Since joining NASA in 1980, Jim Green has seen it all. He has helped the space agency understand Earth’s magnetic field, explore the outer solar system and search for life on Mars. As the new year arrived on Saturday, he bade farewell to the agency.
Over the past four decades, which includes 12 years as the director of NASA’s planetary science division and the last three years as its chief scientist, he has shaped much of NASA’s scientific inquiry, overseeing missions across the solar system and contributing to more than 100 scientific papers across a range of topics. While specializing in Earth’s magnetic field and plasma waves early in his career, he went on to diversify his research portfolio.
One of Dr. Green’s most recent significant proposals has been a scale for verifying the detection of alien life, called the “confidence of life detection,” or CoLD, scale. He has published work suggesting we could terraform Mars, or making it habitable for humans, using a giant magnetic shield to stop the sun from stripping the red planet’s atmosphere, raising the temperature on the surface. He has also long been a proponent of the exploration of other worlds, including a mission to Europa, the icy moon of Jupiter, that is scheduled to launch in 2024.
Ahead of a December meeting of the American Geophysical Union in New Orleans, Dr. Green spoke about some of this wide-ranging work and the search for life in the solar system. Below are edited and condensed excerpts from our interview.
You’ve urged a methodical approach to looking for life with your CoLD scale, ranking possible detections from one to seven. Why do we need such a scale?
A couple of years ago, scientists came out and said they’d seen phosphine in the atmosphere of Venus. At the level they saw it, which was enormous, that led them to believe life was one of the major possibilities. On the CoLD scale, where seven is “we found life,” it is “one.” It didn’t even make it to “two.” They recognized later there was contamination in their signal and it may not even be phosphine and we can’t reproduce it. So we have to do a better job in communicating.
We see methane all over the place on Mars. Ninety-five percent of the methane we find here on Earth comes from life, but there’s a few percent that doesn’t. We’re only at a CoLD Level 3, but if a scientist came to me and said, “Here’s an instrument that will make it a CoLD Level 4,” I’d fund that mission in a minute. They’re not jumping to seven, they’re making that next big step, the right step, to make progress to actually finding life in the solar system. That’s what we’ve got to do, stop screwing around with just crying wolf.
The search for life on Mars has been a focus for NASA for so long, starting in 1976 with the Viking 1 and 2 landers and later with missions from the 1990s onward. Are you surprised we haven’t found life in that time?
Yes and no. What we’re doing now is much more methodical, much more intelligent in the way we recognize what signatures life can produce over time. Our solar system is 4.5 billion years old, and at this time, Earth is covered in life. But if we go back a billion years, we would find that Venus was a blue planet. It had a significant ocean. It might actually have had life, and a lot of it. If we go back another billion years, Mars was a blue planet. We know now Mars lost its magnetic field, the water started evaporating and Mars basically went stagnant about 3.5 billion years ago.
We would like to have found life on the surface. We put the Viking landers in a horrible place because we didn’t know where to put them — we were just trying to put them down on the surface of Mars. It was like putting something down in the Gobi Desert. We should have put them down in Jezero Crater, in this river delta we’re at right now with the Perseverance rover, but we didn’t even know it existed at the time!
One of the Viking experiments indicated there was microbial life in the soils, but only one of the three instruments did, so we couldn’t say we found life. Now we’ll really, definitively know because we’re going to bring back samples. We didn’t know it would need a sample return mission.
You’ve previously suggested it might be possible to terraform Mars by placing a giant magnetic shield between the planet and the sun, which would stop the sun from stripping its atmosphere, allowing the planet to trap more heat and warm its climate to make it habitable. Is that really doable?
Yeah, it’s doable. Stop the stripping, and the pressure is going to increase. Mars is going to start terraforming itself. That’s what we want: the planet to participate in this any way it can. When the pressure goes up, the temperature goes up.
The first level of terraforming is at 60 millibars, a factor of 10 from where we are now. That’s called the Armstrong limit, where your blood doesn’t boil if you walked out on the surface. If you didn’t need a spacesuit, you could have much more flexibility and mobility. The higher temperature and pressure enable you to begin the process of growing plants in the soils.
There are several scenarios on how to do the magnetic shield. I’m trying to get a paper out I’ve been working on for about two years. It’s not going to be well received. The planetary community does not like the idea of terraforming anything. But you know. I think we can change Venus, too, with a physical shield that reflects light. We create a shield, and the whole temperature starts going down.
In 2015, NASA approved the Europa Clipper mission to search for signs of life on Jupiter’s moon Europa, set for launch in 2024, following the detection of plumes erupting from its subsurface ocean in 2013. Did you want to see that mission happen sooner?
Oh, yeah, I would love to have seen it earlier, but it wasn’t going to happen. There are certain series of missions that are so big they’re called strategic missions. For them to actually happen, the stars have to align. You have to propose it, have a solid case work, go to the NASA administration and then pitch it to Congress. Every year, I proposed a Europa mission. Every year. The administration was not interested in going to Europa.
The plumes on Europa are what made the Europa mission happen. I was at an American Geophysical Union meeting in 2013. Several of the scientists were going to give a talk on finding a plume with Hubble on Europa, and I go, “Oh, my God.” I said this is fantastic, I want to do a press conference. I call back to NASA headquarters, and they pulled it off. I took that information back with me to headquarters and added that into the story of Europa. That really turned the corner. They said, “Wow, maybe we should do this.”
Congress decided against putting a lander on the mission. Did you want one?
I would love a lander, but it’s not in the cards. It makes the mission too complicated, but everything we do on Clipper feeds forward to a lander. I insisted that we had a high-resolution imager to the point whereas we fly over certain areas, we’re going to get the information we need to go, “Let’s land right there, and safely.” Europa has got some really hazardous terrains, so if we don’t get the high-resolution imaging, we’ll never be able to land.
You want to take a step, but not a huge step. You fail when you do that. Viking is that example, where we took too big a step. We didn’t know where to go, we didn’t know enough about the soils or the toxins in the soils. We hadn’t really gotten a good idea where water was on the planet in the past. There were 10 things we should have known before we put the two Vikings on the surface.
Are you still going to work on scientific papers in your retirement?
Oh, absolutely. I’ve got the Mars paper to do. I have a Europa paper I’m writing right now. I have an astrobiology book I’m doing. I have an insatiable appetite for science.
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