Robotic Exploration of Mars Before Curiosity

The Mars Science Laboratory (MSL) Curiosity rover continues a legacy of exploration of the martian surface and atmosphere from orbiting and landed robotic missions. The first flyby of Mars was completed by the Mariner 4 mission in 1965, with two more flybys by Mariners 6 and 7 in 1969. The images of the martian surface transmitted by these flybys showed a desolate, cratered terrain, much like the lunar surface, which was likely a disappointing result for those who hypothesized that Mars was much more Earth-like and had liquid water and active volcanos on its surface. The orbiting spacecraft Mariner 9 mapped ~80% of the martian surface in the early 1970s and found evidence of an intriguing geologic history, with ancient river channels, huge extinct volcanos (the largest of which is the size of the state of Arizona), volcanic and impact craters, and a canyon system that is over 3000 kilometers long. Although the martian surface does not currently look very Earth-like, the images from Mariner 9 suggested that Mars may have once looked very much like Earth. This led people to wonder if microbial life could have ever existed on the planet, and the search for life and environments that would have been habitable to microbial life on Mars began.

Following the Mariner missions, the NASA Viking 1 and 2 missions launched for Mars in 1975. Each mission included an orbiter and a lander. The goals of the landers were to image the martian surface and look for evidence for life using biological experiments. The experiments produced no definitive evidence for life at either landing site. Orbiters with high-resolution cameras and infrared spectrometers (for identifying minerals on the surface) were launched in the late 1990s and early 2000s, including the NASA Mars Global Surveyor (launched in 1997), the NASA Mars Odyssey (2001), the European Space Agency (ESA) Mars Express (2003), and the NASA Mars Reconnaissance Orbiter (2006). High-resolution cameras [in particular the High Resolution Imaging Science Experiment (HiRISE) on Mars Reconnaissance Orbiter, which can collect images with a resolution of ~30 centimeters/pixel] have returned stunning images of ancient river channels and lake deposits in impact craters that are 3–4 billion years old. [NOTE: Ages for terrestrial planet surfaces are estimated by counting the number and size of impact craters. This method is based on the premise that a new geologic surface (like a lava flow or landslide deposit) would have zero impact craters, and as the surface ages, meteorites hit the surface and the surface accumulates craters. The ages have been calibrated by samples returned from the Moon, whose ages have been determined from radiometric age dating.] Orbiters dedicated to studying the atmosphere have been launched recently, including NASA’s Mars Atmosphere and Volatile EvolutioN (MAVEN) mission (2013) and ESA’s ExoMars Trace Gas Orbiter (2016).

Infrared spectrometers on Mars Global Surveyor, Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter have shed light on the minerals on the martian surface. Minerals are important because they can tell us about the history of a surface, from igneous processes like the composition of magmas and lavas, to processes involving water like the formation and evolution of lakes and the chemistry of those lake waters. Some minerals discovered in sedimentary rocks deposited ~3–4 billion years ago (Ga) indicate that ancient aqueous environments were diverse and evolved during Mars’ early history. Minerals from the phyllosilicate group (also called clay minerals) are common in very ancient terrains (~3.5–4 Ga), whereas sulfate salts are more common in slightly younger terrains (~3–3.5 Ga). Phyllosilicates require liquid water to form and can form under a range of conditions, but the most common phyllosilicates found on the martian surface (called smectite) suggest formation under near-neutral pH and relatively low temperatures (between ~0° and 100°C). Sulfate salts are found in drier environments where waters containing SO₄^2– in solution have evaporated such that sulfate-bearing salts precipitated. This observation of phyllosilicates in the most ancient terrains and sulfate salts in slightly younger terrains suggests that surface water chemistry on Mars changed ~3.5 Ga, marking a global climate change from a relatively wet early Mars to the very dry Mars we know today.

NASA’s Mars Pathfinder Sojourner (1996) was the first rover sent to Mars and it found rocks deposited by water in a massive ancient outflow channel called Ares Vallis. The twin NASA Mars Exploration Rovers Spirit and Opportunity (2003) were each sent to sites on Mars with evidence for liquid water from orbit, and both rovers found geochemical evidence for a variety of different aqueous environments. The NASA Phoenix lander (2007) landed in the northern plains of Mars to study the biological potential of the martian arctic soils with cameras, a trench-digging robotic arm, and wet chemistry laboratories. Phoenix discovered water ice in the near subsurface, and the chemistry measurements showed the soil was slightly alkaline with modest salinity, but that liquid water had likely not been present for hundreds of millions of years. Curiosity continues this search for habitable environments on the martian surface with a sophisticated science payload.

Curiosity Mission Goals, Instruments, and the Selection of Gale Crater

A main scientific goal of the Curiosity mission is to assess the habitability of ancient martian environments. Habitable environments are defined as those that would support microbial life. Life as we know it on Earth requires liquid water and specific nutrients. The science instruments on the Curiosity rover can look for evidence for ancient liquid water based on geologic context, the types of minerals present, and geochemistry and can detect elements and nutrients necessary for life.

The science cameras include Mastcam, the Mars Hand Lens Imager (MAHLI), and the Remote Micro Imager (RMI) on the Chemistry and Camera (ChemCam) instrument package. Mastcam is made up of two cameras of different focal lengths on the mast of the rover and provides stunning outcrop- to horizon-scale images. The ChemCam RMI generally produces context images of rock and soil targets that the ChemCam Laser Induced Breakdown Spectrometer (LIBS) instrument analyzes for geochemistry, but can also image features that are hundreds of meters to a few kilometers away from the rover. MAHLI produces close-up images of rocks and soils and can resolve features as small as ~12.5 micrometers.

The instruments used for characterizing composition of rocks and soils (i.e., geochemistry and mineralogy) are the ChemCam LIBS instrument, the Alpha Particle X-ray Spectrometer (APXS), the Chemistry and Mineralogy instrument (CheMin), the Sample Analysis at Mars (SAM) instrument package, and the Dynamic Albedo of Neutrons (DAN) instrument. ChemCam and Mastcam also perform spectroscopy in the visible and near-infrared wavelengths, which can be used to identify certain minerals, in particular those that contain iron. ChemCam is located on the mast of the rover and the LIBS instrument fires a laser up to 7 meters away, which creates a plasma out of the target soil or rock. A spectrometer then measures the plasma to determine the chemical composition of the target. The APXS is located on the end of the arm and measures the bulk geochemistry of soil and rock targets up close. CheMin and SAM are laboratories located on the inside of the rover and they both analyze scooped soil and drilled rock samples that are acquired and processed with the Sample Acquisition, Processing, and Handling (SA/SPaH) subsystem on the end of the arm. CheMin can quantify the minerals present in soils and rocks down to a detection limit of ~1 wt.%. SAM is a suite of three instruments (a mass spectrometer, gas chromatograph, and tunable laser spectrometer) that measure gases that are released from soil and rock samples as they are heated to search for carbon-, nitrogen-, hydrogen-, and oxygen-bearing compounds, including organic molecules, and measure isotopes of carbon, hydrogen, oxygen, and noble gases in the atmosphere. DAN is located on the back of the rover and can detect subsurface hydrogen as a proxy of H₂O either in the form of ice or in mineral structures.

Curiosity has two instruments to characterize the modern environment. The Radiation Assessment Detector (RAD) measures high-energy radiation on the martian surface to evaluate the radiation environment for future human exploration of Mars. Finally, the Rover Environmental Monitoring System (REMS) is the rover’s weather station, measuring atmospheric pressure, air and ground temperature, humidity, ultraviolet light, and wind speed and direction.

The Curiosity rover landed in Gale crater on August 6, 2012. Gale crater is an impact crater ~150 kilometers in diameter located near the martian equator along the boundary between older, cratered terrain to the south and younger, smooth plains to the north. It was selected from over 50 initially proposed landing sites during a series of landing site workshops in which NASA solicited input from the planetary science community on the scientific and engineering benefits and drawbacks of each landing site. The final four landing sites also included Mawrth Vallis, Holden crater, and Eberswalde crater. All four landing sites had layered sedimentary rocks containing phyllosilicates, indicating that there was once liquid water at those locations and they may have once been habitable. Although each of the landing sites would have satisfied the science criterion to study a site that has evidence for liquid water in the past, Gale crater was selected because of the diversity of ancient aqueous environments.

Curiosity was sent to Gale crater to study sedimentary rocks that were deposited ~3.5 Ga by rivers and lakes, evaluate changes in environments over time, and determine whether these environments would have been habitable to ancient microbial life. Gale crater contains a ~5 kilometer-tall mound of layered sedimentary rock in the center of the crater called Aeolis Mons (informally known as Mount Sharp). Orbiting spectrometers identified a variety of minerals in the lowest layers of Mount Sharp that formed from water-rock interactions. Phyllosilicates and the iron oxide mineral hematite were identified near the base of Mount Sharp (in the oldest sedimentary rocks) and sulfate salts were identified just above the phyllosilicate- and hematite-bearing rocks (in slightly younger sedimentary rocks). This suggests that the ancient sedimentary rocks in Gale crater preserve a variety of different environments that may have been habitable and that these rocks may help scientists better understand the dramatic climate change that occurred on Mars ~3.5 Ga. By driving up the lower layers of Mount Sharp, Curiosity can study how these environments changed over time.

Evidence for Ancient Rivers and Lakes at Gale Crater

Since landing ~6 years ago, Curiosity has driven over 19 kilometers. The first ~2 years of the mission were spent traversing the crater plains to reach the lower slopes of Mount Sharp. Almost all the rocks that the rover has encountered have been sedimentary and were deposited by either water (rivers, lakes, and deltas) or by wind. Even before landing in Gale crater, ancient streams and alluvial fans were discovered from orbit near the landing site. Some of these stream deposits were studied by Curiosity near the landing site thanks to the descent rockets (part of the incredible, never-before-executed Sky Crane maneuver), which blew away much of the surface dust and sediment. A coarse-grained sedimentary rock called conglomerate made up of rounded pebbles and sand was discovered near the landing site. The size of the pebbles indicates the rock was deposited by a stream that was up to 1 meter deep.

From the landing site, Curiosity took a detour to the east (instead of driving to the southwest toward Mount Sharp) to study three different units identified from orbit at a location named Yellowknife Bay. Curiosity discovered very fine-grained sedimentary rock called mudstone at Yellowknife Bay, indicating that Yellowknife Bay was the site of an ancient lake. Features in the mudstone, like raised ridges and spherules, suggest that groundwater moved through the sediment before it was lithified.

After an extensive campaign at Yellowknife Bay, Curiosity drove to the southwest toward the lower slopes of Mount Sharp. Curiosity did another lengthy campaign at a region known as “The Kimberley,” where many different units identified from orbit were located, including one named the Striated Unit. From the ground, the Striated Unit was made up of sandstone beds that were very gently dipping toward the center of the crater. These dipping beds are signatures of delta deposits, where rivers or streams flowed into lakes and the sediment that was transported by the rivers or streams fell out of suspension.

Curiosity reached the lower slopes of Mount Sharp in September 2014 upon arrival at the Pahrump Hills. The rocks of the Pahrump Hills were predominantly laminated mudstone, where individual layer thicknesses were between ~1 millimeter and 1 centimeter. Laminated mudstone is commonly produced by lake deposits. Since the start of Curiosity’s ascent up Mount Sharp, laminated mudstone has been the most common rock type observed. Rare mud cracks indicate some drying of the lakes, but the persistent laminated mudstone for 300+ meters of vertical stratigraphy suggests that lakes were present for an extended period of time in Gale crater, perhaps for hundreds of thousands to a few millions of years. The sedimentary rocks of lower Mount Sharp also show evidence for groundwater, with the presence of concretions, spherules, and mineralized veins.

Based on the observation of conglomerate and dipping sandstone on the plains of Gale crater and laminated mudstone on lower Mount Sharp, Gale crater was once much wetter and Earth-like. Rivers and streams emanating from the crater rim flowed into lakes on the crater floor. We hypothesize that the rivers and streams formed from the melting of snow and ice on the crater rim because we have not yet discovered evidence that it rained on Mars. For instance, we have not seen rain drop imprints fossilized in the mudstone. In addition to abundant water on the surface, groundwater also moved through sediments before and after they were lithified, indicating Gale crater had a rich history of surface and subsurface water.

What was the Ancient Water like at Gale Crater?

The geochemical and mineralogical composition of the sedimentary rocks in Gale crater can tell us very specific information about the ancient lake waters and groundwater and can help us determine whether or not these environments would have been habitable to microbial life. At the time of writing, Curiosity has drilled 16 rock samples, 12 of which were likely deposited by lakes or rivers. The igneous minerals that CheMin detects in the sedimentary rocks indicate that the igneous rocks surrounding Gale crater are primarily basaltic. Basalt is very low in SiO₂ and is the most common rock type found in Hawaii. It contains abundant plagioclase feldspar and pyroxene.

The minerals that form from water-rock interactions are those that tell us about past aqueous environments. Nearly all the mudstone deposits contain the clay mineral smectite, suggesting formation in water at near-neutral pH (although laboratory research has shown that smectite can form at moderately acidic to moderately basic pH). In addition to basaltic igneous minerals and smectite at Yellowknife Bay, CheMin discovered the iron oxide magnetite, which has both reduced and oxidized iron in its structure, and calcium (Ca)-sulfate in hairline veins (indicating formation after the lake sediments lithified). This mineral assemblage suggests the lake water at Yellowknife Bay had near-neutral pH, low salinity, and was not too oxidizing. The mudstone samples collected from the lower slopes of Mount Sharp have a very diverse mineralogy, indicating they preserve many different environments. Near the base of Mount Sharp, smectite and magnetite are prevalent with little to no Ca-sulfate, suggesting an environment similar to that of Yellowknife Bay. Moving up Mount Sharp into younger and younger rocks, magnetite gives way to the iron oxide mineral hematite, which has oxidized iron and no reduced iron in its structure; Ca-sulfate minerals are also prevalent in the sediment matrix (meaning it likely precipitated before the sediments lithified), and the smectite composition and structure changes slightly, indicating it was more altered by water than the smectite observed in Yellowknife Bay or Pahrump Hills. This change in mineralogy indicates that the water from which these minerals precipitated (lake water and/or groundwater) was more oxidizing and was more saline than the water that precipitated the mineral assemblage at Yellowknife Bay.

It can be difficult to determine whether the minerals that formed from water-rock interaction (like the smectite, iron oxides, and Ca-sulfate minerals) were precipitated from lake waters or from groundwater moving through the sediments. We can use textures in the rock as clues pointing toward the presence of groundwater, but we really need the rock samples in hand so that we can look at them under microscopes and examine the physical and chemical relationships between the minerals. SAM data from one sample drilled from the base of Mount Sharp has shown us that at least one mineral precipitated from groundwater long after the sediments were deposited by lakes. CheMin identified an iron sulfate mineral, jarosite, in the Mojave2 sample from the Pahrump Hills. Jarosite is an important environmental indicator because it forms in acid sulfate solutions with a pH of ~2–4. SAM performed krypton-argon age dating on the jarosite and determined it precipitated 2.12 ± 0.36 Ga, which is nearly 1.5 billion years after the lake sediments were deposited. This incredibly young age for the jarosite in Pahrump Hills tells us that there was a period of ~1.5 billion years during which water was present on the surface or near the subsurface of Gale crater (although the groundwater was likely present intermittently). This has big implications for the habitability of Gale crater.

Was Gale Crater Ever Habitable?

An especially intriguing aspect of the identification of habitable environments in Gale crater ~3.5 Ga is that the oldest microbial fossils on Earth date to around the same time, so it is not unreasonable to think that microbes could have evolved on neighboring terrestrial planets that had similar environmental conditions. To determine whether Gale crater would have been habitable to microbial life, we need to consider the requirements for microbial life to thrive on Earth. Life as we know it on Earth requires liquid water, essential elements and nutrients, food/energy, and shelter from harmful radiation. The RAD instrument has shown that the surface radiation environment at Gale crater is too harsh for microbes to survive today, but ~3.5 Ga, Mars likely had a thicker atmosphere to help protect the surface from high-energy particles coming from space. Microbes could have also lived below the surface in lake sediments or fractures in the rock to protect themselves.

The types of sedimentary rocks we find with Curiosity and the minerals we find in those rocks tell us there was a long history of liquid water at Gale crater. Lakes were likely present on the surface for hundreds of thousands to a few millions of years, and groundwater was present (at least intermittently) for ~1.5 billion years. The minerals we find suggest that some of the lake waters and groundwater would have been relatively fresh and some would have been more saline. However, we have not found evidence for hypersaline waters, and there are some microbes known as halophiles that have evolved to live in very saline conditions on Earth, so the lake and groundwater at Gale crater would have been fresh enough to support microbial activity.

The main elements necessary for life are carbon (C), hydrogen (H), nitrogen (N), oxygen (O), phosphorus (P), and sulfur (S), and these elements or compounds including these elements have been detected by the APXS, ChemCam, CheMin, SAM, and DAN instruments. Important micronutrients for microbes, like manganese (Mn), copper (Cu), zinc (Zn), and nickel (Ni), have also been detected by APXS and ChemCam. Chemotrophic microbes use elements in various states of oxidation as their source of energy, and some of the minerals we find, like magnetite, have elements in different oxidation states that could have been food for microbes. SAM has also detected organic molecules, which are the building blocks of life. Some of these organic molecules could have also been food sources for ancient microbes in Gale crater.

What’s Next for Curiosity?

Curiosity is currently on the Vera Rubin Ridge (named for the famous American astronomer whose research provided evidence for the existence of dark matter). Vera Rubin Ridge was identified from orbit as a target of interest because it is enriched in the iron oxide mineral hematite. Hematite can form under a variety of conditions, many of which involve liquid water, so Curiosity is studying the ridge to determine how the hematite may have formed. Hematite was also studied by the Mars Exploration Rover Opportunity at Meridiani Planum. At Meridiani, hematite was in the form of spherules (or “blueberries”), indicating precipitation from groundwater. Curiosity has not detected similar blueberries, so the hematite is likely present in the sedimentary matrix. Laminated mudstone is very common on the ridge, suggesting deposition by lake water, but we don’t yet know if the hematite precipitated from lake waters or groundwater. Curiosity is poised and ready to take its first drill sample from the Vera Rubin Ridge and deliver that sample to CheMin and SAM. The types of minerals that are found in association with the hematite will help us determine the conditions under which it formed. We anticipate that the Vera Rubin Ridge campaign will last through the summer of 2018, with additional samples drilled from the top of the ridge.

After studying the Vera Rubin Ridge, Curiosity will continue to climb Mount Sharp and will investigate the phyllosilicate-bearing unit then the sulfate-bearing unit that were identified from orbit and were a primary driver for selecting Gale crater as the landing site. In the past 6 years, Curiosity has found evidence for a variety habitable environments based on the sedimentology, mineralogy, and geochemistry. As Curiosity studies these new units, the science team will continue to evaluate their habitability and place them in context with older units to characterize the changing environments on early Mars.

 

Learn More About the Curiosity Mission

To follow Curiosity’s progress and check out the latest images, visit:

https://mars.nasa.gov/msl/mission/overview/

https://mars.nasa.gov/msl/mission/whereistherovernow/

https://mars.nasa.gov/msl/multimedia/raw/

Here is a selection of recent scientific papers and abstracts for those readers who want more details on the scientific findings from the mission. This is not an exhaustive list, and the lead author of this article would be happy to recommend more reading to interested individuals.

Bristow T. F., Rampe E. B., Achilles C. N., et al. (2018) Clay mineral diversity and abundance in sedimentary rocks of Gale crater, Mars. Science Advances, 4(6), eaar3330.

Eigenbrode J. L., Summons R. E., Steele A., et al. (2018) Organic matter preserved in 3-billion-year-old mudstones at Gale crater, Mars. Science, 360(6393), 1096–1101.

Fedo C. M., Grotzinger J. P., Gupta S., et al. (2018) Sedimentology and stratigraphy of the Murray formation, Gale crater, Mars. Lunar and Planetary Science XLIX, Abstract #2078. Lunar and Planetary Institute, Houston.

Fraeman A. A., Edgar L. A., Grotzinger J. P., et al. (2018) Curiosity’s investigation at Vera Rubin Ridge. Lunar and Planetary Science XLIX, Abstract #1557. Lunar and Planetary Institute, Houston.

Grotzinger J. P., Gupta S., Malin M. C., et al. (2015) Deposition, exhumation, and paleoclimate of an ancient lake deposit, Gale crater, Mars. Science, 350(6257), aac7575.

Hurowitz J. A., Grotzinger J. P., Fischer W. W., et al. (2017) Redox stratification of an ancient lake in Gale crater, Mars. Science, 356(6341), eaah6849.

Mahaffy P. R., Webster C. R., Stern J. C., et al. (2015) The imprint of atmospheric evolution in the D/H of Hesperian clay minerals on Mars. Science, 347(6220), 412–414.

Rampe E. B., Ming D. W., Blake D. F., et al. (2017) Mineralogy of an ancient lacustrine mudstone succession from the Murray formation, Gale crater, Mars. Earth and Planetary Science Letters 471 (2017): 172-185.

Stein N., Grotzinger J. P., Schieber J., et al. (2018) Desiccation cracks provide evidence of lake drying on Mars, Sutton Island member, Murray formation, Gale Crater. Geology, 46(6), 515–518.

Thompson L. M., Fraeman A. A., Berger J. A., et al. (2018) APXS determined chemistry of the Vera Rubin (Hematite) Ridge, Gale Crater, Mars: Implications for hematite signature origin. Lunar and Planetary Science XLIV, Abstract #2826. Lunar and Planetary Institute, Houston.

About the Authors

Elizabeth Rampe ([email protected]) is an Exploration Mission Scientist in the Astromaterials Research and Exploration Science Division at the NASA Johnson Space Center in Houston, Texas.

The MSL Science Team includes hundreds of scientists from all over the world. E. Rampe specifically thanks team members J. Bridges, N. Castle, L. Edgar, C. Fedo, C. Freissinet, J. Grotzinger, P. Mahaffy, H. Newsom, J. Johnson, L. Kah, K. Siebach, J. Schieber, V. Sun, A. Vasavada, C. Webster, D. Wellington, and R. Wiens for their thoughtful additions to this article.