The remnants of the snow patches, craters and other similar areas on the surface of the Red Planet may be an excellent place to search for life in future robotic missions. Last article in the series
P. Christensen, Scientific American
Mars' tumultuous past gets most of the attention, but two developments have reignited research into the activity taking place there today. The first is the growing naivete that Mars was geologically active in the recent past. Although most of the large volcanoes and ancient lava plains were formed in the first half of Mars' history.
But the absence of meteor craters on the surface of lava flows in areas such as Athabasca suggests that the young (on a geological scale) flows were formed in eruptions that occurred in the last few million years. Researchers have been examining infrared images taken at night looking for hot or active geothermal spots, but so far to no avail. It seems that Mars has cooled and reached a state where volcanic activity on it is very rare, although lava occasionally rises and erupts to the surface.
frozen water
The second discovery is that Mars has huge reservoirs of frozen water that migrate across the planet as its climate changes. At both poles there are ice deposits, or ice-rich sediments, several kilometers deep and in a total area that reaches almost twice the area of the state of Arizona. Infrared temperature measurements conducted in the 70s determined that the Arctic ice cap is composed of frozen water, but failed to determine the composition of the southern cap.
The surface temperature at the South Pole corresponds to that of frozen carbon dioxide (dry ice), but could there be water ice underneath? Recent temperature measurements using THEMIS have found water ice sticking out in some places, so the answer is probably yes. This was also confirmed in the measurements made by the European spacecraft "Mars Express".
The "Odyssey to Mars" spacecraft also added to the water supply when its gamma ray spectrometer and high energy neutron detector discovered underground ice. Both instruments measure gamma rays and neutrons emitted when cosmic radiation hits the atoms in the ground. The energy distribution of the photons in the gamma radiation and of the neutrons reveals the composition of the chemical elements in the soil down to a depth of several meters.
Hydrogen, for example, absorbs neutrons very efficiently, so a lack of neutrons suggests hydrogen on the surface - most likely the two hydrogen atoms in the water molecule (H2O). In the areas between 60 degrees latitude and the poles it seems that the percentage of water in the soil is higher than 50%. Such a high explosiveness of ice cannot be created just from a simple bubbling of water vapor from the atmosphere into pores in the ground. The ice, therefore, had to accumulate as snow or frost.
The soil formations confirm the findings
Unusual landforms are common throughout the mid-latitude region, and even suggest ice. Between latitude 30 and latitude 50 degrees, in both halves of the planet, there is land whose surface resembles the surface of a basketball. Such soil may form when the soil warms up and the evaporating ice causes the soil to crumble into crumbs.
Another type of sediment, found in cold spaces on the slopes facing the pole, is a layer of material up to 10 meters deep - a possible remnant of snow made of almost pure water. Perhaps the most noteworthy discovery is the small, fresh crevasses found in the middle latitudes, crevasses formed perhaps by spring water, by the melting of ice near the surface or by the melting, from the bottom up, of piles of compacted snow.
All these features, associated with water, imply that Mars, like Earth, goes through cycles of ice ages. The angle at which the axis of rotation of Mars is inclined fluctuates and reaches up to 20 degrees in a cycle of 125,000 years. When the tilt angle is low, the poles are the coldest areas on the planet. So, the amount of snow that falls on them is greater than the snow that evaporates, and the ice keeps accumulating.
As the angle of inclination increases, the poles exposed to more sunlight warm at the expense of the mid-latitudes. The water then tends to move from the poles towards the equator. As the snow accumulates on the surface, running water may drip. Today the middle latitudes are warming and most of the snow cover is gone. If the ice age model is correct, snow will return to Mars in 25,000 to 50,000 years.
The story of the exploration of Mars is similar to the story of the blind man describing an elephant: the geology seems to change depending on where one observes it. The planet is a place whose surface is diverse, its present is surprisingly dynamic and its past is complicated and even paradoxical. Its volcanic rocks are as varied as those of the earth, and water shows an extremely extensive presence.
A rainy planet?
Earlier in its history the planet experienced heavy flooding and possibly even rainfall, yet its ancient rocks still contain minerals that break down quickly in a wet environment. Although today's climate is dry and cold, the Opportunity SUV found itself on the bottom of an ancient sea suggesting a climate that was once very different. Under current conditions, liquid water is unstable, however, crevasses formed there not long ago and may still be forming today.
The variety of conditions on the surface that vary from place to place and from time to time is one of the most encouraging signs about biology on Mars: the diversity provides a rich expanse of environments where life could find a foothold. Long, if intermittent, periods of water were common in the lakes and these periods of time may have been long enough to give life to inanimate matter.
Organisms may still be clinging to life, dormant during the cold periods and awakening when climatic conditions improve. The remains of the snow patches, ravines and other similar areas may be an excellent place to search for life in future robotic missions.
