Mercury Laser Altimeter (MLA)

A topographic map shows relief,
such as mountains or valleys,
and features, including roads and
streams, on a flat piece of paper.
( 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.

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

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.

Contribution to our understanding of 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?

A picture from Mariner 10 of a densely cratered region of Mercury.	Plains terrain east of the Caloris basin is shown in this image.
Hilly and lineated terrain and a patch of smooth plains in a large degraded crater (lower left portion of the image). Small craters are rare in the terrain; most landforms are positive.	The prominent scarp [cliff] that snakes up the image was named Discovery Rupes. Like Hero Rupes, this feature is thought to have been formed as the planet compressed, possibly caused by cooling of the planet.
Image credits for all four images: NASA/JPL/Northwestern University

From Earth we have seen reflective material at the poles of Mercury that appears to be similar to the reflectivity of water ice on other solar system bodies. This could only be the case if large craters produced areas of constant shadow, away from any sunlight. A topographic map would tell us if t here are indeed craters at the poles that could be permanently shadowed. The Gamma-Ray and Neutron Spectrometer (GRNS) instrument onboard the MESSENGER probe may be able to determine if it is water ice or, perhaps, an element like sulfur.

Other applications of this instrument

Laser altimeters have been used to map terrain on Earth , the Moon , Mars, and even an asteroid 433 Eros ! On Earth this technique has been used to measure vegetation cover, the changing thickness of ice sheets, cloud cover, topography or surface relief, and the location, shape, and height of buildings. The laser altimeter experiment aboard the Apollo 15, 16, and 17 missions provided topographic relief data for the moon. The Mars Orbiter Laser Altimeter (MOLA) aboard the Mars Global Surveyor helped to explain the channel and valley network previously observed on Mars; detailed topographic data revealed that the south pole of Mars is higher in elevation than the north pole by about 6 kilometers, resulting in a net slope of 0.036°.

This is a pole-to-pole view of Martian topography from the first MOLA global topographic model [Smith et al., Science, 1999]. The slice runs from the north pole (left) to the south pole (right) along the 0° longitude line. The figure highlights the pole-to-pole slope of 0.036°, such that the south pole has a higher elevation than the north pole by ~6 km.

Working with the other instruments

Most importantly, the laser altimeter can only provide accurate surface topography data if its precise location is known. Therefore, the telecommunication subsystem in concert with the Deep Space Network (DSN) ground station records the exact orbital path of MESSENGER at all times. Additionally, data from the laser altimeter will be used in conjunction with visual images or photographs to interpret the topography. And unlike visual images, the laser altimeter works even when it is dark. Finally, data from the Radio Science (RS) instrument will be used to measure subtle changes in MESSENGER’s velocity as it orbits the planet, which tells us something about its gravity. Combining this data with topographic data from the MLA will help us understand how mass, such as the thickness of the crust, is distributed around Mercury. Scientists can then deduce information about the size and state (liquid or solid) of the core of mercury.

And this is only the information we have planned on gathering! In reality, often missions reveal information that we didn’t even know we were looking for.