Reconnaissance of Mars
Geology equips us with methods to unearth the history. But if the target is beyond our reach for field surveillance, we may have to go take the help of remote sensing equipment to gather information. We have been studying the features of Mars and some surface features have clearly indicated that water once flowed in the red planet. It is also assumed that Mars had a thicker atmosphere once and it might have been geologically active billions of years ago.
Various missions to map the surface features, mineralogy and to capture the images of Mars have been launched to the red planet since the 20th century. Mars is the current home for Pathfinder, Spirit, Opportunity and Curiosity rovers which explore the surface and provide data by direct observation. The images and data gathered by orbiters which explore the red planet have been used to gather information about the early history of Mars.
Mars Reconnaissance Orbiter (MRO) was launched by NASA and it has been one of the most successful missions to study the geologic features of any planet. Equipped with hyperspectral imager such as CRISM, advanced camera such as HiRISE and other advanced equipments, the orbiter has helped to rebuild the history of Mars.
CRISM is an imaging spectrometer with a scannable field of view that can cover wavelengths from 0.362 to 3.92 microns (362 to 3920 nanometers) at 6.55 nanometers/channel. This means that CRISM can observe in both the visible range (0.38 – 0.70 microns) and shorter wavelengths within the infrared wavelength range (the full infrared wavelength range is 0.7- 1000 microns). Being able to detect light in these wavelength ranges enables the CRISM team to identify a broad range of minerals on the Martian surface.
The detectors generate ‘L’ and ‘S’ images based on whether the wavelength lies in visible to near infrared region or visible region respectively. These images are processed through image processing software to remove the effects of atmosphere and to apply the artifact correction. Finally, the spectral plot from the ‘corrected image’ is compared with the mineral signature to identify the mineral.
At visible wavelengths, the way light is reflected is strongly influenced by iron in minerals – for example rust, or iron oxide, appears red. At infrared wavelengths, CRISM can “see” features due to sulfate, carbonate, hydroxyl, and water incorporated into mineral crystals, plus it has greater sensitivity to the kinds of minerals containing iron. None of this is possible at visible wavelengths. It is the coverage of infrared wavelengths that gives CRISM most of its capabilities to map composition of Mars’ surface.
The CRISM instrument consists of:
- The Optical Sensor Unit (OSU), which includes the optics, a gimbal to remove smear due to spacecraft motion, two detectors for visible and infrared images, cryocoolers to cool the infrared detectors, radiators to keep the instrument cold, and electronics.
- The Gimbal Motor Electronics (GME), which commands and powers the gimbal and analyzes data from its angular position encoder in a feedback loop
- The Data Processing Unit (DPU), which accepts and processes commands from the spacecraft and data from the OSU to communicate it to the spacecraft
The data obtained from Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) gives information about the mineral distribution on the surface.
The spectral plot obtained from each pixel can be compared with that of mineral signatures and an estimate of distribution of minerals on the surface can be predicted. This allows us to identify the mineral present on the surface. The distribution of minerals such as (phyllosilicate minerals) can help us to identify whether there was presence of water in the area of observation.
To read more about the technology behind CRISM, you may visit http://crism.jhuapl.edu/ .
(Image Credits- John Hopkins University)