SKY_September2014.pdf

SkyandTelescope.com September 2014 21

Mars

is less than 1% of that at sea level on Earth, equivalent to

being in our stratosphere.

Chemical evidence also indicates that Mars’s atmo-

sphere has mostly disappeared. Elements exist as diff er-

ent isotopes, distinguished by the number of neutrons in

their nuclei. One example is hydrogen (1 proton) and its

heavier and much less common form, deuterium (1 pro-

ton and 1 neutron). An element’s isotopes exist in specifi c

ratios with respect to one another. So when the ratio of

two isotopes in a sample unnaturally favors the heavier,

less common one, scientists can use the ratio to estimate

how much of the more common one must have been lost

in order to produce the levels they detect.

Isotopic studies of Mars’s atmosphere for several ele-

ments, including hydrogen, argon, and carbon, suggest

that the planet has lost between 25% and 90% of its

original atmosphere — with the data leaning toward the

higher end of that range (see the table below).

There are two directions an atmosphere can go when it

disappears: up or down. But as far as down goes, scien-

tists haven’t found a place where all the missing gas could

hide. The two most common compounds outgassed by

rocky planets as they grow their atmospheres are water

(H 2 O) and carbon dioxide (CO 2 ), and these must have

existed in large quantities on early Mars. In the presence

of liquid water, the carbon dioxide would have dissolved,

reacting with rocks to form carbon-bearing minerals

called carbonates. That’s what happened on Earth, and

Martian Isotopic Ratios

& Atmospheric Loss

Chemical isotopes — such as hydrogen and its heavier form,

deuterium — exist in specifi c ratios with one another. When

these ratios are skewed in favor of the less common form, the

change can tell scientists how much of the other isotope was

lost. In this table, the number attached to the element name is

the total number of protons and neutrons in the atom’s nucleus.

Measured Value Amount Lost

Isotopic Ratio (Relative to (%%)

Earth’s)

deuterium/hydrogen 5 60–75

argon-38/argon-36 1.3 50–90

carbon-13/carbon-12 1.05–1.07 50–90

nitrogen-15/nitrogen-14 1.7 90

oxygen-18/oxygen-16 1.025 25–50

The Ancient Atmosphere

Planetary scientists know that ancient Mars looked totally

diff erent than today’s Mars. The oldest craters have had

their edges worn down by precipitation, and gully and

delta features suggest liquid water once fl owed, albeit

perhaps intermittently (S&T: Sept. 2013, p. 16).

Liquid surface water would require higher tempera-

tures and a much higher atmospheric pressure — maybe

between 50 and 200 times the current pressure — than

those on Mars now. These days, the planet’s air pressure

The MAVEN orbiter arrives at Mars in September to investigate

which processes drive the loss of the planet’s atmosphere.

NASA / GSFC

Maven_at_Mars.indd 21 6/23/14 12:17 PM