In a groundbreaking discovery for Mercury science, the MESSENGER spacecraft imaged a plume of material erupting from the surface of the innermost planet. In an MDIS image taken early this morning, a bright source of light may be seen above Mercury's southern hemisphere. Located at approximately 67S, 55E, close to the newly named Alver basin, this light source appears to be an eruptive plume from a previously unrecognized vent.

MESSENGER team members are currently analyzing images of the eruption. One hypothesis under consideration is that the brightness indicates the presence of a "fire fountain," an eruption during which lava is ejected from depth in a jet-like spray of molten rock. There also appears to be a dimmer cloud of material above the central plume.

Under the fire fountain hypothesis, this higher cloud may be composed of smaller droplets of lava of a size that allows them to be blasted to greater altitudes. Most of these droplets will probably fall back to the surface, producing a distinctive "pyroclastic halo" around the vent like those seen elsewhere on Mercury.

An alternative possibility is that the plume is a "geyser" of volatile materials, analogous to the plume at Saturn's moon Enceladus.

Under this scenario, a pocket of volatile-rich material may have been heated by the intrusion of subsurface magma, solar tidal dissipation, or some other process, fueling a geyser-like eruption at the surface. Because of the recent identification of water ice at Mercury's poles, the MESSENGER team is pursuing the idea that water may be involved in the eruption.

Given that possibility and the date of the discovery, the team has already proposed to the International Astronomical Union that the feature be given the name "Poisson d'Avril."

Scientists will continue to acquire and examine as many images of the eruption as possible. The MESSENGER spacecraft has been conducting orbital operations at Mercury for two years, first during its primary mission and then during a year-long extended mission. The MESSENGER team recently submitted a proposal to NASA to extend the mission further by an additional two years of orbital operations.

"One of the objectives of our second extended mission is to seek evidence of recent geological activity, but never did I think we'd see such spectacular evidence this early," enthused MESSENGER Project Scientist Nat MacRulf.

MESSENGER Project Manager Ellen Hinter agreed. "An earlier design of the spacecraft included the Joint Analyzer for Plume Eruptions instrument, intended to collect samples of any plumes active during orbital operations. That instrument was not included in the final design because of mass limitations, but our plasma and energetic particle spectrometers should nonetheless have a field day."