It's clear from geologic features such as the Tharsis volcanoes, analyses of the martian (SNC) meteorites, multispectral data from orbiting spacecraft, and in situ measurements made by landers that the martian surface is composed primarily of basalt. For decades scientists have also had an appreciation that most of the surface has been broken down into friable materials produced by weathering, impact cratering and explosive volcanism. Over time this complex combination of geologic processes would have resulted in a layered landscape containing brecciated and fluvially reworked sediments. Given that layered, brecciated basaltic material can describe most of the martian surface there are several fundamental questions about the history of Mars that can be asked. What are the environmental conditions that could generate runoff and initiate fluvial erosion on such a surface? How long would it take to create a valley network? How would the physical and chemical characteristics of the surface material change as it was transported by wind and water? We are conducting a series of investigations in the Ka'u Desert of Hawaii in an effort to address such questions.

We will analyze fluvial incision on the Keanakako'i Formation and sedimentary deposits in the Ka'u Desert in order to determine

• The environmental conditions necessary to generate runoff and initiate fluvial erosion on basaltic tephra and bedrock;
• The rate of fluvial erosion on basaltic tephra deposits and lava flows; and
• The changes in physical and chemical characteristics of basaltic material during fluvial and eolian transport.

Our work will have implications for understanding the rate of valley network development on Mars as well as the environmental conditions in which these valleys formed. This work is important for determining the duration and intensity of fluvial processes that operated on early Mars, and it has implications for understanding the history of water on Mars. Our work will provide a better understanding of how basaltic materials breakdown through fluvial and eolian sediment transport processes, which can help us to better understand the physical and chemical characteristics of martian surface materials.

The exposed cross-section within this gulley shows the complex emplacement and erosional history associated with the Keanakako'i Formation. The green unit highlighted to the right consists primarily of the lower vitric unit. The red unit is primarily the upper, lithic unit, which was emplaced in a 1790 eruption. Note how the unit mapped as red drapes over the green unit and fills in the floor of a pre-existing gulley. Subsequent erosion, marked in blue, has reactivated this gulley and eroded through the red unit, resulting in a standing terrace against the far wall as well as several islands within the channel.

This Landsat TM image shows a portion of the Keanakako'i Formation in plan view. In 2005 we began to survey the gullies, beginning with the larges, Sand Wash. Using DGPS equipment we collected a number of longitudinal and cross-sectional profiles (yellow lines). We also collected sediment samples for analyses (red dots).

Microscopic images of samples collected from inside Sand Wash. Sample A was collected near the headwater of the gulley, and Sample B was collected about 740 m downstream. The field of view of each image is 9.3 mm at the 20X magnification setting. Focus was selected to maximize the number of grains in focus within the field of view. Basaltic (or lithic) fragments are opaque and occur in a range of colors depending on the amount of alteration that took place prior to emplacement. Basaltic glass (or vitric) fragments are pale green in color and are translucent. Even over these short transport distances it is apparent that the average particle size has become smaller and, as would be expected, the vitric fragments are quickly breaking down.