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Another milestone for the Mars Exploration Rovers: Spirit has been operating on Mars for 2000 sols, or Martian days. Who would have ever thought the rovers would last this long? But here they are, still going, um, pretty strong. Even though she’s got plenty of electrical power, Spirit is currently stuck in loose soil at her location, called Troy. But engineers are working hard to figure out how to set her free. Check out the latest on the efforts at the Free Spirit website.
This view of a rock called "Block Island," the largest meteorite yet found on Mars, comes from the panoramic camera (Pancam) on NASA's Mars Exploration Rover Opportunity. Credit: NASA/JPL
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One of the great things about the Mars Exploration Rovers is that we get to see these scrappy little vehicles ramble across the surface of Mars, and watch science in action. Case in point: the meteorite found by Opportunity, dubbed “Block Island.” Scientists are debating all sorts of things about this watermelon-sized rock. How old is it? What is it made of? Where could it have come from? But not only are we learning about this alien rock, we’re also learning about the Red Planet itself and its environmental history.
See below for a new 3-D version of Block Island created by Stu Atkinson.
3-D Block Island created by Stuart Atkinson.
Scientists calculate Block Island is too massive to have hit the ground without disintegrating unless Mars had a much thicker atmosphere than it has now when the rock fell. An atmosphere slows the descent of meteorites, and with today’s thin Martian atmosphere, this heavy rock would have plummeted to the surface.
Block Island is approximately 60 centimeters (2 feet) in length, half that in height, probably weighs about a half ton, and has a bluish tint that distinguishes it from other rocks in the area.
Opportunity found a smaller iron-nickel meteorite, called “Heat Shield Rock,” in late 2004. Block Island is roughly 10 times as massive as Heat Shield Rock and several times too big to have landed intact without more braking than today’s Martian atmosphere could provide.
“Consideration of existing model results indicates a meteorite this size requires a thicker atmosphere,” said rover team member Matt Golombek of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “Either Mars has hidden reserves of carbon-dioxide ice that can supply large amounts of carbon-dioxide gas into the atmosphere during warm periods of more recent climate cycles, or Block Island fell billions of years ago.”
Additional studies also may provide clues about how weathering has affected the rock since it fell.
“There’s no question that it is an iron-nickel meteorite,” said Ralf Gellert of the University of Guelph in Ontario, Canada. Gellert is the lead scientist for the rover’s alpha particle X-ray spectrometer, an instrument on the arm used for identifying key elements in an object. “We already investigated several spots that showed elemental variations on the surface. This might tell us if and how the metal was altered since it landed on Mars.” The triangular pattern of small ridges seen at the upper right in this image and elsewhere on the rock is characteristic of iron-nickel meteorites found on Earth, especially after they have been cut, polished and etched.
The microscopic imager on the arm revealed a distinctive triangular pattern in Block Island’s surface texture, matching a pattern common in iron-nickel meteorites found on Earth.
“Normally this pattern is exposed when the meteorite is cut, polished and etched with acid,” said Tim McCoy, a rover team member from the Smithsonian Institution in Washington. “Sometimes it shows up on the surface of meteorites that have been eroded by windblown sand in deserts, and that appears to be what we see with Block Island.”
Spectrometer observations have already identified variations in the composition of Block Island at different points on the rock’s surface. The differences could result from interaction of the rock with the Martian environment, where the metal becomes more rusted from weathering with longer exposures to water vapor or liquid.
“We have lots of iron-nickel meteorites on Earth. We’re using this meteorite as a way to study Mars,” said Albert Yen, a rover team member at JPL. “Before we drive away from Block Island, we intend to examine more targets on this rock where the images show variations in color and texture. We’re looking to see how extensively the rock surface has been altered, which helps us understand the history of the Martian climate since it fell.”
When the investigation of Block Island concludes, the team plans to resume driving Opportunity on a route from Victoria Crater, which the rover explored for two years, toward the much larger Endeavour Crater. Opportunity has covered about one-fifth of the 19-kilometer (12-mile) route plotted for safe travel to Endeavour since the rover left Victoria nearly a year ago.
The bright streak is the star Canopus. Credit: NASA/JPL
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When your rover has abundant energy but can’t go anywhere, what’s a scientist to do? How about making observations of the evening and night skies on Mars? With the benefit of a boost in electrical power from a wind gust cleaning off her solar panels, the Spirit rover has more energy available than she’s had for a couple of years. But unfortunately, Spirit is stuck in a patch of loose soil in the Home Plate region on Mars. While the engineers at JPL work hard at figuring out how to “Free Spirit” (see the new website dedicated to their efforts) scientists are making observations of her surroundings to aid in the effort to get her out. But there’s also enough power to do additional observations, and astronomy was a logical choice. “Certainly, a month or more ago, no one was considering astronomy with the rovers,” said Mark Lemmon, planetary scientist at Texas A&M University and member of the rover team. “We thought that was done. With the dust cleanings, though, everyone thinks it is better to use the new found energy on night time science than to just burn it with heaters.” Besides, Lemmon added, using all the energy in the daytime might lead to overheating.
The image above was taken on Spirit’s sol 1943 (June 22 on Earth)showing the night sky above her location.
But most of the “stars” in this raw image are not really stars, just hot pixels. “We use long and multiple exposures to make stars stand out,” Lemmon told Universe Today. “We can only see bright stars, looking through the dust, but can pick out most of the major stars in Orion for instance.”
But a star is visible in this image. “That streak in the 1943 images is the bright star Canopus,” said Jim Bell, planetary scientist at Cornell University and lead for the rovers’ Pancam team. “We’re monitoring stars to search for evidence of night-time clouds, fog, and hazes. We’re also occasionally trying to image Earth and Venus as they set in the west after sunset. We’ve had some success, but the twilight sky is so bright we’re still working on tweaking the exposure times.”
Of course, this isn’t the first time Spirit has done astronomy on Mars. She also made night sky observations back in 2005. In an article Bell wrote for Sky and Telescope in 2006 he described Spirit’s astronomy as “stone-knives and bear-skins backyard astronomy–but from Mars!” And certainly, this is exciting to have an additional opportunity to make astronomical observations from the surface of another world. Spirit's twilight observations from sol 1947. Credit: NASA/JPL
Bell added that the current astronomy campaign with Spirit has many similarities with the one four years ago, and Lemmon said they are focusing on a few different goals for looking at the twilight and night skies.
“The Canopus images may become a regular occurrence, as a way to monitor dust and/or ice in the sky at night–much as we use Sun images in the day,” Lemmon said. “For something like that, we can pick an aim (Canopus, Orion, etc.) and choose filters. We might use color filters to look for any differences that show up, or the clear filter for the most sensitive measurement. Star exposures can go up to 5.5 minutes (compare to 0.1-0.5 sec for a normal day image). We cannot track stars, so they trail after 10 seconds or so–as you see Canopus doing. In longer exposures, hot pixels and cosmic rays show up as points or cluster of light.”
Lemmon said attempting to image Earth and Venus has been challenging. “We’ve imaged both before, farther from the Sun. They are in the twilight, limiting the exposure we can use, and they are in a “bright” part of the sky.”
Lemmon added his personal favorite right now is actually the twilight imaging — not looking at stars but at how fast the twilight glow fades after sunset. “That is proving to be quite helpful in terms of understanding the distribution of dust in the atmosphere –which is closely tied to how weather works on Mars,” he said.
In 2005, the Pancam team was able to capture images of Mars’ two moons, Phobos and Deimos. “They are much brighter and let us use more filters if desired. We may pick this up again. I’m a fan of eclipse imaging, so we would need several quick images to see how fast the moon fades as sunlight is blocked by dust around Mars.”
The moons should start becoming more visible soon, and Lemmon said they will continue to take more images of Canopus and maybe other star fields. The team is not specifically looking for meteors or the orbiters around Mars, but there’s always the prospect of something fascinating showing up on future images.
“We’ve taken some recent images I hope will have new, interesting things in them,” Lemmon said. “But they are still on board the rover so we’ll have to wait and see what they show later.”
Soft soil exposed when wheels of NASA's Mars Exploration Rover Spirit dug into a patch of ground. Image Credit: NASA/JPL-Caltech/Cornell University
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The Spirit rover on has been stuck in Martian soil, going nowhere, but that doesn’t mean she hasn’t been busy or hasn’t been making new discoveries. Just the opposite. Scientists have taken advantage of what could be a bad situation to learn more about the Red Planet’s environmental history. “By serendipity, Troy (the region where Spirit is stuck) is one of the most interesting places Spirit has been,” said Ray Arvidson, deputy principal investigator for the science payloads on the two Mars rovers. “We are able here to study each layer, each different color of the interesting soils exposed by the wheels.”
While the rover team remains optimistic about getting Spirit unstuck, they have also acknowledged the possibility that the rover might not ever be able to leave Troy. But engineers at JPL are pulling out all the stops, and will be conducting test at the Mars Yard with the engineering rover. (More on that below.)
But Spirit is also benefiting from increased power from wind events in April and May that blew away most of the dust accumulated on the rover’s solar panels. “The exceptional amount of power available from cleaning of Spirit’s solar arrays by the wind enables full use of all of the rover’s science instruments,” said Richard Moddis of the Johnson team. “If your rover is going to get bogged down, it’s nice to have it be at a location so scientifically interesting.”
While engineers are working on a plan for getting Spirit extracted from her predicament, scientists have been studying images and data the rover has sent back. One of the rover’s wheels tore into the site, exposing colored sandy materials and a miniature cliff of cemented sands. Some disturbed material cascaded down, evidence of the looseness that will be a challenge for getting Spirit out. But at the edge of the disturbed patch, the soil is cohesive enough to hold its shape as a steep cross-section. Close-up view of the soil at Troy from Spirit's Microscopic imager (sol 1945) Credit: NASA/JPL
Spirit has been using tools on its robotic arm to examine tan, yellow, white and dark-red sandy soil at Troy. Stretched-color images from the panoramic camera show the tints best.
“The layers have basaltic sand, sulfate-rich sand and areas with the addition of silica-rich materials, possibly sorted by wind and cemented by the action of thin films of water. We’re still at a stage of multiple working hypotheses,” said Arvidson. “This may be evidence of much more recent processes than the formation of Home Plate…or is Home Plate being slowly stripped back by wind, and we happened to stir up a deposit from billions of years ago before the wind got to it?”
Water seems to have played a role in the different colors seen at the location, and perhaps differences in tints at Troy observed by the panoramic camera may come from differences in the hydration states of iron sulfates.
Meanwhile back on Earth, the rover team has developed a soil mix for testing purposes that has physical properties similar to those of the soil under Spirit at Troy. This soil recipe combines diatomaceous earth, powdered clay and play sand. A crew is shaping a few tons of that mix this week into contours matching Troy’s. The test rover will be commanded through various combinations of maneuvers during the next few weeks to validate the safest way to proceed on Mars.
Spirit’s right-front wheel has been immobile for more than three years, magnifying the challenge. Diagnostic tests on Spirit in early June provided encouragement that the left-middle wheel remains useable despite an earlier stall.
“With the improved power situation, we have the time to explore all the possibilities to get Spirit out,” said JPL’s John Callas, project manager for Spirit and Opportunity. “We are optimistic. The last time Spirit spun its wheels, it was still making progress. The ground testing will help us avoid doing things that could make Spirit’s situation worse.”
Image from Spirit's front hazcam from sol 1940. Credit: NASA/JPL
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Here’s a rover update: Spirit remains stuck in her location on the west side of Home Plate, and work continues at JPL for testing on how to extract the rover from being embedded in soft soil. A rock may be underneath Spirit, keeping her from moving, but more images are being taken by the microscopic camera at the end of the robotic arm to try and determine exactly what is going on under and around the rover. But with a boosted power supply, Spirit has also been busy making scientific observations of her surroundings. And one more thing, which would be extremely fun, rover driver Scott Maxwell hinted on Twitter that Spirit has so much power now from a recent wind event that cleared off her solar panels that she may attempt to make overnight observations. So stay tuned for PANCAM images of the Martian night sky!
As of Sol 1932 (June 9, 2009), Spirit’s solar array energy production is at 828 watt-hours. Total odometry remains at 7,729.93 meters (4.80 miles).
Meanwhile over on the other side of the planet, Opportunity continues to drive south on the way to Endeavour crater. On Sol 1906 (June 4, 2009), the rover completed a 69-meter (266-foot) drive due south. Elevated actuator currents with the right-front wheel continue to cause concern. On Sol 1910 (June 8, 2009), the planned drive stopped early because a multi-wheel current limit threshold was exceeded. A diagnostic maneuver on the next sol was successful indicating the cause on the previous sol was due to the elevated right-front wheel motor currents. The view from Opportunity on sol 1912. Credit: NASA/JPL
A long, backward drive was performed on Sol 1912 (June 10, 2009). Driving backwards is one technique to mitigate the elevated wheel currents. However, wheel currents continued to be elevated after that 72-meter (236-foot) drive. Further resting of the rover’s actuators is being considered.
The plan ahead includes opening the shroud of the miniature thermal emission spectrometer (Mini-TES) to expose the instrument’s dust-contaminated elevation mirror to the environment. This is an attempt to allow the wind environment to clean dust off the mirror.
As of Sol 1912 (June 10, 2009), Opportunity’s solar array energy production is 431 watt-hours. Opportunity’s total odometry is 16,569.05 meters (10.3 miles).
Carina Nebula. Image credit: Hubble Space Telescope/NASA.
When you look at the amazing pictures captured by the Hubble Space Telescope, or the Mars Exploration Rovers, do you ever wonder: is that what you’d really see with your own eyes? The answer, sadly, is probably not. In some cases, such as with the Mars rovers, scientists try and calibrate the rovers to see in “true color,” but mostly, colors are chosen to yield the most science. Here’s how scientists calibrate their amazing instruments, and the difference between true and false colors.
So, to start off, let’s put this in the form of a true or false question: T or F: When we see the gorgeous, iconic images from Hubble or the stunning panoramas from the Mars rovers, do those pictures represent what human eyes would see if they observed those vistas first hand?
Answer: For the Hubble, mostly false. For the rovers, mostly true, as the rovers provide a combination of so-called “true” and “false” color images. But, it turns out, the term “true color” is a bit controversial, and many involved in the field of extraterrestrial imaging are not very fond of it.
“We actually try to avoid the term ‘true color’ because nobody really knows precisely what the ‘truth’ is on Mars,” said Jim Bell, the lead scientist for the Pancam color imaging system on the Mars Exploration Rovers (MER). In fact, Bell pointed out, on Mars, as well as Earth, color changes all the time: whether it’s cloudy or clear, the sun is high or low, or if there are variations in how much dust is in the atmosphere. “Colors change from moment to moment. It’s a dynamic thing. We try not to draw the line that hard by saying ‘this is the truth!'”
Bell likes to use the term “approximate true color” because the MER panoramic camera images are estimates of what humans would see if they were on Mars. Other colleagues, Bell said, use “natural color.”
Zolt Levay of the Space Telescope Science Institute produces images from the Hubble Space Telescope. For the prepared Hubble images, Levay prefers the term “representative color.”
“The colors in Hubble images are neither ‘true’ colors nor ‘false’ colors, but usually are representative of the physical processes underlying the subjects of the images,” he said. “They are a way to represent in a single image as much information as possible that’s available in the data.”
True color would be an attempt to reproduce visually accurate color. False color, on the other hand, is an arbitrary selection of colors to represent some characteristic in the image, such as chemical composition, velocity, or distance. Additionally, by definition, any infrared or ultraviolet image would need to be represented with “false color” since those wavelengths are invisible to humans.
The cameras on Hubble and MER do not take color pictures, however. Color images from both spacecraft are assembled from separate black & white images taken through color filters. For one image, the spacecraft have to take three pictures, usually through a red, a green, and a blue filter and then each of those photos gets downlinked to Earth. They are then combined with software into a color image. This happens automatically inside off-the-shelf color cameras that we use here on Earth. But the MER Pancams have 8 different color filters while Hubble has almost 40, ranging from ultraviolet (“bluer” than our eyes can see,) through the visible spectrum, to infrared (“redder” than what is visible to humans.) This gives the imaging teams infinitely more flexibility and sometimes, artistic license. Depending on which filters are used, the color can be closer or farther from “reality.”
Stone mountain rock outcrop in true and false colour. Image credit: NASA/JPL
The same rock imaged in true and false color by Opportunity.
In the case of the Hubble, Levay explained, the images are further adjusted to boost contrast and tweak colors and brightness to emphasize certain features of the image or to make a more pleasing picture.
But when the MER Pancam team wants to produce an image that shows what a human standing on Mars would see, how do they get the right colors? The rovers both have a tool on board known as the MarsDial which has been used as an educational project about sundials. “But its real job is a calibration target,” said Bell. “It has grayscale rings on it with color chips in the corners. We measured them very accurately and took pictures of them before launch and so we know what the colors and different shades of grey are.”
One of the first pictures taken by the rovers was of the MarsDial. “We take a picture of the MarsDial and calibrate it and process it through our software,” said Bell. “If it comes out looking like we know it should, then we have great confidence in our ability to point the camera somewhere else, take a picture, do the same process and that those colors will be right, too.”
Hubble can also produce color-calibrated images. Its “UniverseDial” would be standard stars and lamps within the cameras whose brightness and color are known very accurately. However, Hubble’s mission is not to produce images that faithfully reproduce colors. “For one thing that is somewhat meaningless in the case of most of the images,” said Levay, “since we generally couldn’t see these objects anyway because they are so faint, and our eyes react differently to colors of very faint light.” But the most important goal of Hubble is produce images that convey as much scientific information as possible.
The rover Pancams do this as well. “It turns out there is a whole variety of iron-bearing minerals that have different color response at infrared wavelengths that the camera is sensitive to,” said Bell, “so we can make very garish, kind of Andy Warhol-like false color pictures.” Bell added that these images serve double duty in that they provide scientific information, plus the public really enjoys the images.
And so, in both Hubble and MER, color is used as a tool, to either enhance an object’s detail or to visualize what otherwise could not be seen by the human eye. Without false color, our eyes would never see (and we would never know) what ionized gases make up a nebula, for example, or what iron-bearing minerals lie on the surface of Mars.
As for “true color,” there’s a large academic and scholarly community that studies color in areas such as the paint industry that sometimes gets upset when the term “true color” is used by the astronomical imaging group, Bell explained.
“They have a well-established framework for what is true color, and how they quantify color,” he said. “But we’re not really working within that framework at that level. So we try to steer away from using the term ‘true color’.”
Levay noted that no color reproduction can be 100% accurate because of differences in technology between film and digital photography, printing techniques, or even different settings on a computer screen. Additionally, there are variations in how different people perceive color.
“What we’re doing on Mars is really just an estimate,” Bell said, “it’s our best guess using our knowledge of the cameras with the calibration target. But whether it is absolutely 100% true, I think it’s going to take people going there to find that out.”
For more information see http://hubblesite.org/ or check out Jim Bell’s 2006 book “Postcards From Mars.”
