NASA, ESA, STScI, John T. Clarke (Boston University);
Processing: Joseph DePasquale (STScI)
Mars was once a very wet planet as
is evident in its surface geological features. Scientists know that over the
last 3 billion years, at least some water went deep underground, but what
happened to the rest? Now, NASA's Hubble Space Telescope and MAVEN (Mars Atmosphere and Volatile Evolution) missions are helping unlock
that mystery.
"There are only two places
water can go. It can freeze into the ground, or the water molecule can break
into atoms, and the atoms can escape from the top of the atmosphere into
space," explained study leader John Clarke of the Center for Space Physics
at Boston University in Massachusetts. "To understand how much water there
was and what happened to it, we need to understand how the atoms escape into
space."
Clarke and his team combined data
from Hubble and MAVEN to measure the number and current escape rate of the
hydrogen atoms escaping into space. This information allowed them to
extrapolate the escape rate backwards through time to understand the history of
water on the Red Planet.
Escaping Hydrogen and "Heavy Hydrogen"
Water molecules in the Martian
atmosphere are broken apart by sunlight into hydrogen and oxygen atoms.
Specifically, the team measured hydrogen and deuterium, which is a hydrogen
atom with a neutron in its nucleus. This neutron gives deuterium twice the mass
of hydrogen. Because its mass is higher, deuterium escapes into space much more
slowly than regular hydrogen.
Over time, as more hydrogen was
lost than deuterium, the ratio of deuterium to hydrogen built up in the
atmosphere. Measuring the ratio today gives scientists a clue to how much water
was present during the warm, wet period on Mars. By studying how these atoms
currently escape, they can understand the processes that determined the escape
rates over the last four billion years and thereby extrapolate back in time.
Although most of the study's data
comes from the MAVEN spacecraft, MAVEN is not sensitive enough to see the
deuterium emission at all times of the Martian year. Unlike the Earth, Mars
swings far from the Sun in its elliptical orbit during the long Martian winter,
and the deuterium emissions become faint. Clarke and his team needed the Hubble
data to "fill in the blanks" and complete an annual cycle for three
Martian years (each of which is 687 Earth days). Hubble also provided
additional data going back to 1991 – prior to MAVEN's arrival at Mars in 2014.
The combination of data between these missions provided the first holistic view of hydrogen atoms escaping Mars into space.
These are far-ultraviolet Hubble images of Mars near
its farthest point from the Sun, called aphelion, on December 31, 2017 (top),
and near its closest approach to the Sun, called perihelion, on December 19,
2016 (bottom). The atmosphere is clearly brighter and more extended when Mars
is close to the Sun.
Reflected sunlight from Mars at these wavelengths shows scattering by
atmospheric molecules and haze, while the polar ice caps and some surface
features are also visible. Hubble and MAVEN showed that Martian atmospheric
conditions change very quickly. When Mars is close to the Sun, water molecules
rise very rapidly through the atmosphere, breaking apart and releasing atoms at
high altitudes.
NASA, ESA, STScI, John T. Clarke (Boston University);
Processing: Joseph DePasquale (STScI)
A Dynamic and Turbulent Martian Atmosphere
"In recent years scientists
have found that Mars has an annual cycle that is much more dynamic than people
expected 10 or 15 years ago," explained Clarke. "The whole atmosphere
is very turbulent, heating up and cooling down on short timescales, even down
to hours. The atmosphere expands and contracts as the brightness of the Sun at
Mars varies by 40 percent over the course of a Martian year."
The team discovered that the escape
rates of hydrogen and deuterium change rapidly when Mars is close to the Sun.
In the classical picture that scientists previously had, these atoms were
thought to slowly diffuse upward through the atmosphere to a height where they
could escape.
But that picture no longer
accurately reflects the whole story, because now scientists know that
atmospheric conditions change very quickly. When Mars is close to the Sun, the
water molecules, which are the source of the hydrogen and deuterium, rise through
the atmosphere very rapidly releasing atoms at high altitudes.
The second finding is that the changes in hydrogen and deuterium are so rapid that the atomic escape needs added energy to explain them. At the temperature of the upper atmosphere only a small fraction of the atoms have enough speed to escape the gravity of Mars. Faster (super-thermal) atoms are produced when something gives the atom a kick of extra energy. These events include collisions from solar wind protons entering the atmosphere or sunlight that drives chemical reactions in the upper atmosphere.
Mars was once a
very wet planet. Scientists know that over the last 3 billion years, some of
the water went underground, but what happened to the rest? Credit: NASA's
Goddard Space Flight Center; Lead Producer: Paul Morris; Mars Animations
Producer: Dan Gallagher
Serving as a Proxy
Studying the history of water on
Mars is fundamental not only to understanding planets in our own solar system
but also the evolution of Earth-size planets around other stars. Astronomers
are finding more and more of these planets, but they’re difficult to study in
detail. Mars, Earth and Venus all sit in or near our solar system's habitable
zone, the region around a star where liquid water could pool on a rocky planet;
yet all three planets have dramatically different present-day conditions. Along
with its sister planets, Mars can help scientists grasp the nature of far-flung
worlds across our galaxy.
These results appear in the July 26 edition of Science Advances, published by the American Association for the Advancement of Science.
Source: NASA's Hubble, MAVEN Help Solve the Mystery of Mars' Escaping Water - NASA Science
No comments:
Post a Comment