Organic-covered ice deposits discovered on Mercury

Map showing the location of the radar-bright ice deposits discovered by the MESSENGER spacecraft at Mercury’s north pole. Credit: NASA / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington / National Astronomy and Ionosphere Center, Arecibo Observatory

Well, who would have thought? Water ice on Mercury, that scorched little planet closest to the Sun? That’s just what scientists have found, as well as organic material, at Mercury’s north pole, it was announced yesterday.

Actually, previous studies had suggested there could be ice there, and now the MESSENGER spacecraft orbiting the planet has finally confirmed it.

As David Lawrence, a MESSENGER Participating Scientist at the Johns Hopkins University Applied Physics Laboratory and the lead author of one of the papers, explains, “The neutron data indicate that Mercury’s radar-bright polar deposits contain, on average, a hydrogen-rich layer more than tens of centimeters thick beneath a surficial layer 10 to 20 centimeters thick that is less rich in hydrogen. The buried layer has a hydrogen content consistent with nearly pure water ice.”

Even though mostly covered by the darker material, the ice deposits show up as bright patches to the spacecraft’s radar.

But how is this possible? It has to do with Mercury’s axis, which has almost no tilt at all, less than one degree. This means that at the poles, there are places, namely deep craters, which never see sunlight. In this permanent shade, temperatures plunge to be extremely cold, about -184˚ C (-300˚ F) while the rest of the planet in sunlight is still scorching hot, up to about 427˚C (800˚ F) because there is almost no atmosphere to distribute heat.

The north polar deposits are primarily water ice, which in most areas is covered by a darker material thought to be composed of complex organic compounds. These are the same kinds of organics found throughout the solar system, probably brought to Mercury by impacting comets and asteroids.

According to Sean Solomon, principal investigator of the MESSENGER mission, “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. MESSENGER has now supplied a unanimous affirmative verdict.”

Solomon adds, “But the new observations have also raised new questions. Do the dark materials in the polar deposits consist mostly of organic compounds? What kind of chemical reactions has that material experienced?”

Even with water ice and organics however, the chances of any kind of life on Mercury are remote, given the extreme temperatures and lack of atmosphere. As Solomon asks though, “Are there any regions on or within Mercury that might have both liquid water and organic compounds? Only with the continued exploration of Mercury can we hope to make progress on these new questions.”

The finding is important though in that it provides further confirmation that the both water ice and organics, the building blocks of life, are common and widespread in the solar system. That alone is an exciting discovery.

The three new papers were published online yesterday in Science Express.

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