 |
| English: Artist's rendering of the MESSENGER spacecraft orbiting Mercury (Photo credit: Wikipedia) |
NASA’s Messenger spacecraft, in orbit around Mercury for the past four years, came to an abrupt end on April 30 when it crashed into the surface.
That event brought to a close a mission that has painted an unexpected portrait of Mercury, once thought a boring round rock not much different from Earth’s moon. Mercury, the smallest planet in our solar system, is only slightly larger than the moon, although it has more drastic temperature swings – 427 degrees Celsius during the day, minus 184 degrees at night.
“It’s really been exciting to see a planet unfold, a planet that is one of our neighbors,” said Sean C. Solomon, the principal investigator for the mission. “Almost every aspect of Mercury has had its share of surprises.”
Messenger – a shortening of Mercury Surface, Space Environment, Geo-chemistry and Ranging – discovered that Mercury shrank slightly as it cooled over billions of years; mapped ancient lava flows; found enigmatic shallow depressions that are some of the youngest and brightest features on the surface; and confirmed the presence of ice in perpetually dark crates near the poles.
The water ice, perhaps a billion tons, was not a complete surprise. Radio telescope observations from Earth had hinted at something reflective, and the shadowed craters are extremely cold. Beyond confirming the suspected, Messenger also made a new discovery: The ice was covered by an unexplained dark layer.
“We have a reasoned hypothesis,” Dr. Solomon said. The material could be carbon-rich compounds, similar to substances found in certain meteorites and in comets. “It’s tarlike in its consistency,” he said. “It’s as dark as tar.”
The biggest scientific finding of the mission, Dr. Solomon said, was that Mercury was rich in “volatiles,” elements like chlorine, sulfur, potassium and sodium that easily evaporate at moderate temperatures. Mercury is small and dense, full of iron, and the thinking about how it formed was that it must have been heated to temperatures that should have boiled off the volatiles. Yet the volatiles are still there.
Scientists now have to come up with new ideas to explain how Mercury formed. When NASA launched Messenger in 2004, knowledge of Mercury was sparse. The only close-ups had come in three flybys by NASA’s Mariner 10 in the 1970’s
For more than six years, Messenger took a circuitous route through the inner solar system, making flybys of Earth, Venus and Mercury, to slow itself down enough to enter orbit around Mercury. On March 18, 2011, finally arrived.
The orbital portion of the mission was to last one year, then was extended twice.
Even the craft’s destruction could provide insights by digging up what lies beneath the surface- but it will be some time before anyone will see what that is. NASA does not have any follow-up Mercury missions set, but an ambitious European and Japanese collaboration called BepiColombo is scheduled to launch in 2017. It is to arrive at Mercury in 2024.
That mission consists of two orbiters, one focusing on the planet, the other on the surrounding space environment, in greater and finer detail than Messenger.
Johannes Benkhoff, the project scientist for BepiColombo at the European Space Agency, described Messenger as “really a fantastic mission.”
“It’s perfect that we have BepiColombo to follow on” Messenger’s findings, Dr. Benkhoff said. “They have provided a lot of new results, unexpected results.”
BepiColombo is named after Giuseppe Colombo, an Italian scientist nicknamed Bepi who came up with the orbital trajectories that enabled NASA’s Mariner10 flybys.
Dr. Solomon is looking forward to the next trip to Mercury I still want to learn more about, “he said.
Taken from TODAY Saturday Edition, The New York Times International Weekly, May 9, 2015
