SKY_September2014.pdf

SkyandTelescope.com September 2014 23

away. Instead, it’ll fi nd out how the various ways of losing

atmosphere work and interact today. Then scientists can

extrapolate those fi ndings back to the past.

The correspondence between what happened then and

what’s happening now won’t be perfect. The eff ectiveness

of each loss process has likely changed over time — for

example, in the early solar system the Sun put out a

stronger solar wind and more intense ultraviolet radia-

tion, both of which drive atmospheric escape. But the

processes themselves remain the same. If scientists can

fi gure out what the Red Planet’s escape rates are today

and which processes cause the escape, they will be able

to (hopefully) work backward to fi nd out what happened

in the past, explains mission principal investigator Bruce

Jakosky (University of Colorado at Boulder).

The orbiter has two solar-panel-paved wings that curve

slightly to make the craft more aerodynamic. These

stretch from both sides of a thermally shielded box that

wears what looks like a lampshade (its the main commu-

nications antenna). Wingtip to wingtip, the craft spans

11.4 meters (37.5 feet), the length of a school bus.

MAVEN has three instrument packages:

The Remote Sensing Package, an imaging ultraviolet

spectrometer, will determine the global characteristics of

the upper atmosphere and its partially ionized component

(the ionosphere), which are where the solar wind and the

planet’s atmosphere meet.

The Mass Spectrometry Instrument will measure the

composition and isotopic ratios of neutral gas and ions.

The Particles and Fields Package includes six instru-

ments. These will characterize the magnetic-fi eld-infused

solar wind and its interaction with the planet’s upper

atmosphere and localized magnetic fi elds that stick out of

the planet’s crust. Part of this work involves looking at the

characteristics of electrons in the ionosphere and what

the magnetic environment is like.

Daredevil Diving

During its 1-year nominal mission, the orbiter will inves-

tigate how the planet loses ions and neutral gases. During

that year it’ll zip in and out of the atmosphere roughly

2,000 times. When the spacecraft reaches Mars in Sep-

tember (the same month as India’s Mars Orbiter Mission,

or MOM; see page 24), it will enter a highly elliptical orbit

of 4.5 hours that swoops down to 150 km (93 miles) from

the surface — low enough to dip into the upper atmo-

sphere and directly sample its composition. At its farthest

distance of 6,000 km, the spacecraft will image the entire

planet’s disk with its ultraviolet spectrometer.

MAVEN will also take fi ve “deep-dip campaigns”

down to 125 km, diving to the top of the well-mixed lower

atmosphere (a.k.a. the atmosphere in bulk, which Curios-

ity samples from the ground). Each dip is over a diff erent

part of Mars and at a diff erent time of the Martian day,

enabling the team to measure the Sun’s varying infl u-

ence on the atmosphere. But even with the solar panels’

aerodynamic curve the craft can’t take too many dives,

COMETARY TROUBLEMAKER

One complication for MAVEN might be Comet

Siding Spring (C/2013 A1). The comet will pass

138,000 km from Mars on October 19th. Comets

bring lots of debris, and it’s hard to say whether

that will endanger MAVEN, MOM, or the three oth-

er Mars orbiters currently operating. Mission plan-

ners are developing “mitigation plans” — such as

determining which side of the spacecraft is hardiest

so that they can point that side at the dust fl ow and

protect the craft. Read more about the comet at

http://mars.nasa.gov/comets/sidingspring.

Photon Ion

Molecule Electron

Atmosphere

Solar wind Magneticfield lines

One of the ways the solar wind can steal a planet’s atmosphere

is by a process called sputtering. In sputtering, the atmosphere

actually works against itself. First, high-energy ultraviolet pho-

tons from the Sun knock electrons out of atmospheric atoms and

molecules, forming electrically charged ions in the Martian upper

atmosphere (1). These ions are picked up by the solar wind (2),

which is infused with the Sun’s magnetic fi eld. The ions circle

around the magnetic fi eld lines. As the fi eld-carrying solar wind

moves by, it drags these ions with it. Some of these ions, as they

are circling, are fl ung back into the upper atmosphere at high

velocity (3). There, they collide with neutral atoms and molecules

and knock them every which way, like the cue ball scatters the

other billiard balls in a break shot in pool. Some of the atoms are

knocked upward with enough velocity to escape Mars — in other

words, they’re “sputtered.”

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