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Mars is a prominent red "star" in our skies this summer because it is making a rare and relatively close pass to the Earth. Mars is the next planet beyond Earth and completes one orbit around the Sun in 687 days. The Earth moves faster in its orbit around the Sun, and roughly every 26 months the planets pass close together and then the Earth overtakes our celestial neighbor. When this happens, Mars is said to be in "opposition" because it appears to be on the opposite side of the sky from the Sun. During these times, Mars becomes bright and relatively large in telescopic views. However, because the orbit of Mars is quite elliptical, some close approaches are closer than others. At the best possible oppositions, which occur every 15 to 17 years or so, Mars approaches within only 35 million miles (56 million km) of Earth, while in the poorest oppositions Mars is over 63 million miles (100 million km) away. The almost factor of two increase in the apparent size of Mars at the best oppositions explains why telescopic observers concentrate their observations around those times.


The 2001 opposition of Mars is the best in terms of the apparent size and brightness of Mars in the sky since 1988. It is also easily the best since HST started observing Mars in 1990. Mars came within 43 million miles (68 million km) of Earth in late June 2001, when it appeared as a bright red -2.2 magnitude "star" in the constellation Scorpius. Mars will continue to shine brightly in this part of the sky for the rest of the summer.

This close pass of Mars provided an unprecedented opportunity for high spatial resolution telescopic observations from HST. At its closest, HST was able to resolve features as small as about 10 miles (15 km) apart on the Martian surface. This is by far the best resolution ever obtained from an Earth-based telescope, and is the same kind of resolution that was being obtained from Mars spacecraft flybys and orbiters in the 1960s and 1970s! Additionally, HST's high quality digital cameras can obtain images at scientifically important wavelengths that are not being studied by any of the past or present Mars space missions, thus filling an important gap in color coverage and substantially enhancing the scientific return of those missions. HST is, in effect, another NASA mission to Mars--just in a very, very high orbit!


Here we present a color composite of some of the images obtained by HST on June 26, 2001, when Mars was very near its closest approach point to Earth. The composite consists of images obtained through blue (410 nm), green (502 nm), and red (673 nm) filters, as well as several others with the Planetary Camera detector on the WFPC2 instrument. The colors have been balanced to provide an approximate representation of "true color" as would be seen through a backyard telescope, although the contrast of the clouds, hazes, and ices visible primarily at blue wavelengths have been enhanced slightly for better visibility.

The Hubble images and resolution at this opposition are spectacular. A number of interesting surface and atmospheric phenomena are visible, and they remind us that Mars, like the Earth, is a dynamic and ever-changing world. To first order, the surface is divided into bright reddish and darker grayish regions. The reddish color is caused by the presence of oxidized iron minerals (rust) on the Martian surface. Smaller, bluer/whiter areas are seen near the poles (top and bottom in this color composite) and the limbs (left and right sides). These kinds of markings have been observed in roughly this same pattern for at least the last 300 years, since serious telescopic observations of Mars began. However, the markings are known to change with time. Bright regions have been seen to darken, and dark regions to brighten, and many changes have been observed especially along the boundaries between bright and dark regions.

These kinds of changes are caused by the constant movement of bright, fine-grained dust across the surface, carried by the Martian winds from place to place. Two of these dust storms have in fact been caught in the act in these new HST images: one churning high above the north polar cap at the top of this image, and a second swirling within and spilling out of the north end of the giant Hellas impact basin in the lower right portion of this image. These storms, which are also being monitored at lower resolution by a growing cadre of amateur astronomers back on Earth, redistribute dust and soil around the planet and are responsible for the changing patterns of the markings seen from year to year.

These HST images provide high resolution detail showing how dust is being infilled or eroded from individual craters, wind streaks, mountain ridges, and deep valleys. Combined with ground-based monitoring and similar-scale images obtained concurrently by the Mars Global Surveyor spacecraft but in different wavelengths, the HST data will help to constrain both the composition and mineralogy of the dusty and darker regions, as well as meteorological models of the dust storms themselves.

Other prominent features in this image include an extensive south polar cap (the whitish region near the bottom) consisting of mostly dry ice (CO2) that condensed out of the Martian atmosphere during the long south polar winter (the image was obtained very close to the first day of the southern spring season on Mars), a "hood" of blueish-white water ice clouds at high northern latitudes (top of image), prominent early morning (left side) and late evening (right side) blueish-white water ice clouds forming high up in the extremely cold Martian atmosphere, and a thin band of water ice clouds/haze that stretches across the "tropics" of Mars (across the middle of the image). These clouds and polar caps are known to wax and wane as Mars goes through its Earth-like yearly cycle of seasons. In fact, the equatorial cloud belt is much less developed than we have seen in previous HST Mars images, when the planet was much further from the Sun and thus the atmosphere much colder. However, the corresponding decrease in water ice clouds is compensated by the increase in dust storm activity seen here, since dust storms tend to be more prominent during southern spring and summer seasons when the planet is closer to the Sun. These HST images are helping astronomers fill important gaps in the seasonal coverage of observations and models designed to help predict the timing and extent of future dust storm activities on Mars.

Because of their unique temporal and spectral coverage, images like these being obtained from HST are contributing to the selection of landing sites for the next round of NASA Mars rover missions. The NASA Mars Exploration Rovers will launch in 2003 and land in early 2004, exploring two different regions of the surface for at least 90 Martian days each. Images from HST are helping to define the most interesting potential landing sites by providing information on the surface mineralogy and the seasonal/annual meteorological conditions of dozens of candidate landing locations.

These spectacular HST images provide a glimpse of what we can expect to see in 2003, when Mars makes it closest approach to Earth since 1924 (35 million miles, or 56 million km), and is closer than it will be again until the year 2287! During 2003, Mars will be 25% larger in the sky than during 2001, and surface features as small as about 7.5 miles (12 km) will be resolvable by HST. Between now and then, HST and ground-based observers will continue to monitor the red planet for new signs of surface changes and meteorological activity that will further enhance our understanding of this dynamic world.