July 03, 2008

Scientists now have a much clearer view of the surface of Mercury -- as well as data that could lead to new theories about the planet's interior -- from the first direct topographic data sent back from the MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) spacecraft in January. Steven A. Hauck II, assistant professor of geological sciences in the College of Arts and Sciences at Case Western Reserve University, is part of the team collecting and analyzing the data.

A paper, "Laser Altimeter Observations from MESSENGER's First Mercury Flyby," co-authored by Hauck with lead author Maria T. Zuber (Massachusetts Institute of Technology) detailing the analysis is one of 11 articles on the NASA-funded Mercury mission published in the July 4 issue of Science.

The team used data collected by the Mercury Laser Altimeter (MLA) housed on MESSENGER to create a topographic profile of a 3,200-km stretch of Mercury's surface, approximately 20 percent of the circumference of Mercury's equator. When MESSENGER's primary mission is completed, now scheduled for 2012, a complete picture of the northern hemisphere and some areas south of the equator will be collected.

The device directs a laser toward Mercury and measures the time, in milliseconds, it takes for the beam to reflect back to the craft. The higher the altitude of the area covered by the laser, the shorter the reflection time back to MESSENGER. The precision of the altitude measurements is ˜15 cm.

Initial Findings

Findings include the fact that the radius of Mercury apparently decreases by 1.4 km along the equator from 10° to 90° E longitude, corresponding to a 0.02° slope down to the east.

Such a long-wavelength slope, if additional research proves it to be a fundamental feature of the equatorial shape of the planet, might be the result of crustal thickness or crustal density variations, global-scale mantle density, or topography along the planet's core-mantle boundary, located approximately 600 km beneath the planet's surface.

A series of impact craters were profiled. While they are shallower than similar craters on the Moon, due in part to Mercury's higher gravity, the floors of the sampled craters on the planet's surface also varied in roughness and slope. These differences imply complex post-impact modification.

Such modifications may include volcanic resurfacing, tectonic subsidence, wall slumping, viscous relaxation, and emplacement of debris from younger nearby impacts -- all important geological processes.

Just the Beginning

Findings at this point are the result of data compiled from MESSENGER's flyby in January. Two more flybys are scheduled for October 2008 and September 2009. In 2011, MESSENGER will begin orbiting Mercury for approximately one year, collecting data the entire time.

Previous topographic data about Mercury's surface were based on images collected during three flybys by Mariner 10 in 1974 and 1975. Additional insights also were provided by Earth-based radar measurements taken over the last two decades.

"Those results were of lower and more variable resolution than what we're getting from MLA on MESSENGER," Hauck said. "We can pull much more detailed topographic information from the new data."

The team of researchers found that MESSENGER was able to collect MLA data from a position farther from Mercury's surface than originally planned.

"We expected that once MESSENGER reached a distance of 1,200 km from Mercury, it would be difficult to receive reflected laser pulses from the surface," he said. "It turns out that MLA is able to receive reflected laser pulses at a distance of up to 1,500 km. As a result, we will be able to expand our field of study. Previously, we were focusing on the northern hemisphere, but now we are able to see a bit below the equator."

Over the course of the MESSENGER mission, Hauck expects that our understanding of Mercury inside and out will be greatly enhanced.

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