Mars's craters

Examination of Mars’s craters reveals in part the history of the modification of Mars’s surface. Images transmitted from the first spacecraft sent to fly by Mars revealed a cratered surface somewhat resembling the lunar surface rather than one more Earth-like. However, craters on Mars undergo weathering processes that are different from those on either Earth or the Moon.

Overview

There are two classes of craters on any planet or other terrestrial body: impact craters caused by meteors or comets crashing into the surface, and volcanic craters. On all the known bodies of the solar system, including Mars, impact craters outnumber volcanic craters by the thousands.

Early views of the planet Mercury and the detailed knowledge of Earth’s Moon depicted both bodies as heavily cratered. Most of this cratering occurred in the earliest 500 million years after the planets had formed by accretion out of the primordial material that constituted the early solar nebula and sun. During this period, the planets were heavily bombarded from space by meteors, and planetesimals were caught in the gravitational pull of the newly forming planets.

Unlike Mars, Mercury, and its own moon, Earth does not show evidence of having been heavily cratered, although it was subjected to the same rate of incoming meteors; this is in part because Earth’s thick atmosphere burns up and destroys any incoming body of less than 1,000 kilograms in mass. Earth’s widespread weathering processes also quickly erase evidence of meteoritic craters. Any trace of a meteor falling into the ocean is either totally erased or largely mediated by the water. On airless, inactive bodies, however, there are few mechanisms to erase cratering, even though some occurred billions of years ago.

The biggest craters on Mars, typically caused by very large impacting bodies (such as asteroids), are called basins. The largest such basin on Mars is called Hellas Planitia; with a diameter of 2,300 kilometers and a depth of over 7,000 meters, it is the deepest point on the planet’s surface. Other such large features are Isidis (1,500 kilometers in diameter) and Argyre (1,800 kilometers).

The most common type of crater found on Mars is called a rampart crater, first described by planetary geologist J. F. McCauley in 1973. Rampart cratering seems almost unique to Mars. This interesting morphology consists of a central crater wall with an ejecta pattern that resembles a solidified flow pattern radiating outward from the crater wall as a low ridge and radial grooves. This flow pattern hints that the impact liquefied the subsurface materials, causing them to flow away from the impact point and then resolidify as a rampart or gently sloping wall. Causes of this fluidized ejecta are thought to have been entrapment of atmospheric gases in the ejecta or, more significantly, water in the ejecta material from melted permafrost, which acts as a lubricant, allowing the materials to flow away from the impact point. The latter concept is a favorite one of those who hope to discover immense amounts of water locked up in subsurface deposits as permafrost. Since rampart cratering is prevalent on much of Mars, such planetwide deposits of permafrost would be a very positive sign for eventual human exploration of the planet.

One of the most unusual craters found on Mars is the pedestal crater, formed by weathering action that cuts away at the base of the crater until the crater takes on the appearance of a pedestal. The unique meteorology and geology required to produce such craters are found on Mars between 40° and 60° north latitude.

In examining craters on Mars, planetary scientists are able to define the crater’s age, and hence that of the surrounding terrain, through a process of observing certain of its characteristics. The process of a crater eroding away is called degradation. Examining the original impact that formed the crater is a starting point; a new crater with little or no degradation is one with sharp edges and fresh ejecta outlying its central diameter. As a crater degrades through natural processes, its rim loses sharpness, and crater walls slump and lose their definition. Ultimately, the walls may form gullies and eventually become completely degraded to the surface level. In the final stage, the crater is hardly noticeable over the terrain, and some planetary scientists even call such a crater a ghost.

On Earth, the most profound degradation process is weathering, which can erase even large craters relatively quickly, in geologic terms. On Mars, such processes occur much more slowly. Since liquid water does not presently flow on the Martian surface, weathering processes are confined to wind erosion, or aeolian (wind) processes. It is estimated that aeolian erosion is responsible for filling in craters on Mars at the rate of 0.0001 centimeters per year. From these crater studies, there is considerable evidence that such erosion has slowed in the planet’s recent geologic past. The last period of relatively heavier erosion occurred roughly 600 million years ago, according to some crater studies.

The interior morphology of Martian craters is highly variable. It is typically dependent on the mass of the incoming projectile and the composition of the impact site. In smaller craters, the bottom usually assumes a more spherical shape. In larger craters, the central region of the crater flattens out until, in craters greater than twenty-five kilometers in diameter, a central peak is formed. Such flatness is often caused by the impacting body liquefying the crust or by magma welling up from the planet’s interior to fill in the crater.

By examining craters formed within other craters in a process called superimposition, planetary geologists are able to determine the age of whole regions of Mars’s surface. With an overall planetary comparison, which includes mass crater counts, it has been discovered that there are areas that have changed little since Mars was formed, while other regions, notably volcanic regions, have undergone significant change in the recent geologic past.

In some of the most fascinating crater studies accomplished, scientists have attempted to determine the age of what appear to be massive river channels on Mars. Although current conditions on Mars will not allow liquid water to exist on the planet’s surface, it appears that water once flowed in an extensive series of channels. Planetary geologists have examined craters that overlie these enigmatic channels. They range in age from 3.5 billion years to as young as 200 million years or less, strongly suggesting that the channels have been formed on a cyclic basis throughout the geologic history of Mars.

Mars has the largest volcanic craters in the known solar system, resulting from volcanoes between ten and one hundred times larger than the largest volcano on Earth, Mauna Loa in Hawaii. Four very large volcanic craters exist in close proximity in the Tharsis region. The largest volcano is Olympus Mons, a 200-million-year-old volcano nearly twenty-two kilometers high and with a crater eighty kilometers across. These Martian volcanoes, called shield volcanoes after their earthly counterparts, have craters nested inside one another. Three other Tharsis volcanoes are roughly alike in size, all much larger than Mauna Loa. Arsia Mons is almost twenty kilometers high, with a crater 110 kilometers in diameter. The largest Pavonis Mons crater is forty-five kilometers in diameter and five kilometers deep. The largest of the Ascraeus Mons craters is roughly the same diameter as the largest Pavonis Mons crater. The volcanic plains spreading away from the Martian volcanoes are dated according to crater distribution. The youngest is Pavonis Mons, at eighty million years old.

Methods of Study

Earth-based telescopic observations of Mars cannot clearly reveal its distinct surface features. The distance is too great, and the combined effects of Earth’s and Mars’s atmospheres reveal only indistinct and blurred splotches of color. Therefore, aggregate crater data is gathered via orbital photography from spacecraft. Almost all such data have come from flyby spacecraft, orbiters, landers, and rovers. The James T. Webb Space Telescope has also returned spectacular images of Mars’s craters.

The first distinct images of the planet’s surface came from NASA’s Mariner 4 spacecraft on July 14, 1965. This robotic spacecraft transmitted nineteen photographs of the Martian surface back to Earth. In the twenty-two minutes that it took to transmit the photographs, previous speculation about the Martian surface was laid to rest. The most surprising and pronounced of the Mariner 4 revelations was the extensive cratering of the Martian surface, a finding few planetologists had expected. Popular interest in Mars waned at this point, however, because the planet appeared to be rather moon-like instead of perhaps a place where life might have had a chance to develop.

Based on observations from Earth-based telescopes, it was widely expected before 1965 that there was enough weathering on Mars to have erased most of its primordial cratering. Mariner 4 clearly showed that this was not the case. The most immediate implication was that Mars was far less active meteorologically and geologically than was initially thought. Later, both extreme views were mediated by data from spacecraft that gave rise to more detailed studies, which showed that Mars has an active weathering process, though not as vigorous as that of Earth.

The twin Mariner 6 and 7 spacecraft flew past the Red Planet in 1969, only a few weeks after the Apollo 11 astronauts returned to Earth following the first crewed landing on the Moon. At this point, interest in sending humans to Mars was at a precarious stage: some were thrilled that Apollo had achieved its goal but were not interested in pursuing expanded human-based scientific exploration of space; others were eager to build on the success of Apollo and fly to Mars. The supposition of many was that the picture of Mars as revealed by Mariner 4 was atypical of the entire planet. However, Mariner 6 and 7 sent back a catalog of images of the Red Planet that dashed the hopes of those expecting Mars to be more Earth-like. The majority of the images displayed cratered features with no evidence of surface water.

Whereas Mariner 4, 6, and 7 had revealed an inactive world more like the moon, pockmarked with craters, Mariner 9, launched in 1971, returned images with resolutions as high as 0.1 kilometer per pixel and thus revealed an entirely new side of Mars’s surface characteristics. In addition to craters, Mariner 9 provided evidence of large-scale flows of water across the planet’s surface in the distant past. Volcanic craters were imaged to greater resolution, but no evidence was obtained to indicate volcanic activity or hot spots near Mars’s huge volcanoes. The images and data from Mariner 9 helped scientists determine how and when to attempt the next stage of Mars exploration: sending landers to the surface in 1976.

Viking1 and 2 each consisted of a joint orbiter and lander. Whereas the orbiters continued to image as much of the Martian surface as possible—including craters, valleys, volcanoes, and polar regions—the primary focus of the landers was to search for conditions that might support, or have supported in the past, primitive life on Mars. Viking 1 landed on Mars on July 20, 1976, in western Chryse Planitia, and Viking 2 on September 3, approximately 200 kilometers to the west of the crater Mie in Utopia Planitia. Both landers analyzed the soil but failed to find any organic material or evidence of life. It would be twenty-one years before another successful powered landing of a spacecraft, the Mars Pathfinder, on Martian soil. The Phoenix successfully touched down on the Red Planet’s northern arctic region on May 25, 2008, in an area largely devoid of large rocks. The landing site was chosen because it offered a high possibility of the spacecraft encountering permafrost with a layer of ice either on the surface or not very far beneath.

Following the 1976 Viking (1976) landings, NASA refined its plans for Mars exploration at least twice. As a result of the indication that water had once flowed across the cratered Martian surface, a new wave of Mars spacecraft was sent to the Red Planet. Successful missions included Mars Pathfinder, a surface rover (1997); Mars Global Surveyor, an orbiter (1996); Mars Odyssey, an orbiter (2001); twin Mars Exploration Rovers named Spirit and Opportunity (2003); the European Space Agency’s Mars Express (2003); the Mars Reconnaissance Orbiter (2005); and the Mars Phoenix, a powered surface lander (2008). Then, in August 2012, NASA's Mars Science Laboratory landed the rover Curiosity. This was followed in 2020 with the Perseverance rover. The thrust of scientific investigation of all of these second-generation Mars probes has been a search for Martian water, which requires investigation of Martian geology. This search includes the possibility of water ice deposits in craters and the erosional aspect of water on crater walls.

In the process, these spacecraft have provided greatly improved images of Mars. Some returned data over long periods of time, including detailed meteorological observations from both orbit and the surface. From this information, Martian cratering and morphology were assessed in detail. Some of the most elemental assessments of crater morphology include crater size, the composition of ejecta, composition and behavior of impacted terrain, modification processes of the crater, and its effect on surrounding craters and terrain. These findings led to a determination of the ages of the craters, surrounding crater fields, and even the impacted terrain. Assessments were made of crater sizes, numbers, distribution, and ages within selected planetary areas.

Techniques for analysis of Martian craters vary with the aim of the study. For example, a statistical counting method borrowed from other applications, called a frequency distribution, is used to determine the number of craters located over a given area. This information may lead to the determination of the age of the area under study, given a uniform crater deposition rate. Other statistical counting methods include cumulative distribution, logarithmic incremental distributions, and incremental frequency distributions, all of which are specific methods of presenting collected crater population data. Some appear as graphs, others as numerical tables, and others as simple maps showing the locations of craters over a given area. In the largest distribution sampling exhibit, the entire planet of Mars is presented with its crater distributions marked. All of these are unique representations designed to give the researcher information in a specific way so that the concept under study, such as the age of an area, can be efficiently gauged. Inferences are made from these statistical presentations about such broad concepts as crater production and erosion.

Planetologists have arrived at a rate of cratering for Mars that is equivalent to three hundred meteors, of one kilogram or larger, striking each square kilometer every million years. From this baseline of impacts, current photographs can be compared to establish which areas are very old and which are younger geological formations. The logical extension of such knowledge is the ability to date such formations as plains, volcanic flows, and streambeds, as well as to establish a baseline for regional aeolian erosion.

Context

The study of Martian craters allows planetary scientists to determine from orbital photographs many varied characteristics of Mars. From information relating twenty-first conditions on Mars all the way back to its earliest geological history, crater studies have allowed much of the planet’s chronology to be traced. Orbital studies of Mars’s craters may lead to answers about the planet’s most significant geological history, such as when water may have existed on Mars in liquid form, what formed the vast river channels and canyons, and what happened to the water. Answers to such questions will determine what humanity will have to do to survive on Mars should this neighbor planet be visited and perhaps colonized.

Crater studies also address questions of importance relative to Earth’s own history and future. Planetologists seek to understand why Mars is so different from Earth and what indeed happened to its once apparently abundant water. Such questions of planetary history relate to what may one day happen on Earth. If the scientific community is eventually able to draw enough parallels from the study of other planets, it may be able to apply rigid mathematical projections of Earth’s own future based on present conditions and trends.

Scientists continued to make important discoveries regarding craters on Mars in the twenty-first century. In 2021, the Mars Reconnaissance Orbiter sent back images of craters believed to be formed in that year, leading to important data about the planet’s subsurface water ice.

Bibliography

American Geophysical Union. Scientific Results of the Viking Project. Washington: Author, 1978.

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

Barlow, Nadine, and Virgil L. Sharpton. "Stones, Wind, and Ice: A Guide to Martian Impact Craters." Lunar and Planetary Institute. Lunar and Planetary Institute, 2012.

Beatty, J. Kelly, Carolyn Collins Petersen, and Andrew Chaikin, eds. The New Solar System. 4th ed. Cambridge: Sky, 1999.

Collins, Michael. Mission to Mars. New York: Grove, 1990.

De Pater, Imke, and Jack J. Lissauer. Planetary Sciences. New York: Cambridge UP, 2001.

Ezell, Edward, and Linda Ezell. On Mars: Explorations of the Red Planet, 1958–1978. NASA SP-4212. Washington: GPO, 1984.

Fisher, Alise. “Mars Is Mighty in First Webb Observations of Red Planet – James Webb Space Telescope.” NASA Blogs, 19 Sept. 2022, https://blogs.nasa.gov/webb/2022/09/19/mars-is-mighty-in-first-webb-observations-of-red-planet. Accessed 22 Sept. 2023.

Harland, David M. Water and the Search for Life on Mars. New York: Springer, 2005.

Hartmann, William K. Moons and Planets. 5th ed. Belmont: Thomson Brooks/Cole, 2005.

Mars Exploration Program. NASA, 2013.

"NASA Probe Counts Space Rock Impacts on Mars." NASA Jet Propulsion Laboratory. NASA Jet Propulsion Laboratory, 15 May 2013. .

Squyres, Steve. Roving Mars: Spirit, Opportunity, and the Exploration of the Red Planet. New York: Hyperion, 2006.

Tillman, Nola Taylor. “Perseverance Rover: Everything You Need to Know.” Space.com, 24 July 2023, www.space.com/perseverance-rover-mars-2020-mission. Accessed 22 Sept. 2023.

Urrutia, Doris Elín. “NASA Discovers a New, Two-Block-Wide Crater on Mars with Signs of Water Ice.” Inverse, 27 Oct. 2022, www.inverse.com/science/mars-announcement. Accessed 22 Sept. 2023.

Zubrin, Robert. Entering Space: Creating a Spacefaring Civilization. New York: Tarcher, 2000.