RIMFAX

"Seeing" Underground Mars

RIMFAX is a ground-penetrating radar that reveals buried layers of sediment, rock, ice, water, or saltwater. These layers hold the record of ancient climate conditions on Mars.

Different materials (e.g., dense rock, clay, liquid water, ice, or sand) reflect, absorb, or scatter the radar pulses in distinct ways. RIMFAX assembles the returned radar pulses to construct image-like representations of the subsurface structure. RIMFAX images:

• Map buried layers
• Narrow down the possible compositions of underground layers
• Reveal the relationship of underground structures to rocks that are exposed on the surface.

Superman had X-ray vision, but RIMFAX's "superpower" is ground-penetrating radar. RIMFAX can "see" underground and produce images of the hidden layers of geology beneath the Martian surface. It can tell us what those materials are, and whether they hold clues to past environments on Mars, especially those with conditions necessary for supporting life. RIMFAX could also detect water and other resources necessary for future human exploration.

Ground-penetrating radar is at the heart of RIMFAX technology. It provides the ability "to see" underground. Scientists use radar data to produce image-like details of rock and other structures beneath the Martian surface.

"Radar" is an acronym that stands for RAdio Detecting And Ranging. On Earth, familiar types of radar systems detect aircraft in the sky or rain patterns in the atmosphere. Whether looking up at the sky or down inside the Earth, all radar systems essentially work the same way.

RIMFAX measurements start with a transmitter that generates radio waves. Radio waves are one type of electromagnetic radiation in the electromagnetic spectrum. This spectrum ranges from gamma rays and X-rays at one end to radio waves at the other.

All types of electromagnetic radiation form wave patterns as they travel. Each type of radiation has a particular "wavelength." A wavelength is the distance between the peaks of consecutive waves of electromagnetic radiation. Wavelengths across the electromagnetic spectrum range from very short to very long. Gamma waves are the shortest (about the size of a nucleus of an atom). Radio waves are the longest. They range from the length of a pencil to larger than the width of a planet.

The radio waves generated by the RIMFAX transmitter range from 150 to 1200 megahertz. This wavelength ranges from tens of meters to millimeters. Depending on the depth of subsurface penetration desired, the RIMFAX team can adjust the frequency transmitted by the radar from lower (150 megahertz) to higher (1200 megahertz). Lower-frequency radio waves penetrate deeper underground, while higher-frequency waves travel less deeply. The RIMFAX science team can take advantage of this feature of radar signals "to tune" or "to focus" the instrument on deep or shallow terrain of interest. Depending on the depth they want to sample and the size of objects they want "to see" in the radar data, the RIMFAX science team can choose from five different operating modes. A vertical resolution of about 3 to 12 inches (15 to 30 centimeters) in most materials reveals fine layering within rocks and sediments.

The radar energy from RIMFAX is directed downward in pulses. As the rover moves, RIMFAX can collect data every 4 inches (10 centimeters) to build up a subsurface profile. Depending on the properties of the material it encounters, RIMFAX can penetrate from the surface to 30 feet (more than 10 meters).

When the radio waves meet objects in their path, they respond in different ways. Different kinds of rocks, minerals, sediments, water, ice, and brines produce unique radar data. Some materials absorb or partially absorb the radar energy. Others strongly reflect the signal. Still other materials scatter the energy, resulting in a weakened return signal. It depends on their chemical makeup, density, and temperature.

RIMFAX scientists can analyze the radar data from Mars to identify underground structures and materials by their unique radar reflections. They know what the radar signal looks like after it encounters each different type of material. Some materials (e.g., liquid water) strongly reflect the radio signal back to the receiver without changing it very much. Pure water ice is essentially transparent to the radar pulse, which passes straight through. Other materials scatter the signal, much like the way water splashes off a hard surface.

Differences in the speed of the returning signal can also help scientists determine what's underground. For example, radar signals encountering liquid water and saltwater are slower in their return than those passing through pure water ice. These differences in response to the radar signal can be used with other observations to understand the composition of underground materials on Mars.

RIMFAX reveals the detailed makeup and structure of the Martian subsurface. This information allows scientists to probe back to the time when ancient layers of sediments, fluids, and rock once flowed or settled in areas where the rover explores. At shallow depths, RIMFAX can detect fine layering that may include sediments, ice, and rock, including water-bearing minerals. These minerals can provide clues that ancient Martian environments once had conditions suitable for life. This information helps pinpoint areas for deeper study by instruments on the rover that search for chemical, mineral, and structural signs of past microbial life.

Geologists have learned much about Earth's past by drilling boreholes ranging in depth from hundreds of feet to several miles below the surface. Deep drilling and coring has allowed scientists to reach geological records of the bombardment of our planet by debris earlier in the solar system's history, and to gather evidence about the tectonic, climate and biological cycles of past eons. With our small robotic landers and rovers, it is not possible to drill very deeply into the surface of Mars. Ground-penetrating radar is the next best thing for learning about what's underground. The data RIMFAX provides fills in a big blank in the study of Mars. It provides a view of the structure and composition of the ancient layers in areas the rover roams.

RIMFAX provides important reconnaissance for other instruments on the rover. Through RIMFAX's ground-penetrating radar "eye," the Mars 2020 team can "see" below the surface and identify interesting buried bedrock features that may be partly exposed on the surface. This data is important because much of Martian surface is covered by windblown dust that obscures the bedrock. Bedrock contains so much history about habitable past environments that once existed on Mars. RIMFAX gives other Mars 2020 instruments the chance to study and sample rock outcrops that would otherwise be hidden beneath the surface.

The Mars 2020 rover searches for rock outcrops where signs of past life might remain. RIMFAX helps in this effort by identifying outcrops that appear to have spent less time exposed to radiation on the surface. Chemical and mineral signs of past life are best preserved in rocks that have been only recently exposed to the destructive radiation from space that constantly bombards the Martian surface. RIMFAX provides valuable information about the past surface exposure history of sedimentary rock layers that it sees underground.

The RIMFAX team assembles ground-penetrating radar profiles by stacking successive radar soundings to create a two-dimensional subsurface image. They can then analyze and interpret these profiles and features to understand what lies beneath the surface.

The RIMFAX science team can produce preliminary profiles of subterranean Mars within minutes after receiving the data. The rover team can use this information to support mission planning on a daily basis.

It can take days, weeks, or longer for RIMFAX's science team to produce scientifically analyzed geological profiles. They need to calibrate these profiles for the depth of the radar's penetration. Delivery of those findings depends on the complexity of subsurface conditions encountered by the radar.

RIMFAX uses an advanced design based on a series of ground-penetrating radar for seeing Mars-like layers of rock, ice, sediment, and water beneath the frozen glaciers and tundra of Earth's polar regions. Scientists used a recent version of the radar to find buried glaciers in Antarctica.

RIMFAX developer and principal investigator Dr. Svein-Erik Hamran also developed and tested the prototype for the WISDOM ground-penetrating radar instrument onboard the 2020 ExoMars rover mission planned by the European Space Agency (ESA). Dr. Hamran is a co-principal investigator on the WISDOM science team.

Previous radar experiments at Mars have operated from spacecraft in orbit. Orbital radar instruments at Mars include a U.S. instrument called MARSIS, onboard ESA's Mars Express spacecraft. MARSIS penetrated as deep as 2.3 miles (3.7 kilometers) to reveal underground Mars on a broad scale, including layered deposits in the south polar region of Mars. This radar typically detects structures that are hundreds of yards (meters) in size, but under special settings can see structures several yards (meters) in size.

The Shallow Radar (SHARAD) on NASA's Mars Reconnaissance Orbiter looks for liquid or frozen water in the first few hundreds of feet (up to 1 kilometer depth). SHARAD can see stadium-size differences in underground structures up to two-thirds of a mile (1 kilometer) beneath the surface.

Engineers optimized the design of RIMFAX to meet the goal of gathering high-quality radar measurements of small-scale layering and other features in the Martian subsurface.

To meet this goal, they designed RIMFAX to operate over the widest possible range of radio wave frequencies (from 150 to 1200 megahertz). With this capability, the RIMFAX team can select the transmission frequency based on how deeply they want the signal to penetrate.

In the shorter wavelength mode, RIMFAX can focus on studying the materials and structures present in the upper few inches or feet of the surface.

Longer wavelength radio waves are more effective in reaching deeper into the ground than shorter wavelength radio waves. Longer wavelengths are less affected by absorption or scattering. They provide more detail than shorter wavelengths when they encounter rough surfaces. Used in longer-wavelength modes, RIMFAX can see more detail, more deeply under the surface, than is possible at shorter wavelengths. The result is more detailed views of the structures that make up layers below the Martian surface.