MARS: THE RED PLANET

Mars is a prominent red "star" in our skies this winter (in the north hemisphere is 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 day. 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 aproachers are closer than others. At the best possible oppositions, wich 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 constellaton Scorpius.

This close pass of Mars provided an unprecedent 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 a 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 and scientifically important wavelenghts that are not being studied by any of the past or present MArs sapace 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 I 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 consist 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, altough the  contrast of the clouds, hazes and ices visible primiraly at blue wavelenghts 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, blue/wither areas are seen near the poles(top and bottom in this color composite) and the limbs (left and right sides). These kind 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 specially along the boundaries between bright and dark regions