Fine-scale Accumulation Patterns in the Northern Ice Cap of Mars

Thomas Cowan

Thickness Map, Mars North Polar Layered Deposits
Figure 3: ArcGIS gridded thickness map for the vertical distance between reflector a2, and the surface pick (top red line in figure 1).

My project is a study of fine‐scale (typically on the order of 10s of meters) accumulation patterns in the Northern Polar Layered Deposits (NPLD) of Mars. The NPLD makes up the bulk of a large plateau at the north pole of Mars called Palnum Boreum which sits in an enormous sedimentary basin. The deposits are composed almost entirely of water ice and blanket an area about the size of Texas, about 1000 km across and about 3 km thick. Layering within the ice has been attributed to varying amounts of dust and can be seen both with high resolution imagers and with ice penetrating radar, both of which are aboard the Mars Reconnaissance Orbiter (MRO) which has been in Martian orbit since 2006. My honors thesis focuses on using radar data gathered by MROs Shallow Subsurface Radar instrument called SHARAD to map regional accumulation patterns across the entire NPLD.

SHARAD works by emitting an 85 microsecond “chirp” or sweep of radio frequencies from 15 MHz to 25MHz. When the radio waves reach the surface of Mars, some of the energy is reflected and some penetrates the subsurface. SHARAD works particularly well in water ice and, once processed, the data gives us images that look like cross‐sections or slices through the NPLD. These slices are called radargrams. Contrasts in dielectric constant control where reflections occur and SHARAD can resolve vertical changes as small as 9 meters. This is excellent resolution, especially considering the thickness of the NPLD.

Radargram, Mars North Polar Layered Deposits
Figure 1: Radargram showing reflectors mapped in the seismic interpretation software GeoFrame. Thousands of these cross‐sections help characterize the NPLD.

Individual reflectors are mapped across the cap. The time difference between two reflectors is calculated and then converted to depth. This allows the data to be imported into ArcGIS where it can then be gridded into a thickness map. The results of this study indicate that accumulation patterns in the uppermost 250 m of the NPLD are not uniform, and that they are dependent on both elevation and on latitude. It is unclear at this point which of these factors is the dominant determiner of accumulation, but knowing the true accumulation patterns will help climate modelers refine their models. A back‐and‐forth between modeled data and observed data may help constrain the age of the deposits, which is still unknown. The NPLD represents the largest record of Martian paleoclimate,and is the water stored there represents the entire Martian hydrologic system. Understanding the accumulation history is an important step in deducing the climate history of Mars.

Advisor: Jack Holt