The Valles Marineris in equatorial Mars and is one of the the largest canyon in the solar system.  It is surpassed in length only by the rift valleys of Earth. (NASA)

That early Mars was much wetter and warmer than it is today has been well established by numerous missions.  Water ice is visible at the poles and many fossil rivers have been found in the southern highlands of Mars.  The Curiosity rover found as well that the large crater where it landed — Gale Crater – once had a lake and in-flowing streams.

But the presence of water, or proof that water once flowed, has been missing in the equatorial latitudes  of the planet.

However, now a paper based on data from the European/Russian Trace Gas Orbiter (TGO) strongly suggests that the Candor Chasma, located near the heart of the massive canyon system called Valles Marineris, has either large deposits of a kind of permafrost water ice just below its surface or of rocks formed in water and now containing that H2O in their structure.

The article to appear in the journal Icarus says that the discovery of large amounts of hydrogen in the region speaks of this aqueous  past.

“We found a central part of Valles Marineris to be packed full of water – far more water than we expected,” Alexey Malakhov, of the Russian Space Research Institute and a co-author of the study, said in a statement.

“This is very much like Earth’s permafrost regions, where water ice permanently persists under dry soil because of the constant low temperatures.”


Valles Marineris, seen at an angle of 45 degrees to the surface in near-true color and with four times vertical exaggeration. The image covers an area of about 400,000 square miles. The largest portion of the canyon, which spans right across the image, is known as Melas Chasma. Candor Chasma is the connecting trough immediately to the north. The digital terrain model was created from 20 images taken by the High Resolution Stereo Camera of the Mars Express Orbiter. (ESA)

Valles Marineris is 10 times longer and 4 times deeper than our Grand Canyon.  Geologists have theorized that Valles Marineris began to open along geological faults about 3.5 billion years ago. The faulting may have been caused by the tectonic activity that accompanied the growth of the giant volcanoes in Tharsis, lying just to the west.

The apparent discovery of such large water features will not necessarily change that view, since branched channels that most likely once carried surface water and hydrated minerals such as sulfates and iron oxide have been detected before and were seen as surface phenomena.

But if the latest finding is confirmed, it will make our understanding of the vast canyon deeper and far more complex.

The Trace Gas Orbiter is part of the ExoMars mission operated by the European Space Agency (ESA) and its Russian counterpart, Roscosmos.

Among the instruments aboard TGO is one called the Fine Resolution Epithermal Neutron Detector (FREND), which can detect hydrogen, one of the two elements that make up water.  It was new analyses of FREND’s data that showed high levels of hydrogen at a site Candor Chasma.

“With TGO we can look down to one meter below this dusty layer and see what’s really going on below Mars’ surface – and, crucially, locate water-rich ‘oases’ that couldn’t be detected with previous instruments,” said Igor Mitrofanov of the Space Research Institute of the Russian Academy of Sciences and lead author of the new study. in a statement. He is also principal investigator of the FREND (Fine Resolution Epithermal Neutron Detector) neutron telescope.

“FREND revealed an area with an unusually large amount of hydrogen in the colossal Valles Marineris canyon system.” he said. “Assuming the hydrogen we see is bound into water molecules, as much as 40% of the near-surface material in this region appears to be water.”

The water-rich area is about the size of the Netherlands and overlaps with the deep valleys of Candor Chasma, part of the canyon system considered promising in the hunt for water on Mars.

Valles Marineris can be seen stretching across this frame, overlaid by colored shading representing the amount of water mixed into the uppermost meter of soil (ranging from low amounts in orange-red to high in purple-blue tones, as measured by TGO’s FREND instrument). The colored scale at the bottom of the frame shows the amount of ‘water-equivalent hydrogen’ (WEH) by weight. As reflected on these scales, the purple contours in the center of this figure show the most water-rich region. In the area marked with a ‘C’ for Candor Chasma,  up to 40% of the near-surface material appears to be composed of water (by weight). (Mitrofanov et al.),

Water ice usually evaporates in this region of Mars due to the temperature and pressure conditions near the equator. The same applies to chemically bound water; the right combination of temperature, pressure and hydration must be there to keep minerals from losing water.

This suggests that some as-yet-unclear mix of conditions must be present in Valles Marineris to preserve the water – or that it is somehow being replenished

“This finding is an amazing first step, but we need more observations to know for sure what form of water we’re dealing with,” adds study co-author Håkan Svedhem of ESA’s ESTEC in the Netherlands, and former ESA project scientist for TGO.

“The finding demonstrates the unrivaled abilities of TGO’s instruments in enabling us to ‘see’ below Mars’ surface – and reveals a large, not-too-deep, easily exploitable reservoir of water in this region of Mars.”

The FREND observations were taken from from May 2018 to February 2021,  and consisted of mapping the hydrogen content of Mars’ soil by detecting neutrons rather than light.

Neutrons are produced when highly energetic particles known as ‘galactic cosmic rays’ strike Mars.  Malakhov said that “drier soils emit more neutrons than wetter ones, and so we can deduce how much water is in a soil by looking at the neutrons it emits.” He said the FREND observing technique brings far higher spatial resolution than previous measurements of this type, enabling the team to see water features that weren’t spotted before.

Artist’s impression of the ExoMars 2016 Trace Gas Orbiter at Mars. (ESA/ATG medialab)

The Trace Gas Orbiter, which deployed in 2016, has as a primary task to search for gases such as methane and phospine, which are often byproducts of life on Earth.

Methane has been detected in surface bursts on Mars by the Curiosity rover at Gale Crater and also by NASA scientists.  But the TGO has not found it at all.

In a series of 2021 articles in the journals Astronomy & Astrophysics and Icarus, the TGO team reported that it had gathered over two and a half years worth of measurements from one of the instruments (called the Atmospheric Chemistry Suite, or ACS) and over a year’s worth of data from the other (known as Nadir Occultation MArs Discovery, or NOMAD), but had found no trace of their target gases, ESA said in a statement. A separate team, which was looking for another possible signature of life in the atmosphere of the planet, the presence of phosphines, was also unsuccessful.

TGO launched in 2016 as the first of two launches under the ExoMars program. The orbiter will be joined in 2022 by a European rover, Rosalind Franklin, and a Russian surface platform, Kazachok, and all will work together to understand, among other science goals, whether life has ever existed on Mars.