To celebrate NASAs recent release of over 25,000 Mars images snapped between March and August of 1999, Space.com presents our picks for the 20 most beautiful and geologically interesting images of the bunch. Click on thumbnails to see full-sized images.

The martian polar layered caps are centers of present-day geologic activity, and the Red Planets major reservoirs of water. Scientists say the interweaving black streaks are sedimentary deposits of dust, soil, and other debris. This close-up of the martian northern pole is about 82 miles across.

This image, captured in April 1999, shows an early-afternoon scene of a volcano called Apollinaris Patera. The volcano is located near the martian equator, and is thought to be nearly three miles high. The volcanos caldera, the circular pit featured at its summit, is about 50 miles across. The white you see is a patch of white clouds hanging over the volcano.

This image features both the martian northern polar cap and Valles Marineris, a huge chasm spanning 2,500 miles roughly the width of the entire continental United States. The image was taken by Mars Orbiter Cameras global-mapping instrument.

This is an image of an area called Solis Planum, just south of the huge Valles Marineris chasm. The streaks in the image are long volcanic "wrinkle ridges," created by large-scale movement of a cooled lava field. The image spans roughly 140 miles from top to bottom.

This is a close-up of one section of canyons in Valles Marineris. Depending on the spot, canyons in this region of Mars can be up to 10 times as deep as the Grand Canyon in the southwest United States. This image is about 180 miles across.

Olympus Mons is the solar systems largest known volcano five times larger than the largest on Earth. It is about 342 miles in diameter and rises 15.5 miles above the surrounding plain. Thats half the distance between the deepest chasm on Earth (Mariana Trench) to the top of Mt. Everest! At the top of the volcano lies a large caldera, which was produced when the peak of the mountain collapsed as magma flowed out from shallow reservoirs elsewhere in the volcano.

Here, we see two volcanic mounds, Ceraunius Tholus (bottom) and Uranius Tholus (top) in the Tharsis volcanic region of Mars. Scientists think that "small" volcanoes such as these two (the largest is still a resounding 50 miles across) are a result of numerous small vents that abound in this localized region. The squiggly line protruding from Ceranius Tholus is most likely a lava-induced depression.

This is a good example of what impact craters look like on Mars. When an object hits the martian surface, debris from the explosion is spewed out in all directions from the point of impact. In this case, two objects hit an area with a bright surface and a darker underlying layer. The larger crater has a diameter of about 97 yards, while the smaller craters diameter is about 39 yards.

This picture shows a portion of the floor of Melas Chasma in Valles Marineris. Dark sand dunes in the image are spaced about 190 feet apart. Smaller ripples are also visible in the troughs between some of the dunes, perhaps indicating a current windy environment.

This image reveals a close look at Olympica Fossae, a collection of troughs and depressions located in the Tharsis region of Mars. The picture shows several different types of martian land formations, including layered outcrops in canyon walls, evenly-spaced dunes on the canyon floors, dark landslide streaks on the canyon walls, pits formed by ground collapse, and streamlined forms related to the flow of water, mud, or lava.

This is a computer-enhanced composite of nine red and nine blue image strips taken by the Mars Orbiter Camera on successive orbits from pole to pole during the calibration phase of the Mars Global Surveyor mission in March 1999. Bluish-white features are clouds.

Another excellent example of impact craters on Mars. This particular crater is about 4.4 miles across, and lies in the south-central Syria Planum area of Mars. If youve ever been to Meteor Crater in Arizona, you have a good idea of what this martian crater really looks like. But keep in mind that this crater is seven times as large!

This image shows mesas and smaller buttes that occur on the Elysium Plains in the martian eastern hemisphere. Scientists liken this region to the world-famous Monument Valley located in the Navajo Nation on the border of Arizona and Utah. Like their terrestrial counterparts, the mesas and buttes here formed by erosion of layered bedrock.

This image shows a field of dark sand dunes in the Nili Patera area of Syrtis Major, which lies near the martian equator. The odd shape of these sand dunes suggests that wind has moved the sand from the upper right to the lower left area of the image. The resolution of this photo is about 10 feet per pixel.

This image features typical afternoon clouds over each of the large Tharsis volcanoes, or "montes." Olympus Mons is in the upper left corner, with the three Tharsis volcanoes running diagonally from upper center to lower left. Theyre called in order Ascraeus Mons, Pavonis Mons, and Arsia Mons. Together, the Tharsis Montes cover an area that would stretch down the entire west coast of the United States.

This picture was taken in early spring, when the dark sand in this particular region of the north pole was covered by frost left over from the northern winter season. At other times of the martian year, these dunes would most likely look much darker than the rest of the scene. The frost in the area is thought to be frozen water.

Prior to the arrival of the Mars Global Surveyor, some experts thought that the lineated valley floors such as this one in the area of Arabia Terra might be the result of large-scale flows of material, similar to glaciers on Earth. But no proof to support this theory was found, so the origin of this type of terrain remains unknown.

There are many land formations on Mars that are thought to have been formed by water. Scientists say that seepage of underground water may have formed this particular landscape, located in an area called Terra Tyrrhena. The small ridges on the valley floor are windblown sand dunes.

This wide-angle camera view of Mars was snapped in late July 1999, about a week before the start of the planets southern spring. At this time, wintertime frost was slowly melting. A slew of dust storm clouds (grayish-orange) occur just above the southern polar cap at the lower left.

Every now and then, the imaging team at Malin Space Science Systems the company in charge of Mars Global Surveyors Orbiter Camera notices a funny martian formation that resembles an Earthly object or symbol. This time around, it was a crater in the shape of a smiley face. So, this crater, whose official name is Galle Crater, has been appropriately nicknamed the Happy Face Crater!