With blistering days, frigid nights, and parched features, Mercury has long suffered in sun-scorched obscurity. Ancient astronomers strained to observe the diminutive orb through the sun’s glare. Their modern counterparts were tempted more by the terrestrial (“rocky”) planets closer to home, peering through Venus’ toxic clouds or scouring Mars for signs of life. Visits to Mercury by spacecraft long proved elusive, as its swift and close orbit of the sun necessitated risky maneuvers and high fuel consumption. It was not until 1974 that NASA’s Mariner 10 spacecraft revealed the planet’s pockmarked surface of impact craters, volcanic plains, and giant escarpments. With apparently little else to offer, astronomers released Mercury back to its sun-drenched procession.
But in the following decades, interest in Mercury reignited as new observations challenged the planet’s desolate reputation. Evidence emerged that Mercury’s interior may have shrunk. Radio telescopes unexpectedly hinted that its poles contain water ice. Clearly, the planet harbors secrets, and in 1998 NASA unveiled a new Mercury mission to investigate them. Launched in 2004, the MESSENGER spacecraft (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) entered orbit of its target nearly two years ago and began its scientific mission. Since then, data have poured in on Mercury’s magnetic field, sparse atmosphere, geology, and interior.
Three papers recently published in Science shed light on one of Mercury’s most peculiar puzzles, rumors of water ice at its poles. Astrophysicists from NASA’s Goddard Space Flight Center, Johns Hopkins University, and the University of California, Los Angeles used data gathered by two of MESSENGER’s instruments to verify that Mercury’s poles contain water ice, as well as another surprise. The ice resides literally in the shadows, within deep craters and chasms that are shielded permanently from sunlight at the poles and remain cold enough to keep ice stable for extended periods. In the coldest pockets, ice is exposed at the surface. But most of the ice MESSENGER detected is insulated beneath a sheath of dark-colored material.
Unexpectedly, MESSENGER’s data indicate that this dark material could be a layer of carbon-rich organic compounds. If confirmed, these materials would not be signs of life; organic compounds are present in many non-biological settings, and no life, even in the cold shadows, could long withstand the ultraviolet radiation that pummels Mercury. But it is surprising that typically fragile organic chemicals could persist on Mercury’s uninviting surface. Like ice, their time in extreme heat is fleeting. But these cold sunless depths at Mercury’s poles could be a refuge on a planet where daytime temperatures are hot enough to melt lead.
Water ice and organic chemicals likely arrived on Mercury as stowaways on comets and asteroids. Surveys have detected organic compounds within comets and asteroids, and the rocky planets were bombarded with these celestial drifters early in the solar system’s history. On Mercury, most fragile stowaways perished in harsh daytime temperatures. The ice and organic compounds detected by MESSENGER are likely the lucky cargo that accumulated among the cool, sunless corners of the poles.
On Earth, water and organic compounds transported by ancient asteroids and comets had a happier fate. This organic soup accumulated on our planet’s hospitable surface and, about three billion years ago, life evolved in its depths. Thus, MESSENGER’s mission to drag Mercury back out of the shadows also helps astrophysicists and biologists understand why Earth became a sanctuary for life, while Mercury, Venus, and Mars took more hostile paths. Such comparisons are especially relevant in the search for life-bearing planets around distant stars. For that quest, astronomers must divine the rare Earth among a multitude of Mercurys.