[meteorite-list] MESSENGER: Profiling Polar Craters with the Mercury Laser Altimeter

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 27 Apr 2011 15:38:30 -0700 (PDT)
Message-ID: <201104272238.p3RMcUpK022057_at_zagami.jpl.nasa.gov>

http://messenger.jhuapl.edu/soc/highlights.html

MESSENGER Science Highlights from Mercury's Orbit

Profiling polar craters with the Mercury Laser Altimeter

MESSENGER's Mercury Laser Altimeter (MLA) uses a laser to measure the
distance from the spacecraft to Mercury's surface. The instrument sends
a laser pulse to Mercury and measures the time it takes the light to
bounce off the surface and return. Because we know the speed of light,
we can convert the round-trip time to distance. Because we know the
positions of the MESSENGER spacecraft and Mercury, we can determine the
height of the terrain illuminated by the laser spot on the surface.

The laser sends pulses separated in time by about one-eighth of a second
and provides measurements that are usually spaced about 600 meters
(about 660 yards) apart on the surface. The MLA is sufficiently powerful
to measure spacecraft-to-surface distances up to about 1,600 km (1,000
miles). When the laser beam hits Mercury's surface, its footprint (or
spot size) is between 15 and 100 meters in diameter, depending on
distance, so MLA measures the average distance between the spacecraft
and the surface over this area. The relative accuracy between
measurements is better than 10 centimeters, (4 inches). A track from
MLA, shown in Figure 1, contains the height measurements from one pass
over Mercury's surface.

[Figure 1] MLA's first measurements from Mercury orbit. This profile
extends almost 5,000 km across the surface. The colors on the track show
surface altitude relative to Mercury's average radius, estimated to be
2,440 km. The inset shows an expanded view of the very deepest portion
of the profile. The arrow points to the crater shown in Figure 2.

Like all instruments on MESSENGER, MLA provides information for several
different science investigations. The range measurements from MLA will
be used to recover the overall shape of the planet, which helps
determine Mercury's interior structure. When MLA tracks cross
deformational features such as ridges or scarps, the topographic profile
provides information on how the landscape has adjusted in response to
shortening or stretching of the crust. Comparing the change in elevation
from one MLA measurement to the next gives an estimate of the roughness
of the surface.

One of the most important tasks for MLA is to measure the depths of
craters that are near Mercury's north pole. Radar images of Mercury's
polar regions obtained as many as 20 years ago by radio telescopes on
Earth show that the floors of many of these craters contain material
that reflects radio waves very well (Figure 2). Many scientists believe
that these reflective polar deposits consist of water ice, but whether
this is the correct explanation remains to be proved. Because Mercury's
surface reaches temperatures as high as 450? Celsius (800? Fahrenheit),
this explanation may seem surprising. However, the floors of craters
near the poles are thought to be in permanent shadow, shielded from
sunlight throughout the Mercury day and year. This situation arises
because Mercury's axis of rotation is oriented nearly perpendicular to
the planet's orbit, so that sunlight strikes the surface near the poles
at a near-grazing angle. Because Mercury has no appreciable atmosphere,
these areas without sunlight remain extremely cold.

MLA will test whether these craters are sufficiently deep that the
floors are indeed in permanent shadow. Most of the craters are small,
however, and it is challenging to aim MLA with sufficient accuracy to
obtain a profile across the crater floor. The science team decided that
a promising approach would be to obtain as many laser tracks as possible
near the north pole and then to search for those measurements that fall
inside shadowed craters.

[Figure 2] Radar image of polar deposits near Mercury's north pole. The
radar-bright areas coincide with the floors of near-polar impact
craters. The arrow points to the crater crossed by the MLA profile
during its first operations at Mercury. The image is from J. K. Harmon,
M. A. Slade, and M. S. Rice, Icarus, 211, 37-50, 2011.

This plan turned out better than expected. On the very first pass, shown
in Figure 1, the laser track passed directly across a small, deep crater
with a floor that is highly reflective to radar, one of the candidate
locations for water ice (arrow in Figure 2). The low, blue part of the
track (arrow in Figure 1 inset) is the portion within the crater. The
crater floor displays the lowest elevations along the track and is
sufficiently deep for the floor to be permanently shadowed.

Throughout MESSENGER's one-year primary mission, many more MLA
measurements of floor depths of craters near the north pole are
expected. With these data, we will be able to test whether the imaged
locations of strong radar reflections always coincide with areas in
permanent shadow.

For more information on the Mercury Laser Altimeter (MLA), see
http://messenger.jhuapl.edu/instruments/MLA.html.

For more information on polar, radar-bright craters, and possible water
ice on Mercury, see http://messenger.jhuapl.edu/why_mercury/q5.html.
Received on Wed 27 Apr 2011 06:38:30 PM PDT