Throughout the last forty years, including serving as the director of NASA's planetary science division for twelve years and as its chief scientist for the past three years, he has greatly influenced NASA's scientific research, overseeing missions throughout the solar system and contributing to over 100 scientific papers on various topics. While focusing on Earth's magnetic field and plasma waves early in his career, he expanded his research interests. One of his recent significant contributions is the development of a scale called the "confidence of life detection," or CoLD, scale, for confirming the detection of alien life. Additionally, he has proposed the idea of terraforming Mars using a giant magnetic shield to protect the planet's atmosphere and raise surface temperatures. He has also been a strong advocate for exploring other worlds, including a mission to Europa, Jupiter's icy moon, set to launch in 2024. Prior to a December conference of the American Geophysical Union in New Orleans, Green discussed his extensive research and the exploration for extraterrestrial life. The interview excerpts below have been edited and condensed. Green emphasized the importance of a systematic approach in the search for life, introducing the CoLD scale which assigns rankings from one to seven to potential detections. This scale was developed in response to the detection of phosphine in Venus' atmosphere by scientists a few years ago. They observed the possibility of life at an enormous level, but upon further examination, they realized their initial excitement was premature due to contamination in their signal. It may not even be phosphine, and they are unable to reproduce it. Communication about their findings must be improved. Methane is present all over Mars, and while 95% of methane on Earth is produced by life, there is a small percentage that is not. We are currently at a CoLD Level 3, but if a scientist presented an instrument that could elevate it to a CoLD Level 4, I would immediately fund that mission. The goal is to take the right steps towards discovering life in the solar system, instead of just making small advances. NASA has been focused on searching for life on Mars since 1976, but despite missions starting from the 1990s, we have not yet found any signs of life. Response A: Our current methods of identifying signs of life are more systematic and sophisticated compared to before. Earth has been covered in life for a long time, but a billion years ago, Venus had a large ocean and looked very different. It is possible that Mars once had abundant life, as it was a blue planet a billion years ago. However, Mars lost its magnetic field, causing the water to evaporate and the planet to become stagnant around 3.5 billion years ago. It is disappointing that we have not yet found any signs of life on the surface of Mars, as the Viking landers were placed in a random location due to our lack of knowledge about the planet. It felt as though we were placing something in the Gobi Desert, when we should have placed it in Jezero Crater, the river delta where the Perseverance rover is currently located. At the time, we were unaware of its existence. One of the Viking experiments suggested the presence of microbial life in the soil, but only one of the three instruments supported this claim, so we couldn't confirm the discovery of life. Now, with plans to bring back samples, we will finally have conclusive evidence. The need for a sample return mission was unexpected. Question: Can Mars be terraformed by using a large magnetic shield to prevent the sun from stripping its atmosphere, thus allowing the planet to retain more heat and become habitable? 
Answer: Yes, it is possible. By preventing atmospheric stripping, the pressure on Mars will increase, leading to the planet beginning to terraform itself. We desire for the planet to be involved in any way possible. As pressure increases, temperature also rises. The initial stage of terraforming would be at 60 millibars, which is ten times higher than our current level. This is known as the Armstrong limit, where blood would not boil if someone were to step outside without a spacesuit. Without the need for a spacesuit, there would be greater freedom of movement and flexibility. The increased temperature and pressure allow for the possibility of planting crops in the soil. There are multiple approaches to creating a magnetic shield. I am currently finalizing a research paper that I have been working on for two years, although I anticipate 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 Venus by using a physical shield that reflects light, which would lower the temperature. In 2015, NASA approved the Europa Clipper mission to explore Jupiter's moon Europa for signs of life, scheduled for launch in 2024 after discovering plumes erupting from its subsurface ocean in 2013. Were you hoping for the mission to occur earlier? A: I would have liked to see it earlier, but it was not possible. There are specific missions that are so large they are known as strategic missions. In order for them to occur, everything must fall into place. You must make a proposal, present a strong case, approach the NASA administration, and then present it to Congress. Every year, I proposed a mission to Europa. Each year, the administration showed no interest in traveling to Europa. The discovery of plumes on Europa was the driving force behind the Europa mission. In 2013, while attending an American Geophysical Union meeting, I was excited to hear several scientists discuss their findings of a plume using Hubble on Europa. I immediately wanted to organize a press conference to share this fantastic news. I contacted NASA headquarters again and they were successful. I brought that information back to headquarters and included it in the story of Europa, which made a big impact. They were impressed and considered implementing the idea. When asked if I wanted a lander on the mission, Congress ultimately decided against it. A: While I would like to have a lander, it is not currently feasible for the mission as it would complicate things. However, all the work we are doing on Clipper is laying the groundwork for a future lander. I made sure that we included 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 dangerous terrains, so without this imaging, landing would be impossible. It is important to take measured steps rather than rushing into things, as that often leads to failure. Viking serves as a prime example of a situation where we took a significant leap without sufficient knowledge. We lacked understanding of the terrain, soil composition, and toxins present in the soil. Additionally, our understanding of water distribution on the planet was limited. There were ten crucial pieces of information that we should have possessed before sending the Vikings to the surface. As for working on scientific papers in retirement, the question remains unanswered. A: Yes, definitely. I have a paper on Mars that needs to be completed. I am currently working on a paper on Europa. I am also in the process of writing an astrobiology book. I have a strong passion for science. Copyright 2022 by The New York Times Company.