Geoscience Reference
In-Depth Information
12
Mars Dunes
12.1
Introduction
about the Red Planet. The surprising Mariner 4 images
revealed that the surface of Mars was dominated by
numerous large impact craters. While they covered\1%of
the Martian surface, these first close-up images were suffi-
cient to reveal that Mars had a history that appeared to be
more similar to that of Earth's Moon than to the Earth. The
Martian crater morphology was subdued relative to com-
parable craters on the Moon, which was interpreted to
suggest that winds on Mars may have modified the relief of
the rugged Martian crater rims through either deposition or
erosion, or both. As the spacecraft flew past Mars, the
refraction of the radio signals transmitted by the spacecraft
provided strong evidence that the Martian atmosphere was
at most only 1-3 % the density of the atmosphere of Earth,
confirming that the climate on Mars must be substantially
different from that of anything on the surface of the Earth.
The perception of an impact-crater-dominated surface was
further strengthened by the Mariner 6 and 7 fly-bys on July
31 and August 5, 1969, only days after the Apollo 11
landing on the Moon. Hundreds of images with substan-
tially better resolution (*300 m) than the Mariner 4 images
were returned by both of these spacecraft, appearing to
confirm the initial impression that impact craters dominated
the geologic history of Mars, although both the south polar
region and some isolated collapse features hinted that Mars
still held some intriguing features that were distinctly non-
lunar-like.
The prospects for a more diverse and interesting Martian
history improved dramatically with the first spacecraft to be
placed in orbit around another planet. The Mariner 9
spacecraft entered into orbit around Mars on November 14,
1971, but unfortunately it arrived at the height of one of the
most intense dust storms of the last hundred years. But,
unlike a flyby spacecraft, the orbiter could wait out the
storm. In early 1972, the Martian atmosphere cleared suf-
ficiently for Mariner 9 to begin its mapping mission,
eventually returning more than 7300 images (with resolu-
tions down to 100 m/pixel) before the mission concluded in
October
Mars has intrigued humans since this reddish object was
first noted as one of the 'wanderers' (e.g., planets) that
moved among the seemingly fixed stars. The red color is
clearly visible to the unaided eye, particularly when Mars is
closest to the Earth (roughly every 26 months), leading to
the association of this planet with the god of war. As the
positions of the planets were monitored systematically,
careful observers noted that Mars occasionally reversed its
direction of motion, only to revert to its normal progression
among the stars after a few weeks. This perplexing motion
eventually was explained once scientists accepted the fact
that all of the planets orbited the sun, and the unusual
motions of Mars results from its position as the next furthest
planet out from the sun, after the Earth.
Telescopic observations revealed Mars as an ochre-colored
disk displaying both dark and bright (but still reddish-hued)
regions across its surface, with occasional white or rust-col-
ored clouds (Fig. 12.1 ). The polar caps grow and shrink with
the seasons, produced by a tilt of Mars' rotation axis that is
very similar to tilt of the Earth. Telescopic measurements
during the early 20th century began to suggest that the
atmosphere of Mars, which was evident because of the doc-
umented movement of clouds and dust storms across the
planet, was not very substantial in comparison to the atmo-
sphere present around the Earth. Yet even the largest Earth-
based telescopes failed to resolve individual landforms on the
surface of Mars.
The advent of the space age provided the first opportu-
nities to observe planetary surfaces up close using robotic
spacecraft. The first spacecraft to return close-up images of
another planet was Mariner 4, launched November 28,
1964; the spacecraft flew past Mars on July 14, 1965, and
successfully transmitted 22 images back to Earth, taken by a
television camera attached to a small telescope. While crude
by the standards of spacecraft images of today, the Mariner
4 images shattered the hope that Mars might host the
abundant life described in countless science fiction stories
1972.
The
wealth
of
orbiter
images
finally
 
 
Search WWH ::




Custom Search