Tango, Bravo, Echo
Pasadena, Opportunity mission Sol 7
Like steps in an Arthur Murray dance class, the sequence call to the Opportunity rover commanded it to move: "….Tango, Bravo, Echo….". One object of this shorthand sequence was to get the rover into a new location "..Delta..", or position change on Mars. The Opportunity rover needed to drive about 3 meters from its landing perch and get all six wheels into martian soil for the first time.
|Based on pancam spectroscopy, darker material near the bedrock outcrop may show evidence of weathering, or potentially be gray hematite, a rust-like form of iron-oxide that can be associated to water or high-temperature oxidation of volcanic basalt. The white cross-hairs surround a prospective dark rock, and the square, cross-hair identifies the region in context. A test for the presence of another mineral called goethite is the currently favored way to distinguish water formation of iron-oxides. Scientists have referred to the bedrock as the ‘pot of gold’ geologically.
The one-way radio transmission time to send the command sequence was about 11 minutes from Earth to Mars, called the ‘one-way light-time.’
As was a tradition begun during the 1997 Pathfinder mission, the rover team receives a musical wake up call each Sol on Mars. Opportunity’s Sol 7 got this call in the form of a song, Bruce Springstein’s rock ballad, "Born to Run".
Around 8 PM Friday Pacific Standard Time, it was 8:45 local solar time (LST) on Mars, and the Sun was peaking over the crater rim to warm Opportunity’s solar panels for its most important engineering day since impact. Today was egress day.
Egress corresponds to the rover’s roll-off. Unlike the Spirit rover, this phase of Opportunity’s mission was optimally positioned since the landing platform is tilted eleven degrees downhill. A safety margin is built-in for tilts less than a much steeper 40 degrees. The rover landed already facing north-northwest towards its first science target, the bedrock outcrop 10 meters in front of its base location. When the egress was signalled as successful by the return of a single beep from Mars, mission control played the song by "The Who", entitled "Going Mobile".
Leading up to egress, one hour of the rover command sequence captured another set of pancam images from its higher vantage point while still atop the landing platform. The dance steps went in order, "science, egress, science", according to activity lead, Joel Krajewski. The last science was collected from photographing the rover’s wheel track in high-resolution so that soil properties’ scientists could use the texture marks to determine whether the Meridiani site locally is fine-grained and smooth. The dust texture of ‘kitchen flour’, was how science team member, Matt Golombek, speculated the soil might hold wheel imprints earlier. Finer dust will show high fidelity between the soil imprint and the rover’s six wheels.
|The ‘money shot’ for the engineering team. Hazard camera image of Opportunity facing its bedrock science target with wheels in the martian soil. Credit: NASA/JPL|
At the first briefing following the egress, Opportunity scientist Phil Christensen of Arizona State said, "We saw the mineral hematite from orbit, and we went to Meridiani to find it on the surface. I’m pleased to announce that indeed, the mini-TES (infrared) camera has found hematite at Meridiani".
"Honestly, I am not surprised about the hematite signature, but many of my colleagues seem to be", said Christensen. "One of the most exciting experiments so far is: ‘where is the hematite, in the bright soil?’. We don’t see hematite in the airbag bounce mark, so we’re going to have alot of fun finding it. The hematite would then appear to be in the coarse-grain, dark material… Hematite is not in combination with quartz, it is not in carbonates. We’re in the right place. We hit the 350 kilometer hematite region [about the size of Oklahoma], and I’ll leave the rest to the engineers [to explain]."
"One of my worst fears was that the hematite would be in the fine dust, which would make it difficult to track its source," said Christensen. "I believe the hematite in the coarse-material will help us trace it back to where it started, to its origin. I believe it is a layer coating the bedrock. The view from orbit shows this entire area is blanketed in hematite, and not wind-blown from somewhere else. [Hematite] being in that coarse material is a huge first step."
"The spectrum of hematite can have a high-temperature or a low-temperature origin," explained Christensen. "What we’ve found so far fits best to low-temperature formation."
|Pancam color calibration target with its four corners (called ‘color-chips’) which are matched to mineral-like colors predicted to be found on Mars
Polling Egress Subsystems
The complex inner working of the rover can best be understood from terse conversation during the final minutes prior to initiating egress, when the Flight Director polls each station for a status health check.
Flight Director: Begin status poll. Flight software?
When the command sequence to egress is sent to the rover, the phrase from the Flight Director is "You are free to radiate."
Over the mission control communication loop was heard a last request to simulate the egress one more time: "I’m thinking of moving down to the testbed and checking out a few things."
"A way to form hematite is precipitation in water at low temperatures. That’s consistent with the data, that is one scenario. The other is to take magnetite, a volcanic material, and convert it to hematite at high-temperature. I hope Mossbauer will help us see granules of either magnetite or goethite"–a more clear indicator of high vs. low temperature formation.
"There’s ground-water opportunities," summarized Christensen. "There’s hydrothermal opportunities… [to form hematite] may be weathring of volcanic ash to iron oxide. What we’ve determined today is that hematite is there, that it ‘is where it belongs’."
While most mission milestones are reported as ‘firsts’, this particular part of Opportunity’s mission life is celebrated for its ‘seconds’. It is the second time a rover has completed what engineers consider the ‘most complex remote maneuver ever tried on another planet’. It is the only time to have two rovers actually on the surface of Mars. It is the second time that images could be relayed at high transfer rates (128 megabits per second) via a constellation of orbiters.
|The infrared Mini-Thermal Emission Spectrometer (Mini-TES), and pancam that performs color spectroscopy from images alone with 13 filters used to identify composition for rocks and soil. Below, pancam filters number 13, in a right and left view separated by approximately 30 cm to give stereo views, distance measurements, and mineral compositions.
During the traditional lunch-time (on Mars local solar time) lecture given by the rover science team to mission control, Cornell graduate student Jason Soderblom explained how to find rock and soil features using the panoramic camera alone. Soderbom is part of a father-son team working on the science team, as his father, Larry Soderblom, is a US Geological Science colleague.
The title of the lunchtime briefing was "Spectroscopy with Pancam", and was introduced as how to use color filters to determine what minerals on Mars are present in photographic view. Soderblom began his lecture by referring to an internet debate on the website, Slashdot, about why some rover images appear rich in blue and pink shades. Why is the a blue color showing up pink? That online debate started when a blue NASA logo, called the NASA ‘meatball’, first appeared red when imaged on Mars.
The pancam has 13 color filters, ranging in visible shades from blue to near-infrared wavelengths longer than the human eye can perceive. To construct a true color picture of Mars, Soderblom explained, the pancam employs 3 of these filters primarily, labeled L3, L4 and L5, for red, green and blue. Because local illumination can affect comparison between different viewing angles and different times of day during the mission, the camera’s field of view is calibrated against known target colors.
The four corners of the calibration target, explained Soderblom, are called color-chips. These calibration colors were specially selected to correspond to colors predicted as certain mineral types on the two martian sites, one in a crater-lakebed called Gusev and the other in an iron-rich volcanic plain called Meridiani. The "rocks on Mars are wine-red because of their mostly rich in iron-bearing minerals," said Soderblom. "The four corner, color chips cover a red type of hematite, cobalt (a blue mineral), chromium (green) and yellow (the mineral, goethite)."
These color filters are so narrow (around 20 nanometers) spectrally, that to see an intensity peak in a panoramic image can translate to a particular rock or soil region that is very rich in some mineral types, explained Soderblom. By scanning the horizon between red and near-infrared (400 nanometers, the lower blue limit visible to the human eye, to 1009 nanometers, well-beyond the 700 nanometer red limit of human vision), pancam images shot with this color wheel can perform spectroscopy from a single picture frame. "We perform spectroscopy from of all things, a panoramic camera. …Our minds are doing spectroscopy in subtle ways all the time."
"When you see a strange color in an uncalibrated image, no actual information is lost," said Soderblom, "but a filter may be used. Like red-green colorblindness, colors can be exchanged however."
Primary filters are used to hunt for four major mineral types, called hematite, goethite, augite and pigeonite. A good test of whether Mars once had water may depend on whether hematite, a iron-bearing oxide, is found in combination with goethite, another iron-oxide similar to rust. This combination will be the ‘smoking gun’ evidence, if found, of ancient water on Mars.
Soderblom pointed out to the mission control engineers some of the early hypotheses that can be derived from just a single pancam frame of the bedrock.
"There is so much information in a single frame. The plains just beyond the crater rim have a sheen, which we believe is a photometric trick, like a road that is shimmering at certain times of day. That is a question the team is discussing, what is that grey area beyond the crater."
When the rover’s airbag "disturbed the soil, the pancam images shows more red. With color filters, that [soil] is actually less bright in blue, so it appears dark red," said Soderblom.
As shown in the banner image and top right inset view of the bedrock outcrop, Soderblom explained is found "one of the most interesting things ever seen on Mars. The dark rocks are nearly black when seen in a pancam filter between 800-900 nanometers. This dark spot may be a first view of grey hematite. The mini-TES infrared may pick up crystalline hematite."
The day for Opportunity fulfilled all of the verbal commands sent from Earth to Mars, "…tango, bravo, echo, and finally a delta."
Related Web Pages
Mars Exploration Rovers
Spirit Condition Serious
Pancam– Surveying the Martian Scene
Alpha Proton X-ray Spectrometer
Mars Rover: The Owner’s Manual