Mars's polar caps

In 1666, Gian Domenico Cassini observed Mars’s surface and described the planet’s most visible features of two polar caps. These caps have been studied extensively since, telescopically and through theMariner, Viking, Mars Global Surveyor, Mars Odyssey, Mars Reconnaissance Orbiter, and Mars Express space probes.

Overview

When the Italian astronomerGian Domenico Cassini observed the polar caps of Mars in 1666, little was known about the planet’s surface features. A decade before Cassini, the Dutch astronomer Christiaan Huygens suggested the presence of polar caps on Mars, but it was not until 1672 that he saw the planet’s south polar cap, Mars’s most apparent feature, when one views it through a telescope. No one before Cassini had seen the polar caps in detail because no instrument had existed to provide sufficient magnification.

After the invention of the telescope, early astronomers who observed some of the surface features of Mars reached conclusions based on analogies with Earth, the only planet whose atmosphere and chemical composition they knew in detail. It is not surprising, then, that the Martian polar caps were presumed to be like the polar caps found on Earth: composed of ice, snow, hoarfrost, or a combination of these forms of water. This theory presupposed that Mars has water, a prerequisite for life, and therefore water's constituent elements, hydrogen and oxygen. This notion gave rise to the popular theory that Mars could support life in some form, which was subsequently brought into considerable question and remained controversial after the early Mariner and Viking missions to Mars.

In 1719, the Italian astronomer Giacomo Maraldi discovered that polar caps are not centered at Mars’s precise geographical poles. Maraldi also divined from his observations that the south polar region of Mars has a much greater mass than its northern counterpart. Subsequent research and observation have substantiated Maraldi’s theories, revealing the planet’s north polar cap lies about sixty-four kilometers from its geographical north pole and that the south polar cap lies some 400 kilometers from the geographical south pole. This phenomenon occurs because of the planet’s elongated orbit.

Italian astronomer Giovanni Schiaparelli trained telescopes on Mars repeatedly during the 1877 opposition of the Red Planet and drew highly detailed maps of Mars’s surface. Those maps documented networks of linear features Schiaparelli was convinced existed in great numbers between 60 degrees north and south in latitude. Reports of these linear features were taken out of context in public accounts. Schiaparelli had termed these features canali, the Italian word for channels. However, these linear features were most often referred to in English as canals, with the implication that they were artificial structures (in turn implying that they were built by intelligent life forms) to convey water from one location on the planet to another. Thus, the notion of intelligent life on Mars was born and proceeded to take on a life of its own in science fiction as well as in pseudoscience.

The wealthy American astronomer Percival Lowell developed a fascination with the planet Mars. He built the Lowell Observatory in 1894 in Flagstaff, Arizona, atop Mars Hill and spent the final twenty-three years deeply engaged in astronomical investigations, especially focusing on Mars. Lowell sought to use advanced telescope technologies to search for Mars's canals, prominent in Schiaparelli’s maps of the Red Planet. Lowell believed in and popularized the notion that the canals were evidence of alien technology. In three books, Lowell advanced the theory that the Martians had built an elaborate system of canals to bring precious water from the polar ice caps to population centers suffering from increasingly arid conditions.

Lowell’s notions were not well received by the scientific community (although he later would expend considerable effort to locate a body beyond Neptune, which bore fruit after his death with the discovery of Pluto by Clyde Tombaugh). The atmosphere of Mars is known to be such that there can be no accumulated areas of free liquid water, such as lakes, ponds, rivers, or oceans, on the planet’s surface. Much evidence suggests, however, that in the past, the Martian surface was extensively shaped by flowing liquid. Much evidence suggests, however, that the Martian surface was extensively shaped by flowing liquid in the past. Most evidence discounts the theory that this liquid was lava flowing from the planet’s once-active volcanoes. Accepted theory indicates, rather, that in one stage of the planet’s evolution, water in liquid form was plentiful.

When everything astronomers knew about Mars came from their observations through telescopes, they could gather substantial information about the polar caps; they were at odds, however, in their interpretations of these data. They could not always be sure what they were seeing. They also had little means of knowing with any certainty the depth of the polar caps. Some scientists thought they were hundreds of feet deep. Others thought the caps were merely thin coverings of hoarfrost. Common sense supported the latter idea. The argument was that if a thin layer of hoarfrost covered the poles, it would, under warmer conditions, condense and form clouds. On the other hand, if the polar caps were composed of fairly deep layers of solid water, where would the melting residue run during the warmer season? The atmosphere of Mars was then thought to be almost totally dry, yet observations through telescopes clearly showed that in the summer, the polar caps seemed to melt partially. Certainly, they diminished in size, and the areas surrounding them assumed a dark coloration, suggesting that water was mixing with minerals and dust at the periphery of the thawing area and causing what looked like a moist condition.

The south polar cap covers an area of more than ten million square kilometers in winter and, at times, extends almost halfway to the Martian equator. Even a minimal thawing would produce water in great quantity, especially if the water ice ran to great depths, but no water from the polar cap appeared through the telescope to have penetrated other areas of the planet, which seemed dusty and dry.

Faced with this anomaly, scientists could do little to verify their theories until a different sort of evidence, the kind first provided by the Mariner and Viking spacecraft, became available in quantity between 1965 and 1977. Viking 2 measured some of the temperatures of the north polar cap with a high degree of certainty and discovered that temperatures in its dark surrounding areas had a range of between 235 and 240 kelvins and that the white, presumably frozen areas had a range between 210 and 205 kelvins. These temperatures exceed 194 kelvins, at which carbon dioxide sublimates into a solid.

The atmospheric pressure on Mars, thought until the Mariner 4 flyby in 1965 to be about eighty-five millibars, was closer to a thin ten millibars. Earth’s atmosphere at sea level is 1,000 millibars or 1 bar. The Martian atmosphere is comparable to Earth’s atmosphere at about 24,000 meters, where the pull of gravity is all but lost. Such an atmosphere does not permit freestanding water, indicating that if the Martian atmosphere had always been the way it is now, the planet could never have had water. The polar caps would necessarily be composed of some other substance.

This thinking gave rise to the theory that the polar caps were composed of solid (frozen) carbon dioxide or dry ice. Lowell had advanced such a theory as early as 1895. Other eminent astronomers considered it more likely than the water theory as late as 1971 when Mariner 9 was uncovering data that would soon vitiate, although not eliminate, the carbon dioxide theory. Mariner 4, when it made its flyby of Mars in 1965, had returned data suggesting that the Martian atmosphere was largely composed of carbon dioxide under weak pressure. This information caused some astronomers to cast their lots with Lowell’s theory that carbon dioxide in its frozen state covered the polar caps but that, when the temperature rose, it became, through sublimation, gaseous and returned to the atmosphere as mist or fog. This explanation helped to account for the haze often observed over the polar regions.

Mariner 9 data substantiated the theory that Mars’s atmosphere accommodates no accumulations of liquid water. It also presented incontrovertible evidence that the polar caps are composed largely of ice and are minimally 0.8 kilometers deep, indicating clearly that the Martian climate has changed through the eons and that it was once such that liquid water, locked in the polar caps in solid form, was abundant.

The earliest space missions carried out research only concerning the north polar cap, but they managed to dispel a substantial number of misconceptions about Mars, among them the mistaken idea that the planet is totally dry. Data sent back suggested that the atmosphere is quite moist in the northern latitudes above 60 degrees. The atmosphere over the north polar cap has twenty times the water vapor that the atmosphere over the equatorial regions contains. During the Martian summer, surface ice exposed to the Sun evaporates in the morning, causing a mist that seems to condense, resulting later in the day in precipitation. Although these data were gathered exclusively over the north polar cap, there was little reason to think the south polar cap differed significantly. At perihelion, the planet’s closest approach to the Sun, the north polar cap does not face the Sun directly but is tilted away from it. This tilt prevents it from getting the full impact of the solar rays, which would, presumably, melt a polar cap composed of frozen water.

Space probes to Mars indicated that in the ancient past, water must have been quite plentiful on the planet. The climate of Mars must be viewed over eons. The planet is much drier than it once was, and scientists think that it could evolve through this period to one in which conditions resemble what they were when the Martian atmosphere allowed the accumulation of water in bodies similar to the ones on Earth.

It is widely accepted that portions of both of the major early theories about the polar caps were true. Although the evidence is strong that deep layers of water ice cover the polar regions, and although it is known that the deep crater Korolev in the north polar cap is filled with water ice, many astronomers think that during certain seasons, there are thin coatings of carbon dioxide that sublimate into the gaseous state and cause the clouds or mists that have been observed over the polar caps.

Methods of Study

In the three and a half centuries that the polar caps of Mars have been observed, descriptions of the caps have moved from highly speculative to soundly scientific. Astronomers have moved from primitive optical telescopes to incredibly complex telescopes of enormous size capable of detecting invisible radiation, strategically placed to focus on the sky and the planets. Mars has been the most intriguing planet for most astronomers to explore because it is sometimes relatively close to fifty-six million kilometers from Earth. It is also the planet that most resembles Earth in its surface features, although its atmosphere precludes advanced life.

The Mariner and Viking spacecraft first solidified human knowledge of Mars. Telescopic evidence suggested to many that the caps must be composed of water ice because of reflections detected from the polar caps. When the earlier Mariner missions presented evidence favoring the carbon dioxide theory, astronomers were forced to rethink their earlier stands. Later expeditions offered convincing evidence in favor of the water-ice theory. When Viking 1 landed, it photographed the surface extensively and transmitted the pictures to Earth. It also deployed an arm that dug into the Martian surface and analyzed the composition of the materials it uncovered. Finding water locked in Martian rocks clearly established the earlier existence of water on the planet.

Mariner 9 was placed in orbit around Mars in 1971, sending back more than seven thousand pictures. It photographed the south polar cap continuously from November 1971 until March 1972, capturing the waning of the polar cap as summer advanced. These pictures provided extremely varied information and bolstered the water ice theory.

The goal of the Viking program was to land on the surface of the planet and analyze the soil for signs of life. However, it is often overlooked that the landers were each transported to Mars by an orbiter. The Viking 1 orbiter lasted from orbital insertion on June 19, 1976, until August 17, 1980, when it ran out of altitude control propellant. The Viking 2 orbiter lasted from August 7, 1976, until July 25, 1978, at which point it suffered a fuel leak. During those periods, these orbiters produced extensive catalogs of images of a large majority of the Martian surface, including photographs of both polar regions, which, over time, displayed seasonal changes.

The Mars Observer spacecraft was launched on September 25, 1992, and was designed, among other things, to generate a global distribution map of Mars’s elements and minerals. This spacecraft was one of the largest and most sophisticated yet dispatched to Mars. Unfortunately, before it could enter orbit and perform any scientific operations, including studies of the polar caps, Mars Observer was lost on August 21, 1993. The loss of Mars Observer had serious repercussions for studies of the Martian polar regions.

Mars Global Surveyor (MGS) was launched on November 7, 1996, and entered a stable Martian orbit on September 11, 1997. It was the first successful American spacecraft to the red planet in twenty years since the two Vikings. MGS entered a near-polar orbit, so it was also used to study the polar ice caps as it imaged geological features across the planet. Its purpose, to search for evidence of water, went far beyond merely searching for water in the polar regions. MGS provided images of gullies on the walls of craters that could be the result of groundwater eroding the crater walls before freezing. In early November 2006, MGS went silent after experiencing a problem directing its solar arrays toward the Sun to generate sufficient electrical power.

The Mars Climate Orbiter (MCO) was NASA’s next probe sent to Mars after the highly successful Mars Pathfinder rover, which generated tremendous national and international excitement in 1997. MCO was launched on December 11, 1998, primarily to study Mars’s atmosphere. Investigating the transport of water and atmospheric circulation would also have indirectly increased humanity’s understanding of the polar caps. Unfortunately, because of a human miscommunication between engineers, some working using the metric system and others using British units, MCO came in too close to Mars on the final approach to orbital insertion. On September 23, 1999, MCO performed its Mars orbit insertion burn but never came around the back side of the red planet. Entering orbit far too low an altitude, only fifty-seven kilometers above the surface, MCO burned up and was lost.

The Mars Polar Lander (MPL) was a powered lander intended to touch down on the surface of Mars in the southern polar region to search for evidence of water in the soil. The spacecraft was equipped with a sample scoop at the end of a robotic arm to dig up samples. A gas analyzer was included among its scientific instruments to heat soil samples and detect any volatile gases, such as water vapor, given off in the process. On December 3, 1999, MPL began its entry phase at a speed of sixty-eight kilometers per second. Unfortunately, the onboard guidance system appeared to cut off the engines, slowing its speed at an altitude of forty meters and causing the lander to crash on the Martian surface.

Mars Odyssey was launched on April 7, 2001, and entered a nearly polar orbit on October 24, 2001. In addition to being a scientific research platform, the spacecraft was intended to serve as a communications relay for later surface probes (such as Mars Phoenix). Designed primarily to search for evidence of water, perhaps the most important discovery made by Mars Odyssey was the detection of significant amounts of hydrogen, interpreted as an indication of subsurface water. An extended mission was approved to continue observations of the polar ice caps for contemporary changes.

Mars Express was launched by the European Space Agency (ESA) on June 2, 2003. It represented the first planetary mission attempted by ESA and was followed a few years later by a similarly designed probe sent into Venus's orbit, the Venus Express. Mars Express entered Mars orbit on December 25, 2003. It carried a radar system capable of detecting subsurface water ice in permafrost regions. In a near-polar orbit, it was able to study surface changes and make mineralogical maps in addition to monitoring seasonal changes in the polar caps. Mars Express’s Planetary Fourier Spectrometer (PFS) discovered that methane was entering the atmosphere in near-equatorial regions having subsurface ice. Its Visible and Infrared Mineralogical Mapping Spectrometer detailed hydrated sulfates. Radar data provided evidence of underground water ice. Mars Express found the signature of water ice on the planet’s south pole. What some interpreted as a deposit of water ice was discovered on the floor of an otherwise nondescript crater in the vicinity of Mars’s north pole.

Mars Reconnaissance Orbiter (MRO) was launched on August 12, 2005, and injected into orbit about Mars on March 10, 2006, joining five other operational spacecraft already performing scientific investigations of the Red Planet. MRO was outfitted with the largest imaging system, HiRISE, ever sent to another planet, spectrometers, and a penetrating radar. Its primary science objectives were to search for water in the polar regions and a daily determination of meteorological conditions that might lead to changes in Mars’s water deposits. HiRISE images provided evidence of banded terrain, suggesting the action of water on the surface in relatively recent geological times. The spacecraft’s radar was able to detect underground deposits of water ice.

Mars Phoenix successfully touched down on Mars’s northern arctic region on May 25, 2008, in an area largely devoid of large and medium-sized rocks. Its landing marked the first time since the Viking spacecraft that a powered lander had successfully reached the Martian surface. The landing site was chosen to have a high possibility of a layer of ice on the surface or not very far beneath. Phoenix was outfitted with a robotic arm, at the end of which a scoop was attached. The scoop not only could pick up loose surface soils but also could scrape the ice. The idea was to lift soil and potential ice into a special gas analyzer that would process the soil and chemically analyze it for the presence of water and organic molecules. Very soon after Phoenix landed, initial photographs strongly hinted that the lander had touched down near ice. Under the lander, it appeared that exhaust from Phoenix’srockets had kicked up the soil, exposing subsurface ice. Early attempts to analyze the soil were thwarted by communications problems, and the entryway into the gas analyzer was clogged by clumped soil. The first run of the lander’s Thermal Evolution and Gas Analyzer produced a disappointing lack of water signature. However, subsequent data did provide strong evidence of water at the lander’s site.

Scientists studying MRO data revealed that Mars’s crust and upper mantle could be even colder than initially believed. If true, that would mean that liquid water would have to be found even deeper, where internal heat would permit water to exist in that phase. MRO’s Shallow Subsurface Radar (SHARAD) experiment probed the internal structure of ice, sand, and dust layers on the north polar cap and revealed the planet’s lithosphere (combination of crust and mantle) to be thicker and colder than previous models indicated and as such provided greater support against the stress of the weight of the polar cap atop it. Radar imagery indicated four zones of thin ice and dust layers interspersed with rather thick layers of water ice going down into the lithosphere to a geological depth such that it recorded changes in climate on Mars over many millions of years. The thin layers represented climate changes, perhaps lasting only one million years. These data did not shed light on the cause of climate change, but two strong possibilities were alterations of Mars’s rotational axis and/or orbital eccentricity.

Context

Whether there is or could ever be life on Mars has long been a matter of conjecture. If there is, was, or will be life on Mars, that life would likely be confined to organisms much smaller and less complex than anything resembling human beings. To sustain animal or vegetable life, it is presumed that some minimal atmosphere and water must exist. Research into the polar caps of Mars provides evidence that water is plentiful in the planet’s frozen polar caps. The same research suggests that the planet is much drier than Earth, that its atmosphere is so rarefied that it cannot support complex organisms, and that its temperatures (although much less forbidding than those on Jupiter or Venus) are not conducive to life.

In the second quarter of the twentieth century, Eugene M. Antoniadi, a Greek-born astronomer who spent his professional life in France, explained the dark areas on the periphery of the polar caps in summer and designated the Lowell bands for astronomer Percival Lowell. Antoniadi thought that the fringes of the ice fields were reduced in brightness by grasses and bushes that grew in the periphery, presupposing by such a contention the existence of life on Mars and of an atmosphere that would support life. This theory has been disproved, but it reflected the widespread notion that life exists on Mars—although not in the form of the little green men that some works of science fiction describe.

Spacecraft exploration of the polar caps of Mars has revealed that water in some form exists on Mars and has suggested that water in liquid form was once more plentiful there than it is in the modern world. The channels of Mars are generally thought to have been forged through the eons by flowing water, and the presence of the polar ice caps supports this theory. These explorations have also presented evidence that the atmosphere of Mars has changed drastically from what it once was. Climatic change may one day produce a Mars quite different from the present planet. However, astronomers who make such projections caution that they are talking about millions of years, perhaps hundreds of millions. Mars is unlikely to change in easily perceptible ways within the life span of any living human.

In the aftermath of the 2003 Space ShuttleColumbia accident, US President George W. Bush redirected NASA to complete the International Space Station and retire the shuttle fleet by 2010 to move on to human exploration beyond low-Earth orbit. In August 2012, the Mars Science Laboratory mission landed its Curiosity rover on Mars's Gale Crater, with a key goal of further studying water availability on the planet to support a future crewed mission to Mars. Sending humans to Mars is possible only if the robotic search for water bears fruit. The polar ice caps may well play prominent roles in liberating water from the surface of Mars for use by future astronauts. In 2018, the Mars Express used radar pulses to discover liquid salt water near Mars's south pole. Further research both confirmed and disproved this finding, but further research on the polar ice caps was necessary.

Bibliography

Barlow, Nadine. Mars: An Introduction to Its Interior, Surface, and Atmosphere. Cambridge UP, 2008.

Carr, Michael H. The Surface of Mars. Cambridge UP, 2007.

"Dry Ice on Mars." NASA, 2019, solarsystem.nasa.gov/resources/2165/dry-ice-on-mars. Accessed 20 Sept. 2023.

Glasstone, Samuel. The Book of Mars. Washington: NASA, 1968.

Halton, Mary. "Liquid Water 'Lake' Revealed on Mars." BBC, 25 July 2018, www.bbc.com/news/science-environment-44952710. Accessed 20 Sept. 2023.

Hartmann, William K. Moons and Planets. 5th ed. Thomson, 2005.

Hartmann, William K., and Odell Raper. The New Mars: The Discoveries of Mariner 9. Washington: NASA, 1974.

Kargel, Jeffrey S. Mars: A Warmer, Wetter Planet. Springer, 2004.

Lowell, Percival H. Mars and Its Canals. Macmillan, 1906.

"Mars Exploration Program." NASA, 2023, mars.nasa.gov. 20 Sept. 2023.

Moore, Patrick. Guide to Mars. Norton, 1977.

"NASA Orbiter Penetrates Mysteries of Martian Ice Cap." NASA Jet Propulsion Laboratory, 26 May 2010, www.nasa.gov/mission‗pages/MRO/news/mro20100526.html. Accessed 8 July 2013.

Phillips, Roger J., et al. "Massive CO₂ Ice Deposits Sequestered in the South Polar Layered Deposits of Mars." Science, vol. 332, no. 6031, 2011, pp. 838–841.

"Polar Caps." Mars Space Flight Facility, Arizona State University, marsed.asu.edu/mep/ice/polar-caps. Accessed 20 Sept. 2023.

R. Orosei, et al., "Radar Evidence of Subglacial Liquid Water on Mars." Science, vol. 361, no. 6401, 2018, pp. 490-493. DOI:10.1126/science.aar7268.