Jim Green has served as NASA chief scientist for 40 years, but as Green bid the agency farewell on Saturday he left us with one last plan - geoengineer Mars into a planet habitable for humans.

The now retired scientist proposed covering the Red Planet in a giant magnetic shield to block it from the sun's high-energy solar particles.

Speaking with the New York Times, Green explained the protective shield would stop the sun from stripping Mars' atmosphere, allowing the planet to create a human-friendly climate.

'Yeah, it's doable. Stop the stripping, and the pressure is going to increase. Mars is going to start terraforming itself,' Green said.

'The higher temperature and pressure enable you to begin the process of growing plants in the soils.'

Green discussed his Mars plan with the NYT in honor of his retirement, but the former NASA official has been working on it since at least 2017 when he first revealed it to the public.

In a talk at the NASA Planetary Science Vision 2050 Workshop at NASA's headquarters in Washington DC, Green presented simulations, models, and early thinking about how a Martian magnetic field might be re-constituted and the how the climate on Mars could then become more friendly for human exploration and perhaps communities.

'The solar system is ours, let's take it,' Green told the workshop in 2017. 'And that, of course, includes Mars. But for humans to be able to explore Mars, together with us doing science, we need a better environment.'

The proposed plan would put Mars inside a protected 'magnetotail,' which would help it to rebuild the atmosphere that has been stripped from it by an influx of solar particles - 90 percent of the atmosphere has been stripped.