As NASA's MESSENGER spacecraft ends its scientific operations by crashing into Mercury today, Planetary Science Institute researchers looked back at a mission that provided new discoveries on the planet closest to the Sun.

PSI Senior Scientist Catherine Johnson and her group have played key roles in the characterization of Mercury's magnetic field, including the major discovery, with colleague and team member Brian Anderson, that the magnetic field is offset from Mercury's body center. Her group's work has contributed to identifying the large-scale structure of the magnetosphere and the internal field, demonstrating the existence and properties of field aligned currents, and flux of the solar wind to the surface.

PSI Senior Scientist William Feldman was responsible for the conceptual design of the MESSENGER Neutron Spectrometer. This instrument provided the data to confirm the presence of water ice near the surface of permanently shaded craters near the north pole of Mercury.

PSI Senior Scientist Faith Vilas's work identified spectral absorption signatures of surface mineralogy in wide-angle color images of bright geologic surface features called "hollows" in Mercury's craters Dominici and Hopper. Studies of temperature effects on spectral properties of sulfides suggest that the hollows mineralogy incorporates MgS (magnesium sulfide), CaS (calcium sulfide) and darker background material.

Mercury's surface is spectrally bland, and these are the first solidly identified reflectance absorption features for Mercury's surface.

PSI Senior Scientist Deborah Domingue's work focused on photometry, where she used the Mercury Dial Imaging System instrument to study how Mercury's surface reflects light, depending on how you look at it. She contributed photometric standardization for the images, enabling the construction of mosaics, both monochrome and color. One of her key contributions was the review paper published in Space Science Reviews on the space weathering of Mercury's surface in comparison to space weathering on the Moon and asteroids.

PSI Associate Research Scientist Elizabeth Jensen utilized MESSENGER's positioning, radio transmitter, and proximity to the Sun to study the solar wind, specifically the Sun's magnetic field, using Faraday rotation.

These unique set of measurements added a low-resource, high-impact, scientific contribution to the dearth of knowledge on the heating paradox of the Sun: how do you make heat flow from something cool - the 6,000 degree Kelvin surface - to something hot, the 6 million degree Kelvin corona.

Another result from Jensen's work on MESSENGER's Solar Coronal Faraday Rotation observations was the passage of a Coronal Mass Ejection. These are massive plasma structures that can trigger Earth's space weather, or geomagnetic storms. Measuring the magnetic field within one so close to the Sun is extremely rare and valuable to enhancing our ability to predict their effects on our satellites and ionosphere.

And PSI Senior Scientist Robert Gaskell's digital elevation models were used to better understand Mercury's geology, both on the surface and within the planet. He used images acquired by the Mercury Dual Imaging System (MDIS) to derive information on the shape and topography of the inner most planet. This information has been used to model and constrain the characteristics of Mercury's internal structure.

After the spacecraft crashes and ends scientific operations, PSI researchers are funded for another year to study and archive data collected during the mission. They spoke fondly of their MESSENGER experiences.

"This mission was one of the best that I had the opportunity to contribute to," Feldman said.

"It has been an amazing journey of discovery, made possible by the excellence and dedication of the engineering team and an extraordinary science team," Johnson said.

"For me personally it has been four years of outstanding scientific collaborations, incredible hard work (and exhaustion!) and the kind of satisfaction that only ever comes from that level of intensity. I think everyone feels a mixture of pride in what has been accomplished, sadness at the prospect of the imminent end-of-mission, but just sheer joy in what we have been able to learn and what is still to be learned.

"Its mind boggling how much we have accomplished in terms of the scientific research, the technologies developed to get there, and the new tools for mission management and operation," Domingue said. "Its an era coming to a close. There is such a feeling of satisfaction, such a feeling of accomplishment, in having been a part of history."