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NASA Mars Rover Team’s Tilted Winter Strategy Works

View From Within ‘Perseverance Valley’ on Mars

This view from within “Perseverance Valley,” on the inner slope of the western rim of Endurance Crater on Mars, includes wheel tracks from the Opportunity rover’s descent of the valley. The Panoramic Camera (Pancam) on Opportunity’s mast took the component images of the scene during the period Sept. 4 through Oct. 6, 2017, corresponding to sols (Martian days) 4840 through 4871 of the rover’s work on Mars.

Perseverance Valley is a system of shallow troughs descending eastward about the length of two football fields from the crest of the crater rim to the floor of the crater. This panorama spans from northeast on the left to northwest on the right, including portions of the crater floor (eastward) in the left half and of the rim (westward) in the right half. Opportunity began descending Perseverance Valley in mid-2017 (see map) as part of an investigation into how the valley formed.

Rover wheel tracks are darker brown, between two patches of bright bedrock, receding toward the horizon in the right half of the scene.

This view combines multiple images taken through three different Pancam filters. The selected filters admit light centered on wavelengths of 753 nanometers (near-infrared), 535 nanometers (green) and 432 nanometers (violet). The three color bands are combined here to show approximately true color.

Image Credit: NASA/JPL-Caltech/Cornell Univ./Arizona State Univ.


NASA Mars Rover Team’s Tilted Winter Strategy Works

NASA’s senior Mars rover, Opportunity, has just passed the shortest-daylight weeks of the long Martian year with its solar panels in encouragingly clean condition for entering a potential dust-storm season in 2018.

Before dust season will come the 14th Earth-year anniversaries of Mars landings by the twin rovers Spirit and Opportunity in January 2004. Their missions were scheduled to last 90 Martian days, or sols, equivalent to about three months.

“I didn’t start working on this project until about Sol 300, and I was told not to get too settled in because Spirit and Opportunity probably wouldn’t make it through that first Martian winter,” recalls Jennifer Herman, power subsystem operations team lead for Opportunity at NASA’s Jet Propulsion Laboratory in Pasadena, California. “Now, Opportunity has made it through the worst part of its eighth Martian winter.”

The minimum-sunlight period for southern Mars this year was in October and November. Mars takes 1.88 Earth years to orbit the Sun and, like Earth, it has a tilted axis, so it gets seasons resembling Earth’s but nearly twice as long.

Opportunity’s View Downhill Catches Martian Shadows

Late-afternoon shadows include one cast by the rover itself in this look toward the floor of Endeavour Crater by NASA’s Mars Exploration Rover Opportunity. The rover recorded this scene on Nov. 11, 2017, during the 4,911th Martian day, or sol, of the rover’s work on Mars. That was about a week before Opportunity’s eighth Martian winter solstice.

Opportunity’s location is partway down a narrow valley that descends from the crest of the western rim of Endeavour Crater to the crater’s floor. This fluid-carved set of troughs, called “Perseverance Valley,” is the length of about two football fields, at a slope of about 15 to 17 degrees.

The Navigation Camera (Navcam) on Opportunity’s mast took the three component images stitched together into this scene. The images were taken about three minutes apart, long enough to see how the shadow was changing on the slope, at the seams between the images. Wheel tracks in the lower right of the scene were made before the rover climbed back uphill for a closer look at some rocks it had passed. The portions of the rover in the shadow at upper right include the mast with the Navcam and Panoramic Camera (Pancam) on top and the UHF radio antenna, which Opportunity uses to transmit images and other data to overflying orbiters for relay to Earth.

Image Credit: NASA/JPL-Caltech


Both Opportunity and Spirit are in Mars’ southern hemisphere, where the Sun appears in the northern sky during fall and winter, so solar-array output is enhanced by tilting the rover northward. Spirit could not maintain enough energy to survive through its fourth Martian winter, in 2009, after losing use of two wheels, long past their planned lifetime. It became unable to maneuver out of a sand trap to the favorable northward tilt.

Opportunity’s current exploration of fluid-carved “Perseverance Valley” positioned it well for working productively through late fall and early winter this year. The rover has used stops at energy-favorable locations to inspect local rocks, examine the valley’s shape and image the surroundings from inside the valley.

The valley runs downhill eastward on the inner slope of the western rim of Endurance Crater, which is 14 miles (22 kilometers) in diameter. Since entering the top of the valley five months ago, Opportunity’s stops between drives have been at north-facing sites, on the south edge of the channel. The rover team calls the sites “lily pads” and plans routes from each one safely to the next, like a frog hopping from lily pad to lily pad.

Herman’s role includes advising others on the team how much energy is available each sol for activities such as science observations and driving. “Relying on solar energy for Opportunity keeps us constantly aware of the season on Mars and the terrain that the rover is on, more than for Curiosity,” she said. She performs the same role for NASA’s younger Mars rover, Curiosity, which gets its electrical energy from a radioisotope thermoelectric generator instead of solar panels. Wintertime conditions affect use of electrical heaters and batteries on both rovers, but influence Opportunity’s activities much more than Curiosity’s.

Wind’s Marks in ‘Perseverance Valley’ (Enhanced Color)

This patch of rocky Martian ground on the floor of “Perseverance Valley” on the inner slope of the western rim of Endurance Crater slopes steeply downhill from left to right. Some textures seen here, including striations just above and parallel to the edge of a solar panel at far left, may be due to abrasion by wind-driven sand. Researchers interpret them as possible signs of past winds blowing from right to left, up and out of the crater, which currently hosts sand dunes on its central floor.

The view spans about 11.5 feet (3.5 meters) from left to right and is presented in enhanced color to make differences in surface materials easier to see. The Panoramic Camera (Pancam) on NASA’s Mars Exploration Rover Opportunity took the component images of this scene during the period Oct. 13 through Oct. 20, 2017, corresponding to sols (Martian days) 4878 through 4884 of the rover’s work on Mars.

Opportunity entered the upper end of Perseverance Valley in July 2017 for several months of investigating how it formed. The valley is a system of shallow troughs extending about the length of two football fields down the crater rim’s steep inner slope. Endurance Crater is about 14 miles (22 kilometers) in diameter. Opportunity has been exploring features on its western rim since 2011, after investigating a series of smaller craters beginning with the one it landed in on Jan. 25, 2004, Universal Time (Jan. 24, PST).

The origin of Perseverance Valley is unknown, but some observed features suggest that water might have played a role in the past. Opportunity is descending the steep valley, making observations along the way that could help illuminate the origin of this feature.

The bedrock target area in this view is called “La Bajada.” The image combines exposures taken through three Pancam filters, centered at wavelengths of 753 nanometers (near-infrared), 535 nanometers (green) and 432 nanometers (violet).

Image Credit: NASA/JPL-Caltech/Cornell Univ./Arizona State Univ.

Opportunity has not always been on such suitable terrain for winter operations. In its fifth winter, in 2011-2012, it spent 19 weeks at one spot because no other places with favorable tilt were within acceptable driving distance. In contrast, it kept busy its first winter in the southern half of a stadium-size crater, where all of the ground faced north.

Besides tilt and daylight length, other factors in Opportunity’s power status include how much dust is on the solar array and in the sky. Wind can clean some dust off the array, but can also stir up dust storms that block sunlight and then drop dust onto the rover. Southern-hemisphere autumn and winter tend to have clear skies over Opportunity, but the amount of dust on the solar array going into autumn has varied year-to-year, and this year the array was dustier than in all but one of the preceding autumns.

“We were worried that the dust accumulation this winter would be similar to some of the worst winters we’ve had, and that we might come out of the winter with a very dusty array, but we’ve had some recent dust cleaning that was nice to see,” Herman said. “Now I’m more optimistic. If Opportunity’s solar arrays keep getting cleaned as they have recently, she’ll be in a good position to survive a major dust storm. It’s been more than 10 Earth years since the last one and we need to be vigilant.”

Planet-encircling dust storms are most likely in southern spring and summer on Mars, though these storms don’t happen every Martian year. The latest such storm, in 2007, sharply reduced available sunlight for Spirit and Opportunity, prompting emergency cutbacks in operations and communications to save energy. Some atmospheric scientists anticipate that Mars may get its next planet-encircling dust storm in 2018.

In coming months, scientists and engineers plan to continue using Opportunity to investigate how Perseverance Valley was cut into the crater rim. “We have not been seeing anything screamingly diagnostic, in the valley itself, about how much water was involved in the flow,” said Opportunity Project Scientist Matt Golombek, of JPL. “We may get good diagnostic clues from the deposits at the bottom of the valley, but we don’t want to be there yet, because that’s level ground with no more lily pads.”

source: NASA – Jet Propulsion Laboratory – California Institute of Technology

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