Imagine a winter morning when everything is coated in white. Morning frost is evidence of Earth’s water cycle, with dew from the air freezing on the ground in the cold night. Something similar happens on the surface of Mars, 63 million miles or 102 million kilometers away, and scientists can use this to learn more about how water moves across the Red Planet.
One team of scientists, led by Dr. Valantinas at the University of Bern, found what could be morning frost on Mars similar to what we see on Earth. They found this putative frost in bowl-like depressions called calderas at the top of the Tharsis Rise volcanoes. One of these volcanoes, Olympus Mons, is the tallest volcano in our solar system, reaching 21 kilometers or 13 miles in altitude, over 2 and a half times taller than Mount Everest, Earth’s tallest mountain.
Previous researchers estimated that approximately 1 trillion kilograms (around 2.2 trillion pounds) of water vapor circulates in the atmosphere between the northern and southern hemispheres of Mars each year. The Tharsis Rise volcanoes interrupt the movement of this water due to their immense height. The volcanoes create a zone where pressure and windspeed are lower than their surroundings, known as a microclimate. Valantinas’ team focused on this region because the microclimate at the top of the volcanoes creates ideal conditions for frost to form, and the water vapour increases the likelihood of finding water frost.
To spot possible frost, the team looked through thousands of spectral images taken by the Color and Stereo Surface Imaging System, or CaSSIS, on the European Space Agency’s Trace Gas Orbiter, a satellite orbiting Mars. They explained that areas with bright bluish to whitish colors in the CaSSIS images could have frost. They highlighted any images containing frosty colors, then set out to find additional evidence to support the presence of frost.
To do this, the team used a tool that can identify the composition of an object based on wavelengths of light, called a spectrometer. The spectrometer on the Trace Gas Orbiter, called NOMAD, showed ice values at the same time as the CaSSIS images. The researchers combined the CaSSIS images with NOMAD spectrometer values and further imaging from a High Resolution Stereo Camera, to identify 13 cases of frost on the Mars volcanoes.
Valantinas’ team knew the observations indicated frost, but they needed to determine what kind of frost. Mars has a carbon dioxide atmosphere, and carbon dioxide frost can form naturally on the planet’s surface. To determine if the frost was made of carbon dioxide or water, the researchers looked at Martian surface temperatures.
They explained that the frost point of carbon dioxide on Mars is around -130°C or -200°F, meaning that carbon dioxide frost turns from solid directly into gas at warmer temperatures. On the other hand, water has a frost point of only around -90°C or -140°F on Mars. The team used a general circulation model to estimate that the average surface temperature where they spotted frost was approximately -110°C or -170°F, too warm for carbon dioxide frost but cold enough for water frost.
Valantinas’ team observed the frost deposits on the floors and rims of the volcanic calderas. These deposits were absent from the well-lit, warmer regions within the calderas. The team also found that some frost was partially on dust-like particles on the ground. Dust-like particles can get colder at night and warm more slowly in the morning, making them ideal surfaces for frost. They also noticed that frost was only seen in the early Martian morning. The team attributed this to daily warming of the planet’s surface, like on Earth.
Valantinas’ team used images and chemical measurements on Mars to track how water is exchanged between the planet’s atmosphere and surface. The team recommended that future researchers continue to monitor CaSSIS images of these regions to learn more about how morning frost forms on Mars.
For another take on this article, see the summary by University of Delaware student Paige Rebman.
