Pixels TROPOMI methane detector remain almost completely intact

TROPOMI has been flying around the globe every ninety minutes since 2017. As it floats from pole to pole, the world underneath keeps on turning. This allows TROPOMI to quietly scan a new strip of Earth with each orbit. Eventually, that results in a complete world map of atmospheric methane and carbon monoxide, measured by the near-infrared detector.

As peaceful as satellites wander through space, the cosmic rays from the Sun and the Milky Way that strike them are violent. These charged particles are famous for their elegant appearance as they follow the Earth’s magnetic field lines toward the poles and show themselves in the shape of the northern lights. But space detectors run the risk of damaging their materials as they fly over the poles. This risk is even greater when they fly through the South Atlantic Anomaly, above South America, where the protective shield of the Earth’s magnetic field hangs much lower than TROPOMI’s flying altitude.

Despite this space version of the Bermuda Triangle, the pixels in TROPOMI’s methane detector turn out to remain almost undamaged. SRON is responsible for monitoring the detector and concludes in a publication in Measurement Science and Technology that at least 98.7% of the pixels are healthy. This is partly due to spontaneous recovery of the majority of damaged pixels.

‘The detector is made of a semiconductor with a crystal structure,’ says first author Tim van Kempen (SRON). ‘Cosmic rays damage the pixels by disrupting the lattice. Heating and cooling can repair the structure, just like a crack in an ice cube is repaired by melting and refreezing. But our detector operates at 130 degrees below zero so heating up is the last thing you want. Fortunately, we now see that 95% of the pixels recover spontaneously. That is also good news for future space missions, because we did not yet know of this spontaneous recovery at the very low temperatures that space detectors often require.’