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Space Topics: Planetary Analogs

Haughton Impact Crater

Far north in Nunavut Territory in the Canadian High Arctic lies Devon Island, the world's largest uninhabited island. Devon is home to one of the northernmost impact structures on Earth, Haughton Crater. Measuring about 20 kilometers (12.4 miles) in diameter, it formed 23 million years ago when either an asteroid or a comet collided with our planet. At that time, during the Miocene epoch of geologic time, the climate was warmer. Boreal forests experiencing months of continual daylight followed by months of darkness covered the land. Among the thick growths of conifers and birch trees roamed giant rabbits and small rhinoceroids (ancestral cousins to the modern rhinoceros). Streams and lakes teemed with fish.

In an instant, things changed dramatically. A giant meteorite, perhaps 1 kilometer (0.6 mile) in diameter, plowed into the scene. It may have happened in the broad daylight of summer or in the bleak darkness of winter – we may never know. In either case, the impact, delivering an energy equivalent to 100 million kilotons of TNT, would have produced a blinding flash of light, then a monumental air blast that obliterated almost all life for several hundred kilometers around. A colossal shock wave expanded through the ground as the impactor dumped its cosmic momentum into the Earth, blending into the target rocks and vanishing as a superheated gas. The rocks themselves were crushed, melted, vaporized, pushed aside, and ejected. A cavity some 20 kilometers (12.4 miles) wide and 1.7 kilometers (1 mile) deep appeared, only to grow shallow as its unstable walls collapsed inward. Once the dust cleared, a smoldering hole with a vast pool of molten carbonate rocks appeared. Within seconds, Haughton Crater was born.

With Haughton Crater, Devon Island offers the only terrestrial impact structure known to lie in a cold, relatively dry, windy, rocky, dusty, ultraviolet (UV) light-drenched (in the summer), and nearly unvegetated polar desert. From that standpoint, it promises to serve as a parallel to Mars. Although conditions on Devon Island remain significantly milder than those prevailing on Mars (for instance, the average temperature on Devon is -17 degrees Celsius, or 1 degree Fahrenheit, versus -60 degrees Celsius, or -76 degrees Fahrenheit, on Mars), they are a step in the right direction.

Devon Island and Haughton Crater are now being explored as a Mars analog through the NASA Haughton-Mars Project (HMP), led by Pascal Lee, a planetary scientist at the SETI Institute. The HMP is an international, interdisciplinary field research project comprising both a science program -- which focuses on learning more about Earth and Mars, impact cratering, and life in extreme environments -- and an exploration program looking to develop new technologies, strategies, and experience with human factors that will help plan the future exploration of Mars and other planets by robots as well as humans.

Many features outside Haughton Crater itself are also contributing to solving, and sometimes deepening, the mysteries of Mars.  For example, networks of channels found on Devon Island bear similarities to the so-called Martian small valley networks. Most of the latter date from the end of the Heavy Bombardment, while some are also found on more recent terrains, such as the flanks of relatively young volcanoes. The Martian small valley networks are classically thought to be the result of liquid water runoff flowing across the Martian surface (not in the form of gigantic floods, as in the case of the Martian outflow channels, but in more modest trickles) after either localized rainfall, groundwater release, or mud flow.

These interpretations require a fairly warm climate for liquid water to flow at the Martian surface over distances of tens to hundreds of kilometers without freezing. Such a conjecture has forced Mars climate modelers over the past decades to invoke increasingly elaborate mechanisms of climate warming for early Mars, particularly given Mars' relatively great distance from the Sun, small planetary mass, the early Sun's fainter light, and how impacts may have stripped Mars of its early atmosphere(s).

Devon Island is incised by a multitude of small valley networks that bear an uncanny morphologic resemblance, including in their bizarreness, to many of the small valley networks on Mars. The Devon networks formed neither by rainfall, groundwater release, nor mud flow but by the melting of vast ice covers that once occupied the land above the now-exposed surface. Sections of valley floors slant uphill as one hikes downstream, indicating that some valleys are actually channels that formed by confined flow when meltwaters gushed under a wasting ice cover. Because these ancient ice sheets were very cold and mostly static, uplands off to the sides of the channels were spared significant glacial erosion. If anything, the ice cover protected them.

So, is it possible that the many small valley networks on Mars are actually cold climate features instead of evidence that Mars once had a relatively mild climate? Might they have resulted from the subglacial melting of insulating ice covers, which accumulated above the highlands and on the flanks of volcanoes when the ground was warmer and water more readily recycled to the surface by impacts and active volcanism – though in a frigid climate? Might Mars have been cold climatically throughout most of its history, with liquid water at most a local and transient phenomenon at the surface?

The mighty canyons of Devon Island might, independently, reinforce this picture. They seem to have specific morphologic counterparts on Mars, in particular the broad, winding V-shaped valleys of Ius Chasma in western Valles Marineris. Some believe the latter formed by sapping – that is, the slow release of groundwater accompanied by progressive "headward" erosion of rocks in the direction of the source. The canyons on Devon, however, are the result of glacial erosion (the carving done this time by ice, not meltwater, as in the case of the island's channel networks). Might their counterparts on Mars result from glacial carving as well?

While not settling the mystery of past climates on Mars, work on Devon Island is offering new interpretations for many of the Red Planet's so-called fluvial landforms. Research suggests that surface ice deposits have played a much greater role throughout Martian history than classically suspected.

On another front, the ubiquitous presence of ground-ice near the surface in the Arctic is visible, if indirectly, across the landscape on Devon. Terrain features such as rock glaciers, ice-cored mounds, polygons, rock circles, rock stripes, and myriad other forms of "patterned ground" abound. Such features are the trademark of periglacial processes, or processes shaping the landscape in environments that are rich in ground-ice.

Viking orbiter and Mars Global Surveyor (MGS) images of Mars reveal the presence in many locations (mostly at high latitudes but also elsewhere) where similar-looking features occur. One implication is that ground-ice might have been abundant near the Martian surface when the features formed. Given how fresh some of these features look today, might ground-ice still be present at their locations on Mars?

This text is modified from an article written by Pascal Lee for the January/February 2002 issue of The Planetary Report.