A new study indicates astronauts are to blame for rising temperatures on the moon as a decades-old mystery appears to be solved.

The study, recently published in the Journal of Geophysical Research: Planets, indicates astronauts in the Apollo 15 and 17 missions stirred up the moon dust in the area, revealing the darker dust below, called regolith. The darker regolith attracted more of the sun’s rays and heated up the area, the study suggests.

In 1971 and 1972, astronauts with the Apollo 15 and 17 missions installed temperature probes to track heat on the moon. Back on Earth, the temperatures were monitored from 1972-1977, but were only archived until 1974.

The probes indicated the moon’s temperature unexpectedly rose by between 1.8 and 3.4 degrees Fahrenheit in 1974, but without the other three years of data, researchers were left puzzled as to why the temperature had risen by so much. It wasn’t until recently the majority of the missing data was recovered and showed the rising temperatures continued through 1977.

After uncovering the additional data, the research team then used the Lunar Reconnaissance Orbiter Camera, a high-tech camera and mapping tool, to examine the area surrounding the Apollo 15 and 17 probes. The researchers discovered the site had been disturbed and was darker than the surrounding parts of the moon.

The researchers concluded that the darker regolith had been uncovered as the astronauts walked and drove rovers through the region. This darker material attracted the sun and heated up the area surrounding the probes.

“We suggest that, as a result of the astronauts' activities, solar heat intake by the regolith increased slightly on average, and that resulted in the observed warming,” the study’s abstract states.