Mars: Death of a Planet

March 11, 2015… near the peak of its 11-year
magnetic cycle, the Sun emitted a powerful x-class flare. Four days later, on the 15th, it erupted in
a coronal mass ejection, or CME. Two days later, the blast raced past Earth,
producing some of the most colorful auroras in recent memory. Astronauts aboard the international
space station recorded them as they flew over the Indian Ocean. This so-called St. Patrick’s Day storm continued
on to the fourth rock from the Sun… Mars. The newly arrived spacecraft, the Mars Atmosphere
and Volatile Evolution mission, or MAVEN… was there to record its arrival. On our planet, solar plasma is deflected and
channeled by a global magnetic field down toward the poles. There, it interacts with
nitrogen and oxygen molecules in the upper atmosphere to produce the light shows of the
aurora borealis and aurora australis. Mars lacks a global magnetic field, so when
the St. Patrick’s Day storm washed over the planet, it produced a diffuse aurora that
engulfed the planet. It turned out to be an important data point
in Maven’s quest to answer a single question… How did Mars, once lined with the blue tint
of flowing water, go so dry? Maven’s findings take us back to a time
over three and a half billion years ago. On Earth, ponds, hot springs, and undersea
vents were crawling with microbes. Life was taking hold. Earth back then was steadily evolving into
the connected system we know today…. Molten rock welling up from deep below the
surface… surface rock and water circulating into the interior. Volcanoes replenishing the atmosphere… Oceans and atmospheric systems circling the
globe. Add to these, a long-term carbon cycle for
removing the greenhouse gas, carbon dioxide, from the atmosphere… And at the center of it all, a vibrant biosphere. For reasons that are not fully understood,
the connection between Mar’s interior and surface never evolved. Volcanic activity,
concentrated in large structures such as Olympus Mons, came to a halt. The planet became static, with surface water
and rock stuck in place. Scientists estimate that Mars had enough water
to cover its surface in a layer 140 meters deep. That implies that the planet had a much thicker
and denser atmosphere than it does today. The atmosphere began to slowly but surely
bleed away, in a process that is visible to this day. The spacecraft Maven has measured the average
erosion rate at 100 grams per second. That works out to over a metric ton of atmosphere
lost per Earth year. During the St. Patrick’s storm, that rate
jumped by a factor of 10 or 20. This event allowed scientists to model the
erosion of Mars’ atmosphere. When solar particles strike the upper atmosphere,
most are deflected along a boundary called a bow shock. During a solar storm, this boundary
pushes deeper into the atmosphere. When this happens, solar particles accelerate
protons and electrons in the atmosphere, sending them flying out into space. The loss is most
pronounced in a tail flowing away from the planet’s dark side, and out near the pole. This view compares the simulation with data
from the Maven spacecraft. Billions of years ago, when the sun was much
younger, powerful flares and CMEs were much more common. With a declining, or absence of a global magnetic
field, Mars was at the mercy of the sun. Over time, its atmosphere thinned… its oceans
dried up… and any chances of developing a life-supporting climate disappeared. Today, from a planet protected by its own
robust global magnetic field… …we reach out across the void to understand
the Mars that once was. Did life have enough time to begin… and if so, how far did it
get? It may take a geologist, digging and scraping
directly into Martian rocks, to answer that question. Ultimately, the parallel stories of Earth
and Mars can tell us much about what it takes for a planet to forge life… and how common
that really is across a galaxy like ours.

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