It was the luck of the Irish that some of the best displays of the northern lights of the past decade fell on March 17, St. Patrick’s Day. The great shimmering curtains of green electrical light that hung high over the Emerald Isle were a result of Earth being hit by not one but two “coronal mass ejections” at the same time. These great clouds of hot plasma erupted from the atmosphere of the sun on March 14 and tore across interplanetary space toward Earth. They covered the 93 million miles in three days. When they arrived, they dashed upon Earth’s magnetic field, which projects a protective bubble all around us called the magnetosphere. As these coronal mass ejections enveloped the planet, the outer layers of this bubble broke, and particles from the CMEs spiraled in and down along magnetic field lines. These particles, which started their journey in the sun’s atmosphere, ended their voyage by slamming into our atmosphere, causing the air to glow in a beautiful display of the northern (and southern) lights. It took a day or so for the CMEs to pass by, but afterward our magnetosphere regenerated, and everything returned to normal.
But if Earth didn’t have a magnetic field, what would have happened?
Well, as the CME was striking us, it just so happened that I was looking into this very question. I had been looking through data from the European Space Agency’s Venus Express spacecraft and had spotted something very odd. Venus Express arrived at our sister planet back in 2006 and recently burned up in the thick sulfurous clouds. But there is still a mountain of data to look through. Venus is an ideal place to visit if you want to see what happens when a planet that was once possibly habitable and Earthlike gets really, really uninhabitable. Its surface temperature is hot enough to melt lead, and its atmosphere is so thick that you wouldn’t need a parachute to slow your descent—and its atmospheric pressure crushes spacecraft within hours of landing.
One of the big differences between Earth and Venus is that Venus doesn’t have a magnetic field of its own. With no magnetosphere to protect it, the atmosphere of Venus is continuously being blown off by the solar wind in a long, invisible, cometlike tail. Venus is thus a perfect place to go if you want to try to figure out what a planet needs in order for life to flourish (or not, as the case may be). Believe it or not, the jury is still very much out on whether a magnetic field is one of these “must have” things.
CMEs come in two main flavors: fast and slow. Many, including the ones that pummeled us on St. Patrick’s Day, are fast. They come screaming through interplanetary space so quickly that they drive big tsunamilike shock waves in front of them. About a third are slow, lazy CMEs. They gently drift along with the solar wind, a constant stream of particles that flows out from the sun in every direction. Unlike their fast brethren, slow CMEs don’t drive big shock waves in front of them. They are more like a fog that gradually creeps up on you, getting thicker and thicker, and hotter and hotter.
You might get only a dozen or so CMEs hitting your planet in any given year. We are only just beginning to find out what happens when CMEs hit planets like Mars or Venus, which don’t have magnetic fields to protect them. To date, the majority of CMEs seen at these planets have been fast CMEs driving big shock waves, pummeling the poor unprotected atmospheres of Venus and Mars, and tearing chunks of the planets off into space. Although slow CMEs had been seen in the vicinity of Venus before, never had we managed to catch one in the act of actually hitting the planet, and as a result, we haven’t really known what happens when a slow CME comes rolling inexorably in like a rising tide.
So back on St. Patrick’s Day, while the aurorae danced across the nighttime skies, I sat looking at data from Venus. Way back on Christmas Eve eve (Dec. 23) 2006, Venus had clearly been having a bad day. The stream of atmosphere escaping from the planet had become a river, and then a torrent. In all my time as a member of the scientific crew of Venus Express, I have never seen such an outflowing of the atmosphere. It was like someone stuck an invisible knife into the planet, and its skies were pouring blood out into space.
What the heck was going on?
Not knowing quite what to make of things, I went to one of my colleagues at NASA’s Goddard Space Flight Center. Lan Jian hunts CMEs and other giant, invisible phenomena that lurk in the solar wind. Together we looked not just at what was going on at Venus but at what had been going on in interplanetary space nearby for the few days before and after. She carefully went through the evidence and after some very careful analysis was finally able to identify the culprit: Venus had been nailed by a slow CME.
To get final proof, Lan looked through images from the SOHO spacecraft, one of a fleet of solar observatories that keep a constant watch on the sun. After a little bit of searching, she found the faintest impression of a vast ghostly cloud drifting (relatively) slowly away from the sun. Looking at it, it was hard to believe that something so faint could cause so much havoc.
So as it turns out, if your home planet doesn’t have a magnetic field, it looks like all CMEs, great and small, fast and slow, will cause a substantial increase in how much of your atmosphere gets torn off into space. This might make you think that having a magnetic field is a good thing if you want to hold on to an atmosphere. But here’s the rub: We still don’t really fully understand how any planet, including Earth, loses its atmosphere. We certainly have a lot of ideas, but it is very hard to figure out with only one planet on which to test them. We as a species are really only just beginning to take our first exploratory steps into the solar system. But we’re working on it, because the more we understand about these other worlds, the more we can learn about what made Earth habitable, and thus in time, why we are all here.