MSL Picture of the Day: T-11 Days: Feet on Mars: Pathfinder

MSL Picture of the Day: T-11 Days: Feet on Mars: Pathfinder

Mars Pathfinder was launched on December 4, 1996 and arrived on Mars in Ares Vallis, in a region called Chryse Planitia in the Oxia Palus quadrangle on July 4, 1997. It used the bouncing method of airbags to land after having slowed down its landing with a parachute.
Pathfinder carried a rover, the sojourner, weighing only 10.6 kilograms (23 pounds).
The mission was directed by the Jet Propulsion Laboratory (JPL), a division of the California Institute of Technology, responsible for NASA’s Mars Exploration Program.  Tony Spear of JPL was the mission manager. Matt Golombek was the project scientist.

The Mars Global Surveyor had been launched one month prior to the launch of the Pathfinder.

The Pathfinder mission carried a series of scientific instruments to analyze the Martian atmosphere, climate and geology, and the composition of its rocks and soil. It was the second project from NASA’s Discovery Program, which promotes the use of low-cost spacecraft and frequent launches under the motto “cheaper, faster and better” promoted by the then administrator, Daniel Goldin. The Near-Earth Asteroid Rendezvous (NEAR) mission was the first Discovery Program project. NEAR was  launched in February 1996 . It became the first ever to orbit and land on an asteroid.

The primary objective of the Pathfinder was to demonstrate a low-cost method of delivering a set of science instruments and a free-ranging rover to the surface of Mars. The 23-pound Sojourner rover conducted technology experiments and served as an instrument deployment mechanism. This was the first mission that used the concept of airbags to protect the aircraft during the final landing stage.
Slowed in its descent through the thin Martian atmosphere by a parachute, it was in free-fall the last few hundred feet and bounced over the Martian surface on its inflated airbags like a giant beach ball.  Once it rolled to a stop, the airbags deflated, the petals of the lander opened, and the rover descended to begin exploring and analyzing nearby rocks.

Once the lander had landed successfully she was formally named the Carl Sagan Memorial Station.

The science objectives were to:

• Investigate the Martian atmosphere, surface metrology and geology

• Characterize surface features

• Analyze the elemental composition of rocks and soil at the landing site

• Monitor atmospheric conditions as they varied over the course of the mission with photographic and analytical instruments.

The whole mission was planned to last no longer than 1 month.

Those mission objectives were ambitious, but Pathfinder did not disappoint as it surpassed them all. The lander captured over 16,500 images, while the rover snapped 550 pictures. The mission performed more than 15 chemical analyses of rocks and soil and returned extensive data on the winds and other aspects of weather on Mars. The observations suggested that early Mars may once have been more Earth-like than it is today, with liquid water on its surface and a thicker atmosphere.

Soils at the landing site varied from the bright-red dust to darker-red and darker-gray material. It turned out that the soils were lower in silicon than the rocks and richer in sulfur, iron and magnesium than the rocks. The composition of the soil measured was generally the same as those measured at the Viking sites, which are on opposite hemispheres (Viking 1 is 800 kilometers west of Pathfinder; Viking 2 is thousands of kilometers away on the opposite, eastern side of the Northern Hemisphere).  This led to the belief that this soil may be found all over Mars. The similarity in compositions among the soils implies that their differences in color may be due to slight variations in iron mineralogy or in particle size and shape. A bright-red or pink material also covered part of the landing site. This material is similar to the soils in composition, but it seems to be hardened by some process, because the soil was not damaged by extensive scraping using the rover wheels.

The sky on Mars had the same pale pink color as it did when imaged by the Viking landers. Fine-grained, bright-red dust in the atmosphere would explain this color. Before the Pathfinder landed on Mars images by the Hubble Space Telescope suggested a very clear atmosphere. And this led scientists to expect that the atmosphere might even appear blue from the surface, just like the atmosphere on Earth. However Pathfinder found otherwise, suggesting either that the atmosphere always has some dust in it from local dust storms or that the atmospheric opacity varies noticebly over a short time. The inferred dust-particle size (roughly a micron) and shape and the amount of water vapor (equivalent to a pitiful hundred of a millimeter of rainfall) in the atmosphere are also consistent with measurements made by Viking. Even if Mars was once lush, it is now drier and dustier than any desert on Earth.

The meteorological sensors gave further information about the atmosphere. They found patterns of daily and longer-term pressure and temperature fluctuations. The temperature reached its maximum of – 10 degrees Celsius (263 kelvins) every day at 2 p.m. local solar time, and its minimum of –76 degrees Celsius (197 kelvins) just before sunrise. The pressure minimum of just under 6.7 millibars (roughly 0.67 percent of pressure at sea level on Earth) was reached on sol 20, the 20th martian day after landing. On Mars, the air pressure varies with the seasons. During winter, it is so cold that 20 to 30 percent of the entire atmosphere freezes out at the pole, forming a huge pile of solid carbon dioxide. The pressure minimum seen by Pathfinder indicates that the atmosphere was at its thinnest, and the south polar cap its largest, on sol 20.

Morning temperatures fluctuated abruptly with time and height; the sensors positioned 0.25, 0.5, and 1 meter above the spacecraft took different readings. This would mean that if you were standing on Mars, your nose would be at least 20 degrees Celsius colder than your feet. This suggests that cold morning air is warmed by the surface and rises in small eddies, or whirlpools; this is very different from what happens on Earth. Afternoon temperatures, after the atmosphere has warmed, do not show the same variations. In the early afternoon, dust devils repeatedly swept across the lander. They showed up as sharp, short-lived pressure changes and were probably similar to events detected by the Viking landers and orbiters; they may be an important mechanism for raising dust into the martian atmosphere. The prevailing winds were light (less than 10 meters per second, or 36 kilometers per hour) and variable. {br]The dust is highly magnetic. It may consist of small silicate (perhaps clay) particles, with some stain or cement of a highly magnetic mineral known as maghemite. This, too, is consistent with a watery past. The iron may have dissolved out of crustal materials in water, and the maghemite may be a freeze-dried product of condensation of the atmospheric water vapor.

Pathfinder measured atmospheric conditions at higher altitudes during its descent. The upper atmosphere (above 60 kilometers’ altitude) was colder than Viking had measured. This may simply reflect seasonal variations and the time of entry: Pathfinder came in at 3 a.m. local solar time, whereas Viking arrived at 4 p.m. when the atmosphere is naturally warmer. The lower atmosphere was similar to that measured by Viking and its conditions can be attributed to dust mixed uniformly in comparatively warm air.

The tilt of Mars’ spin axis (obliquity) varies cyclically

Mission scientists were able to use the radio communication signals from Pathfinder to measure the rotation of Mars. Daily Doppler tracking and less frequent two-way ranging during communication sessions determined the position of the lander with a precision of 100 meters. The last such positional measurement was done by Viking over 20 years ago. Since the Viking measurements the Martian pole of rotation has precessed — that is, the tilt of the planet has changed, just as a spinning top slowly wobbles.
The difference between the two positional measurements yields the precession rate. The rate is governed by the moment of inertia of the planet, a function of the distribution of mass within the planet.

From Pathfinder’s determination of the moment of inertia we now know that Mars must have a central metallic core of between 1,300 and 2,400 kilometers in radius. With assumptions about the mantle composition — derived from the compositions of the martian meteorites and the rocks measured by the rover — scientists can now start to put estimates about the interior temperatures. Prior to Pathfinder, the composition of the martian meteorites argued for a core, but the size of this core was completely unknown. The new information about the interior will help geophysicists to understand how Mars has evolved over time. In addition to the long-term precession, Pathfinder detected an annual variation in the planet’s rotation rate, which is just what would be expected from the seasonal exchange of carbon dioxide between the atmosphere and ice caps.

The first images gave us a landscape constructed from a mosaic of those images that looked like a rocky plain. About 20 percent of the plain was covered by rocks. Rocky plains like that are seen on Earth in Iceland, where they have been deposited and shaped by catastrophic floods. This was what had been predicted based on remote-sensing data and the location of the landing site (19.13 degrees north, 33.22 degrees west). This site is downstream from the mouth of Ares Vallis in the low area known as Chryse Planitia and great floods might once have come from there. In Viking orbiter images, the area appears analogous to the Channeled Scabland in eastern and central Washington state.

This analogy suggests that Ares Vallis formed when roughly the same volume of water as in the Great Lakes (hundreds of cubic kilometers) was catastrophically released, carving the observed channel in a few weeks. [br']The density of impact craters in the region indicates it formed at an intermediate time in Mars’s history, somewhere between 1.8 and 3.5 billion years ago. The Pathfinder images support this interpretation. They show semi-rounded pebbles, cobbles and boulders similar to those deposited by terrestrial catastrophic floods. Rocks in what we dubbed the Rock Garden — a collection of rocks to the southwest of the lander, with the names Shark, Half Dome, and Moe — are inclined and stacked, as if deposited by rapidly flowing water. Large rocks in the images (0.5 meters or larger) are flat-topped and often perched, also consistent with deposition by a flood. The Twin Peaks, a pair of hills on the southwestern horizon, are streamlined. Viking images suggest that the lander is on the flank of a broad, gentle ridge trending northeast from Twin Peaks; this ridge may be a debris tail deposited in the wake of the peaks. Small channels throughout the scene resemble those in the Channeled Scabland, where drainage in the last stage of the flood preferentially removed fine-grained materials.

example of an conglomerate: a rock consisting of individual clasts within a finer-grained matrix that have become cemented together

Taking all the results together, scientists have deduced that Mars was once more Earth-like than previously appreciated. Some crustal materials on Mars resemble, in silicon content, continental crust on Earth. Moreover, the rounded pebbles and the possible conglomerate, as well as the abundant sand- and dust-sized particles, argue for a previously water-rich planet. The earlier environment seems to have been warmer and wetter, perhaps similar to that of the early Earth. In contrast, since the time that floods produced the landing site 1.8 to 3.5 billion years ago, Mars has been a very un-Earth-like place. The site appears almost unaltered since it was deposited, indicating very low erosion rates — and therefore no water in relatively recent times.

The pathfinder mission functioned on the Martian surface for about three months, well beyond the planned lifetimes of 30 days for the lander and seven days for the rover, coming to an end on Sept. 27, 1997.
On December 10, 1997, the U.S. Postal Service paid tribute to NASA’s Mars Pathfinder mission with a $3 Priority Mail stamp featuring one of the first images transmitted to Earth

Detailed mission results were published in the December 5, 1997 issue of Science, and also in abstracts presented at the 29th Lunar and Planetary Science Conference in March 1998.

Mosaic of Viking orbiter images illustrating the location of the lander (19.17 degrees N, 33.21 degrees W in the USGS reference frame) with respect to surface features. Five prominent features on the horizon include North Knob, Southeast Knob, Far Knob, Twin Peaks, and Big Crater.
credit: Science Magazine, Volume 278, Number 5344, 5 December 1997, ‘Overview of the Mars Pathfinder Mission

Pathfinder also gave us 3D images of the landscape it was parked in. You will need 3D red/cyan glasses to see depth in this image, but if you have those you will enjoy the way 3D enhances the experience of looking at Mars.

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