UPDATE 3-U.S. Supreme Court's Jackson emerges as energetic questioner on first day

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Jackson is Supreme Court's first Black woman justice

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Court hears arguments in environmental case from Idaho

(Recasts first paragraph, adds quotes by Justice Jackson)

By Andrew Chung and Nate Raymond

WASHINGTON, Oct 3 (Reuters) - President Joe Biden's history-making appointee Ketanji Brown Jackson settled into her role as the Supreme Court's newest justice on Monday by posing frequent questions during arguments on her first day of hearing cases, as the top U.S. judicial body launched what promises to be a momentous new nine-month term.

The court, with a 6-3 conservative majority that has shown increasing assertiveness including in June rulings curtailing abortion access and expanding gun rights, heard about three hours of arguments in an important environmental case and a dispute among states over unclaimed property.

Members of the public were allowed into the ornate courtroom to watch the arguments for the first time since early in the COVID-19 pandemic in 2020, but at a reduced capacity.

Jackson, the first Black woman to serve on the court, asked more than 20 probing questions and pointed follow-ups, appearing to demand clarity from the lawyers arguing the two cases. She is the sixth woman ever to serve on the court. For the first time, four women serve together - Jackson, Amy Coney Barrett, Elena Kagan and Sonia Sotomayor - as well as two Black justices - Jackson and Clarence Thomas.

As the newest justice, Jackson sat on the far right of the bench in the ornate courtroom, next to conservative Justice Brett Kavanaugh. She was confirmed by the Senate in April to replace now-retired Justice Stephen Breyer as one of the court's three liberal members.

While Breyer was known for peppering attorneys with queries involving sometimes outlandish hypothetical scenarios as he sorted through complex legal matters, Jackson focused on the meaning of the laws at issue in each case.

In the environmental case, a married couple from Idaho is seeking to limit the scope of the federal Clean Water Act and the EPA's regulatory authority.

Jackson told a lawyer for the plaintiffs that "the objective of the statute is to ensure the chemical, physical and biological integrity of the nation's waters," asking him: "So are you saying that neighboring wetlands can't impact the quality of navigable waters?"

During her confirmation hearings last March, Jackson said she would bring to the Supreme Court her life experiences and perspectives including time as a judge, a court-appointed lawyer for criminal defendants who could not afford an attorney, a member of a federal commission on criminal sentencing and "being a Black woman, lucky inheritor of the civil rights dream."

In the Idaho case, the justices considered for a second time the couple's bid to build on property that the U.S. government has deemed a protected wetland. A lower court ruled against them.

At issue in the case is the test courts should use to determine when property is subject to regulation, requiring owners to obtain permits in order to carry out construction.

Jackson, at times sipping from a white thermos, sparred with the couple's lawyer, Damien Schiff, over his assertion that they did not know the property they had purchased was a wetland.

"Shouldn't they have gathered information about the property prior to purchasing it?" Jackson asked.

In the second case, the justices heard arguments in a dispute between Delaware and a coalition of other states to determine which states are entitled under a 1974 federal law to take possession of certain unclaimed checks issued by MoneyGram, one of the world's largest money transfer companies.

Jackson again asked questions focused on the intent of the U.S. Congress in writing the law.

NEW CASES

Before hearing arguments on Monday, the court announced some new cases it will hear during the term including a challenge to federal protections for internet and social media companies against liability for content posted by users.

The court also turned away other appeals including a bid to reverse a federal ban on devices called "bump stocks" that enable semi-automatic weapons to fire like a machine gun, and a challenge by Missouri and nine other states - mostly Republican-led - to Biden's COVID-19 vaccine mandate for workers in healthcare facilities that receive federal funds.

Mike Lindell, a prominent ally of former President Donald Trump, must face a $1.3 billion lawsuit by Dominion Voting Systems Inc accusing him of defamation, with the justices turning away his appeal.

On the term's second day on Tuesday, the justices are set to hear arguments in an Alabama case that threatens to cripple the 1965 Voting Rights Act, which prohibits racial discrimination in voting. On Oct. 31, the court hears another major race-related case that gives its conservative majority a chance to end affirmative action admissions policies used by many colleges and universities to increase their numbers of Black and Hispanic students.

(Reporting by Andrew Chung and Nate Raymond; Editing by Will Dunham)