Friday, April 29, 2011

Mercury Laser Altimeter (MLA): Profiling Mercury's Topography

Mercury’s interior view

An artist's rendition of what Mercury’s
core might look like. (Image Credit: NASA)

The purpose of the Mercury Laser Altimeter (MLA) is to measure the topography or surface relief of the northern hemisphere of Mercury. MLA data will be used to create topographic maps, which will help characterize the geologic history of the planet.

For example, topographic maps on Earth are used to show relief, such as mountains or valleys, and features including roads and streams, on a flat piece of paper.

Additionally, this data can be combined with other data to tell us something about Mercury’s global shape and spin axis as well as the size and state of its core.

How it works
The MLA measures the range or distance between the MESSENGER probe and the surface of Mercury using a laser transmitter and receiver. This two step process begins as the transmitter generates a brief laser pulse directed toward the planet.

The light travels to the planet and some is reflected back and detected by the receiver. The time it takes for the light to travel round-trip is recorded by the instrument and can then be converted to a distance.

This is accomplished by a very simple calculation; the round-trip time is multiplied by the speed of light, giving us the round-trip distance. The round-trip distance is then divided by 2 to get the distance one way.

Every second 8 laser pulses are transmitted for a pulse rate of 8 Hz. The round-trip transit time is measured with an accuracy of 2.0 nanoseconds (a nanosecond is a billionth of a second!).

Therefore, the resolution of the topographic data is 0.3 meters. Since the probe is traveling as it transmits laser pulses, range measurements are collected every 100 to 300 meters along its path as in the image below.

laser footprint from altimeter

Understanding mercury and beyond
Ultimately, the data we gain from this mission will further our knowledge of how the terrestrial planets—including Earth—formed and evolved. To be more specific, detailed topography along with gravity measurements will help us understand the current and historical geology of the planet.

For example, we could determine the thickness of Mercury’s crust which provides insight into the minerals present and the size of the core.

From previous fly-by missions (Mariner 10 in 1974 and 1975) we know that there are several different types of terrains on Mercury, including regions that are heavily cratered like the Earth’s Moon, vast plains, hilly areas, and features that look like long cliffs that are up to 500 kilometers in length and hundreds of meters in height.

Perhaps we will be able to determine the source of these dominant geologic features; were there active volcanoes, active faults, and did the planet contract or shrink as it cooled?

Read more on the Messenger MLA here

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