What scientific technique later provided evidence suggesting Mars lacked the necessary resources for Lowell's proposed canal system?

Answer

Spectral analysis for atmospheric composition

Later spectral analysis, which better determined atmospheric composition, suggested Mars lacked the necessary water vapor to support large, surface-level liquid canals.

What scientific technique later provided evidence suggesting Mars lacked the necessary resources for Lowell's proposed canal system?
MarsscientistAstronomerobservationplanet