Monday, November 5, 2007

Spirit To Head North For The Winter


With Martian winter approaching, the science and engineering teams have been hard pressed to select a site where Spirit can spend the winter. After previously narrowing the list of candidates to two sites, Spirit's handlers decided to send the rover to the northern edge of the elevated plateau known as "Home Plate," which Spirit has been exploring for many months now.

Previously considered sites included "von Braun," "South Promontory," "Batter's Box" ("West Knoll"), and "North Home Plate." The decision means the rover will move farther away from tantalizing, new terrain to the south, but maximizes the rover's chances of surviving another winter given the excessive coating of dust on the solar arrays.

As Project Manager John Callas announced in an e-mail, "the principal discriminator was the achievable slope at each site. The north side of 'Home Plate' offers slopes of 25 degrees of northerly tilt, while 'South Promontory' offers 20 degrees of northerly tilt. That difference is about 10 watt-hours per sol, which can mean the difference between surviving and not surviving the cold, dark winter."

Meanwhile, Spirit remains healthy and all subsystems are nominal. Energy has been averaging 355 watt-hours (100 watt-hours is the amount of electricity needed to light one 100-watt bulb for one hour) and atmospheric dust measurements (Tau) have been steady at about 0.63.

Plans called for Spirit to head in a northerly direction, toward an area known as "Site 5" on top of Home Plate, starting on sol 1362 (Nov. 2, 2007) . Once there, Spirit may investigate some targets with instruments on the robotic arm before continuing to the north end of Home Plate.

Meanwhile, engineers working on the rover's miniature thermal emission spectrometer have determined that degradation in performance of the spectrometer on both Spirit and its twin, Opportunity, is the result of dust deposition on the scan mirror or in the panoramic camera mast assembly. They have decided not to use the instrument on Opportunity and to use it only for high-priority targets and weekly atmospheric measurements on Spirit while they try to develop strategies for removing the dust.

In addition, tests run on sols 1355, 1358, and 1360 (Oct. 25, Oct. 29, and Oct. 31) determined that the grind motor on Spirit's rock abrasion tool failed on sol 1341 (Oct. 11, 2007) , as it did previously on Opportunity on sol 1045 (Jan. 1, 2007). However, because the rover's handlers have devised an alternate technique for grinding and brushing that takes two Martian days, they are still able to use the brushes on both rock abrasion tools.

Sol-by-sol summary

In addition to receiving morning instructions directly from Earth via the high-gain antenna, sending evening data to Earth at UHF frequencies via the Odyssey orbiter, measuring atmospheric dust levels with the panoramic camera, and surveying the sky and ground with the miniature thermal emission spectrometer, Spirit completed the following activities:

Sol 1355 (Oct. 25, 2007): Spirit unstowed the robotic arm, conducted imaging diagnostics of the rock abrasion tool, and took microscopic images of the capture magnet. The rover placed the alpha-particle X-ray spectrometer on the capture magnet, took panoramic camera images of the rover deck, and transmitted data overnight via the Odyssey orbiter. Spirit monitored dust on the panoramic camera mast assembly, surveyed the horizon with the panoramic camera, acquired a mosaic of images with the navigation camera, and acquired movie frames in search of dust devils with the navigation camera.

Sol 1356: Spirit acquired panoramic camera images of the rover deck and of rock targets nicknamed "Grays Peak," "Elk," and "San Juan." The rover acquired 6 hours worth of data with the alpha-particle X-ray spectrometer and took thumbnail images of the sky with the panoramic camera.

Sol 1357: Spirit used the navigation camera to survey the surface darkened by the rover's shadow. The rover acquired full-color images of its tracks using all 13 filters of the panoramic camera. Spirit acquired another 6 hours of data with the alpha-particle X-ray spectrometer and took spot images of the sky with the panoramic camera.

Sol 1358: Spirit took images of the filter magnet with the microscopic imager, performed diagnostic tests on the rock abrasion tool, and used the panoramic camera to take images of the rover deck and survey the horizon.

Sol 1359: Spirit turned in place for communications relays and performed a "get quick fine attitude" to check for changes in the inertial measurement unit to determine the rover's precise location. Spirit acquired post-drive images with both the navigation and panoramic cameras. In the morning, the rover completed a systematic ground survey with the panoramic camera.

Sol 1360: Spirit unstowed the robotic arm, performed diagnostic tests of the rock abrasion tool, and acquired a mosaic of microscopic images of a soil target known as "Pumpkin Pie" before placing the alpha-particle X-ray spectrometer on the target. Spirit acquired full-color images, using all 13 filters of the panoramic camera, of another soil target known as "Candy Corn." The rover collected data from Pumpkin Pie with the alpha-particle X-ray spectrometer and in the morning, scanned the sky for clouds with the navigation camera. Spirit also surveyed the horizon with the panoramic camera and acquired movie frames in search of dust devils with the navigation camera.

Sol 1361: Spirit stowed the robotic arm in preparation for the next day's drive and took full-color images, using all 13 filters of the panoramic camera, of Elk and San Juan. The rover acquired a mosaic of images with the navigation camera as part of a 360-degree panorama for drive planning. Spirit surveyed the sky at both low sun and high sun with the panoramic camera.

Sol 1362 (Nov. 2, 2007): Plans called for Spirit to drive toward Site 5, acquire full-color, mid-drive images of Pumpkin Pie with all 13 filters of the panoramic camera, and acquire post-drive images with both the navigation and panoramic cameras. The following morning, Spirit was to complete a survey of rock clasts with the panoramic camera and scan the sky for clouds with the navigation camera.

Odometry:

As of sol 1359 (Oct. 30, 2007), Spirit's total odometry was 7,339.70 meters (4.56 miles).

Mars Express Probes The Red Planet's Most Unusual Deposits


The radar system on ESA's Mars Express has uncovered new details about some of the most mysterious deposits on Mars: The Medusae Fossae Formation. It has given the first direct measurement of the depth and electrical properties of these materials, providing new clues about their origin.

The Medusae Fossae Formation (MFF) are unique deposits on Mars. They are also an enigma. Found near the equator, along the divide between the highlands and lowlands, they may represent some of the youngest deposits on the surface of the planet. This is inferred from the marked lack of impact craters dotting this terrain, unlike on older terrain. Mars Express has been collecting data from this region using its Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS). Between March 2006 and April 2007, Mars Express orbited the region many times, taking radar soundings as it went.

For the first time, these radar soundings revealed the depth of the MFF layers, because of the time it took for the radar beam to pass through the top layers and bounce off the solid rock beneath. "We didn't know just how thick the MFF deposits really were" says Thomas Watters, lead author of the results at the Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, USA.

"Some investigators thought they might be a thin veneer overlaying topographic rises in the lowlands. The new data show that the MFF are massive deposits over 2.5 km thick in some places where MARSIS orbits pass over them," Watters added. The MFF deposits intrigue scientists because they are associated with regions that absorb certain wavelengths of Earth-based radar. This had led to them being called 'stealth' regions because they give no radar echo. The affected wavelengths are 3.5 to 12.6 centimetres. MARSIS, however, works at wavelengths of 50 to over 100 metres. At these wavelengths, the radar waves mostly pass through the MFF deposits creating subsurface echoes when the radar signal reflects off the plains material beneath.

A variety of scenarios have been proposed for the origin and composition of these deposits. Firstly, they could be volcanic ash deposits from now-buried vents or other nearby volcanoes. Second, they could be deposits of wind-blown materials eroded from other martian rocks. Thirdly, they could be ice-rich deposits, somewhat similar to the layered ice deposits at the poles of the planet, but formed when the spin axis of Mars tilts over, making the equatorial region colder.

Deciding between these scenarios is not easy, even with the new data. The MARSIS data reveal the electrical properties of the layers. These suggest that the layers could be poorly packed, fluffy or dusty material. However it is difficult to understand how porous material from wind-blown dust can be kilometres thick and yet not be compacted under the weight of the overlying material.

On the other hand, although the electrical properties are consistent with water ice layers, there is no other strong evidence for the presence of ice today in the equatorial regions of Mars. "If there is water ice at the equator of Mars, it must be buried at least several metres below the surface," says Jeffrey Plaut, MARSIS Co-Principal Investigator at the Jet Propulsion Laboratory, USA. This is because the water vapour pressure on Mars is so low that any ice near the surface would quickly evaporate.

So, the mystery of Mars's Medusae Fossae Formation continues. "It is still early in the game. We may get cleverer with our analysis and interpretation or we may only know when we go there with a drill and see for ourselves," says Plaut.

Giovanni Picardi at the University of Rome, La Sapienza, Principal Investigator of the experiment, says, "'Not only is MARSIS providing excellent scientific results but the team is also working on the processing techniques that will allow for more accurate evaluation of the characteristics of the subsurface layers and their constituent material. Hence, the possible extension of the mission will be very important to increase the number of observations over the regions of interest and improve the accuracy of the evaluations.