CRISM (Compact Reconnaissance Imaging Spectrometer for Mars)
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Revealing Questions

Why should we study Mars?

Lots of reasons!  One is that Mars is made up of the same raw solar system materials that came together to form the planet Earth.  However, on Earth, plate tectonics has recycled almost all of the oldest rocks; consequently, we know little about what Earth was like when it was very young - when asteroids and comets were heavily bombarding the planet, and oceans and life were just getting started. Mars, on the other hand, does not have plate tectonics and so the record of that earliest period is still there for us to see. By understanding the geological evolution of Mars we will come to better understand what early Earth could have been like. Another reason is that Mars is a laboratory for studying weather. Like Earth it has seasons, ice caps, and winter frost. Unlike Earth, it lacks oceans. That makes Mars a simpler test case for models of how weather works.

How does CRISM measure the surface and the atmosphere?

CRISM is a hyperspectral reflectance spectrometer. That means that CRISM measures sunlight that passes through Mars’ atmosphere, hits the surface and interacts with the materials there, then “reflects” back through the atmosphere and into space where the instrument records the amount of light received at each of hundreds of different wavelengths.  Different types of minerals on the surface and dust, ice, and gases in the atmosphere absorb the sunlight at particular wavelengths. The wavelengths where the light is absorbed is a fingerprint of the minerals, ices, and gases on the surface and in the atmosphere. Usually CRISM looks "down" at a target as the MRO spacecraft flies over it. Sometimes, CRISM is pointed at the horizon, or limb, of Mars in order to better see the vertical structure of the atmosphere, like clouds!

What questions does CRISM investigate?

The CRISM investigation strives to answer three major questions. First, where and when were there past environments suitable for life? The evidence being sought is the spectral fingerprints of silicate, sulfate, and carbonate minerals that indicate past liquid water. Second, how does Mars' atmosphere work and how is it different from Earth's? CRISM measures and tracks the amount of dust, ice, water vapor and other trace gases in the atmosphere, and how volatile materials migrate back and forth between polar ice and the atmosphere. Third, what processes formed Mars' crust? CRISM studies this by mapping the geology, composition and layering of surface features. Most of the crust was formed by volcanism and impact cratering, but water also shaped it. How these processes formed the crust is recorded by the minerals present.

How do CRISM's discoveries affect future Mars missions?

A major objective of Mars exploration is to find the rock records of past environments that could have supported primitive life, and to search for evidence of whether or not life formed. CRISM “sees” minerals that formed in past water, enabling scientists to find rock formations that preserve past watery environments. This ability makes CRISM images essential tools for selecting the landing sites for landed missions that explore the record of ancient habitable environments. CRISM data helped to pick the landing site for the Curiosity rover in Gale Crater, and they are used to steer the Opportunity rover to new rock outcrops to explore. In the future, CRISM images will even be used to help to select the site for future return of samples from the surface of Mars back to Earth!

What is spectroscopy?

In general, spectroscopy is the science of measuring the relative intensity of light having different wavelengths. Reflectance spectroscopy, the variety used by CRISM's science team, is the science of using the wavelengths of light at which chemical compounds preferentially absorb energy to infer compositions of those compounds.

What are visible and infrared wavelengths?

Visible wavelengths of light are those wavelengths - corresponding to colors - that we can detect with our eyes, ranging from 400 to 700 nanometers on the electromagnetic spectrum. Infrared wavelengths are longer than visible wavelengths and range from 700 nanometers to 1 millimeter.  The wavelengths of infrared light are longer because infrared photons have lower energy than visible photons, and vibrate at lower frequencies. The wavelengths of light that CRISM detects are from about 400 to 3920 nanometers, and so includes the visible and “near” infrared.  CRISM has two separate detectors that return useful data covering 410-1023 nanometers (the "VNIR" detector), and from 1030 to 3920 nanometers (the "IR" detector).

What is the difference between hyperspectral and multispectral images?

The difference is a little fuzzy. In general, a hyperspectral image has many tens to hundreds of different colors, whereas a multispectral image can have a few to a few tens of colors.  For CRISM, hyperspectral generally refers to images with hundreds of wavelengths, all the way through all 544, whereas multispectral refers to images with tens of wavelengths (usually 72) across both detectors.

Why does CRISM need two separate detectors?

At the time CRISM was built, no single detector was sensitive to light across the full 400 to 4000 nanometer wavelength range. It needed two separate spectrometers that share a common telescope, each with its own detector, to observe both the infrared (IR) wavelengths and visible/ near-infrared (VNIR) wavelengths. Some minerals that CRISM detects are only distinctive at VNIR wavelengths but others have the majority of their diagnostic features at IR wavelengths. Since CRISM was built, new technology has been developed that can respond to CRISM's full wavelength range on a single detector.

What is Mars' atmosphere like?

Mars has a very thin atmosphere made of about 95.3% carbon dioxide, 2.7% nitrogen, 1.6% argon, about 0.1% each oxygen and carbon monoxide, and traces of water vapor. Like Earth, Mars has seasons and polar ice caps. Also like Earth, weather systems, clouds and dust storms form at different times of year. Unlike Earth, atmospheric pressure is so low that liquid water is not stable on the surface over more than half the planet - water formed by melting of ice would boil instantly!

What is planetary geology?

Planetary geology is the study of the geological processes that have occurred or are ongoing on planets, moons, asteroids, and comets.

What is a mineral?

A mineral is a naturally occurring crystalline chemical compound that makes up part or all of a rock.

What types of minerals are found on Mars?

Mostly, Mars' surface contains the same minerals that are found on Earth. The most common are pyroxene, olivine, and feldspar that form volcanic rocks, and iron oxides (which make Mars red) formed by weathering of volcanic/basaltic rocks. Aqueous minerals - formed by interaction with water - such as clays, carbonates, and sulfates are also found on Mars, and tend to be more interesting scientifically because they act as markers for wet environments in the planet’s past. CRISM has also found evidence for a mineral - a form of iron sulfate - that does not occur on Earth because no part of Earth is dry enough to allow the mineral to persist!

What is a gimbal?

A gimbal is a mechanical system that allows the rotation around a single axis. CRISM has a gimbal that allows the instrument to pivot back and forth to track a particular location on the surface as the spacecraft flies over it.  Remember, CRISM takes images that are roughly 10 kilometers (6.21 miles) long over about two minutes, as the MRO spacecraft travels at over 3 km/second at an altitude of 300 km! As MRO is overflying it target, the gimbal compensates for over 99% of MRO's along-track velocity.

What is an emission phase function?

An emission phase function (EPF) is a sequence of multiple measurements of the same target from different viewing angles through the atmosphere over a short time while the Sun’s light source is constant.  This allows better characterization of the dust and gas in the atmosphere that contribute to every CRISM targeted image.

What is albedo?

Albedo is a measure of the amount of light that is reflected from a surface (rock, mineral, soil, etc.) at a particular wavelength.  A dark rock reflects only a small portion of sunlight that hits it, thus it has a low albedo. You can think of albedo as the fraction of light that is reflected from a surface as compared to what would reflect from a perfectly diffuse, white surface.