📅 When & How

On May 2, 2025, at dawn, NASA's 2001 Mars Odyssey orbiter captured a remarkable image of Arsia Mons—a massive shield volcano—rising above the morning cloud layer on Mars using its THEMIS camera. This marks the first time this phenomenon has been clearly observed and documented.

🌋 Arsia Mons Overview

Arsia Mons is an ancient shield volcano and the southernmost of the Tharsis Montes trio. Standing approximately 12 miles (20 km) tall—nearly twice the height of Earth's Mauna Loa—it boasts a diameter of about 270 miles (450 km) with a summit caldera spanning 72 miles (120 km).

☁️ Cloud Phenomenon

The striking image reveals how moist air ascends the volcano's slopes, cools, and condenses into clouds—a phenomenon especially pronounced during Mars's aphelion (when it's farthest from the Sun). This creates the aphelion cloud belt near the equator. The greenish haze visible in the image is Mars's thin atmosphere illuminated by the rising sun behind the volcano.

🔭 Scientific Insights

Observations of these cloud formations help scientists study Martian mesospheric processes, including downslope winds and hydraulic jumps that cause rapid cooling and cloud formation at high altitude. The presence of these clouds aligns with Mars's southern summer and equinox, offering valuable clues about seasonal and interannual climate patterns on the Red Planet.

🛰️ Mission & Instrumentation

The 2001 Mars Odyssey is the longest-operating orbiter at Mars, launched in 2001 and currently completing over 69,000 orbits. Its Thermal Emission Imaging System (THEMIS)—an infrared/visible camera system created by ASU in collaboration with Raytheon—continues to provide valuable data about Martian surface and atmospheric conditions.

📝 Summary

This rare view of Arsia Mons protruding through Mars's dawn cloud layer provides scientists with a unique natural "atmospheric litmus test," illuminating how orographic processes interact with seasonal and diurnal cycles. The clarity and timing of the image deepen our understanding of Martian climate and highlight the enduring value of long-term planetary observation programs.