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More secrets of Mercury unveiled in September flyby

Posted: November 3, 2009

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The flyby of Mercury made by a small NASA probe in late September was scientifically rewarding, despite a glitch that squandered half of the planned observations at the innermost planet, scientists said Tuesday.

MESSENGER captured this color image as it flew away from Mercury in late September. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
The primary purpose of the close encounter, to use Mercury's gravity to bend the spacecraft's trajectory, was perfectly executed. It was more accurate than all six planetary flybys conducted by the $446 million MESSENGER mission since its launch in 2004, according to Sean Solomon, the mission's principal investigator at the Carnegie Institution of Washington.

MESSENGER stands for the Mercury Surface, Space Environment, Geochemistry, and Ranging mission.

In its third visit to Mercury Sept. 29, MESSENGER flew 142 miles over the planet's tortured surface. Mercury's gravity changed the craft's velocity in a slingshot-like effect to set up for another encounter in March 2011, when MESSENGER will enter orbit around the innermost planet.

About 98 percent of Mercury has been imaged after the September flyby. A swath more than 350 miles wide at the equator was seen for the first time, allowing researchers to fill in most of the planet's map.

Only Mercury's polar regions remain unexplored, and images of those regions will be collected when MESSENGER is in orbit.

Scientists' knowledge of the planet is entirely based on MESSENGER's three visits and three other encounters by the Mariner 10 probe in 1974 and 1975.

"It's like we've read the first four chapters of a novel, where Mariner 10 was the first chapter and each one of our flybys was another chapter," Solomon said. "We still have a long way to go to understand the full plot of what this innermost planet is telling us about all the inner planets of the solar system, including our own."

The next phase of MESSENGER's mission, orbital operations, will begin exactly 500 days from Tuesday, Solomon said.

Scientists planned a tightly-choreographed sequence of science observations during the September flyby, but the science activities were cut short when a power glitch put the probe in safe mode, shutting down MESSENGER's instruments just before the point of closest approach.

"I want to emphasize that even though we obtained about half of the data we planned to obtain, we saw surprises," Solomon said.

The new territory unveiled in September showed a curious volcanic feature and a double-ring impact basin stretching 180 miles across.

This enhanced-color image shows the volcanic vent and double-ring impact basin uncovered by MESSENGER. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
"This has been a really geologically active area and it's a fascinating place to study," said Brett Denevi, a MESSENGER imaging team member from Arizona State University.

A suspected volcanic vent formed by an ancient explosion spans approximately 19 miles, surrounded by bright colors indicating debris from the volcano was thrown up to 50 miles away.

The double-ring basin, relatively young at 1 billion years old, is filled with smooth terrain left unmarked by the barrage of meteor impacts that crater the rest of Mercury's surface.

"Such an age is quite young for an impact basin, because most basins are about four times older," Denevi said. "The inner floor of this basin is even younger than the basin itself and differs in color from its surroundings. We may have found the youngest volcanic material on Mercury."

Data MESSENGER gathered in September also debunked a common belief about a disparity in iron at Mercury. Scientists know Mercury's massive core is rich in iron, but earlier observations showed the planet's surface was relatively absent of iron and titanium metals.

It turns out surface iron on Mercury is abundant but likely held in minerals like iron titanium oxide instead of more typical silica minerals.

"A consequence of these results is that models of Mercury's formation and evolution are likely going to have to be reassessed to account for the higher iron and titanium," said David Lawrence, MESSENGER participating scientist at the Johns Hopkins University Applied Physics Laboratory.

Earlier models on the formation of Mercury included theories that a massive collision could have peeled away the iron from the surface. Now that scientists have detected higher quantities of metals in the surface, those models will have to be retooled.

"It's not something that we necessarily expected, and it's going to keep people busy," Lawrence said. "It's not that we've necessarily made the problem any easier."

"The iron is in a form that we don't normally encounter in other planetary situations, so it's going to be a volley back to our geochemists and our petrologists in order to come up with a scenario," Solomon said.

Artist's concept of MESSENGER at Mercury. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
As MESSENGER closed in on Mercury, sensors also observed the planet's ultra-thin atmosphere and a comet-like tail made of atoms being stripped away from the surface by solar radiation.

The spacecraft's two previous visits to Mercury revealed a tail of sodium atoms streaming away from the dark side of the planet, but September's flyby showed a sodium signature between 10 and 20 times less instense than previously detected.

The lower amounts of sodium indicate seasonal variations in the tail caused by different levels of solar radiation as Mercury's distance from the sun changes through the planet's 88-day orbit.

"In this third flyby, the net affect of radiation pressure was almost zero, so very little sodium was pushed into the tail," said Ronald Vervack, MESSENGER participating scientist at the Johns Hopkins University Applied Physics Laboratory.

This finding shows Mercury's atmosphere, or exosphere, is one of the most dynamic in the solar system, despite being so thin that its atoms almost never collide.

Levels of calcium and magnesium in the tail actually increased during September's flyby.

"When you put this all together, what it means are the different types of atoms in Mercury's exosphere are going to have their own type of season variations," Vervack said.

Discoveries from MESSENGER's three flybys will keep scientists busy planning more in-depth studies after the probe enters orbit.

"It's going to be a rich and interesting dialogue as we get ready for orbit," Solomon said.