While we are out in the field pretending to be Mars rovers, we need to be able to gather the same kind of data that we acquire on Mars. There are many impressive instruments on both Spirit and Opportunity – the Mars Exploration Rovers (MER) – as well as Curiosity – the Mars Science Laboratory (MSL). For this week, we are limited to those instruments which we are capable of taking out into the field, and many of the MER and MSL instruments do not have field-portable analog instruments here on Earth.
Fortunately, we have two field-portable instruments that are excellent analogs for those on board the current Mars rovers:
Visible Near Infrared Spectroscopy (VNIR)
VNIR spectroscopy measures the spectrum of light that is reflected by a material between 0.35 and 2.5 um (350 – 2500 nm). This range of wavelengths covers all of the visible light spectrum (0.38 to 0.78 um) and into the near-infrared wavelengths.
Every material on Earth (or Mars, in this case) has its own characteristic pattern in the wavelengths of light that it reflects. A spectrometer measures the reflected light coming off a material, and displays it as a reflectance spectra:
The dips in the spectra are called absorption bands, and are due to the material – whatever it may be – absorbing light at that particular wavelength. The wavelengths that a material absorbs are determined primarily by its chemical composition, crystal structure, and grain size. By comparing the reflectance spectra from a material of unknown composition – such as a rock on Mars – to reflectance spectra of known composition (called library or reference spectra), we can match each of the absorption features and determine the composition of our mystery material.
VNIR on Mars
VNIR is a commonly used tool for planetary science, and is used at a variety of scales. VNIR instruments are on satellites, such as the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on the Mars Reconnaissance Orbiter (MRO), and the Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA) on the Mars Express (MEx) orbiter. VNIR from orbit allows planetary scientists to gather compositional data across a wide area, up to a resolution of about 18 m/pixel (for CRISM). The ability to measure large areas provides excellent context for in-situ (ground-based) measurements, allows us to gather compositional data where we don’t have ground-based rovers, and can show large scale variations in surface composition. However, orbital VNIR measurements do not allow us to observe small-scale details, since all the materials in that 18 meter wide pixel add together into one spectra, and often things are lost. Additionally, orbital measurements have to deal with the effects of looking through Mars’ atmosphere. The gas (primarily CO2) and dust in the atmosphere also absorb light energy and need to be corrected for before any surface compositions can be identified.
In addition to orbital measurements, VNIR is also incorporated into each of the current rover cameras. Both the MER PanCam and MSL ChemCam (via the passive LIBS function) are capable of taking images in the VNIR spectral window. These ground-based instruments allow planetary scientists to investigate rock compositions on a much finer scale than what we can see from orbit, which allows us to delve deeper into the geologic history of Mars. These rover-mounted spectrometers are the ones we are simulating in Utah.
Our instrument: Field-portable VNIR
The VNIR spectrometer we will be using this week is the TerraSpec Halo from ASD, Inc. This instrument also measures reflectivity – just like the MER PanCam and MSL ChemCam, and allows us to make the same types of geological interpretations as the planetary scientists who are working with the Mars rover data.
To sample a rock, the sensor of the Halo gun is placed in contact with the material. When the trigger is pulled, the Halo shines a light onto the rock, and measures what wavelengths are reflected back. The internal computer then cycles through all the library (known) spectra and finds those that best match the sample. Very often, more than one spectra can be considered a match, since rocks often consist of more than one mineral. If further analysis (or confirmation) is needed, the spectra are then exported and manually investigated in the spectral analysis software, ENVI.
MER and MSL rovers are capable of gathering VNIR spectra across a wide field of view, such as a large rock outcrop, while the Halo’s contact probe only samples a small circle (about 1 cm diameter). To simulate PanCam and ChemCam’s ability to gather data on the outcrop scale, we will gather several spectra both horizontally and vertically across any outcrops of interest. This will allow our science team to see any spatial variations in the mineralogy, just like we do on Mars.
X-Ray Diffraction (XRD)
X-Ray diffraction also measures the mineralogical composition of a sample, but through a different process, and on a very small scale. To analyze a sample with XRD, it must first be ground into a powder. On Mars, this can be done with the MSL drill. The powdered sample is then loaded into the XRD instrument, where it is bombarded with x-rays.
When a sample is shot with x-rays, the majority of the x-rays are scattered. But when the x-rays hit the sample at just the right angle (which is dependent on the crystal structure of the material), the diffracted x-rays result in constructive interference. (Start here for a more detailed description, including Bragg’s Law.) This constructive interference is recorded as peaks in the resulting diffraction pattern, such as the one seen here:
Just like with VNIR, each mineral has its own characteristic diffraction pattern. For XRD, the diffraction pattern is dependent on the crystal structure of the mineral – specifically, the spacing of the atoms (the “d-spacing”) within the crystal. Because XRD is so sensitive to slight variations in the atomic structure of a mineral, it is useful for identifying elemental substitution, which can subtly alter the d-spacing within a crystal.
XRD is a commonly used tool for sample identification because it provides a bulk rock analysis (the act of grinding up the rock should homogenize the sample), does not require a large amount of material for analysis, and can be used to identify complex samples that may contain several different minerals. One place XRD tends to fall short is phyllosilicate (clay) identification. The complex and often poorly-ordered crystal structure of phyllosilicates, combined with the abundance of elemental substitution within their structures makes identification very difficult with this analytical method. Often, XRD is used in conjunction with VNIR (which is excellent for phyllosilicate identification) in order to get a more complete analysis.
XRD on Mars
The MSL CheMin instrument is currently conducting XRD analysis in Gale Crater. CheMin has the ability to do 74 XRD analyses, and possibly more, since the sample cells (where the powder is loaded to be analyzed) can potentially be re-used beyond their original design.
For analysis, powdered drill tailings are loaded into the body of the rover where CheMin is located. A small amount of sample is loaded into a sample cell and inserted into the instrument, where it is shot with a high-powered x-ray and the diffractogram is gathered.
Our instrument: Field-portable XRD
The XRD instrument we will be using this week is the Terra Portable XRD from Olympus. This instrument was designed in conjunction with CheMin, with the goal of being a functional analog instrument for planetary geologists to use here on Earth.
To analyze a sample with the Terra, it must first be ground into a powder. Once the sample is powdered, a small amount (~ 15 mg) is loaded into one of the sample windows, and then inserted into the instrument. While the sample is being hit with x-rays, the Terra also vibrates it to ensure that the crystals are being hit from every possible angle, which will provide the most accurate diffraction pattern.
During analysis, the diffraction pattern is displayed on the associated laptop, which connects to the Terra via its own wireless signal. Once analysis is complete, the diffraction pattern may be saved, and further analyzed in software such as XPowder.
Other 2016 GHOST rover posts: