NASA's Messenger spacecraft has discovered evidence that the planet Mercury has enough ice on its surface to encase Washington, D.C., in a block two and a half miles deep.

"For more than 20 years the jury has been deliberating on whether the planet closest to the Sun hosts abundant water ice in its permanently shadowed polar regions," writes Sean Solomon of the Columbia University's Lamont-Doherty Earth Observatory, the principal investigator of the Messenger mission. The spacecraft "has now supplied a unanimous affirmative verdict."

"These reflectance anomalies are concentrated on poleward-facing slopes and are spatially collocated with areas of high radar backscatter postulated to be the result of near-surface water ice," Gregory Neumann of NASA's Goddard Space Flight Center writes in the paper. "Correlation of observed reflectance with modeled temperatures indicates that the optically bright regions are consistent with surface water ice."

The study results were published on Wednesday in Science magazine, which explains in its summary, "The buried layer must be nearly pure water ice. The upper layer contains less than 25 wt.% water-equivalent hydrogen. The total mass of water at Mercury's poles is inferred to be 2 × 1016 to 1018 g and is consistent with delivery by comets or volatile-rich asteroids."

Radar imaging of Mercury has long suggested that there could be large deposits on the planet's surface, with reports dating to 1991. But today's report presents harder evidence supporting that theory.

Messenger has fired more than 10 million laser imaging pulses at Mercury's surface since arriving in its orbit in 2011. Feedback from those pulses have helped NASA in its quest to verify whether ice is present in Mercury's poles, which are largely shielded from exposure to the sun's rays.