Surface experiments on Mars

The surface of Mars, as seen in telescopic observations and spacecraft images, hints at a history of past liquid water and possible habitability for life. Experiments conducted by robotic landers and rovers have helped scientists better understand the rocks and dust that form the planet’s surface and the role that water may have played in creating these materials.

Overview

While the surface of Mars has been studied extensively by telescope and spacecraft imaging (from both flybys and orbit), scientists’ understanding of the surface materials and climate has been improved by data collected from robotic spacecraft located on the surface. The National Aeronautics and Space Administration (NASA) has mounted several missions, including the Viking, Mars Pathfinder, and Mars Exploration Rover(MER) missions, that have provided a context for orbital and telescopic observations and have advanced our insights into the geologic history of Mars.

The Viking mission consisted of two identical spacecraft, each with an orbiter and a lander. They were launched in 1975 and arrived at Mars almost a year later. Both landers touched down in the smooth northern plains: Viking 1 in Chryse Planitia (22.3° north, 48.2° west) and Viking 2 in Utopia Planitia (47.7° north, 225.9° west). The landers contained a suite of instruments for studying the environment, a three-meter-long robotic arm to collect surface samples for analysis and two cameras. Images could be acquired in visible or near-infrared wavelengths 360° around the spacecraft from 40° below to 60° above horizontal. The elemental composition of surface materials was measured using an X-ray fluorescence spectrometer, and atmospheric samples were analyzed using a gas chromatograph/mass spectrometer. The latter instrument was designed to search for organic materials as well. Further tests for evidence of life included the pyrolitic release experiment (to identify the metabolism of carbon dioxide and carbon monoxide), the labeled release experiment (to detect the metabolism of nutrients), and the gas exchange experiment (to examine gases released when nutrients and water vapor were added to a sample). Additional instruments included magnets (to identify magnetic components of samples), seismometers, and meteorological stations that measured wind speed and temperature.

Mars Pathfinder, launched in December 1996, bounced onto the surface of Mars (cushioned by airbags) on July 4, 1997. Its landing site (19.3° north, 33.6° west)—800 kilometers east-southeast of the Viking 1 site—was selected for safety and as part of NASA’s “follow the water” strategy for Mars exploration. This location is at the mouth of Ares Vallis, a 1,500-kilometer-long outflow channel likely formed by catastrophic flooding. Rocks deposited there could have originated in any geologic unit crossed by the channel, so a variety of rocks and geologic time periods would likely be present (although source regions for the rocks would not be identifiable). The results were expected to increase knowledge of the geology and geochemistry of the Martian surface.

The Pathfinder mission consisted of a lander (named the Carl Sagan Memorial Station) and a rover (Sojourner). The Imager for Mars Pathfinder (IMP), mounted on the lander’s one-meter-high mast, provided panoramic stereo and multispectral imaging (up to eight Wavelength bands) for rover navigation and science analysis. A set of magnets were also part of the IMP: images of the magnets were acquired throughout the mission to determine how much magnetic material from the Martian dust had been attracted to them. The final instrument system on the lander was the Atmospheric Structure Instrument/Meteorology Package (ASI/MET), which measured atmospheric structure and temperature during descent and meteorological conditions (including temperature, wind speed, and wind direction) during operations.

The Sojourner rover, measuring sixty-five by forty-eight by thirty centimeters, could move up to 500 meters from the lander. The rover used cameras (two front-mounted stereo grayscale cameras and one color camera on the rear) for navigation and science analysis. The rear camera also provided context images for chemical analyses by the Alpha Proton X-ray Spectrometer (APXS) mounted on the rear. This instrument provided elemental compositions of rock targets (excluding hydrogen and helium) by bombarding the rock with alpha particles and measuring what type and energy the particle (alpha particle, proton, or X-ray photon) was backscattered off the rock.

The Mars Exploration Rovers—MER A (Spirit) and MER B (Opportunity)—were launched separately in June 2003 and landed on Mars in January 2004. Landing sites were chosen based on engineering constraints, safety, and continued scientific interest in evidence for past surface water. Spirit landed on the floor of the Gusev crater (14.7° south, 184.5° west), a 170-kilometer-diameter impact crater bordered on the north by the volcano Apollinaris Patera. Gusev’s southern rim is breached by a valley (Ma’adim Vallis) that terminates in the crater. The water that carved the valley may have once formed a lake in the crater. The landing site for Opportunity (2° south, 5.9° west) is located in Meridiani Planum, which was identified in orbital data as containing relatively high concentrations of the mineral hematite (Fe₂ O₃). On Earth, such concentrations are often formed by deposition in standing bodies of water.

The two MER rovers are identical in design. Each rover utilizes multiple imaging systems, including the Panoramic Cameras (PANCAMs) used for navigation and hazard avoidance and a Miniature Thermal Emission Spectrometer (Mini-TES). The PANCAM and Mini-TES optics are mounted on the 1.5-meter-high rover mast. PANCAM provides high-resolution images in fourteen wavelengths (visible and infrared) and stereo coverage in any orientation around the rover (360° panorama with 180° elevation coverage). These images are used for traverse planning and scientific analysis of both surface features and atmospheric conditions. Mini-TES detects thermal infrared radiation emitted by surfaces. Because different minerals emit different amounts of this radiation, Mini-TES can be used both to measure the temperature of a surface and to identify the minerals present.

Each rover has a robotic arm that can extend to reach rock targets up to 0.8 meters away. The arm is equipped with a Rock Abrasion Tool (RAT) to brush away dust or expose unweathered interiors of rocks. Scientific targets are examined using the Microscopic Imager (MI). This instrument acquires images of a thirty-one-millimeter-square area with a resolution of thirty microns per pixel. Compositional analyses are performed using a Mössbauer spectrometer, which is sensitive to minerals containing iron, and an APXS instrument similar to the one on the Mars Pathfinder.

Knowledge Gained

The Viking landers painted a bleak picture of the Martian surface, recording temperatures from 183 to 238 kelvins (-130° to -31° Fahrenheit) and high amounts of ultraviolet radiation. Images of the landing sites show a dusty, rock-strewn surface, with the seasonal formation of extremely thin water-ice frosts at the Viking 2 site. Chemical analyses of the bright red surface sediments are most consistent with iron-rich clay minerals and contain relatively high amounts of sulfur and chlorine, possibly concentrated by evaporation of water. In places, this sediment forms a thin (one- to two-centimeter), hard crust called a duricrust. Rocks at these sites appear dark in color, have a vesicular texture, and are likely basalt. The biological experiments proved ambiguous, as abiotic chemical reactions can explain the observed results. Both landers functioned for several years: Viking 1 until 1982 and Viking 2 until 1980.

Mars Pathfinder images show a similar terrain, with red dust blanketing randomly strewn dark rocks, although some rocks appear to be oriented along the direction of flooding. Compositional analyses, however, suggested more than basalt. Several Pathfinder results are consistent with basaltic andesite, a more silica-rich volcanic rock. Basaltic andesite formation requires recycling basaltic crust, melting (and then erupting) the lighter minerals, and leaving the heavier ones behind. On Earth, this process involves water-rich sediment and plate tectonics. The formation process for basaltic andesites on Mars is unknown. Pathfinder also provided detailed evidence of aeolian (wind) processes on Mars during its eighty-three-day mission, returning images of dust devils, wind streaks, and dunes.

The Mars Exploration Rovers have significantly changed our understanding of the geology of Mars. While initial analyses by Spirit found olivine-bearing basalts along the floor of the Gusev crater (unaltered volcanic rocks, not the expected lakebed deposits), areas studied later in the Columbia Hills, three kilometers from the landing site, included silica-rich sediment (possibly from an ancient hydrothermal system), iron sulfates, and evidence of explosive volcanism (which requires more silica-rich magma than basaltic eruptions, requiring crustal recycling).

Opportunity provided the most conclusive evidence for the presence of past surface water on Mars. Located fortuitously in a small crater, the first images showed an expanse of rock outcrop, not the scattered rocks seen at other sites. This outcrop and others examined later contain fine layers common in sedimentary rocks. Some of these layers are inclined, forming crossbeds that indicate ripples or dunes. Chemical analysis identified light-colored rock in this outcrop as jarosite, an iron sulfate mineral that forms by evaporation of water. Embedded in this layer are spherules of gray hematite (dubbed “blueberries” by the science team), similar to concretions of iron oxide that form in aqueous environments on Earth.

Context

Mars has long piqued the curiosity of observers with hints of past liquid surface water. Early surface experiments suggested a surface dominated by volcanic rock (basalt), while further results point to a more complex geologic history with possible bodies of water, hydrothermal systems, and recycling of the basaltic crust. The Mars Exploration Rovers continue to return data that paint a more detailed geologic history of their local areas of Mars, and the Mars Phoenix lander in 2008 examined the possibility of the existence of subsurface ice and habitats for life in the Martian arctic (near 68° north). Before the arrival of winter at the Mars Phoenix landing site (near the end of calendar year 2008), a gas analysis of samples taken from the subsurface confirmed what photographs of trenches dug by the lander’s robotic arm had strongly suggested, that being that indeed just centimeters below the surface water ice was present. That water ice sublimated into the Martian atmosphere when exposed to ambient conditions. As a result of ongoing Mars exploration by robotic spacecraft in orbit and on the surface, our understanding of the Martian surface is rapidly changing.

In 2009, NASA aired a special Science Update program that revealed that astronomers using a pair of telescope facilities high atop Mauna Kea in Hawaii had detected the infrared spectral signature of methane in the atmosphere of Mars. Given the nature and thinness of the Martian atmosphere, atmospheric methane could not survive the constant irradiation from solar ultraviolet rays. As a result, the implication was that this methane, found in relatively significant amounts, had to be replenished. Two production methods were possible, one geological and the other biological. On Earth, methane can be produced geologically by converting iron oxide to serpentine minerals. This requires carbon dioxide, water, and an internal heat source. Mars has carbon dioxide, and thanks to the results from Mars Phoenix in 2008, it is known to have subsurface water, at least in some locations. With a source of heat, serpentine mineral production on Mars might be possible. A biological origin would involve digestive processes by microorganisms. Data presented at this Science Update could not be used to discern whether biology or geology was responsible for the methane.

At the time of this important announcement, NASA was well into the selection process for identifying suitable landing sites for the 2011 Mars Science Laboratory (MSL)launch. This discovery hoped to shift the three-decade-old paradigm used in Mars exploration: following the water. Instead, consideration was immediately given to the search for a methane vent on the surface of the red planet so that the nuclear-powered MSL could conduct in situ analyses of samples and determine whether Mars’s methane was of geological or biological origin. If the latter proved to be accurate, it was likely that the primitive life forms would exist in subsurface layers in the presence of water and heat.

In August 2013, NASA celebrated the tenth anniversary of launching the Mars rover Opportunity. The rover continues its work on Mars's surface, sending back video and photographic images to scientists, many of which are made available for viewing by the public. Many other twenty-first-century missions gathered further data from Mars. In 2018, the Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport (InSight) mission landed to explore the planet's interior with its two nano spacecraft, MarCO-A and MarCO-B. This exploration continued until 2019. In 2020, NASA launched the first helicopter to Mars. Landing in 2021 with the rover Perseverance, Ingenuity, or Ginny, completed the first aerodynamic flight outside the Earth.

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