Throughout the past 40 years, including a 12-year tenure as the director of NASA's planetary science division and the past three years as its chief scientist, he has significantly influenced NASA's scientific research by overseeing missions throughout the solar system and contributing to over 100 scientific papers covering various topics. Initially focusing on Earth's magnetic field and plasma waves, he later expanded his research interests. One of his recent notable contributions is the development of a scale, known as the "confidence of life detection" (CoLD), for validating the discovery of alien life. He has also proposed the idea of terraforming Mars by using a large magnetic shield to protect the planet's atmosphere from the sun's stripping effects and increase surface temperatures. Additionally, he advocates for the exploration of other celestial bodies, such as Europa, Jupiter's icy moon, with a mission set to launch in 2024. Prior to a meeting of the American Geophysical Union in New Orleans in December, Green discussed his diverse work and the quest for life within our solar system. In our interview, he shared edited and condensed segments. He emphasized the importance of a methodical approach to searching for life, introducing the CoLD scale to rank potential detections from one to seven. This scale became necessary after scientists claimed to have observed phosphine in Venus's atmosphere a few years ago. At the scale at which they observed it, they thought life was a significant possibility. On the CoLD scale, where seven signifies "we found life," it ranked at "one." It didn't even reach "two." They later realized there was contamination in their signal and it might not be phosphine, and they are unable to replicate it. Therefore, there is a need for improved communication. Methane is widespread on Mars. While 95% of the methane found on Earth is from life, there is a small percentage that is not. We are currently at CoLD Level 3, but I would immediately support a scientist if they could bring us to CoLD Level 4 in the search for life in the solar system. Instead of making huge leaps, we need to take the right steps towards progress. The focus should be on actually finding life, rather than just making empty claims. NASA has been searching for life on Mars since 1976, but the fact that we haven't found any yet is not surprising. Response A: Our current approach to identifying signs of life is more systematic and informed compared to before. By studying the signatures that life can leave over time, we have learned that Earth, which is 4.5 billion years old, is now teeming with life. However, if we were to go back a billion years, we would discover that Venus was once a blue planet with a large ocean. It is possible that Mars once had abundant life when it was a blue planet billions of years ago. However, Mars lost its magnetic field, causing water to evaporate and the planet to become stagnant about 3.5 billion years ago. Scientists hoped to discover signs of life on the surface of Mars, but the Viking landers were placed in a random location due to limited knowledge about the planet at the time. It was similar to placing something in the Gobi Desert. We intended to place them in Jezero Crater, the river delta where the Perseverance rover is currently located, but we were unaware of its existence at the time. One of the Viking experiments suggested the presence of microbial life in the soil, but only one of the three instruments confirmed this, so we were unable to definitively confirm the discovery of life. Now, with the plan to bring back samples, we will finally have conclusive evidence. It was unexpected that a sample return mission would be necessary. Question: In the past, you have proposed the idea of using a large magnetic shield to terraform Mars by preventing the sun from stripping its atmosphere, which would potentially warm the planet and make it habitable. Is this idea feasible? 
Answer: Yes, it is feasible. By preventing the stripping of the atmosphere, the pressure on Mars would increase, leading to the planet beginning to terraform itself. This is our desired outcome: for the planet to play a role in any way possible. As pressure increases, so does temperature. The initial stage of terraforming occurs at 60 millibars, which is a tenfold increase from our current conditions. This threshold, known as the Armstrong limit, is where one could walk on the surface without their blood boiling. Without the need for a spacesuit, there would be greater freedom of movement and flexibility. The increased temperature and pressure allow for the initiation of plant growth in soil. There are various methods for creating a magnetic shield. I am currently in the process of finalizing a paper that I have been working on for two years, although I anticipate that it will not be well-received. The planetary community is generally opposed to the concept of terraforming. I believe we have the ability to alter the environment on Venus by using a physical shield that reflects light, leading to a decrease in temperature. NASA approved the Europa Clipper mission in 2015 to search for signs of life on Jupiter's moon Europa, following the detection of plumes in 2013. Would you have preferred for this mission to happen sooner? A: I would have loved to see it earlier, but it wasn't possible. There are certain series of missions known as strategic missions that require extensive planning and approval. In order for these missions to happen, everything has to fall into place - a proposal must be made, a solid case must be built, it must be presented to NASA administration, and then pitched to Congress. I proposed a Europa mission every year. Every year, the administration showed no interest in visiting Europa. The discovery of plumes on Europa was what led to the Europa mission taking place. In 2013, at an American Geophysical Union meeting, several scientists were planning to present their findings on spotting a plume on Europa with the Hubble telescope. I was amazed by this news and immediately wanted to organize a press conference. I phoned NASA headquarters again, and they successfully completed the task. I brought that information back to headquarters and incorporated it into the narrative of Europa. This was a significant turning point. They expressed interest in potentially pursuing this idea. Regarding Congress's decision not to include a lander in the mission, did you personally want one? A: Although I would love to have a lander for the mission, it is not currently feasible. However, all the work we are doing on Clipper is leading us toward eventually having a lander. I made sure that we have a high-resolution imager so that when we fly over certain areas, we will have the necessary information to safely choose a landing spot. Europa has some dangerous terrains, so without this high-resolution imaging, landing would be impossible. It's important to take small steps rather than one big step, as failure often comes from taking on too much at once. Viking serves as a prime example of when we took a significant leap without adequate knowledge. We lacked understanding of the soils, toxins, and water on the planet before sending the Vikings to the surface. There were 10 crucial pieces of information that we should have been aware of beforehand. As for scientific papers in retirement, the question remains whether they will continue to be a focus. A: Yes, definitely. I am currently working on a paper about Mars, another one about Europa, and also writing a book on astrobiology. I have a strong passion for science. Text = Copyright 2022 by The New York Times Company.