CRISM team members are analyzing data from two new pole-to-pole sets of vertical scans of Mars' atmosphere, to provide detailed measurements of how trace gases and clouds vary with height above the Red Planet's surface. The scans of Mars’ limb were completed on Friday, July 10, and are the first in what will be a series of "limb scans" conducted over the course of two orbits, approximately every other month.

By rotating on its gimbal, CRISM can scan a narrow swath of the planet’s edge (the limb) from the surface up to an altitude of about 120 kilometers (75 miles), capturing a detailed profile of the dust and gases throughout the atmosphere.

The vertical profile reveals the scattering and absorption properties of the atmospheric constituents at altitude increments of about 400 meters (1,300 feet or about a quarter-mile). From those properties, we can identify what’s in the atmosphere—such as water vapor, ozone, carbon monoxide, dust and ice clouds—and determine their vertical distribution.

Limb scans are routinely done for environmental monitoring of Earth's atmosphere, but few have been done of Mars’ thin and highly variable atmosphere at the visible and infrared wavelengths measured by CRISM. And no limb scans at any wavelength have had such high resolution.

For the time being, we plan to scan the same regions repeatedly. We’ve chosen two south pole to north pole swathes that cover a variety of terrain features that influence cloud formation, such as high volcanoes and large, low-elevation impact basins.

These two views show an example limb scan in false color, as it appears at infrared wavelengths (top) and visible wavelengths (bottom). On each of the orbits devoted to limb scans, a series of 24 such images of Mars’ limb was acquired as the spacecraft traveled south to north over the planet’s day site. The smallest feature visible is about 400 meters in scale.

Why conduct limb scans?

When making a targeted observation, CRISM looks at the planet’s surface and collects light reflected from the ground, as well as light scattered from all levels of the atmosphere.

We can sort out the contributions of the atmosphere and surface during data processing, but the minor contributions from the thin regions of the upper atmosphere are overwhelmed by reflections from the surface and higher-density regions of the lower atmosphere.

The limb scans will isolate individual altitudes of the atmosphere and clearly show properties of the upper atmosphere. For example, looking straight down through the atmosphere, you might see a cloud but you’d have no way of knowing its altitude. Looking at that same column of air from the side would show where the cloud was, and if it were one of many clouds in cloud decks at different altitudes. Similarly, CRISM can see how much water vapor is in the air, and being able to see how it is distributed vertically will give you information about the humidity and temperature profiles of the atmosphere.

The first limb scan was done early in the mission as a test, and returned impressive results. But since then, the team has been focused mainly on measuring the surface. Now, with more than 10,000 targeted observations of the surface already taken, we’re ready to broaden our measurement strategy to put more emphasis on atmospheric structure.

MRO is currently operating in an extended science phase, and now that all the primary mission objectives have been met, there is greater opportunity for taking different kinds of observations.

Special challenges

Conducting the scans took a great deal of coordination from people across the project because MRO routinely has multiple instruments running around the clock, seven days a week. Before interrupting that routine, the CRISM team wanted to make sure it had figured out all the details of what would be required to aim at Mars’ limb.

Normally, angular constraints built into CRISM's gimbal prevent it from looking at the limb. The flight engineering team got around this limitation by tilting the spacecraft by about 17 degrees. While MRO is in that orientation to conduct a limb scan, some of its other instruments can’t acquire useful data, so we want to minimize interruptions to their operations.

Once the limb scan observations are acquired, they have to flow through the ground system and through the CRISM calibration pipeline. It took a little bit of effort to accommodate the processing of data that were acquired in a non-typical observation geometry. 
 
In a few months, the limb scan data products will become part of a delivery to the Planetary Data System. Browse products may be viewable from the CRISM Web site at some point in the near future.
 

CRISM Team members:
Andy McGovern
Scott Murchie
Hari Nair
Frank Seelos