A Fresh Look at Older Data Yields a Surprise Near the Martian Equator
Scientists taking a new look at older data from NASA’s longest-operating Mars orbiter have discovered evidence of significant hydration near the Martian equator – a mysterious signature in a region of the Red Planet where planetary scientists figure ice shouldn’t exist.
Jack Wilson, a post-doctoral researcher at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, led a team that reprocessed data collected from 2002 to 2009 by the neutron spectrometer instrument on NASA’s Mars Odyssey spacecraft. In bringing the lower-resolution compositional data into sharper focus, the scientists spotted unexpectedly high amounts of hydrogen – which at high latitudes is a sign of buried water ice – around sections of the Martian equator.
An accessible supply of water ice near the equator would be of interest in planning astronaut exploration of Mars. The amount of delivered mass needed for human exploration could be greatly reduced by using Martian natural resources for a water supply and as raw material for producing hydrogen fuel.
By applying image-reconstruction techniques often used to reduce blurring and remove “noise” from medical or spacecraft imaging data, Wilson’s team improved the spatial resolution of the data from around 320 miles to 180 miles (520 kilometers to 290 kilometers). “It was as if we’d cut the spacecraft’s orbital altitude in half,” Wilson said, “and it gave us a much better view of what’s happening on the surface.”
The neutron spectrometer can’t directly detect water, but by measuring neutrons, it can help scientists calculate the abundance of hydrogen – and infer the presence of water or other hydrogen-bearing substances.
Mars Odyssey’s first major discovery, in 2002, was abundant hydrogen just beneath the surface at high latitudes. In 2008, NASA’s Phoenix Mars Lander confirmed that the hydrogen was in the form of water ice.
But at lower latitudes on Mars, water ice is not thought to be thermodynamically stable at any depth. The traces of excess hydrogen that Odyssey’s original data showed at lower latitudes were initially explained as hydrated minerals, which other spacecraft and instruments have since observed.
Wilson’s team concentrated on those equatorial areas, particularly with a 600-mile (1,000-kilometer) stretch of loose, easily erodible material between the northern lowlands and southern highlands along the Medusae Fossae Formation. Radar-sounding scans of the area have suggested the presence of low-density volcanic deposits or water ice below the surface, “but if the detected hydrogen were buried ice within the top meter of the surface, there would be more than would fit into pore space in soil,” Wilson said. The radar data came from both the Shallow Radar on NASA’s Mars Reconnaissance Orbiter and the Mars Advanced Radar for Subsurface and Ionospheric Sounding on the European Space Agency’s Mars Express orbiter and would be consistent with no subsurface water ice near the equator.
How water ice could be preserved there is a mystery. A leading theory suggests an ice and dust mixture from the polar areas could be cycled through the atmosphere when Mars’ axial tilt was larger than it is today. But those conditions last occurred hundreds of thousands to millions of years ago. Water ice isn’t expected to be stable at any depth in that area today, Wilson said, and any ice deposited there should be long gone. Additional protection might come from a cover of dust and a hardened “duricrust” that traps the humidity below the surface, but this is unlikely to prevent ice loss over timescales of the axial tilt cycles.
“Perhaps the signature could be explained in terms of extensive deposits of hydrated salts, but how these hydrated salts came to be in the formation is also difficult to explain,” Wilson added. “So for now, the signature remains a mystery worthy of further study, and Mars continues to surprise us.”
Wilson led the research while at Durham University in the U.K. His team - which includes members from NASA Ames Research Center, the Planetary Science Institute and the Research Institute in Astrophysics and Planetology - published its findings this summer in the journal Icarus.
TOP IMAGE….Taking advantage of Mars’s closest approach to Earth in eight years, astronomers using NASA’s Hubble Space Telescope have taken the space-based observatory’s sharpest views yet of the Red Planet. NASA is releasing these images to commemorate the second anniversary of the Mars Pathfinder landing. The lander and its rover, Sojourner, touched down on the Red Planet’s rolling hills on July 4, 1997, embarking on an historic three-month mission to gather information on the planet’s atmosphere, climate, and geology.
The telescope’s Wide Field and Planetary Camera 2 snapped images between April 27 and May 6, when Mars was 54 million miles (87 million kilometers) from Earth. From this distance the telescope could see Martian features as small as 12 miles (19 kilometers) wide. The telescope obtained four images(see PIA01587), which, together, show the entire planet.
This image is centered on the region of the planet known as Tharsis, home of the largest volcanoes in the solar system. The bright, ring-like feature just to the left of center is the volcano Olympus Mons, which is more than 340 miles (550 kilometers) across and 17 miles(27 kilometers) high. Thick deposits of fine-grained, windblown dust cover most of this hemisphere. The colors indicate that the dust is heavily oxidized (“rusted”), and millions (or perhaps billions) of years of dust storms have homogenized its composition. Prominent late afternoon clouds along the right limb of the planet can be seen.
This color composite is generated from data using three filters: blue (410 nanometers), green (502 nanometers), and red (673 nanometers).
LOWER IMAGE….Re-analysis of 2002-2009 data from a hydrogen-finding instrument on NASA’s Mars Odyssey orbiter increased the resolution of maps of hydrogen abundance. The reprocessed data (lower map) shows more “water-equivalent hydrogen” (darker blue) in some parts of this equatorial region of Mars. Puzzingly, this suggests the possible presence of water ice just beneath the surface near the equator, though it would not be thermodynamically stable there.
The upper map uses raw data from Odyssey’s neutron spectrometer instrument, which senses the energy state of neutrons coming from Mars, providing an indication of how much hydrogen is present in the top 3 feet (1 meter) of the surface. Hydrogen detected by Odyssey at high latitudes of Mars in 2002 was confirmed to be in the form of water ice by the follow-up NASA Phoenix Mars Lander mission in 2008.
A 2017 reprocessing of the older data applied image-reconstruction techniques often used to reduce blurring from medical imaging data. The results are shown here for an area straddling the equator for about one-fourth the circumference of the planet, centered at 175 degrees west longitude. The white contours outline lobes of a formation called Medusae Fossae, coinciding with some areas of higher hydrogen abundance in the enhanced-resolution analysis. The black line indicates the limit of a relatively young lava plain, coinciding with areas of lower hydrogen abundance in the enhanced-resolution analysis.
The color-coding key for hydrogen abundance in both maps is indicated by the horizontal bar, in units expressed as how much water would be present in the ground if the hydrogen is all in the form of water. Units of the equivalent water weight, as a percentage of the material in the ground, are correlated with counts recorded by the spectrometer, ranging from less than 1 weight-percent water equivalent (red) to more than 30 percent (dark blue).
Odyssey’s neutron spectrometer, provided by Los Alamos National Laboratory in New Mexico, is part of the mission’s Gamma Ray Spectrometer suite overseen by the University of Arizona, Tucson. NASA’s Jet Propulsion Laboratory, a division of Caltech in Pasadena, California, manages the Mars Odyssey mission for the NASA Science Mission Directorate, Washington. Lockheed Martin Space Systems, Denver, built the Odyssey spacecraft, which has been studying Mars from orbit since 2001.