"The first astronomical observatory on Mars is open for business!"
This image shows Mars' smaller moon, Deimos, as it appeared to Mars Pathfinder on its third night after landing. Deimos is actually only about two IMP pixels across -- it looks bigger because a set of low resolution, compressed images (that is, they were blurry) were returned to Earth. Observations of Deimos are used to determine its spectrum and composition. Deimos is difficult to observe from Earth or the Hubble Space Telescope because it is always very close to Mars.
This is the first image ever taken from the surface of Mars of an overcast sky. Featured are stratus clouds coming from the northeast at about 15 miles per hour (6.7 meters/second) at an approximate height of ten miles (16 kilometers) above the surface. The "you are here" notation marks where Earth was situated in the sky at the time the image was taken. Scientists had hoped to see Earth in this image, but the cloudy conditions prevented a clear viewing. Similar images will be taken in the future with the hope of capturing a view of Earth. From Mars, Earth would appear as a tiny blue dot as a star would appear to an earthbound observer. Pathfinders' imaging system will not be able to resolve Earths' moon. The clouds consist of water ice condensed on reddish dust particles suspended in the atmosphere. Clouds on Mars are sometimes localized and can sometimes cover entire regions, but have not yet been observed to cover the entire planet. The image was taken about an hour and forty minutes before sunrise by the Imager for Mars Pathfinder (IMP) on Sol 16 at about ten degrees up from the eastern Martian horizon.
Our observations of the Sun using the IMP camera showed more dust in the Martian atmosphere than was expected. In fact, the amount of dust ("optical depth" of 0.4, to scientists) was comparable to Viking observations during clear (non-dust-storm) times. The measurements were made by taking images of the Sun with different colors and with the Sun at different elevations in the sky. As the Sun goes lower in the sky the light passes through more and more dust, becoming fainter and fainter, and allowing the amount of dust to be measured.
On Mars, the dust intercepts essentially the same amount of sunlight in different colors. The reddish color of the sky is because the blue light is absorbed by the dust, but the red light is scattered throughout the sky. By contrast, the molecules in the Earth's atmosphere intercept about as much of the blue sunlight as the Mars dust does, because blue light is scattered easily by Earths' atmosphere (and red light is not, giving the Earth its blue sky).
In future days, scientists will monitor the amount of dust in the atmosphere, and they will try to measure absorption by water vapor with a similar technique. Using other observations of the sky, scientists will measure the size and shape of the dust particles and try to determine how high in the atmosphere the dust extends. On some days, IMP will perform a cloud search, looking for clouds passing over the landing site. The Hubble Space Telescope will also observe Mars on some of the same days, so large clouds--if they are present--may be seen simultaneously from the Earth and from Mars.
Every several days, Mars Pathfinder will image the sunrise and sunset on Mars. Future images will show a larger area - we have a higher data rate than we expected when we planned this image, so we can get more information. Images taken at sunset, like this, and up to two hours later, will be used to investigate the distribution of dust within the Martian atmosphere. Already, we can see some dust layers in the images. By seeing how the twilight fades with time -- it lasts for over two hours -- we can determine that the dust extends high into the atmosphere.