Martian Silk and Sandpaper
Pasadena, Spirit Mission Sol 13
There was nothing unlucky about the thirteenth day on Mars. Rover scientists got their first chance to stretch Spirit’s tool-carrying arm, while taking the highest resolution image ever on another planet.
|This close-up look at a patch of martian soil is the sharpest image ever taken of another planet. The picture was captured by the microscopic imager located on the Mars Exploration Rover Spirit’s instrument deployment device, or "arm." The actual size of the patch is a section of the field of view which was 3 centimeters (1.2 inches) across.
The tool-carrying arm is called the Instrument Deployment Device (IDD) because it houses four diagnostic instruments used to answer question about Mars’ water history. The arm has a rotating head or ‘turret’, that can place various instruments into action against a rock, soil, or boulder. The mission team is transitioning to a science schedule where tomorrow’s activities are governed by results and findings from the previous exploration day, in what is considered one of the most flexible and modular operations tried yet in plantary science planning.
The earliest test on Friday looked at martian soil using the equivalent of a geologists’ hand lens, a common way that rock hunters sample an alien landform on Earth. The science teams used this Microscopic Imager on its robotic arm to "get focus", said Dr. Ray Arvidson of Washington University in St. Louis, deputy principal investigator for the science instruments on Spirit and on its twin Mars Exploration Rover, Opportunity, "and move laterally [to get two images of the same spot up close]. This gives a stereo pair", useful for three-dimensional visualization.
"To a geologist in a new place," said Arvidson,"you get a rock, pull out a hand lens, and look at sizes and shapes of rock grains. On Mars, the microscopic imager will take pictures with each pixel equal to thirty microns [half the width of a human hair]. Those shapes and grain sizes can diagnose rounded grains for an aqueous mineral or sharp grains for ejecta fragments [if the rock was part of crater debris]." Arvidson was enthusiastic about the first chance to see Mars so close, "down and personal in the dirt."
After making sure both the arm and the Microscopic Imager were working well, geologists and soil scientists wanted to find out ‘what bonds soil grains on Mars?’, said Rob Sullivan, a project science team member. The images supplement a picture of cohesive soil, and a weak, but crusty surface layer (duricrust) that may cover even finer dust below. The best analogy on Earth to such a duricrust may be how after a very cold snow, the thin, weak top layer can harden over powdery snow.
|A penny imaged by the Spirit rover during tests at JPL prior to launch. The top view shows the field of view for the Microscopic Imager (MI) camera and the bottom view shows the sharpest resolution, which identifies the tiny statue of Abraham Lincoln seated between the pillars of the Lincoln Memorial. The MI generally resolves about the width of a human hair, or 100 microns, and has been compared to a very good hand lens typically used by field geologists.
But on Mars, instead of snow at the equator, the team is looking at dust layers. "There are abundant fine particles," said Sullivan, and the microscopic imager can resolve sizes down to "the size of table salt, but not smaller." Sullivan is also looking at the rover’s wheel tracks to get a deeper picture of how the soil behaves when disturbed. "If you take your hand and cast a print in talcum powder, you get very high fidelity in the imprint. You might see the detailed grooves in a wedding ring, for instance. But the same hand cast in sand [or coarser material]" then the cast is not detailed. "The rover tracks have a high fidelity to their cast, the wheel cleats leave marks, with lots of pressed details. There is probably a mix of grain sizes…This is the first look at what the rover does under Martian gravity", about one-third the value of terrestrial gravity. "We see no problems with the rover wheels’ sinking," said Sullivan.
"There are various theories about what causes the martian soil to be cohesive," said Ken Herkenoff, science lead on the Microscopic Imager and a geologist at the US Geological Survey (USGS). The duricrust is a weakly cemented layer, described Herkenoff and if soil is "drying out, if evaporation occurs, this thin layer may form. …There are theories that larger particles are form by electrostatic cohesion, or that salts and carbonates may cement grains together. The cohesion seen so far is consistent with small grains forming larger particles, with pits through less cohesive layering. We can’t prove it yet," said Herkenoff, who looks to "the other IDD instruments to confirm what is the elemental chemistry and iron mineralogy of that layer".
"This is by far the highest resolution picture ever taken on another planet," said Herkenoff. Resolution is just beyond the width of a human hair, or 100 microns, like "table salt, or sand sized agglomerates. But we don’t try to resolve fine dust", about 20 to 100 times finer at 1-5 microns.
The rover’s weekend schedule is busy scientifically.
Flight Director, Jessica Collison said Friday that the rover is "ready for Mossbauer placement tomorrow", another instrument on the arm turret to measure the iron minerology of the same soil spot photographed by the microscopic imager at high resolution. The Mossbauer will be "first contact with the martian soil" by the science instruments, as it is placed lightly on the surface and runs for four hours integrating soil spectra.
This weekend introduces a feature of the rover, wherein its deployment arm works while the rest of the rover sleeps at night. "We will rotate the turret," said Collison, "then use the Alpha Proton X-ray Spectrometer (APXS)", another instrument to determine elemental chemical composition, "a few millimeters above the surface. APXS will work overnight [Saturday] on an integration, 20 hours in total, then retract and restow the arm. Mossbauer has a calibration target on the rover, and that test will run for about 15 minutes."
Collison said that the theme for current surface science is about "finding out what it means to be ‘out there’."
More data is expected to be available during overhead passes of relay satellites, since the team will begin using the rover’s high gain antennae "when it is cold in the morning", on Mars. Previous use of these rapid data transfers has been limited to only afternoon sessions, since the rover was locked up on its landing base. Now that the rover has six wheels in the dirt, the thermal environment can support more frequent uses of the high gain antennae for both morning and afternoon relay sessions.
In addition to the "big science weekend", Dr. Joy Crisp said preparations are busily proceeding for the Opportunity rover’s descent in 8 days. Crisp said a "nine-second burn was performed since the rover’s entry was about 100 kilometers beyond its targeted aim point." The planned trajectory correction gives Opportunity "a predicted 80 miles by 7 mile landing ellipse", on the other side of Mars, a target pointing to a grey, basaltic plain called Terra Meridiani. "There is little chance that we will do further maneuvers as things are looking good."