The ESA-Roscosmos ExoMars Trace Gas Orbiter has spotted significant amounts
of water at the heart of Mars’ dramatic canyon system, Valles Marineris.
The water, which is hidden beneath Mars’ surface, was found by the Trace Gas
Orbiter (TGO)’s FREND instrument, which is mapping the hydrogen – a measure
of water content – in the uppermost meter of Mars’ soil.
While water is known to exist on Mars, most is found in the planet’s cold
polar regions as ice. Water ice is not found exposed at the surface near the
equator, as temperatures here are not cold enough for exposed water ice to
be stable.
Missions including ESA’s Mars Express have hunted for near-surface water –
as ice covering dust grains in the soil, or locked up in minerals – at lower
latitudes of Mars, and found small amounts. However, such studies have only
explored the very surface of the planet; deeper water stores could exist,
covered by dust.
“With TGO we can look down to one metre 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,”
says Igor Mitrofanov of the Space Research Institute of the Russian Academy
of Sciences in Moscow, Russia; lead author of the new study; and 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: 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 Chaos, part of the canyon system considered
promising in our hunt for water on Mars.
Tracking neutrons
Igor and colleagues analyzed FREND observations ranging from May 2018 to
February 2021, which mapped 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; 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,” adds co-author Alexey Malakhov, also of the Space Research
Institute of the Russian Academy of Sciences. “FREND’s unique observing
technique brings far higher spatial resolution than previous measurements of
this type, enabling us to now see water features that weren’t spotted
before.
“We found a central part of Valles Marineris to be packed full of water –
far more water than we expected. This is very much like Earth’s permafrost
regions, where water ice permanently persists under dry soil because of the
constant low temperatures.”
This water could be in the form of ice, or water that is chemically bound to
other minerals in the soil. However, other observations tell us that
minerals seen in this part of Mars typically contain only a few percent
water, much less than is evidenced by these new observations. “Overall, we
think this water more likely exists in the form of ice,” says Alexey.
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
special, 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 the ExoMars Trace Gas Orbiter.
“Regardless of the outcome, 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.”
Future exploration
As most future missions to Mars plan to land at lower latitudes, locating
such a reservoir of water here is an exciting prospect for future
exploration.
While Mars Express has found hints of water deeper underground in Mars’
mid-latitudes, alongside deep pools of liquid water under Mars’ south pole,
these potential stores lie up to a few kilometres below ground, making them
less exploitable and accessible to exploration than any found just below the
surface.
The finding also makes Valles Marineris an even more promising target for
future human exploration missions to the planet. The largest canyon in the
Solar System, Valles Marineris is arguably Mars’ most dramatic landscape,
and a feature that is often compared to Earth’s Grand Canyon – despite being
some ten times longer and five times deeper.
“This result really demonstrates the success of the joint ESA-Roscosmos
ExoMars programme,” says Colin Wilson, ESA’s ExoMars Trace Gas Orbiter
project scientist.
“Knowing more about how and where water exists on present-day Mars is
essential to understand what happened to Mars’ once-abundant water, and
helps our search for habitable environments, possible signs of past life,
and organic materials from Mars’ earliest days.”
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 whether life has ever existed on Mars.
Reference:
The evidence for unusually high hydrogen abundances in the central part of
Valles Marineris on Mars, Icarus, Volume 374, 1 March 2022, 114805 DOI: 10.1016/j.icarus.2021.114805
Tags:
Space & Astrophysics