Space Forum / Space Science / August 2007

http://marsrovers.jpl.nasa.gov/spotlight/20070612.html

Extreme Planet Takes Its Toll
Jet Propulsion Laboratory
June 12, 2007

Like Sun Belt retirees who complain about cold weather, NASA's Mars
rovers are becoming less tolerant of temperature changes with age.

Near the martian equator, where the rovers are exploring opposite
sides
of the red planet, highs and lows make Earth temperatures look
downright
tropical. Temperatures often differ by more than 100 degrees Celsius.
That's a change of 180 degrees Fahrenheit -- the equivalent of having
the temperature drop from a high of 70 degrees F. at midday to minus
110
degrees F. the same night. That would be like going from a beach in
Hawaii to the South Pole in mid-winter ... every day!

Air Temperatures -- Spirit
Temperatures in the shade for Spirit ranged from highs of about 35
degrees C. (95 degrees F.) in summer to lows of -90 degrees C. (-130
degrees F.) in winter. In the background is a panoramic camera image
of
sunset <http://marsrovers.jpl.nasa.gov/gallery/press/spirit/
20050610a.html>
on Mars.
Image courtesy: NASA/JPL-Caltech/Cornell/NMMNH

[chart]
This chart shows degrees C. on the left-hand axis and degrees F. on
the
right-hand axis. The horizontal x-axis at the bottom shows the number
of
sols, or Martian days, on the surface.
<http://marsrovers.jpl.nasa.gov/images/
20070612/20070612_Spirit_LFHzczm_plot.jpg>

Extreme Planet

Though both rovers are exploring Mars well beyond their initial 90-day
missions, electrical connections and moving parts are showing signs of
temperature-related fatigue.

"Every day we have a huge thermal cycle," notes Jake Matijevic, chief
of
the rover engineering team. "That causes the solder in electrical
connections to expand and contract until it breaks."

Air Temperatures -- Opportunity
Temperatures in the shade for Opportunity ranged from about 30 degrees
C. (86 degrees F.) in summer to minus 80 degrees C. (-112 degrees F.)
in
winter. The background panorama shows a false-color view of dunes
<http://photojournal.jpl.nasa.gov/catalog/PIA06753> at the bottom of
Endurance Crater.
Credit: NASA/JPL-Caltech/Cornell/NMMNH

[chart]
This chart shows degrees C. on the left axis and degrees F. on the
right-hand axis. The horizontal x-axis at the bottom shows the number
of
sols, or Martian days, on the surface.
<http://marsrovers.jpl.nasa.gov/images/
20070612/20070612_Opportunity_LFHzczm_plot.jpg>

Seasonal Change

During their exploration of Mars, the rovers have recorded
temperatures
ranging from midday highs of about 35 degrees C. (95 degrees F.) in
spring and summer to nighttime lows of about minus 110 degrees C.
(minus
166 degrees F.) in winter. Spirit has experienced greater swings in
temperature because its location is farther from the martian equator,
which puts it seasonally closer to or farther from the Sun than
Opportunity.

Solar Panel Temperatures -- Spirit
Summer temperatures on Spirit's solar arrays have reached summertime
highs of more than 30 degrees C. and winter lows of about minus 110
degrees C. The image in the background shows the rover's view of
"Husband Hill" <http://photojournal.jpl.nasa.gov/catalog/PIA01907>
after
cliimbing down from the top.
Image courtesy: NASA/JPL-Caltech/Cornell/NMMNH

[chart]
This chart shows degrees C. on the left-hand axis and degrees F. on
the
right-hand axis. The horizontal x-axis at the bottom shows the number
of
sols, or Martian days, on the surface.
<http://marsrovers.jpl.nasa.gov/images/
20070612/20070612_Spirit_SA4_plot_2.jpg>

Three Years of Data

Rover engineer Dan Porter has been tracking temperatures recorded by
50
or so sensors on each of the rovers since shortly after they landed on
Mars in January 2004. The results are not only of interest to
scientists, they're a favorite of human audiences as well.

"People ask about this all the time," says New Mexico geologist Larry
Crumpler, a member of the Mars rover science team who created the
charts
showing monthly average temperatures superimposed on panoramic-camera
images from each of the rovers. "These plots of daily temperature are
destined to become an important part of all my public outreach talks."

Solar Panel Temperatures -- Opportunity
Nighttime temperatures on Opportunity's solar panels fell within a
fairly stable range of about minus 90 degrees C. (-130 degrees F.) to
minus 100 degrees C. (-148 degrees F.) most nights. Daytime
temperatures
reached a high of around 30 degrees C. (86 degrees F.) in the summer.
In
the background of the chart is an outcrop known as "Cape St. Mary"
<http://pancam.astro.cornell.edu/pancam_instrument/
1037B_bottomless_bay.html>
in Victoria Crater.
Credit: NASA/JPL-Caltech/Cornell/NMMNH

[chart]
This chart shows degrees C. on the left-hand axis and degrees F. on
the
right-hand axis. The horizontal x-axis at the bottom shows the number
of
sols, or Martian days, on Mars.
<http://marsrovers.jpl.nasa.gov/images/
20070612/20070612_Opportunity_SA4_plot.jpg>

Cold vs. Colder

Just as on Earth, temperatures on Mars are not uniform everywhere.
Objects exposed to direct sunlight are apt to be warmer than those in
the shade. Some objects retain heat better than others. A source of
relative warmth on Mars is rocky terrain, because it retains heat
better
than sandy terrain.

A solar panel is sort of like a "warm wall in mid-winter," says
Crumpler. Components beneath the solar panels, such as the hazard
avoidance cameras, are closer in temperature to the martian air.
Unlike
Earth, Mars does not have a thick, atmospheric blanket to seal in
warmth
by night or minimize the Sun's heat by day.

Colder in the Shade
On Mars as on Earth, surfaces shaded from sunlight are more likely to
develop a veneer of frost. This pair of images taken by the Compact
Reconnaissance Imaging Spectrometer for Mars, on board the Mars
Reconnaissance Orbiter, shows water frost in shaded areas and carbon
dioxide frost in only the coldest of those.
Image courtesy: NASA/JPL-Caltech/JHU APL

[Image]
The top image of this stacked pair of images is a false-color image
showing white frost covering alluvial fans beneath a cliff. The bottom
image is an infrared view of the same surfaces, showing blue areas
covered by frozen water vapor interspersed with aqua patches covered
by
an even colder coating of frozen carbon dioxide.
<http://photojournal.jpl.nasa.gov/catalog/PIA09101>

Wear and Tear

Engineers have traced stalls in a shoulder joint on Opportunity's
robotic arm to a broken electrical wire. The shoulder motor has
experienced greater temperature swings than other parts because of its
location beside a heater that has been stuck in the "on" position
since
shortly after landing.

Notes Matijevic, "Our theory is that one of the wires in the
electrical
coil broke free and disabled the motor."

Frost on Mars Rover
Opportunity discovered frost
<http://marsrovers.jpl.nasa.gov/gallery/press/opportunity/
20041213a.html> on the rover's calibration
target in October 2004.
Credit: NASA/JPL-Caltech/Cornell

[Image]
This photo shows shiny black rocks shaped like masses of thick rope
piled atop each other. In the distance are the rugged cliffs that make
up the slopes of Kilauea, rising gently in elevation from right to
left.
<http://photojournal.jpl.nasa.gov/catalog/PIA07108>

Mechanical Problems

Extremely cold temperatures have also caused mechanical problems.
Recently, Opportunity's right front wheel began drawing excess
electrical current, reminiscent of Spirit's right front wheel in 2004.
The demand for more electrical current suggests that the wheel motor
is
working harder than usual. Engineers suspect that, like motor oil that
can't handle freezing temperatures on Earth, gelid lubricant failed to
flow properly and caused Spirit's right front wheel to fail.
This black-and-white orbital image shows circular sprays of
lighter-colored rocks ejected from within craters onto darker
background
materials. <http://photojournal.jpl.nasa.gov/catalog/PIA03739>

Warmer vs. Colder

[Image}
NASA's Mars Odyssey spacecraft mapped surface changes in nighttime
temperatures on Mars. Lighter areas are warmer than darker areas and
correspond to rock exposures.

Image courtesy: NASA/JPL-Caltech/ASU

Adjusting Expectations

Rover drivers have compensated for Spirit's stuck wheel by driving on
the other five wheels. They have compensated for Opportunity's stalled
shoulder joint by unstowing it before going to sleep at night. This
way,
if the motor fails during the night, when temperatures are at their
lowest, they would still have limited use of the arm by operating the
remaining working joints. Each rover has lost a few temperature
sensors
as well, while dozens more continue operating.

[Image]
This black-and-white image shows a narrow column of light dust rising
from a dark Martian surface.
<http://photojournal.jpl.nasa.gov/catalog/PIA09260>

Warm Air Currents

[Image]
Dust devils form when the surface heats up and creates warm air that
rises in a spinning column. Spirit produced a movie
<http://marsrovers.jpl.nasa.gov/gallery/press/spirit/20070412a.html>
of
this dust devil in February,
2007.

Image courtesy: NASA/JPL-Caltech

Time Will Tell

"How long will it take for us to see other sorts of things related to
temperature swings? We don't know," says Matijevic. "We know it
matters
over time, but we don't know how many of these swings we'll see before
we lose a component.

"Meanwhile, we're applying for a fifth extended mission for both
rovers."

Surface Frost

[Image]
In the 1970s, NASA's Viking landers took images of seasonal ice
coating
rocks and soil on Mars.
Image courtesy: NASA/JPL-Caltech