## 1 Introduction  
There have been a number of successful orbital and landed missions to Mars over the past a few decades. The orbital missions include Mariner 9, Viking orbiter, Mars Global Surveyor (MGS), Mars Odyssey, European Space Agency (ESA)'s Mars Express (MEX), and Mars Reconnaissance Orbiter(MRO) missions. The landed missions include Viking, Pathfinder, Mars Exploration Rover (MER), Phoenix, and the Mars Science Laboratory (MSL) missions. Terabytes of imaging and other data have been acquired from those Mars missions. The size of the image database keeps growing with the current ongoing Odyssey, MRO, MEX, MER and MSL missions. More images are expected from future missions. With the huge amount of high-quality Mars data coming to the planetary science community and the general public, how to help people, especially the geologists, utilize the data has become an important topic. Several interactive tools or online systems have been developed at different institutions to search and visualize the Mars data in 2D [1, 2, 3, 4, 5]. The 2D visualization adds the geographic, visual dimension to the data, but lacks the depth of a 3D visualization. The 3D Visualization of topographic models with commercial tools, such as ArcScene from ESRI(r) ArcGIS, ENVI, ERDAS Imagine, and Surfer provides a better sense of the Mars terrain. These tools let users manipulate 3D models with operations like translation, rotation, zoom in or zoom out. These bring out insights into the data that are not possible with 2D visualizations. For example, observing the subtle changes in elevations with a 3D model. However, the software based 3D visualization on a desktop has no immersion capability and limited interactive functions. The application of virtual reality (VR) techniques to planetary data sets started in early 1985 at NASA AMES Research Center. The Virtual Planetary Exploration (VPE) lab of AMES built a head-mounted, wide-angle, stereoscopic display system to interactively explore the surface of Mars based on Viking images [6, 7]. Building a VR system with navigation, interaction, and/or immersion capabilities for scientific visualization for Mars is cost-effective relative to the high cost and the unfriendly Mars environment of a manned mission to Mars. Using VR techniques to build a computer-simulated 3D environment with remote sensing data at various resolutions will help the study of the environment and evolution of this planet. In the early 1990s, the concept of a Cave Automatic Virtual Environment (CAVE) was developed at the University of Illinois at Chicago's Electronic Visualization Laboratory. CAVE provides an immersive environment by projecting images to multiple walls of a room-sized cube. Various CAVEs have been developed in manufacturing, architecture, urban planning, medicine, simulation, and education. ADVISER (Advanced Visualization in Solar System Exploration and Research) from Brown University was a CAVE application developed for immersive scientific visualization applied to Mars research and exploration [8]. The Fossett Laboratory in the Department of Earth and Planetary science at Washington University in St. Louis also developed a CAVE system to support the planetary studies, including decision making for the MER and Phoenix missions. However, considering the size and cost of a CAVE system, a desktop VR system is more affordable to the researchers and geologists. Desktop VRs have been developed for various planetary applications. [PERSON] _et al._[9] conducted a study about the perceptual errors humans made while judging lunar-like terrain or lunar terrain built from test data and Apollo mission images. They concluded the need to develop a prior-mission VR training tool to calibrate an astronaut's slope, distance, and height perception that would help overcome future navigational difficulties. A virtual microscope library was developed for online delivery of extraterrestrial samples such as lunar samples and Martian meteorites.