ExoMars Spots Water Frost on Ceraunius Tholus Volcano’s Caldera Floor

Scientists have made a surprising discovery of frost on volcanoes near Mars’s equator, a region previously thought too warm for frost to form. This breakthrough was achieved using data from two instruments on the ESA’s ExoMars Trace Gas Orbiter (TGO) – CaSSIS and NOMAD – along with additional images from ESA’s Mars Express.

The image in question shows frost on the caldera floor of the Ceraunius Tholus volcano, highlighted in blue. This blue hue is a result of how CaSSIS captures images, combining near-infrared and visible light channels to create what is known as an ‘NPB’ image, different from the usual RGB (red-green-blue) images. This method uses near-infrared (N), panchromatic (P), and blue (B) filters, providing detailed information about the spectral diversity of features that are invisible to the naked eye.

The four frames in the image sequence include:

• (A) A wide-angle view of Ceraunius Tholus from NASA’s Mars Reconnaissance Orbiter’s Context Camera, with early morning observations by CaSSIS overlaid in a blue-toned rectangle.
• (B) A close-up of the blue rectangle, with a white rectangle marking an even more zoomed-in area.
• (C) A detailed view showing frost on the caldera floor but not on the rim.
• (D) A CaSSIS image of the same area taken at a different time of day, showing no frost.

Both images (B) and (D) were taken using similar settings, indicating that the frost’s appearance is not due to lighting effects. Image (B) was captured in early northern spring, while (D) was taken in late northern winter.

In all frames, north is at the top, and scales are provided in the bottom-right corners. The Local Solar Time (LST), incidence angle (i), and solar longitude (Ls) are also shown. The incidence angle tells us the Sun’s position in the sky, with 0° meaning the Sun is directly overhead and 90° at the horizon. Solar longitude indicates Mars’s position in its orbit around the Sun, marking the seasons for each hemisphere (0° at the start of northern spring, 270° at winter solstice).

According to the study, the frost appears only for a few hours after sunrise before evaporating in the sunlight. It is extremely thin, likely just one-hundredth of a millimeter thick, about the width of a human hair. Despite its thinness, the frost is extensive, with researchers estimating it amounts to at least 150,000 tons of water cycling between the surface and atmosphere daily during the cold seasons. This is equivalent to about 60 Olympic-size swimming pools.

The Tharsis region of Mars, where the frost was found, is home to many volcanoes that rise one to two times higher than Earth’s Mount Everest. Olympus Mons, for example, is as wide as France. The frost is located in the calderas of these volcanoes, which are large hollows at their summits formed by past eruptions. Researchers suggest that the unique air circulation above these mountains creates a special microclimate that allows the thin frost patches to form.

For more information, you can refer to related links about water frost detected on the Tharsis volcanoes and newfound frost atop the Martian volcano Olympus Mons.