Riding into a Martian Sunset

Dust gradually obscures the Sun during a blue-sky martian sunset seen in a sequence of newly processed frames from NASA’s Mars Exploration Rover Opportunity. "It’s inspirational and beautiful, but there’s good science in there, too," said Dr. Jim Bell of Cornell University, Ithaca, N.Y., lead scientist for the panoramic cameras on Opportunity and its twin, Spirit.

A color image of the region known as Charlie.
Image Credit: NASA/JPL

The Opportunity rover captured a still image (top left) from the rover’s panoramic camera showing the Sun just over the horizon. The rover is looking to the southwest. The top right image shows a similar view from the 1997 Pathfinder camera. The amount of dust indicated by Opportunity’s observations of the Sun is about twice as much as NASA’s Mars Pathfinder lander saw from another site on Mars.

In addition to its scientific importance for understanding martian dust and atmospheric effects, sunset on Mars was ranked in Astrobiology Magazine’s reader polls as the most impressive image that could be returned ranked after some evidence for fossil biology itself.

The rapid dimming of the Sun near the horizon is due to the dust in the sky. Thehigher dust content compared to the Pathfinder image causes the Sun to be many times fainter. The sky above the Sun has the same blue tint observed by Pathfinder and also by Viking, which landed on Mars in 1976. This is because dust in the martian atmosphere scatters blue light forward toward the observer much more efficiently than it scatters red light forward. Therefore, a "halo" of blueish sky color is always observed close to the Sun. Only half of this halo can be seen in this image because the other half is below the horizon.

A color image of the region known as El Capitan.
Image Credit: NASA/JPL

The sunset clip uses several of the more than 11,000 raw images that have been received so far from the 18 cameras on the two Mars Exploration Rovers. The Opportunity image is only approximate true color, using infrared, green and violet filters, rather than the human red-green-blue, so that the maximum panoramic camera wavelength range could be covered by the observations, enhancing the scientific value of the measurements. At this week’s JPL press briefing, Bell showed other pictures that combine information from multiple raw frames.

A patch of ground about half the area of a coffee table, imaged with the range of filters available on Opportunity’s panoramic camera, has soil particles with a wide assortment of hues — "more spectral color diversity than we’ve seen in almost any other data set on Mars," Bell said.

Opportunity is partway through several days of detailed observations and composition measurements at a portion of the rock outcrop in the crater where it landed last month. It used its rock abrasion tool this week for the first time, exposing a fresh rock surface for examination. That surface will be studied with its alpha particle X-ray spectrometer for identifying chemical elements and with its Mössbauer spectrometer for identifying iron-bearing minerals. With that rock-grinding session, all the tools have now been used on both rovers.

A panoramic image of the Opportunity landing site at Meridiani Planum, showing the outcrop and landing petal.
Image Credit: NASA/JPL

Dr. Ray Arvidson of Washington University, St. Louis, deputy principal investigator for the rovers’ science work, predicted that in two weeks or so, Opportunity will finish observations in its landing-site crater and be ready to move out to the surrounding flatland. At about that same time, Spirit may reach the rim of a larger crater nicknamed "Bonneville" and send back pictures of what’s inside. "We’ll both be at the rims of craters," he said of the two rovers’ science teams, "one thinking about going in and the other thinking about going out onto the plain."

Not counting occasional backup moves, Spirit has driven 171 meters (561 feet) from its lander. It has about half that distance still to go before reaching the crater rim. The terrain ahead looks different than what’s behind, however. "It’s rockier, but we’re after rocks," Arvidson said.

Spirit can traverse the rockier type of ground in front of it, said Spirit Mission Manager Jennifer Harris of JPL. As it approached the edge of a small depression in the ground earlier this week, the rover identified the slope as a potential hazard, and "did the right thing" by stopping and seeking an alternate route, she said.

However, engineers are also planning to transmit new software to both rovers in a few weeks to improve onboard navigation capabilities. "We want to be more robust for the terrain we’re seeing," Trosper said. The software revisions will also allow engineers to turn off a heater in Opportunity’s arm, which has been wasting some power by going on during cold hours even when not needed.

As it heads toward "Bonneville" to look for older rocks from beneath the region’s current surface layer, Spirit is stopping frequently to examine soil and rocks along the way. Observations with its microscope at one wavy patch of windblown soil allowed scientists to study how martian winds affect the landscape. Coarser grains are concentrated on the crests, with finer grains more dominant in the troughs, a characteristic of "ripples" rather than of dunes, which are shaped by stronger winds. "This gives us a better understanding of the current erosion process due to winds on Mars," said Shane Thompson, a science team collaborator from Arizona State University, Tempe.

Related Web Pages

Mars Rovers: JPL
Spirit’s Sol images and slideshow
Opportunity image gallery and slideshow
NASA’s RATs Go Roving on Mars
Water Signs
Microscopic Imager
Gusev Crater
Pancam– Surveying the Martian Scene
Mössbauer spectrometer
Alpha Proton X-ray Spectrometer