'Mars-shine' Composite
NASA's Mars Exploration Rover Spirit continues to take advantage of
favorable solar power conditions to conduct occasional nighttime
astronomical observations from the summit region of "Husband Hill."
Spirit has been observing the martian moons Phobos and Deimos to learn
more about their orbits and surface properties. This has included
observing eclipses. On Earth, a solar eclipse occurs when the Moon's
orbit takes it exactly between the Sun and Earth, casting parts of Earth
into shadow. A lunar eclipse occurs when the Earth is exactly between the
Sun and the Moon, casting the Moon into shadow and often giving it a
ghostly orange-reddish color. This color is created by sunlight reflected
through Earth's atmosphere into the shadowed region. The primary
difference between terrestrial and martian eclipses is that Mars' moons
are too small to completely block the Sun from view during solar eclipses.
Recently, Spirit observed a "lunar" eclipse on Mars. Phobos, the larger
of the two martian moons, was photographed while slipping into the shadow
of Mars. Jim Bell, the astronomer in charge of the rover's panoramic
camera (Pancam), suggested calling it a "Phobal" eclipse rather than a
lunar eclipse as a way of identifying which of the dozens of moons in our
solar system was being cast into shadow.
With the help of the Jet Propulsion Laboratory's navigation team, the
Pancam team planned instructions to Spirit for acquiring the views shown
here of Phobos as it entered into a lunar eclipse on the evening of the
rover's 639th martian day, or sol (Oct. 20, 2005) on Mars. This image is
a time-lapse composite of eight Pancam images of Phobos moving across the
martian sky. The entire eclipse lasted more than 26 minutes, but Spirit
was able to observe only in the first 15 minutes. During the time closest
to the shadow crossing, Spirit's cameras were programmed to take images
every 10 seconds.
In the first three images, Phobos was in sunlight, moving toward the upper
right. After a 100-second delay while Spirit's computer processed the
first three images, the rover then took the fourth image, showing Phobos
just starting to enter the darkness of the martian shadow. At that point,
an observer sitting on Phobos and looking back toward the Sun would have
seen a spectacular sunset! In the fifth image, Phobos appeared like a
crescent, almost completely shrouded in darkness.
In the last three images, Phobos had slipped entirely into the shadow of
Mars. However, as with our own Moon during lunar eclipses on Earth, it was
not entirely dark. The small amount of light still visible from Phobos is
a kind of "Mars-shine" -- sunlight reflected through Mars' atmosphere and
into the shadowed region.
Rover scientists took some images later in the sequence to try to figure
out if this "Mars-shine" made Phobos colorful while in eclipse, but
they'll need more time to complete the analysis because the signal levels
are so low. Meanwhile, they will use the information on the timing of the
eclipse to refine the orbital path of Phobos. The precise position of
Phobos will be important to any future spacecraft taking detailed pictures
of the moon or landing on its surface. In the near future it might be
possible for one of the rovers to take images of a "Deimal" eclipse to
learn more about Mars' other enigmatic satellite, Deimos, as well.