Much media concern in recent months

has focused on the discovery by

NASA’s 2001 Mars Odyssey space-

craft of enormous quantities of water ice just

under the Martian surface – at least enough to

fill Lake Michigan twice over. The results

appeared in the 31 May issue of Science.

Odyssey’s gamma-ray spectrometer instrument

has detected large amounts of hydrogen, a

strong indication that water ice exists in the

upper metre of soil in a large region surround-

ing the planet’s south pole.

The team found that the hydrogen-rich

regions are in very cold areas where ice should

be stable. This relationship between high hydro-

gen content and regions of predicted ice stabil-

ity led the team to conclude that the hydrogen

is, in fact, in the form of a layer of dirty ice.

Buried beneath a shallow overburden of hydro-

gen-poor soil, the ice-rich layer is about 60 cm

beneath the surface at 60°S latitude, rising to

within about 30 cm of the surface at 75°S lati-

tude. The amount of hydrogen detected indi-

cates 20 to 50% ice by mass in the soil, or more

than 50% water ice by volume. “We were hope-

ful that we could find evidence of ice, but what

we have found is much more ice than we ever

expected,” said William Boynton (University of

Arizona). Seasonal carbon dioxide frost cover-

ing the north polar cap has so far obscured the

signature of water ice, but this is expected to

change in the coming months.

Another new result from similar data is that

large areas of Mars at low to middle latitudes

contain slightly enhanced amounts of hydrogen,

equivalent to several percent water by mass.

The team’s preliminary interpretation is that

this modest amount of hydrogen represents

water chemically bound to the minerals in the

soil and/or hydroxyl radicals – one hydrogen

atom bound to one oxygen atom – rather than

water ice. Comparisons between the neutron

spectrometer data from Lunar Prospector and

Odyssey show that the soil on Mars contains

thousands to millions of times more hydrogen

than the lunar regolith (mars.jpl.nasa.gov/

odyssey/ and grs.lpl.arizona.edu).

Meanwhile, preliminary analysis of the thou-

sands of infrared images taken so far by Mars

Odyssey is providing the first direct evidence

that the Martian surface is made of layers with

different physical properties. A mosaic of day-

time infrared images of the layered Terra

Meridiani region shows a complex geology with

craters and eroded surfaces exposing at least

four distinct layers of rock. The differences in

surface temperature could be caused by the fun-

damental differences in either the size of the rock

fragments in the layer, mineral composition or

density of the layers. Philip Christensen (Arizona

State University) suggests that the layers are

caused not by surface effects, but by a fluctuat-

ing subsurface water table. With no clear evi-

dence for surface water, precipitation or run off,

he believes that changes in levels of underground

water percolating through layers of buried sed-

iments could account for differences in rock

composition between layers (www.jpl.nasa.gov/

images/mars/index.html and themis.asu.edu).

HST’s new camera makes its debut

The Hubble Space Telescope is rarely out of the

news so it was hardly surprising when the media

snapped up the first four images taken by the

HST’s new Advanced Camera for Surveys

(ACS). The camera was installed by Shuttle

astronauts during the fourth HST servicing mis-

sion last March. An ACS image of a colliding

galaxy, the Tadpole (UGC 10214), demonstrated

a five-fold improvement in sensitivity compared

with the Wide Field Planetary Camera 2 as the

result of a doubling of the area and resolution.

An unexpected bonus was the enormous num-

ber of distant galaxies visible, giving it an

appearance that resembled the Hubble Deep

Field (HDF) image taken in 1995. Although it

was taken in one-twelfth the time of the HDF,

the ACS image is so sharp that astronomers can

identify building blocks of galaxies and far-flung

colliding galaxies in the field.

By late April, the new cooler installed during

the servicing mission had successfully pumped

most of the heat from the interior of the Near

Infrared Camera and Multi-Object Spectrometer

(NICMOS), maintaining the target temperature

of 70 K to within a few hundredths of a degree.

The first images taken with NICMOS since

1998 were expected by early June.

Other refurbishment efforts during the

Mission update

4.28 August 2002 Vol 43

Pickof thebunch

Peter Bond reports on a mixed bag

of science from space-based

misssions, including more on

Martian water, X-ray flashes to

match gamma-ray bursts and what

the ionosphere does when Earth is

in the path of a solar storm.

1: This infrared image from the Thermal Emission Imaging System on Mars Surveyor shows daytimesurface temperatures, ranging between –20 and 0 °C. Many of the temperature variations are due to slopeeffects, with sunward slopes warmer than shaded slopes. But several rock layers can be seen to havedistinctly different temperatures, indicating that physical properties vary from layer to layer. Thesedifferences suggest that the environment on this part of Mars varied through time as these layers wereformed. The area shown, in the Terra Meridiani region is about 120 km across, at approximately 358°Elongitude and 3°N latitude (NASA/Arizona State University).

servicing mission have also proved a great suc-

cess. The new rigid solar arrays, working with

the new power control unit, are generating 27%

more electrical power than the previous arrays.

This doubles the power that can be allocated to

the scientific instruments on Hubble. The new

reaction wheel is operating normally and the

Space Telescope Imaging Spectrograph and the

Wide Field and Planetary Camera 2 have been

operational since March.

HST sees double

While astronomers look forward to Hubble’s

future, discoveries continue to be announced

based on previous observations. A team of

astronomers led by Christian Veillet of the

Canada-France-Hawaii Telescope (CFHT)

Corporation has been using the orbital obser-

vatory to investigate binary Kuiper Belt objects

on the fringes of the solar system. In the 17

April issue of Nature, they report that the pair

known as 1998 WW3 waltz around each other

in the most eccentric orbit ever measured for

any binary solar system object or planetary

satellite. The orbital distance between the odd

couple varies by a factor of ten, from 4000 to

40 000 km. Their total mass, calculated from

their mutual 570-day orbit, is about 5000 times

less than that of Pluto and its moon Charon. It

is thought that more than 1% of the known

Kuiper Belt objects are binaries, but their ori-

gins remain unclear.

Unveiling gamma-ray bursts

The Hubble Space Telescope has also played a

key role in helping to unravel the mystery of

gamma-ray bursts (GRBs). At least some

gamma-ray flashes are produced when a star

more than eight times the mass of the Sun

explodes at the end of its life. Two independent

teams of astronomers have combined ground-

based and HST observations to discover a

supernova, designated SN 2001ke, hidden

beneath the powerful glow of a gamma-ray

burst. The gamma-ray burst in question, GRB

011121, was detected by the Italian-Dutch

BeppoSAX satellite on 21 November 2001. Ten

hours later, a team led by Kris Stanek (Harvard-

Smithsonian Center for Astrophysics) and Peter

Garnavich (University of Notre Dame) found

the rapidly fading optical afterglow using a

1.3 m telescope in Chile. The team then made

spectroscopic observations with the Magellan

6.5 m Walter Baade Telescope to find the source

in a galaxy more than 6 billion light years away.

While Stanek’s team continued to monitor the

afterglow with Magellan, a group led by Shri

Kulkarni (California Institute of Technology)

used the HST on four separate occasions to

watch for changes in the light emission from the

GRB’s optical counterpart. More than a week

after the burst, when the optical emission from

the GRB afterglow had faded, they were

rewarded with a “bump” in the light output.

This flash of light peaking a week or two after

the gamma rays is the signature of a supernova.

Spectral data from Magellan – the first spec-

trum of a supernova associated with a GRB –

showed that the supernova was initially bluer

than expected and faded faster than compara-

ble stellar explosions. A paper by Garnavich

and Stanek’s team is online at xxx.lanl.gov/pdf/

astro-ph/0204234. The results of the CalTech

team are presented in an abstract at xxx.lanl

.gov/abs/astro-ph/0203391.

X-ray flashes

Gamma-ray bursts have received a lot of atten-

tion from scientists and the public alike, but

astronomers now have evidence for a poten-

tially new breed of burst, an “X-ray flash”.

Lower energy X-ray transients that typically last

less than a minute have been observed for

decades, and it has long been hypothesized that

these events and gamma-ray bursts are related

to the same phenomenon.

New evidence for this idea is based on a series

of short-lived X-ray events that have been

detected at a rate of approximately four per

year. These flashes were first discovered in 1997

by a team led by John Heise (Netherlands

National Institute for Space Research) using the

Wide Field Cameras (WFC) on the BeppoSAX

satellite. Only the lack of detectable gamma-ray

emission distinguished the X-ray flashes from

the ordinary gamma-ray bursts observed with

BeppoSAX, but there was nothing else to link

the phenomena. Fortunately, the much more

sensitive Burst and Transient Source

Experiment (BATSE) aboard NASA’s Compton

Observatory was able to observe ten of the

WFC X-ray flashes simultaneously. BATSE did

indeed detect weak gamma-ray emission from

nine of the ten observed flashes. In order to con-

firm the link, the team compared the gamma-

ray properties of the flashes to those of the

thousands of gamma-ray bursts observed with

BATSE from 1991–2000. The researchers found

that the detailed spectra were very similar, only

shifted to lower (X-ray) energies. They tenta-

tively conclude that X-ray flashes and gamma-

ray bursts are created by similar mechanisms.

Chandra images galaxy collision

A team of astronomers from the UK, United

States and Spain has used the Chandra X-ray

Observatory to provide the best X-ray image yet

of two galaxies in the midst of a head-on colli-

sion that began only 10 million years ago. The

merger has triggered the formation of hundreds

of millions of stars, in an explosion of activity

that sends a “superwind” rushing out of the

galaxy at hundreds of thousands of kilometres

per hour. At the galactic centre, Chandra pin-

pointed an X-ray source at the nucleus of one of

the pre-merger galaxies. Another fainter X-ray

Mission update

4.29August 2002 Vol 43

Space shorts

� NASA’S EXPLORER MISSION PROPOSALS.

In April, NASA selected four proposals

for Medium-class Explorer (MIDEX)

missions from a total of 42 proposals

submitted in October 2001. Each will

receive $450 000 for a feasibility study.

NASA intends to select two of the pro-

posals for full development by early

2003, with launches envisaged in 2007

and 2008. The proposals are:

The Astrobiology Explorer (ABE) – a

cryogenic telescope to determine the

abundance, distribution and identities of

interstellar organic compounds.

The Next Generation Sky Survey (NGSS)

– an infrared telescope designed to survey

the entire sky with 1000 times more sen-

sitivity than previous missions.

The Time History of Events and Macro-

scale Interactions during Substorms

(THEMIS) mission – five microsatellites

to study the onset of magnetic storms

within the Earth’s magnetotail.

The Advanced Spectroscopic and

Coronagraphic Explorer (ASCE) – will

carry three instruments to study physical

processes in the Sun’s outer atmosphere.

� NASA ASSESSES ISS OBSERVATORY.

NASA has also allocated $250 000 to

study US participation in a European

Space Agency observatory on the

International Space Station. The Extreme

Universe Space Observatory (EUSO)

would detect the highest energy cosmic

rays by using the whole of the Earth’s

atmosphere as a particle detector. EUSO

is under study by ESA for flight on the

Columbus module of the ISS, and NASA

would provide the large Fresnel lens for

the telescope.

� VIKING MANAGER JAMES MARTIN DIES.

James Martin, former NASA project

manager for the Viking missions to Mars,

died on 14 April after a long battle with

cancer. He was 81. Martin joined NASA’s

Langley Research Center in 1964 as assis-

tant project manager for Lunar Orbiter.

In recognition of his contribution to this

project, Martin was awarded the NASA

Exceptional Service Medal in 1967. He

went on to oversee the Viking 1 and 2

missions, sent to Mars in 1975. Martin

left NASA to join Martin Marietta

Aerospace in 1976, after the success of

the Viking Lander programme was

assured. He was asked by NASA to come

out of retirement in 2000 and led the

effort to restructure the Mars Exploration

Programme after two successive failures.

source nearby may coincide with the nucleus of

the other galaxy remnant. The X-ray output of

these point-like sources is greater than expected

for stellar black holes accreting from compan-

ion stars, so they could arise from black holes

at the centres of the merging galaxies. These two

remnant sources are relatively weak, supporting

the theory that the extraordinary luminosity of

Arp 220 – about 100 times that of our Milky

Way galaxy – is due to the rapid rate of star for-

mation rather than an active, supermassive black

hole at its centre. However, this situation may

change in a few hundred million years when the

two existing black holes could merge, perhaps

resulting in an active galactic nucleus (chandra

.harvard.edu and chandra.nasa.gov).

Coronal plasma currents

Observations by SOHO and TRACE (Transition

Region and Coronal Explorer), published in the

March issue of Astrophysical Journal, showed

that coronal loops are not static plasma-filled

structures, as had been thought, but rather

hypervelocity currents of plasma squirted

between the magnetic structures in the corona.

Movies made from TRACE images show bright

blobs of plasma racing up and down the coro-

nal loops. SOHO data confirmed that these

plasma blobs were moving at tremendous speeds

– as much as 320 000 km h–1. The winds and

storms are so intense that they are more impor-

tant than gravity in determining the density of

the solar atmosphere. The researchers observed

plasma flows in approximately half of all coro-

nal loops visible by TRACE. Other flows may

be present, but too faint for TRACE to detect.

One explanation of the plasma current could be

uneven heating at the bases of the loop, with

plasma racing from the hotter end to the cooler

end. But the original cause of coronal loop heat-

ing remains uncertain (www.gsfc.nasa.gov/

topstory/20020515tracewind.htm).

Storms erode the ionosphereFlows of a different kind have been observed

by NASA’s Imager for Magnetopause to

Aurora Global Exploration (IMAGE) space-

craft. The observations, published in the

Journal of Geophysical Research, show how

the outer ionosphere, the electrified gas at

300–1000 km above the Earth’s surface, reacts

during solar storms.

During such storms, the ionosphere absorbs

some of the energy generated by the interaction

between the especially energetic solar wind and

Earth’s magnetosphere. The Low Energy

Neutral Atom imager instrument on IMAGE

showed that electrically charged oxygen atoms

were ejected into space immediately in response

to the bursts of ionosphere heating by power-

ful electric currents. According to the US team

which published the observations, the mass of

ionosphere lost during a typical storm is around

a few hundred tons.

Because of their electric charge, the expelled

oxygen ions are trapped within the Earth’s

magnetosphere. When magnetic reconnection

occurs on the night side of the magnetosphere,

the ions are thrown back towards the planet.

The High Energy Neutral Atom imager instru-

ment showed that these ions, now accelerated

to about 4000 km s–1, appear immediately in

the aurora and in the cloud of hot plasma that

encircles the Earth during geomagnetic storms.

The result is a boost to the cloud of hot plasma

within the magnetosphere formed by the solar

wind – IMAGE has shown that the ionosphere

in fact contributes up to half this plasma, which

is a hazard to satellites. This is confirmation

that Earth’s response to solar wind buffetting

supplies a significant amount of the plasma

cloud (www.gsfc.nasa.gov/topstory/20020509

imagessu.html). �

Peter Bond, RAS Press Officer (Space Science).

Mission update

4.30 August 2002 Vol 43

Space shorts

� CASSINI HAZE DISPERSING. Images of a

star taken in early May suggest that a

haze problem on a Cassini camera lens is

clearing up as anticipated. Images of the

bright star Spica showed that at least

90% of the image diffusion originally

caused by the lens haze had been cor-

rected. The improvement may be greater

than this, as the new images were taken at

a temperature warmer than the camera’s

optimal temperature of about –90 °C.

� GENESIS IN THE LOOP. NASA’s Genesis

spacecraft successfully completed its first

loop around the L1 Lagrange point

between the Sun and Earth on 22 May.

Genesis is following a “halo” orbit

around L1 as it collects samples of the

solar wind. The L1 point is a convenient

location because it allows an uninter-

rupted view of the Sun, is outside the

Earth’s magnetosphere and requires few

manoeuvres to maintain the orbit. In

April, Genesis was bombarded by high-

energy protons from a solar storm which

briefly blinded its star tracker, but the

attitude control software took over and

overall performance was unaffected.

� EXPLORING THE SUN–EARTH

CONNECTION. Although solar maximum

has passed, a flotilla of spacecraft contin-

ues to monitor the mood swings of our

nearest star and their influence on Earth.

On 21 April, for example, a powerful

X-class solar flare and associated coronal

mass ejection were observed by several

satellites as part of the NASA-sponsored

Max Millennium programme. The

Transition Region and Coronal Explorer

(TRACE) spacecraft got a close-up look

at the flare and its aftermath, while the

Reuven Ramaty High Energy Solar

Spectroscopic Imager recorded flashes of

X-rays. At the same time, SOHO cap-

tured the big picture. Observatories on

the ground participated too, for example

the Nobeyama Radio Observatory,

Nagano, Japan, which tracked radio emis-

sion. Data from other spacecraft, includ-

ing the Advanced Composition Explorer,

IMAGE, Polar and Wind spacecraft, were

also gathered in order to determine the

effects on the Earth’s magnetosphere and

particle environment. Atmospheric data

from NASA’s recently launched TIMED

(Thermosphere, Ionosphere, Mesosphere,

Energetics and Dynamics) spacecraft pro-

vided additional data on the final link in

the chain of physical processes that con-

nects the Sun and the Earth.

2: The Chandra image shows a bright central region at the waist of a glowing, hourglass-shaped cloud ofmultimillion-degree gas. Further out, spanning 75 000 light years, are giant lobes of hot gas that could begalactic remnants flung outwards by the early impact of the collision.