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1997 - The year of Mars Pathfinder
Mission captivates the world while setting new standards in
planetary exploration
By DIANE AINSWORTH
Of all the headline news in 1997, Mars Pathfinder's
remarkable landing and performance on the surface of frozen,
nearly airless Mars stole the show. Pathfinder became a landmark
mission and a catalyst for new and affordable ways of exploring
other worlds.
Pathfinder's landing marked America's return to the red
planet after more than 20 years. In addition to a swift, seven-
month cruise to the planet, Pathfinder dived directly into the
Martian atmosphere and landed with the aid of a parachute and
giant cocoon of airbags. This novel entry technique had never
been demonstrated before.
Nor had any spacecraft before Pathfinder carried a roving
vehicle the size of a small microwave oven to the surface of
another planet. Pathfinder's companion rover, named "Sojourner"
after Sojourner Truth, a female abolitionist who lived during
the American Civil War, was the first robotic vehicle ever to
make direct measurements of rocks and soil on Mars.
Over the course of three months -- which was three times the
design lifetime of the spacecraft -- Mars Pathfinder returned
about 2.6 gigabits of data, which included more than 16,000
images of the Martian landscape from the lander camera, 550
images from the rover and about 8.5 million temperature,
pressure and wind measurements. All science objectives had been
fulfilled when the mission ended, 83 days after a nearly perfect
landing on July 4. The only remaining objective was to complete
a high-resolution 360-degree image of the landing site called
the "Super Pan," of which 83 percent had been received. The last
successful data transmission cycle from Pathfinder was completed
at 3:23 a.m. Pacific Daylight Time on Sept. 27, 1997.
Sojourner, built to last seven days, wound up roaming the
floor of an ancient flood basin and exploring about 250 square
meters (820 square feet) of the Martian surface. In all, the
rover traveled a total of about 100 meters (328 feet) in 230
commanded maneuvers, performed more than 16 in-situ chemical
analyses of rocks and soil, and carried out numerous soil
mechanics and technology experiments.
"The mission demonstrated a reliable and low-cost system for
placing science payloads on the surface of Mars," said Project
Manager Brian Muirhead. "We've validated NASA's commitment to
low-cost planetary exploration, shown the usefulness of sending
microrovers to explore Mars, and obtained significant science
data to help understand the structure and meteorology of the
Martian atmosphere and to understand the composition of the
Martian rocks and soil."
"Pathfinder was an unequivocal success and has given us
phenomenal insights into how to operate future landers and
rovers on the surface of Mars," added Dr. Wesley Huntress,
associate administrator for science at NASA Headquarters, when
the mission was officially declared over. "I congratulate the
entire Pathfinder team on their accomplishment, which is a lofty
but wonderful standard for future missions to attempt to
exceed."
Part of NASA's Discovery program of low-cost planetary
missions with highly focused science goals, the spacecraft used
an innovative method of directly entering the Martian
atmosphere. Assisted by an 11-meter (36-foot) diameter
parachute, the spacecraft descended to the surface of Mars and
landed, using airbags to cushion the impact.
This innovative method of diving into the Martian atmosphere
worked like a charm. "Every event during the entry, descent and
landing (EDL) went almost perfectly," said Mission Manager
Richard Cook. "The sequences were executed right on time and
well within our margins."
Pathfinder's descent through the Martian atmosphere was
nearly flawless. After being suspended from a 20-meter (65-foot)
bridle and firing its retro rockets, the spacecraft released a
5.8-meter (19-foot) diameter cluster of airbags intended to
soften the landing. The entry, descent and landing sequence
marked the first time this airbag technique had been used.
Pathfinder hit the ground at a speed of about 18 meters per
second (40 mph) and bounced about 16 times across the landscape
before coming to a halt, Dr. Tim Parker of JPL later reported.
The airbag sustained little damage. To top it off, the
spacecraft landed on its base petal, consequently allowing a
thumb-sized auxiliary antenna to communicate the successful
landing just three minutes after impact.
Once safely on the surface, Pathfinder opened its solar-
powered petals and unveiled the small, 10.5-kilogram (23-pound)
rover and science instruments to their new home. Science
operations got under way within a day of landing, after the
rover had exited the lander using one of two exit ramps.
As the rover ventured out into unexplored territory, the
lander's camera began to image the surroundings, often taking
shots of the rover so that scientists and engineers could
monitor the vehicle's progress. A new portrait of the Martian
environment began to emerge as the spacecraft started to record
weather patterns, atmospheric opacity, winds and a variety of
other Martian conditions. The rover's alpha proton X-ray
spectrometer began studying rocks and making direct measurements
of their chemical compositions, another first in this mission.
Some of the rocks near the landing site were rich in silica,
or quartz, and some were identified as possible conglomerates,
reported Project Scientist Dr. Matthew Golombek and his
colleagues. Conglomerates are usually formed by running water,
which smoothes and rounds pebbles and cobbles found in the
conglomerate. Running water would also be the agent necessary to
deposit these rocks in a sand or clay matrix.
"If you consider all of the evidence we have at Ares Vallis -
- the rounded pebbles and cobbles and the possible conglomerate,
the abundant sand- and dust-sized particles and models for their
origins, in addition to the high silica rocks," Golombek said,
"it suggests a water-rich planet that may have been more Earth-
like than previously recognized, with a warmer and wetter past
in which liquid water was stable and the atmosphere was
thicker."
A panoramic view of Pathfinder's Ares Vallis landing site was
featured on the cover of the Dec. 5, 1997 issue of Science,
showing traces of this warmer, wetter past. The Ares Vallis
flood plain was covered with a variety of rock types, boulders,
rounded and semi-rounded cobbles and pebbles, deposited by
floods which occurred early in Mars' evolution.
"Before the Pathfinder mission, knowledge of the kinds of
rocks present on Mars was based mostly on the Martian meteorites
found on Earth, which are all igneous rocks rich in magnesium
and iron and relatively low in silica," Golombek and his
colleagues reported in Science. Chemical analyses of more than
16 rocks and studies of different regions of soil--along with
spectral imaging of rock colors, textures and structures--
confirmed that these rocks had compositions distinct from those
of the Martian meteorites found on Earth.
"The rocks that were analyzed by the rover's alpha proton X-
ray spectrometer were basaltic or volcanic rocks, with granite-
like origins, known as andesitic rocks," Golombek said. "The
high silica or quartz content of some rocks suggests that they
were formed as the crust of Mars was being recycled, or cooled
and heated up, by the underlying mantle. Analyses of rocks with
lower silica content appear to be rich in sulfur, implying that
they are covered with dust or weathered. Rover images show that
some rocks appear to have small air sacks or cavities, which
would indicate that they may be volcanic. In addition, the soils
are chemically distinct from the rocks measured at the landing
site."
Golombek noted that the rocky surface and rock types found in
Ares Vallis matched the characteristics of a flood plain on
Earth, created when a catastrophic flood washed rocks and
surface materials from another region into the basin. Ares
Vallis was formed in the same way that the 40-kilometer-long
(25-mile) Ephrata Fan of the Channeled Scabland in Washington
state was formed, and the Pathfinder scientists traveled to that
area a year before the landing to study the geology and
experiment with rover prototype hardware.
Additional data from the Pathfinder landing site revealed
that magnetic dust in the Martian atmosphere had been gradually
blanketing most of the magnetic targets on the lander over time.
"The dust is bright red, with magnetic properties that are
similar to that of composite particles," Golombek said. "A small
amount of the mineral maghemite has been deposited almost like a
stain or cement. These results could be interpreted to mean that
the iron was dissolved out of crustal materials in water,
suggesting an active hydrologic cycle on Mars. The maghemite
stain could be a freeze-dried precipitate."
Another team of scientists used daily radio Doppler tracking
and less frequent two-way radio ranging techniques during
communications sessions with the spacecraft to pinpoint the
location of the Pathfinder lander in inertial space and the
direction of Mars' rotational axis.
Dr. William Folkner, an interdisciplinary scientist at JPL,
and co-investigators were able to estimate the Martian polar
moment of inertia, which showed that Mars had a dense metallic
core surrounded by a lighter mantle. The results implied that
the radius of Mars' core was larger than about 1,300 kilometers
(807 miles) and less than about 2,400 kilometers (1,490 miles).
Mars' core and mantle were probably warmer than Earth's at
comparable depths.
"Variations in Mars' rotation around its own spin axis are
thought to be dominated by mass exchange between the polar caps
and the atmosphere," Folkner said. "During winter, part of the
atmosphere condenses at the poles. If the southern cap increased
symmetrically as the northern cap decreased, then there would
not be any change in moment of inertia or rotation rate.
However, because of Mars' orbital eccentricity, differences in
elevation and albedo, the polar caps are not formed
symmetrically.
"The unbalanced waxing and waning of the Martian polar ice
caps results in seasonal changes in air pressure at the
Pathfinder and Viking landing sites," he added. "These changes
in air pressure are correlated with changes in Mars' rotation
rate, which have been observed in our radio tracking
measurements."
The season and time of arrival of Mars Pathfinder in the late
northern summer resulted in some variations in the temperature
of the upper atmosphere compared to Viking data, Dr. Tim
Schofield, JPL team leader of the atmospheric structure and
meteorology instrument, and colleagues reported.
High in the atmosphere, at altitudes of 80 kilometers (50
miles) above the surface, temperatures were cold enough to make
carbon dioxide condense and form carbon dioxide clouds. At
altitudes of between 60 and 120 kilometers (37 and 75 miles),
the Martian atmosphere was an average of 20 degrees colder than
Viking measurements, Schofield said. Seasonal variations and
Pathfinder's entry at 3 a.m. local solar time, compared with
Viking's entry at 4 p.m. local solar time, may account for these
variations. On the surface, however, daytime temperatures were
typically 10 to 12 degrees warmer than Viking surface
temperatures.
Pathfinder measured regular pressure fluctuations twice a
day, which suggested that a moderate amount of dust was being
uniformly mixed in a warm lower atmosphere, as was the case with
Viking data. The daily average pressure reached a minimum on the
20th day of the mission (Sol 20), indicating the winter south
polar cap had reached its maximum size.
Schofield said that surface temperatures followed a regular
daily cycle, with a maximum of 15 degrees Fahrenheit during the
day and a minimum of minus 105 degrees Fahrenheit at night. The
science team also observed rapid daytime temperature
fluctuations of up to 30 degrees Fahrenheit in as little as 25
to 30 seconds. These observations suggested that cold air was
warmed by the surface and convected upward in small eddies.
Among a variety of other science findings, Pathfinder also
observed winds that were light and variable compared to the
winds encountered by the Viking landers. The winds blew steadily
from the south during the Martian nights, but during the day
they rotated in a clockwise direction from south to west to
north to east. Whirlwinds or dust devils were detected
repeatedly from mid-morning through the late afternoons.
Additional scientific findings are likely to result in the
months ahead as researchers continue to analyze data from this
mission. Meanwhile, another mission--Mars Global Surveyor--will be
observing the planet from space, while other missions gear up
for launches in the near term. As part of a sustained program of
exploration, Mars is likely to become a familiar place to
everyone over the next decade.
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