22 December 2015
Mars may have lost its atmosphere from solar winds that sent charged particles into the atmosphere, which “sputtered” the oxygen as it vanished into space
by Bethany Augliere
In March of this year, the sun hurled a giant magnetic solar storm into Mars. The solar wind, full of charged particles, slammed into the red planet’s atmosphere, bouncing or “sputtering” the oxygen into deep space.
Researchers now think the same process could have evaporated Mar’s water several billion years ago, according to a new study presented at the 2015 American Geophysical Union Fall Meeting and recently published in AGU’s publication Geophysical Research Letters.
A team of scientist from NASA’s Mars Atmosphere and Volatile Evolution mission, or MAVEN, used the March 8 solar storm to model how the sun strips away Mars’s atmosphere. The discovery provides a window in the planet’s climate history, including how Mars may have lost its water, said Shannon Curry, a planetary scientist at the University of California-Berkeley and lead author of the study. “We want to try and understand Mars’s early atmospheric history,” she said.
Today, Mars is dry and dusty. Its thin atmosphere drives down atmospheric pressure, so water cannot exist in liquid form. But this wasn’t always the case. Two to three billion years ago, Mars’s atmosphere was thick and giant oceans spanned its surface, similar to Earth today. Gradually, the atmosphere thinned and the water vanished—and scientists want to know why.
MAVEN’s spacecraft launched Nov. 18, 2013 to find out. Researchers used the orbiting spacecraft to measure variables such as solar wind velocity and density, temperature, and magnetic field rotation. In particular, this allowed them to understand how solar storms affect the atmosphere and to model the escape of oxygen, Curry said.
“The spacecraft can’t be everywhere at once, which is why we make the models,” she said. “We simulate the whole Mars space environment.”
An atmosphere and magnetic field protects planets from solar storms. But Mars has a thin atmosphere and lacks a uniform and expansive magnetic field. According to Curry, this means that the charged particles of solar wind can directly interact with the atmosphere, causing it to magnetize: atoms lose an electron and become ions.
Without any protection, Mars can lose its atmospheric oxygen in two ways, she continued. Solar winds can sweep away the oxygen ions that latch onto and travel down rotating magnetic bubbles as they leave the planet. But not all of these ions escape, and some swing back into the atmosphere. As the ions charge in, they slam into the atmosphere and bounce into neutral oxygen causing it to “sputter” out into space.
“It’s like throwing a ball into a ball pit,” Curry said. During a solar storm, the team’s model predicts that Mars loses about 3 kilograms per second of sputtered neutral oxygen. In most cases, atmosphere is depleted through ions getting swept away. But for Mars, it’s probably the sputter, Curry said. That’s because two billion years ago, the sun was younger and much more active, firing off more storms than it does today, according to Curry.
“We think these solar storms may have been driving a lot of atmospheric escape billions of years ago,” she said.
‑ Bethany Augliere is a graduate student in the Science Communication program at UC Santa Cruz. You can follow her on twitter at @BethanyAugliere.