The Case for Spirit's Water
On the heels of the announcement that the Opportunity site on Mars was 'soaked' in water some time in its distant past, the rover teams have identified candidates for trace water at the Gusev crater site. "We're not talking about alot of water," said Dr. Ray Arvidson, the Deputy Principal Investigator and a geologist at Washington University at St. Louis. "The rock to water ratio would be huge [at Spirit] relative to what we see at Opportunity."
|Ray Arvidson, deputy principal investigator for the MER missions. Top banner shows the heat shield debris that is about a half mile from the current Opportunity crater
Credit: Washington University, St. Louis
The new findings center on a 60 cm tall rock called Humphrey that was found on the way from Spirit's landing site towards a crater and eventually towards the far hills to the east. "At Humphrey rock, we did an extended RAT," said Arvidson, referring to the diamond grinding that is done with the robotic arm's rock abrasion tool, or RAT instrument. "We ground a 2.1 mm hole and the dark area around grinding is dust. This look at the interior [because it is so flat], makes geologists very happy. From that flat surface, we see dark specks and hollows. The specks are igneous in a matrix of glass. The bright areas are vugs [or hollows]," said Arvidson.
Rewriting Textbook Science on Mars
"On Earth we would say [Humphrey] is a volcanic rock with a little bit of aqueous flow through it. The minerals formed right after volcanic activity..." continued Arvidson. "We're not talking about gobs of water, but it demonstrates that when rocks are made on Mars, that fluid can run through them."
|Humphrey rock, Spirit site, shows a borehole from RAT tool. The rock is volcanic, but the white mineralization and some hollows point to a late stage crystallization in the presence of some magma interaction with water or other fluids.
"This section will be in every textbook for the next 10 years," said Arvidson.
The unique views below the dust layers on martian rocks have not been possible on any previous trip to Mars. By grinding into the rock, geologists can not only get a cleaner view of the mineral grains and compositions, but also test their mechanical strength. According to Stephen Gorevan, a scientist with Honeybee Robotics and the rock abrasion tool team, "The strength of this [Humphrey] rock is about that of Adirondack [another rock at Gusev that was drilled into]. Measuring the energy it takes to cut this rock, we conclude that Humphrey is not a strong rock. Somewhere between a limestone and weak basalt."
Contrasting the amount of water that may have been at Meridiani, the Opportunity site, and Gusev, the Spirit site, Arvidson concluded: "The rocks at Opportunity are sedimentary rocks, shot through by corrosive water, and alot of minerals were replaced, and sulfates precipitated into the pores."
"The rock Humphrey is definitely a volcanic rock, from crystallization by magma," said Arvidson. "The tantalizing thing is that water was present during last stages of crystallization, and may have been a wet magma or eroded later. We'll look for dark rocks, broken by mother nature to see deeper into the interior."
"The best bet may be that water was in the magma, at the last stages of fluids, that led to these white minerals that filled in the vugs. Probably water that came up with the magma. Or perhaps the magma interacting [with later fluid flows]," said Arvidson.
The First Hundred Days
As Spirit approached its second month and Opportunity neared the 40 day mark, both missions have logged a milestone for surface maneuvers on Mars. Between the two rovers, according to Matt Wallace, JPL's mission manager, this has been a "terrific 100 days on Mars. From an engineering side, we have hit our stride. We are now not just reacting, but can look farther out [in our mission plans]. We are starting to think about software upgrades, optimization and extended mission concepts. Both rovers are healthy, and doing some impressive driving."
|Humphrey rock is about 60 cm tall, and shows the evidence of a Mickey Mouse like pattern from the rock drill. The locations shown indicate where the flat cross-section could be found and possible water flow remodelling of the rock's interior may have shaped the hollows and whiter mineralizations with magma
"Both vehicles are doing alot of robotic arm work, and these are becoming increasingly challenging," summarized Wallace. "The next targets [for Opportunity] are Berry Bowl and Slickrock. Eventually moving over to Last Chance rock, which is in the Big Bend area at Meridiani. Already at places, we have taken 114 microscopic images, requiring over 400 commands, and 200 robotic arm [IDD, or instrument deployment device] positions to get that imagery at the Last Chance [rock, on the borders of the crater's outcrop]."
The next few weeks on the other side of the planet, near the Spirit driving area, a high-value science target is looming increasingly large on the horizon. "Within about a dozen sols," said Arvidson, "we will be at the crater rim, and decide if there are exciting scientific finds there. Then head for the far hills."
Red Crystals of Hematite?
One intriguing result from the grinding experiments has been the large amount of fine red dust kicked up by drilling action at the Opportunity site. The landing location at Meridiani was chosen primarily because orbital observations revealed a locally high concentration of the water-forming mineral hematite. Hematite comes in at least two main types, the coarser grey hematite crystals, and what many believe are finer, red hematite crystals.
One view of the fine red dust or tailings left by the RAT experiments has centered on whether they are the result of cutting what scientists euphemistically call 'blueberry-like' spheres. When the rock drill strikes embedded berries, the tiny BB-like balls may be disintegrating into their inner red dust, according to one theory that would provide a key indicator of where some of the red crystalline hematite may be lodged.
Cornell's lead scientist for the panoramic camera system, Dr. Jim Bell, noted that at the hematite-rich, Meridiani region, "The top portion of El Capitan is a rock about the size of the rover wheels, or a loaf of bread. These are tabular, light-colored rocks found to be very sulfur-rich, with linear features like mineral casts. [The RAT holes create] alot of dust. The tailings and dust look like a tiny impact crater. The center region is free from these tailings and has a similar texture extending down, this 'vuggy', broken up texture."
"We may have very good evidence for fine grain, red hematite crystals," noted Bell. "This is not the gray, coarse hematite elsewhere. If the berries are hematitic, the red dust or the tailings may be created from shaving the berries. We want to test that hypothesis."
"One of the ideas was to dig into laminations. About these dark red areas we have alot of questions. Are these intrinsic to the grains or is RAT throwing dust into the hole?"
Both sites on Mars were chosen because they were relatively flat, free from large rocks to ease driving and not too dusty. The presence of possible water-bearing minerals at Meridiani and a crater lake at Gusev primed the science team to look closer. Actually since Mars is so dusty, picking these sites and including grinding tools became a mission goal. Little however is known about the details of what makes up the dust on Mars.
|Magnetic dust collection experiments Credit: JPL/NASA
One preliminary finding reported from an experiment to test the dust is now taking shape. Both rovers are equipped with strong and weak magnets designed to sort and analyze the magnetic properties of martian dust.
Morten Madsen, a science team member from the Center for Planetary Science, in Copenhagen, said "The atmosphere of Mars is quite dusty. We don't know much about this dust. It is about 2 microns in diameter [or about a tiny fraction of the width of a human hair], but we don't know what minerals are composing them. That is one reason we have magnets. The dust could have been formed by volcanic ash. It could be like the soil, or it may have been produced by precipitation in liqud water."
"The capture magnets," said Madsen, referring to the strongest magnet on the rover, "gets all the dust that is blowing in the wind. Other magnets get rid of weaker magnetic particles but keep only the strongly magnetic ones. The robotic arm will be able to get mineralogical data" of these dust particles.
|Opportunity's bounce path to find itself as a 'hole-in-one' shot inside a crater. According to Jim Bell, the full panorma shows about "100 degrees of azimuth. With a crater to the east we see features in the rim, maybe streaks, about 700 meters away as a potential drive target. The horizon is very flat, incredible for driving. We see the heat shield, less than a half meter tall, and a divot it made when it struck the ground [see banner image]. That is about half a mile, 800 meters, or a nice brisk walk, to see the materials we've left behind. Just off the crater rim, there is a rock about 40 cm long, very dark, and we are not sure what it is made of. Very close we see a bounce mark. With the airbags we hit the only rock in the area!" The only rock anywhere on the extremely flat landscape shows a bounce mark nearby indicating that the airbags took a detour and landed in the crater. The contrast is enhanced, but the panorama also shows the impact mark just on the rim of the crater in the foreground.
One fascinating finding so far is that the dust particles, while much too fine to be imaged by the rovers' microscopes, can be resolved as aggregates. Apparently there is initial evidence that dust is capable of forming fine magnetic needles and agglomerates. Madsen said "Particles arriving on the magnets interact with each other. We see particles that can be resolved with [a future microscopic imager experiment]. They form agglomerates, even needles. We are excited to look at these structures with time and with APX and Mossbauer spectrometers to determine their composition."
The twin rovers operating on opposite sides of Mars are about to enter different phases, as Opportunity gears up for a drive just when Spirit settles into a crater. In the last few weeks, the main tasks have been flipped, as Opportunity explored its own crater and Spirit took on a long and increasingly rugged drive.
Bell summarized where the two missions stand in their timelines, by saying that for Opportunity, "We will continue to tour the crater, then head out. That will be kind of interesting, heading out across the plains, just as Spirit heads into its crater. So far we have obtained over 12,000 images, and everything is healthy and happy."
Related Web Pages
Mars Rovers, JPL
Spirit's Sol images and slideshow
Opportunity image gallery and slideshow
NASA's RATs Go Roving on Mars
Pancam- Surveying the Martian Scene
Alpha Proton X-ray Spectrometer