From the January 2018 issue

Seasonal changes in color or contrast on mars were once thought to indicate the presence of vegetation or water. Are there still seasonal changes, and if so, why?

Paul W. H. Tung Freedom, New Hampshire
By | Published: January 31, 2018
A polar ice cap and variations in the color of Mars’ mid-latitude terrain are readily visible in this Hubble Space Telescope image of the Red Planet, taken March 21, 1995.
NASA, ESA, and The Hubble Heritage Team (STScI/AURA)
In the days before spacecraft observed Mars in detail, we had to rely on what we could see with telescopes on Earth and the flyby Mariner missions of the 1960s. These fuzzy images only gave the barest indication of geologic features on the surface, indicated by light regions and dark regions, as well as the white polar caps. Because there was no real understanding of the planet’s surface, observed changes in color and brightness throughout the year were ascribed to any number of explanations, from water to vegetation. Upon closer inspection, starting with the Viking orbiters in the 1970s, it became clear that Mars is a dry desert world devoid of vegetation or water, which is not stable under current environmental conditions. However, huge canyons and channels reveal that, several billion years ago, Mars was home to flowing water. How it transitioned from a warm, wet planet to a cold, dry one is still up for debate, but the question of seasonal changes is one we can explain.

Like Earth, Mars has an axial tilt (25° for Mars, 23.5° for Earth) that gives the planet seasons. Its polar caps are composed of both water ice and carbon dioxide ice. During the northern or southern summer, the carbon dioxide ice in that polar cap vaporizes (sublimates) into the atmosphere and the polar cap shrinks, revealing the surface beneath. In the winter, the ice is redeposited and the caps grow again. Some of this sublimated carbon dioxide is also redistributed in the atmosphere toward the equator, and then deposited on the surface when the atmosphere cools or the pressure decreases, leaving a covering of frost in the mid- to high latitudes, and at high elevations at low latitudes. This frost can lead to changes in color and brightness, and will sublimate again as spring and summer return.

Sublimation of carbon dioxide frost can also trigger small-scale events such as dust avalanches and debris flows on steep slopes, which have now been observed in real time. Another cause of these changes is global dust storms, which occur every few years and are triggered by seasonal changes in temperature and pressure of the atmosphere.

Dan Berman 
Research Scientist, 
Planetary Science Institute, Tucson, Arizona