This image includes an especially long example of a type of dark marking that advances down some Martian slopes in warmer months and fades away in cooler months.
The features, called "recurrent slope linea," may be seasonal flows of salty water. Red arrows indicate the location of one on this image taken by the High Resolution Imaging Science Experiment (HiRISE) camera aboard NASA's Mars Reconnaissance Orbiter. This one is three-quarters of a mile (1.2 kilometers) long.
NASA's Mars Reconnaissance Orbiter has revealed to scientists slender dark markings -- possibly due to salty water - that advance seasonally down slopes surprisingly close to the Martian equator.
"The equatorial surface region of Mars has been regarded as dry, free of liquid or frozen water, but we may need to rethink that," said Alfred McEwen of the University of Arizona in Tucson, principal investigator for the Mars Reconnaissance Orbiter (MRO) High Resolution Imaging Science Experiment (HiRISE) camera.
Tracking how these features recur each year is one example of how the longevity of NASA orbiters observing Mars is providing insight about changes on many time scales. Researchers at the American Geophysical Union meeting Tuesday in San Francisco discussed a range of current Martian activity, from fresh craters offering glimpses of subsurface ice to multi-year patterns in the occurrence of large, regional dust storms.
The seasonally changing surface flows were first reported two years ago on mid-latitude southern slopes. They are finger-like features typically less than 16 feet (5 meters) wide that appear and extend down steep, rocky slopes during spring through summer, then fade in winter and return the next spring. Recently observed slopes stretch as long as 4,000 feet (1,200 meters).
McEwen and co-authors reported the equatorial flows at the conference and in a paper published online Tuesday by Nature Geoscience. Five well-monitored sites with these markings are in Valles Marineris, the largest canyon system in the solar system. At each of these sites, the features appear on both north- and south-facing walls. On the north-facing slopes, they are active during the part of the year when those slopes get the most sunshine. The counterparts on south-facing slopes start flowing when the season shifts and more sunshine hits their side.
"The explanation that fits best is salty water is flowing down the slopes when the temperature rises," McEwen said. "We still don't have any definite identification of water at these sites, but there's nothing that rules it out, either."
Dissolved salts can keep water melted at temperatures when purer water freezes, and they can slow the evaporation rate so brine can flow farther. This analysis used data from the Compact Reconnaissance Imaging Spectrometer for Mars and the Context Camera on the MRO as well as the Thermal Emission Imaging System experiment on NASA's Mars Odyssey orbiter.
Water ice has been identified in another dynamic process researchers are monitoring with MRO. Impacts of small asteroids or bits of comets dig many fresh craters on Mars every year. Twenty fresh craters have exposed bright ice previously hidden beneath the surface. Five were reported in 2009. The 15 newly reported ones are distributed over a wider range of latitudes and longitudes.
"The more we find, the more we can fill in a global map of where ice is buried," said Colin Dundas of the U.S. Geological Survey in Flagstaff, Ariz. "We've now seen icy craters down to 39 degrees north, more than halfway from the pole to the equator. They tell us that either the average climate over several thousand years is wetter than present or that water vapor in the current atmosphere is concentrated near the surface. Ice could have formed under wetter conditions, with remnants from that time persisting today, but slowly disappearing."
Mars' modern climate becomes better known each year because of a growing set of data from a series of orbiters that have been studying Mars continually since 1997. That has been almost nine Martian years because a year on Mars is almost two years long on Earth. Earlier missions and surface landers have added insight about the dynamics of Mars' atmosphere and its interaction with the ground.
"The dust cycle is the main driver of the climate system," said Robert Haberle of NASA's Ames Research Center in Moffett Field, Calif.
One key question researchers want to answer is why dust storms encircle Mars in some years and not in others. These storms affect annual patterns of water vapor and carbon dioxide in the atmosphere, freezing into polar ice caps in winter and replenishing the atmosphere in spring. Identifying significant variations in annual patterns requires many Martian years of observations.
The data emerging from long-term studies will help future human explorers of Mars know where to find resources such as water, how to prepare for hazards such as dust storms, and where to be extra careful about contamination with Earth microbes.
Launched in 2005, Mars Reconnaissance Orbiter and its six instruments have provided more high-resolution data about the Red Planet than all other Mars orbiters combined. Data are made available for scientists worldwide to research, analyze and report their findings.
NASA's Jet Propulsion Laboratory in Pasadena, Calif., manages the MRO and Mars Odyssey missions for NASA's Science Mission Directorate in Washington. Lockheed Martin Space Systems in Denver built both orbiters. The University of Arizona Lunar and Planetary Laboratory operates the HiRISE camera, which was built by Ball Aerospace & Technologies Corp. of Boulder, Colo.
Recurring "Lineae" on Slopes at Horowitz Crater
The dark, narrow streaks flowing downhill on Mars at sites such as this portion of Horowitz Crater are inferred to be formed by seasonal flow of water on modern-day Mars. The streaks are roughly the length of a football field.
The imaging and topographical information in this processed view come from the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter.
These dark features on the slopes are called "recurring slope lineae" or RSL. Planetary scientists using observations with the Compact Reconnaissance Imaging Spectrometer on the same orbiter detected hydrated salts on these slopes at Horowitz Crater, corroborating the hypothesis that the streaks are formed by briny liquid water.
The image was produced by first creating a 3-D computer model (a digital terrain map) of the area based on stereo information from two HiRISE observations, and then draping an image over the land-shape model. The vertical dimension is exaggerated by a factor of 1.5 compared to horizontal dimensions. The draped image is a red waveband (monochrome) product from HiRISE observation PSP_005787_1475, taken on Oct. 21, 2007, at 32 degrees south latitude, 141 degrees east longitude.
Dark, Recurring Streaks on Walls of Garni Crater on Mars
Dark narrow streaks, called "recurring slope lineae," emanate from the walls of Garni Crater on Mars, in this view constructed from observations by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter.
The dark streaks here are up to few hundred yards, or meters, long. They are hypothesized to be formed by flow of briny liquid water on Mars.
The image was produced by first creating a 3-D computer model (a digital terrain map) of the area based on stereo information from two HiRISE observations, and then draping an image over the land-shape model. The vertical dimension is exaggerated by a factor of 1.5 compared to horizontal dimensions. The draped image is a red waveband (monochrome) product from HiRISE observation ESP_031059_1685, taken on March 12, 2013 at 11.5 degrees south latitude, 290.3 degrees east longitude
The Ares 3 Landing Site: Where Science Fact Meets Fiction
This image from the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter shows a location on Mars associated with the best-selling novel and Hollywood movie, "The Martian."
This area is in the Acidalia Planitia region. In the novel and the movie, it is the landing site of a crewed mission named Ares 3. For the story's central character, Acidalia Planitia is within driving distance from where NASA's Mars Pathfinder, with its Sojourner rover, landed in 1997.
An initial HiRISE image of the site was taken in April 2015 and is online at http://hirise.lpl.arizona.edu/ESP_040776_2115. A second one was taken May 17, 2015, and is shown here. Figure A is a stereo combination of the two, appearing three-dimensional when viewed through blue-red glasses with the red lens on the left
One of the main objectives of the HiRISE camera is to carry out "monitoring science", which involves taking images of certain areas of high scientific interest on regular intervals. The team usually does so to monitor a seasonal or recurring process such as seasonal changes in carbon-dioxide ice near the poles, dune movement or recurring flow-like features on some slopes. HiRISE also takes repeated images of areas with active rovers, such as Curiosity, to help plan safe routes toward areas of high scientific interest.
Another key responsibility for the HiRISE camera is to provide information for use in selection of landing sites for future missions. One technique is to image a site of interest at least twice when the weather conditions are similar, but with a small difference in viewing angle, much like what you would experience if you looked at something with only your right eye, then looked at it again with the left. By doing this, we are able to build a stereo view of the site, providing a chance to identify high and low points in the site more effectively. This resulting 3-D information can combined with elevation data from laser altimeters to create a highly accurate "digital terrain model" or DTM for short.
DTMs allow researchers to view the locations in 3-D and to analyze them by measuring the exact height of features that could be hazardous to the future mission, such as large boulders or small impact craters. DTMs from HiRISE were a key factor in choosing the landing site for NASA's Curiosity Mars rover in Gale Crater and are being used to evaluate sites under consideration for the NASA's 2016 InSight Mars lander and Mars 2020 rover missions.
The location of the site in this image is 31.3 degrees north latitude, 331.3 degrees east latitude. The image is an excerpt from HiRISE observation ESP_041277_2115.
'The Martian' Story's Ares 4 Landing Site
This image from the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter shows a location on Mars associated with the best-selling novel and Hollywood movie, "The Martian." It is the science-fiction tale's planned landing site for the Ares 4 mission.
The novel placed the Ares 4 site on the floor of a very shallow crater in the southwestern corner of Schiaparelli Crater. This HiRISE image shows a flat region there entirely mantled by bright Martian dust. There are no color variations, just uniform reddish dust. A pervasive, pitted texture visible at full resolution is characteristic of many dust deposits on Mars. No boulders are visible, so the dust is probably at least a meter thick.
Past Martian rover and lander missions from NASA have avoided such pervasively dust-covered regions for two reasons. First, the dust has a low thermal inertia, meaning that it gets extra warm in the daytime and extra cold at night, a thermal challenge to survival of the landers and rovers (and people). Second, the dust hides the bedrock, so little is known about the bedrock composition and whether it is of scientific interest.
This view is one image product from HiRISE observation ESP_042014_1760, taken July 14, 2015, at 3.9 degrees south latitude, 15.2 degrees east longitude.
Western Edge of Mars' Marth Crater, a Movie Location
In the best-selling novel "The Martian" and the movie based on it, stranded astronaut Mark Watney's adventures take him to the rim of Mawrth Crater. This image from the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter shows the nature of this terrain.
The crater rim is not very distinct, and from the Martian surface it would be quite difficult to tell that you are even on the rim of a crater. The terrain is hummocky and rolling, punctuated by smaller impact craters and wind-blown drifts of sand or dust.
This view is one image product from HiRISE observation ESP_042252_1930, taken Aug. 1, 2015, at 12.6 degrees north latitude, 355.7 degrees east longitude.