A Tale of Two Missions: Mars Odyssey satellite

The exploration of Mars by unmanned satellites and landers is proceeding at a brisk pace. Over the last few years, several impressive pieces of hardware have been sent to Mars, and the science that they are sending us is filling in many of the gaps in our knowledge of the red planet. Early missions sometimes interact with later ones, continuing to contribute to the overall effort in one way or another.

This is the story of two such missions, so closely linked that they might be viewed as two parts of the same whole. They are the 2001 Mars Odyssey satellite and the Phoenix lander. As we will see, these two projects were designed to work together, with Odyssey paving the way for Phoenix, then serving as its communication relay. They worked together toward a single goal: proving that there are large deposits of frozen water beneath the Martian surface. In this, they were successful.

This article is the first of two parts. In this one, we will look at the 2001 Mars Odyssey satellite and the science it accomplished. In our next article, we will look at the Phoenix lander. As we will see, their combined effect is a new and better understanding of Mars and its water processes.

The 2001 Mars Odyssey was launched on April 7, 2001 from Cape Canaveral, Florida. It was an ungainly assembly of rods, panels, antennae and other devices, but in general terms, it measured 2.2 meters long, 1.7 meters tall and 2.6 meters wide. At launch, it weighed 725 kilograms, which included the 331.8-kilogram spacecraft, 348.7 kilograms of fuel and 44.5 kilograms of instruments. In an effort to keep the weight down, the satellite’s designers built its framework mostly from aluminum and titanium.

Mars Odyssey carried three instruments:

1. Thermal Emission Imaging System (THEMIS)- would acquire high spatial and spectral resolution images of the surface mineralogy, and provide information on the morphology of the Martian surface. Different elements radiate thermal energy in identifiable patterns, so by studying the thermal emission of the Martian surface, it’s possible to determine which elements are present. A thermal survey would also be able to locate areas of volcanic activity, as well as geothermal zones similar to Yellowstone Park on Earth.

2. Gamma Ray Spectrometer (GRS)- would also contribute to a map of the elemental composition of the surface, and determine the abundance of hydrogen in the shallow subsurface. Hydrogen is used as an indicator of the presence of water.

3. Mars Radiation Environment Experiment (MARIE)- would characterize the Martian near-space radiation environment as related to radiation-induced risk to human explorers.

The two-part plan, mentioned earlier, was present from the beginning. Odyssey was intended to locate areas where frozen water might be present in preparation for a lander which would go down and actually take samples. By mapping the surface morphology and mineralogy, rough areas would hopefully be eliminated from the list of possible landing sites. Once the lander was on the ground, Odyssey would act as its relay to send data back to Earth.

In addition to the presence of hydrogen as an indicator of water, it was expected that the thermal survey would find other signs such as sedimentary deposits of water-soluble minerals in areas where underground ice could have melted and come to the surface at some time in the past. This would provide a long-term history of water activity on the Martian surface.

It is now generally accepted that Mars had large amounts of surface water in its distant history; many of the planet’s land-forms were obviously shaped by flowing water. However, recently there has been a growing body of evidence for the presence of deposits of frozen water on Mars now, not just in the past. For instance, data gathered by the European Space Agency’s Mars Express probe, as discussed in our article from a few weeks ago, strongly indicates that there are large amounts of water frozen just under the surface in some parts of Mars.

Once Mars Odyssey had found some interesting areas, a landing site would be chosen for the lander. This was another job where THEMIS would be useful. Big chunks of rock tend to absorb more heat than the surrounding soil, and retain it longer. Because of this, rocky places would be hotter, and would clearly show up in the thermal survey. Rocky areas tend to be rougher than sandy areas, which would make them too dangerous to be considered as landing sites.

The science to be done by this mission could be summed up as four main goals:

1. Determine whether life ever arose on Mars. Odyssey did not carry instruments to directly detect life, but data gathered by this mission would help to determine whether the Martian environment could have ever supported life. For the first time on Mars, a probe was equipped to map the presence of near-surface water and mineral deposits from past water activity.

2. Characterize the climate of Mars. Odyssey would try to understand the evolution of the Martian climate, and how water activity has effected that evolution.

3. Characterize the geology of Mars. Odyssey would determine the chemical elements that make up the Martian surface, and help explain how the planet’s land-forms developed over time. That information should provide clues to the geological and climatic history of Mars and the likelihood of finding past or present life.

4. Prepare for human exploration. Part of the Odyssey mission, as we mentioned earlier, was the Mars Radiation Environment Experiment. This would determine the levels of harmful radiation on the Martian surface, with the thought of preparing future explorers for the hazards they would face.

The Odyssey mission was a huge success, achieving all of these goals and more. Odyssey entered Mars orbit on October 24, 2001. Over the next 76 days, it performed orbital modifications which finally placed it in a two-hour science orbit. Results started coming in almost immediately. Some of the early data from Odyssey’s THEMIS device showed the presence of chloride mineral deposits in the southern Martian highlands. These are salt beds similar to the ones seen in some areas on Earth, and their presence on Mars means the same thing it means here: there was once a lot of water here. In all, THEMIS found about 200 areas with chloride mineral deposits.

The hydrogen mapping part of the mission was also successful, locating areas with elevated hydrogen levels which indicated a high probability of frozen water just underground.

While the early images of the Martian surface were taken from directly above, with Odyssey looking straight downward, later images were obtained by changing the satellite’s orbit and taking pictures of surface features from an oblique angle. By viewing a spot from directly above, and then shifting the orbit and viewing the same spot from an angle, it was possible to construct three-dimensional images of land-forms.

Here’s another important point: these images not only allow the study of the land-forms of Mars, but also of the atmosphere above them. When the light passes through the air, it is modified by the gas molecules and whatever dust and other particles are in the air. Different sizes and types of particles absorb or reflect light in different ways, so if you subtract the information about the actual ground itself, you are left with a picture of the modifications caused by air molecules and suspended particles. This can tell us a great deal about air currents and the movement of dust etc. in the atmosphere of Mars- factors which can have a huge influence on the climate, and which are necessary for a full understanding of the dynamics of the Martian atmosphere.

As we saw earlier, some of this activity had a specific purpose: locating a landing site for a future lander. A suitable site was found, and in time, the Phoenix lander was launched from Earth. This would provide the final, conclusive piece of evidence: an actual sample of ice taken from the Martian soil. Eventually Phoenix arrived and landed in the place selected for it. During its mission, Mars Odyssey provided the communications link which transmitted the data back to Earth.

But that’s another story, as they say. To find out about Phoenix, you’ll have to read our next article.

Odyssey was also the communication relay for the two famous Mars rovers, Spirit and Opportunity, whose ramblings have provided us with such spectacular pictures and data on the Martian surface.

The Odyssey satellite is still working fine, and will undoubtedly perform other jobs relating to future missions. It is the Energizer Bunny of space probes, still going and going even though its official mission is now over. Hang in there, Odyssey!

In our next article, we will take a closer look at the Phoenix lander and the science it has given us. Don’t miss it!

Sources:
Mars Odyssey: Mission Spacecraft at website of the Jet Propulsion Laboratory, California Institute of Technology: mars.jpl.nasa.gov/odyssey/mission/science/
Mars Odyssey: Mission Science at website of the Jet Propulsion Laboratory, California Institute of Technology: mars.jpl.nasa.gov/odyssey/mission/science/

Mars Odyssey: Mission Overview at website of the Jet Propulsion laboratory, California Institute of Technology: mars.jpl.nasa.gov/odyssey/mission/overview/

Mars Odyssey THEMIS: “New Orbit Gives THEMIS Better Looks at Mars Minerals” at website of Arizona State University: themis.asu.edu/news/new-orbit-gives-themis-better-looks-mars-minerals

Mars Odyssey THEMIS: “Sideways Look From THEMIS Probes Mars’ Atmosphere” at website of Arizona State University: themis.asu.edu/sideways

Mars Odyssey THEMIS: THEMIS Helps Phoenix Land Safely on Mars” at website of Arizona State University: themis.asu.edu/news/themis-helps-phoenix-land-safely-mars

Mars Odyssey THEMIS: Mars Salt Deposit Discovery Points to a New Place to Hunt for Life’s Ancient Traces” at website of Arizona State University: themis.asu.edu/news/mars-salt-deposit-discovery-points-new-place-hunt-lifes-ancient-traces