVEN) satellite.
Scheduled to arrive in September, MAVEN carries with it two instruments designed to probe the remaining ionosphere for clues about the past four billion years, and what will happen going forward.
Our first direct glimpse at ions in the upper atmosphere will be courtesy of the Neutral Gas and Ion Mass Spectrometer (NGIMS). Mass spectrometers like NGIMS are ubiquitous in the world of physical science.
They function like the ionosphere itself: by bombarding specimens with electrons and creating ions. This process allows mass spectrometers to divine the contents of a liquid, solid or gas.
Small and durable, as well as extremely useful, mass spectrometers have been placed on dozens of satellites and rovers, including NASA's Mars rover Curiosity.
To find Mars' missing atmosphere, NGIMS will search for certain elements and molecules in the Martian ionosphere: helium, argon, nitrogen, oxygen, carbon monoxide, and carbon dioxide.
It will note how often each occurs in its neutral and ionized states over 170 miles of sky. It will also count the abundance of heavy and light versions of atoms, also known as isotopes.
Counting isotopes may hold the key to atmospheric loss. The Earth, the Sun and Jupiter have balanced amounts of heavy and light argon isotopes.
These bodies have also retained their atmospheres over time. Mars has too much heavy argon and almost no atmosphere. The heavy argon left behind likely represents the original volume of the atmosphere; light argon reflects the lost air.
"Our direct measurement of the vertical distribution of these isotope pairs will let us understand the physics of escape better and ultimately understand how much of the atmosphere has been lost in the past several billions of years."
As NGIMS tries to catch light argon in the act of leaving the planet, it will also watch space weather and dust storms change the composition of the atmosphere: mixing up molecules near the bottom of the ionosphere and sending others on one-way trips into deep space.
While NGIMS picks out particles one by one, the UltraViolet Spectrograph (IUVS) will be making sweeping, planetary-wide maps.
"IUVS and NGIMS are backups for each other," said IUVS Principle investigator Nick Schneider, "They both measure the composition & structure of the atmosphere.
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| Nick Schneider |
As the most powerful ultraviolent spectrograph to ever be sent to another planet, IUVS is exquisitely sensitive to composition and temperature variations of entire upper atmosphere.
The temperature and composition of Mars' atmosphere varies dramatically, not only by altitude, but also by orbit.
At perihelion, when Mars is closest to the Sun, it is 40 million miles closer than at aphelion, when it is farthest away.
The difference in distance means that much more of the Sun's energy will be reaching Mars during certain times of year. As a result, we expect ultraviolet readings at perihelion and aphelion to vary widely.
"But we anticipate seeing changes from other causes too: solar storms like flares and Coronal Mass Ejections (CME's), and dust storms on Mars too," said Schneider.
"These each have the potential to control atmospheric escape on Mars, so we'll be watching them all."
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